ICOS co-stimulation in combination with CTLA4-blockade remodels intra-tumoral T cells to effector T cells and macrophages to critical anti-tumor phenotype

We have previously demonstrated synergy between ICOS co-stimulation (IVAX; ICOSL-transduced B16/F10 cellular vaccine) and CTLA-4 blockade in anti-tumor therapy. In this study, we performed an unbiased protein and gene expression analysis of tumor-infiltrating cells by Cytof and single-cell RNA-sequencing after combination therapy and observed significant remodeling of both the lymphoid and myeloid compartments. Interestingly, compared to tumors treated with anti-CTLA-4 monotherapy, tumors treated with the combination therapy were enriched in Th1 CD4 T cells, effector CD8 T cells, and M1-like macrophages. These M1-like tumor-associated macrophages (TAMs) were critical to the therapeutic efficacy of anti-CTLA-4 combined with IVAX or anti-PD-1. Depletion of macrophages with clodronate caused a significant reduction of effector CD4 and CD8 T cells in the tumor and weakened anti-tumor functions of the remaining effector cells. The recruitment and polarization of M1-like TAMs required IFN-gamma. Therefore, in this study, we show that there is a positive feedback loop between intratumoral effector T cells and TAMs, in which the IFN-gamma produced by the T cells polarizes the TAMs into M1-like phenotype, and the TAMs, in turn, reshape the tumor microenvironment to facilitate T-cell infiltration, immune function, and tumor rejection.