IL-10, T cell exhaustion and viral persistence

Viral infections can have one of two outcomes: control of viral replication and acute infection or viral persistence and chronic infection. It is clear that both pathogen and host characteristics influence the acute versus chronic outcome of viral infection. The early events in the host immune response that favor immunosuppression and viral persistence, however, have remained poorly understood. Using the well-characterized mouse model of acute versus chronic lymphocytic choriomeningitis virus (LCMV) infection, two groups have recently identified the interleukin-10 (IL-10)/IL-10R pathway as a key regulator of acute versus chronic infection. Blockade of IL-10R converted a chronic LCMV infection into a rapidly controlled acute viral infection and prevented the functional exhaustion of memory T cells. These insights into the role of IL-10 in the establishment of chronic infection could lead to new therapeutic opportunities during human infections with pathogens such as HIV, hepatitis C virus (HCV) and hepatitis B virus (HBV).     

Negative Regulation of Type I IFN Expression by OASL1 Permits Chronic Viral Infection and CD8+ T-Cell Exhaustion

The type I interferons (IFN-Is) are critical not only in early viral control but also in prolonged T-cell immune responses. However, chronic viral infections such as those of human immunodeficiency virus (HIV) and hepatitis C virus (HCV) in humans and lymphocytic choriomeningitis virus (LCMV) in mice overcome this early IFN-I barrier and induce viral persistence and exhaustion of T-cell function. Although various T-cell-intrinsic and -extrinsic factors are known to contribute to induction of chronic conditions, the roles of IFN-I negative regulators in chronic viral infections have been largely unexplored. Herein, we explored whether 2′–5′ oligoadenylate synthetase-like 1 (OASL1), a recently defined IFN-I negative regulator, plays a key role in the virus-specific T-cell response and viral defense against chronic LCMV. To this end, we infected Oasl1 knockout and wild-type mice with LCMV CL-13 (a chronic virus) and monitored T-cell responses, serum cytokine levels, and viral titers. LCMV CL-13-infected Oasl1 KO mice displayed a sustained level of serum IFN-I, which was primarily produced by splenic plasmacytoid dendritic cells, during the very early phase of infection (2–3 days post-infection). Oasl1 deficiency also led to the accelerated elimination of viremia and induction of a functional antiviral CD8 T-cell response, which critically depended on IFN-I receptor signaling. Together, these results demonstrate that OASL1-mediated negative regulation of IFN-I production at an early phase of infection permits viral persistence and suppresses T-cell function, suggesting that IFN-I negative regulators, including OASL1, could be exciting new targets for preventing chronic viral infection.     

Inhibition of the type I interferon response by the nucleoprotein of the prototypic arenavirus lymphocytic choriomeningitis virus

The prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a formidable battle horse for the study of viral immunology, as well as viral persistence and associated diseases. Investigations with LCMV have uncovered basic mechanisms by which viruses avoid elimination by the host adaptive immune response. In this study we show that LCMV also disables the host innate defense by interfering with beta interferon (IFN-beta) production in response to different stimuli, including infection with Sendai virus and liposome-mediated DNA transfection. Inhibition of IFN production in LCMV-infected cells was caused by an early block in the IFN regulatory factor 3 (IRF-3) activation pathway. This defect was restored in cells cured of LCMV, indicating that one or more LCMV products are responsible for the inhibition of IRF-3 activation. Using expression plasmids encoding individual LCMV proteins, we found that expression of the LCMV nucleoprotein (NP) was sufficient to inhibit both IFN production and nuclear translocation of IRF-3. To our knowledge, this is the first evidence of an IFN-counteracting viral protein in the Arenaviridae family. Inhibition of IFN production by the arenavirus NP is likely to be a determinant of virulence in vivo.     

Enhanced establishment of a virus carrier state in adult CD4+ T-cell-deficient mice.

CD4+ T cells play an important role in regulating the immune response; their contribution to virus clearance is variable. Mice that lack CD4+ T cells (CD4-/- mice) and are therefore unable to produce neutralizing antibodies cleared viscero-lymphotropic lymphocytic choriomeningitis virus (LCMV) strain WE when infected intravenously with a low dose (2 x 10(2) PFU) because of an effective CD8+ cytotoxic T-cell (CTL) response. In contrast, infection with a high dose (2 x 10(6) PFU) of LCMV strain WE led to expansion of antiviral CTL, which disappeared in CD4-/- mice; in contrast, CD4+ T-cell-competent mice developed antiviral memory CTL. This exhaustion of specific CTL caused viral persistence in CD4-/- mice, whereas CD4+ T-cell-competent mice eliminated the virus. After infection of CD4-/- mice with the faster-replicating LCMV strain DOCILE, abrogation of CTL response and establishment of viral persistence developed after infection with a low dose (5 x 10(2) PFU), i.e., an about 100-fold lower dose than in CD(4+)-competent control mice. These results show that absence of T help enhances establishment of an LCMV carrier state in selected situations.     

Hepatitis C and treatment with pegylated interferon and ribavirin

Hepatitis C is a hepatotropic RNA virus with a propensity to cause chronic infection, causing a worldwide burden of chronic hepatitis, cirrhosis and hepatocellular carcinoma. Both viral elimination and hepatocellular damage are thought to be immune mediated with T-helper and cytotoxic T-lymphocytes central to these events. A delay in the onset of adaptive immunity following infection indicates that a defect in the innate immune response may lead to viral persistence with a combination of other mechanisms subsequently contributing. Antiviral therapy based on interferon-alpha leads to resolution of disease in approximately 50% of patients. Specific anti-hepatitis C anti-viral drugs are being developed.     

Plasmacytoid dendritic cells are productively infected and activated through TLR-7 early after arenavirus infection

The antiviral response is largely mediated by dendritic cells (DCs), including conventional (c) DCs that function as antigen-presenting cells, and plasmacytoid (p) DCs that produce type I interferons, making them an attractive target for viruses. We find that the Old World arenaviruses lymphocytic choriomeningitis virus clone 13 (LCMV Cl13) and Lassa virus bind pDCs to a greater extent than cDCs. Consistently, LCMV Cl13 targets pDCs early after in?vivo infection of its natural murine host and establishes a productive and robust replication cycle. pDCs coproduce type I interferons and proinflammatory cytokines, with the former being induced in both infected and uninfected pDCs, demonstrating?a dissociation from intrinsic virus replication. TLR7?globally mediates pDC responses, limits pDC viral?load, and promotes rapid innate and adaptive immune cell activation. These early events likely help dictate the outcome of infections with arenaviruses and other DC-replicating viruses and shed light on potential therapeutic targets.     

Underwhelming the immune response: effect of slow virus growth on CD8+-T-lymphocyte responses

The speed of virus replication has typically been seen as an advantage for a virus in overcoming the ability of the immune system to control its population growth. Under some circumstances, the converse may also be true: more slowly replicating viruses may evoke weaker cellular immune responses and therefore enhance their likelihood of persistence. Using the model of lymphocytic choriomeningitis virus (LCMV) infection in mice, we provide evidence that slowly replicating strains induce weaker cytotoxic-T-lymphocyte (CTL) responses than a more rapidly replicating strain. Conceptually, we show a "bell-shaped" relationship between the LCMV growth rate and the peak CTL response. Quantitative analysis of human hepatitis C virus infections suggests that a reduction in virus growth rate between patients during the incubation period is associated with a spectrum of disease outcomes, from fulminant hepatitis at the highest rate of viral replication through acute resolving to chronic persistence at the lowest rate. A mathematical model for virus-CTL population dynamics (analogous to predator [CTL]-prey [virus] interactions) is applied in the clinical data-driven analysis of acute hepatitis B virus infection. The speed of viral replication, through its stimulus of host CTL responses, represents an important factor influencing the pathogenesis and duration of virus persistence within the human host. Viruses with lower growth rates may persist in the host because they "sneak through" immune surveillance.     

Immunodominance of an antiviral cytotoxic T cell response is shaped by the kinetics of viral protein expression

Lymphocytic choriomeningitis virus (LCMV) infection induces a protective CTL response consisting of gp- and nucleoprotein (NP)-specific CTL. We find that a small load of LCMV led to immunodominance of NP-CTL, whereas a large viral load resulted in dominance of gp-CTL. This is the first study describing that immunodominance is not fixed after infection with a given pathogen, but varies with the viral load instead. We assumed higher Ag sensitivity for NP-CTL, which would explain their preferential priming at low viral load, as well as their overstimulation resulting in selective exhaustion at high viral load. The higher Ag sensitivity of NP-CTL was due to faster kinetics of NP-epitope presentation. Thus, we uncover a novel factor that impinges upon immunodominance and is related to the kinetics of virus protein expression. We propose that CTL against early viral proteins swiftly interfere with virus replication, resulting in efficient protection. If these "early" CTL fail in immediate virus control, they are activated in the face of higher viral load compared with "late" CTL and are therefore prone to be exhausted. Thus, the observed absence of early CTL in persistent infections might not be the cause, but rather the consequence of viral persistence.     

Critical role for alpha/beta and gamma interferons in persistence of lymphocytic choriomeningitis virus by clonal exhaustion of cytotoxic T cells

Under conditions of high antigenic load during infection with invasive lymphocytic choriomeningitis virus (LCMV) strains, virus can persist by selective clonal exhaustion of antigen-specific CD8(+) T cells. In this work we studied the down-regulation of the virus-specific CD8(+)-T-cell response during a persistent infection of adult mice, with particular emphasis on the contribution of the interferon response in promoting host defense. Studies were conducted by infecting mice deficient in receptors for type I (alpha/beta interferon [IFN-alpha/beta]), type II (IFN-gamma), and both type I and II IFNs with LCMV isolates that vary in their capacity to induce T-cell exhaustion. The main conclusions of this study are as follows. (i) IFNs play a critical role in LCMV infection by reducing viral loads in the initial stages of infection and thus modifying both the extent of CD8(+)-T-cell exhaustion and the course of infection. The importance of IFNs in this context varies with the biological properties of the LCMV strain. (ii) An inverse correlation exists between antigen persistence and responsiveness of virus-specific CD8(+) T cells. This results in distinct programs of activation or tolerance (functional unresponsiveness and/or physical elimination of antigen-specific cells) during acute and chronic virus infections, respectively. (iii) A successful immune response associated with definitive viral clearance requires an appropriate balance between cellular and humoral components of the immune system. We discuss the role of IFNs in influencing virus-specific T cells that determine the outcome of persistent infections.     

Viral escape from the neutralizing antibody response: the lymphocytic choriomeningitis virus model

In addition to CD8+ cytotoxic T lymphocyte (CTL) responses, neutralizing antibodies contribute substantially to the long-term immune control of noncytopathic viruses, as demonstrated during infection with the lymphocytic choriomeningitis virus (LCMV). The high virus load during the initial phase of an infection and the ability of this RNA virus to spontaneously acquire mutations are important prerequisites for escaping an ongoing immune response. In this context, LCMV escape from the humoral response by single point mutations in neutralizing envelope protein determinants may occur, particularly during conditions of CTL deficiency, leading to virus persistence.     

Emergence of distinct multiarmed immunoregulatory antigen-presenting cells during persistent viral infection

During persistent viral infection, adaptive immune responses are suppressed by immunoregulatory factors, contributing to viral persistence. Although this suppression is mediated by inhibitory factors, the mechanisms by which virus-specific T?cells encounter and integrate immunoregulatory signals during persistent infection are unclear. We show that a distinct population of IL-10-expressing immunoregulatory antigen-presenting cells (APCs) is amplified during chronic versus acute lymphocytic choriomeningitis virus (LCMV) infection and suppresses T?cell responses. Although acute LCMV infection induces the expansion of immunoregulatory APCs, they subsequently decline. However, during persistent LCMV infection, immunoregulatory APCs are amplified and parallel the viral replication kinetics. Further characterization demonstrates that immunoregulatory APCs are molecularly and metabolically distinct, and exhibit increased expression of T?cell-interacting molecules and negative regulatory factors that suppress T?cell responses. Thus, immunoregulatory APCs are amplified during viral persistence and deliver inhibitory signals that suppress antiviral T?cell immunity and likely contribute to persistent infection.     

Low level viral persistence after infection with LCMV: a quantitative insight through numerical bifurcation analysis

Many important viruses persist at very low levels in the body in the face of host immunity, and may influence the maintenance of this state of 'infection immunity'. To analyse low level viral persistence in quantitative terms, we use a mathematical model of antiviral cytotoxic T lymphocyte (CTL) response to lymphocytic choriomeningitis virus (LCMV).This model, described by a non-linear system of delay differential equations (DDEs), is studied using numerical bifurcation analysis techniques for DDEs. Domains where low level LCMV coexistence with CTL memory is possible, either as an equilibrium state or an oscillatory pattern, are identified in spaces of the model parameters characterising the interaction between virus and CTL populations. Our analysis suggests that the coexistence of replication competent virus below the conventional detection limit (of about 100 pfu per spleen) in the immune host as an equilibrium state requires the per day relative growth rate of the virus population to decrease at least 5-fold compared to the acute phase of infection. Oscillatory patterns in the dynamics of persisting LCMV and CTL memory, with virus population varying between 1 and 100 pfu per spleen, are possible within quite narrow intervals of the rates of virus growth and precursor CTL population death. Whereas the virus replication rate appears to determine the stability of the low level virus persistence, it does not affect the steady-state level of the viral population, except for very low values.     

Targeted Deletion of FGL2 Leads to Increased Early Viral Replication and Enhanced Adaptive Immunity in a Murine Model of Acute Viral Hepatitis Caused by LCMV WE

Mounting effective innate and adaptive immune responses are critical for viral clearance and the generation of long lasting immunity. It is known that production of inhibitory factors may result in the inability of the host to clear viruses, resulting in chronic viral persistence. Fibrinogen-like protein 2 (FGL2) has been identified as a novel effector molecule of CD4⁺CD25⁺ Foxp3⁺ regulatory T (Treg) cells that inhibits immune activity by binding to FCγRIIB expressed primarily on antigen presenting cells (APC). In this study, we show that infection of mice with Lymphocytic Choriomeningitis Virus WE (LCMV WE) leads to increased plasma levels of FGL2, which were detected as early as 2 days post-infection (pi) and persisted until day 50 pi. Mice deficient in FGL2 (fgl2^−/−) had increased viral titers of LCMV WE in the liver early p.i but cleared the virus by day 12 similar to wild type mice. Dendritic cells (DC) isolated from the spleens of LCMV WE infected fgl2^−/− had increased expression of the DC maturation markers CD80 and MHC Class II compared to wild type (fgl2^+/+). Frequencies of CD8⁺ and CD4⁺ T cells producing IFNγ in response to ex vivo peptide re-stimulation isolated from the spleen and lymph nodes were also increased in LCMV WE infected fgl2 ^−/− mice. Increased frequencies of CD8⁺ T cells specific for LCMV tetramers GP₃₃ and NP₃₉₆ were detected within the liver of fgl2^−/− mice. Plasma from fgl2^−/− mice contained higher titers of total and neutralizing anti-LCMV antibody. Enhanced anti-viral immunity in fgl2^−/− mice was associated with increased levels of serum alanine transaminase (ALT), hepatic necrosis and inflammation following LCMV WE infection. These data demonstrate that targeting FGL2 leads to early increased viral replication but enhanced anti-viral adaptive T & B cell responses. Targeting FGL2 may enhance the efficacy of current anti-viral therapies for hepatotropic viruses.     

Role of ISG15 protease UBP43 (USP18) in innate immunity to viral infection

Innate immune responses provide the host with an early protection barrier against infectious agents, including viruses, and help shape the nature and quality of the subsequent adaptive immune responses of the host. Expression of ISG15 (UCRP), a ubiquitin-like protein, and protein ISGylation are highly increased upon viral infection. We have identified UBP43 (USP18) as an ISG15 deconjugating protease. Protein ISGylation is enhanced in cells deficient in UBP43 (ref. 6). Here we have examined the role of UBP43, encoded by the gene Usp18, in innate immunity to virus infection. Usp18(-/-) mice were resistant to the fatal lymphocytic choriomeningitis and myeloencephalitis that developed in wild-type mice after intracerebral inoculation with lymphocytic choriomeningitis virus (LCMV) or vesicular stomatitis virus (VSV), respectively. Survival of Usp18(-/-) mice after intracerebral LCMV infection correlated with a severe inhibition of LCMV RNA replication and antigen expression in the brain and increased levels of protein ISGylation. Consistent with these findings, mouse embryonic fibroblasts (MEF) and bone marrow-derived macrophages from Usp18(-/-) mice showed restricted LCMV replication. Moreover, MEF from Usp18(-/-) mice showed enhanced interferon-mediated resistance to the cytopathic effect caused by VSV and Sindbis virus (SNV). This report provides the first direct evidence that the ISG15 protease UBP43 and possibly protein ISGylation have a role in innate immunity against viral infection.     

A novel virus carrier state to evaluate immunotherapeutic regimens: regulatory T cells modulate the pathogenicity of antiviral memory cells

Restrictions in the diversity of an adaptive immune repertoire can facilitate viral persistence. Because a host afflicted with an immune deficiency is not likely to purge a persistent infection using endogenous mechanisms, it is important to explore adoptive therapies to supplement the host with a functional immune defense. In this study, we describe a virus carrier state that results from introducing lymphocytic choriomeningitis virus (LCMV) into adult mice possessing a restricted T cell repertoire. On infection of these mice, LCMV establishes systemic persistence, and within the CNS the virus infects astrocytes (and later oligodendrocytes) rather than its traditional parenchymal target neurons. To determine whether LCMV could be purged from a novel target selection in the absence of an endogenous immune repertoire, we adoptively transferred virus-specific memory cells into adult carrier mice. The memory cells purged virus from the periphery as well as the CNS, but they induced fatalities not typically associated with adoptive immunotherapy. When the repertoire of the recipient mice was examined, a deficiency in natural regulatory T cells was noted. We therefore supplemented carrier mice with regulatory T cells and simultaneously performed adoptive immunotherapy. Cotransfer of regulatory T cells significantly reduced mortality while still permitting the antiviral memory cells to purge the persistent infection. These data indicate that regulatory T cells can be used therapeutically to lessen the pathogenicity of virus-specific immune cells in an immunodeficient host. We also propose that the novel carrier state described herein will facilitate the study of immunotherapeutic regimens.     

The LCMV gp33-specific memory T cell repertoire narrows with age

ABSTRACT: BACKGROUND: The memory response to LCMV in mice persists for months to years with only a small decrease in the number of epitope specific CD8 T cells. This long persistence is associated with resistance to lethal LCMV disease. In contrast to studies focused on the number and surface phenotype of the memory cells, relatively little attention has been paid to the diversity of TCR usage in these cells. CD8+ T cell responses with only a few clones of identical specificity are believed to be relatively ineffective, presumably due to the relative ease of virus escape. Thus, a broad polyclonal response is associated with an effective anti-viral CD8+ T cell response. RESULTS: In this paper we show that the primary CD8+ T cell response to the LCMV gp33-41 epitope is extremely diverse. Over time while the response remains robust in terms of the number of gp33-tetramer+ T cells, the diversity of the response becomes less so. Strikingly, by 26 months after infection the response is dominated by a small number TCRbeta sequences. In addition, it is of note the gp33 specific CD8+ T cells sorted by high and low tetramer binding populations 15 and 22 months after infection. High and low tetramer binding cells had equivalent diversity and were dominated by a small number of clones regardless of the time tested. A similar restricted distribution was seen in NP396 specific CD8+ T cells 26 months after infection. The identical TCRVbeta sequences were found in both the tetramerhi and tetramerlo binding populations. Finally, we saw no evidence of public clones in the gp33-specific response. No CDR3 sequences were found in more than one mouse. CONCLUSIONS: These data show that following LCMV infection the CD8+ gp33-specific CD8 T cell response becomes highly restricted with enormous narrowing of the diversity. This narrowing of the repertoire could contribute to the progressively ineffective immune response seen in aging.     

An MHC class Ib-restricted CD8+ T cell response to lymphocytic choriomeningitis virus

Conventional MHC class Ia-restricted CD8(+) T cells play a dominant role in the host response to virus infections, but recent studies indicate that T cells with specificity for nonclassical MHC class Ib molecules may also participate in host defense. To investigate the potential role of class Ib molecules in anti-viral immune responses, K(b-/-)D(b-/-)CIITA(-/-) mice lacking expression of MHC class Ia and class II molecules were infected with lymphocytic choriomeningitis virus (LCMV). These animals have a large class Ib-selected CD8(+) T cell population and they were observed to mediate partial (but incomplete) virus clearance during acute LCMV infection as compared with K(b-/-)D(b-/-)β(2)-microglobulin(-/-) mice that lack expression of both MHC class Ia and class Ib molecules. Infection was associated with expansion of splenic CD8(+) T cells and induction of granzyme B and IFN-γ effector molecules in CD8(+) T cells. Partial virus clearance was dependent on CD8(+) cells. In vitro T cell restimulation assays demonstrated induction of a population of β(2)-microglobulin-dependent, MHC class Ib-restricted CD8(+) T cells with specificity for viral Ags and yet to be defined nonclassical MHC molecules. MHC class Ib-restricted CD8(+) T cell responses were also observed after infection of K(b-/-)D(b-/-)mice despite the low number of CD8(+) T cells in these animals. Long-term infection studies demonstrated chronic infection and gradual depletion of CD8(+) T cells in K(b-/-)D(b-/-)CIITA(-/-) mice, demonstrating that class Ia molecules are required for viral clearance. These findings demonstrate that class Ib-restricted CD8(+) T cells have the potential to participate in the host immune response to LCMV.     

Thymic Tolerance to Only One Viral Protein Reduces Lymphocytic Choriomeningitis Virus-Induced Immunopathology and Increases Survival in Perforin-Deficient Mice

The outcome of viral infections is dependent on the amount of tissue destruction caused either by direct lysis of infected cells and/or by immunopathology resulting from the immune response to the virus. We investigated whether induction of tolerance to only one viral protein could reduce immunopathology caused by nonlytic lymphocytic choriomeningitis virus (LCMV) in perforin-deficient hosts. Earlier studies had shown that LCMV infection results in aplastic anemia and death in most of these mice and that this is associated with bone marrow infiltration by antiviral cytotoxic T lymphocytes (CTL) that secrete inflammatory cytokines. We report here that perforin-deficient mice exhibit severe immunopathology in multiple organs that is characterized by infiltration of anti-LCMV CTL that secrete large amounts of gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha). Importantly, this immunopathology is significantly reduced and long-term survival of LCMV infection is increased in perforin-deficient mice expressing LCMV nucleoprotein (NP) in the thymus (and therefore deleting most of their LCMV-NP CTL) compared to the situation in thymus nonexpressors. This is due to the selective reduction of NP-specific CTL responses and their inflammatory-cytokine (IFN-gamma and TNF-alpha) secretion and to a lack of pathogenetically relevant compensatory responses to other viral proteins. Thus, "selective reduction" of the antiviral immune response to only one viral protein can significantly reduce inflammatory immunopathology and might be a therapeutic possibility for certain nonlytic infections.     

Role of PKR and Type I IFNs in Viral Control during Primary and Secondary Infection

Type I interferons (IFNs) are known to mediate viral control, and also promote survival and expansion of virus-specific CD8⁺ T cells. However, it is unclear whether signaling cascades involved in eliciting these diverse cellular effects are also distinct. One of the best-characterized anti-viral signaling mechanisms of Type I IFNs is mediated by the IFN-inducible dsRNA activated protein kinase, PKR. Here, we have investigated the role of PKR and Type I IFNs in regulating viral clearance and CD8⁺ T cell response during primary and secondary viral infections. Our studies demonstrate differential requirement for PKR, in viral control versus elicitation of CD8⁺ T cell responses during primary infection of mice with lymphocytic choriomeningitis virus (LCMV). PKR-deficient mice mounted potent CD8⁺ T cell responses, but failed to effectively control LCMV. The compromised LCMV control in the absence of PKR was multifactorial, and linked to less effective CD8⁺ T cell-mediated viral suppression, enhanced viral replication in cells, and lower steady state expression levels of IFN-responsive genes. Moreover, we show that despite normal expansion of memory CD8⁺ T cells and differentiation into effectors during a secondary response, effective clearance of LCMV but not vaccinia virus required PKR activity in infected cells. In the absence of Type I IFN signaling, secondary effector CD8⁺ T cells were ineffective in controlling both LCMV and vaccinia virus replication in vivo. These findings provide insight into cellular pathways of Type I IFN actions, and highlight the under-appreciated importance of innate immune mechanisms of viral control during secondary infections, despite the accelerated responses of memory CD8⁺ T cells. Additionally, the results presented here have furthered our understanding of the immune correlates of anti-viral protective immunity, which have implications in the rational design of vaccines.     

Vaccination against persistent viral infection exacerbates CD4+ T-cell-mediated immunopathological disease.

Lymphocytic choriomeningitis virus (LCMV) infection of normal mice results in a fatal immunopathologic meningitis mediated by CD8+ cytotoxic T lymphocytes (CTL). We have previously shown that female beta2-microglobulin-deficient (beta2m-/-) mice, which are also deficient in CD8+ T cells, are susceptible to LCMV-induced immune-mediated meningitis, characterized by significant weight loss and mortality. This LCMV disease in beta2m-/- mice is mediated by CD4+ T lymphocytes. Our previous studies have also demonstrated that male beta2m-/- mice are less susceptible than female beta2m-/- mice to LCMV-induced, immune-mediated mortality and weight loss. In this report, we show that vaccination of male beta2m-/- mice enhances immunopathology following intracranial infection with LCMV. We observed increased production of gamma interferon (IFN-gamma), an increase in CD4+ CTL precursor frequency, and an increased frequency of IFN-gamma-producing cells from spleen cells of vaccinated male beta2m-/- mice. Vaccinated male beta2m-/- mice also had significantly increased inflammation in the cerebrospinal fluid (CSF), characterized by a large CD4+ T-cell infiltrate. CSF cells from vaccinated mice showed increased production of IFN-gamma on day 7 postchallenge. Neither vaccinated nor control beta2m-/- mice were able to clear virus, and the two groups had similarly high levels of virus early after infection. These results suggest that the magnitude of the early immune response is more important than the level of virus in the brain in determining the outcome of immunopathology in beta2m-/- mice. We show here that vaccination can increase CD4+ T-cell-dependent immunopathology to a persistent viral infection.