S100A8 causes a shift toward expression of activatory Fcgamma receptors on macrophages via toll-like receptor 4 and regulates Fcgamma receptor expression in synovium during chronic experimental arthritis

OBJECTIVE: The levels of both Fcgamma receptor (FcgammaR) and the alarmins S100A8 and S100A9 are correlated with the development and progression of cartilage destruction during antigen-induced arthritis (AIA). This study was undertaken to study the active involvement of S100A8, S100A9, and S100A8/S100A9 in FcgammaR regulation in murine macrophages and synovium during AIA. METHODS: Recombinant murine S100A8 (rS100A8) was injected into normal mouse knee joints, and the synovium was isolated for analysis of FcgammaR messenger RNA (mRNA) expression by reverse transcription-polymerase chain reaction (RT-PCR). Macrophages, including bone marrow macrophages derived from Toll-like receptor 4-deficient (TLR-4(-/-)) mice, and polymorphonuclear cells (PMNs) were stimulated with S100 proteins, and levels of FcgammaR mRNA and protein were measured using RT-PCR and fluorescence-activated cell sorting analyses. AIA was induced in the knee joints of S100A9-deficient (S100A9(-/-)) mice, compared with wild-type (WT) controls, and the extent of cartilage destruction was determined using immunohistochemical analysis. RESULTS: Intraarticular injection of rS100A8 into the knee joints of normal mice caused a strong up-regulation of mRNA levels of activating FcgammaRI (64-fold increase) and FcgammaRIV (256-fold increase) in the synovium. Stimulation of macrophages with rS100A8 led to significant up-regulation of mRNA and protein levels of FcgammaRI and FcgammaRIV, but not FcgammaRIII, while the effects of S100A9 or S100A8/S100A9 complexes were less potent. Stimulation of PMNs (32Dcl3 cell line) with S100 proteins had no effect on FcgammaR expression. Up-regulation of FcgammaRI and FcgammaRIV was abrogated in rS100A8-stimulated macrophages from TLR-4(-/-) mice, indicating that the induction of FcgammaR expression by S100A8 is mediated by TLR-4. FcgammaR expression in the inflamed synovium of S100A9(-/-) mice was significantly lower on day 14 after arthritis induction when compared with WT controls, and these findings correlated with reduced severity of matrix metalloproteinase-mediated cartilage destruction. CONCLUSION: S100A8 is a strong promoter of activating FcgammaRI and FcgammaRIV in macrophages through the activation of TLR-4, and acts as a regulator of FcgammaR expression in inflamed synovium in chronic experimental arthritis.