Braking Macrophage Cell-Cell Fusion in the Foreign Body Response

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By virtue of their versatility and plasticity, macrophages have the ability to undergo phenotypic and functional changes in response to microenvironmental cues. One such change is the formation of multinucleated giant cells via intercellular fusion triggered by certain bacterial or parasitic infections or implantation of medical devices to induce a foreign body response. We and others have shown that the transmembrane protein CD13 is a multifunctional molecule that regulates diverse processes such as tumor cell invasion, immune cell trafficking, receptor mediated endocytosis and recycling and organization of the actin cytoskeleton. In our preliminary data, we demonstrate that despite a relatively normal distribution of hematopoietic components in bone marrow and periphery, CD13KO mice have reduced bone mass with increased osteoclast (OC) numbers per bone surface area but normal bone formation parameters. In vitro induction of CD13-deficient myeloid progenitors generated from bone marrow resulted in hyperfusion to generate multinucleated giant cells (MGCs) or OCs that were considerably larger in size, contained many more nuclei than those from wild type progenitors, suggesting that CD13 is a component of common fusion pathways shared by MGC and OC. We observed that while expression of the key fusion proteins, dynamin and DC-STAMP, are typically downregulated in mature cells post-fusion, expression of these proteins is sustained at high levels in multinucleated cells lacking CD13. Thus, CD13 acts as a brake to restrain a common cell-cell fusion pathway and may be a novel therapeutic target in pathological conditions mediated by abnormal cell fusion. In this proposal, we will identify the CD13-dependent mechanisms, molecules and signaling pathways involved in cell- cell fusion in macrophages undergoing giant cell fusion (Aim 1) and the potential for fusion-blocking anti-CD13 monoclonal antibodies as therapeutics in vitro and in vivo (Aim 2). We will use our extensive panels of wild type and CD13 knock out primary cells of mouse and human origin, CD13 blocking or activating antibodies, novel CRISPR-deleted CD13KO myeloid cell lines along with confocal and spinning disk microscopy and CD13 knockout mouse models to address these questions.