Abstract
The subfamily of β2 integrins is implicated in macrophage fusion, a hallmark of chronic inflammation. Among β2 family members, integrin Mac-1 (αMβ2, CD11b/CD18) is abundantly expressed on monocyte/macrophages and mediates critical adhesive reactions of these cells. However, the role of Mac-1 in macrophage fusion leading to the formation of multinucleated giant cells remains unclear. Moreover, the role of integrin αDβ2 (CD11d/CD18), a receptor with recognition specificity overlapping that of Mac-1, is unknown. We found that multinucleated giant cells are formed in the inflamed mouse peritoneum during the resolution phase of inflammation, and their numbers were approximately twofold higher in wild-type mice than in Mac-1(-/-) mice. Analyses of isolated inflammatory peritoneal macrophages showed that IL-4-induced fusion of Mac-1-deficient cells was strongly reduced compared with wild-type counterparts. The examination of adhesive reactions known to be required for fusion showed that spreading, but not adhesion and migration, was reduced in Mac-1-deficient macrophages. Fusion of αDβ2-deficient macrophages was also significantly decreased, albeit to a smaller degree. Deficiency of intercellular adhesion molecule 1, a counter-receptor for Mac-1 and αDβ2, did not alter the fusion rate. The results indicate that both Mac-1 and αDβ2 support macrophage fusion with Mac-1 playing a dominant role and suggest that Mac-1 may mediate cell-cell interactions with a previously unrecognized counter-receptor(s).