Distinct Immunosuppressive Tumor Microenvironment in Gastric Cancer With Peritoneal Metastasis

Immunotherapy combined with chemotherapy is the standard palliative treatment for gastric cancer. However, peritoneal metastases are often resistant to immunotherapy, underscoring the need to better understand the tumor immune microenvironment (TIME). In this study, we aimed to comprehensively analyze the TIME in peritoneal metastases of gastric cancer.

Paired single-cell suspensions from malignant ascites and peripheral blood mononuclear cells (PBMCs) were obtained from 27 patients with gastric cancer for multicolor fluorescence-activated cell sorting (FACS) analysis. Cell-free fluids from malignant ascites and plasma of 15 patients with gastric cancer, along with benign ascites from 15 patients with liver cirrhosis, were analyzed using multiplex enzyme-linked immunosorbent assay (ELISA). Paired samples of primary gastric tumors and metastatic peritoneal tumors from 12 patients were evaluated using multiplex immunohistochemistry (IHC).

FACS analysis revealed that T cells in malignant ascites expressed higher levels of immune checkpoint receptors (programmed death-1, T-cell immunoglobulin and mucin-domain containing-3, T-cell immunoglobulin and ITIM domain, lymphocyte activation gene-3, and cytotoxic T-lymphocyte antigen 4), and that CD8⁺ T cells exhibited terminal exhaustion (Eomes^highT-bet^low). Multiplex ELISA showed that soluble immunosuppressive factors (matrix metalloproteinase [MMP]-1, MMP-2, MMP-7, transforming growth factor-beta 1, hepatocyte growth factor, E-cadherin, vascular endothelial growth factor, and angiopoietin-2) were elevated in malignant ascites. Multiplex IHC showed lower CD4⁺ and CD8⁺ T cell densities in metastatic peritoneal tumors, which predominantly exhibited immunosuppressive TIME subtypes (immune-desert and intrinsic induction).

Our results revealed distinct peritoneal immunosuppressive TIMEs in patients with gastric cancer with peritoneal metastasis.