Cytoskeletal and adhesive structural polarizations accompany IL-13-induced human macrophage fusion

During the inflammatory response to an implanted biomaterial, monocytes undergo a striking phenotypic progression of differentiation into macrophages, which may subsequently fuse to form foreign body giant cells (FBGCs). Taking advantage of an in vitro system of cytokine-induced FBGC formation together with the optical slicing capabilities of a confocal microscope, we investigated the cytoskeletal reorganization and adhesive structure development during this dramatic morphological progression. Human monocytes demonstrated diffuse cytoplasmic staining of adhesive structural proteins. Punctate filamentous (F)-actin structures appeared along the ventral cell membrane of macrophages and were identified as the core of podosome adhesive structures by the distinctive ring staining of vinculin, talin, and paxillin around the F-actin. Cytokine-induced FBGCs were characterized by a restriction of podosomes to the extreme periphery of the ventral cell surface. Although macrophages and FBGC contained equivalent amounts of F-actin, significantly more F-actin was located within 1 micron of the ventral plasma membrane in FBGCs compared to macrophages. Taken together, these results provide new information on the dynamic cytoskeletal reorganization and adhesive structure development that occur during phenotypic progression from human monocytes to macrophages to FBGC. Furthermore, they suggest the acquisition of functional specializations on FBGC formation, which may enhance our understanding of chronic inflammatory processes.