Intratumoral Immune Response to Gastric Cancer Varies by Molecular and Histologic Subtype.

Immune checkpoint inhibition is effective in a subset of patients with advanced gastric cancer. Genomic profiling has revealed the heterogeneity of gastric adenocarcinomas, but the immune microenvironment and predictors of immunotherapy response remain poorly understood. We aimed to better characterize the underlying immune response to gastric cancer. Retrospective review of a prospectively maintained institutional database was performed to identify patients who underwent curative intent resection of gastric adenocarcinoma from 2006 to 2016. Tumors were classified according to modified TCGA subtype: Epstein-Barr virus (EBV)-associated, microsatellite instability (MSI)-high, intestinal as a surrogate for chromosomal instability, diffuse as a surrogate for genomically stable. Tumor-infiltrating leukocytes were measured using immunohistochemistry. Forty-three patients were identified: 6 EBV, 11 MSI, 14 intestinal, 12 diffuse. The most prevalent tumor-infiltrating leukocytes were CD8 T lymphocytes and CD68 macrophages, comprising 15% and 13% of all tumor cells. EBV and MSI tumors were the most infiltrated, harboring 30% to 50% T cells and 20% macrophages. Intestinal tumors contained fewer T cells but disproportionately more macrophages. Diffuse tumors were the least infiltrated. Programmed cell death protein 1 was most frequently expressed in intestinal tumors, whereas 70% of EBV and MSI tumors expressed programmed death-ligand 1. We herein demonstrate a heterogenous immune response to gastric cancer, which varies by tumor subtype and has implications for future immunotherapy trials. Checkpoint inhibition is unlikely to be effective as single-agent therapy against intestinal and diffuse tumors lacking prominent T-cell infiltration or substantial programmed death-ligand 1 expression.