Abstract Immune checkpoint blockade therapy has shown limited efficacy in gastric cancer, with most patients developing resistance through mechanisms that remain incompletely defined. Here, we generate a comprehensive single-cell RNA sequencing atlas of 526,583 cells from gastric cancer patients treated with anti-PD-1 plus chemotherapy, analyzing paired pre- and post-treatment samples to capture dynamic resistance mechanisms. We identify two distinct resistance pathways that emerge during treatment. First, CEACAM5/6+ cancer cells are markedly enriched in pre-treatment non-responders and predict treatment failure. These CEACAM5/6+ epithelial cells show the highest tumor scores and correlate with increased regulatory T cell infiltration expressing CEACAM1, establishing an alternative checkpoint axis that bypasses PD-1/PD-L1 blockade. External validation in independent cohorts confirms CEACAM5/6 expression as a robust predictor of anti-PD-1 resistance. Second, we uncover a macrophage-driven inflammatory cascade central to treatment resistance. IL-1β+ macrophages serve as the primary source of NF-κB pathway activation across the tumor microenvironment, triggering downstream IL-6 production, Th17 cell differentiation, chronic inflammation and epithelial-mesenchymal transition. This macrophage module is significantly enriched in post-treatment non-responders, with TNF-high expressing monocyte-macrophages absent in responders but prevalent in resistant tumors. The resulting inflammatory milieu drives PD-L1 upregulation across multiple cell types, creating a self-reinforcing immunosuppressive niche. Collectively, these findings nominate CEACAM5/6+ epithelial cells and IL-1β+ inflammatory macrophages as actionable therapeutic targets for overcoming anti-PD-1 resistance in gastric cancer, providing a mechanistic framework and rational blueprint for next-generation combination immunotherapy strategies. Citation Format: Liudeng Zhang, Jian Chen, Yikai Luo, Lie Wang, Han Liang. CEACAM5/6+ cancer cell and IL1B+ macrophage-mediated resistance in anti-PD-1 treated gastric cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7746.
Zhang et al. (Fri,) studied this question.