Objective Findings from recent transcriptome analyses of the synovium of patients with rheumatoid arthritis (RA) have revealed that 15‐fold expanded HLA–DR+CD90+ synovial fibroblasts potentially act as key mediators of inflammation. The reasons for the expansion of HLA–DR+CD90+ synovial fibroblasts are unclear, but genetic signatures indicate that interferon‐γ (IFNγ) plays a central role in the generation of this fibroblast subset. The present study was undertaken to investigate the generation, function and therapeutically intended blockage of HLA–DR+CD90+ synovial fibroblasts. Methods We combined functional assays using primary human materials and focused bioinformatic analyses of mass cytometry and transcriptomics patient data sets. Results We detected enriched and activated Fcγ receptor type IIIa–positive (CD16+) NK cells in the synovial tissue from patients with active RA. Soluble immune complexes were recognized by CD16 in a newly described reporter cell model, a mechanism that could be contributing to the activation of natural killer (NK) cells in RA. In vitro, NK cell–derived IFNγ induced HLA–DR on CD90+ synovial fibroblasts, leading to an inflammatory, cytokine‐secreting HLA–DR+CD90+ phenotype. HLA–DR+CD90+ synovial fibroblasts consecutively activated CD4+ T cells upon receptor crosslinking via superantigens. HLA–DR+CD90+ synovial fibroblasts also activated CD4+ T cells in the absence of superantigens, an effect that was initiated by NK cell–derived IFNγ and that was 4 times stronger in patients with RA compared to patients with osteoarthritis. Finally, JAK inhibition in synovial fibroblasts prevented HLA–DR induction and blocked proinflammatory signals to T cells. Conclusion The HLA–DR+CD90+ phenotype represents an activation state of synovial fibroblasts during the process of inflammation in RA that can be induced by IFNγ, likely generated from infiltrating leukocytes such as activated NK cells. The induction of these proinflammatory, interleukin‐6–producing, and likely antigen‐presenting synovial fibroblasts can be targeted by JAK inhibition.
Fc-gamma receptor (FccR) activation by soluble IgG immune complexes (sICs) represents a major mechanism of inflammation in certain autoimmune diseases such as systemic lupus erythematosus (SLE). A robust and scalable test system allowing for the detection and quantification of sIC bioactivity is missing. We developed a comprehensive reporter cell panel detecting activation of FccRs. The reporter cell lines were integrated into an assay that enables the quantification of sIC reactivity via ELISA or a faster detection using flow cytometry. This identified FccRIIA(H) and FccRIIIA as the most sIC-sensitive FccRs in our test system. Reaching a detection limit in the very low nanomolar range, the assay proved also to be sensitive to sIC stoichiometry and size reproducing for the first time a complete Heidelberger-Kendall curve in terms of immune receptor activation. Analyzing sera from SLE patients and mouse models of lupus and arthritis proved that sIC-dependent FccR activation has predictive capabilities regarding severity of SLE disease. The assay provides a sensitive and scalable tool to evaluate the size, amount, and bioactivity of sICs in all settings.
Fcγ-receptor (FcγR) activation by antibody derived soluble immune complexes (sICs) is a major contributor to inflammation in autoimmune diseases such as systemic lupus erythematosus (SLE). A robust and scalable test system allowing for the detection and quantification of sICs with regard to receptor activation is missing. We developed a comprehensive cell-based reporter system capable of measuring the sIC-mediated activation of individual human and mouse FcγRs. We show that compared to human FcγRs IIB and III, human FcγRs I and IIA lack sensitivity to sICs. Further, the assay proved to be sensitive to sIC size enabling us to demonstrate for the first time a complete translation of the Heidelberger-Kendall precipitation curve to FcγR responsiveness. The assay also proved useful to quantify sICs-mediated FcγR activation using sera from SLE patients and mouse models of lupus and arthritis. Thus, in clinical practice, it might be employed to measure FcγR activation as a biomarker for disease activity in immune-complex mediated disease.
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