Background Rheumatoid Arthritis (RA) is characterized by bone destruction in joints caused by enhanced activity of osteoclasts. Ig-G containing immune complexes (ICs) correlate with disease severity and bone erosion and are recognized by Fc gamma receptors (FcγRs) on myeloid cells. In this study we investigated the role of FcγRs in osteoclast-mediated bone erosion comparing development of immune complex mediated antigen induced arthritis (AIA) between mice lacking all FcγRs (FcγRI, II, III, IV–/–) and their wild type controls.
Objectives To investigate the role of FcγRs in bone erosion in experimental arthritis
Methods AIA was induced by injection of 60 μg mBSA into the knee joint of FcγRI, II, III, IV–/– and wild type (WT) control mice previously immunized with mBSA/CFA. Joint inflammation and bone destruction was determined in total knee joints sections. mBSA antibody titers were measured using ELISA and T cell response monitored with a lymphocyte stimulation test. The percentage of osteoclast precursors in the bone marrow was defined through FACS analysis. Gene expression was measured using RT-PCR. Number of mature osteoclasts within the inflamed joints was defined through TRAP staining. Protein expression was measured with bead-based multiplex immunoassay luminex in the synovial wash-out or through immunohistochemistry on knee joint section.
Results In FcγRI, II, III, IV–/– mice the development of bone erosion in knee joints was significantly reduced both at days 7 and 21 after induction of AIA (30% and 25% lower) when compared to WT controls. The immune response against mBSA (serum level of specific anti mBSA (total IgG, IgG1, IgG2a, IgG2b) and mBSA specific T-cell response) was comparable between the two strains. The percentage of osteoclast precursors within the bone marrow (CD11b low-neg/ Ly6C high, described to be upregulated during arthritis) was comparable between FcγRI, II, III, IV–/– and WT controls. At day 7 after AIA induction, the decrease in bone erosion in FcγRI, II, III, IV–/– was associated with a significantly decrease in the number of inflammatory cells present within the joint (infiltrate and exudate 29% and 27% lower respectively compared to WT control). Although less macrophages were detected within the synovium, no differences were found in the number of mature osteoclasts present at locations of bone erosion (32±13 osteoclasts/section in FcγRI, II, II, IV–/– versus 28±9 osteoclasts/section in WT), suggesting that mainly the activity of osteoclasts was affected in the KO animals. In line with this observation levels of macrophage derived factors like IL-1β and S100A8, known to strongly induce osteoclast activity, were reduced both at gene expression level and protein level in the synovium of FcγRI, II, III, IV–/– mice.
Conclusions FcγRs promote bone erosion in AIA by enhancing influx of macrophages within the synovium and release of factors such as IL-1β and S100A8 able to stimulate osteoclast activity.
Disclosure of Interest None declared