The Interaction between Integrin Mac-1 (α_Mβ₂, CD11b/CD18) and Sirpα Mediates Macrophage Fusion

Macrophage fusion leading to the formation of multinucleated giant cells is a hallmark of chronic inflammation. While a number of membrane proteins have been implicated in mediating cell-cell attachement during fusion, their binding partners are not yet clear. Recently, we have demonstrated that IL-4-induced fusion of mouse peritoneal macrophages depends on integrin Mac-1 (α_Mβ₂, CD11b/CD18). Surprisingly, the genetic deficiency of ICAM-1, an established ligand of Mac-1, resulted in no decrease of macrophage fusion, suggesting the involvement of other counter-receptors for Mac-1. In the present study, we demonstrated that SIRPα, which like ICAM-1 belongs to the Ig superfamily and was previously implicated in fusion, interacts with Mac-1. Expression of Mac-1 on the surface of HEK293 cells resulted in their adhesion to recombinant ectodomain of SIRPα and this process was mediated through the α_MI-domain of Mac-1. In addition, adhesion of RAW264.7 mouse macrophages to SIRPα was dependent on Mac-1. Direct interaction between the α_MI-domain and SIRPα was shown by biolayer interferometry. Analyses of the interaction of α_MI-domain with a peptide library spanning the ectodomain of SIRPα showed that recognition of SIRPa conforms to general principles that govern binding by Mac-1 of many of its ligands. SIRPα reportedly interacts with CD47; however, adhesion of Mac-1-expressing HEK293 cells, which express CD47, to SIRPα was not dependent on CD47 and an anti-CD47 function-blocking mAb produced only a limited inhibition of adhesion of RAW264.7 cells. Co-immunoprecipitation experiments revealed that Mac-1 forms a complex with CD47 on the surface of RAW264.7 and Mac-1-expressing HEK293 cells. Finally, co-culturing of SIPRα- and Mac-1-expressing HEK293 cells resulted in cell fusion. Collectively, these data identify SIRPα as a novel counter-receptor for Mac-1 and suggest that the Mac-1-SIRPα interaction may be involved in macrophage fusion.