CD4(+) T cell-mediated control of a gamma-herpesvirus in B cell-deficient mice is mediated by IFN-gamma

A replication-deficient gammaherpesvirus vaccine protects mice from lytic disease and reduces latency establishment

Gammaherpesviruses are oncogenic viruses that establish lifelong infections and are significant causes of morbidity and mortality. Vaccine strategies to limit gammaherpesvirus infection and disease are in development, but there are no FDA-approved vaccines for Epstein-Barr or Kaposi sarcoma herpesvirus. As a new approach to gammaherpesvirus vaccination, we developed and tested a replication-deficient virus (RDV) platform, using murine gammaherpesvirus 68 (MHV68), a well-established mouse model for gammaherpesvirus pathogenesis studies and preclinical therapeutic evaluations. We employed codon-shuffling-based complementation to generate revertant-free RDV lacking expression of the essential replication and transactivator protein encoded by ORF50 to arrest viral gene expression early after de novo infection. Inoculation with RDV-50.stop exposes the host to intact virion particles and leads to limited lytic gene expression in infected cells yet does not produce additional infectious particles. Prime-boost vaccination of mice with RDV-50.stop elicited virus-specific neutralizing antibody and effector T cell responses in the lung and spleen. In contrast to vaccination with heat-inactivated WT MHV68, vaccination with RDV-50.stop resulted in a near complete abolishment of virus replication in the lung 7 days post-challenge and reduction of latency establishment in the spleen 16 days post-challenge with WT MHV68. Ifnar1-/- mice, which lack the type I interferon receptor, exhibit severe disease and high mortality upon infection with WT MHV68. RDV-50.stop vaccination of Ifnar1-/- mice prevented wasting and mortality upon challenge with WT MHV68. These results demonstrate that prime-boost vaccination with a gammaherpesvirus that is unable to undergo lytic replication offers protection against acute replication, impairs the establishment of latency, and prevents severe disease upon the WT virus challenge. Our study also reveals that the ability of a gammaherpesvirus to persist in vivo despite potent pre-existing immunity is an obstacle to obtaining sterilizing immunity.

Indirect CD4+ T cell protection against mouse gamma-herpesvirus infection via interferon gamma

CD4+ T cells play a key role in γ-herpesvirus infection control. However, the mechanisms involved are unclear. Murine herpesvirus type 4 (MuHV-4) allows relevant immune pathways to be dissected experimentally in mice. In the lungs, it colonizes myeloid cells, which can express MHC class II (MHCII), and type 1 alveolar epithelial cells (AEC1), which lack it. Nevertheless, CD4+ T cells can control AEC1 infection, and this control depends on MHCII expression in myeloid cells. Interferon-gamma (IFNγ) is a major component of CD4+ T cell-dependent MuHV-4 control. Here, we show that the action of IFNγ is also indirect, as CD4+ T cell-mediated control of AEC1 infection depended on IFNγ receptor (IFNγR1) expression in CD11c+ cells. Indirect control also depended on natural killer (NK) cells. Together, the data suggest that the activation of MHCII+ CD11c+ antigen-presenting cells is key to the CD4+ T cell/NK cell protection axis. By contrast, CD8+ T cell control of AEC1 infection appeared to operate independently.

IMPORTANCE

CD4+ T cells are critical for the control of gamma-herpesvirus infection; they act indirectly, by recruiting natural killer (NK) cells to attack infected target cells. Here, we report that the CD4+ T cell/NK cell axis of gamma-herpesvirus control requires interferon-γ engagement of CD11c+ dendritic cells. This mechanism of CD4+ T cell control releases the need for the direct engagement of CD4+ T cells with virus-infected cells and may be a common strategy for host control of immune-evasive pathogens.

Replication-dead gammaherpesvirus vaccine protects against acute replication, reactivation from latency, and lethal challenge in mice

Gammaherpesviruses (GHVs) are oncogenic viruses that establish lifelong infections and are significant causes of human morbidity and mortality. While several vaccine strategies to limit GHV infection and disease are in development, there are no FDA-approved vaccines for human GHVs. As a new approach to gammaherpesvirus vaccination, we developed and tested a replication-dead virus (RDV) platform, using murine gammaherpesvirus 68 (MHV68), a well-established mouse model for gammaherpesvirus pathogenesis studies and preclinical therapeutic evaluations. We employed codon-shuffling-based complementation to generate revertant-free RDV lacking expression of the essential replication and transactivator protein (RTA) encoded by ORF50 to arrest viral gene expression early after de novo infection. Inoculation with RDV-50.stop exposes the host to intact virion particles and leads to limited lytic gene expression in infected cells. Prime-boost vaccination of mice with RDV-50.stop elicited virus-specific neutralizing antibody and effector T cell responses in the lung and spleen. Vaccination with RDV-50.stop resulted in a near complete abolishment of virus replication in the lung 7 days post-challenge and virus reactivation from spleen 16 days post-challenge with WT MHV68. Ifnar1-/- mice, which lack the type I interferon receptor, exhibit severe disease upon infection with WT MHV68. RDV-50.stop vaccination of Ifnar1-/- mice prevented wasting and mortality upon challenge with WT MHV68. These results demonstrate that prime-boost vaccination with a GHV that is unable to undergo lytic replication offers protection against acute replication, reactivation, and severe disease upon WT virus challenge.

Membrane association of a model CD4+ T-cell vaccine antigen confers enhanced yet incomplete protection against murid herpesvirus-4 infection

Vaccination against γ-herpesviruses has proved difficult. CD4⁺ T cells are essential to contain infection, but how best to prime them and whether this can reduce viral loads remain unclear. To address these questions, we used ovalbumin (OVA) as a model antigen, delivering it with murine cytomegalovirus (MCMV) to protect mice against OVA-expressing murine herpesvirus-4 (MuHV-4). Membrane-associated OVA (mOVA) was more effective than soluble OVA, both to prime CD4⁺ T cells and as an effector target. It was also a better target than an OVA epitope limited to infected cells, suggesting that protective CD4⁺ T cells recognize infected cell debris rather than infected cells themselves. While MCMV-mOVA protected acutely against MuHV-4-mOVA, long-term protection was incomplete, even when OVA-specific CD8⁺ T cells and B cells were also primed. Thus, even optimized single-target vaccines may poorly reduce long-term γ-herpesvirus infections.

Transcriptional Profiles Associated with Marek's Disease Virus in Bursa and Spleen Lymphocytes Reveal Contrasting Immune Responses during Early Cytolytic Infection

Marek's disease virus (MDV), an alpha herpes virus, causes a lymphoproliferative state in chickens known as Marek's disease (MD), resulting in severe monetary losses to the poultry industry. Because lymphocytes of bursa of Fabricius and spleen are prime targets of MDV replication during the early cytolytic phase of infection, the immune response in bursa and spleen should be the foundation of late immunity induced by MDV. However, the mechanism of the MDV-mediated host immune response in lymphocytes in the early stage is poorly understood. The present study is primarily aimed at identifying the crucial genes and significant pathways involved in the immune response of chickens infected with MDV CVI988 and the very virulent RB1B (vvRB1B) strains. Using the RNA sequencing approach, we analyzed the generated transcriptomes from lymphocytes isolated from chicken bursa and spleen. Our findings validated the expression of previously characterized genes; however, they also revealed the expression of novel genes during the MDV-mediated immune response. The results showed that after challenge with CVI988 or vvRB1B strains, 634 and 313 differentially expressed genes (DEGs) were identified in splenic lymphocytes, respectively. However, 58 and 47 DEGs were observed in bursal lymphocytes infected with CVI988 and vvRB1B strains, respectively. Following MDV CVI988 or vvRB1B challenge, the bursal lymphocytes displayed changes in IL-6 and IL-4 gene expression. Surprisingly, splenic lymphocytes exhibited an overwhelming alteration in the expression of cytokines and cytokine receptors involved in immune response signaling. On the other hand, there was no distinct trend between infection with CVI988 and vvRB1B and the expression of cytokines and chemokines, such as IL-10, IFN-γ, STAT1, IRF1, CCL19, and CCL26. However, the expression profiles of IL-1β, IL-6, IL8L1, CCL4 (GGCL1), and CCL5 were significantly upregulated in splenic lymphocytes from chickens infected with CVI988 compared with those of chickens infected with vvRB1B. Because these cytokines and chemokines are considered to be associated with B cell activation and antigenic signal transduction to T cells, they may indicate differences of immune responses initiated by vaccinal and virulent strains during the early phase of infection. Collectively, our study provides valuable data on the transcriptional landscape using high-throughput sequencing to understand the different mechanism between vaccine-mediated protection and pathogenesis of virulent MDV in vivo.

A CD4+ T Cell-NK Cell Axis of Gammaherpesvirus Control

CD4⁺ T cells are essential to control herpesviruses. Murid herpesvirus 4 (MuHV-4)-driven lung disease in CD4⁺ T-cell-deficient mice provides a well-studied example. Protective CD4⁺ T cells have been hypothesized to kill infected cells directly. However, removing major histocompatibility complex class II (MHCII) from LysM⁺ or CD11c⁺ cells increased MuHV-4 replication not in those cells but in type 1 alveolar epithelial cells, which lack MHCII, LysM, or CD11c. Disruption of MHCII in infected cells had no effect. Therefore, CD4⁺ T cells engaged uninfected presenting cells and protected indirectly. Mice lacking MHCII in LysM⁺ or CD11c⁺ cells maintained systemic antiviral CD4⁺ T cell responses, but recruited fewer CD4⁺ T cells into infected lungs. NK cell infiltration was also reduced, and NK cell depletion normalized infection between MHCII-deficient and control mice. Therefore, NK cell recruitment seemed to be an important component of CD4⁺ T-cell-dependent protection. Disruption of viral CD8⁺ T cell evasion made this defense redundant, suggesting that it is important mainly to control CD8-evasive pathogens.IMPORTANCE Gammaherpesviruses are widespread and cause cancers. CD4⁺ T cells are a key defense. We found that they defend indirectly, engaging uninfected presenting cells and recruiting innate immune cells to attack infected targets. This segregation of CD4⁺ T cells from immediate contact with infection helps the immune system to cope with viral evasion. Priming this defense by vaccination offers a way to protect against gammaherpesvirus-induced cancers.

A Human STAT1 Gain-of-Function Mutation Impairs CD8+ T Cell Responses against Gammaherpesvirus 68

Autosomal dominant STAT1 mutations in humans have been associated with chronic mucocutaneous candidiasis (CMC), as well as with increased susceptibility to herpesvirus infections. Prior studies have focused on mucosal and Th17-mediated immunity against Candida, but mechanisms of impaired antiviral immunity have not previously been examined. To begin to explore the mechanisms of STAT1-associated immunodeficiency against herpesviruses, we generated heterozygous STAT1 R274W knock-in mice that have a frequently reported STAT1 mutation associated in humans with susceptibility to herpesvirus infections. In primary macrophages and fibroblasts, we found that STAT1 R274W had no appreciable effect on cell-intrinsic immunity against herpes simplex virus 1 (HSV-1) or gammaherpesvirus 68 (γHV68) infection. However, intraperitoneal inoculation of mice with γHV68 was associated with impaired control of infection at day 14 in STAT1 R274W mice compared with that in wild-type (WT) littermate control animals. Infection of STAT1 R274W mice was associated with paradoxically decreased expression of IFN-stimulated genes (ISGs) and gamma interferon (IFN-γ), likely secondary to defective CD4⁺ and CD8⁺ T cell responses, including diminished numbers of antigen-specific CD8⁺ T cells. Viral pathogenesis studies in WT and STAT1 R274W mixed bone marrow chimeric mice revealed that the presence of WT leukocytes was sufficient to limit infection and that antigen-specific STAT1 R274W CD8⁺ T cell responses were impaired even in the presence of WT leukocytes. Thus, in addition to regulating Th17 responses against Candida, a STAT1 gain-of-function mutant impedes antigen-specific T cell responses against a common gammaherpesvirus in mice.IMPORTANCE Mechanisms of immunodeficiency related to STAT1 gain of function have not been previously studied in an animal model of viral pathogenesis. Using virological and immunological techniques, we examined the immune response to γHV68 in heterozygous mice that have an autosomal dominant mutation in the STAT1 coiled-coil domain (STAT1 R274W). We observed impaired control of infection, which was associated with diminished production of gamma interferon (IFN-γ), fewer effector CD4⁺ and CD8⁺ T cells, and a reduction in the number of antigen-specific CD8⁺ T cells. These findings indicate that a STAT1 gain-of-function mutation limits production of antiviral T cells, likely contributing to immunodeficiency against herpesviruses.

Immune Control of γ-Herpesviruses

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

Advances in the Characterization of the T-Cell Response to Human Herpesvirus-6

Human herpesvirus (HHV) 6 is thought to remain clinically latent in most individuals after primary infection and to reactivate to cause disease in persons with severe immunosuppression. In allogeneic hematopoietic stem cell transplant recipients, reactivation of HHV-6 species B is a considerable cause of morbidity and mortality. HHV-6B reactivation is the most frequent cause of infectious meningoencephalitis in this setting and has been associated with a variety of other complications such as graft rejection and acute graft versus host disease. This has inspired efforts to develop HHV-6-targeted immunotherapies. Basic knowledge of HHV-6-specific adaptive immunity is crucial for these endeavors, but remains incomplete. Many studies have focused on specific HHV-6 antigens extrapolated from research on human cytomegalovirus, a genetically related betaherpesvirus. Challenges to the study of HHV-6-specific T-cell immunity include the very low frequency of HHV-6-specific memory T cells in chronically infected humans, the large genome size of HHV-6, and the lack of an animal model. This review will focus on emerging techniques and methodological improvements that are beginning to overcome these barriers. Population-prevalent antigens are now becoming clear for the CD4+ T-cell response, while definition and ranking of CD8+ T-cell antigens and epitopes is at an earlier stage. This review will discuss current knowledge of the T-cell response to HHV-6, new research approaches, and translation to clinical practice.

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

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

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

Type I Interferons and NK Cells Restrict Gammaherpesvirus Lymph Node Infection

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

IMPORTANCE

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

The anti-fibrotic effect of inhibition of TGFβ-ALK5 signalling in experimental pulmonary fibrosis in mice is attenuated in the presence of concurrent γ-herpesvirus infection

TGFβ-ALK5 pro-fibrotic signalling and herpesvirus infections have been implicated in the pathogenesis and exacerbation of pulmonary fibrosis. In this study we addressed the role of TGFβ-ALK5 signalling during the progression of fibrosis in a two-hit mouse model of murine γ-herpesvirus 68 (MHV-68) infection on the background of pre-existing bleomycin-induced pulmonary fibrosis. Assessment of total lung collagen levels in combination with ex vivo micro-computed tomography (µCT) analysis of whole lungs demonstrated that MHV-68 infection did not enhance lung collagen deposition in this two-hit model but led to a persistent and exacerbated inflammatory response. Moreover, µCT reconstruction and analysis of the two-hit model revealed distinguishing features of diffuse ground-glass opacities and consolidation superimposed on pre-existing fibrosis that were reminiscent of those observed in acute exacerbation of idiopathic pulmonary fibrosis (AE-IPF). Virally-infected murine fibrotic lungs further displayed evidence of extensive inflammatory cell infiltration and increased levels of CCL2, TNFα, IL-1β and IL-10. Blockade of TGFβ-ALK5 signalling attenuated lung collagen accumulation in bleomycin-alone injured mice, but this anti-fibrotic effect was reduced in the presence of concomitant viral infection. In contrast, inhibition of TGFβ-ALK5 signalling in virally-infected fibrotic lungs was associated with reduced inflammatory cell aggregates and increased levels of the antiviral cytokine IFNγ. These data reveal newly identified intricacies for the TGFβ-ALK5 signalling axis in experimental lung fibrosis, with different outcomes in response to ALK5 inhibition depending on the presence of viral infection. These findings raise important considerations for the targeting of TGFβ signalling responses in the context of pulmonary fibrosis.

Functional heterogeneity in the CD4+ T cell response to murine γ-herpesvirus 68

CD4(+) T cells are critical for the control of virus infections, T cell memory, and immune surveillance. We studied the differentiation and function of murine γ-herpesvirus 68 (MHV-68)-specific CD4(+) T cells using gp150-specific TCR-transgenic mice. This allowed a more detailed study of the characteristics of the CD4(+) T cell response than did previously available approaches for this virus. Most gp150-specific CD4(+) T cells expressed T-bet and produced IFN-γ, indicating that MHV-68 infection triggered differentiation of CD4(+) T cells largely into the Th1 subset, whereas some became follicular Th cells and Foxp3(+) regulatory T cells. These CD4(+) T cells were protective against MHV-68 infection in the absence of CD8(+) T cells and B cells, and protection depended on IFN-γ secretion. Marked heterogeneity was observed in the CD4(+) T cells, based on lymphocyte Ag 6C (Ly6C) expression. Ly6C expression positively correlated with IFN-γ, TNF-α, and granzyme B production; T-bet and KLRG1 expression; proliferation; and CD4(+) T cell-mediated cytotoxicity. Ly6C expression inversely correlated with survival, CCR7 expression, and secondary expansion potential. Ly6C(+) and Ly6C(-) gp150-specific CD4(+) T cells were able to interconvert in a bidirectional manner upon secondary Ag exposure in vivo. These results indicate that Ly6C expression is closely associated with antiviral activity in effector CD4(+) T cells but is inversely correlated with memory potential. Interconversion between Ly6C(+) and Ly6C(-) cells may maintain a balance between the two Ag-specific CD4(+) T cell populations during MHV-68 infection. These findings have significant implications for Ly6C as a surface marker to distinguish functionally distinct CD4(+) T cells during persistent virus infection.

CD4 T cells specific for a latency-associated γ-herpesvirus epitope are polyfunctional and cytotoxic

The oncogenic γ-herpesviruses EBV and Kaposi sarcoma-associated herpesvirus are ubiquitous human pathogens that establish lifelong latent infections maintained by intermittent viral reactivation and reinfection. Effector CD4 T cells are critical for control of viral latency and in immune therapies for virus-associated tumors. In this study, we exploited γHV68 infection of mice to enhance our understanding of the CD4 T cell response during γ-herpesvirus infection. Using a consensus prediction approach, we identified 16 new CD4 epitope-specific responses that arise during lytic infection. An additional epitope encoded by the M2 protein induced uniquely latency-associated CD4 T cells, which were not detected at the peak of lytic infection but only during latency and were not induced postinfection with a latency-deficient virus. M2-specific CD4 T cells were selectively cytotoxic, produced multiple antiviral cytokines, and sustained IL-2 production. Identification of latency-associated cytolytic CD4 T cells will aid in dissecting mechanisms of CD4 immune control of γ-herpesvirus latency and the development of therapeutic approaches to control viral reactivation and pathology.

IFNγ and perforin cooperate to control infection and prevent fatal pathology during persistent gammaherpesvirus infection in mice

Infection with murine gammaherpesvirus 68 has become an accepted model for studying the virus/host interactions with regard to gammaherpesvirus infections. Previous studies using gene-deficient mice have revealed that neither IFNγ nor perforin is essential in controlling the outcome of infection or the virus load during chronic infection in C57BL/6 mice. However, pronounced multiorgan fibrosis and splenic atrophy are observed in mice lacking IFNγ or the IFNγ receptor. To study the interplay between perforin and IFNγ in controlling the virus-induced pathology and the viral load during chronic gammaherpesvirus infection, we infected IFNγ/perforin double-deficient C57BL/6 mice and followed the course of infection. While absence of perforin prevented the splenic atrophy in IFNγ-deficient mice, fibrosis did not disappear. Moreover, double-deficient mice developed extreme splenomegaly, were unable to control the viral load and displayed chronic immune activation. Thus, IFNγ and perforin act in concert to minimize pathology and control the viral load in mice chronically infected with MHV68. Furthermore, while certain aspect of the virus-induced pathology in IFNγ-deficient mice may be alleviated in double-deficient mice, other aspects are exaggerated, and the normal architecture of the spleen is completely destroyed. We believe that these findings add to the understanding of the virus/host interaction during chronic gammaherpes virus infection.

Phagocytic cells contribute to the antibody-mediated elimination of pulmonary-infected SARS coronavirus

While the 2002–2003 outbreak of severe acute respiratory syndrome (SARS) resulted in 774 deaths, patients who were affected with mild pulmonary symptoms successfully recovered. The objective of the present work was to identify, using SARS coronavirus (SARS-CoV) mouse infection models, immune factors responsible for clearing of the virus. The elimination of pulmonary SARS-CoV infection required the activation of B cells by CD4⁺ T cells. Furthermore, passive immunization (post-infection) with homologous (murine) anti-SARS-CoV antiserum showed greater elimination efficacy against SARS-CoV than that with heterologous (rabbit) antiserum, despite the use of equivalent titers of neutralizing antibodies. This distinction was mediated by mouse phagocytic cells (monocyte-derived infiltrating macrophages and partially alveolar macrophages, but not neutrophils), as demonstrated both by adoptive transfer from donors and by immunological depletion of selected cell types. These results indicate that the cooperation of anti-SARS-CoV antibodies and phagocytic cells plays an important role in the elimination of SARS-CoV.

Is murine gammaherpesvirus-68 (MHV-68) a suitable immunotoxicological model for examining immunomodulatory drug-associated viral recrudescence?

Immunosuppressive agents are used for treatment of a variety of autoimmune diseases including rheumatoid arthritis (RA), systemic lupus erythematosis (SLE), and psoriasis, as well as for prevention of tissue rejection after organ transplantation. Recrudescence of herpesvirus infections, and increased risk of carcinogenesis from herpesvirus-associated tumors are related with immunosuppressive therapy in humans. Post-transplant lymphoproliferative disorder (PTLD), a condition characterized by development of Epstein Barr Virus (EBV)-associated B-lymphocyte lymphoma, and Kaposi's Sarcoma (KS), a dermal tumor associated with Kaposi Sarcoma-associated virus (KSHV), may develop in solid organ transplant patients. KS also occurs in immunosuppressed Acquired Immunodeficiency (AIDS) patients. Kaposi Sarcoma-associated virus (KSHV) is a herpes virus genetically related to EBV. Murine gammaherpes-virus-68 (MHV-68) is proposed as a mouse model of gammaherpesvirus infection and recrudescence and may potentially have relevance for herpesvirus-associated neoplasia. The pathogenesis of MHV-68 infection in mice mimics EBV/KSHV infection in humans with acute lytic viral replication followed by dissemination and establishment of persistent latency. MHV-68-infected mice may develop lymphoproliferative disease that is accelerated by disruption of the immune system. This manuscript first presents an overview of gammaherpesvirus pathogenesis and immunology as well as factors involved in viral recrudescence. A description of different types of immunodeficiency then follows, with particular focus on viral association with lymphomagenesis after immunosuppression. Finally, this review discusses different gammaherpesvirus animal models and describes a proposed MHV-68 model to further examine the interplay of immunomodulatory agents and gammaherpesvirus-associated neoplasia.

CD4 T cell responses in latent and chronic viral infections

The spectrum of tasks which is fulfilled by CD4 T cells in the setting of viral infections is large, ranging from support of CD8 T cells and humoral immunity to exertion of direct antiviral effector functions. While our knowledge about the differentiation pathways, plasticity, and memory of CD4 T cell responses upon acute infections or immunizations has significantly increased during the past years, much less is still known about CD4 T cell differentiation and their beneficial or pathological functions during persistent viral infections. In this review we summarize current knowledge about the differentiation, direct or indirect antiviral effector functions, and the regulation of virus-specific CD4 T cells in the setting of persistent latent or active chronic viral infections with a particular emphasis on herpes virus infections for the former and chronic lymphocytic choriomeningitis virus infection for the latter.

CD4 and CD8 T cells directly recognize murine gammaherpesvirus 68-immortalized cells and prevent tumor outgrowth

There has been extensive research regarding T cell recognition of Epstein-Barr virus-transformed cells; however, less is known regarding the recognition of B cells immortalized by gamma-2 herpesviruses. Here we show that B cells immortalized by murine gammaherpesvirus 68 (MHV-68, γHV-68) can be controlled by either CD4 or CD8 T cells in vivo. We present evidence for the direct recognition of infected B cells by CD4 and CD8 T cells. These data will help in the development of immunotherapeutic approaches combating gamma-2 herpesvirus-related disease.

Herpesviruses placating the unwilling host: manipulation of the MHC class II antigen presentation pathway

Lifelong persistent infection by herpesviruses depends on the balance between host immune responses and viral immune evasion. CD4 T cells responding to antigens presented on major histocompatibility complex class II (MHC-II) molecules are known to play an important role in controlling herpesvirus infections. Here we review, with emphasis on human herpesvirus infections, the strategies evolved to evade CD4 T cell immunity. These viruses target multiple points on the MHC class II antigen presentation pathway. The mechanisms include: suppression of CIITA to inhibit the synthesis of MHC class II molecules, diversion or degradation of HLA-DR molecules during membrane transport, and direct targeting of the invariant chain chaperone of HLA-DR.

Kaposi's sarcoma-associated herpesvirus inhibits expression and function of endothelial cell major histocompatibility complex class II via suppressor of cytokine signaling 3

Endothelial cells (EC) can present antigen to either CD8(+) T lymphocytes through constitutively expressed major histocompatibility complex class I (MHC-I) or CD4(+) T lymphocytes through gamma interferon (IFN-γ)-induced MHC-II. Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma (KS), an EC neoplasm characterized by dysregulated angiogenesis and a substantial inflammatory infiltrate. KSHV is understood to have evolved strategies to inhibit MHC-I expression on EC and MHC-II expression on primary effusion lymphoma cells, but its effects on EC MHC-II expression are unknown. Here, we report that the KSHV infection of human primary EC inhibits IFN-γ-induced expression of the MHC-II molecule HLA-DR at the transcriptional level. The effect is functionally significant, since recognition by an HLA-DR-restricted CD4(+) T-cell clone in response to cognate antigen presented by KSHV-infected EC was attenuated. Inhibition of HLA-DR expression was also achieved by exposing EC to supernatant from KSHV-inoculated EC before IFN-γ treatment, revealing a role for soluble mediators. IFN-γ-induced phosphorylation of STAT-1 and transcription of CIITA were suppressed in KSHV-inoculated EC via a mechanism involving SOCS3 (suppressor of cytokine signaling 3). Thus, KSHV infection resulted in transcriptional upregulation of SOCS3, and treatment with RNA interference against SOCS3 relieved virus-induced inhibition of IFN-γ-induced STAT-1 phosphorylation. Since cell surface MHC-II molecules present peptide antigens to CD4(+) T lymphocytes that can function either as direct cytolytic effectors or to initiate and regulate adaptive immune responses, inhibition of this antigen-presenting pathway would provide a survival advantage to the virus.

Destruction of lymphoid organ architecture and hepatitis caused by CD4+ T cells

Immune responses have the important function of host defense and protection against pathogens. However, the immune response also causes inflammation and host tissue injury, termed immunopathology. For example, hepatitis B and C virus infection in humans cause immunopathological sequel with destruction of liver cells by the host's own immune response. Similarly, after infection with lymphocytic choriomeningitis virus (LCMV) in mice, the adaptive immune response causes liver cell damage, choriomeningitis and destruction of lymphoid organ architecture. The immunopathological sequel during LCMV infection has been attributed to cytotoxic CD8(+) T cells. However, we now show that during LCMV infection CD4(+) T cells selectively induced the destruction of splenic marginal zone and caused liver cell damage with elevated serum alanin-transferase (ALT) levels. The destruction of the splenic marginal zone by CD4(+) T cells included the reduction of marginal zone B cells, marginal zone macrophages and marginal zone metallophilic macrophages. Functionally, this resulted in an impaired production of neutralizing antibodies against LCMV. Furthermore, CD4(+) T cells reduced B cells with an IgM(high)IgD(low) phenotype (transitional stage 1 and 2, marginal zone B cells), whereas other B cell subtypes such as follicular type 1 and 2 and germinal center/memory B cells were not affected. Adoptive transfer of CD4(+) T cells lacking different important effector cytokines and cytolytic pathways such as IFNγ, TNFα, perforin and Fas-FasL interaction did reveal that these cytolytic pathways are redundant in the induction of immunopathological sequel in spleen. In conclusion, our results define an important role of CD4(+) T cells in the induction of immunopathology in liver and spleen. This includes the CD4(+) T cell mediated destruction of the splenic marginal zone with consecutively impaired protective neutralizing antibody responses.

Pathogenesis and host control of gammaherpesviruses: lessons from the mouse

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

Expression of perforin-granzyme pathway genes in the bursa of infectious bursal disease virus-infected chickens

Infectious bursal disease (IBD) is an economically important immunosuppressive disease of chickens. The IBD virus (IBDV) actively replicates in B cells and causes severe bursal damage. Generally, T cells are refractory to infection with IBDV but are known to promote virus clearance. However, the mechanisms of T cell mediated viral clearance are not well understood. In this study, we evaluated the molecular mechanisms of cytotoxic T cell responses in the pathogenesis of IBD in chickens. Infection of chickens with IBDV was accompanied by the infiltration of CD4(+) and CD8(+) T cells in the bursa. There was an upregulation in the gene expression of important cytolytic molecules; perforin (PFN), granzyme-A (Gzm-A), DNA repair and apoptotic proteins; high mobility proteins group (HMG) and poly (ADP-ribose) polymerase (PARP) in the bursa of Fabricius (BF) whereas expression of NK (natural killer) lysin was downregulated. Importantly, PFN producing CD4(+) and CD8(+) T cells were also detected in the bursa of IBDV-infected chickens by immunohistochemistry. The Th1 cytokines, IL-2 and IFN-γ expression was also strongly upregulated, suggesting the activation of T cells. The findings of this study highlight the mechanisms of IBD pathogenesis and the role of cytotoxic T cells in the clearance of virus-infected cells.

Kaposi's sarcoma-associated herpesvirus viral interferon regulatory factor 3 inhibits gamma interferon and major histocompatibility complex class II expression

Kaposi's sarcoma-associated herpesvirus (KSHV) carries four genes with homology to human interferon regulatory factors (IRFs). One of these IRFs, the viral interferon regulatory factor 3 (vIRF-3), is expressed in latently infected primary effusion lymphoma (PEL) cells and required for their continuous proliferation. Moreover, vIRF-3 is known to be involved in modulation of the type I interferon (IFN) response. We now show that vIRF-3 also interferes with the type II interferon system and antigen presentation to the adaptive immune system. Starting with an analysis of the transcriptome, we show that vIRF-3 inhibits expression of major histocompatibility complex class II (MHC II) molecules: small interfering RNA (siRNA)-mediated knockdown of vIRF-3 in KSHV-infected PEL cell lines resulted in increased MHC II levels; overexpression of vIRF-3 in KSHV-negative B cells leads to downmodulation of MHC II. This regulation could be traced back to inhibition of class II transactivator (CIITA) transcription by vIRF-3. Reporter assays revealed that the gamma interferon (IFN-γ)-sensitive CIITA promoters PIV and PIII were inhibited by vIRF-3. Consistently, IFN-γ levels increased upon vIRF-3 knockdown in PEL cells. IFN-γ regulation by vIRF-3 was confirmed in reporter assays as well as by upregulation of typical IFN-γ target genes upon knockdown of vIRF-3 in PEL cells. In summary, we conclude that vIRF-3 contributes to the viral immunoevasion by downregulation of IFN-γ and CIITA and thus MHC II expression.

Prospects of a novel vaccination strategy for human gamma-herpesviruses

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

Clearance of HIV type 1 envelope recombinant sendai virus depends on CD4+ T cells and interferon-gamma but not B cells, CD8+ T cells, or perforin

T cell-mediated viral clearance is classically attributed to the CD8(+) T cell subset, but CD4(+) T cells can sometimes assume this role. One such instance was illustrated by the immunization of C57BL/6 mice with HIV-1 envelope, followed by challenge with a recombinant Sendai virus (rSeV-env) carrying a gene for secreted HIV-1 envelope protein. Vaccinated mice that lacked both B cells (microMT) and CD8(+) T cells controlled virus, but control was lost when CD4(+) T cells were depleted. To explain this activity, we questioned whether CD4(+) T cells might utilize perforin for killing of MHC class II-positive targets. We also asked if the process might depend on IFN-gamma, which can upregulate MHC expression and enhance T cell recruitment to sites of virus challenge. To address these possibilities, we vaccinated perforin-KO mice with HIV-1 envelope and challenged them with rSeV-env. We found that perforin was not required for (1) CD4(+) T cell homing to the site of virus challenge, (2) expression of Th1 and Th2 cytokines (including IFN-gamma), or (3) virus clearance. To determine if IFN-gamma was required for protection, we repeated experiments in IFN-gamma-KO animals. In this case, significant protection was lost, although the CD4(+) T cells trafficked readily to the site of infection. In fact, local CD4(+) T cell numbers in vaccinated IFN-gamma- KO mice exceeded those in wild type animals. In both cases, cells were alphass TCR(+), NK-1.1(-), and CD44(+), typifying an activated CD4(+) T cell subset. Taken together, our results showed that HIV-1 envelope recombinant virus clearance was dependent on CD4(+) T cells and IFN-gamma, but occurred in the absence of B cells, CD8(+) T cells, or perforin.

CD4 T-cell help programs a change in CD8 T-cell function enabling effective long-term control of murine gammaherpesvirus 68: role of PD-1-PD-L1 interactions

We previously showed that agonistic antibodies to CD40 could substitute for CD4 T-cell help and prevent reactivation of murine gammaherpesvirus 68 (MHV-68) in the lungs of major histocompatibility complex (MHC) class II(-/-) (CII(-/-)) mice, which are CD4 T cell deficient. Although CD8 T cells were required for this effect, no change in their activity was detected in vitro. A key question was whether anti-CD40 treatment (or CD4 T-cell help) changed the function of CD8 T cells or another cell type in vivo. To address this question, in the present study, we showed that adoptive transfer of CD8 T cells from virus-infected wild-type mice or anti-CD40-treated CII(-/-) mice caused a significant reduction in lung viral titers, in contrast to those from control CII(-/-) mice. Anti-CD40 treatment also greatly prolonged survival of infected CII(-/-) mice. This confirms that costimulatory signals cause a change in CD8 T cells enabling them to maintain effective long-term control of MHV-68. We investigated the nature of this change and found that expression of the inhibitory receptor PD-1 was significantly increased on CD8 T cells in the lungs of MHV-68-infected CII(-/-), CD40(-/-), or CD80/86(-/-) mice, compared with that in wild-type or CD28/CTLA4(-/-) mice, correlating with the level of viral reactivation. Furthermore, blocking PD-1-PD-L1 interactions significantly reduced viral reactivation in CD4 T-cell-deficient mice. In contrast, the absence of another inhibitory receptor, NKG2A, had no effect. These data suggest that CD4 T-cell help programs a change in CD8 T-cell function mediated by altered PD-1 expression, which enables effective long-term control of MHV-68.

The de novo methyltransferases DNMT3a and DNMT3b target the murine gammaherpesvirus immediate-early gene 50 promoter during establishment of latency

The role of epigenetic modifications in the regulation of gammaherpesvirus latency has been a subject of active study for more than 20 years. DNA methylation, associated with transcriptional silencing in mammalian genomes, has been shown to be an important mechanism in the transcriptional control of several key gammaherpesvirus genes. In particular, DNA methylation of the functionally conserved immediate-early replication and transcription activator (RTA) has been shown to regulate Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus Rta expression. Here we demonstrate that the murine gammaherpesvirus (MHV68) homolog, encoded by gene 50, is also subject to direct repression by DNA methylation, both in vitro and in vivo. We observed that the treatment of latently MHV68-infected B-cell lines with a methyltransferase inhibitor induced virus reactivation. In addition, we show that the methylation of the recently characterized distal gene 50 promoter represses activity in a murine macrophage cell line. To evaluate the role of de novo methyltransferases (DNMTs) in the establishment of these methylation marks, we infected mice in which conditional DNMT3a and DNMT3b alleles were selectively deleted in B lymphocytes. DNMT3a/DNMT3b-deficient B cells were phenotypically normal, displaying no obvious compromise in cell surface marker expression or antibody production either in naïve mice or in the context of nonviral and viral immunogens. However, mice lacking functional DNMT3a and DNMT3b in B cells exhibited hallmarks of deregulated MHV68 lytic replication, including increased splenomegaly and the presence of infectious virus in the spleen at day 18 following infection. In addition, total gene 50 transcript levels were elevated in the spleens of these mice at day 18, which correlated with the hypomethylation of the distal gene 50 promoter. However, by day 42 postinfection, aberrant virus replication was resolved, and we observed wild-type frequencies of viral genome-positive splenocytes in mice lacking functional DNMT3a and DNMT3b in B lymphocytes. The latter correlated with increased CpG methylation in the distal gene 50 promoter, which was restored to levels similar to those of littermate controls harboring functional DNMT3a and DNMT3b alleles in B lymphocytes, suggesting the existence of an alternative mechanism for the de novo methylation of the MHV68 genome. Importantly, this DNMT3a/DNMT3b-independent methylation appeared to be targeted specifically to the gene 50 promoter, as we observed that the promoters for MHV68 gene 72 (v-cyclin) and M11 (v-bcl2) remained hypomethylated at day 42 postinfection. Taken together, these data provide the first evidence of the importance of DNA methylation in regulating gammaherpesvirus RTA/gene 50 transcription during virus infection in vivo and provide insight into the hierarchy of host machinery required to establish this modification.

Persistent gamma-herpesvirus infection induces a CD4 T cell response containing functionally distinct effector populations

The direct effector mechanisms of CD4 T cells during gamma-herpesvirus 68 (gammaHV68)-persistent infection are less well understood than those of their CD8 T cell counterparts, although there is substantial evidence that CD4 T cells are critical for the control of persistent gamma-herpesvirus infection. Our results show that in gammaHV68-persistently infected mice, CD4 T cells are not cytokine polyfunctional, but there is a division of labor in the CD4 T cell compartment in which CD4 T cells polarize toward two distinct populations with different effector functions: IFN-gamma producers and CD107(+) cytolytic effectors. These two CD4 T cell effector populations degranulate and produce IFN-gamma during steady state without need for exogenous antigenic restimulation, which is fundamentally different from that observed with gammaHV68-specific CD8 T cells. By using anti-IFN-gamma Ab depletions and IFN-gamma-deficient mice, we show that CD4 T cell-mediated cytotoxicity in vivo is not dependent on IFN-gamma activity. In addition, our data show that purified CD4 T cells isolated from gammaHV68-latently infected mice have the capacity to inhibit gammaHV68 reactivation from latency. Our results support the concept that CD4 T cells are critical effectors for the control of gamma-herpesvirus latent infection, and they mediate this effect by two independent mechanisms: IFN-gamma production and cytotoxicity.

Immune control of mammalian gamma-herpesviruses: lessons from murid herpesvirus-4

Many acute viral infections can be controlled by vaccination; however, vaccinating against persistent infections remains problematic. Herpesviruses are a classic example. Here, we discuss their immune control, particularly that of gamma-herpesviruses, relating the animal model provided by murid herpesvirus-4 (MuHV-4) to human infections. The following points emerge: (i) CD8(+) T-cell evasion by herpesviruses confers a prominent role in host defence on CD4(+) T cells. CD4(+) T cells inhibit MuHV-4 lytic gene expression via gamma-interferon (IFN-gamma). By reducing the lytic secretion of immune evasion proteins, they may also help CD8(+) T cells to control virus-driven lymphoproliferation in mixed lytic/latent lesions. Similarly, CD4(+) T cells specific for Epstein-Barr virus lytic antigens could improve the impact of adoptively transferred, latent antigen-specific CD8(+) T cells. (ii) In general, viral immune evasion necessitates multiple host effectors for optimal control. Thus, subunit vaccines, which tend to prime single effectors, have proved less successful than attenuated virus mutants, which prime multiple effectors. Latency-deficient mutants could make safe and effective gamma-herpesvirus vaccines. (iii) The antibody response to MuHV-4 infection helps to prevent disease but is suboptimal for neutralization. Vaccinating virus carriers with virion fusion complex components improves their neutralization titres. Reducing the infectivity of herpesvirus carriers in this way could be a useful adjunct to vaccinating naive individuals with attenuated mutants.

Resistance of common carp (Cyprinus carpio L.) to Cyprinid herpesvirus-3 is influenced by major histocompatibility (MH) class II B gene polymorphism

The role of MH class II B (Cyca-DAB1-like) genes in resistance of common carp (Cyprinus carpio L.) to Cyprinid herpesvirus-3 (CyHV-3), also known as koi herpesvirus (KHV) was analysed. The material consisted of 934 fish from six carp crosses. Fish were challenged with CyHV-3 at an age of 7 and 10 months. During challenge experiments the peak of mortality caused by CyHV-3 was observed at days 8-12 p.i. and the overall cumulative mortality reached 79.9%. Among six Cyca-DAB1-like genotypes, revealed by PCR-RF-SSCP analysis, one genotype (E) was found associated with higher resistance to CyHV-3. Three other genotypes (B, H and J) could be linked to higher susceptibility to CyHV-3. Analysis of the alleles that compose the Cyca-DAB1-like genotypes linked one particular allele (Cyca-DAB1*05) to significantly increased, and two alleles (Cyca-DAB1*02 and Cyca-DAB1*06) to significantly decreased resistance to CyHV-3. Our data indicate that MH class II B genes could be used as potential genetic markers in breeding of common carp for resistance to this virus.

gamma-Herpesvirus-induced protection against bacterial infection is transient

Herpesviruses are widely disseminated in the population and establish lifelong latency, which is associated with a variety of pathological consequences. A recent report showed that mice latently infected with either murine gamma-herpesvirus-68 (gammaHV68) or murine cytomegalovirus (mCMV), mouse pathogens genetically similar to the human herpesviruses, Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and cytomegalovirus, had enhanced resistance to subsequent bacterial infection, suggesting protective as well as deleterious effects of latency. Here we confirm that latent gammaHV68 infection confers protection against subsequent infection with Listeria monocytogenes. However, the effect is transient, lasting only a few months.

CD4 T cells mediate killing during persistent gammaherpesvirus 68 infection

CD4 T cells are critical for the control of gammaherpesvirus persistence, but their direct effector mechanisms of virus control in vivo are still poorly understood. In this study, we use murine gammaherpesvirus 68 (gammaHV68) in in vitro and in vivo cytotoxicity assays to show CD4-dependent killing of gammaHV68-loaded cells in mice persistently infected with gammaHV68. Our results underscore the cytotoxic capacity of CD4 T cells during gammaHV68 persistence.

Dendritic cells and B cells maximize mucosal Th1 memory response to herpes simplex virus

Although the importance of cytotoxic T lymphocytes and neutralizing antibodies for antiviral defense is well known, the antiviral mechanism of Th1 remains unclear. We show that Th1 cells mediate noncytolytic antiviral protection independent of direct lysis through local secretion of IFN-gamma after herpes simplex virus (HSV) 2 infection. IFN-gamma acted on stromal cells, but not on hematopoietic cells, to prevent further viral replication and spread throughout the vaginal mucosa. Importantly, unlike other known Th1 defense mechanisms, this effector function did not require recognition of virally infected cells via MHC class II. Instead, recall Th1 response was elicited by MHC class II(+) antigen-presenting cells at the site of infection. Dendritic cells (DCs) were not required and only partially sufficient to induce a recall response from memory Th1 cells. Importantly, DCs and B cells together contributed to restimulating memory CD4 T cells to secrete IFN-gamma. In the absence of both DCs and B cells, immunized mice rapidly succumbed to HSV-2 infection and death. Thus, these results revealed a distinct mechanism by which memory Th1 cells mediate noncytolytic IFN-gamma-dependent antiviral protection after recognition of processed viral antigens by local DCs and B cells.

Primary clearance of murine gammaherpesvirus 68 by PKCtheta-/- CD8 T cells is compromised in the absence of help from CD4 T cells

CD4 T cells are dispensable for acute control of murine gammaherpesvirus 68 (MHV-68) but are necessary for effective long-term control of the virus by CD8 T cells. In contrast, protein kinase C theta (PKCtheta) is not essential for either acute or long-term viral control. However, we found that while either CD4 or CD8 T cells could mediate the clearance of MHV-68 from the lungs of PKCtheta(+/+) mice, PKCtheta(-/-) mice depleted of either subset failed to clear the virus. These data suggest that there are two alternative pathways for MHV-68 clearance, one dependent on CD4 T cells and the other on PKCtheta. Protection mediated by the latter appears to be short-lived. These observations may help to explain the differential requirement for PKCtheta in various models of CD8 T-cell activation and differences in the costimulatory requirements for acute and long-term viral control.

Heterogeneous memory T cells in antiviral immunity and immunopathology

Memory T cells are generated following an initial viral infection, and have the potential for mediating robust protective immunity to viral re-challenge due to their rapid and enhanced functional responses. In recent years, it has become clear that the memory T cell response to most viruses is remarkably diverse in phenotype, function, and tissue distribution, and can undergo dynamic changes during its long-term maintenance in vivo. However, the role of this variegation and compartmentalizationof memory T cells in protective immunity to viruses remains unclear. In this review,we discuss the diverse features of memory T cells that can delineate different subsets, the characteristics of memory T cells thus far identified to promote protective immune responses, and how the heterogeneous nature of memory T cells may also promote immunopathology during antiviral responses. We propose that given the profound heterogeneity of memory T cells, regulation of memory T cells during secondary responses could focus the response to participation of specific subsets,and/or inhibit memory T-cell subsets and functions that can lead to immunopathology.

Identification of an N-linked glycosylation in the C4 region of HIV-1 envelope gp120 that is critical for recognition of neighboring CD4 T cell epitopes

The heavy glycosylation of HIV-1 envelope gp120 shields this important Ag from recognition by neutralizing Abs and cytolytic CD8 T cells. However, very little work has been done to understand the influence of glycosylation on the generation of gp120 epitopes and their recognition by MHC class II-restricted CD4 T cells. In this study, three conserved glycans (linked to N406, N448, and N463) flanking the C4 region of gp120 that contains many known CD4 T cell epitopes were disrupted individually or in combination by asparagine-to-glutamine substitutions. The mutant proteins lacking the N448 glycan did not effectively stimulate CD4 T cells specific for the nearby C4 epitopes, although the same mutants were recognized well by CD4 T cells specific for epitopes located in the distant C1 and C2 regions. The loss of recognition was not due to amino acid substitutions introduced to the mutant proteins. Data from trypsin digestion and mass spectrometry analyses demonstrated that the N448 glycan removal impeded the proteolytic cleavage of the nearby C4 region, without affecting more distant sites. Importantly, this inhibitory effect was observed only in the digestion of the native nondenatured protein and not in that of the denatured protein. These data indicate that the loss of the N448 glycan induces structural changes in the C4 region of gp120 that make this specific region more resistant to proteolytic processing, thereby restricting the generation of CD4 T cell epitopes from this region. Hence, N-linked glycans are critical determinants that can profoundly influence CD4 T cell recognition of HIV-1 gp120.

IFN-gamma is the only anti-rotavirus cytokine found after in vitro stimulation of memory CD4+ T cells from mice immunized with a chimeric VP6 protein

CD4+ T cells are the only lymphocytes required for protection of mice against rotavirus shedding after mucosal immunization with chimeric VP6 (MBP::VP6) and the adjuvant LT(R192G). One possible effector of protection is CD4+ T-cell cytokines. To determine if memory CD4+ T cells of immunized mice produce cytokines with direct anti-rotavirus activity, an in vitro infection model was developed using mouse CMT-93 cells and rhesus rotavirus (RRV). Spleen and lamina propria (LP) cells, as well as purified splenic CD4T cells obtained after intranasal immunization of BALB/c mice with MBP::VP6/LT(R192G) released large quantities of two cytokines (IL-17 and IFN-gamma) into cell supernatants when stimulated with MBP::VP6. Production of these same cytokines is rapidly upregulated in intestinal lymphocytes after rotavirus inoculation of immunized mice. IL-17 pretreatment of CMT-93 cells had no effect on subsequent RRV replication, but IFN-gamma was the most potent inhibitor within a panel of nine cytokines tested. Supernatants obtained after in vitro stimulation of splenic CD4+ T cells of immunized mice had high levels of anti-RRV activity and their pretreatment with mAb against IFN-gamma caused essentially complete loss of activity. Thus, IFN-gamma was the only cytokine identified in stimulated CD4+ T cells from immunized mice that directly inhibited rotavirus replication.

A replication-deficient murine gamma-herpesvirus blocked in late viral gene expression can establish latency and elicit protective cellular immunity

The human gamma-herpesviruses, EBV and Kaposi's sarcoma-associated herpesvirus, are widely disseminated and are associated with the onset of a variety of malignancies. Thus, the development of prophylactic and therapeutic vaccination strategies is an important goal. The experimental mouse gamma-herpesvirus, gammaHV68 (or MHV-68), has provided an in vivo model for studying immune control of these persistent viruses. In the current studies, we have examined infectivity, immunogenicity, and protective efficacy following infection with a replication-deficient gammaHV68 blocked in late viral gene expression, ORF31STOP. The data show that ORF31STOP was able to latently infect B cells. However, the anatomical site and persistence of the infection depended on the route of inoculation, implicating a role for viral replication in viral spread but not the infectivity per se. Furthermore, i.p. infection with ORF31STOP elicited strong cellular immunity but a non-neutralizing Ab response. In contrast, intranasal infection was poorly immunogenic. Consistent with this, mice infected i.p. had enhanced control of both the lytic and latent viral loads following challenge with wild-type gammaHV68, whereas intranasal infected mice were not protected. These data provide important insight into mechanisms of infection and protective immunity for the gamma-herpesviruses and demonstrate the utility of replication-deficient mutant viruses in direct testing of "proof of principal" vaccination strategies.

A recombinant Sendai virus is controlled by CD4+ effector T cells responding to a secreted human immunodeficiency virus type 1 envelope glycoprotein

The importance of antigen-specific CD4(+) helper T cells in virus infections is well recognized, but their possible role as direct mediators of virus clearance is less well characterized. Here we describe a recombinant Sendai virus strategy for probing the effector role(s) of CD4(+) T cells. Mice were vaccinated with DNA and vaccinia virus recombinant vectors encoding a secreted human immunodeficiency virus type 1 (HIV-1) envelope protein and then challenged with a Sendai virus carrying a homologous HIV-1 envelope gene. The primed mice showed (i) prompt homing of numerous envelope-primed CD4(+) T cell populations to the virus-infected lung, (ii) substantial production of gamma interferon, and interleukin-2 (IL-2), IL-4, and IL-5 in that site, and (iii) significantly reduced pulmonary viral load. The challenge experiments were repeated with immunoglobulin(-/-) microMT mice in the presence or absence of CD8(+) and/or CD4(+) T cells. These selectively immunodeficient mice were protected by primed CD4(+) T cells in the absence of antibody or CD8(+) T cells. Together, these results highlight the role of CD4(+) T cells as direct effectors in vivo and, because this protocol gives such a potent response, identify an outstanding experimental model for further dissecting CD4(+) T-cell-mediated immunity in the lung.

IgG fc receptors provide an alternative infection route for murine gamma-herpesvirus-68

Background

Herpesviruses can be neutralized in vitro but remain infectious in immune hosts. One difference between these settings is the availability of immunoglobulin Fc receptors. The question therefore arises whether a herpesvirus exposed to apparently neutralizing antibody can still infect Fc receptor⁺ cells.

Principal Findings

Immune sera blocked murine gamma-herpesvirus-68 (MHV-68) infection of fibroblasts, but failed to block and even enhanced its infection of macrophages and dendritic cells. Viral glycoprotein-specific monoclonal antibodies also enhanced infection. MHV-68 appeared to be predominantly latent in macrophages regardless of whether Fc receptors were engaged, but the infection was not abortive and new virus production soon overwhelmed infected cultures. Lytically infected macrophages down-regulated MHC class I-restricted antigen presentation, endocytosis and their response to LPS.

Conclusions

IgG Fc receptors limit the neutralization of gamma-herpesviruses such as MHV-68.

Disregulated influenza A virus-specific CD8+ T cell homeostasis in the absence of IFN-gamma signaling

Recent studies indicate that IFN-gamma may influence both the expansion and the trafficking of virus-specific CD8+ CTL, though the effects are not necessarily consistent for different models of viral and bacterial disease. Influenza A virus infection of mice deficient for IFN-gamma (IFN-gamma(-/-)) or deficient for the IFN-gamma receptor 1 (IFNGR1(-/-)) was, when compared with the wild-type (WT) B6 controls, associated with increased Ag-specific CD8+ T cell counts in the spleen and mediastinal lymph nodes. At the same time, fewer of these CTL effectors were found in the bronchoalveolar lavage population recovered from the IFN-gamma(-/-) mice. Comparable effects were observed for WT mice treated with a neutralizing IFN-gamma-specific mAb. Transfer of WT memory Thy1.1(+) CD8+ populations into Thy1.2+ B6 IFN-gamma(-/-) or IFNGR1(-/-) mice followed by intranasal virus challenge demonstrated both that IFN-gamma produced by the host was important for the regulation of Ag-specific CTL numbers and that IFN-gamma was likely to act directly on the T cells themselves. In addition, the prevalence of CTLs undergoing apoptosis in spleen was lower when measured directly ex vivo for IFN-gamma(-/-) vs WT B6 mice. The present analysis is the first comprehensive demonstration that IFN-gamma signaling can differentially regulate both Ag-specific CTL homeostasis in secondary lymphoid organs and trafficking to a site of virus-induced pathology.

Association of gamma interferon and interleukin-17 production in intestinal CD4+ T cells with protection against rotavirus shedding in mice intranasally immunized with VP6 and the adjuvant LT(R192G)

Mucosal immunization of mice with chimeric, Escherichia coli-expressed VP6, the protein that comprises the intermediate capsid layer of the rotavirus particle, together with attenuated E. coli heat-labile toxin LT(R192G) as an adjuvant, reduces fecal shedding of rotavirus antigen by >95% after murine rotavirus challenge, and the only lymphocytes required for protection are CD4+ T cells. Because these cells produce cytokines with antiviral properties, the cytokines whose expression is upregulated in intestinal memory CD4+ T cells immediately after rotavirus challenge of VP6/LT(R192G)-immunized mice may be directly or indirectly responsible for the rapid suppression of rotavirus shedding. This study was designed to identify which cytokines are significantly upregulated in intestinal effector sites and secondary lymphoid tissues of intranasally immunized BALB/c mice after challenge with murine rotavirus strain EDIM. Initially, this was done by using microarray analysis to quantify mRNAs for 96 murine common cytokines. With this procedure, the synthesis of mRNAs for gamma interferon (IFN-gamma) and interleukin-17 (IL-17) was found to be temporarily upregulated in intestinal lymphoid cells of VP6/LT(R192G)-immunized mice at 12 h after rotavirus challenge. These cytokines were also produced in CD4+ T cells obtained from intestinal sites specific to VP6/LT(R192G)-immunized mice after in vitro exposure to VP6 as determined by intracellular cytokine staining and secretion of cytokines. Although genetically modified mice that lack receptors for either IFN-gamma or IL-17 remained protected after immunization, these results provide suggestive evidence that these cytokines may play direct or indirect roles in protection against rotavirus after mucosal immunization of mice with VP6/LT(R192G).

Gamma interferon blocks gammaherpesvirus reactivation from latency

Establishment of latent infection and reactivation from latency are critical aspects of herpesvirus infection and pathogenesis. Interfering with either of these steps in the herpesvirus life cycle may offer a novel strategy for controlling herpesvirus infection and associated disease pathogenesis. Prior studies show that mice deficient in gamma interferon (IFN-gamma) or the IFN-gamma receptor have elevated numbers of cells reactivating from murine gammaherpesvirus 68 (gammaHV68) latency, produce infectious virus after the establishment of latency, and develop large-vessel vasculitis. Here, we demonstrate that IFN-gamma is a powerful inhibitor of reactivation of gammaHV68 from latency in tissue culture. In vivo, IFN-gamma controls viral gene expression during latency. Importantly, depletion of IFN-gamma in latently infected mice results in an increased frequency of cells reactivating virus. This demonstrates that IFN-gamma is important for immune surveillance that limits reactivation of gammaHV68 from latency.

CD4+ T cells specific for a model latency-associated antigen fail to control a gammaherpesvirusin vivo

CD4(+) T cells play a major role in containing herpesvirus infections. However, their cellular targets remain poorly defined. In vitro CD4(+) T cells have been reported to kill B cells that harbor a latent gammaherpesvirus. We used the B cell-tropic murine gammaherpesvirus-68 (MHV-68) to test whether this also occurred in vivo. MHV-68 that expressed cytoplasmic ovalbumin (OVA) in tandem with its episome maintenance protein, ORF73, stimulated CD8(+) T cells specific for the H2-K(b)-restricted OVA epitope SIINFEKL and was rapidly eliminated from C57BL/6 (H2(b)) mice. However, the same virus failed to stimulate CD4(+) T cells specific for the I-A(d)/I-A(b)-restricted OVA(323-339) epitope. We overcame any barrier to the MHC class II-restricted presentation of an endogenous epitope by substituting OVA(323-339) for the CLIP peptide of the invariant chain (ORF73-IRES-Ii-OVA), again expressed in tandem with ORF73. This virus presented OVA(323-339) but showed little or no latency deficit in either BALB/c (H2(d)) or C57BL/6 mice. Latent antigen-specific CD4(+) T cells therefore either failed to recognize key virus-infected cell populations in vivo or lacked the effector functions required to control them.

Induction of a virus-specific effector-memory CD4+ T cell response by attenuated SIV infection

We investigated simian immunodeficiency virus (SIV)-specific CD4⁺ T cell responses in rhesus macaques chronically infected with attenuated or pathogenic SIV strains. Analysis of SIVΔnef-infected animals revealed a relatively high frequency of SIV-specific CD4⁺ T cells representing 4–10% of all CD4⁺ T lymphocytes directed against multiple SIV proteins. Gag-specific CD4⁺ T cells in wild-type SIV-infected animals were 5–10-fold lower in frequency and inversely correlated with the level of plasma viremia. SIV-specific CD4⁺ cells from SIVΔnef animals were predominantly CD27⁻CD28⁻CD45RA^lowCCR7⁻CCR5⁻, consistent with an effector–memory subset, and included a fully differentiated CD45RA⁺CCR7⁻ subpopulation. In contrast, SIV-specific CD4⁺ T cells from SIV-infected animals were mostly CD27⁺CD28⁺CD45RA⁻CCR7⁺CCR5⁺, consistent with an early central memory phenotype. The CD45RA⁺CCR7⁻CD4⁺ subset from SIVΔnef animals was highly enriched for effector CD4⁺ T cells, as indicated by the perforin expression and up-regulation of the lysosomal membrane protein CD107a after SIV Gag stimulation. SIV-specific CD4⁺ T cells in attenuated SIV-infected animals were increased in frequency in bronchioalveolar lavage and decreased in lymph nodes, consistent with an effector–memory T cell population. The ability of SIVΔnef to induce a high frequency virus-specific CD4⁺ T cell response with direct effector function may play a key role in protective immunity produced by vaccination with attenuated SIV strains.