IgG Immune Complexes Drive Rapid Differentiation Of Human Monocytes Towards Osteoclasts In A RANKL-independent Fashion

Osteoclast(OC)is a macrophage polykaryon formed by the fusion of precursor cells of the monocyte/macrophage linage.As the only specialized bone-resorbing cells in the body,they are critical for the constantly ongoing bone remodeling.Overt osteoclastogenesis is one of the main reasons for systemic bone loss in systemic autoimmune disorders such as rheumatoid arthritis(RA),but its underlying molecular mechanisms are not fully understood.In the canonical pathway of osteoclastogenesis,osteoclast precursor cells fuse and differentiate to form tartrate resistant acidic phosphatase(TRAP)-positive multinucleated cells(MNCs)in the presence of receptor activator for nuclear factor kappa B ligand(RANKL)and macrophage colony stimulating factor(M-CSF).In addition to the RANKL-dependent canonical pathway,non-canonical pathways of osteoclastogenesis have been described in which several cytokines and growth factors(e.g.TNF-? and TGF-?)are able to facilitate,or even substitute,RANKL in in vitro cultures.Additionally,signaling through ITAM-coupled Fc gamma receptors(Fc?Rs)or other Ig superfamily receptors associated with the Fc R ? chain(Fc R?)or DNAX-activating protein 12 k Da(DAP12)can provide critical co-stimulatory signals for OC generation.The Fc?R-mediated pathway is generally considered supplementary to the RANKL-dependent pathway of osteoclastogenesis.Producing autoantibodies is a hallmark of RA,and synovial accumulation of IgG immunocomplexes(ICs)containing a variety of auto-antigens in RA patients has been observed,and accumulating evidence suggests a contribution by such ICs in overt osteoclastogenesis in RA through interaction with Fc?Rs on the surface of osteoprecursors.However,so far studies in this field have been designed under the concept that ICs only play supporting roles in OC generation by fueling the RANKL-dependent pathway.In the present study,we provide evidence showing that IgG ICs,represented by plate-coated IgG(c IgG)and soluble complex between human lactoferrin(LTF)and specific IgG,are far more powerful than RANKL in driving osteoclast differentiation from human monocytes in vitro.Importantly,circulating IgG ICs isolated from sera of patients with rheumatoid arthritis(RA)were similarly effective in eliciting OC generation.Minimum time required for the formation of tartrate resistant acidic phosphatase(TRAP)-positive multinucleated cells(osteoclasts)in human monocyte cultures under stimulation with IgG IC alone was 3 days,much less than that(7-9 days)treated with RANKL plus inflammatory cytokines.IC-driven osteoclastogenesis was unsusceptible to inhibition of osteoprotegerin,a decoy receptor for RANKL,but sensitive to IL-4 and IL-10 or blocking antibodies against Fc?RII.Transcriptomic profiling on human monocytes after a 24 h treatment with c IgG or LTF-IC revealed significantly enhanced expression of molecules key to OC generation,including DC-STAMP,OC-STAMP,implying that IgG ICs are able to steer the differentiation of human monocytes towards OC at very early stage of contact.These results strongly suggest that,in local tissues(e.g.affected RA synovia)with substantial IgG IC deposition,IC-mediated signaling through Fc?Rs may play a leading rather than a supporting role in driving overt osteoclastogenesis and consequently bone loss.Taken together,this study firstly demonstrates that IgG ICs can drive RANKL-independent osteoclastogenesis.It suggests the potential therapeutic effects on the treatment of autoimmune-mediated bone loss and joint erosion by targeting IgG ICs.