FGL1 facilitates rather than suppresses anticancer immunity against microsatellite instable gastric cancer.

Microsatellite instability (MSI) is a phenotype characterized by changes in the sequence length of microsatellites in tumor cells and is closely linked to tumorigenesis and prognosis. Immune checkpoint inhibitors have shown good therapeutic effects in gastric cancer (GC) with MSI-high (MSI-H). However, the role of the novel immune checkpoint fibrinogen-like protein 1 (FGL1) in GC treatment has not been fully investigated. FGL1 expression in GC tissues and the difference in FGL1 immune infiltration between MSI/ microsatellite stability (MSS) patients were analyzed by bioinformatics and were verified in clinical samples. Xenograft models of MSS and MSI GC were constructed in human immune reconstitution mice, and FGL1 expression in tumors was detected. Immunofluorescence and immunohistochemistry were used to assay the infiltration of immune cells in the two types of mice. Cytotoxicity and chemotaxis tests were used to detect the toxicity and chemotaxis of CD8⁺T cells to GC cells, respectively. The cytokine content was detected by enzyme-linked immunosorbent assay. The therapeutic effects of FGL1 antibody on different types of GC were analyzed by xenograft mouse models. FGL1 exhibited significantly higher expression in GC, and its expression and immune cell infiltration levels were significantly higher in MSI GC than in MSS GC. CD8⁺T cells were significantly more effective in killing and chemotaxis of MSI GC cells than MSS GC cells. The FGL1 antibody was more effective in treating MSI GC.The novel immunosuppressor FGL1 antibody exerts a good therapeutic influence on MSI GC. These findings provide a basis for the development of drugs targeting FGL1 for MSI GC treatment.