Immunotherapy-activated T cells install a subset of late-stage activated M1-like macrophages that are critical for therapeutic efficacy

Total tumor clearance through immunotherapy is associated with a fully coordinated innate and adaptive immune response, but knowledge on the exact contribution of each immune cell subset is limited. We show that therapy-induced intratumoral CD8 T cells recruited and installed late-stage activated M1-like macrophages, which were critical for effective tumor control in two different murine models of cancer immunotherapy. The activated CD8 T cells summon these macrophages into the tumor and their close vicinity via CCR5-signaling. Exposure of non-polarized macrophages to activated T-cell supernatant and tumor lysate recapitulates the late-stage activated and tumoricidal phenotype in vitro.The transcriptomic signature of these macrophages is also detected in a similar macrophage population present in human tumors and coincides with clinical response to immune checkpoint inhibitors. The requirement of a functional co-operation between CD8 T cells and effector macrophages for effective immunotherapy gives warning to combinations with broad macrophage targeting strategies. To gain an understanding of different subsets of intratumoral M1-like macrophages, we performed single cell RNA-sequencing (scRNAseq) on MHCII+ and MHCII- macrophage populations (CD11b+F4/80+) isolated from RMA-Qa1b-/- tumors that responded to therapeutic vaccination.