Abstract 4360: Inhibition of HSP90 enhances T cell-mediated antitumor immune responses through expression of interferon-alpha response Genes

Recently, T cell based immunotherapies have moved to the forefront of cancer immunotherapy with the success of Adoptive T cell therapy (ACT) and Immune checkpoint blockade. ACT, where patients are treated with tumor infiltrating T cells (TILs), conferred a clinical response rate of ∼50%. Treatment with anti-CTLA4 therapy, Ipilimumab, conferred response rates of 10-20%, greatly improving the overall survival of patients with advanced melanoma. Despite the encouraging outcomes, there are relatively low response rates coupled with the delay of weeks to months before tumor shrinkage can be appreciated. Thus, understanding mechanisms of resistance to immune therapies, to improve response rates, shorten time to treatment effect and developing predictive biomarkers of response are vital to the care of melanoma patients. In order to identify possible resistance mechanisms to immunotherapy, a high-throughput in vitro screen with 850 different bio-active compounds (Selleckchem), was designed to search for agents that could either increase or decrease the resistance of melanoma tumor cells to T cell mediated killing. Paired tumor samples and TILs from melanoma patients were used to assess which compounds when used to treat the melanoma cell lines can enhance the cytotoxic activity of the TILs against the paired melanoma sample, using a flow cytometry based assay in which active caspase 3 was used as a read out of apoptosis. We identified heat shock protein 90 (HSP90) inhibitors amongst compounds that improved T cell mediated cytotoxicity. We show that treatment with the HSP90 inhibitor ganetespib (Synta) greatly improves T cell mediated cytotoxicity of both human and murine cancer cells lines in vitro. Furthermore, in vivo murine studies using the MC38/gp100 tumor model show that ganestespib in combination with anti-CTLA4, resulted in superior antitumor effect and survival compared to either treatment alone (Average tumor volume at day 21 of treatment: Vehicle 294.3mm3, α-CTLA4 193 mm3, Ganetespib 237.5 mm3 and Ganetespib + α-CTLA4 105.8 mm3, P < 0.0001). Microarray analysis of human cell lines treated with ganetespib in vitro revealed an increase in interferon alpha (IFN-α) response genes including IFIT1, IFIT2, IFIT3 and IFIH1. Silencing IFIT2 abrogated the synergy observed with ganetespib treatment and T cell mediated killing, suggesting that the IFN-α response pathway plays an important role in this combination therapy. We are further elucidating the role of thes