Macrophage fusion resulting in the formation of multinucleated giant cells occurs in a variety of chronic inflammatory diseases， yet the mechanism responsible for initiating this process is unknown. Here， we used live cell imaging to show that actin-based protrusions at the leading edge initiate macrophage fusion. Phase-contrast video microscopy demonstrated that in the majority of events， short protrusions (∼3 µm) between two closely apposed cells initiated fusion， but occasionally we observed long protrusions (∼12 µm). Using macrophages isolated from LifeAct mice and imaging with lattice light sheet microscopy， we further found that fusion-competent protrusions formed at sites enriched in podo... More
Macrophage fusion resulting in the formation of multinucleated giant cells occurs in a variety of chronic inflammatory diseases， yet the mechanism responsible for initiating this process is unknown. Here， we used live cell imaging to show that actin-based protrusions at the leading edge initiate macrophage fusion. Phase-contrast video microscopy demonstrated that in the majority of events， short protrusions (∼3 µm) between two closely apposed cells initiated fusion， but occasionally we observed long protrusions (∼12 µm). Using macrophages isolated from LifeAct mice and imaging with lattice light sheet microscopy， we further found that fusion-competent protrusions formed at sites enriched in podosomes. Inducing fusion in mixed populations of GFP- and mRFP-LifeAct macrophages showed rapid spatial overlap between GFP and RFP signal at the site of fusion. Cytochalasin B strongly reduced fusion and when rare fusion events occurred， protrusions were not observed. Fusion of macrophages deficient in Wiskott-Aldrich syndrome protein and Cdc42， key molecules involved in the formation of actin-based protrusions and podosomes， was also impaired both in vitro and in vivo. Finally， inhibiting the activity of the Arp2/3 complex decreased fusion and podosome formation. Together these data suggest that an actin-based protrusion formed at the leading edge initiates macrophage fusion.
