Role of CD28 for the generation and expansion of antigen-specific CD8(+) T lymphocytes during infection with Listeria monocytogenes.

Infection of mice with the intracellular bacterium Listeria monocytogenes results in a strong CD8(+) T cell response that is critical for efficient control of infection. We used CD28-deficient mice to characterize the function of CD28 during Listeria infection, with a main emphasis on Listeria-specific CD8(+) T cells. Frequencies and effector functions of these T cells were determined using MHC class I tetramers, single cell IFN-gamma production and Listeria-specific cytotoxicity. During primary Listeria infection of CD28(-/-) mice we observed significantly reduced numbers of Listeria-specific CD8(+) T cells and only marginal levels of specific IFN-gamma production and cytotoxicity. Although frequencies were also reduced in CD28(-/-) mice during secondary response, we detected a considerable population of Listeria-specific CD8(+) T cells in these mice. In parallel, IFN-gamma production and cytotoxicity were observed, revealing that Listeria-specific CD8(+) T cells in CD28(-/-) mice expressed normal effector functions. Consistent with their impaired CD8(+) T cell activation, CD28(-/-) mice suffered from exacerbated listeriosis both after primary and secondary infection. These results demonstrate participation of CD28 signaling in the generation and expansion of Ag-specific CD8(+) T cells in listeriosis. However, Ag-specific CD8(+) T cells generated in the absence of CD28 differentiated into normal effector and memory T cells.     

Depletion of CD4+ T cells during immunization with nonviable Listeria monocytogenes causes enhanced CD8+ T cell-mediated protection against listeriosis

Immunization of mice with nonviable Listeria monocytogenes generates an insufficient CD8(+) T cell response and consequently only limited protection against subsequent L. monocytogenes infection. We have recently demonstrated that depletion of regulatory CD4(+) T cells during immunization significantly enhances CD8(+) T cell responses. In the present study, we determined the impact of CD4(+) T cell depletion on the CD8(+) T cell response against heat-killed LISTERIA: Treatment of mice with anti-CD4 mAb during boost immunization with heat-killed Listeria significantly increased numbers of Listeria-specific CD8(+) T cells and improved protection against subsequent infection with L. monocytogenes. During challenge infection, numbers of Listeria-specific CD8(+) T cells were enhanced, and these cells expressed effector functions in terms of IFN-gamma production. In summary, we demonstrate that combining nonviable L. monocytogenes vaccination and CD4(+) T cell depletion improves generation of long-lasting and functional Listeria-specific CD8(+) memory T cells.     

Inducible costimulator protein controls the protective T cell response against Listeria monocytogenes

The inducible costimulator protein (ICOS) was recently identified as a costimulatory molecule for T cells. Here we analyze the role of ICOS for the acquired immune response of mice against the intracellular bacterium Listeria monocytogenes. During oral L. monocytogenes infection, low levels of ICOS expression were detected by extracellular and intracellular Ab staining of Listeria-specific CD4(+) and CD8(+) T cells. Blocking of ICOS signaling with a soluble ICOS-Ig fusion protein markedly impaired the Listeria-specific T cell responses. Compared with control mice, the ICOS-Ig treated mice generated significantly reduced numbers of Listeria-specific CD8(+) T cells in spleen and liver, as determined by tetramer and intracellular cytokine staining. In contrast, the specific CD8(+) T cell response in the intestinal mucosa did not appear to be impaired by the ICOS-Ig treatment. Analysis of the CD4(+) T cell response revealed that ICOS-Ig treatment also affected the specific CD4(+) T cell response. When restimulated with listerial Ag in vitro, reduced numbers of CD4(+) T cells from infected and ICOS-Ig-treated mice responded with IFN-gamma production. The impaired acquired immune response in ICOS-Ig treated mice was accompanied by their increased susceptibility to L. monocytogenes infection. ICOS-Ig treatment drastically enhanced bacterial titers, and a large fraction of mice succumbed to the otherwise sublethal dose of infection. Thus, ICOS costimulation is crucial for protective immunity against the intracellular bacterium L. monocytogenes.     

CD8+ T lymphocytes control murine cytomegalovirus replication in the central nervous system of newborn animals

Human CMV infection of the neonatal CNS results in long-term neurologic sequelae. To define the pathogenesis of fetal human CMV CNS infections, we investigated mechanisms of virus clearance from the CNS of neonatal BALB/c mice infected with murine CMV (MCMV). Virus titers peaked in the CNS between postnatal days 10-14 and infectious virus was undetectable by postnatal day 21. Congruent with virus clearance was the recruitment of CD8(+) T cells into the CNS. Depletion of CD8(+) T cells resulted in death by postnatal day 15 in MCMV-infected animals and increased viral loads in the liver, spleen, and the CNS, suggesting an important role for these cells in the control of MCMV replication in the newborn brain. Examination of brain mononuclear cells revealed that CD8(+) T cell infiltrates expressed high levels of CD69, CD44, and CD49d. IE1(168)-specific CD8(+) T cells accumulated in the CNS and produced IFN-gamma and TNF-alpha but not IL-2 following peptide stimulation. Moreover, adoptive transfer of brain mononuclear cells resulted in decreased virus burden in immunodepleted MCMV-infected syngeneic mice. Depletion of the CD8(+) cell population following transfer eliminated control of virus replication. In summary, these results show that functionally mature virus-specific CD8(+) T cells are recruited to the CNS in mice infected with MCMV as neonates.     

The role of B cells in the establishment of T cell response in mice infected with an intracellular bacteria, Listeria monocytogenes.

To clarify the role of B cells in the establishment of T cell response against intracellular bacteria, B-cell-deficient (muMT-/-) mice were infected with an intracellular bacteria, Listeria monocytogenes, and T cell response against the bacteria was analyzed. On day 6 of primary Listeria infection, spleen T cells of the muMT-/- mice showed significantly lower levels of proliferative response and IFN-gamma production than those of normal infected mice after in vitro stimulation with listerial antigen. Even in the secondary Listeria infection after immunization with viable bacteria, spleen T cells of the muMT-/- mice proliferated and produced IFN-gamma against listerial antigen at significantly lower levels than those of normal immunized mice. These results demonstrate participation of B cells in priming of Listeria-specific T cells in vivo. However, B cells failed to present Listeria antigen to Listeria-specific T cells in vitro unless Listeria antigen was solubilized. Furthermore, transfer of immune serum from Listeria-infected normal mice failed to enhance the Listeria-specific T cell response of muMT-/- mice. The results indicate that B cells support the T cell response against intracellular bacteria through a mechanism other than their Ig production or antigen presentation function.     

IFN-gamma induces the erosion of preexisting CD8 T cell memory during infection with a heterologous intracellular bacterium

Memory T cells are critical for the control of intracellular pathogens and require few signals for maintenance; however, erosion of established preexisting memory CD8(+) T cells has been shown to occur during infection with heterologous viral infections. We evaluated whether this also occurs during infection with various intracellular bacteria and what mechanisms may be involved. We demonstrate that erosion of established memory is also induced during infection of mice with various intracellular bacteria, such as Listeria monocytogenes, Salmonella typhimurium, and Mycobacterium bovis (bacillus Calmette-Guérin). The extent of erosion of established CD8(+) T cell memory was dependent on the virulence of the heterologous pathogen, not persistence. Furthermore, when antibiotics were used to comprehensively eliminate the heterologous pathogen, the numbers of memory CD8(+) T cells were not restored, indicating that erosion of preexisting memory CD8(+) T cells was irreversible. Irrespective of the initial numbers of memory CD8(+) T cells, challenge with the heterologous pathogen resulted in a similar extent of erosion of memory CD8(+) T cells, suggesting that cellular competition was not responsible for erosion. After challenge with the heterologous pathogen, effector memory CD8(+) T cells were rapidly eliminated. More importantly, erosion of preexisting memory CD8(+) T cells was abrogated in the absence of IFN-gamma. These studies help reveal the paradoxical role of IFN-gamma. Although IFN-gamma promotes the control of intracellular bacterial replication during primary infection, this comes at the expense of erosion of preexisting memory CD8(+) T cells in the wake of infection with heterologous pathogens.     

Cytosolic localization of Listeria monocytogenes triggers an early IFN-gamma response by CD8+ T cells that correlates with innate resistance to infection

IFN-gamma is critical for innate immunity against Listeria monocytogenes (L. monocytogenes), and it has long been thought that NK cells are the major source of IFN-gamma during the first few days of infection. However, it was recently shown that a significant number of CD44highCD8+ T cells also secrete IFN-gamma in an Ag-independent fashion within 16 h of infection with L. monocytogenes. In this report, we showed that infection with other intracellular pathogens did not trigger this early IFN-gamma response and that cytosolic localization of Listeria was required to induce rapid IFN-gamma production by CD44highCD8+ T cells. Infection of C57BL/6 mice with an Escherichia coli strain expressing listeriolysin O (LLO), a pore-forming toxin from L. monocytogenes, also resulted in rapid IFN-gamma expression by CD8+ T cells. These results suggest that LLO expression is essential for induction of the early IFN-gamma response, although it is not yet clear whether LLO plays a direct role in triggering a signal cascade that leads to cytokine production or whether it is required simply to release other bacterial product(s) into the host cell cytosol. Interestingly, mouse strains that displayed a rapid CD8+ T cell IFN-gamma response (C57BL/6, 129, and NZB) all had lower bacterial burdens in the liver 3 days postinfection compared with mouse strains that did not have an early CD8+ T cell IFN-gamma response (BALB/c, A/J, and SJL). These data suggest that participation of memory CD8+ T cells in the early immune response against L. monocytogenes correlates with innate host resistance to infection.     

IL-12 and type-I IFN synergize for IFN-gamma production by CD4 T cells, whereas neither are required for IFN-gamma production by CD8 T cells after Listeria monocytogenes infection

Differentiation of Ag-specific T cells into IFN-gamma producers is essential for protective immunity to intracellular pathogens. In addition to stimulation through the TCR and costimulatory molecules, IFN-gamma production is thought to require other inflammatory cytokines. Two such inflammatory cytokines are IL-12 and type I IFN (IFN-I); both can play a role in priming naive T cells to produce IFN-gamma in vitro. However, their role in priming Ag-specific T cells for IFN-gamma production during experimental infection in vivo is less clear. In this study, we examine the requirements for IL-12 and IFN-I, either individually or in combination, for priming Ag-specific T cell IFN-gamma production after Listeria monocytogenes (Lm) infection. Surprisingly, neither individual nor combined defects in IL-12 or IFN-I signaling altered IFN-gamma production by Ag-specific CD8 T cells after Lm infection. In contrast, individual defects in either IL-12 or IFN-I signaling conferred partial ( approximately 50%) reductions, whereas combined deficiency in both IL-12 and IFN-I signaling conferred more dramatic (75-95%) reductions in IFN-gamma production by Ag-specific CD4 T cells. The additive effects of IL-12 and IFN-I signaling on IFN-gamma production by CD4 T cells were further demonstrated by adoptive transfer of transgenic IFN-IR(+/+) and IFN-IR(-/-) CD4 T cells into normal and IL-12-deficient mice, and infection with rLm. These results demonstrate an important dichotomy between the signals required for priming IFN-gamma production by CD4 and CD8 T cells in response to bacterial infection.     

Adaptive immunity and enhanced CD8+ T cell response to Listeria monocytogenes in the absence of perforin and IFN-gamma

Single Ag-specific CD8+ T cells from IFN-gamma-deficient (GKO) or perforin-deficient (PKO) mice provide substantial immunity against murine infection with Listeria monocytogenes. To address the potential for redundancy between perforin and IFN-gamma as CD8+ T cell effector mechanisms, we generated perforin/IFN-gamma (PKO/GKO) double-deficient mice. PKO/GKO-derived CD8+ T cells specific for the immunodominant listeriolysin O (LLO91-99) epitope provide immunity to LM infection similar to that provided by Ag-matched wild-type (WT) CD8+ T cells in the liver but reduced in the spleen. Strikingly, polyclonal CD8+ T cells from immunized PKO/GKO mice were approximately 100-fold more potent in reducing bacterial numbers than the same number of polyclonal CD8+ T cells from immunized WT mice. This result is probably quantitative, because the frequency of the CD8+ T cell response against the immunodominant LLO91-99 epitope is >4.5-fold higher in PKO/GKO mice than WT mice at 7 days after identical immunizations. Moreover, PKO/GKO mice can be immunized by a single infection with attenuated Listeria to resist >80,000-fold higher challenges with virulent organisms than naive PKO/GKO mice. These data demonstrate that neither perforin nor IFN-gamma is required for the development or expression of adaptive immunity to LM. In addition, the results suggest the potential for perforin and IFN-gamma to regulate the magnitude of the CD8+ T cell response to infection.     

The role of CD1d in the immune response against Listeria infection

To address the role of CD1d in mucosal immune regulation in bacterial infection, we infected CD1d KO mice with Listeria monocytogenes (Lm). A higher systemic bacterial burden associated with inflammatory lymphocytic infiltrations within the intestine was found in CD1d KO compared with wild type (WT) mice. Lm induced strong IFN-gamma mRNA expression in the liver of WT and the intestine of CD1d KO mice, thus demonstrating the dual, opposing immune activities of IFN-gamma in Lm infection that is dependent on CD1d and/or NKT cells. Analysis of hepatic T cell population demonstrated a reduction of NK1.1(+)TCRbeta+ cells in both mice, followed by recovery only in WT mice. Last, the proportion of alpha4beta1 integrin on lung lymphocytes from CD1d KO was dramatically increased compared with WT mice. Thus, the absence of CD1d resulted in increased susceptibility towards Listeria infection, induced changes in NKT cells, and increased trafficking of alpha4beta1 molecule to inflamed lung.     

Efficient in vivo presentation of Listeria monocytogenes- derived CD4 and CD8 T cell epitopes in the absence of IFN-gamma

IFN-gamma is an essential component of the early Listeria monocytogenes-specific immune response, and is also an important regulator of Ag processing and presentation. Ag presentation is required for the induction and also the effector function of antimicrobial T cells. To evaluate the effect of IFN-gamma on bacterial Ag presentation in vivo, macrophages and dendritic cells were separated from L. monocytogenes-infected tissues and analyzed with peptide-specific CD4 and CD8 T cell lines in a sensitive ELISPOT-based ex vivo Ag presentation assay. The comparison of professional APCs isolated from infected IFN-gamma-deficient and wild-type mice revealed different peptide presentation patterns of L. monocytogenes-derived CD8 T cell epitopes, while the presentation pattern of CD4 T cell epitopes remained unchanged. The further in vitro analysis of the generation of CD8 T cell epitopes revealed a peptide-specific effect of IFN-gamma on MHC class I-restricted Ag presentation. These results show that despite this modulation of the Ag presentation pattern of CD8 T cell epitopes, IFN-gamma is not generally required for the MHC class I- and MHC class II-restricted presentation of L. monocytogenes-derived antigenic peptides by professional APCs in vivo.     

Tyk2 signaling in host environment plays an important role in contraction of antigen-specific CD8+ T cells following a microbial infection

Tyrosine kinase 2 (Tyk2), a member of JAK signal transducer family contributes to the signals triggered by IL-12 for IFN-gamma production. To elucidate potential roles of Tyk2 in generation and maintenance of Ag-specific CD8+ T cells, we followed the fate of OVA-specific CD8+ T cells in Tyk2-deficient (-/-) mice after infection with recombinant Listeria monocytogenes expressing OVA (rLM-OVA). Results showed that the numbers of OVA(257-264)/K(b) tetramer-positive CD8+ T cells in Tyk2(-/-) mice were almost the same as those in Tyk2(+/+) mice at the expansion phase on day 7 but were significantly larger in Tyk2(-/-) mice than those in Tyk2(+/+) mice at the contraction phase on day 10 and at the memory phase on day 60 after infection. The intracellular expression level of active caspase-3 was significantly decreased in the OVA-specific CD8+ T cells of Tyk2(-/-) mice on day 7 compared with those of Tyk2(+/+) mice. Adaptive transfer experiments revealed that Tyk2 signaling in other factors rather than CD8+ T cells played a regulatory role in CD8+ T cell contraction following infection. Administration of exogenous IFN-gamma from day 6 to day 9 restored the CD8+ T cell contraction in Tyk2(-/-) mice after infection with rLM-OVA. These results suggest that Tyk2 signaling for IFN-gamma production in host environment plays an important role in contraction of effector CD8+ T cells following a microbial infection.     

Perforin and gamma interferon-mediated control of coronavirus central nervous system infection by CD8 T cells in the absence of CD4 T cells

Infection of the central nervous system (CNS) with the neurotropic JHM strain of mouse hepatitis virus produces acute and chronic demyelination. The contributions of perforin-mediated cytolysis and gamma interferon (IFN-gamma) secretion by CD8(+) T cells to the control of infection and the induction of demyelination were examined by adoptive transfer into infected SCID recipients. Untreated SCID mice exhibited uncontrolled virus replication in all CNS cell types but had little or no demyelination. Memory CD8(+) T cells from syngeneic wild-type (wt), perforin-deficient, or IFN-gamma-deficient (GKO) donors all trafficked into the infected CNS in the absence of CD4(+) T cells and localized to similar areas. Although CD8(+) T cells from all three donors suppressed virus replication in the CNS, GKO CD8(+) T cells expressed the least antiviral activity. A distinct viral antigen distribution in specific CNS cell types revealed different mechanisms of viral control. While wt CD8(+) T cells inhibited virus replication in all CNS cell types, cytolytic activity in the absence of IFN-gamma suppressed the infection of astrocytes, but not oligodendroglia. In contrast, cells that secreted IFN-gamma but lacked cytolytic activity inhibited replication in oligodendroglia, but not astrocytes. Demyelination was most severe following viral control by wt CD8(+) T cells but was independent of macrophage infiltration. These data demonstrate the effective control of virus replication by CD8(+) T cells in the absence of CD4(+) T cells and support the necessity for the expression of distinct effector mechanisms in the control of viral replication in distinct CNS glial cell types.     

Chlamydia trachomatis infection alters the development of memory CD8+ T cells

The obligate intracellular bacterium Chlamydia trachomatis is the most common cause of bacterial sexually transmitted disease in the United States and the leading cause of preventable blindness worldwide. Prior exposure to C. trachomatis has been shown to provide incomplete protection against subsequent infection. One possible explanation for the limited immunity afforded by prior C. trachomatis infection is poor activation of Chlamydia-specific memory CD8+ T cells. In this study, we examined the development of CD8+ memory T cell responses specific for the Chlamydia Ag CrpA. The percentage of CrpA63-71-specific T cells expressing an effector memory T cell phenotype (IL-7R+ CD62low) was dramatically diminished in mice immunized with C. trachomatis, compared with mice immunized with vaccinia virus expressing the CrpA protein. These alterations in memory T cell development were correlated with a significant reduction in the capacity of convalescent mice to mount an enhanced recall response to Chlamydia Ags, compared with the primary response. CrpA-specific memory T cells primed during VacCrpA infection also failed to respond to a challenge with Chlamydia. We therefore investigated whether C. trachomatis infection might have a global inhibitory effect on CD8+ T cell activation by coinfecting mice with C. trachomatis and Listeria monocytogenes and we found that the activation of Listeria-specific naive and memory CD8+ T cells was reduced in the presence of C. trachomatis. Together, these results suggest that Chlamydia is able to alter the development of CD8+ T cell responses during both primary and secondary infection, perhaps accounting for the incomplete protection provided by prior Chlamydia infection.     

High-magnitude, virus-specific CD4 T-cell response in the central nervous system of coronavirus-infected mice

The neurotropic JHM strain of mouse hepatitis virus (MHV) causes acute encephalitis and chronic demyelinating encephalomyelitis in rodents. Previous results indicated that CD8 T cells infiltrating the central nervous system (CNS) were largely antigen specific in both diseases. Herein we show that by 7 days postinoculation, nearly 30% of the CD4 T cells in the acutely infected CNS were MHV specific by using intracellular gamma interferon (IFN-gamma) staining assays. In mice with chronic demyelination, 10 to 15% of the CD4 T cells secreted IFN-gamma in response to MHV-specific peptides. Thus, these results show that infection of the CNS is characterized by a large influx of CD4 T cells specific for MHV and that these cells remain functional, as measured by cytokine secretion, in mice with chronic demyelination.     

Listeria monocytogenes infection overcomes the requirement for CD40 ligand in exogenous antigen presentation to CD8(+) T cells.

In vivo priming of CD8(+) T lymphocytes against exogenously processed model Ags requires CD4(+) T cell help, specifically interactions between CD40 ligand (CD40L) expressed by activated CD4(+) T cells and CD40, which is present on professional APC such as dendritic cells (DCs). To address this issue in the context of bacterial infection, we examined CD40L-CD40 interactions in CD8(+) T cell priming against an exogenously processed, nonsecreted bacterial Ag. CD40L interactions were blocked by in vivo treatment with anti-CD40L mAb MR-1, which inhibited germinal center formation and CD8(+) T cell cross-priming against an exogenous model Ag, OVA. In contrast, MR-1 treatment did not interfere with CD8(+) T cell priming against a nonsecreted or secreted recombinant Ag expressed by Listeria monocytogenes. Memory and secondary responses of CD8(+) T cells against nonsecreted and secreted bacterial Ags were also largely unimpaired by transient MR-1 treatment. When MR-1-treated mice were concurrently immunized with L. monocytogenes and OVA-loaded splenocytes, cross-priming of OVA-specific naive CD8(+) T cells occurred. No significant decline in cross-priming against OVA was measured when either TNF or IFN-gamma was neutralized in L. monocytogenes-infected animals, demonstrating that multiple signals exist to overcome CD40L blockade of CD8(+) T cell cross-priming during bacterial infection. These data support a model in which DCs can be stimulated in vivo through signals other than CD40, becoming APC that can effectively stimulate CD8(+) T cell responses against exogenous Ags during infection.     

Antigen presentation by nonhemopoietic cells amplifies clonal expansion of effector CD8 T cells in a pathogen-specific manner

Professional APCs of hemopoietic-origin prime pathogen-specific naive CD8 T cells. The primed CD8 T cells can encounter Ag on infected nonhemopoietic cell types. Whether these nonhemopoietic interactions perpetuate effector T cell expansion remains unknown. We addressed this question in vivo, using four viral and bacterial pathogens, by comparing expansion of effector CD8 T cells in bone marrow chimeric mice expressing restricting MHC on all cell types vs mice that specifically lack restricting MHC on nonhemopoietic cell types or radiation-sensitive hemopoietic cell types. Absence of Ag presentation by nonhemopoietic cell types allowed priming of naive CD8 T cells in all four infection models tested, but diminished their sustained expansion by approximately 10-fold during lymphocytic choriomeningitis virus and by < or =2-fold during vaccinia virus, vesicular stomatitis virus, or Listeria monocytogenes infections. Absence of Ag presentation by a majority (>99%) of hemopoietic cells surprisingly also allowed initial priming of naive CD8 T cells in all the four infection models, albeit with delayed kinetics, but the sustained expansion of these primed CD8 T cells was markedly evident only during lymphocytic choriomeningitis virus, but not during vaccinia virus, vesicular stomatitis virus, or L. monocytogenes. Thus, infected nonhemopoietic cells can amplify effector CD8 T cell expansion during infection, but the extent to which they can amplify is determined by the pathogen. Further understanding of mechanisms by which pathogens differentially affect the ability of nonhemopoietic cell types to contribute to T cell expansion, how these processes alter during acute vs chronic phase of infections, and how these processes influence the quality and quantity of memory cells will have implications for rational vaccine design.     

A role for IFN-gamma from antigen-specific CD8+ T cells in protective immunity to Listeria monocytogenes

Whether IFN-gamma contributes to the per-cell protective capacity of memory CD8(+) T cells against Listeria monocytogenes (LM) has not been formally tested. In this study, we generated LM Ag-specific memory CD8(+) T cells via immunization of wild-type (WT) and IFN-gamma-deficient (gamma knockout (GKO)) mice with LM peptide-coated dendritic cells and compared them phenotypically and functionally. Immunization of WT and GKO mice resulted in memory CD8(+) T cells that were similar in number, functional avidity, TCR repertoire use, and memory phenotype. The protective capacity of memory CD8(+) T cells from immunized WT and GKO mice was evaluated after adoptive transfer of equal numbers of WT or GKO cells into naive BALB/c mice followed by LM challenge. The adoptively transferred CD8(+) T cells from GKO donors exhibited a decreased ability to reduce bacterial numbers in the organs of recipient mice when compared with an equivalent number of Ag-matched WT CD8(+) T cells. This deficiency was most evident early (day 3) after infection if a relatively low infectious dose was used; however, transferring fewer memory CD8(+) T cells or increasing the LM challenge dose revealed a more pronounced defect in protective immunity mediated by the CD8(+) T cells from GKO mice. Our studies identified a decrease in Ag-specific target cell lysis in vivo by CD8(+) T cells from GKO mice as the mechanism for the decreased protective immunity after LM challenge. Further studies suggest that the lack of IFN-gamma production by the Ag-specific CD8 T cells themselves diminishes target cell sensitivity to cytolysis, thereby reducing the lytic potency of IFN-gamma-deficient LM-specific memory CD8(+) T cells.     

Molecular mimicry between neurons and an intracerebral pathogen induces a CD8 T cell-mediated autoimmune disease

To identify basic mechanisms of how infections may induce a neuron-specific autoimmune response, we generated mice expressing OVA as neuronal autoantigen under control of the neuron-specific enolase promoter (NSE-OVA mice). Intracerebral, but not systemic, infection with attenuated Listeria monocytogenes-secreting OVA induced an atactic-paretic neurological syndrome in NSE-OVA mice after bacterial clearance from the brain, whereas wild-type mice remained healthy. Immunization with attenuated Listeria monocytogenes-secreting OVA before intracerebral infection strongly increased the number of intracerebral OVA-specific CD8 T cells aggravating neurological disease. T cell depletion and adoptive transfer experiments identified CD8 T cells as decisive mediators of the autoimmune disease. Importantly, NSE-OVA mice having received OVA-specific TCR transgenic CD8 T cells developed an accelerated, more severe, and extended neurological disease. Adoptively transferred pathogenic CD8 T cells specifically homed to OVA-expressing MHC class I(+) neurons and, corresponding to the clinical symptoms, approximately 30% of neurons in the anterior horn of the spinal cord became apoptotic. Thus, molecular mimicry between a pathogen and neurons can induce a CD8 T cell-mediated neurological disease, with its severity being influenced by the frequency of specific CD8 T cells, and its induction, but not its symptomatic phase, requiring the intracerebral presence of the pathogen.     

Innate immune activation of CD4 T cells in salmonella-infected mice is dependent on IL-18

Production of IFN-gamma by CD4 T cells is generally thought to be mediated by TCR triggering, however, Ag-nonspecific activation of effector CD8 T cells has been reported in infection models. In this study, we demonstrate that Ag-experienced CD4 T cells in the spleen of Salmonella-infected mice acquire the capacity to rapidly secrete IFN-gamma in response to stimulation with bacterial lysate or LPS. This innate responsiveness of T cells was transient and most apparent during, and immediately following, active Salmonella infection. Furthermore, innate T cell production of IFN-gamma in response to bacterial lysate or LPS was Ag independent and could be induced in Listeria-infected mice and in the absence of MHC class II expression. IL-18 was required for maximal innate responsiveness of CD4 T cells in Salmonella-infected mice and for optimal bacterial clearance in vivo. These data demonstrate that CD4 T cells acquire the capacity to respond to innate stimuli during active bacterial infection, a process that may contribute significantly to amplifying effector responses in vivo.