S100A8 Causes a Shift Toward Expression of Activatory Fcγ Receptors on Macrophages via Toll-like Receptor 4 and Regulates Fcγ Receptor Expression in Synovium During Chronic Experimental Arthritis

Objective. The levels of both Fc gamma receptor (Fc gamma R) 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 Fc gamma R 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 Fc gamma R 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 Fc gamma R 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 Fc gamma RI (64-fold increase) and Fc gamma RIV (256-fold increase) in the synovium. Stimulation of macrophages with rS100A8 led to significant up-regulation of mRNA and protein levels of Fc gamma RI and Fc gamma RIV, but not Fc gamma RIII, 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 Fc gamma R expression. Up-regulation of Fc gamma RI and Fc gamma RIV was abrogated in rS100A8-stimulated macrophages from TLR-4(-/-) mice, indicating that the induction of Fc gamma R expression by S100A8 is mediated by TLR-4. Fc gamma R 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 Fc gamma RI and Fc gamma RIV in macrophages through the activation of TLR-4, and acts as a regulator of Fc gamma R expression in inflamed synovium in chronic experimental arthritis.