Abstract
BACKGROUND: In rheumatoid arthritis (RA) macrophages play a major role in amplifying synovial inflammation. Important activating signals are those induced by Toll-like receptor (TLR) ligands and by activated T cells. The balance between activating and inhibitory Fc gamma receptors (FcgammaRs) on macrophages might be crucial in modulating these inflammatory responses. The purpose of this study was to determine FcgammaR expression on pro- and anti-inflammatory macrophages (gmMphi and mMphi, respectively) and identify functional consequences on immune complex uptake and macrophage activation. METHODS: Human monocytes were isolated and differentiated into gmMphi and mMphi. A full FcgammaR characterization of both macrophage subtypes was performed and uptake of fluorescent immune complexes (ICs) was determined. FcgammaRIIb isoforms were determined by qPCR. Macrophages were stimulated via different TLRs or cytokine activated T cells in the presence or absence of ICs and cytokine production was determined. Blocking studies were performed to look into the pathways involved. RESULTS: mMphi expressed high levels of the activating FcgammaRIIa and FcgammaRIII and low levels of the inhibitory FcgammaRIIb, while the FcgammaR balance on gmMphi was shifted towards the inhibitory FcgammaRIIb. This was accompanied by a clear increase in FcgammaRIIb1 mRNA expression in gmMphi. This resulted in higher IC uptake by mMphi compared to gmMphi. Furthermore, FcgammaR-mediated stimulation of gmMphi inhibited TLR2, 3, 4 and 7/8 mediated cytokine production via FcgammaRIIb and PI3K signaling. In addition, gmMphi but not mMphi produced TNFalpha upon co-culture with cytokine activated T cells, which was reduced by IC binding to FcgammaRIIb. The latter was dependent on PI3K signaling and COX2. CONCLUSIONS: FcgammaR expression patterns on gmMphi and mMphi are significantly different, which translates in clear functional differences further substantiating FcgammaRIIb as an interesting target for inflammation control in RA and other autoimmune/inflammatory diseases.
