Abrogation of tolerance to a chronic viral infection

This study documents failure of peripheral tolerance mechanisms in a chronic viral infection and shows that T cell tolerance to a viral Ag seen as self from fetal life can be broken despite the presence of this Ag in extrathymic tissues. Congenital infection of mice with lymphocytic choriomeningitis virus (LCMV) results in T cell tolerance to the virus. Such mice become carriers for life harboring virus in many tissues including the thymus and exhibit no LCMV-specific CTL responses. Our previous studies have documented the curing of this congenitally acquired chronic infection after adoptive transfer of CD8+ T cells from LCMV-immune mice and the presence of host-derived, LCMV-specific CTL in these "cured" carriers. In this study we have examined the mechanism by which these carriers acquired T cell competence and show that these CTL differentiated from the bone marrow after elimination of viral Ag from the thymus. These results demonstrate that even when a chronic infection has been established in utero, the adult thymus retains the ability to restore immunocompetence to the host and to provide protection against reinfection. Surprisingly, these LCMV specific CTL were acquired at a time when infectious virus and intracellular viral Ag, although cleared from the thymus, were readily detectable in organs such as the kidney, testes, and brain. In fact, active viral replication in peripheral tissues was ongoing when these mice acquired new virus-specific T cells. These results show that clearance of virus form the thymus was sufficient to abrogate tolerance to a congenitally acquired chronic infection and that Ag in peripheral tissues did not tolerize newly developing T cells. These findings suggest that mechanisms that operate on immature cells within the thymus to silence self-reactive T cells are effective in induction of tolerance to viruses, but mechanisms of tolerizing mature T cells are likely to breakdown. This has implications for virus-induced autoimmunity and for treatment of chronic infections.     

FTY720 immunosuppression impairs effector T cell peripheral homing without affecting induction, expansion, and memory

FTY720 (2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol hydrochloride) prolongs survival of solid organ allografts in animal models. Mechanisms of FTY720 immunomodulation were studied in mice infected with lymphocytic choriomeningitis virus (LCMV) to assess T cell responses or with vesicular stomatitis virus to evaluate Ab responses. Oral FTY720 (0.3 mg/kg/day) did not affect LCMV replication and specific CTL and B cells were induced and expanded normally. Moreover, the anti-viral humoral immune responses were normal. However, FTY720 treatment showed first a shift of overall distribution of CTL from the spleen to peripheral lymph nodes and lymphocytopenia was observed. This effect was reversible within 7-21 days. Together with unimpaired T and B cell memory after FTY720 treatment, this finding rendered enhancement of lymphocyte apoptosis by FTY720 in vivo unlikely. Secondly, the delayed-type hypersensitivity reaction to a viral MHC class I-presented peptide was markedly reduced by FTY720. These results were supported by impaired circulation of LCMV specific TCR transgenic effector lymphocytes in the peripheral blood and reduced numbers of tissue infiltrating CTL in response to delayed-type hypersensitivity reaction. Thirdly, in a CD8+ T cell-mediated diabetes model in a transgenic mouse expressing the LCMV glycoprotein in the islets of the pancreas, FTY720 delayed or prevented disease by reducing islet-infiltrating CTL. Thus, FTY720 effectively reduced recirculation of CD8+ effector T cells and their recruitment to peripheral lesions without affecting the induction and expansion of immune responses in secondary lymphoid organs. These properties may offer the potential to treat ongoing organ-specific T cell-mediated immunopathologic disease.     

In vivo selection of neutralization-resistant virus variants but no evidence of B cell tolerance in lymphocytic choriomeningitis virus carrier mice expressing a transgenic virus-neutralizing antibody

B cell tolerance is maintained by active deletion and functional anergy of self-reactive B cells depending on the time, amount, and site of the self-antigen expression. To study B cell tolerance toward a transplacentally transmitted viral Ag, we crossed transgenic mice expressing the mu heavy and the kappa light chain of the lymphocytic choriomeningitis virus (LCMV)-neutralizing mAb KL25 (HL25-transgenic mice) with persistently infected LCMV carrier mice. Although HL25-transgenic LCMV carrier mice exhibited the same high virus titers as nontransgenic LCMV carrier mice, no evidence for B cell tolerance was found. In contrast, enhanced LCMV-neutralizing Ab titers were measured that, however, did not clear the virus. Instead, LCMV isolates from different tissues turned out to be neutralization resistant Ab escape variants expressing different substitutions of amino acid Asn119 of the LCMV-glycoprotein 1 that displays the neutralizing B cell epitope. Virus variants with the same mutations were also selected in vitro in the presence of the transgenic mAb KL25 confirming that substitutions of Asn119 have been selected by LCMV-neutralizing Abs. Thus, despite abundant expression of viral neo-self-antigen in HL25-transgenic LCMV carrier mice, transgenic B cells expressing LCMV-neutralizing Abs were rather stimulated than tolerized and neutralization resistant Ab escape variants were selected in vivo.     

Low 2-dimensional CD4 T cell receptor affinity for myelin sets in motion delayed response kinetics

T cells recognizing self-peptides that mediate autoimmune disease and those that are responsible for efficacious immunity against pathogens may differ in affinity for antigen due to central and peripheral tolerance mechanisms. Here we utilize prototypical self-reactive (myelin) and viral-specific (LCMV) T cells from T cell receptor (TCR) transgenic mice (2D2 and SMARTA, respectively) to explore affinity differences. The T cells responsive to virus possessed >10,000 fold higher 2D affinity as compared to the self-reactive T cells. Despite their dramatically lower affinity for their cognate ligand, 2D2 T cells respond with complete, albeit delayed, activation (proliferation and cytokine production). SMARTA activation occurs rapidly, achieving peak phosphorylation of p38 (1 minute), Erk (30 minutes), and Jun (3 hours) as well as CD69 and CD25 upregulation (3 and 6 hours, respectively), with a corresponding early initiation of proliferation. 2D2 stimulation with MOG results in altered signaling--no phospho-Erk or phospho-p38 accumulation, significantly delayed activation kinetics of Jun (12 hours), and delayed but sustained SHP-1 activity--as well as delayed CD69 and CD25 expression (12-24 hours), and slow initiation of proliferation. This delay was not intrinsic to the 2D2 T cells, as a more potent antigen with >100-fold increased 2D affinity restored rapid response kinetics in line with those identified for the viral antigen. Taken together, these data demonstrate that time can offset low TCR affinity to attain full activation and suggest a mechanism by which low affinity T cells participate in autoimmune disease.     

Cyclosporine A and peripheral tolerance. Inhibition of veto cell-mediated clonal deletion of postthymic precursor cytotoxic T lymphocytes.

Veto cell-mediated suppression of CTL responses has been proposed as one mechanism by which self tolerance is maintained in mature T cell populations. We have reported that murine bone marrow cells cultured in the presence of high-dose IL-2 (activated bone marrow cells) mediate strong veto suppressor function in vitro and in vivo, and that such veto activity is effected through clonal deletion of cytotoxic T cell precursors. In our studies, we have determined that bone marrow cell populations from athymic NCr-nu mice (H-2d) mediate strong veto cell activity without exposure to exogenous IL-2 in vitro. To examine mechanisms by which these naturally occurring veto cell populations in BM suppress precursor CTL (pCTL) responses, we used as a responding cell population in MLC, spleen cells of transgenic mice expressing at high frequency TCR specific for H-2 Ld encoded Ag with stimulation by H-2d-expressing cells in culture. Flow cytometric analysis was performed by staining the responding MLC cell population with the mAb 1B2 specific for the transgene-encoded TCR and determined changes of 1B2+ T cells. Such experiments demonstrated that the anti-H-2d cytotoxic response by these cell populations was specifically suppressed by NCr-nu (H-2d) bone marrow, and that 1B2+ pCTL were in fact specifically deleted from the responding cell population by incubation with such naturally occurring veto cell populations expressing the appropriate target Ag. In addition, to further understand the interactions of pCTL and veto cells and possible contributions by the latter to peripheral tolerance, we evaluated the effect of cyclosporine A (CsA) on veto cell-mediated suppression of pCTL of the transgenic mice. CsA inhibited veto cell-mediated suppression of cytotoxic T cell responses, and this inhibition correlated with a lack of clonal deletion of pCTL by veto cells in the presence of CsA. Furthermore, CsA exerted its effect through pCTL and not through veto cells, indicating that pCTL may play an active role in their own deletion by veto cells.     

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.     

Requirement for T-cell apoptosis in the induction of peripheral transplantation tolerance.

The mechanisms of allograft tolerance have been classified as deletion, anergy, ignorance and suppression/regulation. Deletion has been implicated in central tolerance, whereas peripheral tolerance has generally been ascribed to clonal anergy and/or active immunoregulatory states. Here, we used two distinct systems to assess the requirement for T-cell deletion in peripheral tolerance induction. In mice transgenic for Bcl-xL, T cells were resistant to passive cell death through cytokine withdrawal, whereas T cells from interleukin-2-deficient mice did not undergo activation-induced cell death. Using either agents that block co-stimulatory pathways or the immunosuppressive drug rapamycin, which we have shown here blocks the proliferative component of interleukin-2 signaling but does not inhibit priming for activation-induced cell death, we found that mice with defective passive or active T-cell apoptotic pathways were resistant to induction of transplantation tolerance. Thus, deletion of activated T cells through activation-induced cell death or growth factor withdrawal seems necessary to achieve peripheral tolerance across major histocompatibility complex barriers.     

Functional tolerance of CD8+ T cells induced by muscle-specific antigen expression

Skeletal muscles account for more than 30% of the human body, yet mechanisms of immunological tolerance to this tissue remain mainly unexplored. To investigate the mechanisms of tolerance to muscle-specific proteins, we generated transgenic mice expressing the neo-autoantigen OVA exclusively in skeletal muscle (SM-OVA mice). SM-OVA mice were bred with OT-I or OT-II mice that possess a transgenic TCR specific for OVA peptides presented by MHC class I or class II, respectively. Tolerance to OVA did not involve clonal deletion, anergy or an increased regulatory T cell compartment. Rather, CD4+ T cell tolerance resulted from a mechanism of ignorance revealed by their response following OVA immunization. In marked contrast, CD8+ T cells exhibited a loss of OVA-specific cytotoxic activity associated with up-regulation of the immunoregulatory programmed death-1 molecule. Adoptive transfer experiments further showed that OVA expression in skeletal muscle was required to maintain this functional tolerance. These results establish a novel asymmetric model of immunological tolerance to muscle autoantigens involving Ag ignorance for CD4+ T cells, whereas muscle autoantigens recognized by CD8+ T cells results in blockade of their cytotoxic function. These observations may be helpful for understanding the breakage of tolerance in autoimmune muscle diseases.     

T-Cell Tolerance: Central and Peripheral

Somatic recombination of TCR genes in immature thymocytes results in some cells with useful TCR specificities, but also many with useless or potentially self-reactive specificities. Thus thymic selection mechanisms operate to shape the T-cell repertoire. Thymocytes that have a TCR with low affinity for self-peptide–MHC complexes are positively selected to further differentiate and function in adaptive immunity, whereas useless ones die by neglect. Clonal deletion and clonal diversion (Treg differentiation) are the major processes in the thymus that eliminate or control self-reactive T cells. Although these processes are thought to be efficient, they fail to control self-reactivity in all circumstances. Thus, peripheral tolerance processes exist wherein self-reactive T cells become functionally unresponsive (anergy) or are deleted after encountering self-antigens outside of the thymus. Recent advances in mechanistic studies of central and peripheral T-cell tolerance are promoting the development of therapeutic strategies to treat autoimmune disease and cancer and improve transplantation outcome.T cells recognize pathogen fragments in the context of surface MHC molecules on host cells. As such, they have the potential to do enormous damage to healthy tissue when they are not appropriately directed, that is, when they respond to self-antigens as opposed to foreign antigens. T lymphocyte tolerance is particularly important, because it impacts B-cell tolerance as well, through the requirement of T cell help in antibody responses. Thus, failure of T-cell tolerance can lead to many different autoimmune diseases. The tolerance of T cells begins as soon as a T-cell receptor is formed and expressed on the cell surface of a T-cell progenitor in the thymus. Tolerance mechanisms that operate in the thymus before the maturation and circulation of T cells are referred to as “central tolerance.” However, not all antigens that T cells need to be tolerant of are expressed in the thymus, and thus central tolerance mechanisms alone are insufficient. Fortunately, additional tolerance mechanisms exist that restrain the numbers and or function of T cells that are reactive to developmental or food antigens, which are not thymically expressed. These mechanisms act on mature circulating T cells and are referred to as “peripheral tolerance.”     

Melanoma-induced suppression of tumor antigen-specific T cell expansion is comparable to suppression of global T cell expansion

We have observed that in vivo interaction between melanoma and resting T cells promotes suppression of antigen-driven proliferative T cell expansion. We hypothesized that this suppression would affect tumor antigen-specific T cell populations more potently than tumor-unrelated T cell populations. A B16F10 cell line was stably transfected to express low levels of the lymphocytic choriomeningitis virus (LCMV) glycoprotein GP33 (B16GP33). Mice bearing B16F10 or B16GP33 tumors were infected with LCMV, and proliferative expansion of LCMV epitope-specific T cell populations was quantified. In vitro and in vivo assays confirmed low levels of antigenic GP33 expression by B16GP33 tumors. Suppressed expansion of GP33-specific T cells was equivalent between mice bearing B16F10 and B16GP33 tumors. These observations suggest that the ability of growing melanoma tumors to impair antigen-driven proliferative expansion of activated T cells is global and not antigen-specific, and provide further insight into the influence of cancer on activated T cell homeostasis.     

Peripheral deletion of mature CD8+ antigen-specific T cells after in vivo exposure to male antigen.

It has been well established that T cell tolerance to self Ag occurs primarily via clonal deletion of immature thymocytes in the thymus. Evidence also exists that there are additional mechanisms operative on mature T cells for establishing and maintaining tolerance in the periphery. To follow the fate of mature Ag-specific T cells in vivo, we used female transgenic mice, which contain a large population of male H-Y Ag-specific T cells that can be identified by immunostaining with mAb directed against CD8 and the transgenic TCR. H-Y Ag was introduced into these mice by injecting Ag-bearing male lymphocytes using conditions known to induce CTL precursor response reduction. The number of Ag-reactive CD8+ transgenic T cells in the periphery started to decrease after 2 days of in vivo exposure to male Ag. Decline was maximum (up to 80% of total) by 7 days, and stayed at this level for at least 6 wk. CD4+ cells and those CD8+ cells that did not carry the transgenic TCR were not affected. Most or all of the remaining Ag-reactive CD8+ cells in the periphery were fully responsive when stimulated by male Ag in vitro. Maturation of transgenic T cells in the thymus of injected mice remained the same as that of control animals. Our data provide direct evidence that mature Ag-reactive CD8+ cells are susceptible to clonal deletion in the periphery when exposed to the Ag in vivo. These findings suggest the presence of two types of APC in the periphery: stimulatory APC (e.g., macrophages and dendritic cells) required for initiating an active immune response; and functionally deleting APC (or veto cells) capable of deleting mature T lymphocytes that recognize Ag presented on their surface. Functionally deleting APC that present self Ag to peripheral T cells may provide a fail-safe mechanism against autoreactive cells that escaped deletion during differentiation in the thymus.     

Spontaneous large-scale lymphoid neogenesis and balanced autoimmunity versus tolerance in the stomach of H+/K+-ATPase-reactive TCR transgenic mouse

Autoimmunity is often accompanied by the development of ectopic lymphoid tissues in the target organ, and these tissues have been believed to have close relevance to the severity of the disease. However, the true relationship between the extent of such lymphoid structures and the intensity or type of immune responses mediated by self-reactive T cells has remained unclear. In the present study, we generated transgenic mice expressing TCR from an autoimmune gastritis (AIG)-inducing Th1 cell clone specific for one of the major stomach self-Ags, H(+)/K(+)-ATPase alpha subunit. The transgenic mice spontaneously develop massive lymphoid neogenesis with a highly organized tissue structure in the gastric mucosa, demonstrating Ag-specific, T cell-mediated induction of the lymphoid tissues. Nevertheless, the damage of surrounding tissue and autoantibody production were considerably limited compared with those in typical AIG induced by neonatal thymectomy. Such a moderate pathology is likely due to the locally restricted activation and Th2 skewing of self-reactive T cells, as well as the accumulation of naturally occurring regulatory T cells in the target organ. Altogether, the findings suggest that lymphoid neogenesis in chronic autoimmunity does not simply correlate with the destructive response; rather, the overall activation status of the T cell network, i.e., the balance of self-reactivity and tolerance, in the local environment has an impact.     

Overexpressed Bcl-x(L) prevents bacterial superantigen-induced apoptosis of thymocytes in vitro

bcl-x, a homologous gene of bcl-2, has an anti-apoptotic function and appears to play a critical role in the development of lymphoid systems. To investigate the effect of overexpressed Bcl-x(L) on the development of T lymphocytes, we established two lines of transgenic mice by using Emu-chicken bcl-x(L) (cbcl-x(L)) transgene, where the cBcl-x(L) protein was expressed mainly in lymphoid cells. Although thymocytes and splenocytes from cbcl-x(L) transgenic mice are resistant to apoptosis in vitro, clonal deletion of thymocytes, recognizing endogenous self-superantigens in the thymus, still normally proceeded and no self-reactive T cells were found in the spleen of the transgenic mice. To dissect clonal deletion, we utilized two in vitro models, thymocytes/antigen presenting cells co-culture system and fetal thymus organ culture system. In both, bacterial superantigen staphylococcus aureus enterotoxin B (SEB) induces apoptosis of T cells with Vbeta8+ T cell receptor (TCR) reacting to SEB, which mimics clonal deletion of self-reactive thymocytes in vivo. SEB-induced depletion of Vbeta8+ T cells from thymocytes when taken from the transgenic mice was effectively inhibited. The data might raise the possibility that cell death process involved in clonal deletion in the thymus is a form of apoptosis inhibited by Bcl-x(L).     

The central tolerance response to male antigen in normal mice is deletion and not receptor editing

It is widely accepted that developing T cells can undergo clonal deletion in the thymus in response to a high affinity self-Ag. This is largely based on studies of TCR transgenics. However, encounter with high affinity self-Ag can also result in receptor editing in TCR transgenic models. Because all TCR transgenics display ectopic receptor expression, the tolerance mechanism that predominates in normal mice remains an open question. When self-Ag drives receptor editing during T cell development, one expects to find in-frame, self-reactive TCRalpha joins on TCR excision circles (TRECs), which are the products of secondary V/J recombination in the TCRalpha locus. Such joins are not expected if clonal deletion occurs, because the progenitor cell would be eliminated by apoptosis. To test the relative utilization of receptor editing vs clonal deletion, we determined the frequency of in-frame, male-specific joins on TRECs in male and female HYbeta transgenic mice. In comparison with female HYbeta transgenic mice, our analysis showed a lower frequency of TRECs with male-reactive V17J57 joins in male mice. Thus, it would appear that receptor editing is not a predominant tolerance mechanism for this self-Ag.     

Abnormal thymocyte development and production of autoreactive T cells in T cell receptor transgenic autoimmune mice.

Development of a C57BL/6-+/+ TCR transgenic mouse containing the rearranged TCR alpha- and beta-chain specific for the Db + HY male Ag results in production of a nearly monoclonal population of early thymocytes expressing the Db + HY reactive TCR. These thymocytes are autoreactive in H-2Db male mice and undergo clonal deletion and down-regulation of CD8. To study the effect of the lpr gene on development of autoreactive T cells, these transgenic mice were backcrossed with C57BL/6-lpr/lpr mice. T cell populations in the thymus and spleen were analyzed by three-color flow cytometry for expression of CD4, CD8, and TCR. The thymus of TCR transgenic H-2b/b lpr/lpr male mice had an increase in percent and absolute number of CD8dull thymocytes compared to TCR transgenic H-2b/b +/+ male mice. However, there was not a complete defect in clonal deletion, because clonal deletion and down-regulation of CD8 was apparent in both +/+ and lpr/lpr H-2Db HY+ male mice compared to H-2Db HY- female mice. The phenotype of splenic T cells was almost identical in TCR transgenic +/+ and lpr/lpr males with about 50% CD4-CD8- T cells and 50% CD8+ T cells. However, there was a dramatic increase in the SMLR proliferative response of splenic T cells from TCR transgenic lpr/lpr males compared to TCR transgenic +/+ males. To determine the specificity of this response, spleen cells from TCR transgenic lpr/lpr and +/+ mice were cultured with irradiated H-2b/b and H-2k/k male and female spleen cells. T cells from TCR transgenic C57BL/6-lpr/lpr male mice had an increased proliferative response to H-2b/b male spleen cells compared to T cells from TCR transgenic C57BL/6(-)+/+ male mice, but both lpr/lpr and +/+ mice had a minimal response to irradiated H-2b/b female or H-2k/k male or female stimulator cells. The splenic T cells from TCR transgenic lpr/lpr mice also had an increased specific cytotoxic activity against H-2b/b male target cells compared to TCR transgenic +/+ mice. These results demonstrate that there is a defect in negative selection of self-reactive T cells in the thymus of lpr/lpr mice and a defect in induction or maintenance of clonal anergy of self-reactive T cells in the periphery of lpr/lpr mice.     

Defects in the Bcl-2-regulated apoptotic pathway lead to preferential increase of CD25 low Foxp3+ anergic CD4+ T cells

Defects in the Bcl-2-regulated apoptotic pathway inhibit the deletion of self-reactive T cells. What is unresolved, however, is the nature and fate of such self-reactive T cells escaping deletion. In this study, we report that mice with such defects contained increased numbers of CD25(low)Foxp3(+) cells in the thymus and peripheral lymph tissues. The increased CD25(low)Foxp3(+) population contained a large fraction of cells bearing self-reactive TCRs, evident from a prominent increase in self-superantigen-specific Foxp3(+)Vβ5(+)CD4(+) T cells in BALB/c Bim(-/-) mice compared with control animals. The survival rate of the expanded CD25(low)Foxp3(+) cells was similar to that of CD25(high)Foxp3(+) CD4 T cells in vitro and in vivo. IL-2R stimulation, but not TCR ligation, upregulated CD25 on CD25(low)Foxp3(+)CD4(+) T cells in vitro and in vivo. The expanded CD25(low)Foxp3(+)CD4(+) T cells from Bim(-/-) mice were anergic but also had weaker regulatory function than CD25(high)Foxp3(+) CD4(+) T cells from the same mice. Analysis of Bim(-/-) mice that also lacked Fas showed that the peripheral homeostasis of this expanded population was in part regulated by this death receptor. In conclusion, these results show that self-reactive T cell escapes from thymic deletion in mice defective in the Bcl-2-regulated apoptotic pathway upregulate Foxp3 and become unresponsive upon encountering self-Ag without necessarily gaining potent regulatory function. This clonal functional diversion may help to curtail autoaggressiveness of escaped self-reactive CD4(+) T cells and thereby safeguard immunological tolerance.     

IFN-induced attrition of CD8 T cells in the presence or absence of cognate antigen during the early stages of viral infections

Profound lymphopenia has been observed during many acute viral infections, and our laboratory has previously documented a type I IFN-dependent loss of CD8 T cells immediately preceding the development of the antiviral T cell response. Most memory (CD44(high)) and some naive (CD44(low)) CD8 T cells are susceptible to IFN-induced attrition, and we show in this study that the IFN-induced attrition of CD8(+)CD44(high) T cells is associated with elevated activation of caspase-3 and caspase-8. We questioned whether TCR engagement by Ag would render CD8 T cells resistant to attrition. We tested whether a high concentration of Ag (GP33 peptide) would protect lymphocytic choriomeningitis (LCMV)-specific naive CD8 T cells (TCR transgenic P14 cells specific for the GP33 epitope of LCMV) and memory CD8 T cells (GP33-specific LCMV-immune cells) from depletion. Both naive P14 and memory GP33-specific donor CD8 T cells decreased substantially 16 h after inoculation with the Toll receptor agonist and IFN inducer, poly(I:C), regardless of whether a high concentration of GP33 peptide was administered to host mice beforehand. Moreover, donor naive P14 and LCMV-specific memory cells were depleted from day 2 LCMV-infected hosts by 16 h posttransfer. These results indicate that Ag engagement does not protect CD8 T cells from the IFN-induced T cell attrition associated with viral infections. In addition, computer models indicated that early depletion of memory T cells may allow for the generation for a more diverse T cell response to infection by reducing the immunodomination caused by cross-reactive T cells.     

The role of peripheral T-cell deletion in transplantation tolerance

The apoptotic deletion of thymocytes that express self-reactive antigen receptors is the basis of central (thymic) self-tolerance. However, it is clear that some autoreactive T cells escape deletion in the thymus and exist as mature lymphocytes in the periphery. Therefore, peripheral mechanisms of tolerance are also crucial, and failure of these peripheral mechanisms leads to autoimmunity. Clonal deletion, clonal anergy and immunoregulation and/or suppression have been suggested as mechanisms by which 'inappropriate' T-lymphocyte responses may be controlled in the periphery. Peripheral clonal deletion, which involves the apoptotic elimination of lymphocytes, is critical for T-cell homeostasis during normal immune responses, and is recognized as an important process by which self-tolerance is maintained. Transplantation of foreign tissue into an adult host represents a special case of 'inappropriate' T-cell reactivity that is subject to the same central and peripheral tolerance mechanisms that control reactivity against self. In this case, the unusually high frequency of naive T cells able to recognize and respond against non-self-allogeneic major histocompatibility complex (MHC) antigens leads to an exceptionally large pool of pathogenic effector lymphocytes that must be controlled if graft rejection is to be avoided. A great deal of effort has been directed toward understanding the role of clonal anergy and/or active immunoregulation in the induction of peripheral transplantation tolerance but, until recently, relatively little progress had been made towards defining the potential contribution of clonal deletion. Here, we outline recent data that define a clear requirement for deletion in the induction of peripheral transplantation tolerance across MHC barriers, and discuss the potential implications of these results in the context of current treatment modalities used in the clinical transplantation setting.     

Intestinal epithelial antigen induces mucosal CD8 T cell tolerance, activation, and inflammatory response

Intestinal autoimmune diseases are thought to be associated with a breakdown in tolerance, leading to mucosal lymphocyte activation perhaps as a result of encounter with bacterium-derived Ag. To study mucosal CD8(+) T cell activation, tolerance, and polarization of autoimmune reactivity to self-Ag, we developed a novel (Fabpl(4x at -132)-OVA) transgenic mouse model expressing a truncated form of OVA in intestinal epithelia of the terminal ileum and colon. We found that OVA-specific CD8(+) T cells were partially tolerant to intestinal epithelium-derived OVA, because oral infection with Listeria monocytogenes-encoding OVA did not elicit an endogenous OVA-specific MHC class I tetramer(+)CD8(+) T cell response and IFN-gamma-, IL-4-, and IL-5-secreting T cells were decreased in the Peyer's patches, mesenteric lymph nodes, and intestinal mucosa of transgenic mice. Adoptive transfer of OVA-specific CD8(+) (OT-I) T cells resulted in their preferential expansion in the Peyer's patches and mesenteric lymph nodes and subsequently in the epithelia and lamina propria but failed to cause mucosal inflammation. Thus, CFSE-labeled OT-I cells greatly proliferated in these tissues by 5 days posttransfer. Strikingly, OT-I cell-transferred Fabpl(4x at -132)-OVA transgenic mice underwent a transient weight loss and developed a CD8(+) T cell-mediated acute enterocolitis 5 days after oral L. monocytogenes-encoding OVA infection. These findings indicate that intestinal epithelium-derived "self-Ag" gains access to the mucosal immune system, leading to Ag-specific T cell activation and clonal deletion. However, when Ag is presented in the context of bacterial infection, the associated inflammatory signals drive Ag-specific CD8(+) T cells to mediate intestinal immunopathology.