CD4+ T Cell-mediated Cytotoxicity Eliminates Primary Tumor Cells in Metastatic Melanoma Through High MHC Class II Expression and Can Be Enhanced by Inhibitory Receptor Blockade

Metastatic melanoma is a rapidly progressing disease with high mortality rate and limited treatment options. Immunotherapy based on tumor-targeting cytotoxic T cell responses represents a promising strategy. To assist in its development, we examined the possibility and efficacy of using CD4 cytotoxic T cells. The regulatory mechanisms controlling CD4 T cell-mediated cytotoxicity were also investigated. We found that naturally occurring granzyme B and perforin-expressing CD4 cytotoxic T cells can be recovered from metastatic melanoma patients at significantly elevated frequencies compared to those from healthy controls. These CD4 cytotoxic T cells were also capable of killing autologous tumor cells harvested from metastatic melanoma, independent of CD8 T cells or any other cell types. However, several restricting factors were observed. First, the cytolytic activity by CD4 T cells required high MHC class II expression on melanoma cells, which was not satisfied in a subset of melanomas. Second, the granzyme B and perforin release by activated CD4 cytotoxic T cells was reduced after coculturing with autologous melanoma cells, characterized by low LAMP-1 expression and low granzyme B and perforin secretion in the supernatant. This suggested that inhibitory mechanisms were present to suppress CD4 cytotoxic T cells. Indeed, blockade of PD-1 and CTLA-4 had increased the cytolytic activity of CD4 T cells but was only effective in MHC class II high but not MHC class II low melanomas. Together, our study showed that CD4 T cell-mediated cytotoxicity could eliminate primary melanoma cells but the efficacy depended on MHC class II expression.