Abstract A95: Distinct roles of type I and type II interferons in radiation-induced antitumor immunity against B16 melanoma.

Endogenous type I interferons (IFN-α/β) and interferon gamma (IFN-γ) have both been implicated in immune responses against cancer. Interestingly, studies done using viral infection models have demonstrated that IFN-α/β and IFN-γ play non-redundant roles in the immune system and they act synergistically, enhancing responses that are induced by the two different signaling pathways. However, this phenomenon, known as priming, has not been well-characterized in the context of anti-tumor immunity. Our lab previously demonstrated using a B16 murine melanoma model that single local high dose (15Gy) radiation therapy induced elevated levels of intratumoral IFN-γ, which had several downstream effects like increased MHC class I expression on tumor cells that in turn enhanced recognition by CD8+ T cells. Overall, these IFN-γ;-dependent effects led to increased tumor control, evident by significant decreased rate of tumor growth following treatment. In the current study, we hypothesize that local radiation therapy (RT) increases the levels of IFN-α/β within the tumor, and this is required to enhance IFN-γ responses so that maximal therapeutic potential can be achieved. Indeed, local RT resulted in an early, transient spike in intratumoral IFN-α levels about two days after treatment. Interestingly, an increase in IFN-γ levels was observed four days post-treatment and remained elevated for at least five days. When B16 tumors were grown in mice that lack functional IFN-α/β receptors (IFNABR KO), the subsequent increase in IFN-γ following RT was completely abrogated. The ability of RT in reducing tumor growth was also less effective in IFNABR KO mice than in wild type (WT) mice. To investigate the mechanism involved, we examined the transcription of interferon-inducible genes, including the CXCR3 chemokines CXCL9 (MIG) and CXCL10 (IP-10). Following RT, intratumoral transcript levels of MIG and IP-10 were increased to similar extents in both WT and IFNγKO mice, but remained at basal levels in IFNABR KO mice. This suggests that IFN-α/β, but not IFN-γ, may be contributing to the initial chemoattraction of lymphocytes into radiation-treated tumors. Using flow cytometry analysis of immune cell populations within the tumors, we confirmed that the levels of intratumoral CXCR3 chemokines correlate strongly with tumor infiltration by T lymphocytes. Our data suggest that IFN-α/β play a critical role in the increase of CXCR3 chemokine levels within the tumor after RT. These chemokines are important for the recruitment of activated T cells into the tumor, which then produce IFN-γ, increasing IFNγ-induced immune responses locally. To investigate whether exogenous IFN-α will improve the efficacy of radiation therapy, we genetically-engineered a B16 cell line that can be induced to express IFN-α by exposing these cells, named B16-iIFN-α, to a rapamycin-analog. When inoculated in mice, B16-iIFN-α cells form established tumors similar to that of parental B16 cells. Production of IFN-α by B16-iIFN-α cells can be switched on at specific time points in tumor-bearing mice by treating the mice with the rapamycin-analog. This is a novel and invaluable tool for examining the anti-tumor effects of a local increase in IFN-α, and can be modified to study the effects of other cytokines as well. Our preliminary data demonstrated that radiation and IFN-α combination therapy resulted in promising improvement in tumor growth control compared to either of the treatments alone. We are currently studying the immunological mechanisms involved, as well as fine-tuning the treatment regimen to harness the maximal therapeutic potential of this treatment strategy. Citation Format: Joanne Y.H. Lim, Scott A. Gerber, Edith M. Lord. Distinct roles of type I and type II interferons in radiation-induced antitumor immunity against B16 melanoma. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr A95.