Longitudinal remodeling of the tumor-immune microenvironment during regorafenib, nivolumab, and chemotherapy in advanced gastric cancer

Previous studies combining the multikinase inhibitor regorafenib with a PD-1 inhibitor showed promising activity in gastric cancer. We conducted a phase 1b/2 trial (NCT05394740) of first-line regorafenib, nivolumab, and chemotherapy in 30 patients with mismatch repair–proficient gastric cancer and investigated its impact on the tumor microenvironment. The primary end point was met; 22 of 30 patients showed objective tumor response (73%; 80% confidence interval, 60.3 to 83.8). Single-cell RNA and T cell receptor sequencing were performed on biopsy samples at baseline, during treatment, and at disease progression. The combination treatment increased tumor antigen–reactive CD8 ⁺ T cells and M1-like proinflammatory macrophages and decreased immunosuppressive and tumor-promoting populations, including activated CCR8 ⁺ regulatory T cells, LRRC15 ⁺ cancer-associated fibroblasts (CAFs), RGS5 ⁺ pericytes, and APLN ⁺ tip cells. At progression, LRRC15 ⁺ CAFs and APLN ⁺ tip cells remained reduced, activated CCR8 ⁺ regulatory T cells reexpanded, and additional treatment-resistant subsets emerged, including APOC1 ⁺ lipid-associated macrophages. In murine models of acquired resistance, therapeutic targeting of these suppressive immune cell populations with antibodies against CCR8 or TREM2 in combination with PD-1 blockade restored antitumor immunity. These findings demonstrate dynamic remodeling of the tumor microenvironment and suggest potential therapeutic targets.