Selective gene expression of latent murine gammaherpesvirus 68 in B lymphocytes

Age-associated B cells are long-lasting effectors that impede latent γHV68 reactivation

Age-associated B cells (ABCs; CD19⁺CD11c⁺T-bet⁺) are a unique population that are increased in an array of viral infections, though their role during latent infection is largely unexplored. Here, we use murine gammaherpesvirus 68 (γHV68) to demonstrate that ABCs remain elevated long-term during latent infection and express IFNγ and TNF. Using a recombinant γHV68 that is cleared following acute infection, we show that ABCs persist in the absence of latent virus, though their expression of IFNγ and TNF is decreased. With a fluorescent reporter gene-expressing γHV68 we demonstrate that ABCs are infected with γHV68 at similar rates to other previously activated B cells. We find that mice without ABCs display defects in anti-viral IgG2a/c antibodies and are more susceptible to reactivation of γHV68 following virus challenges that typically do not break latency. Together, these results indicate that ABCs are a persistent effector subset during latent viral infection that impedes γHV68 reactivation.

Age-associated B cells are long-lasting effectors that impede latent γHV68 reactivation

Age-associated B cells (ABCs; CD19⁺CD11c⁺T-bet⁺) are a unique population that are increased in an array of viral infections, though their role during latent infection is largely unexplored. Here, we use murine gammaherpesvirus 68 (γHV68) to demonstrate that ABCs remain elevated long-term during latent infection and express IFNγ and TNF. Using a recombinant γHV68 that is cleared following acute infection, we show that ABCs persist in the absence of latent virus, though their expression of IFNγ and TNF is decreased. With a fluorescent reporter gene-expressing γHV68 we demonstrate that ABCs are infected with γHV68 at similar rates to other previously activated B cells. We find that mice without ABCs display defects in anti-viral IgG2a/c antibodies and are more susceptible to reactivation of γHV68 following virus challenges that typically do not break latency. Together, these results indicate that ABCs are a persistent effector subset during latent viral infection that impedes γHV68 reactivation.

De Novo Polycomb Recruitment: Lessons from Latent Herpesviruses

The Human Herpesviruses persist in the form of a latent infection in specialized cell types. During latency, the herpesvirus genomes associate with cellular histone proteins and the viral lytic genes assemble into transcriptionally repressive heterochromatin. Although there is divergence in the nature of heterochromatin on latent herpesvirus genomes, in general, the genomes assemble into forms of heterochromatin that can convert to euchromatin to permit gene expression and therefore reactivation. This reversible form of heterochromatin is known as facultative heterochromatin and is most commonly characterized by polycomb silencing. Polycomb silencing is prevalent on the cellular genome and plays a role in developmentally regulated and imprinted genes, as well as X chromosome inactivation. As herpesviruses initially enter the cell in an un-chromatinized state, they provide an optimal system to study how de novo facultative heterochromatin is targeted to regions of DNA and how it contributes to silencing. Here, we describe how polycomb-mediated silencing potentially assembles onto herpesvirus genomes, synergizing what is known about herpesvirus latency with facultative heterochromatin targeting to the cellular genome. A greater understanding of polycomb silencing of herpesviruses will inform on the mechanism of persistence and reactivation of these pathogenic human viruses and provide clues regarding how de novo facultative heterochromatin forms on the cellular genome.

KSHV LANA acetylation-selective acidic domain reader sequence mediates virus persistence

Viruses modulate biochemical cellular pathways to permit infection. A recently described mechanism mediates selective protein interactions between acidic domain readers and unacetylated, lysine-rich regions, opposite of bromodomain function. Kaposi´s sarcoma (KS)-associated herpesvirus (KSHV) is tightly linked with KS, primary effusion lymphoma, and multicentric Castleman's disease. KSHV latently infects cells, and its genome persists as a multicopy, extrachromosomal episome. During latency, KSHV expresses a small subset of genes, including the latency-associated nuclear antigen (LANA), which mediates viral episome persistence. Here we show that LANA contains two tandem, partially overlapping, acidic domain sequences homologous to the SET oncoprotein acidic domain reader. This domain selectively interacts with unacetylated p53, as evidenced by reduced LANA interaction after overexpression of CBP, which acetylates p53, or with an acetylation mimicking carboxyl-terminal domain p53 mutant. Conversely, the interaction of LANA with an acetylation-deficient p53 mutant is enhanced. Significantly, KSHV LANA mutants lacking the acidic domain reader sequence are deficient for establishment of latency and persistent infection. This deficiency was confirmed under physiological conditions, on infection of mice with a murine gammaherpesvirus 68 chimera expressing LANA, where the virus was highly deficient in establishing latent infection in germinal center B cells. Therefore, LANA's acidic domain reader is critical for viral latency. These results implicate an acetylation-dependent mechanism mediating KSHV persistence and expand the role of acidic domain readers.

Dangerous Liaisons: Gammaherpesvirus Subversion of the Immunoglobulin Repertoire

A common biologic property of the gammaherpesviruses Epstein–Barr Virus and Kaposi sarcoma herpesvirus is their use of B lymphocytes as a reservoir of latency in healthy individuals that can undergo oncogenic transformation later in life. Gammaherpesviruses (GHVs) employ an impressive arsenal of proteins and non-coding RNAs to reprogram lymphocytes for proliferative expansion. Within lymphoid tissues, the germinal center (GC) reaction is a hub of B cell proliferation and death. The goal of a GC is to generate and then select for a pool of immunoglobulin (Ig) genes that will provide a protective humoral adaptive immune response. B cells infected with GHVs are detected in GCs and bear the hallmark signatures of the mutagenic processes of somatic hypermutation and isotype class switching of the Ig genes. However, data also supports extrafollicular B cells as a reservoir engaged by GHVs. Next-generation sequencing technologies provide unprecedented detail of the Ig sequence that informs the natural history of infection at the single cell level. Here, we review recent reports from human and murine GHV systems that identify striking differences in the immunoglobulin repertoire of infected B cells compared to their uninfected counterparts. Implications for virus biology, GHV-associated cancers, and host immune dysfunction will be discussed.

Murine gammaherpesvirus infection is skewed toward Igλ+ B cells expressing a specific heavy chain V-segment

One of the defining characteristics of the B cell receptor (BCR) is the extensive diversity in the repertoire of immunoglobulin genes that make up the BCR, resulting in broad range of specificity. Gammaherpesviruses are B lymphotropic viruses that establish life-long infection in B cells, and although the B cell receptor plays a central role in B cell biology, very little is known about the immunoglobulin repertoire of gammaherpesvirus infected cells. To begin to characterize the Ig genes expressed by murine gammaherpesvirus 68 (MHV68) infected cells, we utilized single cell sorting to sequence and clone the Ig variable regions of infected germinal center (GC) B cells and plasma cells. We show that MHV68 infection is biased towards cells that express the Igλ light chain along with a single heavy chain variable gene, IGHV10-1*01. This population arises through clonal expansion but is not viral antigen specific. Furthermore, we show that class-switching in MHV68 infected cells differs from that of uninfected cells. Fewer infected GC B cells are class-switched compared to uninfected GC B cells, while more infected plasma cells are class-switched compared to uninfected plasma cells. Additionally, although they are germinal center derived, the majority of class switched plasma cells display no somatic hypermutation regardless of infection status. Taken together, these data indicate that selection of infected B cells with a specific BCR, as well as virus mediated manipulation of class switching and somatic hypermutation, are critical aspects in establishing life-long gammaherpesvirus infection.

Gammaherpesvirus-infected germinal center cells express a distinct immunoglobulin repertoire

Germinal center B cells infected with gammaherpesvirus display altered repertoire with biased usage of lambda light chain and skewed utilization of IGHV genes.

The gammaherpesviruses (γHVs), human Kaposi sarcoma-associated herpesvirus (KSHV), EBV, and murine γHV68 are prevalent infections associated with lymphocyte pathologies. After primary infection, EBV and γHV68 undergo latent expansion in germinal center (GC) B cells and persists in memory cells. The GC reaction evolves and selects antigen-specific B cells for memory development but whether γHV passively transients or manipulates this process in vivo is unknown. Using the γHV68 infection model, we analyzed the Ig repertoire of infected and uninfected GC cells from individual mice. We found that infected cells displayed the hallmarks of affinity maturation, hypermutation, and isotype switching but underwent clonal expansion. Strikingly, infected cells displayed distinct repertoire, not found in uninfected cells, with recurrent utilization of certain Ig heavy V segments including Ighv10-1. In a manner observed with KSHV, γHV68 infected cells also displayed lambda light chain bias. Thus, γHV68 subverts GC selection to expand in a specific B cell subset during the process that develops long-lived immunologic memory.

Vaccine protection against murid herpesvirus-4 is maintained when the priming virus lacks known latency genes

γ-Herpesviruses establish latent infections of lymphocytes and drive their proliferation, causing cancers and motivating a search for vaccines. Effective vaccination against murid herpesvirus-4 (MuHV-4)-driven lymphoproliferation by latency-impaired mutant viruses suggests that lytic access to the latency reservoir is a viable target for control. However, the vaccines retained the immunogenic MuHV-4 M2 latency gene. Here, a strong reduction in challenge virus load was maintained when the challenge virus lacked the main latency-associated CD8⁺ T-cell epitope of M2, or when the vaccine virus lacked M2 entirely. This protection was maintained also when the vaccine virus lacked both episome maintenance and the genomic region encompassing M1, M2, M3, M4 and ORF4. Therefore, protection did not require immunity to known MuHV-4 latency genes. As the remaining vaccine virus genes have clear homologs in human γ-herpesviruses, this approach of deleting viral latency genes could also be applied to them, to generate safe and effective vaccines against human disease.

A comparative epigenome analysis of gammaherpesviruses suggests cis-acting sequence features as critical mediators of rapid polycomb recruitment

Latent Kaposi sarcoma-associated herpesvirus (KSHV) genomes rapidly acquire distinct patterns of the activating histone modification H3K4-me3 as well as repressive H3K27-me3 marks, a modification linked to transcriptional silencing by polycomb repressive complexes (PRC). Interestingly, PRCs have recently been reported to restrict viral gene expression in a number of other viral systems, suggesting they may play a broader role in controlling viral chromatin. If so, it is an intriguing possibility that latency establishment may result from viral subversion of polycomb-mediated host responses to exogenous DNA. To investigate such scenarios we sought to establish whether rapid repression by PRC constitutes a general hallmark of herpesvirus latency. For this purpose, we performed a comparative epigenome analysis of KSHV and the related murine gammaherpesvirus 68 (MHV-68). We demonstrate that, while latently replicating MHV-68 genomes readily acquire distinct patterns of activation-associated histone modifications upon de novo infection, they fundamentally differ in their ability to efficiently attract H3K27-me3 marks. Statistical analyses of ChIP-seq data from in vitro infected cells as well as in vivo latency reservoirs furthermore suggest that, whereas KSHV rapidly attracts PRCs in a genome-wide manner, H3K27-me3 acquisition by MHV-68 genomes may require spreading from initial seed sites to which PRC are recruited as the result of an inefficient or stochastic recruitment, and that immune pressure may be needed to select for latency pools harboring PRC-silenced episomes in vivo. Using co-infection experiments and recombinant viruses, we also show that KSHV's ability to rapidly and efficiently acquire H3K27-me3 marks does not depend on the host cell environment or unique properties of the KSHV-encoded LANA protein, but rather requires specific cis-acting sequence features. We show that the non-canonical PRC1.1 component KDM2B, a factor which binds to unmethylated CpG motifs, is efficiently recruited to KSHV genomes, indicating that CpG island characteristics may constitute these features. In accord with the fact that, compared to MHV-68, KSHV genomes exhibit a fundamentally higher density of CpG motifs, we furthermore demonstrate efficient acquisition of H2AK119-ub by KSHV and H3K36-me2 by MHV-68 (but not vice versa), furthermore supporting the notion that KSHV genomes rapidly attract PRC1.1 complexes in a genome-wide fashion. Collectively, our results suggest that rapid PRC silencing is not a universal feature of viral latency, but that some viruses may rather have adopted distinct genomic features to specifically exploit default host pathways that repress epigenetically naive, CpG-rich DNA.

Immune Control of γ-Herpesviruses

Vaccination against γ-herpesviruses has been hampered by our limited understanding of their normal control. Epstein–Barr virus (EBV)-transformed B cells are killed by viral latency antigen-specific CD8⁺ T cells in vitro, but attempts to block B cell infection with antibody or to prime anti-viral CD8⁺ T cells have protected poorly in vivo. The Doherty laboratory used Murid Herpesvirus-4 (MuHV-4) to analyze γ-herpesvirus control in mice and found CD4⁺ T cell dependence, with viral evasion limiting CD8⁺ T cell function. MuHV-4 colonizes germinal center (GC) B cells via lytic transfer from myeloid cells, and CD4⁺ T cells control myeloid infection. GC colonization and protective, lytic antigen-specific CD4⁺ T cells are now evident also for EBV. Subunit vaccines have protected only transiently against MuHV-4, but whole virus vaccines give long-term protection, via CD4⁺ T cells and antibody. They block infection transfer to B cells, and need include no known viral latency gene, nor any MuHV-4-specific gene. Thus, the Doherty approach of in vivo murine analysis has led to a plausible vaccine strategy for EBV and, perhaps, some insight into what CD8⁺ T cells really do.

Conserved Gammaherpesvirus Protein Kinase Selectively Promotes Irrelevant B Cell Responses

Gammaherpesviruses are ubiquitous pathogens that are associated with B cell lymphomas. In the early stages of chronic infection, these viruses infect naive B cells and subsequently usurp the B cell differentiation process through the germinal center response to ensure latent infection of long-lived memory B cells. A unique feature of early gammaherpesvirus chronic infection is a robust differentiation of irrelevant, virus-nonspecific B cells with reactivities against self-antigens and antigens of other species. In contrast, protective, virus-specific humoral responses do not reach peak levels until a much later time. While several host factors are known to either promote or selectively restrict gammaherpesvirus-driven germinal center response, viral mechanisms that contribute to the irrelevant B cell response have not been defined. In this report we show that the expression and the enzymatic activity of the gammaherpesvirus-encoded conserved protein kinase selectively facilitates the irrelevant, but not virus-specific, B cell responses. Further, we show that lack of interleukin-1 (IL-1) receptor attenuates gammaherpesvirus-driven B cell differentiation and viral reactivation. Because germinal center B cells are thought to be the target of malignant transformation during gammaherpesvirus-driven lymphomagenesis, identification of host and viral factors that promote germinal center responses during gammaherpesvirus infection may offer an insight into the mechanism of gammaherpesvirus pathogenesis.IMPORTANCE Gammaherpesviruses are ubiquitous cancer-associated pathogens that usurp the B cell differentiation process to establish life-long latent infection in memory B cells. A unique feature of early gammaherpesvirus infection is the robust increase in differentiation of B cells that are not specific for viral antigens and instead encode antibodies that react with self-antigens and antigens of other species. Viral mechanisms that are involved in driving such irrelevant B cell differentiation are not known. Here, we show that gammaherpesvirus-encoded conserved protein kinase and host IL-1 signaling promote irrelevant B cell responses and gammaherpesvirus-driven germinal center responses, with the latter thought to be the target of viral transformation.

In Vivo Persistence of Chimeric Virus after Substitution of the Kaposi's Sarcoma-Associated Herpesvirus LANA DNA Binding Domain with That of Murid Herpesvirus 4

The latency-associated nuclear antigen from Kaposi's sarcoma-associated herpesvirus (KSHV), kLANA, and its homolog from the murid herpesvirus 4 (MuHV-4), mLANA, are essential for viral latency. kLANA is nearly four times the size of mLANA, mainly due to an extensive central repeat region that is absent in mLANA. Both proteins harbor a C-terminal DNA binding domain (DBD). The DBD binds the terminal repeat (TR) DNA sequences of the viral genome to mediate persistence. Despite structural conservation, the kLANA and mLANA DBDs differ in sequence and mode of oligomerization. kLANA DBD oligomers are flexible and bent, while mLANA DBD oligomers bind DNA in a rigid, linear conformation. We previously reported that kLANA and mLANA acted reciprocally on TR sequences. Furthermore, a MuHV-4 expressing kLANA instead of mLANA (v-kLANA) established latency in mice, albeit at a lower magnitude than the wild-type (WT) virus. Here, we asked if kLANA can accommodate the mLANA DBD and generated a fusion protein which contains kLANA but with the mLANA C-terminal region in place of that of kLANA. We report a recombinant MuHV-4 (v-KM) encoding this LANA fusion protein instead of mLANA. The fusion protein was expressed in lytic infection in vitro and assembled nuclear LANA dots in infected splenocytes. Results demonstrated that kLANA functionally accommodated mLANA's mode of DNA binding, allowing MuHV-4 chimeric virus to establish latency in vivo Notably, v-KM established latency in germinal center B cells more efficiently than did v-kLANA, although levels were reduced compared to WT MuHV-4.IMPORTANCE KSHV is a human oncogenic virus for which there is no tractable, immunocompetent animal model of infection. MuHV-4, a related rodent gammaherpesvirus, enables pathogenesis studies in mice. In latency, both viruses persist as extrachromosomal, circular genomes (episomes). LANA proteins encoded by KSHV (kLANA) and MuHV-4 (mLANA) contain a C-terminal DNA binding domain (DBD) that acts on the virus terminal repeats to enable episome persistence. mLANA is a smaller protein than kLANA. Their DBDs are structurally conserved but differ strikingly in the conformation of DNA binding. We report a recombinant, chimeric MuHV-4 which contains kLANA in place of mLANA, but in which the DBD is replaced with that of mLANA. Results showed that kLANA functionally accommodated mLANA's mode of DNA binding. In fact, the new chimeric virus established latency in vivo more efficiently than MuHV-4 expressing full-length kLANA.

Gammaherpesvirus Colonization of the Spleen Requires Lytic Replication in B Cells

Gammaherpesviruses infect lymphocytes and cause lymphocytic cancers. Murid herpesvirus-4 (MuHV-4), Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus all infect B cells. Latent infection can spread by B cell recirculation and proliferation, but whether this alone achieves systemic infection is unclear. To test the need of MuHV-4 for lytic infection in B cells, we flanked its essential ORF50 lytic transactivator with loxP sites and then infected mice expressing B cell-specific Cre (CD19-Cre). The floxed virus replicated normally in Cre^- mice. In CD19-Cre mice, nasal and lymph node infections were maintained; but there was little splenomegaly, and splenic virus loads remained low. Cre-mediated removal of other essential lytic genes gave a similar phenotype. CD19-Cre spleen infection by intraperitoneal virus was also impaired. Therefore, MuHV-4 had to emerge lytically from B cells to colonize the spleen. An important role for B cell lytic infection in host colonization is consistent with the large CD8⁺ T cell responses made to gammaherpesvirus lytic antigens during infectious mononucleosis and suggests that vaccine-induced immunity capable of suppressing B cell lytic infection might reduce long-term virus loads.IMPORTANCE Gammaherpesviruses cause B cell cancers. Most models of host colonization derive from cell cultures with continuous, virus-driven B cell proliferation. However, vaccines based on these models have worked poorly. To test whether proliferating B cells suffice for host colonization, we inactivated the capacity of MuHV-4, a gammaherpesvirus of mice, to reemerge from B cells. The modified virus was able to colonize a first wave of B cells in lymph nodes but spread poorly to B cells in secondary sites such as the spleen. Consequently, viral loads remained low. These results were consistent with virus-driven B cell proliferation exploiting normal host pathways and thus having to transfer lytically to new B cells for new proliferation. We conclude that viral lytic infection is a potential target to reduce B cell proliferation.

Viral FGARAT ORF75A promotes early events in lytic infection and gammaherpesvirus pathogenesis in mice

Gammaherpesviruses encode proteins with homology to the cellular purine metabolic enzyme formyl-glycinamide-phosphoribosyl-amidotransferase (FGARAT), but the role of these viral FGARATs (vFGARATs) in the pathogenesis of a natural host has not been investigated. We report a novel role for the ORF75A vFGARAT of murine gammaherpesvirus 68 (MHV68) in infectious virion production and colonization of mice. MHV68 mutants with premature stop codons in orf75A exhibited a log reduction in acute replication in the lungs after intranasal infection, which preceded a defect in colonization of multiple host reservoirs including the mediastinal lymph nodes, peripheral blood mononuclear cells, and the spleen. Intraperitoneal infection rescued splenic latency, but not reactivation. The 75A.stop virus also exhibited defective replication in primary fibroblast and macrophage cells. Viruses produced in the absence of ORF75A were characterized by an increase in the ratio of particles to PFU. In the next round of infection this led to the alteration of early events in lytic replication including the deposition of the ORF75C tegument protein, the accelerated kinetics of viral gene expression, and induction of TNFα release and cell death. Infecting cells to deliver equivalent genomes revealed that ORF75A was required for initiating early events in infection. In contrast with the numerous phenotypes observed in the absence of ORF75A, ORF75B was dispensable for replication and pathogenesis. These studies reveal that murine rhadinovirus vFGARAT family members ORF75A and ORF75C have evolved to perform divergent functions that promote replication and colonization of the host.

Gammaherpesviruses are infectious agents that cause cancer. The study of viral genes unique to this subfamily may offer insight into the strategies that these viruses use to persist in the host and drive disease. The vFGARATs are a family of viral proteins found only in gammaherpesviruses, and are critical for replication in cell culture. Here we report that a rhadinovirus of rodents requires a previously uncharacterized vFGARAT family member, ORF75A, to support viral growth and persistence in mice. In addition, viruses lacking ORF75A are defective in the production of infectious viral particles. Thus, duplications and functional divergence of the various vFGARATs in the rhadinovirus lineage have likely been driven by selective pressures to disseminate within and colonize the host. Identification of the shared host processes that are targeted by the diverse family of vFGARATs may reveal novel targets for therapeutic agents to prevent life-long infections by these oncogenic viruses.

Conditional mutagenesis in vivo reveals cell type- and infection stage-specific requirements for LANA in chronic MHV68 infection

Gammaherpesvirus (GHV) pathogenesis is a complex process that involves productive viral replication, dissemination to tissues that harbor lifelong latent infection, and reactivation from latency back into a productive replication cycle. Traditional loss-of-function mutagenesis approaches in mice using murine gammaherpesvirus 68 (MHV68), a model that allows for examination of GHV pathogenesis in vivo, have been invaluable for defining requirements for specific viral gene products in GHV infection. But these approaches are insufficient to fully reveal how viral gene products contribute when the encoded protein facilitates multiple processes in the infectious cycle and when these functions vary over time and from one host tissue to another. To address this complexity, we developed an MHV68 genetic platform that enables cell-type-specific and inducible viral gene deletion in vivo. We employed this system to re-evaluate functions of the MHV68 latency-associated nuclear antigen (mLANA), a protein with roles in both viral replication and latency. Cre-mediated deletion in mice of loxP-flanked ORF73 demonstrated the necessity of mLANA in B cells for MHV68 latency establishment. Impaired latency during the transition from draining lymph nodes to blood following mLANA deletion also was observed, supporting the hypothesis that B cells are a major conduit for viral dissemination. Ablation of mLANA in infected germinal center (GC) B cells severely impaired viral latency, indicating the importance of viral passage through the GC for latency establishment. Finally, induced ablation of mLANA during latency resulted in complete loss of affected viral genomes, indicating that mLANA is critically important for maintenance of viral genomes during stable latency. Collectively, these experiments provide new insights into LANA homolog functions in GHV colonization of the host and highlight the potential of a new MHV68 genetic platform to foster a more complete understanding of viral gene functions at discrete stages of GHV pathogenesis.

Gammaherpesviruses (GHVs), including the human pathogens Epstein-Barr virus and Kaposi sarcoma-associated herpesvirus, establish lifelong infections that can lead to cancer. Defining the functions of viral gene products in acute replication and chronic infection is important for understanding how these viruses cause disease. Infection of mice with the related GHV, murine gammaherpesvirus 68 (MHV68), provides a tractable small animal model for defining how viral gene products function in chronic infection and understanding how host factors limit disease. Here we describe the development of a new viral genetic platform that enables the targeted deletion of specific genes from the viral genome in discrete host cells or at distinct times during infection. We utilize this system to better define requirements for the conserved latency-associated nuclear antigen in MHV68 lytic replication and latency in mice. This work highlights the utility of this MHV68 genetic platform for defining mechanisms of GHV infection and disease.

Type I Interferon Signaling to Dendritic Cells Limits Murid Herpesvirus 4 Spread from the Olfactory Epithelium

Murid herpesvirus 4 (MuHV-4) is a B cell-tropic gammaherpesvirus that can be studied in vivo Despite viral evasion, type I interferons (IFN-I) limit its spread. After MuHV-4 inoculation into footpads, IFN-I protect lymph node subcapsular sinus macrophages (SSM) against productive infection; after peritoneal inoculation, they protect splenic marginal zone macrophages, and they limit MuHV-4 replication in the lungs. While invasive infections can be used to test specific aspects of host colonization, it is also important to understand natural infection. MuHV-4 taken up spontaneously by alert mice enters them via olfactory neurons. We determined how IFN-I act in this context. Blocking IFN-I signaling did not increase neuronal infection but allowed the virus to spread to the adjacent respiratory epithelium. In lymph nodes, a complete IFN-I signaling block increased MuHV-4 lytic infection in SSM and increased the number of dendritic cells (DC) expressing viral green fluorescent protein (GFP) independently of lytic infection. A CD11c⁺ cell-directed signaling block increased infection of DC only. However, this was sufficient to increase downstream infection, consistent with DC providing the main viral route to B cells. The capacity of IFN-I to limit DC infection indicated that viral IFN-I evasion was only partly effective. Therefore, DC are a possible target for IFN-I-based interventions to reduce host colonization.IMPORTANCE Human gammaherpesviruses infect B cells and cause B cell cancers. Interventions to block virus binding to B cells have not stopped their infection. Therefore, we must identify other control points that are relevant to natural infection. Human infections are difficult to analyze. However, gammaherpesviruses colonize all mammals. A related gammaherpesvirus of mice reaches B cells not directly but via infected dendritic cells. We show that type I interferons, an important general antiviral defense, limit gammaherpesvirus B cell infection by acting on dendritic cells. Therefore, dendritic cell infection is a potential point of interferon-based therapeutic intervention.

Murine gammaherpesvirus M2 antigen modulates splenic B cell activation and terminal differentiation in vivo

Murine gammaherpesvirus 68 (MHV68) infection of laboratory strains of mice has provided a tractable small animal model for dissecting gammaherpesvirus pathogenesis. The MHV68 latency associated antigen M2 promotes viral latency establishment in germinal center (GC) B cells and plays an important role in virus infection of plasma cells (PCs), which is linked to virus reactivation. More recently, M2 has been highlighted as a potent immunomodulatory molecule capable of hindering both cell-mediated and humoral immunity to MHV68 infection and subsequent challenges. M2 expression in B cells results in activation of B cell receptor signaling pathways that promote proliferation, differentiation, and cytokine production—a hallmark of gammaherpesviruses. In this study, we utilized an adoptive transfer model to explore the biological consequence of M2 expression in activated B cells in vivo. Secondly, we engineered and validated two independent MHV68 M2 reporter viruses that track M2 protein expression in latently infected B cells during infection. Here we demonstrate that upon adoptive transfer into naive mice, M2 expression promotes activated primary B cells to competitively establish residency in the spleen as either a GC B cell or a PC, most notably in the absence of an ongoing GC reaction. Moreover, M2 antigen drives robust PC differentiation and IL10 production in vivo in the absence of other viral factors. Lastly, we confirm that M2 expression during MHV68 infection is localized to the GC compartment, which is a long term latency reservoir for gammaherpesviruses. Overall, these observations are consistent with, and extend upon previous reports of M2 function in B cells and within the context of MHV68 infection. Moreover, this work provides support for a model by which M2-driven dysregulation of B cell function compromises multiple aspects of antiviral immunity to achieve persistence within the infected host.

Gammaherpesvirus (GHVs), which primarily infect B cells, are capable of exploiting B cell biology to achieve a stable and persistent infection for the lifetime of the host. GHV infections traffick to germinal center (GC) B cells and plasma cells (PCs), which are important immune effectors that promote the generation of protective antibodies in response to pathogens. The mechanism by which murine gammaherpesvirus 68 (MHV68) M2 latency protein activates B cell receptor signaling pathways to modulate the immune response to infection and further promote viral pathogenesis within the GC B cell and PC compartments is not completely understood. Here we demonstrate that M2 expression alone, in the absence of other viral factors, drives robust PC differentiation and IL10 production in vivo. Moreover, M2 promotes the accumulation of splenic GC B cells, which was subsequently verified as the site for potent M2 expression during latent MHV68 infection. Our work further substantiates a model in which a viral protein dysregulates B cell activation, differentiation, and cytokine production to create a permissive environment for viral persistence in the infected host. This work justifies further investigations addressing the impact of GHV latency antigen function within the GC reaction and overall host response to infection.

CD8+ T cell evasion mandates CD4+ T cell control of chronic gamma-herpesvirus infection

Gamma-herpesvirus infections are regulated by both CD4⁺ and CD8⁺ T cells. However clinical disease occurs mainly in CD4⁺ T cell-deficient hosts. In CD4⁺ T cell-deficient mice, CD8⁺ T cells control acute but not chronic lung infection by Murid Herpesvirus-4 (MuHV-4). We show that acute and chronic lung infections differ in distribution: most acute infection was epithelial, whereas most chronic infection was in myeloid cells. CD8⁺ T cells controlled epithelial infection, but CD4⁺ T cells and IFNγ were required to control myeloid cell infection. Disrupting the MuHV-4 K3, which degrades MHC class I heavy chains, increased viral epitope presentation by infected lung alveolar macrophages and allowed CD8⁺ T cells to prevent disease. Thus, viral CD8⁺ T cell evasion led to niche-specific immune control, and an essential role for CD4⁺ T cells in limiting chronic infection.

Gamma-herpesviruses chronically infect most people. While infection is usually asymptomatic, disease occurs if the immune system is weakened. Understanding how immune control normally works should provide a basis for preventing disease. In mice, CD8⁺ T cells can control acute gamma-herpesvirus infection but not chronic infection. We show that acute and chronic infections involve different cell types. CD8⁺ T cells controlled epithelial cell infection, which predominated acutely, but they could not control chronic macrophage infection unless viral immune evasion was disabled. Instead CD4⁺ T cells were required. Thus, viral evasion made host defence cell type-specific: CD8⁺ T cells controlled epithelial cell infection; CD4⁺ T cells controlled macrophage infection; and comprehensive control required both T cell subsets.

Type I Interferons and NK Cells Restrict Gammaherpesvirus Lymph Node Infection

Gammaherpesviruses establish persistent, systemic infections and cause cancers. Murid herpesvirus 4 (MuHV-4) provides a unique window into the early events of host colonization. It spreads via lymph nodes. While dendritic cells (DC) pass MuHV-4 to lymph node B cells, subcapsular sinus macrophages (SSM), which capture virions from the afferent lymph, restrict its spread. Understanding how this restriction works offers potential clues to a more comprehensive defense. Type I interferon (IFN-I) blocked SSM lytic infection and reduced lytic cycle-independent viral reporter gene expression. Plasmacytoid DC were not required, but neither were SSM the only source of IFN-I, as IFN-I blockade increased infection in both intact and SSM-depleted mice. NK cells restricted lytic SSM infection independently of IFN-I, and SSM-derived virions spread to the spleen only when both IFN-I responses and NK cells were lacking. Thus, multiple innate defenses allowed SSM to adsorb virions from the afferent lymph with relative impunity. Enhancing IFN-I and NK cell recruitment could potentially also restrict DC infection and thus improve infection control.

IMPORTANCE

Human gammaherpesviruses cause cancers by infecting B cells. However, vaccines designed to block virus binding to B cells have not stopped infection. Using a related gammaherpesvirus of mice, we have shown that B cells are infected not via cell-free virus but via infected myeloid cells. This suggests a different strategy to stop B cell infection: stop virus production by myeloid cells. Not all myeloid infection is productive. We show that subcapsular sinus macrophages, which do not pass infection to B cells, restrict gammaherpesvirus production by recruiting type I interferons and natural killer cells. Therefore, a vaccine that speeds the recruitment of these defenses might stop B cell infection.

Latency-Associated Nuclear Antigen E3 Ubiquitin Ligase Activity Impacts Gammaherpesvirus-Driven Germinal Center B Cell Proliferation

Viruses have evolved mechanisms to hijack components of cellular E3 ubiquitin ligases, thus modulating the ubiquitination pathway. However, the biological relevance of such mechanisms for viral pathogenesis in vivo remains largely unknown. Here, we utilized murid herpesvirus 4 (MuHV-4) infection of mice as a model system to address the role of MuHV-4 latency-associated nuclear antigen (mLANA) E3 ligase activity in gammaherpesvirus latent infection. We show that specific mutations in the mLANA SOCS box (V199A, V199A/L202A, or P203A/P206A) disrupted mLANA's ability to recruit Elongin C and Cullin 5, thereby impairing the formation of the Elongin BC/Cullin 5/SOCS (EC5S(mLANA)) complex and mLANA's E3 ligase activity on host NF-κB and Myc. Although these mutations resulted in considerably reduced mLANA binding to viral terminal repeat DNA as assessed by electrophoretic mobility shift assay (EMSA), the mutations did not disrupt mLANA's ability to mediate episome persistence. In vivo, MuHV-4 recombinant viruses bearing these mLANA SOCS box mutations exhibited a deficit in latency amplification in germinal center (GC) B cells. These findings demonstrate that the E3 ligase activity of mLANA contributes to gammaherpesvirus-driven GC B cell proliferation. Hence, pharmacological inhibition of viral E3 ligase activity through targeting SOCS box motifs is a putative strategy to control gammaherpesvirus-driven lymphoproliferation and associated disease.

IMPORTANCE

The gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) cause lifelong persistent infection and play causative roles in several human malignancies. Colonization of B cells is crucial for virus persistence, and access to the B cell compartment is gained by virus-driven proliferation in germinal center (GC) B cells. Infection of B cells is predominantly latent, with the viral genome persisting as a multicopy episome and expressing only a small subset of viral genes. Here, we focused on latency-associated nuclear antigen (mLANA) encoded by murid herpesvirus-4 (MuHV-4), which exhibits homology in sequence, structure, and function to KSHV LANA (kLANA), thereby allowing the study of LANA-mediated pathogenesis in mice. Our experiments show that mLANA's E3 ubiquitin ligase activity is necessary for efficient expansion of latency in GC B cells, suggesting that the development of pharmacological inhibitors of LANA E3 ubiquitin ligase activity may allow strategies to interfere with gammaherpesvirus-driven lymphoproliferation and associated disease.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

Type I Interferons Direct Gammaherpesvirus Host Colonization

Gamma-herpesviruses colonise lymphocytes. Murid Herpesvirus-4 (MuHV-4) infects B cells via epithelial to myeloid to lymphoid transfer. This indirect route entails exposure to host defences, and type I interferons (IFN-I) limit infection while viral evasion promotes it. To understand how IFN-I and its evasion both control infection outcomes, we used Mx1-cre mice to tag floxed viral genomes in IFN-I responding cells. Epithelial-derived MuHV-4 showed low IFN-I exposure, and neither disrupting viral evasion nor blocking IFN-I signalling markedly affected acute viral replication in the lungs. Maximising IFN-I induction with poly(I:C) increased virus tagging in lung macrophages, but the tagged virus spread poorly. Lymphoid-derived MuHV-4 showed contrastingly high IFN-I exposure. This occurred mainly in B cells. IFN-I induction increased tagging without reducing viral loads; disrupting viral evasion caused marked attenuation; and blocking IFN-I signalling opened up new lytic spread between macrophages. Thus, the impact of IFN-I on viral replication was strongly cell type-dependent: epithelial infection induced little response; IFN-I largely suppressed macrophage infection; and viral evasion allowed passage through B cells despite IFN-I responses. As a result, IFN-I and its evasion promoted a switch in infection from acutely lytic in myeloid cells to chronically latent in B cells. Murine cytomegalovirus also showed a capacity to pass through IFN-I-responding cells, arguing that this is a core feature of herpesvirus host colonization.

Murid Gammaherpesvirus Latency-Associated Protein M2 Promotes the Formation of Conjugates between Transformed B Lymphoma Cells and T Helper Cells

Establishment of persistent infection in memory B cells by murid herpesvirus-4 (MuHV-4) depends on the proliferation of latently infected germinal center B cells, for which T cell help is essential. Whether the virus is capable of modulating B-T helper cell interaction for its own benefit is still unknown. Here, we investigate if the MuHV-4 latency associated M2 protein, which assembles multiprotein complexes with B cell signaling proteins, plays a role. We observed that M2 led to the upregulation of adhesion and co-stimulatory molecules in transduced B cell lines. In an MHC-II restricted OVA peptide-specific system, M2 polarized to the B-T helper contact zone. Furthermore, it promoted B cell polarization, as demonstrated by the increased proximity of the B cell microtubule organizing center to the interface. Consistent with these data, M2 promoted the formation of B-T helper cell conjugates. In an in vitro competition assay, this translated into a competitive advantage, as T cells preferentially conjugated with M2-expressing B cells. However, expression of M2 alone in B cells was not sufficient to lead to T cell activation, as it only occurred in the presence of specific peptide. Taken together, these findings support that M2 promotes the formation of B-T helper cell conjugates. In an in vivo context this may confer a competitive advantage to the infected B cell in acquisition of T cell help and initiation of a germinal center reaction, hence host colonization.

Interleukin 21 signaling in B cells is required for efficient establishment of murine gammaherpesvirus latency

The human gammaherpesviruses take advantage of normal B cell differentiation pathways to establish life-long infection in memory B cells. Murine gammaherpesvirus 68 (MHV68) infection of laboratory strains of mice also leads to life-long infection in memory B cells. To gain access to the memory B cell population, MHV68 infected B cells pass through the germinal center reaction during the onset of latency and require signals from T follicular helper (T_FH) cells for proliferation. Interleukin 21 (IL-21), one of the secreted factors produced by T_FH cells, plays an important role in both the maintenance of the germinal center response as well as in the generation of long-lived plasma cells. Using IL-21R deficient mice, we show that IL-21 signaling is required for efficient establishment of MHV68 infection. In the absence of IL-21 signaling, fewer infected splenocytes are able to gain access to either the germinal center B cell population or the plasma cell population – the latter being a major site of MHV68 reactivation. Furthermore, the germinal center B cell population in IL-21R^-/- mice is skewed towards the non-proliferating centrocyte phenotype, resulting in reduced expansion of infected B cells. Additionally, the reduced frequency of infected plasma cells results in a significant reduction in the frequency of splenocytes capable of reactivating virus. This defect in establishment of MHV68 infection is intrinsic to B cells, as MHV68 preferentially establishes infection in IL-21R sufficient B cells in mixed bone marrow chimeric mice. Taken together, these data indicate that IL-21 signaling plays multiple roles during establishment of MHV68 infection, and identify IL-21 as a critical T_FH cell-derived factor for efficient establishment of gammaherpesvirus B cell latency.

Gammaherpesviruses establish life-long infection in B cells by taking advantage of the host immune response that is generated during primary infection. During initial infection, the immune system responds by inducing rapid proliferation of responding B cells during the germinal center reaction. This response is highly coordinated and relies on the interplay of multiple cell types. CD4 T helper cells are an important component of the germinal center reaction in that they communicate with B cells by providing both proliferation and survival signals. Gammaherpesviruses infect B cells that receive these signals, resulting in proliferation and survival of infected cells, allowing the virus to establish life-long infection. Here we show that interleukin 21 (IL-21), one of the signaling factors produced by CD4 T cells, is required for efficient establishment of infection in a mouse model of gammaherpesvirus infection. In the absence of IL-21 signaling, the viral load is markedly reduced and the composition of the infected cell population is altered to cell types that are less proliferative and produce less virus. These results demonstrate how gammaherpesviruses are able to take advantage of the immune response being generated against it to establish lifelong infection.

Rhadinovirus host entry by co-operative infection

Rhadinoviruses establish chronic infections of clinical and economic importance. Several show respiratory transmission and cause lung pathologies. We used Murid Herpesvirus-4 (MuHV-4) to understand how rhadinovirus lung infection might work. A primary epithelial or B cell infection often is assumed. MuHV-4 targeted instead alveolar macrophages, and their depletion reduced markedly host entry. While host entry was efficient, alveolar macrophages lacked heparan - an important rhadinovirus binding target - and were infected poorly ex vivo. In situ analysis revealed that virions bound initially not to macrophages but to heparan⁺ type 1 alveolar epithelial cells (AECs). Although epithelial cell lines endocytose MuHV-4 readily in vitro, AECs did not. Rather bound virions were acquired by macrophages; epithelial infection occurred only later. Thus, host entry was co-operative - virion binding to epithelial cells licensed macrophage infection, and this in turn licensed AEC infection. An antibody block of epithelial cell binding failed to block host entry: opsonization provided merely another route to macrophages. By contrast an antibody block of membrane fusion was effective. Therefore co-operative infection extended viral tropism beyond the normal paradigm of a target cell infected readily in vitro; and macrophage involvement in host entry required neutralization to act down-stream of cell binding.

All viral infections start with host entry. Entry into cells is studied widely in isolated cultures; entry into live hosts is more complicated and less well understood: our tissues have specific anatomical structures and our cells differ markedly from most cultured cells in size, shape and behaviour. The respiratory tract is a common site of virus infection. Size dictates where inhaled particles come to rest, and virus-sized particles can reach the lungs. Rhadinoviruses chronically infect both humans and economically important animals, and cause lung disease. We used a well-characterized murine example to determine how a rhadinovirus enters the lungs. At its peak, infection was prominent in epithelial cells lining the lung air spaces. However it started in macrophages, which normally clear the lungs of inhaled debris. Only epithelial cells expressed the molecules required for virus binding, but only macrophages internalized virus particles after binding; infection involved interaction between these different cell types. Blocking epithelial infection with an antibody did not stop host entry because attached antibodies increase virus uptake by lung macrophages; but an antibody that blocks macrophage infection was effective. Thus, understanding how rhadinovirus infections work in normal tissues provided important information for their control.

A murid gamma-herpesviruses exploits normal splenic immune communication routes for systemic spread

Gamma-herpesviruses (γHVs) are widespread oncogenic pathogens that chronically infect circulating lymphocytes. How they subvert the immune check-point function of the spleen to promote persistent infection is not clear. We show that Murid Herpesvirus-4 (MuHV-4) enters the spleen by infecting marginal zone (MZ) macrophages, which provided a conduit to MZ B cells. Relocation of MZ B cells to the white pulp allowed virus transfer to follicular dendritic cells. From here the virus reached germinal center B cells to establish persistent infection. Mice lacking MZ B cells, or treated with a sphingosine-1-phosphate receptor agonist to dislocate them, were protected against MuHV-4 colonization. MuHV-4 lacking ORF27, which encodes a glycoprotein necessary for efficient intercellular spread, could infect MZ macrophages but was impaired in long-term infection. Thus, MuHV-4, a γHV, exploits normal immune communication routes to spread by serial lymphoid/myeloid exchange.

The murine gammaherpesvirus immediate-early Rta synergizes with IRF4, targeting expression of the viral M1 superantigen to plasma cells

MHV68 is a murine gammaherpesvirus that infects laboratory mice and thus provides a tractable small animal model for characterizing critical aspects of gammaherpesvirus pathogenesis. Having evolved with their natural host, herpesviruses encode numerous gene products that are involved in modulating host immune responses to facilitate the establishment and maintenance of lifelong chronic infection. One such protein, MHV68 M1, is a secreted protein that has no known homologs, but has been shown to play a critical role in controlling virus reactivation from latently infected macrophages. We have previous demonstrated that M1 drives the activation and expansion of Vβ4⁺ CD8⁺ T cells, which are thought to be involved in controlling MHV68 reactivation through the secretion of interferon gamma. The mechanism of action and regulation of M1 expression are poorly understood. To gain insights into the function of M1, we set out to evaluate the site of expression and transcriptional regulation of the M1 gene. Here, using a recombinant virus expressing a fluorescent protein driven by the M1 gene promoter, we identify plasma cells as the major cell type expressing M1 at the peak of infection in the spleen. In addition, we show that M1 gene transcription is regulated by both the essential viral immediate-early transcriptional activator Rta and cellular interferon regulatory factor 4 (IRF4), which together potently synergize to drive M1 gene expression. Finally, we show that IRF4, a cellular transcription factor essential for plasma cell differentiation, can directly interact with Rta. The latter observation raises the possibility that the interaction of Rta and IRF4 may be involved in regulating a number of viral and cellular genes during MHV68 reactivation linked to plasma cell differentiation.

Through coevolution with their hosts, gammaherpesviruses have acquired unique genes that aid in infection of a particular host. Here we study the regulation of the MHV68 M1 gene, which encodes a protein that modulates the host immune response. Using a strategy that allowed us to identify MHV68 infected cells in mice, we have determined that M1 expression is largely limited to the antibody producing plasma cells. In addition, we show that M1 gene expression is regulated by both cellular and viral factors, which allow the virus to fine-tune gene expression in response to environmental signals. These findings provide insights into M1 function through a better understanding of how M1 expression is regulated.

Establishment of murine gammaherpesvirus latency in B cells is not a stochastic event

Murid γ-herpesvirus-4 (MuHV-4) promotes polyclonal B cell activation and establishes latency in memory B cells via unclear mechanisms. We aimed at exploring whether B cell receptor specificity plays a role in B cell susceptibility to viral latency and how this is related to B cell activation. We first observed that MuHV-4-specific B cells represent a minority of the latent population, and to better understand the influence of the virus on non-MuHV-4 specific B cells we used the SW_HEL mouse model, which produce hen egg lysozyme (HEL)-specific B cells. By tracking HEL⁺ and HEL⁻ B cells, we showed that in vivo latency was restricted to HEL⁻ B cells while the two populations were equally sensitive to the virus in vitro. Moreover, MuHV-4 induced two waves of B cell activation. While the first wave was characterized by a general B cell activation, as shown by HEL⁺ and HEL⁻ B cells expansion and upregulation of CD69 expression, the second wave was restricted to the HEL⁻ population, which acquired germinal center (GC) and plasma cell phenotypes. Antigenic stimulation of HEL⁺ B cells led to the development of HEL⁺ GC B cells where latent infection remained undetectable, indicating that MuHV-4 does not benefit from acute B cell responses to establish latency in non-virus specific B cells but relies on other mechanisms of the humoral response. These data support a model in which the establishment of latency in B cells by γ-herpesviruses is not stochastic in terms of BCR specificity and is tightly linked to the formation of GCs.

Murid γ-herpesvirus-4 (MuHV-4) is a good model to study infectious mononucleosis in mice, in which the virus ultimately establishes life-long latency in B cells. Whereas several viral proteins have been shown to modulate B cell behavior, in the present study we aimed at clarifying the parameters that dictate the establishment of viral latency from the B cell perspective. Indeed, the B cell repertoire is highly diverse and it remains unknown whether latency takes place randomly in B cells. To study this question, we isolated latently infected B cells in which we observed a low frequency of virus-specific B cells, suggesting that viral latency is not restricted to this population. To better understand MuHV-4 influence on non-virus specific B cells, we then followed the fate of B cells specific for a foreign antigen, hen egg lysozyme (HEL). While in vitro experiments showed that HEL-specific B cells could be acutely infected by MuHV-4, these cells were resistant to MuHV-4 latent infection in vivo. These results suggest that while establishment of γ-herpesvirus latency is not restricted to virus-specific B cells, it does not take place randomly in B cells and relies on mechanisms that remain to be identified.

Defining immune engagement thresholds for in vivo control of virus-driven lymphoproliferation

Persistent infections are subject to constant surveillance by CD8⁺ cytotoxic T cells (CTL). Their control should therefore depend on MHC class I-restricted epitope presentation. Many epitopes are described for γ-herpesviruses and form a basis for prospective immunotherapies and vaccines. However the quantitative requirements of in vivo immune control for epitope presentation and recognition remain poorly defined. We used Murid Herpesvirus-4 (MuHV-4) to determine for a latently expressed viral epitope how MHC class-I binding and CTL functional avidity impact on host colonization. Tracking MuHV-4 recombinants that differed only in epitope presentation, we found little latitude for sub-optimal MHC class I binding before immune control failed. By contrast, control remained effective across a wide range of T cell functional avidities. Thus, we could define critical engagement thresholds for the in vivo immune control of virus-driven B cell proliferation.

Chronic viral infections cause huge morbidity and mortality worldwide. γ-herpesviruses provide an example relevant to all human demographics, causing, inter alia, Hodgkin's disease, Burkitt's lymphoma, Kaposi's Sarcoma, and nasopharyngeal carcinoma. The proliferation of latently infected B cells and their control by CD8⁺ T cells are central to pathogenesis. Although many viral T cell targets have been identified in vitro, the functional impact of their engagement in vivo remains ill-defined. With the well-established Murid Herpesvirus-4 infection model, we used a range of recombinant viruses to define functional thresholds for the engagement of a latently expressed viral epitope. These data advance significantly our understanding of how the immune system must function to control γ-herpesvirus infection, with implications for vaccination and anti-cancer immunotherapy.

Expansion of murine gammaherpesvirus latently infected B cells requires T follicular help

X linked lymphoproliferative disease (XLP) is an inherited immunodeficiency resulting from mutations in the gene encoding the slam associated protein (SAP). One of the defining characteristics of XLP is extreme susceptibility to infection with Epstein-Barr virus (EBV), a gammaherpesvirus belonging to the genus Lymphocryptovirus, often resulting in fatal infectious mononucleosis (FIM). However, infection of SAP deficient mice with the related Murine gammaherpesvirus 68 (MHV68), a gammaherpesvirus in the genus Rhadinovirus, does not recapitulate XLP. Here we show that MHV68 inefficiently establishes latency in B cells in SAP deficient mice due to insufficient CD4 T cell help during the germinal center response. Although MHV68 infected B cells can be found in SAP-deficient mice, significantly fewer of these cells had a germinal center phenotype compared to SAP-sufficient mice. Furthermore, we show that infected germinal center B cells in SAP-deficient mice fail to proliferate. This failure to proliferate resulted in significantly lower viral loads, and likely accounts for the inability of MHV68 to induce a FIM-like syndrome. Finally, inhibiting differentiation of T follicular helper (T_FH) cells in SAP-sufficient C57Bl/6 mice resulted in decreased B cell latency, and the magnitude of the T_FH response directly correlated with the level of infection in B cells. This requirement for CD4 T cell help during the germinal center reaction by MHV68 is in contrast with EBV, which is thought to be capable of bypassing this requirement by expressing viral proteins that mimic signals provided by T_FH cells. In conclusion, the outcome of MHV68 infection in mice in the setting of loss of SAP function is distinct from that observed in SAP-deficient patients infected with EBV, and may identify a fundamental difference between the strategies employed by the rhadinoviruses and lymphocryptoviruses to expand B cell latency during the early phase of infection.

During an immune response, B cells respond to invading pathogens by undergoing massive expansion during the germinal center reaction. This proliferation requires signals from CD4 T cells, with some B cells then maturing into antibody secreting plasma cells, while others mature into memory B cells that may persist for the life of the host. Gammaherpesviruses take advantage of this immune response by infecting B cells, resulting in expansion of the pool of infected cells during the germinal center reaction. The human gammaherpesvirus Epstein-Barr virus (EBV) is thought to be able to accomplish this without the need for CD4 T cell help by expressing viral proteins that mimic signals from CD4 T cells. Here we show in a mouse model of gammaherpesvirus infection that infected B cells require signals from CD4 T cells for proliferation. Since the mouse gammaherpesvirus and EBV belong to different subgroups of gammaherpesviruses, this suggests that these subgroups utilize fundamentally different strategies to expand the pool of infected B cells during the establishment of latency. These different strategies may explain the different outcome of infection by these different subgroups of gammaherpesviruses in the context of defective germinal center responses that result from defective CD4 T cell help.

A gammaherpesvirus Bcl-2 ortholog blocks B cell receptor-mediated apoptosis and promotes the survival of developing B cells in vivo

Gammaherpesviruses such as Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV, HHV-8) establish lifelong latency in their hosts and are associated with the development of several types of malignancies, including a subset of B cell lymphomas. These viruses are thought to co-opt the process of B cell differentiation to latently infect a fraction of circulating memory B cells, resulting in the establishment of a stable latency setpoint. However, little is known about how this infected memory B cell compartment is maintained throughout the life of the host. We have previously demonstrated that immature and transitional B cells are long-term latency reservoirs for murine gammaherpesvirus 68 (MHV68), suggesting that infection of developing B cells contributes to the maintenance of lifelong latency. During hematopoiesis, immature and transitional B cells are subject to B cell receptor (BCR)-mediated negative selection, which results in the clonal deletion of autoreactive B cells. Interestingly, numerous gammaherpesviruses encode homologs of the anti-apoptotic protein Bcl-2, suggesting that virus inhibition of apoptosis could subvert clonal deletion. To test this, we quantified latency establishment in mice inoculated with MHV68 vBcl-2 mutants. vBcl-2 mutant viruses displayed a marked decrease in the frequency of immature and transitional B cells harboring viral genome, but this attenuation could be rescued by increased host Bcl-2 expression. Conversely, vBcl-2 mutant virus latency in early B cells and mature B cells, which are not targets of negative selection, was remarkably similar to wild-type virus. Finally, in vivo depletion of developing B cells during chronic infection resulted in decreased mature B cell latency, demonstrating a key role for developing B cells in the maintenance of lifelong latency. Collectively, these findings support a model in which gammaherpesvirus latency in circulating mature B cells is sustained in part through the recurrent infection and vBcl-2-mediated survival of developing B cells.

BAFF receptor deficiency limits gammaherpesvirus infection

Lymphocyte colonization by gammaherpesviruses (γHVs) is an important target for cancer prevention. However, how it works is not clear. Epstein-Barr virus drives autonomous B cell proliferation in vitro but in vivo may more subtly exploit the proliferative pathways provided by lymphoid germinal centers (GCs). Murid herpesvirus 4 (MuHV-4), which realistically infects inbred mice, provides a useful tool with which to understand further how a γHV colonizes B cells in vivo. Not all γHVs necessarily behave the same, but common events can with MuHV-4 be assigned an importance for host colonization and so a potential as therapeutic targets. MuHV-4-driven B cell proliferation depends quantitatively on CD4(+) T cell help. Here we show that it also depends on T cell-independent survival signals provided by the B cell-activating factor (BAFF) receptor (BAFF-R). B cells could be infected in BAFF-R(-/-) mice, but virus loads remained low. This corresponded to a BAFF-R-dependent defect in GC colonization. The close parallels between normal, antigen-driven B cell responses and virus-infected B cell proliferation argue that in vivo, γHVs mostly induce infected B cells into normal GC reactions rather than generating large numbers of autonomously proliferating blasts.

IMPORTANCE

γHVs cause cancers by driving the proliferation of infected cells. B cells are a particular target. Thus, we need to know how virus-driven B cell proliferation works. Controversy exists as to whether viral genes drive it directly or less directly orchestrate the engagement of normal, host-driven pathways. Here we show that the B cell proliferation driven by a murid γHV requires BAFF-R. This supports the idea that γHVs exploit host proliferation pathways and suggests that interfering with BAFF-R could more generally reduce γHV-associated B cell proliferation.

Shared mechanisms in stemness and carcinogenesis: lessons from oncogenic viruses

A rise in technologies for epigenetic reprogramming of cells to pluripotency, highlights the potential of understanding and manipulating cellular plasticity in unprecedented ways. Increasing evidence points to shared mechanisms between cellular reprogramming and the carcinogenic process, with the emerging possibility to harness these parallels in future therapeutics. In this review, we present a synopsis of recent work from oncogenic viruses which contributes to this body of knowledge, establishing a nexus between infection, cancer, and stemness.

Crystal structure of the gamma-2 herpesvirus LANA DNA binding domain identifies charged surface residues which impact viral latency

Latency-associated nuclear antigen (LANA) mediates γ2-herpesvirus genome persistence and regulates transcription. We describe the crystal structure of the murine gammaherpesvirus-68 LANA C-terminal domain at 2.2 Å resolution. The structure reveals an alpha-beta fold that assembles as a dimer, reminiscent of Epstein-Barr virus EBNA1. A predicted DNA binding surface is present and opposite this interface is a positive electrostatic patch. Targeted DNA recognition substitutions eliminated DNA binding, while certain charged patch mutations reduced bromodomain protein, BRD4, binding. Virus containing LANA abolished for DNA binding was incapable of viable latent infection in mice. Virus with mutations at the charged patch periphery exhibited substantial deficiency in expansion of latent infection, while central region substitutions had little effect. This deficiency was independent of BRD4. These results elucidate the LANA DNA binding domain structure and reveal a unique charged region that exerts a critical role in viral latent infection, likely acting through a host cell protein(s).

Herpesviruses establish life-long latent infections. During latency, gammaherpesviruses, such as Kaposi's sarcoma-associated herpesvirus (KSHV), persist as multicopy, circularized genomes in the cell nucleus and express a small subset of viral genes. KSHV latency-associated nuclear antigen (LANA) is the predominant gene expressed during latent infection. C-terminal LANA binds KSHV terminal repeat (TR) DNA to mediate DNA replication. TR DNA binding also allows tethering of the viral genome to mitotic chromosomes to mediate DNA segregation to daughter nuclei. We describe here the crystal structure of the murine gammaherpesvirus 68 LANA DNA binding domain, which is homologous to that of KSHV LANA. The structure revealed a dimer and we identified residues involved in the interaction with viral DNA. Mutation of these residues abolished DNA binding and viable latency establishment in a mouse model of infection. We also identified a positively charged patch on the dimer surface opposite to the DNA binding region and found this patch exerts an important role in the virus's ability to expand latent infection in vivo. This work elucidates the structure of the LANA DNA binding domain and identifies a novel surface feature that is critical for viral latent infection, likely by acting through a host cell protein.

Stabilization of Myc through heterotypic poly-ubiquitination by mLANA is critical for γ-herpesvirus lymphoproliferation

Host colonization by lymphotropic γ-herpesviruses depends critically on expansion of viral genomes in germinal center (GC) B-cells. Myc is essential for the formation and maintenance of GCs. Yet, the role of Myc in the pathogenesis of γ-herpesviruses is still largely unknown. In this study, Myc was shown to be essential for the lymphotropic γ-herpesvirus MuHV-4 biology as infected cells exhibited increased expression of Myc signature genes and the virus was unable to expand in Myc defficient GC B-cells. We describe a novel strategy of a viral protein activating Myc through increased protein stability resulting in increased progression through the cell cycle. This is acomplished by modulating a physiological post-translational regulatory pathway of Myc. The molecular mechanism involves Myc heterotypic poly-ubiquitination mediated via the viral E3 ubiquitin-ligase mLANA protein. EC5S(mLANA) modulates cellular control of Myc turnover by antagonizing SCF(Fbw7) mediated proteasomal degradation of Myc, mimicking SCF(β-TrCP). The findings here reported reveal that modulation of Myc is essential for γ-herpesvirus persistent infection, establishing a link between virus induced lymphoproliferation and disease.

The absence of M1 leads to increased establishment of murine gammaherpesvirus 68 latency in IgD-negative B cells

The secreted M1 protein of murine gammaherpesvirus 68 (MHV68) promotes effector Vβ4(+) CD8(+) T cell expansion to impact virus control and immune-mediated pathologies in C57BL/6 mice, but not BALB/c mice. We report a striking increase in the number of genome-positive, IgD(-) B cells during chronic infection of both mouse strains. This suggests a novel role for M1 in influencing long-term maintenance in a major latency reservoir irrespective of the degree of Vβ4(+) CD8(+) T cell expansion.

Myeloid infection links epithelial and B cell tropisms of Murid Herpesvirus-4

Gamma-herpesviruses persist in lymphocytes and cause disease by driving their proliferation. Lymphocyte infection is therefore a key pathogenetic event. Murid Herpesvirus-4 (MuHV-4) is a rhadinovirus that like the related Kaposi's Sarcoma-associated Herpesvirus persists in B cells in vivo yet infects them poorly in vitro. Here we used MuHV-4 to understand how virion tropism sets the path to lymphocyte colonization. Virions that were highly infectious in vivo showed a severe post-binding block to B cell infection. Host entry was accordingly an epithelial infection and B cell infection a secondary event. Macrophage infection by cell-free virions was also poor, but improved markedly when virion binding improved or when macrophages were co-cultured with infected fibroblasts. Under the same conditions B cell infection remained poor; it improved only when virions came from macrophages. This reflected better cell penetration and correlated with antigenic changes in the virion fusion complex. Macrophages were seen to contact acutely infected epithelial cells, and cre/lox-based virus tagging showed that almost all the virus recovered from lymphoid tissue had passed through lysM⁺ and CD11c⁺ myeloid cells. Thus MuHV-4 reached B cells in 3 distinct stages: incoming virions infected epithelial cells; infection then passed to myeloid cells; glycoprotein changes then allowed B cell infection. These data identify new complexity in rhadinovirus infection and potentially also new vulnerability to intervention.

Rhadinoviruses cause lymphocytic cancers. Their infection of lymphocytes is therefore an important therapeutic target. How this occurs is unclear. One prevalent hypothesis has been that virions directly infect lymphocytes when they enter new hosts. Here we show that host entry by Murid Herpesvirus-4, a close relative of the Kaposi's Sarcoma-associated Herpesvirus, is an epithelial rather than a lymphocyte infection: the mucosal lymphoid colonization typical of acute infectious mononucleosis only occurred later. Macrophages were closely associated with the acutely infected epithelium, and most if not all of the virus reaching B cells showed evidence of previous myeloid cell infection. Macrophage-derived virions showed a greatly enhanced capacity for lymphocyte infection that was associated with antigenic changes in the viral fusion proteins. Thus host colonization required epithelial and myeloid infections before there was lymphocyte infection. The implication is that each of these infection events could be independently targeted to limit viral persistence.

Murine gammaherpesvirus 68 LANA acts on terminal repeat DNA to mediate episome persistence

Murine gammaherpesvirus 68 (MHV68) ORF73 (mLANA) has sequence homology to Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA). LANA acts on the KSHV terminal repeat (TR) elements to mediate KSHV episome maintenance. Disruption of mLANA expression severely reduces the ability of MHV68 to establish latent infection in mice, consistent with the possibility that mLANA mediates episome persistence. Here we assess the roles of mLANA and MHV68 TR (mTR) elements in episome persistence. mTR-associated DNA persisted as an episome in latently MHV68-infected tumor cells, demonstrating that the mTR elements can serve as a cis-acting element for MHV68 episome maintenance. In some cases, both control vector and mTR-associated DNAs integrated into MHV68 episomal genomes. Therefore, we also assessed the roles of mTRs as well as mLANA in the absence of infection. DNA containing both mLANA and mTRs in cis persisted as an episome in murine A20 or MEF cells. In contrast, mTR DNA never persisted as an episome in the absence of mLANA. mLANA levels were increased when mLANA was expressed from its native promoters, and episome maintenance was more efficient with higher mLANA levels. Increased numbers of mTRs conferred more efficient episome maintenance, since DNA containing mLANA and eight mTR elements persisted more efficiently in A20 cells than did DNA with mLANA and two or four mTRs. Similar to KSHV LANA, mLANA broadly associated with mitotic chromosomes but relocalized to concentrated dots in the presence of episomes. Therefore, mLANA acts on mTR elements to mediate MHV68 episome persistence.

Tracking murine gammaherpesvirus 68 infection of germinal center B cells in vivo

Infection of mice with murine gammaherpesvirus 68 (MHV68) provides a tractable small animal model to study various aspects of persistent gammaherpesvirus infection. We have previously utilized a transgenic MHV68 that expresses enhanced yellow fluorescent protein (EYFP) to identify infected cells. While this recombinant MHV68 has been useful for identifying infected cell populations by flow cytometry, it has been suboptimal for identification of infected cells in tissue sections due to the high solubility of EYFP. Efficient detection of EYFP expressed from the MHV68 genome in tissue sections requires fixation of whole organs prior to sectioning, which frequently leads to over-fixation of some cellular antigens precluding their detection. To circumvent this issue, we describe the generation and characterization of a transgenic MHV68 harboring a fusion gene composed of the EYFP coding sequence fused to the histone H2B open reading frame. Because the H2bYFP fusion protein is tightly bound in nucleosomes in the nucleus it does not freely diffuse out of unfixed tissue sections, and thus eliminates the need for tissue fixation. We have used the MHV68-H2bYFP recombinant virus to assess the location and distribution of virus infected B cells in germinal centers during the peak of MHV68 latency in vivo. These analyses show that the physical location of distinct populations of infected germinal center B cells correlates well with their surface phenotype. Furthermore, analysis of the distribution of virus infection within germinal center B cell populations revealed that ca. 70% of MHV68 infected GC B cells are rapidly dividing centroblasts, while ca. 20% have a clear centrocyte phenotype. Finally, we have shown that marking of infected cells with MHV68-H2bYFP is extended long after the onset of latency – which should facilitate studies to track MHV68 latently infected cells at late times post-infection.

Tiled microarray identification of novel viral transcript structures and distinct transcriptional profiles during two modes of productive murine gammaherpesvirus 68 infection

We applied a custom tiled microarray to examine murine gammaherpesvirus 68 (MHV68) polyadenylated transcript expression in a time course of de novo infection of fibroblast cells and following phorbol ester-mediated reactivation from a latently infected B cell line. During de novo infection, all open reading frames (ORFs) were transcribed and clustered into four major temporal groups that were overlapping yet distinct from clusters based on the phorbol ester-stimulated B cell reactivation time course. High-density transcript analysis at 2-h intervals during de novo infection mapped gene boundaries with a 20-nucleotide resolution, including a previously undefined ORF73 transcript and the MHV68 ORF63 homolog of Kaposi's sarcoma-associated herpesvirus vNLRP1. ORF6 transcript initiation was mapped by tiled array and confirmed by 5' rapid amplification of cDNA ends. The ∼1.3-kb region upstream of ORF6 was responsive to lytic infection and MHV68 RTA, identifying a novel RTA-responsive promoter. Transcription in intergenic regions consistent with the previously defined expressed genomic regions was detected during both types of productive infection. We conclude that the MHV68 transcriptome is dynamic and distinct during de novo fibroblast infection and upon phorbol ester-stimulated B cell reactivation, highlighting the need to evaluate further transcript structure and the context-dependent molecular events that govern viral gene expression during chronic infection.

Viral cyclins mediate separate phases of infection by integrating functions of distinct mammalian cyclins

Gammaherpesvirus cyclins have expanded biochemical features relative to mammalian cyclins, and promote infection and pathogenesis including acute lung infection, viral persistence, and reactivation from latency. To define the essential features of the viral cyclin, we generated a panel of knock-in viruses expressing various viral or mammalian cyclins from the murine gammaherpesvirus 68 cyclin locus. Viral cyclins of both gammaherpesvirus 68 and Kaposi's sarcoma-associated herpesvirus supported all cyclin-dependent stages of infection, indicating functional conservation. Although mammalian cyclins could not restore lung replication, they did promote viral persistence and reactivation. Strikingly, distinct and non-overlapping mammalian cyclins complemented persistence (cyclin A, E) or reactivation from latency (cyclin D3). Based on these data, unique biochemical features of viral cyclins (e.g. enhanced kinase activation) are not essential to mediate specific processes during infection. What is essential for, and unique to, the viral cyclins is the integration of the activities of several different mammalian cyclins, which allows viral cyclins to mediate multiple, discrete stages of infection. These studies also demonstrated that closely related stages of infection, that are cyclin-dependent, are in fact genetically distinct, and thus predict that cyclin requirements may be used to tailor potential therapies for virus-associated diseases.

Measurement of γHV68 infection in mice

γ-Herpesviruses (γ-HVs) are notable for their ability to establish latent infections of lymphoid cells(1). The narrow host range of human γ-HVs, such as EBV and KSHV, has severely hindered detailed pathogenic studies. Murine γ-herpesvirus 68 (γHV68) shares extensive genetic and biological similarities with human γ-HVs and is a natural pathogen of murid rodents(2). As such, evaluation of γHV68 infection of mice inbred strains at different stages of viral infection provides an important model for understanding viral lifecycle and pathogenesis during γ-HVs infection. Upon intranasal inoculation, γHV68 infection results in acute viremia in the lung that is later resolved into a latent infection of splenocytes and other cells, which may be reactivated throughout the life of the host(3,4). In this protocol, we will describe how to use the plaque assay to assess infectious virus titer in the lung homogenates on Vero cell monolayers at the early stage (5 - 7 days) of post-intranasal infection (dpi). While acute infection is largely cleared 2 - 3 weeks postinfection, a latent infection of γHV68 is established around 14 dpi and maintained later on in the spleen of the mice. Latent infection usually affects a very small population of cells in the infected tissues, whereby the virus stays dormant and shuts off most of its gene expression. Latently-infected splenocytes spontaneously reactivate virus upon explanting into tissue culture, which can be recapitulated by an infectious center (IC) assay to determine the viral latent load. To further estimate the amount of viral genome copies in the acutely and/or latently infected tissues, quantitative real-time PCR (qPCR) is used for its maximal sensitivity and accuracy. The combined analyses of the results of qPCR and plaque assay, and/or IC assay will reveal the spatiotemporal profiles of viral replication and infectivity in vivo.

Murid herpesvirus-4 exploits dendritic cells to infect B cells

Dendritic cells (DCs) play a central role in initiating immune responses. Some persistent viruses infect DCs and can disrupt their functions in vitro. However, these viruses remain strongly immunogenic in vivo. Thus what role DC infection plays in the pathogenesis of persistent infections is unclear. Here we show that a persistent, B cell-tropic gamma-herpesvirus, Murid Herpesvirus-4 (MuHV-4), infects DCs early after host entry, before it establishes a substantial infection of B cells. DC-specific virus marking by cre-lox recombination revealed that a significant fraction of the virus latent in B cells had passed through a DC, and a virus attenuated for replication in DCs was impaired in B cell colonization. In vitro MuHV-4 dramatically altered the DC cytoskeleton, suggesting that it manipulates DC migration and shape in order to spread. MuHV-4 therefore uses DCs to colonize B cells.

We detect invading viruses with dendritic cells and eliminate them with lymphocytes. A key interaction is lymphocyte activation by dendritic cells presenting viral antigens. Not all viruses can be eliminated, and some that persist deliberately colonize lymphocytes and dendritic cells, such that parasitism and host defence co-exist within the same sites. Once established, these infections are very hard to eliminate. Therefore to vaccinate against them we must determine how infection first occurs. Here we show that a gamma-herpesvirus relation of the Kaposi's Sarcoma-associated Herpesvirus and Epstein-Barr virus - B cell-tropic human pathogens that cause cancers - uses dendritic cells to reach and infect B lymphocytes. Dendritic cells were infected before B cells; viruses marked genetically in dendritic cells were recovered from B cells; and a virus unable to replicate in dendritic cells infected B cells poorly. Thus dendritic cells not only present viral antigens to lymphocytes, but can be exploited by evasive viruses to infect lymphocytes. Therefore targeting dendritic cell infection could be an effective means of vaccine-primed host defence.

Global mRNA degradation during lytic gammaherpesvirus infection contributes to establishment of viral latency

During a lytic gammaherpesvirus infection, host gene expression is severely restricted by the global degradation and altered 3' end processing of mRNA. This host shutoff phenotype is orchestrated by the viral SOX protein, yet its functional significance to the viral lifecycle has not been elucidated, in part due to the multifunctional nature of SOX. Using an unbiased mutagenesis screen of the murine gammaherpesvirus 68 (MHV68) SOX homolog, we isolated a single amino acid point mutant that is selectively defective in host shutoff activity. Incorporation of this mutation into MHV68 yielded a virus with significantly reduced capacity for mRNA turnover. Unexpectedly, the MHV68 mutant showed little defect during the acute replication phase in the mouse lung. Instead, the virus exhibited attenuation at later stages of in vivo infections suggestive of defects in both trafficking and latency establishment. Specifically, mice intranasally infected with the host shutoff mutant accumulated to lower levels at 10 days post infection in the lymph nodes, failed to develop splenomegaly, and exhibited reduced viral DNA levels and a lower frequency of latently infected splenocytes. Decreased latency establishment was also observed upon infection via the intraperitoneal route. These results highlight for the first time the importance of global mRNA degradation during a gammaherpesvirus infection and link an exclusively lytic phenomenon with downstream latency establishment.

Pathogenesis and host control of gammaherpesviruses: lessons from the mouse

Gammaherpesviruses are lymphotropic viruses that are associated with the development of lymphoproliferative diseases, lymphomas, as well as other nonlymphoid cancers. Most known gammaherpesviruses establish latency in B lymphocytes. Research on Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV68/γHV68/MHV4) has revealed a complex relationship between virus latency and the stage of B cell differentiation. Available data support a model in which gammaherpesvirus infection drives B cell proliferation and differentiation. In general, the characterized gammaherpesviruses exhibit a very narrow host tropism, which has severely limited studies on the human gammaherpesviruses EBV and Kaposi's sarcoma-associated herpesvirus. As such, there has been significant interest in developing animal models in which the pathogenesis of gammaherpesviruses can be characterized. MHV68 represents a unique model to define the effects of chronic viral infection on the antiviral immune response.

Chemokine binding protein M3 of murine gammaherpesvirus 68 modulates the host response to infection in a natural host

Murine γ-herpesvirus 68 (MHV-68) infection of Mus musculus-derived strains of mice is an attractive model of γ-herpesvirus infection. Surprisingly, however, ablation of expression of MHV-68 M3, a secreted protein with broad chemokine-binding properties in vitro, has no discernable effect during experimental infection via the respiratory tract. Here we demonstrate that M3 indeed contributes significantly to MHV-68 infection, but only in the context of a natural host, the wood mouse (Apodemus sylvaticus). Specifically, M3 was essential for two features unique to the wood mouse: virus-dependent inducible bronchus-associated lymphoid tissue (iBALT) in the lung and highly organized secondary follicles in the spleen, both predominant sites of latency in these organs. Consequently, lack of M3 resulted in substantially reduced latency in the spleen and lung. In the absence of M3, splenic germinal centers appeared as previously described for MHV-68-infected laboratory strains of mice, further evidence that M3 is not fully functional in the established model host. Finally, analyses of M3's influence on chemokine and cytokine levels within the lungs of infected wood mice were consistent with the known chemokine-binding profile of M3, and revealed additional influences that provide further insight into its role in MHV-68 biology.

Infection of inbred strains of laboratory mice (Mus musculus) with the rodent γ-herpesvirus MHV-68 continues to be developed as an attractive experimental model of γ-herpesvirus infection. In this regard, the MHV-68 protein M3 has been shown to selectively bind and inhibit chemokines involved in the antiviral immune response, a property expected to contribute significantly to virus infection and host colonization. However, inactivation of the M3 gene has no discernable consequence on infection in this animal host. Prompted by recent evidence that natural hosts of MHV-68 are members of the genus Apodemus, and that MHV-68 infection in laboratory-bred wood mice (Apodemus sylvaticus) differs significantly from that which has been described in standard strains of laboratory mice, we addressed whether M3 functions in a host-specific manner. Indeed, we find that M3 is responsible for host-specific differences observed for MHV-68 infection, that its influence on infection within wood mice is consistent with its chemokine-binding properties, and that in its absence, persistent latent infection - a hallmark of herpesvirus infections - is attenuated. This highlights the importance of host selection when investigating specific roles of pathogenesis-related viral genes, and advances our understanding of this model and its potential application to human γ-herpesvirus infections.

Prospects of a novel vaccination strategy for human gamma-herpesviruses

Due to the oncogenic potential associated with persistent infection of human gamma-herpesviruses, including Epstein-Barr virus (EBV or HHV-4) and Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8), vaccine development has focused on subunit vaccines. However, the results using an animal model of mouse infection with a related rodent virus, murine gamma-herpesvirus 68 (MHV-68, γHV-68, or MuHV-4), have shown that the only effective vaccination strategy is based on live attenuated viruses, including viruses engineered to be incapable of establishing persistence. Vaccination with a virus lacking persistence would eliminate many potential complications. Progress in understanding persistent infections of EBV and KSHV raises the possibility of engineering a live attenuated virus without persistence. Therefore, we should keep the option open for developing a live EBV or KSHV vaccine.

Immature and transitional B cells are latency reservoirs for a gammaherpesvirus

Gammaherpesviruses, including Kaposi's sarcoma-associated herpesvirus (KSHV; also known as human herpesvirus 8 [HHV-8]), Epstein-Barr virus (EBV), and murine gammaherpesvirus 68 (MHV68; also known as gammaherpesvirus 68 [γHV68] or murine herpesvirus 4 [MuHV-4]), establish lifelong latency in the resting memory B cell compartment. However, little is known about how this reservoir of infected mature B cells is maintained for the life of the host. In the context of a normal immune system, the mature B cell pool is naturally maintained by the renewable populations of developing B cells that arise from hematopoiesis. Thus, recurrent infection of these developing B cell populations could allow the virus continual access to the B cell lineage and, subsequent to differentiation, the memory B cell compartment. To begin to address this hypothesis, we examined whether MHV68 establishes latency in developing B cells during a normal course of infection. In work described here, we demonstrate the presence of viral genome in bone marrow pro-pre-B cells and immature B cells during early latency and immature B cells during long-term latency. Further, we show that transitional B cells in the spleen are latently infected and express the latency-associated nuclear antigen (LANA) throughout chronic infection. Because developing B cells normally exhibit a short life span and a high rate of turnover, these findings suggest a model in which gammaherpesviruses may gain access to the mature B cell compartment by recurrent seeding of developing B cells.

Important role for the murid herpesvirus 4 ribonucleotide reductase large subunit in host colonization via the respiratory tract

Viral enzymes that process small molecules provide potential chemotherapeutic targets. A key constraint-the replicative potential of spontaneous enzyme mutants-has been hard to define with human gammaherpesviruses because of their narrow species tropisms. Here, we disrupted the murid herpesvirus 4 (MuHV-4) ORF61, which encodes its ribonucleotide reductase (RNR) large subunit. Mutant viruses showed delayed in vitro lytic replication, failed to establish infection via the upper respiratory tract, and replicated to only a very limited extent in the lower respiratory tract without reaching lymphoid tissue. RNR could therefore provide a good target for gammaherpesvirus chemotherapy.

Interference with dendritic cell populations limits early antigen presentation in chronic γ-herpesvirus-68 infection

A critical factor influencing the ability of the host to mount a robust immune response against a virus depends on the rapid recruitment of dendritic cells (DCs) presenting Ags. From the outset, this step sets the tempo for subsequent activation of virus-specific T cells. Despite this, how induction of the immune response might be modified by pathogens with the capacity to establish persistence is unclear. In this study, we have characterized the in vivo influence of murine gamma-herpesvirus K3-mediated interference with MHC class I in DCs that drive the initial adaptive immune response. We observed that gamma-herpesvirus could interfere with the very earliest phase of Ag presentation through K3 by directly targeting migratory and lymph node-resident DCs. These results show that a pathogen with the capacity to interfere with early Ag presentation can establish suboptimal conditions for rapid induction of the adaptive immune response and thus favor establishment of viral persistence.