Regulatory B cells drive immune evasion in the tumor microenvironment and are involved peritoneal metastasis in gastric cancer

Abstract Background: Peritoneal metastasis (PM) of gastric cancer (GC) has an immune escape environment. Regulatory B cells (Bregs), characterized by IL-10 production, play an important role in the tumor immunity; however, the function of Bregs in PM remains unclear. We investigated the frequency and effects of Bregs on other immune cells in the PM using clinical specimens and mouse models of PM. Methods: Breg frequency was assessed in the blood and ascites from healthy subjects and patients with early GC, advanced GC, and PM by flow cytometry, and in resected specimens of primary GC and PM by immunohistochemistry. Additionally, we analyzed Bregs and other immune cells in C57BL/6J and B cell-specific phosphatase and tensin homolog (PTEN)-deficient PM mouse models. Results: In the peripheral blood and ascites, Breg frequency was significantly higher in patients with GC with PM than in those without PM. In clinical PM samples, Breg frequency was an independent prognostic factor. In the mouse PM model, peritoneal tumors showed higher Breg infiltration than subcutaneous tumors. In the PTEN-deficient PM model, activation of Bregs promoted ascites and peritoneal tumor growth, decreased the infiltration of CD8+ T cells, and increased the infiltration of M2 macrophages. In contrast, treatment with wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, suppressed Breg infiltration, resulting in decreased M2 macrophage infiltration and increased CD8+ T cell infiltration. Conclusions: Bregs are indicated to be involved in immunosuppression of PM and are promising targets for improving the efficacy of immunotherapy against PM.