Spatial organisation of tumor-infiltrating B-cells informs immune checkpoint inhibitor response in Claudin 18.2-expressing gastric cancer

PURPOSE: Claudin 18.2 (CLDN18.2), a tight junction protein normally expressed in gastric epithelium and rendered accessible during malignant transformation, has emerged as a key therapeutic target. Here, we seek to characterize the immune microenvironment of CLDN18.2-expressing gastric cancer and identify immune features associated with CLDN18.2 expression status. PATIENTS AND METHODS: Tumor immune features were profiled in 103 patients with HER2-neg/low advanced GC treated with first-line immune checkpoint inhibitor (ICI) using multiplex immunohistochemistry (7,423,483 cells across 2,315 regions of interest [ROIs]). Findings were validated in five published whole-transcriptome sequencing/microarray cohorts (1,521 samples), including the CheckMate 649 phase 3 trial. Spatial architecture was assessed by cellular neighborhood analysis. Digital spatial profiling (DSP) was performed on 480 ROIs from a CLDN18.2-annotated tissue microarray (75 patients). RESULTS: CLDN18.2high tumors showed reproducible enrichment of humoral pathways, with higher CD20+ B-cell density and enrichment of B-cell/plasma-cell signatures. CLDN18.2 status alone was not associated with survival on first-line ICI therapy; in CLDN18.2low tumors, B-cell enrichment was associated with ICI benefit, unlike CLDN18.2high tumors. Spatial analyses indicated that CLDN18.2high tumors preferentially harbored tumor-infiltrating B-cells, rather than tertiary lymphoid structure/lymphoid aggregate-like neighborhoods, and this tumor-compartment localization was associated with diminished ICI benefit. DSP confirmed that CD20+ B-cell enrichment was most pronounced within tumor regions, alongside compartment-specific activation of humoral and extra-follicular response programs and interferon-associated and remodeling signatures. CONCLUSIONS: CLDN18.2high GC exhibits a humoral-enriched microenvironment with increased tumor-infiltrating B-cells and spatial immune organization associated with reduced ICI benefit. Spatial and humoral-state biomarkers may inform ongoing CLDN18.2-directed combination trials.