Human Fc?RIIa mediates osteoclastogenesis through a novel noncanonical pathway independent of RANKL and inflammatory cytokines

Under physiological conditions, osteoclasts (OCs) are generated from monocytic osteoprecursors under the stimulation of RANKL and M-CSF, requiring co-stimulatory signals from Fc receptor common ? chain (FcR?) or DNAX-activated protein 12. It has been proposed that immune complexes (ICs) directly potentiate RANKL-mediated osteoclastogenesis by triggering Fc?R-coupled Fc? receptors (Fc?Rs), thereby linking IC accumulation and pathological osteolysis under various disease conditions. However, whether ICs possess pro-osteoclastogenic potential independent of RANKL is unknown. Here we demonstrate that IgG ICs alone can drive the differentiation of human blood monocytes into nonclassical OCs (NOCs) phenotypically and functionally distinguishable from RANKL-induced classical OCs (COCs). This novel noncanonical osteoclastogenesis pathway is triggered by full crosslinking of human Fc?RIIa (hFc?RIIa), or co-ligation of hFc?RIIa and TLR4, and signals through the Src family kinase-STAT5 axis without inducing the expression of NFATc1, a master transcription factor for the previously described osteoclastogenesis pathways. Surprisingly, IgG ICs strongly overrule the generation of COCs driven by RANKL in vitro. More importantly, TRAP+ OCs found in the inflammatory joint tissues of hFc?RIIa-transgenic mice with collagen-induced arthritis bear the NFATc1- phenotype. Our results unmask the “double faces” of IgG ICs in health and disease, and suggest a novel and important pathway for ICs contributing to pathological bone erosion in inflammatory arthritis. Overall design: Monocytes were pre-incubated either with cell culture medium (Medium), LFIC (10µg/ml) or cIgG (10 µg/ml) for 24 hrs.After wash-out, cells were harvested and processed into RNA extraction.

在生理条件下，破骨细胞（osteoclasts, OCs）由单核系骨前体细胞在核因子κB受体活化因子配体（Receptor Activator of Nuclear Factor κB Ligand, RANKL）与巨噬细胞集落刺激因子（Macrophage Colony-Stimulating Factor, M-CSF）的刺激下分化生成，该过程需Fc受体共同γ链（Fc receptor common γ chain, FcRγ）或DNAX活化蛋白12（DNAX-activated protein 12）提供共刺激信号。已有研究表明，免疫复合物（immune complexes, ICs）可通过触发FcγR偶联的Fcγ受体（FcγRs）直接增强RANKL介导的破骨细胞生成，从而将多种疾病状态下的免疫复合物蓄积与病理性骨溶解关联起来。然而，免疫复合物是否具备不依赖RANKL的促破骨细胞生成潜能，目前仍未明确。
本研究证实，仅IgG型免疫复合物即可诱导人外周血单核细胞分化为非经典型破骨细胞（nonclassical OCs, NOCs），该细胞在表型与功能上均与RANKL诱导产生的经典型破骨细胞（classical OCs, COCs）存在显著差异。这一全新的非经典破骨细胞生成通路可通过人FcγRIIa（human FcγRIIa, hFcγRIIa）的完全交联，或hFcγRIIa与Toll样受体4（Toll-like receptor 4, TLR4）的共连接所触发，并通过Src家族激酶-STAT5信号轴传导信号，且不会诱导此前报道的破骨细胞生成通路核心转录因子活化T细胞核因子c1（Nuclear Factor of Activated T Cells c1, NFATc1）的表达。
令人意外的是，IgG型免疫复合物可在体外显著抑制RANKL介导的经典型破骨细胞生成。更为关键的是，在携带人FcγRIIa的胶原诱导性关节炎（collagen-induced arthritis, CIA）模型小鼠的炎性关节组织中，所检测到的抗酒石酸酸性磷酸酶阳性（tartrate-resistant acid phosphatase positive, TRAP+）破骨细胞均呈现NFATc1阴性表型。
本研究揭示了IgG型免疫复合物在生理与病理状态下的双重特性，并提出了一条全新且重要的通路，用以阐释免疫复合物如何参与炎性关节炎中的病理性骨侵蚀。
总体实验设计：将单核细胞分别于细胞培养基（Medium）、脂多糖结合免疫复合物（lipopolysaccharide-containing immune complexes, LFIC，10μg/ml）或常规IgG（cIgG，10μg/ml）中预孵育24小时。清洗去除处理因素后，收集细胞并进行RNA提取。