ABSTRACT: Osteoclast-mediated bone erosion is a central feature of rheumatoid arthritis (RA). Immune complexes, present in a large percentage of patients, bind to Fc? receptors (Fc?Rs), thereby modulating the activity of immune cells. In this study, we investigated the contribution of Fc?Rs, and Fc?RIV in particular, during antigen-induced arthritis (AIA).AIA was induced in knee joints of wild-type (WT), Fc?RI,II,III-/-, and Fc?RI,II,III,IV-/- mice. Bone destruction, numbers of tartrate-resistant acid phosphatase-positive (TRAP+) osteoclasts, and inflammation were evaluated using histology; expression of the macrophage marker F4/80, neutrophil marker NIMPR14, and alarmin S100A8 was evaluated using immunohistochemistry. The percentage of osteoclast precursors in the bone marrow was determined using flow cytometry. In vitro osteoclastogenesis was evaluated with TRAP staining, and gene expression was assessed using real-time PCR.Fc?RI,II,III,IV-/- mice showed decreased bone erosion compared with WT mice during AIA, whereas both the humoral and cellular immune responses against methylated bovine serum albumin were not impaired in Fc?RI,II,III,IV-/- mice. The percentage of osteoclast precursors in the bone marrow of arthritic mice and their ability to differentiate into osteoclasts in vitro were comparable between Fc?RI,II,III,IV-/- and WT mice. In line with these observations, numbers of TRAP+ osteoclasts on the bone surface during AIA were comparable between the two groups. Inflammation, a process that strongly activates osteoclast activity, was reduced in Fc?RI,II,III,IV-/- mice, and of note, mainly decreased numbers of neutrophils were present in the joint. In contrast to Fc?RI,II,III,IV-/- mice, AIA induction in knee joints of Fc?RI,II,III-/- mice resulted in increased bone erosion, inflammation, and numbers of neutrophils, suggesting a crucial role for Fc?RIV in the joint pathology by the recruitment of neutrophils. Finally, significant correlations were found between bone erosion and the number of neutrophils present in the joint as well as between bone erosion and the number of S100A8-positive cells, with S100A8 being an alarmin strongly produced by neutrophils that stimulates osteoclast resorbing activity.Fc?Rs play a crucial role in the development of bone erosion during AIA by inducing inflammation. In particular, Fc?RIV mediates bone erosion in AIA by inducing the influx of S100A8/A9-producing neutrophils into the arthritic joint.
