Herpesvirus-specific CD8 T cell immunity in old age: cytomegalovirus impairs the response to a coresident EBV infection

Aging in humans is associated with increased infections and the reduced proliferative capacity of T cells, part of the more global phenomenon termed immune senescence. The etiology of immune senescence is unknown but the accumulation of virus-specific memory T cells may be a contributory factor. We have examined CD8 T cell responses to two persistent herpesvirus infections, CMV and EBV, and to a recurrent virus infection, influenza, in different age cohorts of healthy donors using HLA-peptide tetramers and intracellular cytokine detection. Of these, CMV appears to be the most immunogenic, with the CD8 T cell response representing over 10% of the CD8 pool in many elderly donors. Interestingly, the effect of age upon EBV-specific responses depends upon donor CMV sero-status. In CMV seropositive donors, the magnitude of the EBV-specific immune response is stable with age, but in CMV seronegative donors, the response to EBV increases significantly with age. By contrast, the influenza-specific CD8 T cell immune response decreases with age, independent of CMV status. The functional activity of the herpesvirus-specific immune response decreases in elderly donors, although the characteristic phenotypes of CMV- and EBV-specific memory populations are retained. This demonstrates that aging is associated with a marked accumulation of CMV-specific CD8 T cells together with a decrease in immediate effector function. Moreover, infection with CMV can reduce prevailing levels of immunity to EBV, another persistent virus. These results suggest that carriage of CMV may be detrimental to the immunocompetent host by suppressing heterologous virus-specific immunity during aging.     

Differential evolution and stability of epitope-specific CD8(+) T cell responses in EBV infection

Murine models of lymphocytic choriomeningitis virus infection suggest that the memory CD8(+) T cell repertoire is reflective of the CD8(+) T cell repertoire generated during acute infection. Less is known regarding the evolution of CD8(+) T cell repertoires during human viral infections. We therefore examined epitope-specific CD8(+) T cell responses in a large cohort of individuals with acute through latent Epstein-Barr virus infection. Using 16 of 20 published EBV epitopes restricted by HLA-A2, HLA-A3 or HLA-B7, we showed that lytic cycle-specific CD8(+) T cell responses predominated during acute EBV infection. However, whereas HLA-A2(+)-restricted BMLF-1-specific CD8(+) T cell responses were maintained through latency, HLA-A2(+)- and HLA-B7(+)-restricted BZLF-1, as well as HLA-A3(+)-restricted BRLF-1 CD8(+) T cell responses, were generated but not readily maintained. Analyses of CD8(+) T cell responses to EBV latent cycle Ags showed delayed detection and lower frequencies of latent epitope-specific CD8(+) T cell responses during acute EBV infection, with maintenance of these responses 1 yr post-EBV infection. Early BMLF-1 and EBNA-3A epitope-specific CD8(+) T cell frequencies did not correlate with their frequencies at 1 yr postinfection. Interestingly, populations of EBV-specific CD8(+) T cells were stable during 20 mo in our long term EBV-seropositive populations, suggesting homeostasis between virus and the host immune system. This study demonstrates that CD8(+) T cell repertoires generated during persistent viral infections are not simply reflective of the initial pool of CD8(+) T cells and provides evidence that the generation of CD8(+) T cell responses to a persistent infection is a dynamic process.     

Comprehensive early and lasting loss of memory CD8 T cells and functional memory during acute and persistent viral infections

Viral infections have been shown to induce lymphopenias that lower memory CD8 T cell frequencies, and they also have been shown to cause a permanent loss of memory cells specific to previously encountered pathogens. In this study, the patterns and significance of virus-induced memory CD8 T cell depletion were examined in mice immune to heterologous (Pichinde, vesicular stomatitis, vaccinia) viruses and subsequently challenged with acute or persistent lymphocytic choriomeningitis virus infections. Memory CD8 T cell loss was comprehensive and occurred in both lymphoid and peripheral tissues of the immune host. The impact of the loss of memory T cells was reflected by in vivo cytotoxicity assays, which showed decreased clearance of epitope-expressing targets. Memory CD8 T cell loss occurred very early (day 2) after infection, and was thereafter sustained, consistent more with an active deletion model than with a competition model. Cross-reactive T cells, in contrast, increased in number, but memory cells were reduced whether or not there was competition from cross-reactive T cells. Memory T cell loss was more profound during persistent infection than after acute infection. Adoptive transfer studies showed that, unlike the resolved acute infection, in which the reduced memory frequencies became stable, memory T cell loss was a continuously ongoing process during persistent infection. This study therefore links an early virus-induced lymphopenia to a subsequent long-term loss of CD8 T cell memory and offers a new mechanism for immune deficiency during persistent viral infections.     

Cells en route to apoptosis are characterized by the upregulation of c-fos,c-myc,c-jun,cdc2, and RB phosphorylation,resembling events of early cell-cycle traverse

Apoptosis is an important mechanism enabling the selection of the non-self-reactive T cell repertoire and for maintaining homeostasis in the immune system after it has expanded to combat infections. Highly activated, proliferating T cells become susceptible to apoptosis driven by a number of stimuli, and T cells activated during a viral infection become susceptible to “activation induced cell death” after repeated stimulation through the T cell receptor (TcR). This is a major mechanism for the immune deficiencies observed during many viral infections. During infections with a high antigen load this can lead to a selective deletion of virus-specific cytotoxic T lymphocytes (CTL) and to the establishment of persistent infection. More commonly, the CTL control the infection first, and high levels of apoptosis in the expanded lymphocyte population occur after antigen and growth factors become limiting. This cell death does not seem to depend on TcR specificity, as the residual population contains a remarkably stable population of memory CTL precursors that approximate the frequency per CD8 cell of that seen during the peak of the acute infection. Subsequent infections with heterologous viruses result in an expansion and then an apoptotic elimination of T cells, with the consequence being a reduction in precursor CTL specific for the first virus. Thus, apoptosis shapes the quality and quantity of T cell memory. © 1995 Wiley-Liss, Inc.     

Suppressive CD8+ T cells arise in the absence of CD4 help and compromise control of persistent virus

There is an urgent need to develop novel therapies for controlling chronic virus infections in immunocompromised patients. Disease associated with persistent γ-herpesvirus infection (EBV, human herpesvirus 8) is a significant problem in AIDS patients and transplant recipients, and clinical management of these conditions is difficult. Immune surveillance failure followed by γ-herpesvirus recrudescence can be modeled using murine γ-herpesvirus (MHV)-68 in mice lacking CD4(+) T cells. In contrast with other chronic infections, no obvious defect in the functional capacity of the viral-specific CD8(+) T cell response was detected. We show in this article that adoptive transfer of MHV-68-specific CD8(+) T cells was ineffective at reducing the viral burden. Together, these indicate the potential presence of T cell extrinsic suppressive factors. Indeed, CD4-depleted mice infected with MHV-68 express increased levels of IL-10, a cytokine capable of suppressing the function of both APCs and T cells. CD4-depleted mice developed a population of CD8(+) T cells capable of producing IL-10 that suppressed viral control. Although exhibiting cell surface markers indicative of activation, the IL-10-producing cells expressed increased levels of programmed death-1 but were not enriched in the MHV-68-specific compartment, nor were they uniformly CD44(hi). Therapeutic administration of an IL-10R blocking Ab enhanced control of the recrudescent virus. These data implicate IL-10 as a promising target for the restoration of immune surveillance against chronic γ-herpesvirus infection in immunosuppressed individuals.     

CD8 T cells recruited early in mouse polyomavirus infection undergo exhaustion

Repetitive Ag encounter, coupled with dynamic changes in Ag density and inflammation, imparts phenotypic and functional heterogeneity to memory virus-specific CD8 T cells in persistently infected hosts. For herpesvirus infections, which cycle between latency and reactivation, recent studies demonstrate that virus-specific T cell memory is predominantly derived from naive precursors recruited during acute infection. Whether functional memory T cells to viruses that persist in a nonlatent, low-level infectious state (smoldering infection) originate from acute infection-recruited naive T cells is not known. Using mouse polyomavirus (MPyV) infection, we previously showed that virus-specific CD8 T cells in persistently infected mice are stably maintained and functionally competent; however, a sizeable fraction of these memory T cells are short-lived. Further, we found that naive anti-MPyV CD8 T cells are primed de novo during persistent infection and contribute to maintenance of the virus-specific CD8 T cell population and its phenotypic heterogeneity. Using a new MPyV-specific TCR-transgenic system, we now demonstrate that virus-specific CD8 T cells recruited during persistent infection possess multicytokine effector function, have strong replication potential, express a phenotype profile indicative of authentic memory capability, and are stably maintained. In contrast, CD8 T cells recruited early in MPyV infection express phenotypic and functional attributes of clonal exhaustion, including attrition from the memory pool. These findings indicate that naive virus-specific CD8 T cells recruited during persistent infection contribute to preservation of functional memory against a smoldering viral infection.     

The antiviral CD8+ T cell response is differentially dependent on CD4+ T cell help over the course of persistent infection

Although many studies have investigated the requirement for CD4(+) T cell help for CD8(+) T cell responses to acute viral infections that are fully resolved, less is known about the role of CD4(+) T cells in maintaining ongoing CD8(+) T cell responses to persistently infecting viruses. Using mouse polyoma virus (PyV), we asked whether CD4(+) T cell help is required to maintain antiviral CD8(+) T cell and humoral responses during acute and persistent phases of infection. Though fully intact during acute infection, the PyV-specific CD8(+) T cell response declined numerically during persistent infection in MHC class II-deficient mice, leaving a small antiviral CD8(+) T cell population that was maintained long term. These unhelped PyV-specific CD8(+) T cells were functionally unimpaired; they retained the potential for robust expansion and cytokine production in response to Ag rechallenge. In addition, although a strong antiviral IgG response was initially elicited by MHC class II-deficient mice, these Ab titers fell, and long-lived PyV-specific Ab-secreting cells were not detected in the bone marrow. Finally, using a minimally myeloablative mixed bone marrow chimerism approach, we demonstrate that recruitment and/or maintenance of new virus-specific CD8(+) T cells during persistent infection is impaired in the absence of MHC class II-restricted T cells. In summary, these studies show that CD4(+) T cells differentially affect CD8(+) T cell responses over the course of a persistent virus infection.     

Costimulation requirements for antiviral CD8+ T cells differ for acute and persistent phases of polyoma virus infection

The requirement for costimulation in antiviral CD8+ T cell responses has been actively investigated for acutely resolved viral infections, but it is less defined for CD8+ T cell responses to persistent virus infection. Using mouse polyoma virus (PyV) as a model of low-level persistent virus infection, we asked whether blockade of the CD40 ligand (CD40L) and CD28 costimulatory pathways impacts the magnitude and function of the PyV-specific CD8+ T response, as well as the humoral response and viral control during acute and persistent phases of infection. Costimulation blockade or gene knockout of either CD28 or CD40L substantially dampened the magnitude of the acute CD8+ T cell response; simultaneous CD28 and CD40L blockade severely depressed the acute T cell response, altered the cell surface phenotype of PyV-specific CD8+ T cells, decreased PyV VP1-specific serum IgG titers, and resulted in an increase in viral DNA levels in multiple organs. CD28 and CD40L costimulation blockade during acute infection also diminished the memory PyV-specific CD8+ T cell response and serum IgG titer, but control of viral persistence varied between mouse strains and among organs. Interestingly, we found that CD28 and CD40L costimulation is dispensable for generating and/or maintaining PyV-specific CD8+ T cells during persistent infection; however, blockade of CD27 and CD28 costimulation in persistently infected mice caused a reduction in PyV-specific CD8+ T cells. Taken together, these data indicate that CD8+ T cells primed within the distinct microenvironments of acute vs persistent virus infection differ in their costimulation requirements.     

IRF4 Regulates the Ratio of T-Bet to Eomesodermin in CD8+ T Cells Responding to Persistent LCMV Infection

CD8⁺ T cell exhaustion commonly occurs in chronic infections and cancers. During T cell exhaustion there is a progressive and hierarchical loss of effector cytokine production, up-regulation of inhibitory co-stimulatory molecules, and eventual deletion of antigen specific cells by apoptosis. A key factor that regulates T cell exhaustion is persistent TCR stimulation. Loss of this interaction results in restoration of CD8⁺ T cell effector functions in previously exhausted CD8⁺ T cells. TCR stimulation is also important for the differentiation of Eomes^hi anti-viral CD8⁺ effector T cells from T-bet^hi precursors, both of which are required for optimal viral control. However, the molecular mechanisms regulating the differentiation of these two cell subsets and the relative ratios required for viral clearance have not been described. We show that TCR signal strength regulates the relative expression of T-bet and Eomes in antigen-specific CD8⁺ T cells by modulating levels of IRF4. Reduced IRF4 expression results in skewing of this ratio in the favor of Eomes, leading to lower proportions and numbers of T-bet⁺ Eomes^- precursors and poor control of LCMV-clone 13 infection. Manipulation of this ratio in the favor of T-bet restores the differentiation of T-bet⁺ Eomes^- precursors and the protective balance of T-bet to Eomes required for efficient viral control. These data highlight a critical role for IRF4 in regulating protective anti-viral CD8⁺ T cell responses by ensuring a balanced ratio of T-bet to Eomes, leading to the ultimate control of this chronic viral infection.     

Late priming and variability of epitope-specific CD8+ T cell responses during a persistent virus infection

Control of persistently infecting viruses requires that antiviral CD8(+) T cells sustain their numbers and effector function. In this study, we monitored epitope-specific CD8(+) T cells during acute and persistent phases of infection by polyoma virus, a mouse pathogen that is capable of potent oncogenicity. We identified several novel polyoma-specific CD8(+) T cell epitopes in C57BL/6 mice, a mouse strain highly resistant to polyoma virus-induced tumors. Each of these epitopes is derived from the viral T proteins, nonstructural proteins produced by both productively and nonproductively (and potentially transformed) infected cells. In contrast to CD8(+) T cell responses described in other microbial infection mouse models, we found substantial variability between epitope-specific CD8(+) T cell responses in their kinetics of expansion and contraction during acute infection, maintenance during persistent infection, as well as their expression of cytokine receptors and cytokine profiles. This epitope-dependent variability also extended to differences in maturation of functional avidity from acute to persistent infection, despite a narrowing in TCR repertoire across all three specificities. Using a novel minimal myeloablation-bone marrow chimera approach, we visualized priming of epitope-specific CD8(+) T cells during persistent virus infection. Interestingly, epitope-specific CD8(+) T cells differed in CD62L-selectin expression profiles when primed in acute or persistent phases of infection, indicating that the context of priming affects CD8(+) T cell heterogeneity. In summary, persistent polyoma virus infection both quantitatively and qualitatively shapes the antiviral CD8(+) T cell response.     

Influenza A virus infection results in a robust, antigen-responsive, and widely disseminated Foxp3+ regulatory T cell response

Foxp3(+) CD4(+) regulatory T cells (Tregs) represent a highly suppressive T cell subset with well-characterized immunosuppressive effects during immune homeostasis and chronic infections, although the role of these cells in acute viral infections is poorly understood. The present study sought to examine the induction of Foxp3(+) CD4(+) Tregs in a nonlethal murine model of pulmonary viral infection by the use of the prototypical respiratory virus influenza A. We establish that influenza A virus infection results in a robust Foxp3(+) CD4(+) T cell response and that regulatory T cell induction at the site of inflammation precedes the effector T cell response. Induced Foxp3(+) CD4(+) T cells are highly suppressive ex vivo, demonstrating that influenza virus-induced Foxp3(+) CD4(+) T cells are phenotypically regulatory. Influenza A virus-induced regulatory T cells proliferate vigorously in response to influenza virus antigen, are disseminated throughout the site of infection and primary and secondary lymphoid organs, and retain Foxp3 expression in vitro, suggesting that acute viral infection is capable of inducing a foreign-antigen-specific Treg response. The ability of influenza virus-induced regulatory T cells to suppress antigen-specific CD4(+) and CD8(+) T cell proliferation and cytokine production correlates closely to their ability to respond to influenza virus antigens, suggesting that virus-induced Tregs are capable of attenuating effector responses in an antigen-dependent manner. Collectively, these data demonstrate that primary acute viral infection is capable of inducing a robust, antigen-responsive, and suppressive regulatory T cell response.     

Phenotypic and functional heterogeneity of EBV epitope-specific CD8+ T cells

High frequencies of EBV-specific CD8(+) T cells have been detected during acute EBV infection, yet persistent infection inevitably results. To address this issue, we characterized the phenotype and function of epitope-specific CD8(+) T cell populations from presentation with acute through latent infection. Considerable phenotypic and functional heterogeneity within, as well as between, two different epitope-specific populations was observed over time following acute infection. B7 EBV-encoded nuclear Ag (EBNA)-3A-specific CD8(+) T cells expressed only CD45RO from acute through latent EBV infection. A2 BMLF-1-specific CD8(+) T cells expressed CD45RO during acute infection and either CD45RA or CD45RO during latent EBV infection. This difference in CD45 isoform expression between the two epitope-specific populations did not translate into differences in perforin content, the ability to produce IFN-gamma, or the ability to proliferate in response to Ag in vitro. In individuals with latent EBV infection, the frequencies of A2 BMLF-1- or B7 EBNA-3A-specific CD8(+) T cells that expressed CD45RA, CD45RO, CD62 ligand, CCR7, and perforin were stable over time. However, the expression of CD62 ligand and CCR7 was significantly higher among EBNA-3A-specific CD8(+) T cells than among BMLF-1-specific CD8(+) T cells. Further work is necessary to understand how phenotypic and functional differences between EBV epitope-specific CD8(+) T cells are related to the biology of the virus and to the equilibrium between the virus and the host during persistent infection.     

Persistent depots of influenza antigen fail to induce a cytotoxic CD8 T cell response

Encounter with Ag during chronic infections results in the generation of phenotypically and functionally heterogeneous subsets of Ag-specific CD8 T cells. Influenza, an acute infection, results in the generation of similar CD8 T cell heterogeneity, which may be attributed to long-lived depots of flu Ags that stimulate T cell proliferation well after virus clearance. We hypothesized that the heterogeneity of flu-specific CD8 T cells and maintenance of T cell memory required the recruitment of new CD8 T cells to persistent depots of flu Ag, as was the case for flu-specific CD4 T cell responses. However, robust expansion and generation of highly differentiated cytolytic effectors and memory T cells only occurred when naive CD8 T cells were primed during the first week of flu infection. Priming of new naive CD8 T cells after the first week of infection resulted in low numbers of poorly functional effectors, with little to no cytolytic activity, and a negligible contribution to the memory pool. Therefore, although the presentation of flu Ag during the late stages of infection may provide a mechanism for maintaining an activated population of CD8 T cells in the lung, few latecomer CD8 T cells are recruited into the functional memory T cell pool.     

Human Cytomegalovirus Latency-Associated Proteins Elicit Immune-Suppressive IL-10 Producing CD4+ T Cells

Human cytomegalovirus (HCMV) is a widely prevalent human herpesvirus, which, after primary infection, persists in the host for life. In healthy individuals, the virus is well controlled by the HCMV-specific T cell response. A key feature of this persistence, in the face of a normally robust host immune response, is the establishment of viral latency. In contrast to lytic infection, which is characterised by extensive viral gene expression and virus production, long-term latency in cells of the myeloid lineage is characterised by highly restricted expression of viral genes, including UL138 and LUNA. Here we report that both UL138 and LUNA-specific T cells were detectable directly ex vivo in healthy HCMV seropositive subjects and that this response is principally CD4⁺ T cell mediated. These UL138-specific CD4⁺ T cells are able to mediate MHC class II restricted cytotoxicity and, importantly, show IFNγ effector function in the context of both lytic and latent infection. Furthermore, in contrast to CD4⁺ T cells specific to antigens expressed solely during lytic infection, both the UL138 and LUNA-specific CD4⁺ T cell responses included CD4⁺ T cells that secreted the immunosuppressive cytokine cIL-10. We also show that cIL-10 expressing CD4⁺ T-cells are directed against latently expressed US28 and UL111A. Taken together, our data show that latency-associated gene products of HCMV generate CD4⁺ T cell responses in vivo, which are able to elicit effector function in response to both lytic and latently infected cells. Importantly and in contrast to CD4⁺ T cell populations, which recognise antigens solely expressed during lytic infection, include a subset of cells that secrete the immunosuppressive cytokine cIL-10. This suggests that HCMV skews the T cell responses to latency-associated antigens to one that is overall suppressive in order to sustain latent carriage in vivo.     

CD4+ T-Cell Help Is Required for Effective CD8+ T Cell-Mediated Resolution of Acute Viral Hepatitis in Mice

Cytotoxic CD8+ T cells are essential for the control of viral liver infections, such as those caused by HBV or HCV. It is not entirely clear whether CD4+ T-cell help is necessary for establishing anti-viral CD8+ T cell responses that successfully control liver infection. To address the role of CD4+ T cells in acute viral hepatitis, we infected mice with Lymphocytic Choriomeningitis Virus (LCMV) of the strain WE; LCMV-WE causes acute hepatitis in mice and is cleared from the liver by CD8+ T cells within about two weeks. The role of CD4+ T-cell help was studied in CD4+ T cell-lymphopenic mice, which were either induced by genetic deficiency of the major histocompatibility (MHC) class II transactivator (CIITA) in CIITA−/− mice, or by antibody-mediated CD4+ cell depletion. We found that CD4+ T cell-lymphopenic mice developed protracted viral liver infection, which seemed to be a consequence of reduced virus-specific CD8+ T-cell numbers in the liver. Moreover, the anti-viral effector functions of the liver-infiltrating CD8+ T cells in response to stimulation with LCMV peptide, notably the IFN-γ production and degranulation capacity were impaired in CIITA−/− mice. The impaired CD8+ T-cell function in CIITA−/− mice was not associated with increased expression of the exhaustion marker PD-1. Our findings indicate that CD4+ T-cell help is required to establish an effective antiviral CD8+ T-cell response in the liver during acute viral infection. Insufficient virus control and protracted viral hepatitis may be consequences of impaired initial CD4+ T-cell help.     

Effect of Staphylococcus enterotoxin B on the concurrent CD8(+) T cell response to influenza virus infection

Bacterial superantigens have potent in vivo effects. Respiratory viral infections are often associated with secondary bacterial infections, raising the likelihood of exposure to bacterial superantigens after the initiation of the anti-viral immune response. In this study, the general and V beta-specific effects of exposure to Staphylococcal enterotoxin B (SEB) during influenza virus infection on both the ongoing acute and the subsequent recall CD8(+) T cell responses were analyzed, using the well-characterized murine influenza model system and tetrameric MHC/peptide reagents to directly identify virus-specific T cells. The results show that although superantigen exposure during the primary viral infection caused delayed viral clearance, there was remarkably little effect of SEB on the magnitude or TCR repertoire of the ongoing cytolytic T cell response or on the recall response elicited by secondary viral infection. Thus, despite the well-characterized immunomodulatory effects of SEB, there was surprisingly little interference with concurrent anti-viral immunity.     

Antibody-independent control of gamma-herpesvirus latency via B cell induction of anti-viral T cell responses

B cells can use antibody-dependent mechanisms to control latent viral infections. It is unknown whether this represents the sole function of B cells during chronic viral infection. We report here that hen egg lysozyme (HEL)-specific B cells can contribute to the control of murine gamma-herpesvirus 68 (gammaHV68) latency without producing anti-viral antibody. HEL-specific B cells normalized defects in T cell numbers and proliferation observed in B cell-/- mice during the early phase of gammaHV68 latency. HEL-specific B cells also reversed defects in CD8 and CD4 T cell cytokine production observed in B cell-/- mice, generating CD8 and CD4 T cells necessary for control of latency. Furthermore, HEL-specific B cells were able to present virally encoded antigen to CD8 T cells. Therefore, B cells have antibody independent functions, including antigen presentation, that are important for control of gamma-herpesvirus latency. Exploitation of this property of B cells may allow enhanced vaccine responses to chronic virus infection.     

Memory CD8+ T cells vary in differentiation phenotype in different persistent virus infections

The viruses HIV-1, Epstein-Barr virus (EBV), cytomegalovirus (CMV) and hepatitis C virus (HCV) are characterized by the establishment of lifelong infection in the human host, where their replication is thought to be tightly controlled by virus-specific CD8+ T cells. Here we present detailed studies of the differentiation phenotype of these cells, which can be separated into three distinct subsets based on expression of the costimulatory receptors CD28 and CD27. Whereas CD8+ T cells specific for HIV, EBV and HCV exhibit similar characteristics during primary infection, there are significant enrichments at different stages of cellular differentiation in the chronic phase of persistent infection according to the viral specificity, which suggests that distinct memory T-cell populations are established in different virus infections. These findings challenge the current definitions of memory and effector subsets in humans, and suggest that ascribing effector and memory functions to subsets with different differentiation phenotypes is no longer appropriate.     

γ-Herpesvirus reactivation differentially stimulates epitope-specific CD8 T cell responses

The γ-herpesviruses are characterized by their ability to establish lifelong latency. Subsequent immune suppression leads to viral reactivation from latency and the onset of a variety of pathologies, including lymphoproliferative disease and cancers. CD8 T cells play a key role in preventing reactivation of latent virus. Therefore, to develop effective therapeutic immune strategies, it is essential to understand the maintenance of CD8 T cell responses during latency. Because the γ-herpesviruses are highly species-specific and mice cannot be infected with the human pathogens, EBV or Kaposi's sarcoma-associated herpesvirus, we have used a natural rodent γ-herpesvirus experimental infection model, γ-herpesvirus-68. In this report, we show that during long-term latent infection, naive CD8 T cells are recruited into the ongoing immune response in an epitope-specific manner. When virus reactivation is induced in vivo, the recruitment of CD8 T cells for some, but not all, epitopes is enhanced. The variation in recruitment is not due to differences in epitope presentation. We also show that CD8 T cells that are newly stimulated during reactivation are functionally impaired compared with acutely stimulated cells in terms of cytokine production. Thus, our results demonstrate unexpected complexity in the response of CD8 T cells specific for different viral epitopes that were stimulated during acute infection, quiescent latency, and reactivation.     

Sustained dysfunction of antiviral CD8+ T lymphocytes after infection with hepatitis C virus

Hepatitis C virus (HCV) sets up persistent infection in the majority of those exposed. It is likely that, as with other persistent viral infections, the efficacy of T-lymphocyte responses influences long-term outcome. However, little is known about the functional capacity of HCV-specific T-lymphocyte responses induced after acute infection. We investigated this by using major histocompatibility complex class I-peptide tetrameric complexes (tetramers), which allow direct detection of specific CD8+ T lymphocytes ex vivo, independently of function. Here we show that, early after infection, virus-specific CD8+ T lymphocytes detected with a panel of four such tetramers are abnormal in terms of their synthesis of antiviral cytokines and lytic activity. Furthermore, this phenotype is commonly maintained long term, since large sustained populations of HCV-specific CD8+ T lymphocytes were identified, which consistently had very poor antiviral cytokine responses as measured in vitro. Overall, HCV-specific CD8+ T lymphocytes show reduced synthesis of tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) after stimulation with either mitogens or peptides, compared to responses to Epstein-Barr virus and/or cytomegalovirus. This behavior of antiviral CD8+ T lymphocytes induced after HCV infection may contribute to viral persistence through failure to effectively suppress viral replication.