Innate Immune-Like Function of Osteogenic Cells and Their Effects on Inflammatory Osteolysis

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Abstract

The immune response is an essential defense mechanism that protects the human body from foreign infection. Nevertheless, excessive immune response results in high levels of inflammation leading to destruction of healthy tissues. In orthopaedic fields, this duality of the immune response has been a challenge to the success of arthroplasties. Wear particle-induced inflammatory osteolysis is considered a major culprit of implant failure. Resident osteogenic bone cells, such as osteoprogenitors and osteoblasts, are exposed to wear particles. Although osteogenic cells are responsible for the initial osseointegration of implants and ongoing bone regeneration, their response to wear particles has been underestimated. Thus, the goal of this dissertation is to explore the immune mechanisms of osteogenic cells and to identify the role of osteogenic cells in particle mediated osteolysis.

To meet this end, we evaluated the immune capacity of osteogenic cells by exploring their ability to phagocytose wear particles and subsequently express pro-inflammatory cytokines. We developed a customized JAVA program and confocal microscopy methodology to quantify the phagocytic activity of osteogenic cells. Osteoprogenitors and osteoblasts were able to phagocytose Titanium (Ti) particles with aggressive actin remodeling. The actin remodeling to engulf particles activated ERK-CEBP/b pathway leading to Cox2 and IL6 gene expression. Interestingly, equibiaxial strain also increased inflammatory gene expression such as MCSF, IL6, and Cox2 through the ERK pathway. Physiological and super-physiological levels of strain were applied to osteogenic cells and macrophage-like cells via Flexcell system. Super-physiological strain exaggerated Ti particle induced inflammatory gene expression from osteogenic cells, while macrophage-like cells were not affected by strain. Taken together, these data suggest actin and ERK-CEBP/b signaling mediates phagocytosis-induced innate immune responses of osteogenic cells.

Next, we confirmed the role of osteogenic cells in inflammatory osteolysis. Although we observed that osteogenic cells secrete inflammatory cytokines after phagocytosis of wear particles, it was difficult to discern whether osteogenic cells have a major role in inflammatory osteolysis because there are a multitude of cells exposed to wear particles at the site of bone implant. Thus, we developed an osteogenic cell line specific ERK-dysfunctional mouse using an osterix-promoter-driven CRE-loxp system (CRE/dn-MEK1). An in vivo mouse calvaria model was utilized to induce inflammatory osteolysis. Ti particles were implanted on top of pericranium layer without invasive incision. This approach allowed observation of osteogenic cell responses to Ti particles in the pericranium. With this model, we observed severe calvarial osteolysis with increased osteoclastogenesis and pro-inflammatory cytokine release of IL6, PGE2 and MCSF. Significantly decreased inflammatory cytokine release and macrophage migration were observed in the CRE/dn-MEK1 mouse in in vivo mouse calvaria model and in vitro experiments. Similar trends were detected in mouse calvaria treated with AZD6244, a potent ATP-uncompetitive inhibitor of MAPK/ERK kinase.

In summary, this study supports hypothesis that (1) osteogenic cells are able to initiate inflammatory responses through well established innate immune function and (2) the ERK pathway could be a clinically important therapeutic target for preventing inflammatory osteolysis.

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