Macrophage polarization state influences fusion and phenotype of human foreign body giant cells

Introduction: Macrophages and their fused counterpart, foreign body giant cells (FBGCs), have a determining influence on the long-term functionality of implanted biomaterials, coordinating biomaterial resorption and fibrous encapsulation. Modulating macrophage phenotype into favorable polarization states is a widely adopted strategy to attempt steering the foreign body response. However, the links between macrophage phenotype and FBGC formation and function are unknown. The goal of this study was to determine the fusion potential of well-defined macrophage polarization states, and to explore whether prior polarization state translates to FBGC phenotype in primary human macrophages. Methods: Peripheral blood-derived human macrophages were first stimulated toward M0/M1/M2a/M2c polarizations after which they were stimulated to fuse into FBGCs using interleukin-4 (IL-4). After fusion, these cells were re-stimulated to investigate the effect of the respective cytokine cocktails on FBGC phenotype. Results: The results showed that M1 macrophages had increased IL-4 receptor expression, an altered cytoskeleton, and significantly increased expression of fusion-associated genes, when compared to unstimulated, M2a and M2c macrophages. FBGCs could form from all polarization states. Although M1 polarization led to a decreased fusion percentage, the FBGCs that did form were larger in terms of cell size and number of nuclei. FBGCs showed significant changes in their cytoskeletal structure by forming rings of actin around the periphery of the cell, which was consistent for the different macrophage polarizations. Expression of select genes and proteins associated with macrophage polarization states were either maintained during fusion, or reacquired by FBGCs upon restimulation with polarizing factors. Discussion: These results provide evidence for the existence of FBGC polarization states, emphasizing the importance of the macrophage population and microenvironment in regulating FBGC formation. This suggests that FBGC function may be steerable, for example using biomaterial design parameters, in order to improve long-term biomaterial functionality and integration.