Despite studies based on experimental models and clinical trials, the impact of T cell avidity on tumor immunity remains incompletely understood. We previously reported that sensitization of mice to a melanoma differentiation antigen, tyrosinase-related protein-2 (TRP-2, dopachrome tautomerase, dct), results in weak T cell responses unless combined with GM-CSF 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) epitope, that differ in their functional avidity. We cloned the T cell antigen receptor genes and used them to make transgenic mice. This generated a unique model system as it consists of distinct T cell receptor gene combinations with common antigenic specificity. The goal of this project is to use novel transgenic mouse models that we have developed at the NCI to test the hypothesis that T cell avidity is a critical determinant of tumor immunity. These studies will identify how T cell avidity correlates with tumor immunity and autoimmunity. We have recently reported on the intermediate avidity transgenic (Tg) mouse line. These mice 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 nave phenotype and proliferated in response to TRP-2 in vitro. In addition, transfer of in vitro-activated Tg T cells reduced B16 pulmonary tumor burden. In contrast, nave TcR Tg T cells did not become activated by the tumor-derived TRP-2 antigen. We next sought to determine the in vivo responses of the Tg T cells to endogenous and tumor-derived TRP-2. Adoptive transfer of nave 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 begun to study 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. In addition, B16 melanoma tumor growth is reduced in the Tg mice. When the high avidity T cells are transferred into wild-type mice and primed with a peptide-pulsed DC vaccine, B16 tumor growth is significantly slowed. Interestingly, the 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 stimulation, the tumor cells lose their susceptibility to killing by the T cells and evade immune responsiveness. Our on-going studies will address the mechanisms by which the higher avidity T cells become tolerized and how this influences the tumor growth.
