The CIS association of CD47 with integrin Mac-1 regulates macrophage responses by stabilizing the extended integrin conformation

CD47 is a ubiquitously expressed cell surface integrinassociated protein. Recently, we have demonstrated that integrin Mac-1 (alpha M beta 2, CD11b/CD18, CR3), the major adhesion receptor on the surface of myeloid cells, can be coprecipitated with CD47. However, the molecular basis for the CD47-Mac-1 interaction and its functional consequences remain unclear. Here, we demonstrated that CD47 regulates macrophage functions directly interacting with Mac-1. In particular, adhesion, spreading, migration, phagocytosis, and fusion of CD47deficient macrophages were significantly decreased. We validated the functional link between CD47 and Mac-1 by coimmunoprecipitation analysis using various Mac-1- expressing cells. In HEK293 cells expressing individual alpha M and beta 2 integrin subunits, CD47 was found to bind both subunits. Interestingly, a higher amount of CD47 was recovered with the free beta 2 subunit than in the complex with the whole integrin. Furthermore, activating Mac-1-expressing HEK293 cells with phorbol 12-myristate 13-acetate (PMA), Mn2+, and activating antibody MEM48 increased the amount of CD47 in complex with Mac-1, suggesting CD47 has a greater affinity for the extended integrin conformation. Notably, on the surface of cells lacking CD47, fewer Mac-1 molecules could convert into an extended conformation in response to activation. Additionally, we identified the binding site in CD47 for Mac-1 in its constituent IgV domain. The complementary binding sites for CD47 in Mac-1 were localized in integrin epidermal growth factor-like domains 3 and 4 of the beta 2 and calf-1 and calf-2 domains of the alpha M subunits. These results indicate that Mac1 forms a lateral complex with CD47, which regulates essential macrophage functions by stabilizing the extended integrin conformation.