Abstract 636: Sub-lethal irradiation of diverse human carcinoma cells imparts enhanced and sustained expression of important modulators of effector CTL activity

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Current cancer immunotherapy (CIT) approaches can efficiently introduce anti-tumor effector T-cells in patients, however, tumor cells can be immunosuppressive and directly inhibitory to T-cell activity. Previous data suggests that human tumor cells surviving radiation are phenotypically modulated making them better targets for CTLs. We examined several tumor cell lines (colorectal, breast and prostate) for their response to various doses of radiation (0-10Gy). Experiments quantified changes in the expression of genes that could result in altered effector CTL activity (OX40L, 41BBL, PD-L1, ICOSL and CD70) against tumor cells. Following irradiation, changes in expression of effector costimulatory molecules was examined in surviving and proliferating tumor cells. In human tumor cells expressing similar levels of major histocompatibility complex class I (MHC-I) and tumor associated antigens (TAA) following radiation, differences in CTL activity correlated with modulation of two positive costimulators, OX40L and 41BBL. The genes upregulated in our study (41BBL and OX40L) provide positive signals to killer T-cells that can attack tumor cells. Increased expression occurred when radiation was delivered either as a single dose or in fractions and could be observed as long as 7-days post-irradiation. We also found that radiation could alter gene expression in tumor cells via epigenetic mechanisms such as histone modifications of specific promoters. Furthermore we discovered that altered expression of OX40L and 41BBL (but not CD70, ICOSL or PD-L1) correlated with enhanced killing of irradiated tumor cells by both CEA and MUC-specific CTLs in cytotoxicity assays and this lysis was reversed by costimulatory blocking or gene-knockdown. We also observed enhanced activation and survival of CTLs exposed to irradiated tumor cell lines as well as exposed to ex vivo irradiated CD326+ tumor cells isolated from patient tumors. Overall, the results of our studies suggest that radiation can be used to make human tumors more amenable to effector CTL attack. Significance: The full role of IR as an independent immune-enhancer, in the absence of cell death or in radio-resistant tumors, remains unclear. Our data addresses a critical barrier to progress in improving combination radiation-cancer immunotherapy (RT-CIT) strategies by identifying mechanisms responsible for increased killing of irradiated tumor cells by effector CTLs. This “immunogenic modulation” of tumor cells is a mechanism different from ICD (requiring death of tumor cells and subsequent antigen processing and presentation by dendritic cells) that could be invoked independently, or to complement ICD approaches. This approach represents an alternate way of triggering important T-cell signal pathways that does not require the use of agonist antibodies. Citation Format: Anita Kumari, Orpha Rachel Mott, Ercan Cacan, Susanna F. Greer, Charlie T. Garnett. Sub-lethal irradiation of diverse human carcinoma cells imparts enhanced and sustained expression of important modulators of effector CTL activity. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 636. doi:10.1158/1538-7445.AM2014-636