TNF-a mediated inflammatory macrophage polarization contributes to the pathogenesis of steroid-induced osteonecrosis in mice

The phenotypic polarization of macrophages are involved in steroid-induced osteonecrosis (ON). This study tried to investigate the detrimental and beneficial roles of M1/M2 macrophages associated with TNF-a in ON. Mice ON model was induced by the injection of methylprednisolone. After that, flow cytometry technique, immunohistochemistry, immunofluorescence, ELISA, and RT-PCR methods were used to investigate the expression pattern of macrophages and the expression of inflammatory cytokines. During the progression of ON, massive chronic inflammatory cells infiltrated into the necrotic zone, represented by the infiltration of macrophages. In the early stage of ON, there was high TNF-a activity; and a large population of M1 macrophages infiltrated into the necrotic zone. On the contrary, the expression of TNF-a gradually decreased; simultaneously, a larger M2 cell population presented in the necrotic zone in the late stage of ON. The increased M2 macrophages could be beneficial for resolving inflammation and promoting tissue repair, confirmed by the histologic findings of appositional new bone formation around the necrotic bone. Thus, it showed that TNF-a-mediated alteration of M1/M2 macrophage polarization contributed to the pathogenesis of steroid-induced osteonecrosis. M1-polarized macrophages appeared to be disruptive in the early stage of ON, while M2-polarized macrophages played an important role in the late stage during the pathogenesis of ON.