The pattern-recognition molecule mindin binds integrin Mac-1 to promote macrophage phagocytosis via Syk activation...

Mindin has a broad spectrum of roles in the innate immune system, including in macrophage migration, antigen phagocytosis and cytokine production. Mindin functions as a pattern-recognition molecule for microbial pathogens. However, the underlying mechanisms of mindin-mediated phagocytosis and its exact membrane receptors are not well established. Herein, we generated mindin-deficient mice using the CRISPR-Cas9 system and show that peritoneal macrophages from mindin-deficient mice were severely defective in their ability to phagocytize E  coli. Phagocytosis was enhanced when E  coli or fluorescent particles were pre-incubated with mindin, indicating that mindin binds directly to bacteria or non-pathogen particles and promotes phagocytosis. We defined that ¹³¹ I-labelled mindin binds with integrin Mac-1 (CD11b/CD18), the F-spondin (FS)-fragment of mindin binds with the α_M -I domain of Mac-1 and that mindin serves as a novel ligand of Mac-1. Blockade of the α_M -I domain of Mac-1 using either a neutralizing antibody or si-Mac-1 efficiently blocked mindin-induced phagocytosis. Furthermore, mindin activated the Syk and MAPK signalling pathways and promoted NF-κB entry into the nucleus. Our data indicate that mindin binds with the integrin Mac-1 to promote macrophage phagocytosis through Syk activation and NF-κB p65 translocation, suggesting that the mindin/Mac-1 axis plays a critical role during innate immune responses.