Macrophage fusion leading to the formation of multinucleated giant cells (MGCs) is a hallmark of many chronic inflammatory reactions. MGCs are an invariable constituent of tuberculoid lesions and also found in a variety of conditions leading to granulomatous inflammation as well as the foreign body reaction. Despite the prominent phenotype, the molecular mechanisms underlying macrophage fusion are not well understood. MGCs originate from macrophages that are recruited to sites of chronic inflammation. The major myelo-monocytic integrin αMβ2 (CD11b/CD18, Mac-1), together with a related integrin αDβ2 (CD11d/CD18), mediate critical adhesive reactions of monocyte/macrophages. The function of β2 integrins in macrophage fusion remains controversial. Some studies using function blocking antibodies implicated αMβ2 in the cell/substrate adhesive interactions that are required for MGC formation, whereas one recent report indicated that this integrin plays a minor role. Moreover, the contribution of αDβ2, a receptor with recognition specificity overlapping that of αMβ2, to macrophage fusion is unknown. To evaluate the role of αMβ2 and αDβ2 in MGC formation, we examined fusion of inflammatory peritoneal macrophages isolated from wild-type mice and mice with deficiency of αM or αD integrin subunits. Macrophages were isolated at day 3 after thioglycollate injection and cultured for 24–72 hours in the presence of IL-4 to induce fusion. Percentage fusion was quantified as the number of giant cell nuclei (≥2 nuclei) to the number of total nuclei. The number of fused macrophages isolated from wild-type mice gradually increased and ∼40–50% macrophages formed MGCs after IL-4 stimulation by day 3. Analyses of fusion of αMβ2-deficient macrophages demonstrated that fusion was significantly reduced. By day 3, macrophage fusion of αMβ2-deficient macrophages was 23 ± 2% of wild-type macrophages. Fusion of αDβ2-deficient macrophages was also decreased and the change was statistically significant, albeit to a smaller degree (75 ± 4%). Using a mouse model of sterile peritonitis induced by thioglycollate injection, we also examined the formation of MGCs in vivo. In wild-type mice, the number of MGCs gradually increased from 2.5 ± 0.3% at day 0 (resident macrophages) to 17 ± 2% at day 18 (the resolution phase of inflammation). Moreover, expression of αMβ2 and αDβ2 on peritoneal macrophages increased by ∼2 and 1.6-fold, respectively, on 18th day after induction of inflammation. In αMβ2-deficient mice, the number of MGCs was reduced by 2.1-fold compared to wild-type mice. Furthermore, while the size of wild-type and αMβ2-deficient giant cells was the same, the number of cells with 3 and more nuclei in fused αMβ2-deficient MGCs was 4-fold less than in wild-type cells. The results indicate that both αMβ2 and αDβ2 integrins support macrophage fusion with αMβ2 playing a dominant role.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.