TIM4 enables large peritoneal macrophages to cross-present tumor antigens at early stages of tumorigenesis

Cross-presentation of ingested tumor antigens by cells of the myeloid lineage is critical to shape anti-tumoral immune responses. However, the identity of cross-presenting macrophages, the cellular mechanism and the impact on anti-cancer CD8 T cell responses along tumor progression needs to be clarified. Here we examined the capacity of TIM4+ large peritoneal macrophages (LPM) to capture and cross-present antigens of incipient peritoneal metastasis in a model of ovarian cancer. We show that TIM4+ LPM, the most abundant myeloid population in the cavity, avidly engulf cancer cells and cross-present tumor-associated antigens, specifically at tumor inception. The phosphatidylserine (PS) receptor TIM4 promotes maximal uptake and triggers inflammatory and metabolic gene programs coupled to cytoskeletal remodeling and upregulation transcriptional signatures related to antigen processing. At the cellular levels, TIM4 is recruited with F-actin at the phagocytic cup and translocates in nascent phagosomes, controlling the kinetic of phagosomal acidification and cargo degradation. TIM4 deletion abrogates cross-presentation of tumor-associated antigens and blunts expansion of effector CD8 T cells at tumor inception. In addition, targeting tumor antigens to LPM by PS liposomes can trigger CD8 T cell activation. Together these results suggest that TIM4 enables LPMs to scan the antigenic content of incoming tumor cells to promote immune surveillance by CD8 T cells and, at defined temporal windows, may be exploited for therapeutic purposes. Overall design: To understand phagocytosis-mediated transcriptional changes in LPM in presence or absence of scavenger receptor Tim4, we performed sequencing of LPM 24 hours after challenge with ID8 expressing ZsGreen and OVA. In detail, we sorted LPM (F4/80high MHCIIlow) from naïve WT and TIM4 KO mice, and LPM ZsGreen+ (F4/80high MHCIIlow ZsGreen+)from WT ID8 and TIM4 KO ID8 mice.