Coordinated macrophage and T cell interactions mediate response to checkpoint blockade in colorectal cancer

Colorectal cancer (CRC) accounts for 10% of all cancer cases and is the second leading cause of cancer-related deaths worldwide. Immunotherapies have significantly advanced over the past decades, marking a major breakthrough in cancer treatment. While anti-PD-1 therapy is utilized in both local and advanced disease, up to 50% of mismatch repair deficient (MMRd) and most mismatch repair proficient (MMRp) CRC fail to respond. Using orthotopic animal and patient-derived models of CRC, along with single cell and spatial analyses, we determined that colocalization and interactions between MHC + C1Q + CXCL9 + macrophages and TCF + PRF1 + T cell subsets are associated with control of tumor growth during anti-PD-1 treatment. In contrast, resistance is associated with upregulation of TIM3, LAG3, TIGIT, and PD-1 expression on T cells, and tumor infiltration by immunosuppressive TREM2 + macrophages and monocytes in T cell excluded zones. A novel combinatorial checkpoint blockade targeting TREM2, LAG3, CTLA4, and PD-1 achieves up to 100% tumor clearance in MMRd CRC and > 70% in MMRp CRC models, compared to 0% with anti-PD-1 monotherapy. This approach induces durable anti-tumor immune memory, mediated by coordinated interactions among MHC + macrophages, CD4+/CD8 + T cells, and TCF + T cells. It also reduced infiltration by immunosuppressive myeloid cells and T cell exhaustion. Together this study identifies key T cell and macrophage subsets mediating the efficacy of immunotherapy in overcoming immune escape in both MMRd and MMRp CRC settings.