Dynamic modulation of claudin18.2 expression and remodeling of the tumor microenvironment in gastric cancer during chemo-immunotherapy

Background Claudin 18.2 (CLDN18.2) is a tight junction protein retained in malignant transformation of gastric cancer (GC) and is a promising therapeutic target. Despite the clinical benefit of zolbetuximab in CLDN18.2-positive tumors, the dynamic expression of CLDN18.2 during chemoimmunotherapy and its implications in the tumor microenvironment (TME) remain poorly understood. Methods In a prospective single-arm phase II trial, we evaluated serial tumor biopsies from patients with advanced GC receiving first-line chemotherapy (capecitabine/oxaliplatin) with sequential pembrolizumab. CLDN18.2 expression was assessed by immunohistochemistry, and integrated molecular analyses—including whole transcriptome, whole exome, and single-cell RNA sequencing—were performed to explore TME alterations and molecular correlates. Results Among 57 patients, 40.4% were CLDN18.2-positive at baseline. CLDN18.2 positivity was associated with diffuse-type histology, higher programmed death-ligand 1 (PD-L1) combined positive scores, and an immune-inflamed, stroma-rich TME characterized by enhanced T cell infiltration, transforming growth factor-β signaling, and matrix remodeling. Gene set enrichment analyses revealed immune activation and stromal remodeling in CLDN18.2-positive tumors. Single-cell analysis showed increased regulatory T cells and galectin-3-CD44 signaling in CLDN18.2-positive tumors. After one cycle of chemotherapy, CLDN18.2 expression was lost in 40% of initially positive tumors, while 10% of initially negative tumors gained expression—particularly in fibrotic TMEs. Survival outcomes were not significantly different between CLDN18.2-positive and CLDN18.2-negative groups; however, patients negative for both CLDN18.2 and PD-L1 showed poorer prognosis. Conclusions CLDN18.2 expression is dynamically regulated during chemoimmunotherapy and is associated with a distinct immunosuppressive and fibrotic TME. These findings highlight the importance of repeated biomarker assessment and suggest potential combinatorial therapeutic strategies targeting both epithelial and stromal compartments in GC.