Molecular Regulation of Foreign Body Giant Cell Formation During the Foreign Body Response

The foreign body response begins with injury acquired during implantation of a biomaterial and is detrimental due to the eventual encapsulation of the implant. A hallmark of the FBR, fusion of macrophages to create a foreign body giant cell is the consequence of a multistep mechanism believed to be induced by IL-4 and followed by the acquisition of a fusion competent state, chemotaxis, and subsequent cytoskeletal rearrangements during and after fusion. However, the precise mechanism, regulation, and interplay among molecular mediators to generate FBGCs are insufficiently understood. In this study we investigated the activation state of macrophages undergoing fusion as well as novel mediators of this phenomenon. Based on previous studies indicating that alternatively activated macrophages are associated with more favorable outcomes during the FBR, we characterized the polarization state of macrophages undergoing fusion. Through use of in vitro and in vivo fusion assays and analysis of classical (Ml) and alternative (M2) gene expression profiles we found that fusing macrophages exhibited a unique state that included markers of both Ml and M2 polarization. In addition, we have identified an essential role for TNF- induced NF-&kgr;B pathway during the FBR. Seeking novel mediators of fusion that might be regulated at the post-transcriptional level, we examined the role of microRNAs (miRs) in this process. Through a miR microarray approach we identified miR-223 as a negative regulator of macrophage fusion and demonstrated that when this miR is upregulated through in vitro transfection, fusion is attenuated. Furthermore, studies utilizing cells from miR-223 KO mice in vitro and in vivo implantation in miR-223 KO mice, showed that ablation of miR223 resulted in an augmentation of macrophage fusion. Finally, we developed a method for in vivo delivery of miR-223 mimic utilizing PLGA nanoparticles, which inhibited macrophage fusion in a peritoneal implant model. Collectively, our findings suggest a unique activation state of macrophages undergoing fusion and demonstrate the previously unidentified role of the NF-&kgr;B pathway and miR-223 in this process. Furthermore, we present miR-223 mimic as a potential therapeutic inhibitor of macrophage fusion, demonstrating its ability to successfully attenuate FBGC formation in vivo.