Fibroblasts regulate tissue homeostasis, coordinate inflammatory responses, and mediate tissue damage. In rheumatoid arthritis (RA), synovial fibroblasts maintain chronic inflammation which leads to joint destruction. Little is known about fibroblast heterogeneity or if aberrations in fibroblast subsets relate to pathology. Here, we show functional and transcriptional differences between fibroblast subsets from human synovial tissues using bulk transcriptomics of targeted subpopulations and single-cell transcriptomics. We identify seven fibroblast subsets with distinct surface protein phenotypes, and collapse them into three subsets by integrating transcriptomic data. One fibroblast subset, characterized by the expression of proteins podoplanin, THY1 membrane glycoprotein and cadherin-11, but lacking CD34, is threefold expanded in patients with RA relative to patients with osteoarthritis. These fibroblasts localize to the perivascular zone in inflamed synovium, secrete proinflammatory cytokines, are proliferative, and have an in vitro phenotype characteristic of invasive cells. Our strategy may be used as a template to identify pathogenic stromal cellular subsets in other complex diseases.

Glomerulonephritis is a major cause of morbidity in patients with systemic lupus erythematosus. Although substantial progress has been made in the identification of pathogenic triggers that result in autoantibody production, little is known about the pathogenesis of aggressive proliferative processes that lead directly to irreversible glomerular damage and compromise of renal function. In this study, we describe a model of polyinosinic: polycytidylic acid-accelerated lupus nephritis in NZB/W mice that is characterized by severe glomerular proliferative lesions with de novo crescent formation, findings that are linked with decreased survival and adverse outcomes in lupus. Proliferative glomerulonephritis was associated with infiltrating kidney macrophages and renal expression of IFN-inducible genes, matrix metalloproteinases (MMPs), and growth factors. Crescent formation and renal MMP and growth factor expression were dependent on renal macrophages that expressed Il10 , MMPs, osteopontin , and growth factors, including Pdgfc and Hbegf . Infiltrating macrophages and renal MMP expression were induced by type I IFN. These findings reveal a role for type I IFNs and alternatively activated macrophages in aggressive proliferative lesions of lupus nephritis.

Objective: The nonresolving character of synovial inflammation in rheumatoid arthritis (RA) is a conundrum. To identify the contribution of fibroblast‐like synoviocytes (FLS) to the perpetuation of synovitis, we investigated the molecular mechanisms that govern the tumor necrosis factor α (TNFα)–driven inflammatory program in human FLS.Methods: FLS obtained from the synovial tissues of patients with RA or osteoarthritis were stimulated with TNFα and assayed for gene expression and cytokine production by real‐time quantitative reverse transcription–polymerase chain reaction analysis and enzyme‐linked immunosorbent assay. NF‐κB signaling was evaluated by Western blotting. Histone acetylation, chromatin accessibility, and NF‐κB p65 and RNA polymerase II (Pol II) occupancy at the interleukin‐6 (IL‐6) promoter were measured by chromatin immunoprecipitation and restriction enzyme accessibility assays.Results: In FLS, TNFα induced prolonged transcription of messenger RNA (mRNA) for IL‐6 and progressive accumulation of IL‐6 protein over 4 days. Similarly, induction of mRNA for CXCL8/IL‐8, CCL5/RANTES, matrix metalloproteinase 1 (MMP‐1), and MMP‐3 after TNFα stimulation was sustained for several days. This contrasted with the macrophage response to TNFα, which characteristically involved a transient increase in the expression of proinflammatory genes. In FLS, TNFα induced prolonged activation of NF‐κB signaling and sustained transcriptional activity, as indicated by increased histone acetylation, chromatin accessibility, and p65 and Pol II occupancy at the IL‐6 promoter. Furthermore, FLS expressed low levels of the feedback inhibitors A20‐binding inhibitor of NF‐κB activation 3 (ABIN‐3), IL‐1 receptor–associated kinase M (IRAK‐M), suppressor of cytokine signaling 3 (SOCS‐3), and activating transcription factor 3 (ATF‐3), which terminate inflammatory responses in macrophages.Conclusion: TNFα signaling is not effectively terminated in FLS, which leads to an uncontrolled inflammatory response. The results suggest that prolonged and sustained inflammatory responses by FLS in response to synovial TNFα contribute to the persistence of synovial inflammation in RA.

IRHOM2, encoded by the gene Rhbdf2, regulates the maturation of the TNF-α convertase (TACE), which controls shedding of TNF-α and its biological activity in vivo. TACE is a potential target to treat TNF-α–dependent diseases, such as rheumatoid arthritis, but there are concerns about potential side effects, because TACE also protects the skin and intestinal barrier by activating EGFR signaling. Here we report that inactivation of Rhbdf2 allows tissue-specific regulation of TACE by selectively preventing its maturation in immune cells, without affecting its homeostatic functions in other tissues. The related iRHOM1, which is widely expressed, except in hematopoietic cells, supported TACE maturation and shedding of the EGFR ligand TGF-α in Rhbdf2-deficient cells. Remarkably, mice lacking Rhbdf2 were protected from K/BxN inflammatory arthritis to the same extent as mice lacking TACE in myeloid cells or Tnfa-deficient mice. In probing the underlying mechanism, we found that two main drivers of K/BxN arthritis, complement C5a and immune complexes, stimulated iRHOM2/TACE-dependent shedding of TNF-α in mouse and human cells. These data demonstrate that iRHOM2 and myeloid-expressed TACE play a critical role in inflammatory arthritis and indicate that iRHOM2 is a potential therapeutic target for selective inactivation of TACE in myeloid cells.