Gastric cancer tertiary lymphoid structures in the muscularis propria are associated with therapy resistance and promote immunosuppression.

Tertiary lymphoid structures (TLS) are local immune microenvironments within tumors housing T cells and B cells, are often coordinating anti-tumor immunity and associated with better responses to immunotherapy. Here, we use H&E morphological assessment and multiplex immunofluorescence to analyze surgical tumor specimens from 71 patients with gastric cancer (GC) after treatment with neoadjuvant anti-PD-1 therapy, chemotherapy, and subsequent gastrectomy. We find increased TLS density and area in the muscularis propria of treatment-resistant GCs. These muscularis TLS are structurally and functionally impaired, with increased naïve B cell presence but reduced germinal center B cell infiltration. Mechanistically, PLA2G2A+ tumor cells accumulate adjacent to disrupted muscularis TLS, and PLA2G2A treatment in vitro increases differentiation of a fibroblast cell line into PDGFRA+ inflammatory cancer-associated fibroblasts (iCAF), which are linked to CXCL14 secretion and recruitment of naïve B cells. In vitro, PLA2G2A also impairs B cell differentiation and induces T follicular helper cell death, potentially contributing to muscularis TLS dysfunction. Taken together, we propose a tumor cell-iCAF-B cell axis disrupting TLS function, with this axis serving as a potential therapeutic target to overcome immunotherapy resistance.