Constitutive Activation of Integrin α9 Augments Self-Directed Hyperplastic and Proinflammatory Properties of Fibroblast-like Synoviocytes of Rheumatoid Arthritis

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Objective. The purpose of this study was to evaluate the role of integrin α9 and its ligand, tenascin-C (Tn-C), on the proliferative and inflammatory response of fibroblast-like synoviocytes (FLSs) from RA patients grown in 3D-micromass culture. Methods. We maintained RA-FLSs in 3D-micromass culture system and examined the expression levels of integrin α9 and its ligand tenascin-C (Tn-C). We also used RNA interference and integrin α9 blocking antibody in this system, and examined the functional involvement of integrin α9 in disease-related behaviors of RA-FLSs including lining cell condensation and the proinflammatory phenotypes. Results. FLSs from osteoarthritis patients, when grown in the 3D-culture system formed self-directed lining-like structures whereas FLSs from RA patients (RA-FLSs) formed an abnormal condensed cellular accumulation that was consistent with the pathogenic features seen in RA synovial tissues. In addition, RA-FLSs grown in 3D-culture showed autonomous production of proinflammatory mediators. Expression of integrin α9 and Tn-C was observed in the condensed lining and knock-down of these molecules abrogated the abnormal lining-like structure formation as well as suppressed the spontaneous expression of MMPs, IL-6, TNFSF11/RANKL and cadherin-11. Disruption of integrin α9 also inhibited the expression of Tn-C, suggesting a positive feedback loop in which Tn-C via integrin α9 self-amplifies its signaling and promotes further synovial hyperplasia. Depletion of integrin α9 also suppressed the PDGF-induced hyperplastic response of RA-FLSs and blunted the TNF-α-induced expression of MMPs and IL-6. Finally, integrin α9 blocking antibody concentration-dependently suppressed the formation of the condensed cellular lining by RA-FLSs in 3D-cultures. Conclusion. Integrin α9 plays a central role in pathogenic behaviors of RA-FLSs and is a potential treatment target for RA associated synovitis.