Homotypic cell fusion occurs in several cell types including macrophages in the formation of foreign body giant cells. Previously, monocyte chemoattractant protein-1 (MCP-1) was demonstrated to be required for foreign body giant cell formation in the foreign body response. The present study investigated the fusion defect in MCP-1-null macrophages by implanting biomaterials intraperitoneally in wild-type and MCP-1-null mice and monitoring the macrophage response at 12 hours to 4 weeks. MCP-1-null mice exhibited reduced accumulation and fusion of macrophages on implants, which was associated with attenuation of the foreign body response. Consistent with previous in vitro findings, the level of matrix metalloproteinase-9 (MMP-9) was reduced in MCP-1-null macrophages adherent to implants. In contrast, CCR2 expression was unaffected. In vitro studies revealed reduced tumor necrosis factor-alpha (TNF-alpha) production and abnormal subcellular redistribution of E-cadherin and beta-catenin during fusion in MCP-1-null macrophages. Exogenous TNF-alpha caused an increase in the production of MMP-9 and rescued the fusion defect. Addition of GM6001 (MMP inhibitor) or NSC23766 (Rac1 inhibitor) indicated two distinct induction pathways, one for E-cadherin/beta-catenin and one for MCP-1, TNF-alpha, and MMP-9. Considered together, these observations demonstrate that induction of E-cadherin/beta-catenin is not sufficient for fusion in the absence of MCP-1 or the downstream mediators TNF-alpha and MMP-9. Moreover, attenuation of the foreign body response in intraperitoneal implants in MCP-1-null mice demonstrates that the process depends on tissue-specific factors.
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