Pathogenesis of murine gammaherpesvirus-68 infection in interleukin-6-deficient mice

Intrinsic p53 Activation Restricts Gammaherpesvirus-Driven Germinal Center B Cell Expansion during Latency Establishment

Gammaherpesviruses (GHV) are DNA tumor viruses that establish lifelong latent infections in lymphocytes. For viruses such as Epstein-Barr virus (EBV) and murine gammaherpesvirus 68 (MHV68), this is accomplished through a viral gene-expression program that promotes cellular proliferation and differentiation, especially of germinal center (GC) B cells. Intrinsic host mechanisms that control virus-driven cellular expansion are incompletely defined. Using a small-animal model of GHV pathogenesis, we demonstrate in vivo that tumor suppressor p53 is activated specifically in B cells that are latently infected by MHV68. In the absence of p53, the early expansion of MHV68 latency was greatly increased, especially in GC B cells, a cell-type whose proliferation was conversely restricted by p53. We identify the B cell-specific latency gene M2, a viral promoter of GC B cell differentiation, as a viral protein sufficient to elicit a p53-dependent anti-proliferative response caused by Src-family kinase activation. We further demonstrate that EBV-encoded latent membrane protein 1 (LMP1) similarly triggers a p53 response in primary B cells. Our data highlight a model in which GHV latency gene-expression programs that promote B cell proliferation and differentiation to facilitate viral colonization of the host trigger aberrant cellular proliferation that is controlled by p53.

IMPORTANCE

Gammaherpesviruses cause lifelong infections of their hosts, commonly referred to as latency, that can lead to cancer. Latency establishment benefits from the functions of viral proteins that augment and amplify B cell activation, proliferation, and differentiation signals. In uninfected cells, off-schedule cellular differentiation would typically trigger anti-proliferative responses by effector proteins known as tumor suppressors. However, tumor suppressor responses to gammaherpesvirus manipulation of cellular processes remain understudied, especially those that occur during latency establishment in a living organism. Here we identify p53, a tumor suppressor commonly mutated in cancer, as a host factor that limits virus-driven B cell proliferation and differentiation, and thus, viral colonization of a host. We demonstrate that p53 activation occurs in response to viral latency proteins that induce B cell activation. This work informs a gap in our understanding of intrinsic cellular defense mechanisms that restrict lifelong GHV infection.

Proteomics of Bronchoalveolar Lavage Fluid Reveals a Lung Oxidative Stress Response in Murine Herpesvirus-68 Infection

Murine herpesvirus-68 (MHV-68) productively infects mouse lungs, exhibiting a complex pathology characteristic of both acute viral infections and chronic respiratory diseases. We sought to discover proteins differentially expressed in bronchoalveolar lavage (BAL) from mice infected with MHV-68. Mice were infected intranasally with MHV-68. After nine days, as the lytic phase of infection resolved, differential BAL proteins were identified by two-dimensional (2D) electrophoresis and mass spectrometry. Of 23 unique proteins, acute phase proteins, vitamin A transport, and oxidative stress response factors Pdx6 and EC-SOD (Sod3) were enriched. Correspondingly, iNOS2 was induced in lung tissue by seven days post-infection. Oxidative stress was partly a direct result of MHV-68 infection, as reactive oxygen species (ROS) were induced in cultured murine NIH3T3 fibroblasts and human lung A549 cells infected with MHV-68. Finally, mice infected with a recombinant MHV-68 co-expressing inflammatory cytokine murine interleukin 6 (IL6) showed exacerbated oxidative stress and soluble type I collagen characteristic of tissue recovery. Thus, oxidative stress appears to be a salient feature of MHV-68 pathogenesis, in part caused by lytic replication of the virus and IL6. Proteins and small molecules in lung oxidative stress networks therefore may provide new therapeutic targets to ameliorate respiratory virus infections.

The replication and transcription activator of murine gammaherpesvirus 68 cooperatively enhances cytokine-activated, STAT3-mediated gene expression

Gammaherpesviruses (γHVs) have a dynamic strategy for lifelong persistence, involving productive infection, latency, and intermittent reactivation. In latency reservoirs, such as B lymphocytes, γHVs exist as viral episomes and express few viral genes. Although the ability of γHV to reactivate from latency and re-enter the lytic phase is challenging to investigate and control, it is known that the γHV replication and transcription activator (RTA) can promote lytic reactivation. In this study, we provide first evidence that RTA of murine γΗV68 (MHV68) selectively binds and enhances the activity of tyrosine-phosphorylated host STAT3. STAT3 is a transcription factor classically activated by specific tyrosine 705 phosphorylation (pTyr⁷⁰⁵-STAT3) in response to cytokine stimulation. pTyr⁷⁰⁵-STAT3 forms a dimer that avidly binds a consensus target site in the promoters of regulated genes, and our results indicate that RTA cooperatively enhances the ability of pTyr⁷⁰⁵-STAT3 to induce expression of a STAT3-responsive reporter gene. As indicated by coimmunoprecipitation, in latently infected B cells that are stimulated to reactivate MHV68, RTA bound specifically to endogenous pTyr⁷⁰⁵-STAT3. An in vitro binding assay confirmed that RTA selectively recognizes pTyr⁷⁰⁵-STAT3 and indicated that the C-terminal transactivation domain of RTA was required for enhancing STAT3-directed gene expression. The cooperation of these transcription factors may influence both viral and host genes. During MHV68 de novo infection, pTyr⁷⁰⁵-STAT3 promoted the temporal expression of ORF59, a viral replication protein. Our results demonstrate that MHV68 RTA specifically recognizes and recruits activated pTyr⁷⁰⁵-STAT3 during the lytic phase of infection.

Ablation of STAT3 in the B Cell Compartment Restricts Gammaherpesvirus Latency In Vivo

A challenging property of gammaherpesviruses is their ability to establish lifelong persistence. The establishment of latency in B cells is thought to involve active virus engagement of host signaling pathways. Pathogenic effects of these viruses during latency or following reactivation can be devastating to the host. Many cancers, including those associated with members of the gammaherpesvirus family, Kaposi’s sarcoma-associated herpesvirus and Epstein-Barr virus, express elevated levels of active host signal transducer and activator of transcription-3 (STAT3). STAT3 is activated by tyrosine phosphorylation in response to many cytokines and can orchestrate effector responses that include proliferation, inflammation, metastasis, and developmental programming. However, the contribution of STAT3 to gammaherpesvirus pathogenesis remains to be completely understood. This is the first study to have identified STAT3 as a critical host determinant of the ability of gammaherpesvirus to establish long-term latency in an animal model of disease. Following an acute infection, murine gammaherpesvirus 68 (MHV68) established latency in resident B cells, but establishment of latency was dramatically reduced in animals with a B cell-specific STAT3 deletion. The lack of STAT3 in B cells did not impair germinal center responses for immunoglobulin (Ig) class switching in the spleen and did not reduce either total or virus-specific IgG titers. Although ablation of STAT3 in B cells did not have a global effect on these assays of B cell function, it had long-term consequences for the viral load of the host, since virus latency was reduced at 6 to 8 weeks postinfection. Our findings establish host STAT3 as a mediator of gammaherpesvirus persistence.

The insidious ability of gammaherpesviruses to establish latent infections can have detrimental consequences for the host. Identification of host factors that promote viral latency is essential for understanding latency mechanisms and for therapeutic interventions. We provide the first evidence that STAT3 expression is needed for murine gammaherpesvirus 68 to establish latency in primary B cells during an active immune response to infection. STAT3 deletion in B cells does not impair adaptive immune control of the virus, but loss of STAT3 in B cells has a long-lasting impact on viral persistence. These results indicate a potential therapeutic benefit of STAT3 inhibitors for combating gammaherpesvirus latency and, thereby, associated pathologies.

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.

Phenotypic complementation of genetic immunodeficiency by chronic herpesvirus infection

Variation in the presentation of hereditary immunodeficiencies may be explained by genetic or environmental factors. Patients with mutations in HOIL1 (RBCK1) present with amylopectinosis-associated myopathy with or without hyper-inflammation and immunodeficiency. We report that barrier-raised HOIL-1-deficient mice exhibit amylopectin-like deposits in the myocardium but show minimal signs of hyper-inflammation. However, they show immunodeficiency upon acute infection with Listeria monocytogenes, Toxoplasma gondii or Citrobacter rodentium. Increased susceptibility to Listeria was due to HOIL-1 function in hematopoietic cells and macrophages in production of protective cytokines. In contrast, HOIL-1-deficient mice showed enhanced control of chronic Mycobacterium tuberculosis or murine γ-herpesvirus 68 (MHV68), and these infections conferred a hyper-inflammatory phenotype. Surprisingly, chronic infection with MHV68 complemented the immunodeficiency of HOIL-1, IL-6, Caspase-1 and Caspase-1;Caspase-11-deficient mice following Listeria infection. Thus chronic herpesvirus infection generates signs of auto-inflammation and complements genetic immunodeficiency in mutant mice, highlighting the importance of accounting for the virome in genotype-phenotype studies.

DOI:http://dx.doi.org/10.7554/eLife.04494.001

The immune system protects an individual from invading bacteria, viruses and parasites, as well as malfunctioning or cancerous host cells. However, some people inherit genetic defects that cause part of the immune system to be missing or to not work properly. This is called a genetic immunodeficiency, and puts individuals at a higher risk of infection and disease.

The symptoms of immunodeficiencies can vary substantially between individuals, even when they have defects in the same gene. For example, only some of the individuals who have defects in both of their copies of a gene called HOIL-1—which has been linked to several roles in the body's immune response—are reported to suffer from an altered susceptibility to bacterial infections and chronic (persistent) inflammation. Gaining a clear understanding of the possible factors that influence such variations in the symptoms of genetic immune deficiencies could help to speed up their diagnosis, as well as helping to develop more effective treatments.

MacDuff et al. studied mice that had mutations in both copies of the mouse equivalent of the HOIL-1 gene. These mice, when raised in a clean barrier facility that reduces their exposure to viruses, were severely immunodeficient and died when infected by certain bacteria and parasites, including Listeria monocytogenes. However, they were able to tolerate infections with a herpesvirus or the bacterium that causes tuberculosis. The immunodeficiency to L. monocytogenes was linked to problems producing protective molecules called cytokines, which form a crucial part of the immune response. Unexpectedly, MacDuff et al. found that a chronic herpesvirus infection substantially protected these very immunodeficient animals from infection with Listeria monocytogenes, and the mice were able to efficiently produce protective cytokines.

Mice with two other distinct genetic deficiencies that affect their immune system were also better able to survive otherwise lethal bacterial infections if they had a long-term herpesvirus infection. Macduff et al. suggest that the chronic herpesvirus infection stimulates the immune system, and so allows it to compensate for the lack of cytokine production associated with various immunodeficiencies, including those caused by mutations in the HOIL-1 gene. This suggests that the presence of viruses or other long-term infections may be responsible for some of the variability seen in the symptoms of different individuals with the same genetic immunodeficiency. This is an important concept since essentially all humans have life-long chronic infections from various herpesviruses, as well as other viruses that form the human virome.

DOI:http://dx.doi.org/10.7554/eLife.04494.002

Murine gamma herpesvirus 68 hijacks MAVS and IKKβ to abrogate NFκB activation and antiviral cytokine production

Upon viral infection, mitochondrial antiviral signaling (MAVS) protein serves as a key adaptor to promote cytokine production. We report here that murine gamma herpesvirus 68 (γHV68), a model virus for oncogenic human gamma herpesviruses, subverts cytokine production via the MAVS adaptor. During early infection, γHV68 hijacks MAVS and IKKβ to induce the site-specific phosphorylation of RelA, a crucial subunit of the transcriptionally active NFκB dimer, which primes RelA for the proteasome-mediated degradation. As such, γHV68 efficiently abrogated NFκB activation and cytokine gene expression. Conversely, uncoupling RelA degradation from γHV68 infection promoted NFκB activation and elevated cytokine production. Loss of MAVS increased cytokine production and immune cell infiltration in the lungs of γHV68-infected mice. Moreover, exogenous expression of the phosphorylation- and degradation-resistant RelA variant restored γHV68-induced cytokine production. Our findings uncover an intricate strategy whereby signaling via the upstream MAVS adaptor is intercepted by a pathogen to nullify the immediate downstream effector, RelA, of the innate immune pathway.

Innate immunity represents the first line of defense against invading pathogens chiefly through anti-viral cytokines. The mitochondrial antiviral signaling (MAVS)-dependent innate immune pathways are critical for inflammatory cytokine production. Deficiency in essential innate immune components, such as MAVS, severely impairs cytokine production and host defense that are enabled by the master transcription factor, NFκB. Here we show that murine gamma herpesvirus 68 (γHV68), a model herpesvirus for human Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus, hijacks MAVS and IKKβ to abrogate NFκB activation and cytokine production. Uncoupling RelA degradation from γHV68 infection restored NFκB-dependent cytokine gene expression and elevated cytokine production. Thus, our results demonstrate that upstream innate immune activation can be harnessed by pathogens to inactivate the downstream effector and subvert cytokine production.

The MHV68 M2 protein drives IL-10 dependent B cell proliferation and differentiation

Murine gammaherpesvirus 68 (MHV68) establishes long-term latency in memory B cells similar to the human gammaherpesvirus Epstein Barr Virus (EBV). EBV encodes an interleukin-10 (IL-10) homolog and modulates cellular IL-10 expression; however, the role of IL-10 in the establishment and/or maintenance of chronic EBV infection remains unclear. Notably, MHV68 does not encode an IL-10 homolog, but virus infection has been shown to result in elevated serum IL-10 levels in wild-type mice, and IL-10 deficiency results in decreased establishment of virus latency. Here we show that a unique MHV68 latency-associated gene product, the M2 protein, is required for the elevated serum IL-10 levels observed at 2 weeks post-infection. Furthermore, M2 protein expression in primary murine B cells drives high level IL-10 expression along with increased secretion of IL-2, IL-6, and MIP-1alpha. M2 expression was also shown to significantly augment LPS driven survival and proliferation of primary murine B cells. The latter was dependent on IL-10 expression as demonstrated by the failure of IL10-/- B cells to proliferate in response to M2 protein expression and rescue of M2-associated proliferation by addition of recombinant murine IL-10. M2 protein expression in primary B cells also led to upregulated surface expression of the high affinity IL-2 receptor (CD25) and the activation marker GL7, along with down-regulated surface expression of B220, MHC II, and sIgD. The cells retained CD19 and sIgG expression, suggesting differentiation to a pre-plasma memory B cell phenotype. These observations are consistent with previous analyses of M2-null MHV68 mutants that have suggested a role for the M2 protein in expansion and differentiation of MHV68 latently infected B cells-perhaps facilitating the establishment of virus latency in memory B cells. Thus, while the M2 protein is unique to MHV68, analysis of M2 function has revealed an important role for IL-10 in MHV68 pathogenesis-identifying a strategy that appears to be conserved between at least EBV and MHV68.

TLR9 contributes to antiviral immunity during gammaherpesvirus infection

The human gammaherpesviruses Kaposi's sarcoma-associated herpesvirus and EBV cause important infections. As pathogenetic studies of the human infections are restricted, murine gammaherpesvirus 68 serves as a model to study gammaherpesvirus pathogenesis. TLRs are a conserved family of receptors detecting microbial molecular patterns. Among the TLRs, TLR9 recognizes unmethylated CpG DNA motifs present in bacterial and viral DNA. The aim of this study was to assess the role of TLR9 in gammaherpesvirus pathogenesis. Upon stimulation with murine gammaherpesvirus 68, Flt3L-cultured bone marrow cells (dendritic cells) from TLR9-/- mice secreted reduced levels of IL-12, IFN-alpha, and IL-6, when compared with dendritic cells from wild-type mice. Intranasal infection of TLR9-/- and wild-type mice did not reveal any differences during lytic and latent infection. In contrast, when infected i.p., TLR9-/- mice showed markedly higher viral loads both during lytic and latent infection. Thus, we show for the first time that TLR9 is involved in gammaherpesvirus pathogenesis and contributes to organ-specific immunity.

Inhibition of NF-kappaB activation in vivo impairs establishment of gammaherpesvirus latency

A critical determinant in chronic gammaherpesvirus infections is the ability of these viruses to establish latency in a lymphocyte reservoir. The nuclear factor (NF)-κB family of transcription factors represent key players in B-cell biology and are targeted by gammaherpesviruses to promote host cell survival, proliferation, and transformation. However, the role of NF-κB signaling in the establishment of latency in vivo has not been addressed. Here we report the generation and in vivo characterization of a recombinant murine gammaherpesvirus 68 (γHV68) that expresses a constitutively active form of the NF-κB inhibitor, IκBαM. Inhibition of NF-κB signaling upon infection with γHV68-IκBαM did not affect lytic replication in cell culture or in the lung following intranasal inoculation. However, there was a substantial decrease in the frequency of latently infected lymphocytes in the lung (90% reduction) and spleens (97% reduction) 16 d post intranasal inoculation. Importantly, the defect in establishment of latency in lung B cells could not be overcome by increasing the dose of virus 100-fold. The observed decrease in establishment of viral latency correlated with a loss of activated, CD69^hi B cells in both the lungs and spleen at day 16 postinfection, which was not apparent by 6 wk postinfection. Constitutive expression of Bcl-2 in B cells did not rescue the defect in the establishment of latency observed with γHV68-IκBαM, indicating that NF-κB–mediated functions apart from Bcl-2–mediated B-cell survival are critical for the efficient establishment of gammaherpesvirus latency in vivo. In contrast to the results obtained following intranasal inoculation, infection of mice with γHV68-IκBαM by the intraperitoneal route had only a modest impact on splenic latency, suggesting that route of inoculation may alter requirements for establishment of virus latency in B cells. Finally, analyses of the pathogenesis of γHV68-IκBαM provides evidence that NF-κB signaling plays an important role during multiple stages of γHV68 infection in vivo and, as such, represents a key host regulatory pathway that is likely manipulated by the virus to establish latency in B cells.

A central aspect of chronic infection of a host by herpesviruses is the ability of these viruses to establish a quiescent infection (latent infection) in some cell type(s) in which there is only intermittent production of progeny virus (virus reactivation). The establishment of a latent infection in the antibody producing cells of the host immune system (B lymphocytes) is critical for life-long persistence of gammaherpesviruses, as well as the development of virus-associated lymphoproliferative diseases (e.g., B-cell lymphomas). Nuclear factor (NF)-κB transcription factors are a family of cellular proteins that play an important role regulating gene expression in B cells, and it has been shown that gammaherpesviruses have evolved multiple strategies for manipulating NF-κB activity. However, to date there has been no reported examination of the role of NF-κB in the establishment of chronic gammaherpesvirus infection in vivo. Murine gammaherpesvirus 68 (γHV68) infects rodents and shares genetic and biologic properties with the human gammaherpesviruses, Epstein-Barr virus and Kaposi sarcoma–associated herpesvirus. To selectively block the function of NF-κB in infected cells, we engineered a transgenic virus that expresses a repressor of NF-κB activation (IκBαM). Notably, this recombinant virus was defective in the establishment of latency in B cells in the lungs and spleen following intranasal inoculation. We also observed that the decrease in B-cell infection could not be rescued by forced expression of the cellular Bcl-2 protein, which is normally upregulated by NF-κB and serves to protect B cells from some forms of cell death. Thus, we conclude that NF-κB is an important host factor for the successful establishment of a chronic infection by gammaherpesviruses, and likely requires functions of NF-κB apart from its role in B-cell survival.

Limited IL-6 production following infection with murine gammaherpesvirus 68

Murine gammaherpesvirus 68 (gammaHV-68) was found to induce IL-6 secretion following in vitro infection of macrophages, but not cultured dendritic or epithelial cells. A detectable, but very limited IL-6 response was observed in the lungs and mediastinal lymph nodes following intranasal infection. Surprisingly, no detectable in vivo IL-6 production was observed in the spleen or sera of infected mice despite observable systemic leukocytosis. These studies demonstrate that endogenous IL-6 production contributes little to the host response, or to the viral-induced mononucleosis-like disease, due to the fact that limiting amounts of this cytokine are produced in vivo during gammaHV-68 infection.

Murine gammaherpesvirus 68 open reading frame 45 plays an essential role during the immediate-early phase of viral replication

Murine gammaherpesvirus 68 (MHV-68) has been developed as a model for the human gammaherpesviruses Epstein-Barr virus and human herpesvirus 8/Kaposi's sarcoma-associated herpesvirus (HHV-8/KSHV), which are associated with several types of human diseases. Open reading frame 45 (ORF45) is conserved among the members of the Gammaherpesvirinae subfamily and has been suggested to be a virion tegument protein. The repression of ORF45 expression by small interfering RNAs inhibits MHV-68 viral replication. However, the gene product of MHV-68 ORF45 and its function have not yet been well characterized. In this report, we show that MHV-68 ORF45 is a phosphorylated nuclear protein. We constructed an ORF45-null MHV-68 mutant virus (45STOP) by the insertion of translation termination codons into the portion of the gene encoding the N terminus of ORF45. We demonstrated that the ORF45 protein is essential for viral gene expression immediately after the viral genome enters the nucleus. These defects in viral replication were rescued by providing ORF45 in trans or in an ORF45-null revertant (45STOP.R) virus. Using a transcomplementation assay, we showed that the function of ORF45 in viral replication is conserved with that of its KSHV homologue. Finally, we found that the C-terminal 23 amino acids that are highly conserved among the Gammaherpesvirinae subfamily are critical for the function of ORF45 in viral replication.

Initiation of the Host Response Against Murine Gammaherpesvirus Infection in Immunocompetent Mice

Murine gammaherpesvirus 68 (gammaHV-68) provides a useful model for understanding the initiation of the host response against the gammaherpesviruses. Its value as a model for such studies lies in large part with the inherent difficulties in investigating human responses against EBV and HHV-8 during the first few days following infection. While studies aimed at defining the initiation of gammaHV-68 infection are far from complete, an unexpected trend in this early host response has already emerged. Despite viral replication and the beginnings of viral latency at the site of infection during the first few days following infection, the early host response seems surprisingly inadequate. For example, the pro-inflammatory response is quite limited, and with the exception of the type I interferons, it is not at all clear what innate responses are necessary to provide protection from acute infection. This confusion results from the lack of any significant effect on acute viral replication in several strains of mice which have been made genetically deficient in the expression of particular pro-inflammatory molecules. It is likely that these unexpected results reflect the ability of gammaherpesviruses to carefully control the initial response so that they are efficacious pathogens even in immunocompetent hosts.

Cytokines and Costimulatory Molecules in the Immune Response to Murine Gammaherpesvirus-68

Murine gammaherpesvirus 68 (MHV-68) infection of mice provides a useful small animal model for studying gammaherpesvirus pathogenesis and immunity. Recent work has elucidated the cytokine and chemokine profiles during MHV-68 infection and has identified some of the costimulatory interactions that are important for an effective immune response to this virus. Several themes emerge from this work. There is a differential requirement for certain cytokines and costimulatory molecules in the acute and long-term control of MHV-68, and for control of the virus in different anatomical sites. CD4 T cell help is not required for short-term control of MHV-68 in the lung by cytotoxic CD8 T cells, but is essential for effective long-term control. Stimulation via CD40 is an important component of this CD4 T cell help, and interestingly, some of its effects appear to be independent of CD28. MHV-68 infection also increases the expression of several chemokines, which could potentially play important roles in leukocyte trafficking to sites of infection. However, to counter this response, MHV-68 has evolved strategies that enable it to evade or subvert the host chemokine system. Studying the role of cytokines and costimulatory molecules in immunity to MHV-68 may provide useful insights for the development of agents to control gammaherpesviruses that cause human disease.

Murine γ-Herpesvirus-68-Induced IL-12 Contributes to the Control of Latent Viral Burden, but Also Contributes to Viral-Mediated Leukocytosis

Early IFN-alpha/beta production, followed by the development of a viral-specific CTL response, are critical factors in limiting the level of murine gamma-herpesvirus-68 (gammaHV-68) infection. Development of a long-lived CTL response requires T cell help, and these CTLs most likely function to limit the extent of infection following reactivation. The importance of IL-12 in the development and/or activity of Th1 cells and CTLs is well documented, and we investigated the kinetics and magnitude of gammaHV-68-induced IL-12 production. Following intranasal infection, IL-12 and IL-23 mRNA expression was up-regulated in lung and spleen and lung, respectively, followed by increased levels of IL-12p40 in lung homogenates and sera. Exposure of cultured macrophages or dendritic cells to gammaHV-68 induced secretion of IL-12, suggesting that these cells might be responsible for IL-12 production in vivo. gammaHV-68 infection of mice made genetically deficient in IL-12p40 expression (IL-12p40(-/-)) resulted in a leukocytosis and splenomegaly that was significantly less than that observed in syngeneic C57BL/6 mice. IL-12p40(-/-) mice showed increased levels of infectious virus in the lung, but only at day 9 postinfection. Increased levels of latent virus in the spleen at day 15 postinfection were also observed in IL-12p40(-/-) mice when compared with syngeneic C57BL/6 mice. An overall reduction in gammaHV-68-induced IFN-gamma production was observed in IL-12p40(-/-) mice, suggesting that most of the viral-induced IFN-gamma in C57BL/6 mice was IL-12 dependent. Taken together, these results suggest that gammaHV-68-induced IL-12 contributes to the pathophysiology of viral infection while also functioning to limit viral burden.

Reduced CTL response and increased viral burden in substance P receptor-deficient mice infected with murine gamma-herpesvirus 68

One component of the protective host response against mucosal pathogens includes the local production and increased expression of certain neuropeptides and their receptors. The present study further demonstrates this fact by investigating the contribution that substance P receptor expression makes toward immunity against a gamma-herpesvirus infection. Following intragastric inoculation with murine gamma-herpesvirus 68 (gamma HV-68), expression of substance P and its receptor was increased in mucosal and peripheral lymphoid organs in wild-type strains of mice. These results suggested that this receptor/ligand pair might be an important component of the host response against this viral infection. Such a hypothesis was supported by the demonstration that mice, genetically deficient in substance P receptor expression, showed an increased viral burden when compared with syngeneic C57BL/6 mice. Furthermore, substance P receptor-deficient mice showed a reduced CTL response against gamma HV-68, suggesting a mechanism to explain this increased viral burden. Such limitations in the Ag-specific CTL response in substance P receptor-deficient mice could result from lowered expression of IL-12 during viral infection. Consistent with this hypothesis, increases in mRNA encoding IL-12 and secretion of this cytokine into sera of infected, wild-type animals were markedly reduced in substance P receptor-deficient mice. These studies demonstrate that genetic elimination of substance P receptors in mice results in an increased gamma-herpesvirus burden and an altered host response.

Interleukin-6 protects anterior horn neurons from lethal virus-induced injury

We evaluated the role of interleukin-6 (IL-6) in neuronal injury after CNS infection. IL-6-/- and IL-6+/+ mice of resistant major histocompatibility complex (MHC) H-2b haplotype intracerebrally infected with Theiler's virus cleared the infection normally without development of viral persistence, lethal neuronal infection, or late phase demyelination. In contrast, infection of IL-6-/- mice on a susceptible H-2q haplotype resulted in frequent deaths and severe neurologic deficits within 2 weeks of infection as compared with infected IL-6+/+ H-2q littermate controls. Morphologic analysis demonstrated dramatic injury to anterior horn neurons of IL-6-/- H-2q mice at 12 d after infection. Infectious viral titers in the CNS (brain and spinal cord combined) were equivalent between IL-6-/- H-2q and IL-6+/+ H-2q mice. In contrast, more viral RNA was detected in the spinal cord of IL-6-/- mice compared with IL-6+/+ H-2q mice. Virus antigen was localized predominantly to anterior horn cells in infected IL-6-/- H-2q mice. IL-6 deletion did not affect the humoral response directed against virus, nor did it affect the expression of CD4, CD8, MHC class I, or MHC class II in the CNS. Importantly, IL-6 was expressed by astrocytes of infected IL-6+/+ mice but not in astrocytes of IL-6-/- mice or uninfected IL-6+/+ mice. Furthermore, expression of various chemokines was robust at 12 d after infection in both H-2b and H-2q IL-6-/- mice, indicating that intrinsic CNS inflammatory responses did not depend on the presence of IL-6. Finally, in vitro analysis of virus-induced death in neuroblastoma-spinal cord-34 motor neurons and primary anterior horn cell neurons showed that IL-6 exerted a neuroprotective effect. These data support the hypothesis that IL-6 plays a critical role in protecting specific populations of neurons from irreversible injury.

Kinetics of murine gammaherpesvirus 68 gene expression following infection of murine cells in culture and in mice

A model system to study the pathogenesis of gammaherpesvirus infections is the infection of mice with murine gammaherpesvirus 68 (MHV-68). To define the kinetics of infection, we developed an RNase protection assay to quantitate gene expression from lytic (K3, Rta, M8, DNA polymerase [DNA pol], and gB) and candidate latency (M2, M3, M9, M11, ORF73, and ORF74) genes. All candidate latency genes were expressed during lytic infection of 3T3 cells. Four kinetic classes of transcripts were observed following infection of 3T3 cells: immediate-early (K3, Rta, M8, and ORF73), early (DNA pol), early-late (M3, M11, and ORF74), and late (M2, M9, and gB). To assess the kinetics of viral gene expression in vivo, lungs, spleens, and mediastinal lymph nodes (MLN) were harvested from MHV-68-infected mice. All transcripts were expressed between 3 and 6 days postinfection (dpi) in the lungs. In the spleen, K3, M3, M8, and M9 transcripts were expressed between 10 and 16 dpi when latency is established. The K3, M3, M8, M9, and M11 transcripts were detected in the MLN from 2 through 16 dpi. This is the first demonstration of MHV-68 gene expression in the MLN. Importantly, our data showed that MHV-68 has different kinetics of gene expression at different sites of infection. Furthermore, we demonstrated that K3, a gene recently shown to encode a protein that downregulates major histocompatibility complex class I on the surface of cells, is expressed during latency, which argues for a role of K3 in immune evasion during latent infection.

Dissecting the host response to a gamma-herpesvirus

The murine gamma-herpesvirus 68 (MHV-68) provides a unique experimental model for dissecting immunity to large DNA viruses that persist in B lymphocytes. The analysis is greatly facilitated by the availability of genetically disrupted (-/-) mice that lack key host-response elements, and by the fact that MHV-68 is a lytic virus that can readily be manipulated for mutational analysis. The mutant virus strategy is being used, for example, to characterize the part played in vivo by an MHV-68-encoded chemokine-binding protein that may ultimately find an application in human therapeutics. Experiments with various -/- mice and monoclonal antibody depletion protocols have shown very clearly that type I interferons (IFNs) are essential for the early control of MHV-68 replication, while CD4+ T cells producing IFN-gamma function to limit the consequences of viral persistence. Virus-specific CD8+ effectors acting in the absence of the CD4+ subset seem initially to control the lytic phase in the lung following respiratory challenge, but are then unable to prevent the reactivation of replicative infection in epithelia and the eventual death of CD4+ T-cell-deficient mice. This could reflect the fact that the interaction between the CD8+ T cells and the virus-infected targets is partially compromised by the MHV-68 K3 protein, which inhibits antigen presentation by MHC class I glycoproteins. Immunization strategies focusing on the CD8+ T-cell response to epitopes expressed during the lytic phase of MHV-68 infection can limit virus replication, but are unable to prevent the establishment of latency. Other experiments with mutant viruses also suggest that there is a disconnection between lytic MHV-68 infection and latency. The massive nonspecific immunoglobulin response and the dramatic expansion of Vbeta4+ CD8+ T cells, which is apparently MHC independent, could represent some sort of 'smoke screen' used by MHV-68 to subvert immunity. Although MHV-68 is neither Epstein-Barr virus nor human herpesvirus-8, the results generated from this system suggest possibilities that may usefully be addressed with these human pathogens. Perhaps the main lesson learned to date is that all the components of immunity are likely to be important for the control of these complex viruses.

Natural history of murine gamma-herpesvirus infection

Murine gamma-herpesvirus 68 (MHV-68) is a natural pathogen of small rodents and insectivores (mice, voles and shrews). The primary infection is characterized by virus replication in lung epithelial cells and the establishment of a latent infection in B lymphocytes. The virus is also observed to persist in lung epithelial cells, dendritic cells and macrophages. Splenomegaly is observed two weeks after infection, in which there is a CD4+ T-cell-mediated expansion of B and T cells in the spleen. At three weeks post-infection an infectious mononucleosis-like syndrome is observed involving a major expansion of Vbeta4+CD8+ T cells. Later in the course of persistent infection, ca. 10% of mice develop lymphoproliferative disease characterized as lymphomas of B-cell origin. The genome from MHV-68 strain g2.4 has been sequenced and contains ca. 73 genes, the majority of which are collinear and homologous to other gamma-herpesviruses. The genome includes cellular homologues for a complement-regulatory protein, Bcl-2, cyclin D and interleukin-8 receptor and a set of novel genes M1 to M4. The function of these genes in the context of latent infections, evasion of immune responses and virus-mediated pathologies is discussed. Both innate and adaptive immune responses play an active role in limiting virus infection. The absence of type I interferon (IFN) results in a lethal MHV-68 infection, emphasizing the central role of these cytokines at the initial stages of infection. In contrast, type II IFN is not essential for the recovery from infection in the lung, but a failure of type II IFN receptor signalling results in the atrophy of lymphoid tissue associated with virus persistence. Splenic atrophy appears to be the result of immunopathology, since in the absence of CD8+ T cells no pathology occurs. CD8+ T cells play a major role in recovery from the primary infection, and also in regulating latently infected cells expressing the M2 gene product. CD4+ T cells have a key role in surveillance against virus recurrences in the lung, in part mediated through 'help' in the genesis of neutralizing antibodies. In the absence of CD4+ T cells, virus-specific CD8+ T cells are able to control the primary infection in the respiratory tract, yet surprisingly the memory CD8+ T cells generated are unable to inhibit virus recurrences in the lung. This could be explained in part by the observations that this virus can downregulate major histocompatibility complex class I expression and also restrict inflammatory cell responses by producing a chemokine-binding protein (M3 gene product). MHV-68 provides an excellent model to explore methods for controlling gamma-herpesvirus infection through vaccination and chemotherapy. Vaccination with gp150 (a homologue of gp350 of Epstein-Barr virus) results in a reduction in splenomegaly and virus latency but does not block replication in the lung, nor the establishment of a latent infection. Even when lung virus infection is greatly reduced following the action of CD8+ T cells, induced via a prime-boost vaccination strategy, a latent infection is established. Potent antiviral compounds such as the nucleoside analogue 2'deoxy-5-ethyl-beta-4'-thiouridine, which disrupts virus replication in vivo, cannot inhibit the establishment of a latent infection. Clearly, devising strategies to interrupt the establishment of latent virus infections may well prove impossible with existing methods.

Human herpesvirus 8 infections in the Amsterdam Cohort Studies (1984-1997): analysis of seroconversions to ORF65 and ORF73

We have shown previously that human herpesvirus 8 (HHV8) seroconversion for antibodies to the latency-associated nuclear antigen encoded by ORF73 and/or the lytic capsid antigen (vp19) encoded by ORF65 is associated with orogenital contact and is strongly linked to the development of Kaposi's sarcoma among HIV-infected individuals in the Amsterdam Cohort Studies. Here, we investigate the relationship between seroconversion to these antigens and primary HHV8 infection. Between 1984 and 1997, 215 HHV8 seroconversions to ORF73 (106 cases or 49%) and/or to ORF65 (159 cases or 74%) were recorded in the cohort of homosexual men. The HHV8 seroconversion rate among HIV-infected homosexual men (6.2 per 100 person years) was consistently higher than among HIV-uninfected men (2.6 per 100 person years). In HIV-infected but not in uninfected individuals, seroconversion to ORF73/latency-associated nuclear antigen precedes that to ORF65/vp19. Antibody levels to both ORF65- and ORF73-encoded antigens were higher in HIV-infected than in HIV-uninfected men, and among HIV-seropositives, antibody levels to ORF65/vp19 rise even higher with declining CD4 cell counts and peak with Kaposi's sarcoma development, suggesting continuing and increasing viral replication. In 10.3% of HHV8 seroconversions, transient serum viremia could be demonstrated before or at seroconversion. Together with the previously reported link between unprotected orogenital sex and HHV8 seroconversion, our observations suggest that HHV8 seroconversions result from primary infections.

Host and viral genetics of chronic infection: a mouse model of gamma-herpesvirus pathogenesis

A general association of human and primate lymphotropic herpesviruses (gamma-herpesviruses) with the development of lymphomas, as well as other tumors, especially in immunocompromised hosts, has been well documented. The lack of relevant small animal models for human gamma-herpesviruses has impeded progress in understanding the role of these viruses in the development of chronic disease. Recent research characterizing infection of inbred strains of mice with a murine gamma-herpesvirus, gamma-herpesvirus 68 (gammaHV68), is providing insights into viral and host factors involved in the establishment and control of chronic gamma-herpesvirus infection.

Non-antigen-specific B-cell activation following murine gammaherpesvirus infection is CD4 independent in vitro but CD4 dependent in vivo

The murine gammaherpesvirus MHV-68 multiplies in the respiratory epithelium after intranasal inoculation, then spreads to infect B cells in lymphoid germinal centers. Exposing B cells to MHV-68 in vitro caused an increase in cell size, up-regulation of the CD69 activation marker, and immunoglobulin M (IgM) production. The infectious process in vivo was also associated with increased CD69 expression on B cells in the draining lymph nodes and spleen, together with a rise in total serum Ig. However, whereas the in vitro effect on B cells was entirely T-cell independent, evidence of in vivo B-cell activation was minimal in CD4(+) T-cell-deficient (I-Ab-/-) or CD4(+) T-cell-depleted mice. Furthermore, the Ig present at high levels in serum was predominantly of the IgG class. Surprisingly, the titer of influenza virus-specific serum IgG in previously immunized mice fell following MHV-68 infection, suggesting that there was relatively little activation of memory B cells. Thus, CD4(+) T cells seemed both to amplify a direct viral activation of B cells in lymphoid tissue and to promote new Ig class switching despite a lack of obvious cognate antigen.