A selective sphingosine-1-phosphate receptor 1 agonist SEW-2871 aggravates gastric cancer by recruiting myeloid-derived suppressor cells.

The immune status of tumor microenvironment in gastric cancer is poorly understood, which limits the development of novel strategies in this field. Sphingosine-1-phosphate (S1P) acts as an immune modulator, but the role of S1P in gastric cancer is elusive. Here, we aim to investigate S1P receptor 1 (S1P1)-mediated effect of S1P in gastric cancer. We generated a xenograft mouse model and used SEW-2871, a S1P1 specific agonist to activate S1P1 signalling. Tumor-infiltrating lymphocytes (TILs) were isolated and analysed using flow cytometry. Chemokine expression of tumor cells was evaluated using quantitative real-time polymerase chain reaction. Myeloid-derived suppressor cells (MDSCs) migration was assessed using Transwell chambers. SEW-2871 promoted tumor growth in our mouse model, and induced a higher level of MDSC and a reduced level of CD8+CD69+ T cells within tumor. Consistently, the anti-tumoral function of cytotoxic T lymphocytes was impaired in mice with SEW-2871 treatment. Additionally, SEW-2871 enhanced expression of several MDSC recruitment-associated chemokines (CXCL12, CXCL5 and CCL2) in tumor cells. These chemokines facilitated MDSC migration by interaction with CCR2, CXCR2 and CXCR4. S1P1 signalling promoted gastric cancer by enhancing chemokine expression in tumor cells and recruiting MDSC to tumor microenvironment, which impaired anti-tumoral function of TILs.