Latency versus persistence or intermittent recurrences: evidence for a latent state of murine cytomegalovirus in the lungs

Promoter control over foreign antigen expression in a murine cytomegalovirus vaccine vector

Previous studies have reported on the development of a recombinant murine cytomegalovirus (rMCMV) containing the mouse zona pellucida 3 (mZP3) gene for use as a virally vectored immunocontraceptive (VVIC). This study aimed to alter promoter control over foreign antigen expression and cellular localisation of the antigen expressed in order to overcome virus attenuation previously encountered. Early studies reported on the mZP3 gene expressed by a strong constitutive human cytomegalovirus immediate-early 1 promoter (pHCMV IE1). This virus was able to induce >90% infertility in BALB/c mice despite being heavily attenuated in vivo. In this study the mZP3 was placed under the control of the MCMV early 1 (pMCMV E1) promoter and the inducible tetracycline promoter (Tet-On). In both instances the recombinant virus was able to induce infertility in directly infected mice. However, the viruses remained attenuated. This study demonstrated the capacity to manipulate the nature of the immune response by altering promoter control over foreign antigen expression and cellular localisation of the expressed antigen. We were able to demonstrate that by using the MCMV E1 promoter it was still possible to sterilize female BALB/c mice with an MCMV vector expressing mZP3. The use of the MCMV E1 promoter provides an added level of safety to any MCMV based VVIC approach as it only allows for transgene expression in MCMV permissive cells.

A novel transmembrane domain mediating retention of a highly motile herpesvirus glycoprotein in the endoplasmic reticulum

Gene m164 of murine cytomegalovirus belongs to the large group of 'private' genes that show no homology to those of other cytomegalovirus species and are thought to represent 'host adaptation' genes involved in virus-host interaction. Previous interest in the m164 gene product was based on the presence of an immunodominant CD8 T-cell epitope presented at the surface of infected cells, despite interference by viral immune-evasion proteins. Here, we provide data to reveal that the m164 gene product shows unusual features in its cell biology. A novel strategy of mass-spectrometric analysis was employed to map the N terminus of the mature protein, 107 aa downstream of the start site of the predicted open reading frame. The resulting 36.5 kDa m164 gene product is identified here as an integral type-I membrane glycoprotein with exceptional intracellular trafficking dynamics, moving within the endoplasmic reticulum (ER) and outer nuclear membrane with an outstandingly high lateral membrane motility, actually 100 times higher than those published for cellular ER-resident proteins. Notably, gp36.5/m164 does not contain any typical ER-retention/retrieval signals, such as the C-terminal motifs KKXX or KXKXX, and does not pass the Golgi apparatus. Instead, it belongs to the rare group of viral glycoproteins in which the transmembrane domain (TMD) itself mediates direct ER retention. This is the first report relating TMD usage of an ER-resident transmembrane protein to its lateral membrane motility as a paradigm in cell biology. We propose that TMD usage for ER retention facilitates free and fast floating in ER-related membranes and between ER subdomains.

Antiviral prevention of sepsis induced cytomegalovirus reactivation in immunocompetent mice

INTRODUCTION

Immunocompetent patients can reactivate latent cytomegalovirus (CMV) during critical illness and reactivation is associated with significantly worse outcomes. Prior to clinical trials in humans to prove causality, we sought to determine an optimal antiviral treatment strategy.

METHODS

Mice latently infected with murine CMV (MCMV) received a septic reactivation trigger and were randomized to receive one of four ganciclovir regimens or saline. Lungs were evaluated for viral transcriptional reactivation and fibrosis after each regimen. Influences of ganciclovir on early sepsis-induced pulmonary inflammation and T-cell activation were studied after sepsis induction.

RESULTS

All ganciclovir regimens reduced measurable MCMV transcriptional reactivation, and 10mg/day for 7 or 21 days was most effective. Lower dose (5mg/kg/day) or delayed therapy was associated with significant breakthrough reactivation. Higher doses of ganciclovir given early were associated with the lowest incidence of pulmonary fibrosis, and delay of therapy for 1 week was associated with significantly worse pulmonary fibrosis. Although bacterial sepsis induced activation of MCMV-specific pulmonary T-cells, this activation was not influenced by ganciclovir.

CONCLUSION

These results suggest that antiviral treatment trials in humans should use 10mg/kg/day ganciclovir administered as early as possible in at-risk patients to minimize reactivation events and associated pulmonary injury.

Immune evasion proteins of murine cytomegalovirus preferentially affect cell surface display of recently generated peptide presentation complexes

For recognition of infected cells by CD8 T cells, antigenic peptides are presented at the cell surface, bound to major histocompatibility complex class I (MHC-I) molecules. Downmodulation of cell surface MHC-I molecules is regarded as a hallmark function of cytomegalovirus-encoded immunoevasins. The molecular mechanisms by which immunoevasins interfere with the MHC-I pathway suggest, however, that this downmodulation may be secondary to an interruption of turnover replenishment and that hindrance of the vesicular transport of recently generated peptide-MHC (pMHC) complexes to the cell surface is the actual function of immunoevasins. Here we have used the model of murine cytomegalovirus (mCMV) infection to provide experimental evidence for this hypothesis. To quantitate pMHC complexes at the cell surface after infection in the presence and absence of immunoevasins, we generated the recombinant viruses mCMV-SIINFEKL and mCMV-Deltam06m152-SIINFEKL, respectively, expressing the K(b)-presented peptide SIINFEKL with early-phase kinetics in place of an immunodominant peptide of the viral carrier protein gp36.5/m164. The data revealed approximately 10,000 K(b) molecules presenting SIINFEKL in the absence of immunoevasins, which is an occupancy of approximately 10% of all cell surface K(b) molecules, whereas immunoevasins reduced this number to almost the detection limit. To selectively evaluate their effect on preexisting pMHC complexes, cells were exogenously loaded with SIINFEKL peptide shortly after infection with mCMV-SIINFEKA, in which endogenous presentation is prevented by an L174A mutation of the C-terminal MHC-I anchor residue. The data suggest that pMHC complexes present at the cell surface in advance of immunoevasin gene expression are downmodulated due to constitutive turnover in the absence of resupply.

Synergism between the components of the bipartite major immediate-early transcriptional enhancer of murine cytomegalovirus does not accelerate virus replication in cell culture and host tissues

Major immediate-early (MIE) transcriptional enhancers of cytomegaloviruses are key regulators that are regarded as determinants of virus replicative fitness and pathogenicity. The MIE locus of murine cytomegalovirus (mCMV) shows bidirectional gene-pair architecture, with a bipartite enhancer flanked by divergent core promoters. Here, we have constructed recombinant viruses mCMV-ΔEnh1 and mCMV-ΔEnh2 to study the impact of either enhancer component on bidirectional MIE gene transcription and on virus replication in cell culture and various host tissues that are relevant to CMV disease. The data revealed that the two unipartite enhancers can operate independently, but synergize in enhancing MIE gene expression early after infection. Kick-start transcription facilitated by the bipartite enhancer configuration, however, did not ultimately result in accelerated virus replication. We conclude that virus replication, once triggered, proceeds with a fixed speed and we propose that synergism between the components of the bipartite enhancer may rather increase the probability for transcription initiation.

Liver sinusoidal endothelial cells are a site of murine cytomegalovirus latency and reactivation

Latent cytomegalovirus (CMV) is frequently transmitted by organ transplantation, and its reactivation under conditions of immunosuppressive prophylaxis against graft rejection by host-versus-graft disease bears a risk of graft failure due to viral pathogenesis. CMV is the most common cause of infection following liver transplantation. Although hematopoietic cells of the myeloid lineage are a recognized source of latent CMV, the cellular sites of latency in the liver are not comprehensively typed. Here we have used the BALB/c mouse model of murine CMV infection to identify latently infected hepatic cell types. We performed sex-mismatched bone marrow transplantation with male donors and female recipients to generate latently infected sex chromosome chimeras, allowing us to distinguish between Y-chromosome (gene sry or tdy)-positive donor-derived hematopoietic descendants and Y-chromosome-negative cells of recipients' tissues. The viral genome was found to localize primarily to sry-negative CD11b(-) CD11c(-) CD31(+) CD146(+) cells lacking major histocompatibility complex class II antigen (MHC-II) but expressing murine L-SIGN. This cell surface phenotype is typical of liver sinusoidal endothelial cells (LSECs). Notably, sry-positive CD146(+) cells were distinguished by the expression of MHC-II and did not harbor latent viral DNA. In this model, the frequency of latently infected cells was found to be 1 to 2 per 10(4) LSECs, with an average copy number of 9 (range, 4 to 17) viral genomes. Ex vivo-isolated, latently infected LSECs expressed the viral genes m123/ie1 and M122/ie3 but not M112-M113/e1, M55/gB, or M86/MCP. Importantly, in an LSEC transfer model, infectious virus reactivated from recipients' tissue explants with an incidence of one reactivation per 1,000 viral-genome-carrying LSECs. These findings identified LSECs as the main cellular site of murine CMV latency and reactivation in the liver.

NK cell receptors and their MHC class I ligands in host response to cytomegalovirus: insights from the mouse genome

The complex interaction between natural killer (NK) cells and cytomegalovirus is a paradigm of the co-evolution between genomes of large DNA viruses and their host immune systems. Both human and mouse cytomegalovirus posses numerous mechanisms to avoid NK cell detection. Linkage studies, positional cloning and functional studies in mice and cells, have led to the identification of key genes governing resistance to cytomegalovirus, including various NK cell activating receptors of major histocompatibility complex (MHC) class I. These receptors, however, seem to require either viral or host MHC class I molecules to operate recognition and elimination of the cytomegalovirus-infected cell leading to host resistance. Here we will review the genes and molecules involved in these mechanisms while contrasting their function with that of other NK cell receptors. Activating receptors of MHC class I may represent a window of therapeutic intervention during human infection with viruses, of which cytomegalovirus remains an important health threat.

The immune evasion paradox: immunoevasins of murine cytomegalovirus enhance priming of CD8 T cells by preventing negative feedback regulation

Cytomegaloviruses express glycoproteins that interfere with antigen presentation to CD8 T cells. Although the molecular modes of action of these "immunoevasins" differ between cytomegalovirus species, the convergent biological outcome is an inhibition of the recognition of infected cells. In murine cytomegalovirus, m152/gp40 retains peptide-loaded major histocompatibility complex class I molecules in a cis-Golgi compartment, m06/gp48 mediates their vesicular sorting for lysosomal degradation, and m04/gp34, although not an immunoevasin in its own right, appears to assist in the concerted action of all three molecules. Using the L(d)-restricted IE1 epitope YPHFMPTNL in the BALB/c mouse model as a paradigm, we provide here an explanation for the paradox that immunoevasins enhance CD8 T-cell priming although they inhibit peptide presentation in infected cells. Adaptive immune responses are initiated in the regional lymph node (RLN) draining the site of pathogen exposure. In particular for antigens that are not virion components, the magnitude of viral gene expression providing the antigens is likely a critical parameter in priming efficacy. We have therefore focused on the events in the RLN and have related priming to intranodal viral gene expression. We show that immunoevasins enhance priming by downmodulating an early CD8 T-cell-mediated "negative feedback" control of the infection in the cortical region of the RLN, thus supporting the model that immunoevasins improve antigen supply for indirect priming by uninfected antigen-presenting cells. As an important consequence, these findings predict that deletion of immunoevasin genes in a replicative vaccine virus is not a favorable option but may, rather, be counterproductive.

Establishment of murine cytomegalovirus latency in vivo is associated with changes in histone modifications and recruitment of transcriptional repressors to the major immediate-early promoter

Human cytomegalovirus (CMV) is a ubiquitous herpesvirus with the ability to establish a lifelong latent infection. The mechanism by which this occurs is not well understood. Regulation of, for example, immediate-early (IE) gene expression is thought to be a critical control point in transcriptional control of the switch between latency and reactivation. Here, we present evidence that supports previous studies showing that the majority of genomes are quiescent with respect to gene expression. To study the possible role of epigenetic factors that may be involved in repression of ie gene expression in latency, we have analyzed changes in the patterns of modifications of histones bound to the major IE promoter (MIEP) in the kidneys of acutely and latently infected mice. Our studies show that, like herpes simplex virus, murine CMV genomes become relatively enriched in histones in latent infection. There are dramatic changes in modifications of histones associated with the MIEP when latency is established: H3 and H4 become hypoacetylated and H3 is hypomethylated at lysine 4, while H3 lysine 9 is hypermethylated in latently infected mice. These changes are accompanied by a relative loss of RNA polymerase and gain of heterochromatin protein 1gamma and Yin-Yang 1 bound to the MIEP. Our studies suggest that, in the majority of cells, CMV establishes a true latent infection, defined as the lack of expression of genes associated with productive infection, and that this occurs through changes in histone modifications and recruitment of transcriptional silencing factors to the MIEP.

Manifestations of human cytomegalovirus infection: proposed mechanisms of acute and chronic disease

Infections with human cytomegalovirus (HCMV) are a major cause of morbidity and mortality in humans with acquired or developmental deficits in innate and adaptive immunity. In the normal immunocompetent host, symptoms rarely accompany acute infections, although prolonged virus shedding is frequent. Virus persistence is established in all infected individuals and appears to be maintained by both a chronic productive infections as well as latency with restricted viral gene expression. The contributions of the each of these mechanisms to the persistence of this virus in the individual is unknown but frequent virus shedding into the saliva and genitourinary tract likely accounts for the near universal incidence of infection in most populations in the world. The pathogenesis of disease associated with acute HCMV infection is most readily attributable to lytic virus replication and end organ damage either secondary to virus replication and cell death or from host immunological responses that target virus-infected cells. Antiviral agents limit the severity of disease associated with acute HCMV infections, suggesting a requirement for virus replication in clinical syndromes associated with acute infection. End organ disease secondary to unchecked virus replication can be observed in infants infected in utero, allograft recipients receiving potent immunosuppressive agents, and patients with HIV infections that exhibit a loss of adaptive immune function. In contrast, diseases associated with chronic or persistent infections appear in normal individuals and in the allografts of the transplant recipient. The manifestations of these infections appear related to chronic inflammation, but it is unclear if poorly controlled virus replication is necessary for the different phenotypic expressions of disease that are reported in these patients. Although the relationship between HCMV infection and chronic allograft rejection is well known, the mechanisms that account for the role of this virus in graft loss are not well understood. However, the capacity of this virus to persist in the midst of intense inflammation suggests that its persistence could serve as a trigger for the induction of host-vs-graft responses or alternatively host responses to HCMV could contribute to the inflammatory milieu characteristic of chronic allograft rejection.

Transactivation of cellular genes involved in nucleotide metabolism by the regulatory IE1 protein of murine cytomegalovirus is not critical for viral replicative fitness in quiescent cells and host tissues

Despite its high coding capacity, murine CMV (mCMV) does not encode functional enzymes for nucleotide biosynthesis. It thus depends on cellular enzymes, such as ribonucleotide reductase (RNR) and thymidylate synthase (TS), to be supplied with deoxynucleoside triphosphates (dNTPs) for its DNA replication. Viral transactivation of these cellular genes in quiescent cells of host tissues is therefore a parameter of viral fitness relevant to pathogenicity. Previous work has shown that the IE1, but not the IE3, protein of mCMV transactivates RNR and TS gene promoters and has revealed an in vivo attenuation of the mutant virus mCMV-DeltaIE1. It was attractive to propose the hypothesis that lack of transactivation by IE1 and a resulting deficiency in the supply of dNTPs are the reasons for growth attenuation. Here, we have tested this hypothesis with the mutant virus mCMV-IE1-Y165C expressing an IE1 protein that selectively fails to transactivate RNR and TS in quiescent cells upon transfection while maintaining the capacity to disperse repressive nuclear domains (ND10). Our results confirm in vivo attenuation of mCMV-DeltaIE1, as indicated by a longer doubling time in host organs, whereas mCMV-IE1-Y165C replicated like mCMV-WT and the revertant virus mCMV-IE1-C165Y. Notably, the mutant virus transactivated RNR and TS upon infection of quiescent cells, thus indicating that IE1 is not the only viral transactivator involved. We conclude that transactivation of cellular genes of dNTP biosynthesis is ensured by redundancy and that attenuation of mCMV-DeltaIE1 results from the loss of other critical functions of IE1, with its function in the dispersal of ND10 being a promising candidate.

Prior infection with murine cytomegalovirus (MCMV) limits the immunocontraceptive effects of an MCMV vector expressing the mouse zona-pellucida-3 protein

We have developed a murine cytomegalovirus (MCMV)-vectored vaccine expressing the mouse zona-pellucida-3 gene (rMCMV-ZP3), which successfully induces infertility in experimentally inoculated laboratory or wild-derived mice. However, the future success of this vector as a fully disseminating vaccine in free-living mice may be compromised by pre-existing immunity since there is a high prevalence of naturally acquired MCMV infection in these mice. To evaluate the effect of prior immunity to MCMV on vaccine efficacy, we constructed two new biologically effective recombinant MCMV vectors expressing the mouse ZP3 protein from two MCMV strains (N1 and G4) derived from free-living mice. In wild mice, mixed MCMV infection is common and could be acquired either by simultaneous coinfection or sequential infection with different MCMV strains. Interestingly, while coinfection with both wild-type and rMCMV-ZP3 via the intraperitoneal route reduced the impact of the rMCMV-ZP3, prior infection with the same wild-type strain as that used to construct the rMCMV-ZP3 abrogated the immunocontraceptive effects of either N1-ZP3 or G4-ZP3. However, prior infection with G4 28 days before the introduction of N1-ZP3 had a reduced influence on the efficacy of the rMCMV-ZP3. Thus, the strain of virus and the timing of prior infection are factors that may influence the efficacy of the rMCMV-ZP3. Given that mixed infection of mice with MCMV is common, it is possible that prior immunity acquired by natural mucosal infection may have less a less inhibitory effect on the immunocontraceptive outcome.

Role of the cytomegalovirus major immediate early enhancer in acute infection and reactivation from latency

The cytomegalovirus (CMV) major immediate early (MIE) enhancer-containing promoter regulates the expression of the downstream MIE genes, which have critical roles in reactivation from latency and acute infection. The enhancer consists of binding sites for cellular transcription factors that are repeated multiple times. The primate and nonprimate CMV enhancers can substitute for one another. The enhancers are not functionally equivalent, but they do have overlapping activities. The CMV MIE enhancers are located between divergent promoters where the leftward genes are critical and essential for reactivation from latency and acute infection and the rightward gene is nonessential. The rightward transcription unit is controlled by an enhancer for murine CMV. In contrast, human CMV has a set of repressor elements that prevents enhancer effects on the rightward viral promoter. The human CMV enhancer that controls the leftward transcription unit has a distal component that is nonessential at high multiplicity of infection (MOI), but has a significant impact on the MIE gene expression at low MOI. The proximal enhancer influences directly the level of transcription of the MIE genes and contains an essential Sp-1 site. The MIE promoter has a site adjacent to the transcription start site that is essential at the earliest stage of infection. The MIE enhancer-containing promoter responds to signal transduction events and to cellular differentiation. The role of the CMV MIE enhancer-containing promoter in acute infection and reactivation from latency are reviewed.

Hematopoietic stem cell transplantation with latently infected donors does not transmit virus to immunocompromised recipients in the murine model of cytomegalovirus infection

Hematopoietic stem cell transplantation (HSCT) bears a risk of reactivating latent cytomegalovirus (CMV) in either the transplanted hematopoietic donor cells or in parenchymal and stromal tissue cells of the immunocompromised recipient, or in both. While reactivated human CMV in recipients of organ transplantations is frequently the virus variant of the donor, this is not usually the case in HSCT recipients. Here we have used experimental sex-mismatched HSCT in the BALB/c mouse model to test if latent murine CMV from CMV-immune donors is transmitted with bone marrow cells to naive immunocompromised recipients.

MHC class I immune evasion in MCMV infection

Murine cytomegalovirus (MCMV) is a well-studied model of natural beta-herpesvirus infection. However, many questions remain regarding its control by and evasion of the immune response it generates. CD8 and CD4 T cells have both unique and redundant roles in control of the virus that differ based on the immunocompetence of the infected mice. MCMV encodes major histocompatibility complex (MHC) class I immune evasion genes that can have an impact in vitro, but their role in infection of immunocompetent mice has been difficult to identify. This review addresses the evidence for their in vivo function and suggests why they may be evolutionarily conserved.

A mouse model for cytomegalovirus infection

This unit describes procedures for infecting newborn and adult mice with murine cytomegalovirus (mCMV). Methods are included for propagating mCMV in cell cultures and for preparing a more virulent form of mCMV from salivary glands of infected mice. A plaque-forming cell (PFC) assay is provided for measuring mCMV titers of infected tissues or virus stocks. In addition, a method is described for preparing the murine embryonic fibroblasts used for propagating mCMV and for the PFC assay.

Subdominant CD8 T-cell epitopes account for protection against cytomegalovirus independent of immunodomination

Cytomegalovirus (CMV) infection continues to be a complication in recipients of hematopoietic stem cell transplantation (HSCT). Preexisting donor immunity is recognized as a favorable prognostic factor for the reconstitution of protective antiviral immunity mediated primarily by CD8 T cells. Furthermore, adoptive transfer of CMV-specific memory CD8 T (CD8-T(M)) cells is a therapeutic option for preventing CMV disease in HSCT recipients. Given the different CMV infection histories of donor and recipient, a problem may arise from an antigenic mismatch between the CMV variant that has primed donor immunity and the CMV variant acquired by the recipient. Here, we have used the BALB/c mouse model of CMV infection in the immunocompromised host to evaluate the importance of donor-recipient CMV matching in immundominant epitopes (IDEs). For this, we generated the murine CMV (mCMV) recombinant virus mCMV-DeltaIDE, in which the two memory repertoire IDEs, the IE1-derived peptide 168-YPHFMPTNL-176 presented by the major histocompatibility complex class I (MHC-I) molecule L(d) and the m164-derived peptide 257-AGPPRYSRI-265 presented by the MHC-I molecule D(d), are both functionally deleted. Upon adoptive transfer, polyclonal donor CD8-T(M) cells primed by mCMV-DeltaIDE and the corresponding revertant virus mCMV-revDeltaIDE controlled infection of immunocompromised recipients with comparable efficacy and regardless of whether or not IDEs were presented in the recipients. Importantly, CD8-T(M) cells primed under conditions of immunodomination by IDEs protected recipients in which IDEs were absent. This shows that protection does not depend on compensatory expansion of non-IDE-specific CD8-T(M) cells liberated from immunodomination by the deletion of IDEs. We conclude that protection is, rather, based on the collective antiviral potential of non-IDEs independent of the presence or absence of IDE-mediated immunodomination.

Murine cytomegalovirus influences Foxj1 expression, ciliogenesis, and mucus plugging in mice with allergic airway disease

We have followed throughout time the development of allergic airway disease (AAD) in both uninfected mice and mice infected intranasally with murine cytomegalovirus (MCMV). Histological evaluation of lung tissue from uninfected mice with AAD demonstrated mucus plugging after 14 and 21 days of ovalbumin-aerosol challenge, with resolution of mucus plugging occurring by 42 days. In MCMV/AAD mice, mucus plugging was observed after 7 days of ovalbumin-aerosol challenge and remained present at 42 days. The level of interleukin-13 in bronchoalveolar lavage fluid from MCMV/AAD mice was decreased compared with AAD mice and was accompanied by increased levels of interferon-gamma. Levels of Muc5A/C, Muc5B, or Muc2 mucin mRNA in the lungs of MCMV/AAD mice were not elevated compared with AAD mice. MCMV was able to infect the airway epithelium, resulting in decreased expression of Foxj1, a transcription factor critical for ciliogenesis, and a loss of ciliated epithelial cells. In addition, an increase in the number of epithelial cells staining positive for periodic acid-Schiff was observed in MCMV/AAD airways. Together, these findings suggest that MCMV infection of the airway epithelium enhances goblet cell metaplasia and diminishes efficient mucociliary clearance in mice with AAD, resulting in increased mucus plugging.

Murine model of cytomegalovirus latency and reactivation

Efficient resolution of acute cytopathogenic cytomegalovirus infection through innate and adaptive host immune mechanisms is followed by lifelong maintenance of the viral genome in host tissues in a state of replicative latency, which is interrupted by episodes of virus reactivation for transmission. The establishment of latency is the result of aeons of co-evolution of cytomegaloviruses and their respective host species. Genetic adaptation of a particular cytomegalovirus to its specific host is reflected by private gene families not found in other members of the cytomegalovirus group, whereas basic functions of the viral replicative cycle are encoded by public gene families shared between different cytomegaloviruses or even with herpesviruses in general. Private genes include genes coding for immunoevasins, a group of glycoproteins specifically dedicated to dampen recognition by the host's innate and adaptive immune surveillance to protect the virus against elimination. Recent data in the mouse model of cytomegalovirus latency have indicated that viral replicative latency established in the immunocompetent host is a dynamic state characterized by episodes of viral gene desilencing and immune sensing of reactivated presentation of antigenic peptides at immunological checkpoints by CD8 T cells. This sensing maintains viral replicative latency by triggering antiviral effector functions that terminate the viral gene expression program before infectious viral progeny are assembled. According to the immune sensing hypothesis of latency control, immunological checkpoints are unique for each infected individual in reflection of host MHC (HLA) polymorphism and the proteome(s) of the viral variant(s) harbored in latency.

Flt3 ligand expands lymphoid progenitors prior to recovery of thymopoiesis and accelerates T cell reconstitution after bone marrow transplantation

Deficient thymopoiesis and retarded recovery of newly developed CD4(+) T cells is one of the most important determinants of impaired immunocompetence after hemopoietic stem cell transplantation. Here we evaluated whether Fms-like tyrosine kinase 3 (Flt3) ligand (FL) alone or combined with IL-7 affects T cell recovery, thymopoiesis, and lymphoid progenitor expansion following bone marrow transplantation in immunodeficient mice. FL strongly accelerated and enhanced the recovery of peripheral T cells after transplantation of a low number of bone marrow cells. An additive effect on T cell recovery was not observed after coadministration of IL-7. Lineage(-)sca-1(+)c-kit(+)flt3(+) lymphoid progenitor cell numbers were significantly increased in bone marrow of FL-treated mice before recovery of thymopoiesis. Thymocyte differentiation was advanced to more mature stages after FL treatment. Improved T cell recovery resulted in better immunocompetence against a post-bone marrow transplantation murine CMV infection. Collectively, our data suggest that FL promotes T cell recovery by enhanced thymopoiesis and by expansion of lymphoid progenitors.

Role of the indigenous microbiota in maintaining the virus-specific CD8 memory T cells in the lung of mice infected with murine cytomegalovirus

The potent role of indigenous microbiota in maintaining murine CMV (MCMV)-specific memory T cells, which were measured by multimer staining, was investigated using germfree (GF) mice. When the BALB/c mice bred under specific pathogen-free (SPF) conditions were i.p. infected with 0.2 LD(50) of MCMV, high frequencies of CD69(+)/CD44(+) MCMV-specific CD8 T cells were noted in the lungs even at 6-12 mo after infection (11.1 +/- 3.2 and 9.8 +/- 0.9%, respectively). In contrast, even though the viral load and expression levels of mRNA of such cytokines as IL-2, IL-7, IL-15, and IFN-gamma in the lungs of MCMV-infected GF mice were comparable to those of infected SPF mice, the frequencies of MCMV-specific CD8 T cells in the lungs of infected GF mice were kept lower than 1% at 6-12 mo after infection. In addition, the reconstitution of microbiota of MCMV-infected GF mice by orally administering a fecal suspension prepared from SPF mice restored the frequencies of both CD8(+)/multimer(+) and CD8(+)/multimer(-) T cells to levels similar to those found in SPF mice. These results suggested the indigenous microbiota to play a crucial role in the expansion and maintenance of viral-specific CD8 memory T cells, probably by cross-reactivity between the antigenic epitope of the MCMV-specific memory T cells and the variety of peptides derived from the members of the microbiota. Such cross-reactivity may thus be a major feature of those cells.

A flow cytometry-based method for detecting antibody responses to murine cytomegalovirus infection

An assay based on target cells infected with green fluorescent protein labeled murine cytomegalovirus (GFP-MCMV) and dual color flow cytometry for detecting antibody to MCMV is described. After optimizing conditions for this technique, kinetics of anti-MCMV IgG antibody response was tested in susceptible (BALB/c) and resistant (C57BL/6) mouse strains following primary MCMV infection. Previously published antibody kinssetics were confirmed in susceptible mice, with peak IgG response seen approximately 8 weeks after primary infection, decreasing by 20 weeks after infection. In contrast, MCMV resistant C57BL/6 mice showed significantly lower IgG antibody responses than susceptible mice. Although several techniques have been previously described to detect murine antibody responses to MCMV, including nuclear anti-complement immunofluorescence, viral immunoblotting, complement fixation, indirect immunofluorescence, indirect hemagglutination, and enzyme-liked immunosorbent assay techniques, these techniques are all time consuming and laborious. The technique presented is a simple time efficient alternative to detect previous MCMV antibody responses in experimentally infected mice.

Pentraxin 3 protects from MCMV infection and reactivation through TLR sensing pathways leading to IRF3 activation

Reactivation of latent human cytomegalovirus (HCMV) following allogeneic transplantation is a major cause of morbidity and mortality and predisposes to severe complications, including superinfection by Aspergillus species (spp). Antimicrobial polypeptides, including defensins and mannan-binding lectin, are known to block viral fusion by cross-linking sugars on cell surface. Pentraxin 3 (PTX3), a member of the long pentraxin family, successfully restored antifungal immunity in experimental hematopoietic transplantation. We assessed here whether PTX3 binds HCMV and murine virus (MCMV) and the impact on viral infectivity and superinfection in vivo. We found that PTX3 bound both viruses, reduced viral entry and infectivity in vitro, and protected from MCMV primary infection and reactivation as well as Aspergillus superinfection. This occurred through the activation of interferon (IFN) regulatory factor 3 (IRF3) in dendritic cells via the TLR9/MyD88-independent viral recognition sensing and the promotion of the interleukin-12 (IL-12)/IFN-γ–dependent effector pathway.

Lipopolysaccharide, tumor necrosis factor alpha, or interleukin-1beta triggers reactivation of latent cytomegalovirus in immunocompetent mice

We have previously shown that cytomegalovirus (CMV) can reactivate in lungs of nonimmunosuppressed patients during critical illness. Our recent work has shown that polymicrobial bacterial sepsis can trigger reactivation of latent murine CMV (MCMV). We hypothesize that MCMV reactivation following bacterial sepsis may be caused by inflammatory mediators. To test this hypothesis, BALB/c mice latently infected with Smith strain MCMV received sublethal intraperitoneal doses of lipopolysaccharide (LPS), tumor necrosis factor alpha (TNF-alpha), interleukin-1beta (IL-1beta), or saline. Lung tissue homogenates were evaluated for viral reactivation 3 weeks after mediator injection. Because LPS is known to signal via Toll-like receptor 4 (TLR-4) in mice, further studies blocking this signaling mechanism were performed using monoclonal MTS510. Finally, mice were tested with intravenous TNF-alpha to determine whether this would cause reactivation. All mice receiving sublethal intraperitoneal doses of LPS, TNF-alpha, or IL-1beta had pulmonary reactivation of latent MCMV 3 weeks following injection, and LPS caused MCMV reactivation with kinetics similar to those for sepsis. When TLR-4 signaling was blocked, exogenous LPS did not reactivate latent MCMV. Intravenous TNF-alpha administration at near-lethal doses did not reactivate MCMV. Exogenous intraperitoneal LPS, TNF-alpha, and IL-1beta are all capable of reactivating CMV from latency in lungs of previously healthy mice. LPS reactivation of MCMV appears dependent on TLR-4 signaling. Interestingly, intravenous TNF-alpha did not trigger reactivation, suggesting possible mechanistic differences that are discussed. We conclude that inflammatory disease states besides sepsis may be capable of reactivating CMV from latency.

Innate antiviral resistance influences the efficacy of a recombinant murine cytomegalovirus immunocontraceptive vaccine

Recombinant betaherpesviruses are attractive vaccine candidates because of their persistence in the host. A recombinant murine cytomegalovirus expressing the mouse ovarian glycoprotein zona pellucida 3 induces long lasting sterility in female BALB/c mice. Using inbred mouse strains selected for their innate resistance or susceptibility to MCMV, we show that genetically determined innate resistance to MCMV can reduce immunocontraceptive success. The Cmv1 locus that controls natural killer cell mediated responses to MCMV was implicated in determining vaccine efficacy. However, the role of the H-2 haplotype was less clear. Interestingly, Mus domesticus from an outbred colony of wild-derived mice were readily sterilised by vaccination, consistent with observations that strong innate immunity to MCMV is not common in Australian wild mice.

CD8 T cells control cytomegalovirus latency by epitope-specific sensing of transcriptional reactivation

During murine cytomegalovirus (mCMV) latency in the lungs, most of the viral genomes are transcriptionally silent at the major immediate-early locus, but rare and stochastic episodes of desilencing lead to the expression of IE1 transcripts. This low-frequency but perpetual expression is accompanied by an activation of lung-resident effector-memory CD8 T cells specific for the antigenic peptide 168-YPHFMPTNL-176, which is derived from the IE1 protein. These molecular and immunological findings were combined in the "silencing/desilencing and immune sensing hypothesis" of cytomegalovirus latency and reactivation. This hypothesis proposes that IE1 gene expression proceeds to cell surface presentation of the IE1 peptide by the major histocompatibility complex (MHC) class I molecule L(d) and that its recognition by CD8 T cells terminates virus reactivation. Here we provide experimental evidence in support of this hypothesis. We generated mutant virus mCMV-IE1-L176A, in which the antigenic IE1 peptide is functionally deleted by a point mutation of the C-terminal MHC class I anchor residue Leu into Ala. Two revertant viruses, mCMV-IE1-A176L and the wobble nucleotide-marked mCMV-IE1-A176L*, in which Leu is restored by back-mutation of Ala codon GCA into Leu codons CTA and CTT, respectively, were constructed. Pulmonary latency of the mutant virus was found to be associated with an increased prevalence of IE1 transcription and with events of IE3 transactivator splicing. In conclusion, IE1-specific CD8 T cells recognize and terminate virus reactivation in vivo at the first opportunity in the reactivated gene expression program. The perpetual gene expression and antigen presentation might represent the driving molecular force in CMV-associated immunosenescence.

Lymphoma cell apoptosis in the liver induced by distant murine cytomegalovirus infection

Cytomegalovirus (CMV) poses a threat to the therapy of hematopoietic malignancies by hematopoietic stem cell transplantation, but efficient reconstitution of antiviral immunity prevents CMV organ disease. Tumor relapse originating from a minimal residual leukemia poses another threat. Although a combination of risk factors was supposed to enhance the incidence and severity of transplantation-associated disease, a murine model of a liver-adapted B-cell lymphoma has previously shown a survival benefit and tumor growth inhibition by nonlethal subcutaneous infection with murine CMV. Here we have investigated the underlying antitumoral mechanism. Virus replication proved to be required, since inactivated virions or the highly attenuated enhancerless mutant mCMV-DeltaMIEenh did not impact the lymphoma in the liver. Surprisingly, the dissemination-deficient mutant mCMV-DeltaM36 inhibited tumor growth, even though this virus fails to infect the liver. On the other hand, various strains of herpes simplex viruses consistently failed to control the lymphoma, even though they infect the liver. A quantitative analysis of the tumor growth kinetics identified a transient tumor remission by apoptosis as the antitumoral effector mechanism. Tumor cell colonies with cells surviving the CMV-induced "apoptotic crisis" lead to tumor relapse even in the presence of full-blown tissue infection. Serial transfer of surviving tumor cells did not indicate a selection of apoptosis-resistant genetic variants. NK cell activity of CD49b-expressing cells failed to control the lymphoma upon adoptive transfer. We propose the existence of an innate antitumoral mechanism that is triggered by CMV infection and involves an apoptotic signal effective at a distant site of tumor growth.

Mixed infection with multiple strains of murine cytomegalovirus occurs following simultaneous or sequential infection of immunocompetent mice

As with human cytomegalovirus (HCMV) infection of humans, murine CMV (MCMV) infection is widespread in its natural host, the house mouse Mus domesticus, and may consist of mixed infection with different CMV isolates. The incidence and mechanisms by which mixed infection occurs in free-living mice are unknown. This study used two approaches to determine whether mixed infection with MCMV could be established in laboratory mice. The first utilized two naturally occurring MCMV strains, N1 and G4, into which the lacZ gene was inserted by homologous recombination. The lacZ gene was used to track recombinant and parental viruses in simultaneously coinfected mice. In the second approach, a real-time quantitative PCR (qPCR) assay was used to detect viral immediate-early 1 (ie1) gene sequences in mice successively coinfected with G4 and then with the K181 MCMV strain. In both systems, mixed infection was detected in the salivary glands and lungs of experimentally infected mice. MCMV-specific antibody in sera and G4 IE1-specific cytotoxic lymphocyte responses in the spleens of twice-infected mice did not prevent reinfection. Finally, the prevalence of mixed infection in free-living mice trapped in four Australian locations was investigated using real-time qPCR to detect ie1 DNA sequences of N1, G4 and K181. Mixed infection with MCMVs containing the G4 and K181 ie1 sequences was detected in the salivary glands of 34·2 % of trapped mice. The observations that mixed infections are common in free-living M. domesticus and are acquired by immunocompetent mice through simultaneous or successive infections are important for vaccine development.

Pulmonary cytomegalovirus reactivation causes pathology in immunocompetent mice

OBJECTIVE

Cytomegalovirus (CMV) is a ubiquitous herpes virus that persists in the host in a latent state following primary infection. We have recently observed that CMV reactivates in lungs of critically ill surgical patients and that this reactivation can be triggered by bacterial sepsis. Although CMV is a known pathogen in immunosuppressed transplant patients, it is unknown whether reactivated CMV is a pathogen in immunocompetent hosts. Using an animal model of latency/reactivation, we studied the pathobiology of CMV reactivation in the immunocompetent host.

DESIGN

Laboratory study.

SETTING

University laboratory.

SUBJECTS

Cohorts of immunocompetent BALB/c mice with or without latent murine CMV (MCMV+/MCMV-).

INTERVENTIONS

Mice underwent cecal ligation and puncture. Lung tissue homogenates were evaluated after cecal ligation and puncture for tumor necrosis factor-alpha, interleukin-1beta, neutrophil chemokine KC, and macrophage inflammatory protein-2 messenger RNA by polymerase chain reaction and real-time quantitative reverse transcription-polymerase chain reaction. Because pulmonary tumor necrosis factor-alpha expression is known to cause pulmonary fibrosis, trichrome-stained sections of lung tissues were analyzed using image analysis to quantitate pulmonary fibrosis. In a second experiment, a cohort of MCMV+ mice received ganciclovir (10 mg/kg/day subcutaneously) following cecal ligation and puncture. Tumor necrosis factor-alpha messenger RNA and pulmonary fibrosis were evaluated as described previously.

MEASUREMENTS AND MAIN RESULTS

All MCMV+ mice had CMV reactivation beginning 2 wks after cecal ligation and puncture. Following reactivation, these mice had abnormal tumor necrosis factor-alpha, interleukin-1beta, neutrophil chemokine KC, and macrophage inflammatory protein-2 messenger RNA expression compared with controls. Image analysis showed that MCMV+ mice had significantly increased pulmonary fibrosis compared with MCMV- mice 3 wks after cecal ligation and puncture. Ganciclovir treatment following cecal ligation and puncture prevented MCMV reactivation. Furthermore, ganciclovir-treated mice did not demonstrate abnormal pulmonary expression of tumor necrosis factor-alpha messenger RNA. Finally, ganciclovir treatment prevented pulmonary fibrosis following MCMV reactivation.

CONCLUSIONS

This study shows that CMV reactivation causes abnormal tumor necrosis factor-alpha expression, and that following CMV reactivation, immunocompetent mice have abnormal pulmonary fibrosis. Ganciclovir blocks MCMV reactivation, thus preventing abnormal tumor necrosis factor-alpha expression and pulmonary fibrosis. These data may explain a mechanism by which critically ill surgical patients develop fibroproliferative acute respiratory distress syndrome. These data suggest that human studies using antiviral agents during critical illness are warranted.

A murine model of dual infection with cytomegalovirus and Pneumocystis carinii: Effects of virus-induced immunomodulation on disease progression

Despite the use of antimicrobial prophylaxis, cytomegalovirus (CMV) and Pneumocystis carinii (PC) pneumonia (PCP) are both leading causes of morbidity and mortality in immunocompromised patients. It has previously been reported that CMV infection modulates host immune responses with a variety of mechanisms which include the suppression of helper T cell functions and antigen presenting cell (APC) functions, both of which are critical for PCP resolution. However, the mechanisms of these interactions and other possible immune regulatory effects are not clearly understood. In this study, we investigated the impact of murine CMV (MCMV) induced immunomodulation on the progression of PCP in a co-infection model. Initial results show that dually infected mice had evidence of more severe PC disease, which include a greater loss of body weight, an excess lung PC burden and delayed clearance of PC from lungs, compared to mice with PC infection alone. At day 7 post-infection, dually infected mice had reduced numbers of MHC-II expressing cells in the lung interstitium and lymph nodes and reduced migration of CD11c+ cells to both the tracheobronchial lymph nodes and alveolar spaces. Dual infected mice showed elevated numbers of specific CD8 responses concomitant with a decrease in activated CD4+ T cells in both the lymph nodes and in alveolar spaces when compared to mice infected with MCMV alone. These data suggest that MCMV infection inhibits the immune responses generated against PC which contribute to the delayed clearance of the organism.

Evolution of diverse antiviral CD8+ T cell populations after murine cytomegalovirus infection

Cytomegalovirus (CMV) is a major human pathogen normally controlled by cellular immune responses. The infection can be modeled in the mouse using murine CMV (MCMV). During the latent phase of infection, two different patterns of CD8(+) T cell responses have been observed: some specificities show increasing frequencies over time ("memory inflation"), while others, which are present acutely, are barely detectable at later time points. This distinction is independent of initial immunodominance. We analyzed the extent to which such responses differ functionally and tracked both their population distribution and their evolution over time. We observed two clear patterns of memory development that diverged early after infection. Acutely, CD8(+) T cells directed against all epitopes showed similar activation, phenotype and distribution. Thereafter, one set of responses ("inflationary") increased in frequency over time, was found in high numbers in non-lymphoid organs and was associated with an activated (CD28(low) CD27(low)CD122(low)) phenotype. In contrast, CD8(+) T cells responses specific for other MCMV epitopes ("non-inflationary") showed a slow reversion to a classical "central" memory phenotype without enrichment in non-lymphoid organs. A simple model to describe the equilibrium state in MCMV is presented, which may point to previously unexplored antiviral populations present after human CMV infection.

Role for tumor necrosis factor alpha in murine cytomegalovirus transcriptional reactivation in latently infected lungs

Interstitial pneumonia is a major clinical manifestation of primary or recurrent cytomegalovirus (CMV) infection in immunocompromised recipients of a bone marrow transplant. In a murine model, lungs were identified as a prominent site of CMV latency and recurrence. Pulmonary latency of murine CMV is characterized by high viral genome burden and a low incidence of variegated immediate-early (IE) gene expression, reflecting a sporadic activity of the major IE promoters (MIEPs) and enhancer. The enhancer-flanking promoters MIEP1/3 and MIEP2 are switched on and off during latency in a ratio of approximately 2:1. MIEP1/3 latency-associated activity generates the IE1 transcript of the ie1/3 transcription unit but not the alternative splicing product IE3 that encodes the essential transactivator of early gene expression. Splicing thus appeared to be an important checkpoint for maintenance of latency. In accordance with previous work of others, we show here that signaling by the proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha) activates IE1/3 transcription in vivo. As an addition to current knowledge, Poisson distribution analysis revealed an increased incidence of IE1/3 transcriptional events as well as a higher amount of transcripts per event. Notably, TNF-alpha promoted the splicing to IE3 transcripts, but transcription did not proceed to the M55/gB early gene. Moreover, the activated transcriptional state induced by TNF-alpha did not predispose latently infected mice to a higher incidence of virus recurrence after hematoablative treatment. In conclusion, TNF-alpha is an important inductor of IE gene transcriptional reactivation, whereas early genes downstream in the viral replicative cycle appear to be the rate-limiting checkpoint(s) for virus recurrence.

Development and use of an internal standard for oyster herpesvirus 1 detection by PCR

Oyster samples were examined using a competitive PCR method in order to detect and quantify oyster herpesvirus 1 (OsHV-1) DNA. Quantitation of viral DNA by competitive PCR assay was based on co-amplification of OsHV-1 DNA and a competitor where a known amount of competitor DNA was present in the same reaction mixture. The competitor was engineered so that it differs in length (deletion of 76 base pairs) from the viral DNA. The assay allowed the detection of 1 fg of viral DNA among 0.5 mg of oyster tissues. The method was used to demonstrate the absence of PCR inhibitors in oyster spat ground tissues. PCR inhibition was observed in adult oyster samples when the same tissue preparation procedure was used. On the contrary, classical phenol/chloroform DNA extraction from adult oyster tissues allowed co-amplification of the internal standard competitor and the viral DNA. The method was successfully used to demonstrate the presence of viral DNA in asymptomatic adult oysters. Quantitation of OsHV-1 DNA in infected spat and asymptomatic adult oysters was also carried out. Viral DNA (1.5-325 pg) were detectable in 0.5 mg of oyster tissues in adults. The amounts of viral DNA in infected oyster spat varied from 750 pg to 35 ng per 0.5 mg of ground tissues.

Murine cytomegalovirus infection directs macrophage differentiation into a pro-inflammatory immune phenotype: implications for atherogenesis

We have previously demonstrated that mouse cytomegalovirus (MCMV) aggravates atherosclerosis in apolipoprotein E knockout (apoE(-/-)) mice, most likely by enhancing both systemic and local (e.g. in the vascular wall) cytokine production. However, until now it was unclear which cell type is responsible for this enhanced pro-inflammatory cytokine production. In this study we focused on the macrophage (mPhi), which besides being an important source of such cytokines, is known to be an important player in both atherosclerosis and viral clearance. We investigated whether MCMV could induce a pro-inflammatory immune mPhi phenotype, which ultimately may contribute to the development of atherosclerosis. To this end, peritoneal exudate cells (PEC) were elicited in apoE(-/-) mice by either MCMV or thioglycolate injection, and mPhi were phenotyped at 1 week post-intraperitoneal injection. MCMV-induced peritoneal mPhi contained MCMV DNA but had limited MCMV mRNA expression, indicating latent infection. These mPhi showed increased production of interferon-gamma (IFNgamma), exclusive production of interleukin-18 (IL-18) and increased expression of major histocompatibility complex (MHC) class II, CD40, CD80 and CD86, when compared with thioglycolate-induced mPhi. From these results, we conclude that intraperitoneal injection of MCMV induces an immune-responsive exudate in which at 7 days post-infection, MCMV-infected mPhi express a pro-inflammatory immune phenotype. As such, the MCMV-induced mPhi may be an important player in aggravating atherosclerosis through systemic and/or local immune activation.

Murine cytomegalovirus interference with antigen presentation has little effect on the size or the effector memory phenotype of the CD8 T cell response

As with most herpesviruses, CMVs encode viral genes that inhibit Ag presentation to CD8 T cells (VIPRs). VIPR function has been assumed to be essential for CMV to establish its characteristic lifetime infection of its host. We compared infection of C57BL/6 mice with wild-type murine CMV (MCMV) and a virus lacking each of MCMV's three known VIPRs: m4, m6, and m152. During acute infection, there was very little difference between the two viruses with respect to the kinetics of viral replication and clearance, or in the size and kinetics of the virus-specific CD8 T cell response. During chronic infection, a large, effector memory, virus-specific CD8 T cell population (CD8(low)CD62L(-)CD11c(+)NKG2A(+)) was maintained in both infections; the size and phenotype of the CD8 T cell response to both viruses was remarkably similar. The characteristic effector memory phenotype of the CD8 T cells suggested that both wild-type and Deltam4+m6+m152 virus continued to present Ag to CD8 T cells during the chronic phase of infection. During the chronic phase of infection, MCMV cannot be isolated from immunocompetent mice. However, upon immunosuppression, both Deltam4+m6+m152 and wild-type virus could be reactivated from mice infected for 6 wk. Thus, restoring the ability of CD8 T cells to detect MCMV had little apparent effect on the course of MCMV infection and on the CD8 T cell response to it. These results challenge the notion that VIPR function is necessary for CMV persistence in the host.

Impact of Cytomegalovirus Match on Survival after Cardiac and Lung Transplantation

Acute cytomegalovirus (CMV) disease and indirect effects caused by the virus alter the outcome after solid organ transplantation. Long-term results after 54 lung and 139 cardiac transplants at a single center have been retrospectively analyzed with regard to CMV status. Standard CMV prophylaxis consisted of ganciclovir for 100 days. Lung recipients were pretransplant CMV negative in 32 per cent as compared to heart recipients with 23 per cent. Patient survival after mismatch transplants (donor positive, recipient negative) was significantly reduced as compared to the other match groups (42% vs 76% at five years, P = 0.01). In heart recipients, CMV positive patients receiving a CMV negative graft showed best survival, whereas in the group of lung recipients negative/negative matched transplants produced best results. In both groups, CMV negative grafts had a better outcome than CMV positive grafts, and a survival difference between heart and lung recipients was only observed in recipients of a CMV positive grafts. Despite ganciclovir prophylaxis, CMV match remains an important factor for survival follwing heart and, even more profoundly, lung transplantation. Because survival was least favorable in the mismatched group, prophylactic regimens warrant improvement. For CMV negative lung recipients, CMV matching might be considered.

Improved detection and quantification of mouse cytomegalovirus by real-time PCR

During latent cytomegalovirus (CMV) infection, viral presence cannot be detected by plaque assay. Therefore, we assessed the applicability of real-time PCR for temporal determination of virus dissemination in two different mouse strains. Eight-week-old BALB/c and C57BL/6J mice were infected with mouse CMV (MCMV) and sacrificed at 1, 2, 4, 6, 14 and 28 days post infection. Real-time PCR was used to determine MCMV copy number in the heart, bone marrow cells, aorta and blood. In lung, liver, salivary gland and spleen the presence of MCMV was determined both by plaque assay and real-time PCR. In analogy with the plaque assay, the real-time PCR technique revealed higher numbers of MCMV genomic copies in all organs obtained from BALB/c mice when compared with C57BL/6J mice, demonstrating the applicability of the technique. A significant correlation was observed between both assays when a positive test result was seen with both assays. Nonetheless, lower viral infectivity titers were found compared to real-time PCR data. Thus, the real-time PCR technique is more sensitive in detecting the presence of MCMV and is therefore well suited for (dose-response) intervention studies aimed at studying virus eradication.

Pathogenesis of murine cytomegalovirus infection

Infection of mice with murine cytomegalovirus (MCMV) is an established model for studying human cytomegalovirus (HCMV) infection. Similarly to HCMV infection, pathological changes and disease manifestations during MCMV infection are mainly dependent on the immune status of the mouse host. This review focuses mainly on the pathogenesis of MCMV infection in immunocompetent and immunodeficient and/or immature mice and discusses the principles of immunosurveillance of infection and the mechanisms by which this virus evades immune control.

Major histocompatibility complex class I allele-specific cooperative and competitive interactions between immune evasion proteins of cytomegalovirus

Cytomegaloviruses (CMVs) deploy a set of genes for interference with antigen presentation in the major histocompatibility complex (MHC) class I pathway. In murine CMV (MCMV), three genes were identified so far: m04/gp34, m06/gp48, and m152/gp40. While their function as immunoevasins was originally defined after their selective expression, this may not necessarily reflect their biological role during infection. The three immunoevasins might act synergistically, but they might also compete for their common substrate, the MHC class I complexes. To approach this question in a systematic manner, we have generated a complete set of mutant viruses with deletions of the three genes in all seven possible combinations. Surface expression of a set of MHC class I molecules specified by haplotypes H-2(d) (K(d), D(d), and L(d)) and H-2(b) (K(b) and D(b)) was the parameter for evaluation of the interference with class I trafficking. The data show the following: first, there exists no additional MCMV gene of major influence on MHC class I surface expression; second, the strength of the inhibitory effect of immunoevasins shows an allele-specific hierarchy; and third, the immunoevasins act not only synergistically but can, in certain combinations, interact antagonistically. In essence, this work highlights the importance of studying the immunosubversive mechanisms of cytomegaloviruses in the context of gene expression during the viral replicative cycle in infected cells.

Persisting murine cytomegalovirus can reactivate and has unique transcriptional activity in ocular tissue

Cytomegalovirus (CMV) retinitis is an important ocular complication in human immunodeficiency virus-infected individuals and the leading cause of blindness in those not undergoing highly active antiretroviral therapy. Murine CMV (MCMV) infection of mice has been shown to be a useful small-animal model for the study of CMV pathogenesis in the eye. The purpose of this study was to evaluate CMV persistence in ocular tissue and to determine the potential for reactivation. Following subretinal inoculation of immunocompetent BALB/c mice, tissues were tested for infectious virus by plaque assay and for the presence of viral DNA and RNA by PCR. The latent phase of the infection in mouse tissues was analyzed by plaque assay, PCR, and explantation cocultivation in both immunocompetent and cyclophosphamide-treated mice. The acute phase of the infection was resolved by 2 to 3 weeks postinfection, while viral DNA persisted beyond 12 months. Immediate-early 1 transcripts were detected in 100% of the ocular samples tested, and glycoprotein H transcripts were detected in 86% of the samples, but no difference in viral DNA or RNA levels between immunocompetent and immunosuppressed animals was measured. Irrespective of immune status, no in vivo reactivation was detected; however, reactivated virus was observed in 76 to 82% of the eyes following explantation onto a permissive cell layer. The transcriptional activity and relatively high frequency of explantation-induced reactivation in both immunocompetent and immunosuppressed mice suggest that control of MCMV latency in ocular tissue might involve other regulatory events that are not entirely dependent on intact specific immunity.

Mouse models of cytomegalovirus latency: overview

BACKGROUND

The molecular regulation of viral latency and reactivation is a central unsolved issue in the understanding of cytomegalovirus (CMV) biology. Like human CMV (hCMV), murine CMV (mCMV) can establish a latent infection in cells of the myeloid lineage. Since mCMV genome remains present in various organs after its clearance from hematopoietic cells first in bone marrow and much later in blood, there must exist one or more widely distributed cell type(s) representing the cellular site(s) of enduring mCMV latency in host tissues. Endothelial cells and histiocytes are candidates, but the question is not yet settled. Another long debated problem appears to be solved: mCMV establishes true molecular latency rather than a low-level persistence of productive infection. This conclusion is based on two recent advances. First, on a highly improved assay of infectivity, and second, on very sensitive RT PCRs for detecting viral transcripts during latency. In essence, infectious virus and productive cycle transcripts, such as transcripts of early-phase gene M55 (gB) and ie3 transcripts specifying the essential transactivator protein IE3, were found to be absent during mCMV latency in the lungs.

OBJECTIVES

We will here review recent data on the variegated expression of IE-phase genes ie1 and ie2 during mCMV latency in the lungs, and on the expression patterns found in transcriptional foci during induced reactivation. We will discuss immunological implications of ie1 gene expression during latency and will speculate a bit on how CD8 T cells might trigger latency-associated ie1 gene expression in a regulatory circuit.

A model for reactivation of CMV from latency

BACKGROUND

Reactivation of CMV from latency results in serious morbidity and mortality in immunocompromised transplant recipients. The mechanism by which CMV reactivates from latency has not been well understood.

OBJECTIVE

In this review we discuss three models for reactivation from latency and present evidence in favor of the model that reactivation is a multi-step process which is initiated by the allogeneic response to the transplanted organ. Study design (J. Virol. 75 (2001) 4814). Mice latently infected with murine cytomegalovirus (MCMV) were used as donors for allogeneic or syngeneic kidney transplants into immunocompetent recipients. The contralateral donor kidneys were used as controls. Transplanted kidneys were removed at various times after transplant and analyzed for expression of viral genes associated with productive infection and for expression of inflammatory cytokines. Electrophoretic mobility shift assay was performed on nuclear extracts of control and transplanted kidneys to examine activation of AP-1 and NFkappaB. Latently infected mice were also injected with tumor necrosis factor (TNF) to examine the effect of TNF alone on induction of MCMV immediate-early (IE) gene expression. Transgenic major immediate early promoter-lacZ mice carrying a beta-galactosidase reporter gene under the control of the human cytomegalovirus (HCMV) IE promoter/enhancer were used as donors for allogeneic kidney transplants to study the effect of allogeneic transplantation on induction of HCMV IE gene expression.

RESULTS

Allogeneic, but not syngeneic transplantation induces MCMV IE-1 expression and expression of inflammatory cytokines, including TNF. Allogeneic transplantation activates transcription factors, including NFkappaB and AP-1. TNF alone can induce MCMV IE-1 gene expression and activation of NFkappaB and AP-1 in some tissues.

CONCLUSIONS

We propose that induction of IE-1 gene expression is the first step in reactivation of the virus in an immunocompromised transplant recipient, and that it occurs as a result of the allogeneic response, which induces expression of TNF and subsequent activation of NFkappaB, and ischemia/reperfusion injury, which induces activation of AP-1. We speculate that the natural stimulus for reactivation in an immunocompetent host is an inflammatory immune response to infection and that allogeneic transplantation mimics this process.

Characterisation of murine cytomegalovirus myocarditis: cellular infiltration of the heart and virus persistence

Myocarditis triggered by a viral infection has integral viral and immunological aspects associated with the pathogenesis of disease. The present study was performed to analyse the cellular inflammatory response in the heart and cytomegalovirus replication during the development of myocarditis in vivo. We examined murine cytomegalovirus in an animal model of myocarditis using both susceptible BALB/c and resistant C57BL/6 mice. The heart infiltrating cells of BALB/c mice were found to comprise predominantly CD8+ T cells, with other cells of the CD4+ T cell, macrophage, B cell and neutrophil phenotype. Infectious MCMV titres in the heart were low and replicative virus could not be isolated beyond the first week post-infection (p.i.). Direct viral lysis of myocytes in vitro and apoptosis of cardiac cells in vivo was observed. Furthermore, viral DNA was detected in the heart of both mouse strains throughout the development of chronic disease. Viral gB RNA was detected during the first 35 days p.i. However, viral transcript for ie1 RNA but not gB RNA was found in the heart during the late stage of disease, suggesting latent viral infection of the heart. Our findings suggest that maintenance of the chronic phase of myocarditis involving post-viral immunological responses can occur in the presence of little infectious virus replication in the heart.

Processing and presentation of murine cytomegalovirus pORFm164-derived peptide in fibroblasts in the face of all viral immunosubversive early gene functions

CD8 T cells are the principal effector cells in the resolution of acute murine cytomegalovirus (mCMV) infection in host organs. This undoubted antiviral and protective in vivo function of CD8 T cells appeared to be inconsistent with immunosubversive strategies of the virus effected by early (E)-phase genes m04, m06, and m152. The so-called immune evasion proteins gp34, gp48, and gp37/40, respectively, were found to interfere with peptide presentation at different steps in the major histocompatibility complex (MHC) class I pathway of antigen processing and presentation in fibroblasts. Accordingly, they were proposed to prevent recognition and lysis of infected fibroblasts by cytolytic T lymphocytes (CTL) during the E phase of viral gene expression. We document here that the previously identified MHC class I D(d)-restricted antigenic peptide (257)AGPPRYSRI(265) encoded by gene m164 is processed as well as presented for recognition by m164-specific CTL during the E and late phases of viral replication in the very same cells in which the immunosubversive viral proteins are effectual in preventing the presentation of processed immediate-early 1 (m123-exon 4) peptide (168)YPHFMPTNL(176). Thus, while immunosubversion is a reality, these mechanisms are apparently not as efficient as the term immune evasion implies. The pORFm164-derived peptide is the first noted peptide that constitutively escapes the immunosubversive viral functions. The most important consequence is that even the concerted action of all immunosubversive E-phase proteins eventually fails to prevent immune recognition in the E phase. The bottom-line message is that there exists no immune evasion of mCMV in fibroblasts.

Tumor control in a model of bone marrow transplantation and acute liver-infiltrating B-cell lymphoma: an unpredicted novel function of cytomegalovirus

Tumor relapse and cytomegalovirus (CMV) infection are major concerns in the therapy of hematopoietic malignancies by bone marrow transplantation (BMT). Little attention so far has been given to a possible pathogenetic interplay between CMV and lymphomas. CMV inhibits stem cell engraftment and hematopoietic reconstitution. Thus, by causing maintenance of bone marrow aplasia and immunodeficiency, CMV could promote tumor relapse. Alternatively, CMV could aid tumor remission. One might think of cytopathogenic infection of tumor cells, induction of apoptosis or inhibitory cytokines, interference with tumor cell extravasation or tumor vascularization, or bystander stimulation of an antitumoral immune response. To approach these questions, the established model of experimental BMT and murine CMV infection was extended by the introduction of liver-infiltrating, highly tumorigenic variant clone E12E of BALB/c-derived B-cell lymphoma A20. We document a remarkable retardation of lymphoma progression. First-guess explanations were ruled out: (i) lymphoma cells were not infected; (ii) lymphoma cells located next to infected hepatocytes did not express executioner caspase 3 but were viable and proliferated; (iii) an inhibitory effect of virus on the formation of tumor nodules in the liver became apparent by day 7 after BMT, long before the reconstitution of immune cells; and (iv) recombinant tumor necrosis factor alpha (TNF-alpha) did not substitute for virus; accordingly anti-TNF-alpha did not prevent the inhibition. Notably, while the antitumoral effect required replicative virus, prevention of cytopathogenic infection of the liver by antiviral CD8 T cells did not abolish lymphoma control. These findings are paradigmatic for a novel virus-associated antitumoral mechanism distinct from oncolysis.

Two antigenic peptides from genes m123 and m164 of murine cytomegalovirus quantitatively dominate CD8 T-cell memory in the H-2d haplotype

The importance of CD8 T cells for the control of cytomegalovirus (CMV) infection has raised interest in the identification of immunogenic viral proteins as candidates for vaccination and cytoimmunotherapy. The final aim is to determine the viral "immunome" for any major histocompatibility complex class I molecule by antigenicity screening of proteome-derived peptides. For human CMV, there is a limitation to this approach: the T cells used as responder cells for peptide screening are usually memory cells that have undergone in vivo selection. On this basis, pUL83 (pp65) and pUL123 (IE1 or pp68 to -72) were classified as immunodominant proteins. It is an open question whether this limited "memory immunome" really reflects the immunogenic potential of the human CMV proteome. Here we document an analogous focus of the memory repertoire on two proteins of murine CMV. Specifically, ca. 80% of all memory CD8 T cells in the spleen as well as in persisting pulmonary infiltrates were found to be specific for the known IE1 peptide 168YPHFMPTNL176 and for the peptide 257AGPPRYSRI265, newly defined here, derived from open reading frame m164. Notably, CD8 T-cell lines of both specificities protected against acute infection upon adoptive transfer. In contrast, the natural immune response to acute infection in draining lymph nodes and in the lungs indicated a somewhat broader specificity repertoire. We conclude that the low number of antigenic peptides identified so far for CMVs reflects a focused memory repertoire, and we predict that more antigenic peptides will be disclosed by analysis of the acute immune response.

MCMV induces neointima in IFN-gammaR-/- mice: intimal cell apoptosis and persistent proliferation of myofibroblasts

BACKGROUND

CMV infections have been linked to vasculopathies like atherosclerosis and Scleroderma. CMV infects vascular endothelium with intermittent shedding of the virus and the development of latency.

METHODS

We adopted a model of arteritis, developed by Presti et al. (1998), triggered by murine cytomegalovirus (MCMV) infection. Our studies focused on neointima formation. Groups of mice include: 1) immunocompetent 129S, 2) immunocompetent 129S receiving whole body irradiation and MCMV, 3) IFN-gammaR-/- receiving MCMV, and 4) IFN-gammaR-/- receiving MCMV and whole body irradiation.

RESULTS

Mice were inoculated with MCMV (5 x 10(4) or 1 x 10(5) PFU's) by i.p. injection; hearts and abdominal aortas were collected and histopathology evaluated. Infected immunocompetent animals exhibited widespread perivascular inflammation, which subsided by 8 weeks. Intimal pathology was not observed in any control group. Immunocompetent animals receiving MCMV and irradiation developed mild to moderate intimal lesions associated with medial and adventitial inflammation. IFN-gammaR-/- mice infected for 4 months and receiving whole body irradiation 2 months after infection developed pathology characterized by extensive adventitial and medial infiltrate and significant neointima, suggesting that infection and immunosuppression were co-requisites of neointima formation. Immunohistochemical analysis revealed myofibroblasts as a major component of neointima. The disease is characterized by up-regulation of growth factors (TGF-beta1, PDGF-A and B). Apoptosis was detected in the intimal layer of affected aortas. Active proliferation of myofibroblasts and infiltrating cells was also detected.

CONCLUSION

These results indicate that CMV infections may lead to intimal injury that results in the formation of neointima characteristic of autoimmune vasculopathies.