Despite studies based on experimental models and clinical trials, the impact of T cell avidity on tumor immunity remains incompletely understood. It is generally believed that higher avidity T cells would be more successful for cancer immunotherapy, but concern remains about loss of specificity, self-reactivity, and increased susceptibility to immune suppression. We previously reported that sensitization of mice to a melanoma differentiation antigen, tyrosinase-related protein-2 (TRP-2, dopachrome tautomerase, Dct), resulted in weak T cell responses unless combined with GM-CSF administration and CTLA-4 blockade. In collaboration with Enzo Bronte (Univ. Padua), we subsequently established three T cell lines with specificity for the TRP-2(180-188) peptide epitope, yet differed in their 'avidity'. We cloned the T cell antigen receptor (TcR) genes from these cell lines and used them to make transgenic mice. This generated a unique model system as it consists of distinct TcR gene combinations with common antigenic specificity. The goal of this project is to use these novel transgenic mouse models to test the hypothesis that T cell avidity is a critical determinant of tumor immunity and regulates susceptibility to immune suppression. These studies will identify how T cell avidity correlates with tumor immunity and autoimmunity and lead to more effective cancer therapies. We previously reported that the intermediate avidity transgenic (Tg) mouse line did not spontaneously develop autoimmune depigmentation, despite systemic expression of TRP-2 in the skin. Peripheral T cells from these TCR Tg mice exhibited a naive phenotype and proliferated in response to TRP-2 in vitro. However, transfer of in vitro-activated Tg T cells reduced B16 pulmonary tumor burden. Adoptive transfer of naive TcR Tg T cells into wild-type C57BL/6 mice, in combination with a TRP-2-pulsed dendritic cell (DC) vaccine, induced proliferation of the Tg T cells and resulted in migration of the Tg T cells into a subcutaneous B16 melanoma tumor. Although these tumor-infiltrating Tg T cells remained reactive against TRP-2, they did not reduce growth of the primary tumor. These data demonstrate that despite in vivo priming and retention of effector function, tumor-infiltrating T cells may fail to reduce tumor burden. This lack of tumor reduction suggests that the T cells are """"""""ignorant"""""""" of the developing tumor.More recently, we have studied the higher avidity Tg T cells. Mice bearing these T cells spontaneously develop autoimmune depigmentation, suggesting that they are primed by the endogenous TRP-2 antigen expressed by melanocytes. In addition, B16 melanoma tumor growth is reduced in the TcR Tg mice. When the high avidity Tg T cells are transferred into wild-type mice and primed with a peptide-pulsed DC vaccine, B16 tumor growth is significantly slowed. Interestingly, unlike the intermediate avidity T cells, the high avidity T cells become tolerized inside the tumor, as they lose their ability to secrete interferon-gamma. In parallel, there is a loss of MHC expression on the tumor cells, suggesting that in the absence of interferon-gamma production by Tg T cells, the tumor cells lose their susceptibility to killing and evade immune responsiveness. Interestingly, only the higher avidity Tg T cells are susceptible to tolerance induction by tumor-associated DC. In addition, we have noted that provision of CD4+ T cells preferentially helps the higher avidity T cells, improving immunity to recently established tumors, at the cost of increased autoimmune depigmentation. Our ongoing studies are aimed at understanding how these different T cell populations can be used to generate more effective anti-tumor responses without the adverse effects of autoimmunity and loss of responsiveness due to tolerance induction. We will also study the signaling patterns induced by both tolerogenic and competent antigen presenting cells.
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