Altered functional and biochemical response by CD8+ T cells that remain after tolerance

Fas-mediated T cell deletion potentiates tumor antigen-specific tolerance in a mouse model of prostate cancer

A pivotal obstacle to cancer immunotherapy is peripheral T cell tolerance to tumor-associated antigens (TAAs). Tolerance induction among mature T cells in the periphery operates through a variety of mechanisms, including anergy and apoptosis. Although Fas-FasL-mediated apoptosis is a well-defined tolerance inducing mechanism, direct evidence of its interference with TAA-specific immunity in vivo is still lacking. In this report, we used the TRAMP mouse, which expresses SV40 large T antigen (Tag) preferentially in the prostate and develops prostate tumors, as a model system to address the role of Fas-mediated apoptosis in regulating peripheral T cell tolerance. Using RT-PCR and tetramer staining to quantify TAA-specific TCR-expressing cytolytic T lymphocytes (CTLs), we have shown the presence of TAA-specific CTLs at higher levels in TRAMP mice than in syngeneic C57Bl/6 mice. Tag-specific immunization led to the expansion of Tag-specific CTLs in C57Bl/6 mice, and to their elimination in TRAMP mice. Interestingly, in TRAMP mice with deficient Fas (Hybrid TRAMP-lpr/lpr), Tag-specific CTL elimination in response to Tag immunization did not take place. The results of cytolytic-function assays were consistent with induction and elimination patterns of TAA-specific CTLs and those of RT-PCR and tetramer staining. In conclusion, our data show that Fas-mediated TAA-specific CTL apoptosis contributes to T cell tolerance and suggest that such tolerance could be potentiated following TAA-specific immunization.

Peripheral CD8+ T cell tolerance to self-proteins is regulated proximally at the T cell receptor

CD8(+) T cell tolerance, although essential for preventing autoimmunity, poses substantial obstacles to eliciting immune responses to tumor antigens, which are generally overexpressed normal proteins. Development of effective strategies to overcome tolerance for clinical applications would benefit from elucidation of the immunologic mechanism(s) regulating T cell tolerance to self. To examine how tolerance is maintained in vivo, we engineered dual-T cell receptor (TCR) transgenic mice in which CD8(+) T cells recognize two distinct antigens: a foreign viral-protein and a tolerizing self-tumor protein. Encounter with peripheral self-antigen rendered dual-TCR T cells tolerant to self, but these cells responded normally through the virus-specific TCR. Moreover, proliferation induced by virus rescued function of tolerized self-tumor-reactive TCR, restoring anti-tumor activity. These studies demonstrate that peripheral CD8(+) T cell tolerance to self-proteins can be regulated at the level of the self-reactive TCR complex rather than by central cellular inactivation and suggest an alternate strategy to enhance adoptive T cell immunotherapy.

Deletion is neither sufficient nor necessary for the induction of peripheral tolerance in mature CD8+ T cells

Previous reports have demonstrated clonal deletion of CD8(+) T cells during peripheral tolerance induction to tissue antigens. However, direct evidence demonstrating a causal connection between deletion and tolerance has not been reported because of model limitations in which the tissue antigens were expressed in vital organs. Thus, studies were initiated in a mouse model where expression of a membrane-bound ovalbumin fusion protein (mOVA) was driven by a prostate specific androgen regulated probasin promotor, providing restricted expression in a non-vital organ where antigen levels can be abrogated through androgen deprivation. Adoptive transfer of mOVA specific CD8(+) T cells (OT-I) was used to assess the development of peripheral tolerance. Proliferation of OT-I cells was observed, as was partial deletion of transferred OT-I cells. Although deletion occurred, the long-term persistence of a stable level of OT-I cells was observed. Importantly, the persistent OT-I cells lost antigen responsiveness within 3 weeks of transfer. Castration resulted in loss of high-level prostate mOVA expression, with a resultant abrogation of tolerance induction, but surprisingly did not affect the deletion rate of OT-I cells. In contrast, abrogation of deletion through the adoptive transfer of OT-I cells from third generation CD95-deficient mice had no effect on tolerance induction. These data demonstrate the necessity for continued expression of tissue antigen throughout the establishment of peripheral tolerance. Furthermore, these findings demonstrate that deletion is neither sufficient nor required for CD8(+) T-cell tolerance to tissue antigens, suggesting that regulatory events independent of deletion are necessary for peripheral tolerance induction to prostate antigens.

The Fate of Low Affinity Tumor-Specific CD8+ T Cells in Tumor-Bearing Mice

A major challenge in tumor immunology is how best to activate the relatively low avidity self-specific and tumor-specific T cells that are available in the self-tolerant repertoire. To address this issue, we produced a TCR transgenic mouse expressing a class I-restricted hemagglutinin (HA)-specific TCR (clone 1 TCR) derived from a mouse that expressed HA as a self-Ag in the insulin-producing beta cells of the pancreatic islets (InsHA) mice. Upon transfer of clone 1 TCR CD8(+) T cells into InsHA mice, very few cells were activated by cross-presented HA, indicating that the cells were retained in InsHA mice because they ignored the presence of Ag, and not because they were functionally inactivated by anergy or tuning. Upon transfer into recipient mice in which HA is expressed at high concentrations as a tumor-associated Ag in spontaneously arising insulinomas (RIP-Tag2-HA mice), a high proportion of clone 1 cells were activated when they encountered cross-presented tumor Ag in the pancreatic lymph nodes. However, the activated cells exhibited very weak effector function and were soon tolerized. The few activated cells that did migrate to the tumor were unable to delay tumor progression. However, when HA-specific CD4 helper cells were cotransferred with clone 1 cells into RIP-Tag2-HA recipients and the mice were vaccinated with influenza, clone 1 cells were found to exert a significant level of effector function and could delay tumor growth. This tumor model should prove of great value in identifying protocols that can optimize the function of low avidity tumor-specific T cells.

The T-cell repertoire available for recognition of self-antigens

Autoimmunity can be viewed as the price we pay in order to have a highly diverse TCR repertoire. This is not to say that every effort isn't made by the immune system to prevent autoimmune disease. During thymic development, and later in the periphery, the TCR repertoire is continuously purged of cells that are activated by self-antigens. Other lymphocytes are recruited to join a legion of T regulatory cells that assist in preventing 'friendly fire'. Recent studies continue to reveal the importance of the composition of the T cell repertoire in predisposing an individual to autoimmune disease.