Pathogenesis of an infectious mononucleosis-like disease induced by a murine gamma-herpesvirus: role for a viral superantigen?

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.

Immunophenotypic study of atypical lymphocytes. Generated in peripheral blood and spleen of nude mice After MHV-72 infection

Inbred athymic nude mice (BALB/c) were injected subcutaneously with the wild-type murine gammaherpesvirus 72 (MHV-72), which has been shown to induce the infectious mononucleosis (IM)-like syndrome in immunocompetent mice. The mice were also injected with UV-irradiated MHV-72. We studied the pattern of acute and chronic infection in the blood cells of the nude mice and detected viral DNA sequences in the infected leukocytes by polymerase chain reaction (PCR) technique up to when the animal died, close to 1 month postinfection. Using the UV-irradiated virus that induces an increase in mouse survival time, the viral sequences were present in the blood up to 3 months postinfection, then disappeared. We detected atypical lymphocytes in the blood of mice infected with both wt and UV-irradiated viruses. These atypical cells were similar in shape to those present in the blood of patients with IM induced by Epstein-Barr virus (EBV). Via Unscheduled DNA Synthesis (UDS), DNA synthesis was demonstrated in the atypical cells whose phenotype is identical to that of B cells, as shown with a panel of monoclonal antibodies. By double immunofluorescence staining, using an hyperimmune anti-MHV-72 serum and an anti-IgG + IgM + IgA monoclonal antibody, we demonstrated that these atypical B cells express some viral antigens. Contrary to the immunocompetent mice, the nude mice did not develop splenomegaly after infection with wt virus, probably due to the lack of T cell subsets. However, we observed an increase of nude mice B cells in the spleen. The nude mice died 1 month postinfection showing a high frequency (40%) of atypical lymphoblast-like B-cells in the blood; the increase in natural killer (NK) cell number was not detected after infection. Such findings suggest that NK cells probably did not play an important role in immune response to the MHV infection in nude mice. Finally, this mouse model could play an important role in antigammaherpesviral therapy of immunocompromised patients.

Control of gammaherpesvirus latency by latent antigen-specific CD8(+) T cells

The contribution of the latent antigen-specific CD8(+) T cell response to the control of gammaherpesvirus latency is currently obscure. Some latent antigens induce potent T cell responses, but little is known about their induction or the role they play during the establishment of latency. Here we used the murine gammaherpesvirus system to examine the expression of the latency-associated M2 gene during latency and the induction of the CD8(+) T cell response to this protein. M2, in contrast to the M3 latency-associated antigen, was expressed at day 14 after infection but was undetectable during long-term latency. The induction of the M2(91-99)/K(d) CD8(+) T cell response was B cell dependent, transient, and apparently induced by the rapid increase in latently infected cells around day 14 after intranasal infection. These kinetics were consistent with a role in controlling the initial "burst" of latently infected cells. In support of this hypothesis, adoptive transfer of an M2-specific CD8(+) T cell line reduced the initial load of latently infected cells, although not the long-term load. These data represent the first description of a latent antigen-specific immune response in this model, and suggest that vaccination with latent antigens such as M2 may be capable of modulating latent gammaherpesvirus infection.

Quantitative analysis of the CD8+ T-cell response to readily eliminated and persistent viruses

The recent development of techniques for the direct staining of peptide-specific CD8+ T cells has revolutionized the analysis of cell-mediated immunity (CMI) in virus infections. This approach has been used to quantify the acute and long-term consequences of infecting laboratory mice with the readily eliminated influenza A viruses (fluA) and a persistent gammaherpesvirus (gammaHV). It is now, for the first time, possible to work with real numbers in the analysis of CD8+ T CMI, and to define various characteristics of the responding lymphocytes both by direct flow cytometric analysis and by sorting for further in vitro manipulation. Relatively little has yet been done from the latter aspect, though we are rapidly accumulating a mass of numerical data. The acute, antigen-driven phases of the fluA and gammaHV-specific response look rather similar, but CD8+ T-cell numbers are maintained in the long term at a higher 'set point' in the persistent infection. Similarly, these 'memory' T cells continue to divide at a much greater rate in the gammaHV-infected mice. New insights have also been generated on the nature of the recall response following secondary challenge in both experimental systems, and the extent of protection conferred by large numbers of virus-specific CD8+ T cells has been determined. However, there are still many parameters that have received little attention, partly because they are difficult to measure. These include the rate of antigen-specific CD8+ T-cell loss, the extent of the lymphocyte 'diaspora' to other tissues, and the diversity of functional characteristics, turnover rates, clonal life spans and recirculation profiles. The basic question for immunologists remains how we reconcile the extraordinary plasticity of the immune system with the mechanisms that maintain a stable milieu interieur. This new capacity to quantify CD8+ T-cell responses in readily manipulated mouse models has obvious potential for illuminating homeostatic control, particularly if the experimental approaches to the problem are designed in the context of appropriate predictive models.

Murine gamma-herpesvirus infection causes V(beta)4-specific CDR3-restricted clonal expansions within CD8(+) peripheral blood T lymphocytes

Infection of mice with the gamma-herpesvirus MHV-68 results in lytic infection in the lung cleared by CD8(+) cells and establishment of lifelong latency. An Epstein-Barr virus (EBV)-like infectious mononucleosis (IM) syndrome emerges approximately 3 weeks after infection. In human IM, the majority of T cells in the peripheral blood are monoclonal or oligoclonal and are frequently specific for lytic or latent viral epitopes. However, a unique feature of MHV-68-induced IM is a prominent MHC haplotype-independent expansion of CD8(+) T cells, the majority of which utilize V(beta)4 chains in their alphabetaTCR. The ligand driving the V(beta)4 expansion is unknown, but the V(beta) bias and MHC haplotype independence raised the possibility that these cells were responding to a virally encoded or a virally induced endogenous superantigen (sAg). The aim of this study was to determine whether this rapidly proliferating subset is composed of polyclonally or clonally expanded T cells. Complementarity-determining region (CDR)-3 size analysis of V(beta)4(+)CD8(+) cells in infected mice demonstrated CDR3-restricted expansions in the V(beta)4 family as a whole. More refined analysis demonstrated major distortions in every J(beta) subfamily. V-D-J junctional region sequencing indicated that these CDR3 size-restricted expansions were composed of clonal or oligoclonal populations. The sequences were largely unique in individual mice, although evidence for 'public' or highly conserved T cell expansions was also seen between different mice. Taken together with previous studies showing an apparent MHC independence, the data suggest that a novel ligand, distinct from conventional sAg and peptide-MHC, drives proliferation of V(beta)4(+)CD8(+) T cells.

Accessing complexity: the dynamics of virus-specific T cell responses

The cellular dynamics of the immune system are complex and difficult to measure. Access to this problematic area has been greatly enhanced by the recent development of tetrameric complexes of MHC class I glycoprotein + peptide (tetramers) for the direct staining of freshly isolated, antigen-specific CD8(+ )T cells. Analysis to date with both naturally acquired and experimentally induced infections has established that the numbers of virus-specific CD8(+) T cells present during both the acute and memory phases of the host response are more than tenfold in excess of previously suspected values. The levels are such that the virus-specific CD8(+) set is readily detected in the human peripheral blood lymphocyte compartment, particularly during persistent infections. Experimentally, it is now possible to measure the extent of cycling for tetramer (+)CD8(+) T cells during the acute and memory phases of the host response to viruses. Dissection of the phenotypic, functional, and molecular diversity of CD8(+) T cell populations has been greatly facilitated. It is hoped it will also soon be possible to analyze CD4(+) T cell populations in this way. Though these are early days and there is an enormous amount to be done, our perceptions of the shape of virus-specific cell-mediated immunity are changing rapidly.

Latent murine gamma-herpesvirus infection is established in activated B cells, dendritic cells, and macrophages

Intranasal infection of mice with the murine gamma-herpesvirus MHV-68 results in an acute lytic infection in the lung, followed by the establishment of lifelong latency. Development of an infectious mononucleosis-like syndrome correlates with the establishment of latency and is characterized by splenomegaly and the appearance of activated CD8+ T cells in the peripheral blood. Interestingly, a large population of activated CD8+ T cells in the peripheral blood expresses the V beta 4+ element in their TCR. In this report we show that MHV-68 latency in the spleen after intranasal infection is harbored in three APC types: B cells, macrophages, and dendritic cells. Surprisingly, since latency has not previously been described in dendritic cells, these cells harbored the highest frequency of latent virus. Among B cells, latency was preferentially associated with activated B cells expressing the phenotype of germinal center B cells, thus formally linking the previously reported association of latency gene expression and germinal centers to germinal center B cells. Germinal center formation, however, was not required for the establishment of latency. Significantly, although three cell types were latently infected, the ability to stimulate V beta 4+CD8+ T cell hybridomas was limited to latently infected, activated B cells.

Lymphotoxin-alpha-deficient mice can clear a productive infection with murine gammaherpesvirus 68 but fail to develop splenomegaly or lymphocytosis

Respiratory challenge with murine gammaherpesvirus 68 (MHV-68) leads to an acute productive infection of the lung and a persistent latent infection in B lymphocytes, epithelia, and macrophages. The virus also induces splenomegaly and an increase in the number of activated CD8 T cells in the circulation. Lymphotoxin- alpha-deficient (LTalpha(-/-)) mice have no lymph nodes and have disrupted splenic architecture. Surprisingly, in spite of the severe defect in secondary lymphoid tissue, LTalpha(-/-) mice could clear a productive MHV-68 infection, although with delayed kinetics compared to wild-type mice, and could control latent infection. Cytotoxic T-cell activity was comparable in the lungs and spleens of LTalpha(-/-) and wild-type mice. However, splenic gamma interferon responses were substantially reduced in LTalpha(-/-) mice. Furthermore, LTalpha(-/-) mice failed to develop splenomegaly or lymphocytosis. Although germinal centers were absent, LTalpha(-/-) mice were able to class switch and showed significant virus-specific antibody titers. This work demonstrates that organized secondary lymphoid tissue is not an absolute requirement for the generation of immune responses to viral infections.

Analysis of the virus-specific and nonspecific B cell response to a persistent B-lymphotropic gammaherpesvirus

Respiratory challenge of mice with murine gammaherpesvirus 68 (gammaHV68) results in acute replication in respiratory epithelial cells and persistent, latent infection of B cells and macrophages. gammaHV68 elicits virus-specific Ab, and also nonspecifically activates B cells to Ab production through a CD4+ T cell-dependent process. The current analysis characterizes virus-specific and nonspecific Ab production at the single cell level and investigates the requirements and nature of the nonspecific response. Virus-specific Ab-forming cell (AFC) numbers were dwarfed by the increase in total AFC in all sites examined, indicating substantial nonspecific Ab production. Clear increases and decreases in specific and total AFC numbers occurred in the lymph nodes draining the respiratory tract and the spleen, but AFC numbers in the bone marrow (BM) increased to a plateau and remained constant. The longevity of the BM response was reflected in a sustained increase in virus-specific and total serum Ab levels. Generally, the IgG2a and IgG2b isotypes predominated. Analysis of cytokine-deficient mice, CD40 ligand-deficient mice, and radiation BM chimeras lacking MHC class II expression specifically on B cells indicated that nonspecific Ab production is independent of IL-6 or IFN-gamma, and dependent on cognate CD4+ T cell help. Several observations were consistent with polyclonal B cell activation by gammaHV68, including the induction of durable serum levels of IgG reactive with mammalian dsDNA and murine type II collagen. Our findings indicate new directions for studies of this valuable model of gamma-herpesvirus pathogenesis.

Infection of intestinal epithelial cells and development of systemic disease following gastric instillation of murine gammaherpesvirus-68

Murine gammaherpesvirus-68 (gammaHV-68) induces a lymphocytosis in mice and establishes a latent infection of B lymphocytes following intranasal administration in anaesthetized animals. Because gammaHV-68 is a gammaherpesvirus, it has been used as a model to understand the pathogenesis of Epstein-Barr virus (EBV) and human herpesvirus-8 (HHV-8) infections. In this study, we investigated the unlikely possibility that gammaHV-68 could survive the harsh gastrointestinal environment to efficiently infect intestinal epithelial cells, and then disseminate from mucosal sites to cause systemic disease. Surprisingly, oral administration, or gastric instillation which by-passed the oral cavity, readily caused a systemic lymphocytosis and established a latent infection in splenic leukocytes. The finding that gammaHV-68 could readily infect adult mice following gastric instillation strongly suggested that intestinal epithelial cells could be productively infected. Unlike the more routinely used method of intranasal inoculation, gammaHV-68 given intragastrically resulted in lytic virus, viral RNA and viral DNA being present in isolated intestinal epithelial cells. Furthermore, gammaHV-68 RNA and DNA, but not latent virus, could be detected in epithelial cells as long as 30 days post-infection, suggesting that some of these cells might be persistently infected. Taken together, these studies demonstrate that gammaHV-68 can survive passage through the gastrointestinal tract and infect intestinal epithelial cells. Following infection of gut epithelial cells, gammaHV-68 can disseminate from mucosal sites to induce a systemic lymphocytosis which is similar to the disease induced following intranasal inoculation.

T-cell vaccination alters the course of murine herpesvirus 68 infection and the establishment of viral latency in mice

Diseases caused by gammaherpesviruses such as Epstein-Barr virus are a major health concern, and there is significant interest in developing vaccines against this class of viral infections. However, the requirements for effective control of gammaherpesvirus infection are only poorly understood. The recent development of the murine herpesvirus MHV-68 model provides an experimental tool to dissect the immune response to gammaherpesvirus infections. In this study, we investigated the impact of priming T cells specific for class I- and class II-restricted epitopes on the acute phase of the infection and the subsequent establishment of latency and infectious mononucleosis. The data show that vaccination with either major histocompatibility complex class I- or class II-restricted T-cell epitopes derived from lytic cycle proteins significantly reduced lung viral titers during the acute infection. Moreover, the peak level of latently infected spleen cells was significantly reduced following vaccination with immunodominant CD8(+) T-cell epitopes. However, this vaccination approach did not prevent the long-term establishment of latency or the development of the infectious mononucleosis-like syndrome in infected mice. Thus, the virus is able to establish latency efficiently despite strong immunological control of the lytic infection.

Requirement for CD40 ligand, CD4(+) T cells, and B cells in an infectious mononucleosis-like syndrome

Respiratory challenge with the murine gammaherpesvirus 68 (gammaHV-68) results in productive infection of the lung, the establishment of latency in B lymphocytes and other cell types, transient splenomegaly, and prolonged clonal expansion of activated CD8(+) CD62L(lo) T cells, particularly a Vbeta4(+) CD8(+) population that is found in mice with different major histocompatibility complex (MHC) haplotypes. Aspects of the CD8(+)-T-cell response are substantially modified in mice that lack B cells, CD4(+) T cells, or the CD40 ligand (CD40L). The B-cell-deficient mice show no increase in Vbeta4(+) CD8(+) T cells. Similar abrogation of the Vbeta4(+) CD8(+) response is seen following antibody-mediated depletion of the CD4(+) subset, through the numbers of CD8(+) CD62L(lo) cells are still significantly elevated. Virus-specific CD4(+)-T-cell frequencies are minimal in the CD40L(-/-) mice, and the Vbeta4(+) CD8(+) population remains unexpanded. Apparently B-cell-CD4(+)-T-cell interactions play a part in the gammaHV-68 induction of both splenomegaly and non-MHC-restricted Vbeta4(+) CD8(+)-T-cell expansion.

In vivo models for Epstein-Barr virus (EBV)-associated B cell lymphoproliferative disease (BLPD)

EBV infects B lymphocytes in vivo and establishes a life-long persistent infection in the host. The latent infection is controlled by EBV-specific MHC class 1-restricted CTL. Immunosuppression reduces CTL activity, and this facilitates outgrowth of EBV+ve B cell lymphoproliferative disease (BLPD). BLPD are aggressive lesions with high mortality. This review presents some key facets in the development of EBV-associated BLPD and in vivo studies on its pathogenesis. The animal models used to date include the common marmoset (Callithrix jacchus), the cottontop tamarin (Saguinus oedipus oedipus), rhesus monkey, murine herpesvirus 68 (MHV68), and the severe combined immunodeficient (scid) mouse, each of which has been used to address particular aspects of EBV biology and BLPD development. Scid mice inoculated i.p. with PBMC from EBV-seropositive individuals develop EBV+ve BLPD-like tumours. Thus this small animal model (hu-PBMC-scid) is currently used by many laboratories to investigate EBV-associated diseases. We and others have studied BLPD pathogenesis in the hu-PBMC-scid model and shown that EBV+ve B cells on their own do not give rise to tumours in this model without inclusion of autologous T cell subsets in the inoculum. Based on the findings that (1) established tumours do not contain T cells and (2) tumour cells express a variety of B cell growth factors, a stepwise model of lymphomagenesis in the scid mouse model can be defined. Additionally, the hu-PBMC-scid model can be used to assess novel therapeutic regimes against BLPD before introduction into a clinical setting.

Kinetic and phenotypic changes in murine lymphocytes infected with murine gammaherpesvirus-68 in vitro

Primary infection with murine gammaherpesvirus-68 (MHV-68), as with other members of the gammaherpesvirus subfamily, is characterized by a lymphoproliferative phase. MHV-68 causes acute splenomegaly and an infectious mononucleosis-like syndrome in which there is expansion of the CD8+ T cell subset. In long-term infections, MHV-68 is associated with lymphoma development. In order to elucidate the mechanisms underlying the proliferative processes, the events following infection of murine splenocytes or purified murine B lymphocytes in vitro have been examined. MHV-68 infection prolonged the viability of murine splenocytes and stimulated cellular proliferation. Unlike Epstein-Barr virus and herpesvirus saimiri, MHV-68 did not cause growth transformation. Growth transformation did not occur even when cells with a predisposition to transformation were infected or when culture conditions were selected to enhance the viability of the cells. Following MHV-68 infection, the latency-associated viral tRNAs were transcribed. However, transcription of the other known latency-associated gene, M2, was not observed. In addition, there was no evidence of productive virus replication either by staining with antibodies specific for late virus antigens or by in situ hybridization for early and late mRNAs. In contrast to Epstein-Barr virus- and herpesvirus saimiri-infected lymphocytes, where episomal genomes are seen, Gardella gel analysis indicated that the primary lymphocytes infected by MHV-68 in vitro contained only linear virus DNA. This DNA was nuclease sensitive, indicating that, while MHV-68 was efficiently uncoated, its circularization in vitro was extremely inefficient. These results are discussed in terms of the host-virus interaction.

Requirement for CD4+ T Cells in Vβ4+CD8+ T Cell Activation Associated with Latent Murine Gammaherpesvirus Infection

A CD8+ T cell lymphocytosis in the peripheral blood is associated with the establishment of latency following intranasal infection with murine gammaherpesvirus-68. Remarkably, a large percentage of the activated CD8+ T cells of mice expressing different MHC haplotypes express Vβ4+ TCR. Identification of the ligand driving the Vβ4+CD8+ T cell activation remains elusive, but there is a general correlation between Vβ4+CD8+ T cell stimulatory activity and establishment of latency in the spleen. In the current study, the role of CD4+ T cells in the Vβ4+CD8+ T cell expansion has been addressed. The results show that CD4+ T cells are essential for expansion of the Vβ4+CD8+ subset, but not other Vβ subsets, in the peripheral blood. CD4+ T cells are required relatively late in the antiviral response, between 7 and 11 days after infection, and mediate their effect independently of IFN-γ. Assessment of Vβ4+CD8+ T cell stimulatory activity using murine gammaherpesvirus-68-specific T cell hybridomas generated from latently infected mice supports the idea that CD4+ T cells control levels of the stimulatory ligand that drives the Vβ4+CD8+ T cells. As Vβ4+CD8+ T cell expansion also correlates with levels of activated B cells, these data raise the possibility that CD4+ T cell-mediated B cell activation is required for optimal expression of the stimulatory ligand. In addition, in cases of low ligand expression, there may also be a direct role for CD4+ T cell-mediated help for Vβ4+CD8+ T cells.

Type I interferons and IRF-1 play a critical role in the control of a gammaherpesvirus infection

The murine gammaherpesvirus 68 (MHV-68) is an ideal model system for the study of interactions between gammaherpesviruses and their hosts. Intranasal infection of mice with MHV-68 results in replication of the virus in the lung epithelium followed by latent infection of B cells. Resolution of productive MHV-68 infection depends on the adaptive immune system, but little is known about the role of innate immune mechanisms and the early interaction between the host and the virus. In this report, we have used mice that are deficient in components of the early defence system, the common type I interferon (IFN) receptor (IFN R), the transcriptional activator IRF-1, and the inducible nitric oxide synthase, to investigate the contribution of these mechanisms to control of MHV-68 infection. We show that while wild-type mice are highly resistant to infection with MHV-68, mice unresponsive to type I IFNs (IFN-alpha/beta R(-/-) ) are highly susceptible to the virus. At high multiplicities of infection (m.o.i. ; 4 x 10(6) PFU), 80-90% of IFN-alpha/beta R(-/-) mice succumb to infection, and at low m.o.i. (4 x 10(3) PFU), 50% mortality rates occur. Both high and low doses of virus lead to 100- to 1000-fold higher lung virus titres in IFN-alpha/beta R(-/-) mice than are found in wild-type mice and result in systemic dissemination of the virus. Latently infected cells are detectable in the spleens of IFN-alpha/beta R(-/-) mice earlier than in wild-type mice, and the numbers of latently infected cells are 10-fold higher in the IFN-alpha/beta R(-/-) mice during the acute phase of infection. We find IRF-1 has a critical role in protection from fatal disease, whereas inducible nitric oxide synthase does not appear to be important. The results indicate that innate immune mechanisms are critical for the early control of MHV-68 and may play a role in the establishment of latency.

Turnover of T cells in murine gammaherpesvirus 68-infected mice

Respiratory challenge of C57BL/6 mice with murine gammaherpesvirus 68 induces proliferation of T lymphocytes early after infection, as evidenced by incorporation of the DNA precursor bromodeoxyuridine. Using pulse-chase analysis, splenic and peripheral blood activated T lymphocytes were found to continue dividing for at least a month after the initial virus challenge. The results are in accord with the idea that T cells are stimulated for a substantial time after the acute, lytic phase of virus infection is resolved.

A gamma-herpesvirus sneaks through a CD8(+) T cell response primed to a lytic-phase epitope

To determine whether established CD8(+) T cell memory to an epitope prominent during the replicative phase of a gamma-herpesvirus infection protects against subsequent challenge, mice were primed with a recombinant vaccinia virus expressing the p56 peptide and then boosted by intranasal exposure to an influenza A virus incorporating p56 in the neuraminidase protein. Clonally expanded populations of functional, p56-specific CD8(+) T cells were present at high frequency in both the lung and the lymphoid tissue 1 month later, immediately before respiratory challenge with gammaHV-68. This prime-and-boost regime led to a massive reduction of productive gammaHV-68 infection in the respiratory tract and, initially, to much lower levels of latency in both the regional lymph nodes and the spleen. The CD8(+) T cell response to another epitope (p79) was diminished, there was less evidence of B cell activation, and the onset of the CD4(+) T cell-dependent splenomegaly was delayed. Within 3-4 weeks of the gammaHV-68 challenge, however, the extent of latent infection in the lymph nodes and spleen was equivalent, and both groups developed the prominent infectious mononucleosis-like syndrome that is characteristic of this infection. The reverse protocol (influenza then vaccinia) seemed to be slightly less effective. Even though immune CD8(+) T cells may be present at the time and site of virus challenge, establishing a high level of CD8(+) T cell memory to lytic-phase epitopes alone does not protect against the longer-term consequences of this gammaHV infection.

Apparent MHC-Independent Stimulation of CD8+ T Cells In Vivo During Latent Murine Gammaherpesvirus Infection

Like EBV-infected humans with infectious mononucleosis, mice infected with the rodent gammaherpesvirus MHV-68 develop a profound increase in the number of CD8+ T cells in the circulation. In the mouse model, this lymphocytosis consists of highly activated CD8+ T cells strikingly biased toward Vβ4 TCR expression. Moreover, this expansion of Vβ4+CD8+ T cells does not depend on the MHC haplotype of the infected animal. Using a panel of lacZ-inducible T cell hybridomas, we have detected Vβ4-specific T cell stimulatory activity in the spleens of MHV-68-infected mice. We show that the appearance and quantity of this activity correlate with the establishment and magnitude of latent viral infection. Furthermore, on the basis of Ab blocking studies as well as experiments with MHC class II, β2-microglobulin (β2m) and TAP1 knockout mice, the Vβ4-specific T cell stimulatory activity does not appear to depend on conventional presentation by classical MHC class I or class II molecules. Taken together, the data indicate that during latent infection, MHV-68 may express a T cell ligand that differs fundamentally from both conventional peptide Ags and classical viral superantigens.

Unraveling immunity to gamma-herpesviruses: a new model for understanding the role of immunity in chronic virus infection

Murine gamma-herpesvirus 68 (gammaHV68) infection is a new model for understanding how immunity and chronic gamma-herpesvirus infection inter-relate. gammaHV68 is closely related to the human Epstein-Barr virus and Kaposi's sarcoma herpesvirus and is associated with tumors, vasculitis of the great elastic arteries and splenic fibrosis. Advances in the past year have provided an even stronger foundation for believing that gammaHV68 infection of normal and mutant mice will become the pre-eminent animal model for understanding gamma-herpesvirus pathogenesis and immunity. gammaHV68 latency has been characterized employing new assays for quantitating cells carrying the gammaHV68 genome and cells that reactivate gammaHV68 and for detecting the presence of preformed infectious virus in tissues. These advances have fostered the first steps towards a molecular definition of gammaHV68 latency. It appears that gammaHV68 shares latency programs with human gamma-herpesviruses - including the loci for gene 73, v-bcl-2 and the viral homolog of the G-protein coupled receptor. This provides candidate antigens for analysis of the role of T and B cells in regulating latency. Multiple cellular reservoirs for gammaHV68 latency were uncovered with the demonstration that gammaHV68 latently infects macrophages in addition to B cells. A critical role for B cells in regulating the nature of gammaHV68 latency was discovered and the mechanism was shown to be via alteration of the efficiency of reactivation. Studies of the response of CD4(+) and CD8(+) cells during acute and chronic gammaHV68 were performed. These new studies provide key building blocks for further development of this novel and interesting model system.

Lytic Cycle T Cell Epitopes Are Expressed in Two Distinct Phases During MHV-68 Infection

Murine herpesvirus-68 (MHV-68) is a type 2 γ herpesvirus that productively infects alveolar epithelial cells during the acute infection and establishes long-term latency in B cells and lung epithelial cells. In C57BL/6 mice, T cells specific for lytic cycle MHV-68 epitope p56/Db dominate the acute phase of the infection, whereas T cells specific for another lytic cycle epitope, p79/Kb, dominate later phases of infection. To further understand this response, we analyzed the kinetics of Ag presentation in vivo using a panel of lacZ-inducible T cell hybridomas specific for several lytic cycle epitopes, including p56/Db and p79/Kb. Two distinct peaks of Ag presentation were observed. The first peak was at day 6 in the mediastinal lymph nodes and correlated with the initial pulmonary lytic infection. The second peak was at day 18 in both the mediastinal lymph nodes and spleen and correlated with the peak of latent infection. Interestingly, the p56 epitope was detected only in the mediastinal lymph nodes at day 6 after infection whereas the p79 epitope was predominantly presented in the spleen at day 18, suggesting that differential epitope presentation drives the switch in T cell responses to this virus. Phenotypic analysis of APCs at day 18 postinfection revealed that dendritic cells, macrophages, and B cells all presented lytic cycle epitopes. Taken together, the data suggest that there is a resurgence of lytic cycle Ags in the spleen, which explains the kinetics and specificity of the T cell response to MHV-68.

Changing patterns of dominance in the CD8+ T cell response during acute and persistent murine gamma-herpesvirus infection

The murine gamma-herpesvirus MHV-68 causes an acute, transient pneumonitis, followed by an infectious mononucleosis (IM)-like illness with splenomegaly, widespread latent infection of B lymphocytes and an expansion of Vbeta4+ CD8+ T cells. CD8+ T cells specific for an H-2Db-restricted epitope were prominent during the acute respiratory infection, but their prevalence declined rapidly during the mononucleosis. In contrast, CD8+ T cells specific for an H-2Kb-restricted epitope, apparently expressed by virus-infected B lymphocytes, were most numerous during the mononucleosis illness and were maintained at relatively high frequencies thereafter. The prevalence of all peptide-specific CD8+ T cells decreased during the expansion of the Vbeta4+ CD8+ population, which did not recognize any peptide epitopes identified and was apparent also in an MHC class I-deficient environment. The CD8+ T cell population recognizing productively infected epithelial cells thus differed substantially from that responding during the IM illness.

Understanding the mechanism of action of bacterial superantigens from a decade of research

In the face of the unique diversity and plasticity of the immune system pathogenic organisms have developed multiple mechanisms in adaptation to their hosts, including the expression of a particular class of molecules called superantigens. Bacterial superantigens are the most potent stimulators of T cells. The functional consequences of the expression of superantigens by bacteria can be extended not only to T lymphocytes, but also to B lymphocytes and to cells of the myeloid compartment, including antigen-presenting cells and phagocytes. The biological effects of bacterial superantigens as well as their molecular aspects have now been studied for a decade. Although there is still a long way to go to clearly understand the role these molecules play in the establishment of disease, recently acquired knowledge of their biochemistry now offers unique experimental opportunities in defining the molecular rules of T-cell activation. Here, we present some of the most recent functional and molecular aspects of the interaction of bacterial superantigens with MHC class II molecules and the T-cell receptor.

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.

Murine gamma-herpesvirus 68 glycoprotein 150 protects against virus-induced mononucleosis: a model system for gamma-herpesvirus vaccination

Murine gamma-herpesvirus 68 (MHV-68) is a model for the study of the pathogenesis of gamma-herpesviruses. Epstein-Barr virus (EBV) is a highly related gamma-herpesvirus that causes significant disease in humans. The major membrane antigen gp350 of EBV is a candidate vaccine antigen for protection against EBV-related disease. An MHV-68 glycoprotein, gp150, has significant homology to EBV gp350. We have therefore used the MHV-68 gp150 to model the potential efficacy of EBV gp350 in protecting from virus-associated disease. A recombinant vaccinia virus expressing MHV-68 gp150 was constructed. This recombinant vaccinia virus was used to infect mice via the subcutaneous route. This vaccination resulted in production of MHV-68-neutralising antibodies. Mice were then challenged intra-nasally with MHV-68. MHV-68-associated mononucleosis was virtually abrogated in immunised mice. However, mice did establish MHV-68 latency. The results suggest that gp350 may be effective as an immunogen to prevent EBV-associated infectious mononucleosis in humans that are EBV-seronegative.

Virus-specific CD8(+) T cell numbers are maintained during gamma-herpesvirus reactivation in CD4-deficient mice

The murine gamma-herpesvirus 68 replicates in epithelial sites after intranasal challenge, then persists in various cell types, including B lymphocytes. Mice that lack CD4(+) T cells (I-Ab-/-) control the acute infection, but suffer an ultimately lethal recrudescence of lytic viral replication in the respiratory tract. The consequences of CD4(+) T cell deficiency for the generation and maintenance of murine gamma-herpesvirus 68-specific CD8(+) set now have been analyzed by direct staining with viral peptides bound to major histocompatibility complex class I tetramers and by a spectrum of functional assays. Both acutely and during viral reactivation, the CD8(+) T cell responses in the I-Ab-/- group were no less substantial than in the I-Ab+/+ controls. Indeed, virus-specific CD8(+) T cell numbers were increased in the lymphoid tissue of clinically compromised I-Ab-/- mice, although relatively few of the potential cytotoxic T lymphocyte effectors were recruited back to the site of pathology in the lung. Thus the viral reactivation that occurs in the absence of CD4(+) T cells was not associated with any exhaustion of the virus-specific cytotoxic T lymphocyte response. It seems that CD8(+) T cells alone are insufficient to maintain long-term control of this persistent gamma-herpesvirus.

A role for herpesvirus saimiri orf14 in transformation and persistent infection

The product of open reading frame 14 (orf14) of herpesvirus saimiri (HVS) exhibits significant homology with mouse mammary tumor virus superantigen. orf14 encodes a 50-kDa secreted glycoprotein, as shown previously (Z. Yao, E. Maraskovsky, M. K. Spriggs, J. I. Cohen, R. J. Armitage, and M. R. Alderson, J. Immunol. 156:3260-3266, 1996). orf14 expressed from recombinant baculovirus powerfully induces proliferation of CD4-positive cells originating from several different species. To study the role of orf14 in transformation, a mutant form of HVS (HVS Deltaorf14) was constructed with a deletion in the orf14 gene. The transforming potential of HVS Deltaorf14 was tested in cell culture and in common marmosets. Parental HVS subgroup C strain 488 immortalized common marmoset T lymphocytes in vitro to interleukin-2-independent growth, while the HVS Deltaorf14 mutant did not produce such a growth transformation. In addition, HVS Deltaorf14 was nononcogenic in common marmosets. In contrast to other nononcogenic HVS mutant viruses which were repeatedly isolated from peripheral blood mononuclear cells of infected marmosets for more than 5 months, HVS Deltaorf14 did not persist at a high level in vivo. These results demonstrate that orf14 of HVS is not required for replication but is required for transformation and for high-level persistence in vivo.

Murine gammaherpesvirus 68: a model for the study of gammaherpesvirus pathogenesis

Murine gammaherpesvirus 68 (MHV-68) is a naturally occurring herpesvirus of wild rodents and is genetically related to human herpesvirus 8 and Epstein-Barr virus. The ability of MHV-68 to establish acute and persistent infection within laboratory mice offers a unique opportunity to investigate immunological and virological aspects of gammaherpesvirus pathogenesis.

Mtv-1 superantigen trafficks independently of major histocompatibility complex class II directly to the B-cell surface by the exocytic pathway

Presentation of the Mtv-1 superantigen (vSag1) to specific Vbeta-bearing T cells requires association with major histocompatibility complex class II molecules. The intracellular route by which vSag1 trafficks to the cell surface and the site of vSag1-class II complex assembly in antigen-presenting B lymphocytes have not been determined. Here, we show that vSag1 trafficks independently of class II to the plasma membrane by the exocytic secretory pathway. At the surface of B cells, vSag1 associates primarily with mature peptide-bound class II alphabeta dimers, which are stable in sodium dodecyl sulfate. vSag1 is unstable on the cell surface in the absence of class II, and reagents that alter the surface expression of vSag1 and the conformation of class II molecules affect vSag1 stimulation of superantigen reactive T cells.

A dominant V beta bias in the CTL response after HSV-1 infection is determined by peptide residues predicted to also interact with the TCR beta-chain CDR3

Many T cell responses are dominated by restricted TCR expression and can range from repeated usage of particular TCR Vbeta- and/or Valpha-elements, to the preferential usage of both V- and J-elements, often in conjunction with conserved V-D-J or V-J junctional sequences. Cytotoxic T lymphocytes specific for a Kb-restricted determinant from the herpes simplex virus glycoprotein B (gB) preferentially express a dominant TCRBV10 beta-chain with sequence conservation of a tryptophan-glycine located in the V-D junction. Here we have examined whether immunisation of C57BL/6 mice with the gB-peptide can mimic the CTL response seen after HSV-1 infection. Immunisation with the gB-peptide resulted in the generation of gB-specific CTL that showed a similar TCRBV10 bias to that observed after HSV-1 infection. When the gB-determinant was expressed as a part of a fusion protein, immunised mice again exhibited the TCRBV10 bias with the junctional sequence conservation in the responding CTL. C57BL/6 mice were then immunised with variants of the gB-peptide that contained amino acid substitutions at positions previously predicted to contact the TCR beta-chain CDR3. Analysis of the TCRBV usage of variant specific CTL lines showed that substitutions at the TCR-contact positions 4, 6 and 7 of the gB-peptide resulted in a loss of the TCRBV10 bias. These results suggest that the TCRBV10 bias seen in gB-specific CTL after HSV-1 infection is due to antigenic selection by the minimal peptide and is determined by residues proposed to contact the TCR beta-chain CDR3.

Mouse mammary tumor virus and its interaction with the immune system

Mouse mammary tumor virus (MMTV) is a retrovirus that is transmitted through milk to offspring. Gut-associated B cells are the first cells to be infected during virus transmission, and these cells present a virus-encoded superantigen to cognate T cells. This allows MMTV to replicate and amplify in activated lymphocytes and ultimately results in virus transmission to the mammary epithelial cells. Because the superantigen has profound effects on the T cell repertoire and because MMTV replicates in lymphoid cells, loss of immune response to the virus may also play a role in its ability to persist within its host. Transcriptional control of MMTV expression also plays an important part in this pathway and DNA recognition sequences for transcription factors that allow its expression in lymphoid organs and mammary epithelia are encoded within the virus. Thus, this virus has evolved to take maximum advantage of its host's biology.

Kinetic analysis of the specific host response to a murine gammaherpesvirus

Respiratory infection of BALB/c mice with the murine gammaherpesvirus 68 (MHV-68) induces the clonal expansion of virus-specific cytotoxic T-lymphocyte (CTL) precursors (CTLp) in the regional, mediastinal lymph nodes (MLN). Some of these CTLps differentiate to become fully functional CTL effectors, which can be detected in both the lymphoid tissue and in the site of pathology in the lung. Though the lymph nodes and spleen harbor substantial populations of latently infected B cells for life, the level of virus-specific CTL activity decreases rapidly in all sites. The CD8+ CTLp numbers fall to background levels in the MLN within several months of the termination of the productive phase of MHV-68 infection in the respiratory epithelium but are maintained at relatively low frequency in the spleen. The continued presence of a gamma interferon-producing, MHV-68-specific CD4+ set can also be demonstrated in cultured spleen cells. The virus-specific immunoglobulin G (IgG) response is slow to develop, with serum neutralizing antibody and enzyme-linked immunosorbent assay titers continuing to rise for several months. The level of total serum IgG increases dramatically within 2 weeks of infection, probably as a consequence of polyclonal B-cell activation, and remains high. The immune response profile is clearly influenced by the persistence of this DNA virus.

Effector CD4+ and CD8+ T-cell mechanisms in the control of respiratory virus infections

The rules for T-cell-mediated control of viruses that infect via the respiratory mucosae show both common themes and differences, depending on the nature of the pathogen. Virus-specific CD8+ cytotoxic T lymphocytes (CTLs) are the key effectors of virus clearance in mice infected with both negative strand RNA viruses (influenza and Sendai) and a DNA virus, the murine gamma-herpesvirus-68 (MHV-68). Recently completed experiments establish that these activated CD8+ T cells indeed operate primarily via contact-dependent lysis. Perforin-mediated cytotoxicity seems to be the preferred mode, though a Fas-based mechanism can apparently serve as an alternative mechanism. Immune CD4+ T cells functioning in the absence of the CD8+ subset cannot eliminate MHV-68 from lung epithelial cells, are somewhat less efficient than the CD8+ CTLs at clearing the RNA viruses, and are generally ineffectual in mice that lack B lymphocytes. Though cytokine secretion by CD4+ and CD8+ T cells in the virus-infected lung may promote both T-cell extravasation and macrophage activation, such processes are not alone sufficient to deal consistently with any of these infections. However, CD4+ T help is mandatory for an effective B-cell response, and can operate to promote the clonal expansion of virus-specific CD8+ T cells in the lymph nodes and spleen. Furthermore, a concurrent CD4+ T-cell response seems to be essential for maintaining continued CD8+ T-cell surveillance and effector capacity through the persistent, latent phase of MHV-68 infection in B cells. Thus, the evidence to date supports a very traditional view; CD8+ T cells function mainly as killers and the CD4+ T cells as helpers in these respiratory virus infections.

Tuning into immunological dissonance: an experimental model for infectious mononucleosis

Virus infections cause a much more profound perturbation of the lymphoid tissue than can be accounted for by the exigencies of the antigen-specific response. The extent of this 'immunological dissonance' is seen most dramatically in mice infected with a persistent gamma-herpesvirus, MHV-68. A profile of massive, continuing proliferation of both T and B cells in the lymph nodes and spleen leads to a dramatic increase in the prevalence of a CD62Llow CD8+ T cell subset in the blood, a pattern first detected two to three weeks after intranasal exposure to the inducing virus. This syndrome, which seems identical to human infectious mononucleosis (IM), persists for a further month or more. Part of the IM-like phase of MHV-68 infection reflects the selective expansion of Vbeta4+ CD8+ T cells, with the Vbeta4 effect being apparent for several different MHC class I H-2 types but not in mice that are deficient in MHC class II glycoprotein expression. Depleting CD4(+) T helper cells in MHV-68-infected mice leads to the decreased proliferation of the CD8+ T cells in the spleen and fewer CD62Llow CD8+ T lymphocytes than would be expected in peripheral blood, but fails to diminish the prominence of the V4beta+ CD8+ population. The results so far of this unique experimental mouse model of IM suggest that both cytokine-mediated effects and a viral superantigen are operating to promote the dramatic expansion and persistence of activated CD8+ T cells in the vascular compartment.