Baseline Tumor Features and Systemic Immune Dynamics Underlying Efficacy in MSS Metastatic Colorectal Cancer Treated with Regorafenib, Ipilimumab, and Nivolumab.

Microsatellite-stable metastatic colorectal cancer (MSS mCRC) remains resistant to conventional immunotherapies. In a phase I trial, we observed encouraging efficacy of the combination of regorafenib, ipilimumab, and nivolumab (RIN), with a 27.6% overall response rate and a median overall survival of 20 months. The most pronounced benefits were observed in patients without liver metastases. To uncover immunologic mechanisms underlying response and resistance, we performed correlative studies of the tumor microenvironment (TME) and systemic immune features. Tumor biopsies from 8 patients and peripheral blood samples from 29 patients with MSS mCRC were collected and analyzed at baseline and during treatment. At baseline, tumors from good responders exhibited enhanced proliferation, DNA repair pathways, and STING expression, whereas poor responders showed enrichment of complement and metabolism pathways. In peripheral blood, good responders had a higher CD4/CD8 T-cell ratio, increased dendritic cells, and intact type 1 cytokine responses. In contrast, poor responders exhibited more effector T-cell differentiation, elevated immune checkpoint molecule expression, and increased DNA damage in lymphocytes. In good responders, RIN therapy increased tumor-infiltrating lymphocytes and upregulated immune activation genes, accompanied by heightened T-cell proliferation and activation in peripheral blood, including the expansion of low-frequency T-cell receptor clones in CD8+ T cells. Patients with liver metastases exhibited T-cell senescence and metabolic hyperactivation, correlating with therapeutic resistance. These findings highlight that preexisting tumor immunogenicity and T-cell functional capacity are associated with response to RIN therapy and that RIN treatment may facilitate both systemic T-cell activation and local TME modulation.