Antibodies to citrulline-modified proteins have a high diagnostic value in rheumatoid arthritis (RA). However, their biological role in disease development is still unclear. To obtain insight into this question, a panel of mouse monoclonal antibodies was generated against a major triple helical collagen type II (CII) epitope (position 359–369; ARGLTGRPGDA) with or without arginines modified by citrullination. These antibodies bind cartilage and synovial tissue, and mediate arthritis in mice. Detection of citrullinated CII from RA patients' synovial fluid demonstrates that cartilage-derived CII is indeed citrullinated in vivo. The structure determination of a Fab fragment of one of these antibodies in complex with a citrullinated peptide showed a surprising β-turn conformation of the peptide and provided information on citrulline recognition. Based on these findings, we propose that autoimmunity to CII, leading to the production of antibodies specific for both native and citrullinated CII, is an important pathogenic factor in the development of RA.

Collagen type II (CII) is a relevant joint‐specific autoantigen in the pathogenesis of rheumatoid arthritis (RA). Whereas the reasons for the breakage of self tolerance to this major cartilage component are still enigmatic, T cell responses to glycosylated CII determinants in RA patients indicate that post‐translational modifications play a role. Since the conversion of arginine into citrulline by peptidylarginine deiminases (PAD) in some non‐joint‐specific antigens such as filaggrin or fibrin has been shown to give rise to RA‐specific humoral immune responses, we investigated whether PAD modification of cartilage‐specific CII might affect its recognition by circulating autoantibodies in early RA. In vitro treatment with purified PAD led to arginine deimination of native CII or of synthetic CII peptides as evidenced by amino acid analysis. The citrullination resulted in modified recognition of the immunodominant CII epitope C1III (amino acid residues 359–369) by murine and human antibodies. In a cohort of early RA patients (n=286), IgG antibodies directed toward a synthetic citrullinated C1III peptide (citC1III‐P) were detectable with a prevalence of 40.4%. The partial autoantibody cross‐reactivity between citC1III‐P and citrullinated peptides mimicking epitopes of the cytoskeletal autoantigen filaggrin suggests that autoimmunity to cartilage‐specific modified self might be a critical intermediate bridging recognition of PAD‐modified extra‐articular autoantigens with the disruption of tolerance to native cartilage constituents.

IdeS is a streptococcal protease that cleaves IgG antibodies into F(ab’)2 and Fc fragments with a unique degree of specificity, thereby providing a novel treatment opportunity of IgG-driven autoimmune conditions and antibody mediated transplant rejection. Here we report the results from a first in man, double blinded and randomized study with single ascending doses of IdeS in healthy, male subjects. Twenty healthy subjects were given intravenous single ascending doses of IdeS. With impressive efficacy IdeS cleaved the entire plasma IgG-pool only minutes after dosing. IgG reached nadir 6-24 hours after dosing and then slowly recovered. The half-life of IdeS was 4.9 (±2.8) hours at 0.24 mg/kg with the main fraction eliminated during 24 hours. Already two hours after IdeS-dosing, the phagocytic capacity of IgG/IgG-fragments was reduced to background levels. Importantly, IdeS has the capacity to inactivate Fc-mediated effector function in vivo, was considered safe with no serious adverse events, and without dose limiting toxicity in this study. The complete, rapid, but temporary removal of IgG provides a new potent therapeutic opportunity in IgG-mediated pathogenic conditions.Trial RegistrationClinicalTrials.gov NCT01802697

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Mesenchymal stem cells (MSCs) are multipotent cells that have the capacity to differentiate into various different cell lineages and can generate bone, cartilage and adipose tissue. MSCs are presently characterized using a broad range of different cell-surface markers that are not exclusive to MSCs and not sensitive to culture conditions or differentiation capacity. We show that the integrin subunits alpha10 and alpha11 of the collagen binding integrins alpha10beta1 and alpha11beta1 are expressed by human MSCs in monolayer cultures. We also demonstrate that the expression of alpha10 increases, while alpha1 and alpha11 decrease, during aggregate culture of MSCs in chondrogenic medium. Alpha10beta1 is expressed by chondrocytes in cartilage, whereas alpha11beta1 integrins are predominantly expressed by subsets of the fibroblastic lineage. In extensive monolayer cultures of MSCs, alpha10 expression is down-regulated. We show that this down-regulation is reversed by fibroblast growth factor-2 (FGF-2) treatment. Addition of FGF-2 to MSCs not only results in increased alpha10 expression, but also in decreased alpha11 expression. FGF-2 treatment of MSCs has been shown to keep the cells more multipotent and also induces cell proliferation and Sox-9 up-regulation. We demonstrate improved chondrogenecity as well as increased collagen-dependant migratory potential of FGF-2-treated MSCs having a high alpha10 expression. We also demonstrate expression of alpha10 and alpha11 integrin subunits in the endosteum and periosteum of mice, but very low or not detectable expression levels in freshly aspired human or mouse BM. We show that MSCs with high chondrogenic differentiation potential are highly alpha10 positive and propose alpha10 as a potential marker to predict the differentiation state of MSCs.