Abstract 2339: High fat diet modulates host immunity and impairs the efficacy of combined radiation and immunotherapy

Introduction: Obesity is associated with a higher incidence of cancer, increased cancer mortality, impaired responses to radiation therapy (RT), and worse toxicity after RT. Paradoxically, recent studies suggest that obese patients may actually have improved outcomes with immunotherapy (IO), such as programmed death (PD)-1 inhibition, highlighting a link between the host’s metabolic state and immune homeostasis. We therefore hypothesized that a high fat diet could be used to alter the host’s immune response to improve the efficacy of combined RT and IO. Materials/Methods: 12-week old female Balb/c mice were injected with 5x104 syngeneic, luciferase tagged, triple-negative, murine breast cancer cells (4T1). Upon development of a palpable tumor, the mice were randomized to either an ad lib (AL) or high fat (HF), defined as 45% calories from fat, diet. Additionally, each cohort was treated with either diet alone (D), diet plus RT (4 Gy in 3 daily fractions), diet plus PD-1 inhibitor, or a combination of all three (Combo). On day 26, the mice were injected with luciferin and tumor growth was measured using the IVIS bioluminescence in vivo imaging system. At time of sacrifice, primary tumors and spleen were harvested for downstream analysis of immune markers via RNA microarray and flow cytometry. Results: The median overall survival (OS) for animals fed either an HF or AL diet was 36 and 39 days, respectively (NS). The difference in OS however became much more apparent between the individual treatment arms: HF+αPD1 vs AL+αPD1 (37d vs 41.5d, p=0.041); HF+RT vs AL+RT (36d vs 45d, p=0.039); HF+Combo vs AL+Combo (39d vs 48d, p=0.037). HF fed mice also developed metastases sooner than AL fed mice across all arms (D: 22d vs 30d; RT: 24d vs 32d; αPD1: 24d vs 34d; Combo: 27d vs 37d, p=0.047). Microarray analysis of primary tumor tissue revealed an at least 1.5 fold increase expression of cytokines associated with inflammation including IFN-y, TNF-α, IL-17F, IL-22 and IL-1β in the HF diet groups while AL fed mice demonstrated increased expression of the anti-inflammatory cytokines IL-10 and TGF-β. Consistent with the increase in inflammatory markers after a HF diet, flow cytometric analysis of splenocytes revealed a significant decrease in the number of CD4+CD25+Foxp3+ regulatory T cells in the HF group compared with AL fed mice (3.3% versus 7.68%, respectively; p = 0.006). Notably, these Tregs also produced less IL-10 compared with those from AL mice (24.4% vs 54.5%, p=0.037). Conclusions: We demonstrate that a HF diet impairs the efficacy of RT, IO, and combined therapy. This is associated with the development of a pro-inflammatory state demonstrated by increased expression of pro-inflammatory cytokines and a decrease in the number and activity of regulatory T cells. Based on these findings, we conclude that diet can directly impact treatment efficacy and should be considered as a variable in future clinical studies. Note: This abstract was not presented at the meeting. Citation Format: Gregor Manukian, Brittany A. Simone, Tiziana DeAngelis, Kevin Ko, Charles Kivolowitz, Sylvester Jusu, Nicole L. Simone. High fat diet modulates host immunity and impairs the efficacy of combined radiation and immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2339.