Although there have been major advances in the treatment of rheumatoid arthritis with the advent of biological agents, the mechanisms that drive cytokine production and sustain disease chronicity remain unknown. Tenascin-C (encoded by Tnc) is an extracellular matrix glycoprotein specifically expressed at areas of inflammation and tissue damage in inflamed rheumatoid joints. Here we show that mice that do not express tenascin-C show rapid resolution of acute joint inflammation and are protected from erosive arthritis. Intra-articular injection of tenascin-C promotes joint inflammation in vivo in mice, and addition of exogenous tenascin-C induces cytokine synthesis in explant cultures from inflamed synovia of individuals with rheumatoid arthritis. Moreover, in human macrophages and fibroblasts from synovia of individuals with rheumatoid arthritis, tenascin-C induces synthesis of proinflammatory cytokines via activation of Toll-like receptor 4 (TLR4). Thus, we have identified tenascin-C as a novel endogenous activator of TLR4-mediated immunity that mediates persistent synovial inflammation and tissue destruction in arthritic joint disease.

Toll-like receptors (TLRs) and their downstream signaling pathways have been comprehensively characterized in innate immunity. In addition to this function, these receptors have also been suggested to be involved in the pathogenesis of many autoimmune diseases, including rheumatoid arthritis (RA). Murine in vivo models and human in vitro tissue models of RA have provided a wealth of information on the potential activity of TLRs and components of the downstream signaling pathways. Whilst most early work investigated the cell surface TLRs, more recently the focus has moved to the endosomal TLRs 3, 7, 8, and 9. These receptors recognize self and foreign double-stranded RNA and single-stranded RNA and DNA. The development of therapeutics to inhibit the endosomal TLRs or components of their signaling cascades may represent a way to target inflammation upstream of cytokine production. This may allow for greater specificity than existing therapies including cytokine blockade. Here, we review the current information suggesting a role for the endosomal TLRs in RA pathogenesis and the efforts to target these receptors therapeutically.

The mechanism by which oxidative stress induces inflammation and vice versa is unclear but is of great importance, being apparently linked to many chronic inflammatory diseases. We show here that inflammatory stimuli induce release of oxidized peroxiredoxin-2 (PRDX2), a ubiquitous redox-active intracellular enzyme. Once released, the extracellular PRDX2 acts as a redoxdependent inflammatory mediator, triggering macrophages to produce and release TNF-α. The oxidative coupling of glutathione (GSH) to PRDX2 cysteine residues (i.e., protein glutathionylation) occurs before or during PRDX2 release, a process central to the regulation of immunity. We identified PRDX2 among the glutathionylated proteins released in vitro by LPS-stimulated macrophages using mass spectrometry proteomic methods. Consistent with being part of an inflammatory cascade, we find that PRDX2 then induces TNF-α release. Unlike classical inflammatory cytokines, PRDX2 release does not reflect LPS-mediated induction of mRNA or protein synthesis; instead, PRDX2 is constitutively present in macrophages, mainly in the reduced form, and is released in the oxidized form on LPS stimulation. Release of PRDX2 is also observed in human embryonic kidney cells treated with TNF-α. Importantly, the PRDX2 substrate thioredoxin (TRX) is also released along with PRDX2, enabling an oxidative cascade that can alter the -SH status of surface proteins and thereby facilitate activation via cytokine and Toll-like receptors. Thus, our findings suggest a model in which the release of PRDX2 and TRX from macrophages can modify the redox status of cell surface receptors and enable induction of inflammatory responses. This pathway warrants further exploration as a potential novel therapeutic target for chronic inflammatory diseases.cysteine oxidation | thiol oxidation | redox proteomics

IntroductionThe development of novel therapies for sepsis depends on the understanding of the basic mechanisms of the disease. 1 The principal active agent involved in the pathogenesis of sepsis is bacterial lipopolysaccharide (LPS), an essential component of the surface of gram-negative bacteria. LPS exerts its toxic effects by potently activating macrophages and endothelial cells, and inducing the expression of inflammatory cytokines such as tumor necrosis factor ␣ (TNF␣) and interleukin 6 (IL-6). [2][3][4][5] Thus, elucidating how LPS signals through cell-surface receptors to induce inflammatory gene expression in humans is of major importance.Central to the recognition of LPS and also many other microbial products by the host is a family of transmembrane proteins that have leucine-rich repeats in their extracellular domains known as the toll-like receptors (TLRs). 6 LPS interacts with a heterologous receptor that contains TLR4 7,8 as well as CD14 9,10 and MD2. [11][12][13] As CD14 is a glycosyl phosphatidylinositol-anchored protein and MD2 is on the cell surface, transduction of the LPS signal across the membrane is mediated by TLR4. TLR4, as all TLR family members, contains a cytoplasmic domain that is homologous to a cytoplasmic domain found in the IL-1 receptor known as the Toll/IL-1 receptor (IL-1R) homology (TIR) domain that is essential for downstream signaling. [14][15][16] The presence of the TIR domain in both TLR and IL-1 receptor family members suggested that these receptors use an identical framework of signaling molecules to exert their downstream effects. This was supported by subsequent studies in mouse and human cell lines. Thus, IL-1R and TLR4 were shown to engage the TIR-containing cytosolic adaptor molecule myeloid differentiation protein 88 (MyD88) through homotypic interactions, [17][18][19] with subsequent recruitment of IL-1R-associated kinase (IRAK) and IRAK2, IRAK4, and TRAF6. 17,18,20,21 TRAF6 is thought to subsequently activate nuclear factor (NF)-B either through the IB kinase (IKK) complex and the kinases TAB-1 and TAK-1, 22 or through evolutionarily conserved signaling intermediate in Toll pathways (ECSIT) and mitogen-activated protein kinase/ERK kinase kinase 1 (MEKK-1). 23 The recent derivation of MyD88 Ϫ/Ϫ mice, however, challenged a universal role for MyD88 in LPS signaling. Although there was the expected complete ablation of IL-1 signaling, LPS still activated NF-B although the ability to induce TNF␣ from macrophages was lost. 24 In addition, LPS-induced NF-B activation and up-regulation of costimulatory molecules in bone marrowderived dendritic cells from these mice was not compromised. 25 To account for a MyD88-independent pathway of NF-B activation, a novel MyD88 homologue termed MyD88 adaptor-like (Mal) 26 or TIR domain-containing adaptor protein (TIRAP) 27 was described. This was shown to act as an adaptor protein specifically involved in TLR4 but not other TLRs or IL-1R-induced NF-B activation. 26,27 As Mal/TIRAP does not contain the death domain (DD) found in MyD88...