Cathepsin B controls the persistence of memory CD8+ T lymphocytes

The persistence of memory T lymphocytes confers lifelong protection from pathogens. Memory T cells survive and undergo homeostatic proliferation (HSP) in the absence of Ag, although the cell-intrinsic mechanisms by which cytokines drive the HSP of memory T cells are not well understood. In this study we report that lysosome stability limits the long-term maintenance of memory CD8(+) T cell populations. Serine protease inhibitor (Spi) 2A, an anti-apoptotic cytosolic cathepsin inhibitor, is induced by both IL-15 and IL-7. Mice deficient in Spi2A developed fewer memory phenotype CD44(hi)CD8(+) T cells with age, which underwent reduced HSP in the bone marrow. Spi2A was also required for the maintenance of central memory CD8(+) T cell populations after acute infection with lymphocytic choriomeningitis virus. Spi2A-deficient Ag-specific CD8(+) T cell populations declined more than wild-type competitors after viral infection, and they were eroded further after successive infections. Spi2A protected memory cells from lysosomal breakdown by inhibiting cathepsin B. The impaired maintenance of Spi2A-deficient memory CD8(+) T cells was rescued by concomitant cathepsin B deficiency, demonstrating that cathepsin B was a physiological target of Spi2A in memory CD8(+) T cell survival. Our findings support a model in which protection from lysosomal rupture through cytokine-induced expression of Spi2A determines the long-term persistence of memory CD8(+) T cells.     

A role for the granzyme B inhibitor serine protease inhibitor 6 in CD8+ memory cell homeostasis

Generation and maintenance of protective immunological memory is the goal of vaccination programs. It has recently become clear that CD8+ memory T cells are derived directly from CTLs. The mechanisms underlying this transformation and the subsequent survival of memory cells are not completely understood. However, some effector molecules required by CTLs to eliminate infected cells have also been shown to control the number of Ag-specific cells. We report that memory cells express high levels of serine protease inhibitor (Spi) 6, an inhibitor of the effector molecule granzyme B, and that Spi6 can protect T cells from granzyme B-mediated apoptosis. In mouse models, both elevated expression of Spi6 and the complete absence of granzyme B in CD8+ T cells led to an increase in memory cells after infection with lymphocytic choriomeningitis virus. This was not the result of increased levels of antilymphocytic choriomeningitis virus CD8+ T cells during the expansion or contraction phases, but rather transgenic Spi6 directly influenced the survival of CD8+ memory T cells. We propose that expression of protective molecules, like Spi6, serves to shield metabolically active CD8+ memory T cells from their own effector molecules.     

Impact of CCR7 on priming and distribution of antiviral effector and memory CTL

The chemokine receptor CCR7 is a key factor in the coordinate migration of T cells and dendritic cells (DC) into and their localization within secondary lymphoid organs. In this study we investigated the impact of CCR7 on CD8(+) T cell responses by infecting CCR7(-/-) mice with lymphocytic choriomeningitis virus (LCMV). We found that the absence of CCR7 affects the magnitude of an antiviral CTL response during the acute phase, with reduced numbers of virus-specific CTL in all lymphoid and nonlymphoid organs tested. On the single cell level, CCR7-deficient CTL gained full effector function, such that antiviral protection in CCR7-deficient mice was complete, but delayed. Similarly, adoptive transfer experiments using DC from CCR7-deficient or competent mice for the priming of CCR7-positive or CCR7-negative CD8(+) T cells, respectively, revealed that ectopic positioning of DC and CTL outside organized T cell zones results in reduced priming efficacy. In the memory phase, CCR7-deficient mice maintained a stable LCMV-specific CTL population, predominantly in nonlymphoid organs, and rapidly mounted protective CTL responses against a challenge infection with a vaccinia virus recombinant for the gp33 epitope of LCMV. Taken together, the CCR7-dependent organization of the T cell zone does not appear to be a prerequisite for antiviral effector CTL differentiation and the sustenance of antiviral memory responses in lymphoid or peripheral tissues.     

In vivo induction of cytotoxic T cell response by a free synthetic peptide requires CD4+ T cell help.

Most attempts to induce CTL responses by in vivo priming with free synthetic peptides have been unsuccessful so far. However, two separate studies have recently succeeded in inducing antiviral CTL responses by immunizing mice with unmodified free synthetic peptides derived from nucleoproteins from either lymphocytic choriomeningitis virus or Sendai virus. In the present study, we have analyzed the cellular mechanisms by which the lymphocytic choriomeningitis virus synthetic peptide induced CTL responses. We demonstrated that this peptide, which was previously shown to be recognized by CD8+ T cells, also contains a helper CD4+ T cell epitope. It stimulates in vivo both CD4+ T cell-mediated CTL response. The in vivo elimination of CD4+ T cells by treatment with a mAb was shown to strongly reduce the antipeptide CTL response. This study therefore demonstrates that to be able to induce CTL responses, a peptide has to stimulate both CD4+ and CD8+ T cell subset.     

Priming of CTLs by lymphocytic choriomeningitis virus depends on dendritic cells

Appropriate activation of naive CD8(+) T cells depends on the coordinated interaction of these cells with professional APC that present antigenic peptides in the context of MHC class I molecules. It is accepted that dendritic cells (DC) are efficient in activating naive T cells and are unique in their capacity to prime CD8(+) T cell responses against exogenous cell-associated Ags. Nevertheless, it is unclear whether epitopes, derived from endogenously synthesized proteins and presented by MHC class I molecules on the surface of other APC including B cells and macrophages, can activate naive CD8(+) T cells in vivo. By infecting transgenic CD11c-DTR/GFP mice that allow conditional depletion of DC with lymphocytic choriomeningitis virus (LCMV), which infects all types of APC and elicits a vigorous CTL response, we unambiguously show that priming of LCMV-specific CD8(+) T cells is crucially dependent on DC, despite ample presence of LCMV-infected macrophages and B cells in secondary lymphoid organs.     

CpG-DNA activates in vivo T cell epitope presenting dendritic cells to trigger protective antiviral cytotoxic T cell responses

MHC class I-restricted T cell epitopes lack immunogenicity unless aided by IFA or CFA. In an attempt to circumvent the known inflammatory side effects of IFA and CFA, we analyzed the ability of immunostimulatory CpG-DNA to act as an adjuvant for MHC class I-restricted peptide epitopes. Using the immunodominant CD8 T cell epitopes, SIINFEKL from OVA or KAVYNFATM (gp33) from lymphocytic choriomeningitis virus glycoprotein, we observed that CpG-DNA conveyed immunogenicity to these epitopes leading to primary induction of peptide-specific CTL. Furthermore, vaccination with the lymphocytic choriomeningitis virus gp33 peptide triggered not only CTL but also protective antiviral defense. We also showed that MHC class I-restricted peptides are constitutively presented by immature dendritic cells (DC) within the draining lymph nodes but failed to induce CTL responses. The use of CpG-DNA as an adjuvant, however, initiated peptide presenting immature DC progression to professional licensed APC. Activated DC induced cytolytic CD8 T cells in wild-type mice and also mice deficient of Th cells or CD40 ligand. CpG-DNA thus incites CTL responses toward MHC class I-restricted T cell epitopes in a Th cell-independent manner. Overall, these results provide new insights into CpG-DNA-mediated adjuvanticity and may influence future vaccination strategies for infectious and perhaps tumor diseases.     

CD8alpha+ dendritic cells selectively present MHC class I-restricted noncytolytic viral and intracellular bacterial antigens in vivo

CD8alpha(+) dendritic cells (DCs) have been shown to be the principal DC subset involved in priming MHC class I-restricted CTL immunity to a variety of cytolytic viruses, including HSV type 1, influenza, and vaccinia virus. Whether priming of CTLs by CD8alpha(+) DCs is limited to cytolytic viruses, which may provide dead cellular material for this DC subset, or whether these DCs selectively present intracellular Ags, is unknown. To address this question, we examined Ag presentation to a noncytolytic virus, lymphocytic choriomeningitis virus, and to an intracellular bacterium, Listeria monocytogenes. We show that regardless of the type of intracellular infection, CD8alpha(+) DCs are the principal DC subset that initiate CD8(+) T cell immunity.     

A marked reduction in priming of cytotoxic CD8+ T cells mediated by stress-induced glucocorticoids involves multiple deficiencies in cross-presentation by dendritic cells

Protracted psychological stress elevates circulating glucocorticoids, which can suppress CD8(+) T cell-mediated immunity, but the mechanisms are incompletely understood. Dendritic cells (DCs), required for initiating CTL responses, are vulnerable to stress/corticosterone, which can contribute to diminished CTL responses. Cross-priming of CD8(+) T cells by DCs is required for initiating CTL responses against many intracellular pathogens that do not infect DCs. We examined the effects of stress/corticosterone on MHC class I (MHC I) cross-presentation and priming and show that stress/corticosterone-exposed DCs have a reduced ability to cross-present OVA and activate MHC I-OVA(257-264)-specific T cells. Using a murine model of psychological stress and OVA-loaded β(2)-microglobulin knockout "donor" cells that cannot present Ag, DCs from stressed mice induced markedly less Ag-specific CTL proliferation in a glucocorticoid receptor-dependent manner, and endogenous in vivo T cell cytolytic activity generated by cross-presented Ag was greatly diminished. These deficits in cross-presentation/priming were not due to altered Ag donation, Ag uptake (phagocytosis, receptor-mediated endocytosis, or fluid-phase uptake), or costimulatory molecule expression by DCs. However, proteasome activity in corticosterone-treated DCs or splenic DCs from stressed mice was partially suppressed, which limits formation of antigenic peptide-MHC I complexes. In addition, the lymphoid tissue-resident CD11b(-)CD24(+)CD8α(+) DC subset, which carries out cross-presentation/priming, was preferentially depleted in stressed mice. At the same time, CD11b(-)CD24(+)CD8α(-) DC precursors were increased, suggesting a block in development of CD8α(+) DCs. Therefore, glucocorticoid-induced changes in both the cellular composition of the immune system and intracellular protein degradation contribute to impaired CTL priming in stressed mice.     

CD4-deficient mice have reduced levels of memory cytotoxic T lymphocytes after immunization and show diminished resistance to subsequent virus challenge.

Although primary antiviral CD8+ cytotoxic T lymphocytes (CTL) can be induced in mice depleted of CD4+ T cells, the role of CD4+ T lymphocytes in the generation and maintenance of antiviral memory CTL is uncertain. This question, and the consequences upon vaccine-mediated protection, were investigated in transgenic CD4 knockout (CD4ko) mice, which lack CD4+ T lymphocytes. Infection of immunocompetent C57BL/6 mice with lymphocytic choriomeningitis virus (LCMV), or with recombinant vaccinia viruses bearing appropriate LCMV sequences, induces long-lasting protective immunity, mediated mainly by antiviral CD8+ CTL. Here we report two important findings. First, LCMV-specific CD8+ memory CTL are maintained at considerably lower levels in CD4ko mice than in normal C57BL/6J mice; we demonstrate a reduction in precursor CTL evident as soon as 30 days postimmunization and declining, by day 120, to levels 1 to 2 log units below those in normal mice. Thus, CD4+ T cells appear to be important to the generation and maintenance of their CD8+ counterparts. Second, this reduction has an important biological consequence; compared with immunocompetent mice, CD4ko mice immunized with vaccinia virus recombinants expressing nucleoprotein or glycoprotein of LCMV are less effectively protected from subsequent LCMV challenge. Thus, this study underscores the potential importance of CD4+ T lymphocytes in generation of appropriate levels of CD(8+)-cell-mediated immunoprotective memory and has implications for vaccine efficacy in individuals with immune defects in which CD4 levels may be reduced, such as AIDS.     

CD4 T cell-dependent CD8 T cell maturation

We have investigated the contribution of CD4 T cells to the optimal priming of functionally robust memory CD8 T cell subsets. Intranasal infection of CD4 T cell-deficient (CD4(-/-)) mice with lymphocytic choriomeningitis virus resulted in the elaboration of virus-specific CD8 T cell responses that cleared the infection. However, by comparison with normal mice, the virus-specific CD8 T cells in CD4(-/-) mice were quantitatively and qualitatively different. In normal mice, lymphocytic choriomeningitis virus-specific memory CD8 T cells are CD44(high), many are CD122(high), and a majority of these cells regain expression of CD62L overtime. These cells produce IFN-gamma and TNF-alpha, and a subset also produces IL-2. In the absence of CD4 T cell help, a distinct subset of memory CD8 T cells develops that remains CD62L(low) up to 1 year after infection and exhibits a CD44(int)CD122(low) phenotype. These cells are qualitatively different from their counterparts in normal hosts, as their capacity to produce TNF-alpha and IL-2 is diminished. In addition, although CD4-independent CD8 T cells can contain the infection following secondary viral challenge, their ability to expand is impaired. These findings suggest that CD4 T cell responses not only contribute to the optimal priming of CD8 T cells in chronically infected hosts, but are also critical for the phenotypic and functional maturation of CD8 T cell responses to Ags that are more rapidly cleared. Moreover, these data imply that the development of CD62L(high) central memory CD8 T cells is arrested in the absence of CD4 T cell help.     

NKT cells provide help for dendritic cell-dependent priming of MHC class I-restricted CD8+ T cells in vivo

Dendritic cells (DC) are potent APCs for naive T cells in vivo. This is evident by inducing T cell responses through adoptive DC transfer. Priming specific CTL responses in vivo often requires "help". We study alternative sources of help in DC-dependent priming of MHC class I-restricted CTL. Priming an anti-viral CTL response in naive B6 mice by adoptive transfer of antigenic peptide-pulsed DC required CD4(+) T cell help. CTL priming was facilitated by providing MHC class II-dependent specific help. Furthermore, transfers of MHC class II-deficient pulsed DC into naive, normal hosts, or DC transfers into naive, CD4(+) T cell-depleted hosts primed CTL inefficiently. Pretreatment of DC with immune-stimulating oligodeoxynucleotides rendered them more efficient for CD4(+) T cell-independent priming of CTL. DC copresenting a K(b)-binding antigenic peptide and the CD1d-binding glycolipid alpha-galactosyl-ceramide efficiently primed CTL in a class II-independent way. To obtain NKT cell-dependent help in CTL priming, the same DC had to present both the peptide and the glycolipid. CTL priming by adoptive DC transfer was largely NK cell-dependent. The requirement for NK cells was only partially overcome by recruiting NKT cell help into DC-dependent CTL priming. NKT cells thus are potent helper cells for DC-dependent CTL priming.     

Delayed contraction of the CD8+ T cell response toward lymphocytic choriomeningitis virus infection in mice lacking serglycin

We previously reported that the lack of serglycin proteoglycan affects secretory granule morphology and granzyme B (GrB) storage in in vitro generated CTLs. In this study, the role of serglycin during viral infection was studied by infecting wild-type (wt) mice and serglycin-deficient (SG(-/-)) mice with lymphocytic choriomeningitis virus (LCMV). Wt and SG(-/-) mice cleared 10(3) PFU of highly invasive LCMV with the same kinetics, and the CD8(+) T lymphocytes from wt and SG(-/-) animals did not differ in GrB, perforin, IFN-gamma, or TNF-alpha content. However, when a less invasive LCMV strain was used, SG(-/-) GrB(+) CD8(+) T cells contained approximately 30% less GrB than wt GrB(+) CD8(+) T cells. Interestingly, the contraction of the antiviral CD8(+) T cell response to highly invasive LCMV was markedly delayed in SG(-/-) mice, and a delayed contraction of the virus-specific CD8(+) T cell response was also seen after infection with vesicular stomatitis virus. BrdU labeling of cells in vivo revealed that the delayed contraction was associated with sustained proliferation of Ag-specific CD8(+) T cells in SG(-/-) mice. Moreover, wt LCMV-specific CD8(+) T cells from TCR318 transgenic mice expanded much more extensively in virus-infected SG(-/-) mice than in matched wt mice, indicating that the delayed contraction represents a T cell extrinsic phenomenon. In summary, the present report points to a novel, previously unrecognized role for serglycin proteoglycan in regulating the kinetics of antiviral CD8(+) T cell responses.     

Antigen presentation by an immature myeloid dendritic cell line does not cause CTL deletion in vivo, but generates CD8+ central memory-like T cells that can be rescued for full effector function

Immature dendritic cells (DC), in contrast to their mature counterparts, are incapable of mobilizing a CD8+ CTL response, and, instead, have been reported to induce CTL tolerance. We directly addressed the impact of immature vs mature DC on CTL responses by infusing adenovirus peptide-loaded DC (of the D1 cell line) into mice that had received adenovirus-specific naive TCR-transgenic CD8+ T cells. Whereas i.v. injection of mature DC triggered vigorous CTL expansion, immature DC elicited little proliferation involving only a minority of the TCR-transgenic CTL. Even though the latter CTL developed effector functions, including cytolytic activity and proinflammatory cytokine secretion, these cells differed significantly from CTL primed by mature DC in that they did not exhibit down-regulation of CD62L and CCR7, receptors involved in trapping of T cells in the lymphoid organs. Interestingly, adoptive transfer of CTL effector cells harvested after priming by either mature or immature DC into naive recipient mice, followed by exposure to adenovirus, yielded quantitatively and qualitatively indistinguishable CTL memory responses. Therefore, in vivo priming of naive CD8+ T cells by immature DC, although failing to induce a full-blown, systemic CTL response, resulted in the formation of central memory-like T cells that were able to expand and produce IFN-gamma upon secondary antigenic stimulation.     

Depletion of CD4+ T cells precipitates immunopathology in immunodeficient mice infected with a noncytocidal virus

IFN-gamma-deficient (IFN-gamma(-/-)) mice inoculated with intermediate doses of a slowly replicating strain of lymphocytic choriomeningitis virus become chronically infected. In such mice a hypercompensated CTL response is observed that partially controls virus replication. Here we have investigated whether CD4(+) Th cells are required to establish and maintain this new equilibrium. The absence of IFN-gamma does not impair the generation of IL-2-producing CD4(+) cells, and depletion of these cells precipitates severe CD8(+) T cell-mediated immunopathology in IFN-gamma(-/-) mice, indicating an important role of CD4(+) T cells in preventing this syndrome. Analysis of organ virus levels revealed a further impairment of virus control in IFN-gamma(-/-) mice following CD4(+) cell depletion. Initially the antiviral CTL response did not require CD4(+) cells, but with time an impaired reactivity toward especially the glycoprotein 33--41 epitope was noted. Enumeration of epitope-specific (glycoprotein 33--41 and nucleoprotein 396--404) CD8(+) T cells by use of tetramers gave similar results. Finally, limiting dilution analysis of CTL precursors reveal an impaired capacity to sustain this population in CD4(+)-depleted mice, especially in mice also deficient in IFN-gamma. Thus, our findings disclose that T cell help is required to sustain the expanded CTL precursor pool required in IFN-gamma(-/-) mice. This interpretation is supported by mathematical modeling that predicts an increased requirement for help in IFN-gamma(-/-) hosts similar to what is found with fast replicating virus strains in normal hosts. Thus, the functional integrity of CD8(+) effector T cells is one important factor influencing the requirement for T cell help during viral infection.     

Cutting edge: increased expression of Bcl-2 in antigen-specific memory CD8+ T cells

Bcl-2 plays a critical role in regulating cell survival and apoptosis. We examined Bcl-2 expression in virus-specific CD8 T cells during the expansion, death, and memory phases of the T cell response following infection of mice with lymphocytic choriomeningitis virus (LCMV). Naive CD8 T cells expressed a basal level of Bcl-2 that was down-regulated in effector CD8 T cells just before the death phase. Bcl-2 levels remained low during the death phase but surviving memory CD8 T cells expressed higher levels of Bcl-2 than naive cells. These changes were shown to occur in LCMV TCR transgenic cells as well as virus-specific CD8 T cells in C57BL/6 and BALB/c mice identified by MHC class I tetramers. In all instances, memory CD8 T cells expressed higher levels of Bcl-2, suggesting that increased Bcl-2 expression plays a role in the long-term maintenance of memory CD8 T cells in vivo.     

4-1BB ligand, a member of the TNF family, is important for the generation of antiviral CD8 T cell responses.

4-1BB (CD137) is a costimulatory molecule expressed on activated T cells and interacts with 4-1BB ligand (4-1BBL) on APCs. To investigate the role of 4-1BB costimulation for the development of primary immune responses, 4-1BBL-deficient (4-1BBL-/-) mice were infected with lymphocytic choriomeningitis virus (LCMV). 4-1BBL-/- mice were able to generate CTL and eliminate acute LCMV infection with normal kinetics, but CD8 T cell expansion was 2- to 3-fold lower than in wild-type (+/+) mice. In the same mice, virus-specific CD4 Th and B cell responses were minimally affected, indicating that 4-1BB costimulation preferentially affects CD8 T cell responses. This result contrasts with our earlier work with CD40L-deficient (CD40L-/-) mice, in which the CD8 T cell response was unaffected and the CD4 T cell response was markedly impaired. When both 4-1BBL- and B7-dependent signals were absent, CD8 T cell expansion was further reduced, resulting in lower CTL activity and impairing their ability to clear LCMV. Altogether, these results indicate that T cells have distinct costimulatory requirements: optimal CD8 responses require 4-1BBL-dependent interactions, whereas CD4 responses are minimally affected by 4-1BB costimulation, but require CD40-CD40L and B7-dependent interactions.     

Selectively impaired CD8+ but not CD4+ T cell cycle arrest during priming as a consequence of dendritic cell interaction with plasmodium-infected red cells

Individuals living in malaria-endemic areas show generally low T cell responses to malaria Ags. In this study, we show murine dendritic cell (DC) interaction with parasitized erythrocytes (pRBC) arrested their maturation, resulting in impaired ability to stimulate naive, but not recall T cell responses in vitro and in vivo. Moreover, within the naive T cell population, pRBC-treated DC were selectively deficient in priming CD8(+) but not CD4(+) T cells. Indeed, DC that had taken up pRBC were shown for the first time to efficiently prime CD4(+) T cell responses to a known protective merozoite Ag, MSP4/5. In contrast, impaired priming resulted in decreases in both proliferation and cytokine production by CD8(+) T cells. Deficient priming was observed to both a model and a Plasmodium berghei-specific CD8(+) T cell epitope. The mechanisms underlying the inability of parasite-treated DC to prime CD8(+) T cells were explored. pRBC treatment of DC from wild-type C57BL/6, but not from IL-10 knockout animals, suppressed DC-mediated T cell priming across a Transwell, suggesting active IL-10-dependent suppression. CD8(+) T cells were arrested at the G(0) stage of the cell cycle after two cell divisions post-Ag stimulation. The proliferation arrest was partially reversible by the addition of IL-2 or IL-7 to responder cultures. These results suggest that in malaria-endemic areas, priming of CD8(+) T cell responses may be more difficult to induce via vaccination than the priming of CD4(+) T cells. Moreover, pathogens may selectively target the CD8(+) T cell arm of protective immunity for immune evasion.     

Priming MHC-I-restricted cytotoxic T lymphocyte responses to exogenous hepatitis B surface antigen is CD4+ T cell dependent.

MHC-I (Ld)-restricted, S28-39-specific CTL responses are efficiently primed in H-2d BALB/c mice injected with low doses of native hepatitis B surface Ag (HBsAg) lipoprotein particles without adjuvants. Priming of this CTL response by exogenous HBsAg required CD4+ T cell "help" and IL-12: this CTL response could be neither induced in mice depleted of CD4+ T cells by in vivo Ab treatment, nor in (CD4+ T cell-competent or CD4+ T cell-depleted) IL-12-unresponsive STAT4-/- knockout BALB/c mice. Codelivery of oligonucleotides (ODN) with immunostimulating CpG sequences (ISS) with exogenous HBsAg reconstituted the CTL response to exogenous HBsAg in CD4+ T cell-depleted normal mice and in CD4+ T cell-competent and CD4+ T cell-depleted STAT4-/- BALB/c mice. Injection (by different routes) of "naked" pCI/S plasmid DNA encoding HBsAg into IL-12-responsive or -unresponsive BALB/c mice efficiently primed the MHC-I-restricted, HBsAg-specific CTL response. CTL priming was not detectable when CD4+ T cell-depleted animals were subjected to genetic immunization. In vivo priming of the well-characterized CD8+ CTL response to HBsAg in "high responder" BALB/c mice either by exogenous surface lipoprotein particles or by DNA vaccination is thus CD4+ T cell dependent. CTL priming by exogenous HBsAg, but not by genetic immunization, is IL-12 dependent. The dependence of CTL priming by exogenous HBsAg on CD4+ T cells can be overcome by codelivering ODN with ISS motifs, and this "adjuvants effect" operates efficiently in IL-12-unresponsive mice. The data characterize a feature of the adjuvant effect of ISS-containing ODN on CTL priming that may be of major interest for the design of CTL-stimulating vaccines with efficacy in immunodeficiency conditions.     

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.     

Break of T cell ignorance to a viral antigen in the liver induces hepatitis

To study peripheral tolerance of CD8 T cells to a classically MHC-restricted peptide Ag expressed in hepatocytes, ALB1 transgenic (tg) mice expressing the CTL epitope GP33 of the lymphocytic choriomeningitis virus glycoprotein under control of the mouse albumin promoter were generated. ALB1 mice exclusively expressed the GP33 transgene in the liver and, at a 100- to 1000-fold lower level, in the thymus. TCR-tg mice specific for the GP33 epitope were used to directly follow GP33-specific T cells in vivo. These experiments revealed that 1) thymic expression of the GP33 transgene led to incomplete central deletion of TCR-tg cells; and 2) peripheral TCR-tg cells in ALB1 mice ignored the GP33 transgene expressed in hepatocytes. Ignorance of adoptively transferred TCR-tg cells in ALB1 mice was broken by infection with lymphocytic choriomeningitis virus, leading to induction of hepatitis in ALB1, but not in control, mice. Taken together, we have established a novel model of virus-induced CD8 T cell-mediated autoimmune hepatitis in mice and demonstrate that naive CD8 T cells may ignore Ags expressed in the liver.