TNF is a pleiotropic cytokine with important functions in homeostasis and disease pathogenesis. Recent discoveries have provided insights into TNF biology that introduce new concepts for the development of therapeutics for TNF-mediated diseases. The model of TNF receptor signalling has been extended to include linear ubiquitination and the formation of distinct signalling complexes that are linked with different functional outcomes, such as inflammation, apoptosis and necroptosis. Our understanding of TNF-induced gene expression has been enriched by the discovery of epigenetic mechanisms and concepts related to cellular priming, tolerization and induction of ‘short-term transcriptional memory’. Identification of distinct homeostatic or pathogenic TNF-induced signalling pathways has introduced the concept of selectively inhibiting the deleterious effects of TNF while preserving its homeostatic bioactivities for therapeutic purposes. In this Review, we present molecular mechanisms underlying the roles of TNF in homeostasis and inflammatory disease pathogenesis, and discuss novel strategies to advance therapeutic paradigms for the treatment of TNF-mediated diseases.

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.

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.

IL-27 is a member of the IL-12 family of cytokines that activates the Jak-STAT signaling pathway in a context-dependent manner and has pleiotropic effects on acquired immunity. IL-27 has the capacity to promote early stages of Th1 generation, but recent evidence has suggested a predominant suppressive effect on Th1, Th2, and Th17 differentiation. Although modest suppressive effects of IL-27 on myeloid lineage cells have been observed, there is limited knowledge about the role of IL-27 in the regulation of innate immunity. In this study we report that although in resting murine macrophages IL-27 had minimal if any effects, in resting human monocytes IL-27 had profound proinflammatory functions. IL-27 activated a STAT1-dominant pattern of signaling in human monocytes with the consequent activation of STAT1-dependent inflammatory target genes. IL-27 primed monocytes for augmented responses to TLR stimulation in a STAT1-dependent manner, altered IL-10 signaling, and attenuated IL-10-induced gene expression. Strikingly, IL-27 strongly suppressed TLR-induced IL-10 production in human monocytes. However, the proinflammatory effects of IL-27 on human monocytes were rapidly abrogated by LPS via a p38-mediated mechanism that inhibited IL-27 signaling. Our findings identify a predominantly proinflammatory function for IL-27 in human monocytes and suggest a mechanism by which the activating effects of IL-27 on innate immunity are attenuated as an immune response proceeds and IL-27 transitions to predominantly suppressive effects on acquired immunity.

SUMMARY An important function of immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors is cross-regulation of heterologous receptor signaling, but mechanisms of cross-inhibition are poorly understood. We show that high avidity ligation of ITAM-coupled β2 integrins and FcγRs in macrophages inhibited type I interferon receptor and Toll-like receptor (TLR) signaling and induced expression of interleukin-10 (IL-10), signaling inhibitors SOCS3, ABIN-3 and A20, and repressors of cytokine gene transcription STAT3 and Hes1. Induction of inhibitors was dependent on a pathway comprised of signaling molecules DAP12, Syk, and Pyk2 that coupled to downstream kinases p38 and MSKs, and required integration of IL-10-dependent and independent signals. ITAM-induced inhibitors abrogated TLR responses by cooperatively targeting distinct steps in TLR signaling. Inhibitory signaling was suppressed by IFN-γ and attenuated in inflammatory arthritis synovial macrophages. These results provide an indirect mechanism of cross-inhibition of TLRs and delineate a signaling pathway important for deactivation of macrophages.

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 IFNinducible 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.alternative macrophages | kidney disease | polyinosinic: polycytidylic acid | systemic lupus erythematosus | tissue repair

Objective The non resolving 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 TNFα-driven inflammatory program in human FLS. Methods FLS obtained from synovial tissues of patients with RA or osteoarthritis were stimulated with TNFα and assayed for gene expression and cytokine production by qPCR and ELISA. NF-κB signaling was evaluated using Western blotting. Histone acetylation, chromatin accessibility, and NF-κB p65 and RNA polymerase II (Pol II) occupancy at the IL6 promoter were measured by chromatin immunoprecipitation and restriction enzyme accessibility assays. Results In FLS, TNFα induced prolonged transcription of IL6 and progressive accumulation of IL-6 protein over four days. Similarly, induction of CXCL8/IL-8, CCL5/RANTES, MMP1 and MMP3 mRNA 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 pro-inflammatory genes. In FLS, TNFα induced prolonged activation of NF-κB signaling and sustained transcriptional activity indicated by increased histone acetylation, chromatin accessibility, and p65 and Pol II occupancy at the IL6 promoter. Furthermore, FLS expressed low levels of the feedback inhibitors ABIN3, IRAK-M, SOCS3 and ATF3 that terminate inflammatory responses in macrophages. Conclusions TNFα signaling is not effectively terminated in FLS, leading 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.

Type I interferons are pleiotropic cytokines with antiviral, antitumor and immunoregulatory functions. An aspect of their complex biology is the paradox that, depending on context, type I interferons can be anti-inflammatory and tissue protective or can be proinflammatory and promote autoimmunity. Along these lines, the activation of type I interferon pathways is effective in suppressing disease activity in patients with multiple sclerosis and in animal models of arthritis and colitis, while there is an expectation that blockade of the same pathways will be beneficial in the treatment of patients with systemic lupus erythematosus.