T cell Migration and Target-Killing after a Potent Stimulation

It has been suggested that anti-tumor T cells specifically traffic to the tumor site where they effect tumor destruction. Furthermore, non-mutated tissue differentiation antigens expressed by tumors are attractive targets for cancer immunotherapy, but the consequences of a highly effective antitumor immune response on self-tissue have not been fully characterized. To test whether tumor-reactive CD8+ T cells specifically home to tumor and are involved in "on-target" toxicities we employed the melanoma/melanocyte-specific TCR-transgenic pmel-1 model. We assessed the trafficking of the gp100-specific pmel-1 cells to large, vascularized tumors that express or do not express the target antigen. Activation of tumor-specific CD8+ pmel-1 T cells with IL-2 and vaccination with an altered peptide ligand caused regression of gp100-positive tumors (B16) but not gp100-negative tumors (MCA-205). Surprisingly, we found approximately equal, and very large numbers of pmel-1 T cells (>25% of all lymphocytes) infiltrating into both the antigen-positive and antigen-negative tumors. We also found evidence of massive infiltration and proliferation of activated anti-tumor pmel-1 cells in a variety of peripheral tissues, including lymph nodes, liver, spleen, eye, and lungs. Most importantly, evidence for T cell function, as measured by the production of interferon-gamma, release of perforin and activation of caspase-3 in the target cells, was confined to antigen-expressing tumor. We thus conclude that CD8+ T cell-mediated destruction of tumor is the result of specific T cell triggering at the tumor site. The ability to induce ubiquitous homing and specific tumor destruction may be important in the case of non-inflammatory metastatic tumor foci though may have negative implication in the case target antigen-rich immune-privileged eye. Indeed further investigation revealed that the infusion of ex vivo expanded adoptively transferred melanoma/melanocyte-specific CD8+ T cells that mediated robust tumor killing also induced autoimmune destruction of melanocytes in the eye. This severe autoimmunity was associated with the upregulation of MHC class I molecules in the eye and high levels of IFN-gamma derived from both adoptively transferred CD8+ T cells and the host. Furthermore, ocular autoimmunity required the presence of the IFN-gamma receptor on target tissues. Data compiled from >200 eyes and tumors in 10 independently performed experiments revealed a highly significant correlation (P < 0.0001) between the efficacy of tumor immunotherapy and the severity of ocular autoimmunity. While non-mutated shared antigens might be burdened with potential on-target toxicities, their relative broad and high level of expression amongst melanoma patients still make them valuable targets for adoptive immunotherapy. Finding a means to block migration of tumor/self-reactive T cells into critical tissues such as the eye, while still maintaining tumor-homing capabilities might prove vital. Many factors involved in T cell homing into the CNS and eye have been described. However many of these factors also appear to be important in T cell homing to tumors. One molecule, NCAM (CD56), is expressed in the eye and might be of particular interest. Important in ocular development, NCAM appears to play a role in cell-cell adhesion via homo interactions and is expressed on T cells found during an active ocular inflammatory event. In concert with this notion we found NCAM to be expressed on treatment TIL of some melanoma patients experiencing ocular autoimmunity. We found that the administration of a NCAM blocking antibody prevented T cell migration into the eye but did not effect infiltration into the tumor or inhibit immunotherapy. Thus blocking NCAM might provide a means in which to redirect tumor/self reactive T cells from critical tissues while still targeting broadly shared tumor antigens. The findings may have important implications in immunotherapies targeting antigens expressed in sensitive tissues.