Early immune remodeling steers clinical response to frontline chemoimmunotherapy in advanced gastric cancer

Abstract Adding anti–programmed cell death protein 1 (anti–PD-1) to 5-fluorouracil (5-FU)/platinum improves survival in some advanced gastroesophageal adenocarcinomas (GEA). To understand the effects of chemotherapy and immunotherapy, we conducted a phase II first-line trial (n = 47) sequentially adding pembrolizumab to 5-FU/platinum in advanced GEA. Using serial biopsy of the primary tumor at baseline, after one cycle of 5-FU/platinum, and after the addition of pembrolizumab, we transcriptionally profiled 358,067 single cells to identify evolving multicellular tumor microenvironment (TME) networks. Chemotherapy induced early on-treatment multicellular hubs with tumor-reactive T-cell and M1-like macrophage interactions in slow progressors. Faster progression featured increased MUC5A and MSLN containing treatment resistance programs in tumor cells and M2-like macrophages with immunosuppressive stromal interactions. After pembrolizumab, we observed increased CD8 T-cell infiltration and development of an immunity hub involving tumor-reactive CXCL13 T-cell program and epithelial interferon-stimulated gene programs. Strategies to drive increases in antitumor immune hub formation could expand the portion of patients benefiting from anti–PD-1 approaches. Significance: The benefit of 5-FU/platinum with anti–PD-1 in first-line advanced gastric cancer is limited to patient subgroups. Using a trial with sequential anti–PD-1, we show coordinated induction of multicellular TME hubs informs the ability of anti–PD-1 to potentiate T cell–driven responses. Differential TME hub development highlights features that underlie clinical outcomes.