Abstract 4363: Loss of PTEN promotes resistance to T cell-mediated immunotherapy

T cell-mediated immunotherapies are promising cancer treatments. However, most patients still fail to respond to these therapies. The molecular determinants of immune resistance are poorly understood. Here, we interrogated the role of loss of expression of the tumor suppressor, PTEN, in immune resistance. In preclinical studies, we found that silencing PTEN in tumor cells inhibited T cell-mediated tumor killing and decreased T cell trafficking into tumors. In clinical studies, we observed that tumors with loss of PTEN had significantly less CD8+ T cell infiltration than PTEN-present tumors. In addition, 26% of melanomas that did not yield successful TIL growth demonstrated PTEN loss, which was more frequent than was observed in tumors that yielded successful TIL growth (11%). We further validated the association between reduced number and impaired function of TIL with PTEN loss using another independent cohort, TCGA dataset for SKCM. More importantly, we analyzed clinical outcomes of metastatic melanoma patients treated with the FDA-approved anti-PD-1 antibodies. Our analysis demonstrates that a greater reduction in tumor burden was achieved by PD-1 blockade in PTEN present patients, when compared with PTEN absent patients. To decipher the factors mediating the immunosuppressive effects of PTEN loss, we determined the expression profiles of tumor cells with or without PTEN expression. Our results indicated that PTEN loss increased the production of immunosuppressive factors, including CCL2 and VEGF. Anti-VEGF blocking antibody improved anti-tumor activity of transferred tumor-reactive T cells and enhanced tumor infiltration of transferred T cells in PTEN-silenced tumors. These results suggest that loss of PTEN can facilitate the resistance of T cell-mediated immune responses by increasing the expression of immunosuppressive factors. Given that PTEN loss results in activation of the PI3K pathway, we evaluated the efficacy of immunotherapy in combination with a selective PI3Kβinhibitor to treat spontaneously developed BRAF mutant, PTEN null melanomas in genetically engineered mouse models. Our result showed that the combination of PI3Kβ inhibitor and anti-PD-1 significantly delayed tumor growth in tumor-bearing mice. Mice treated with this combination had a median survival time of 28 days, which is longer than the survival time of mice treated with either therapy. Increased numbers of T cells at tumor sites were found in mice receiving the combination therap