Abstract 254: Dietary fish oil and aspirin to augment anti-PD-1 immunotherapy in metastatic melanoma

Introduction: First line anti-PD-1 immune checkpoint inhibition (αPD-1 ICI) induces objective responses in less than 50% of metastatic melanoma patients. Aspirin (ASA) use is associated with improved responsiveness to ICI, with inhibition of myeloid-induced immunosuppression in the tumor microenvironment (TME) a purported mechanism. Cyclooxygenase modulators ASA (pharmacologic inhibitor) and dietary fish oil (FO, substrate) shift myeloid and T cell populations to anti-inflammatory, pro-resolution phenotypes in immunity and cancer. Methods: C57-BL6/J mice were injected with YUMM 1.7 melanoma in the flank. Mice were fed control chow and water (CTL) or omega-3 rich (FO) chow (10% w/w, 30%kcal/kcal), given ASA in drinking water (600 ug/mL), or FO/ASA combined (COMBO) starting 12 days post tumor implantation, the same day as starting intraperitoneal αPD1 or IgG2a vehicle (q3-4 days). An alternative cohort received pre-αPD-1 treatment ASA and FO starting day 7. Tumors were assessed for growth, harvested, and characterized via flow cytometry on day 32. Results: When diets and water were started concurrent with αPD-1/IgG2a, FO decreased tumor volume vs. CTL for both vehicle (28%) and αPD-1 (27%) treatments; ASA and COMBO (p = 0.0501) did not. Compared to CTL, FO and COMBO increased monocytes (CD45+, CD11b+, Ly6C+, Ly6G-) and PD-L1+ monocytes in the TME with no effect on tumor associated macrophages (TAMS); ASA decreased the percent of Arginase 1+, CD206+ TAMs. ASA increased PD1+ CD8 T cells, while COMBO increased CD4 and CD8 T cells including LAG3+ and PD1+ CD8 T cells. When diets and water were started on day 7, FO + αPD-1 decreased tumor volume (21%) vs. αPD-1 controls; ASA and COMBO did not. Compared to CTL, FO and COMBO similarly altered immune populations as above, with FO additionally increasing total CD3+ T-cells, PD-L1+ CD4 T cells, and PD-1+ CD8 T cells; COMBO increased dendritic cells (CD11c+, MHCII+, F4/80-). TAMS were not assessed. Results significant (p< 0.05) by ANOVA or t-test unless specified. Food eaten, water imbibed, and weight did not vary between groups. Conclusions: Dietary fish oil impaired in vivo melanoma tumor growth with and without αPD-1 ICI despite increases in classically deleterious monocyte populations, while aspirin decreased M2-like macrophages in the TME without alterations in tumor growth. Modulation of the cyclooxygenase axis may be a cost-effective method to skew TME immune populations to favor αPD-1 immunotherapy responses in