Effect of SLAMF8 on the phagocytosis of tumor-associated macrophages in colorectal cancer.

Abstract

e14571 Background: Phagocytosis plays an essential role in innate immunity and is key in bridging innate immunity and adaptive immunity. Phagocytosis checkpoints promote tumor immune escape by inhibiting phagocytosis by tumor-associated macrophages (TAMs). SLAMF8, which is mainly expressed on the surface of TAMs, may be involved in regulating the tumor immune microenvironment. However, its regulation of the phagocytosis of TAMs remains unclear. The present study aims to investigate the effect of SLAMF8 on the phagocytosis of TAMs and to provide a new target for tumor immunotherapy. Methods: The dataset including 458 colorectal cancer (CRC) tissues from the TCGA database was used to analyze the correlation between the expression of SLAMF8 and several classical phagocytosis checkpoint molecules, and the above correlation was further verified by the RNA-sequencing data of 20 CRC tissues in our center. In vitro, the effects of SLAMF8 expression level on tumor cell proliferation and macrophage phagocytosis were also evaluated. RNA sequencing was used to analyze the effect of SLAMF8 on the expression of genes related to promoting macrophage phagocytosis. The effect of SLAMF8 on the expression levels of proteins related to FcR-mediated phagocytosis was detected by flow cytometry. Results: The results from the TCGA dataset and our validation dataset both showed that the SLAMF8 expression was significantly positively correlated with the expression levels of classical phagocytosis checkpoint molecules such as SIGLEC11, SIGLEC10, SIGLEC9, CD274and LILRB1. In vitro, functional tests showed that the phagocytosis of CT26 CRC cells by macrophages was significantly enhanced after SLAMF8 knockdown. RNA sequencing results suggested that the expression levels of molecules related to FcR-mediated phagocytosis were significantly upregulated after the decreased expression of SLAMF8.Flow cytometry analysis confirmed that SLAMF8 inhibited the expression levels of FcR-mediated phagocytosis related proteins such as CD16/32, CD64, and CD11b. Conclusions: SLAMF8 may release the “don’t-eat me” signal to inhibit FcR-mediated phagocytosis of tumor cells by macrophage. Further studies are needed to investigate the underlying mechanism of SLAMF8 on the regulation of macrophage phagocytosis and to test its possibility as a new immunotherapy target.