Tumor-derived retinoic acid promotes immune suppression in sarcoma

Abstract

Abstract Sarcoma is a rare but heterogeneous collection of fatal malignancies that arise from mesenchymal tissue such as fat, muscle, cartilage, etc. Recent efforts to utilize immunotherapies such as immune checkpoint blockade in sarcoma have demonstrated efficacy only in a small percentage of patients, underscoring the importance of elucidating additional immune evasion mechanisms. Immunosuppressive tumor associated macrophages (TAMs) are abundant in the solid tumor microenvironment (TME) and pose a major barrier to effective anti-tumor immunity. Though circulating monocytes are a major reservoir for TAMs, how the TME governs intratumoral monocyte differentiation is poorly understood. In multiple types of mouse and human sarcoma, we found that the TME induced tumor cells to produce high levels of the tissue metabolite retinoic acid (RA). RA promoted intratumoral monocytes to differentiate into immunosuppressive TAMs and inhibited monocyte differentiation into dendritic cells. Sarcomas genetically modified to produce limited RA harbored a more stimulatory myeloid compartment and enhanced T cell dependent anti-tumor immunity. Notably, RA inhibition demonstrated robust synergy with immune checkpoint blockade therapy. Together, our findings suggest that RA is a local tissue metabolite that promotes myeloid-mediated immune suppression in sarcoma.