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

Despite advances in the treatment of rheumatoid arthritis (RA), currently approved medications can have significant side effects due to their direct immunosuppressive activities. Additionally, current therapies do not address residual synovial inflammation. In this study, we evaluated the role of integrin alpha 9 and its ligand, tenascin-C (Tn-C), on the proliferative and inflammatory response of fibroblast-like synoviocytes (FLSs) from RA patients grown in three-dimensional (3D)-micromass culture. FLSs from osteoarthritis patients, when grown in the 3D-culture system, formed self-directed lining-like structures, whereas FLSs from RA tissues (RA-FLSs) developed an abnormal structure of condensed cellular accumulation reflective of the pathogenic features of RA synovial tissues. Additionally, RAFLSs grown in 3D culture showed autonomous production of proinflammatory mediators. Predominant expression of alpha 9 and Tn-Cwas observed in the condensed lining, and knockdown of these molecules abrogated the abnormal lining-like structure formation and suppressed the spontaneous expression of matrix metalloproteinases, IL-6, TNFSF11/RANKL, and cadherin-11. Disruption of alpha 9 also inhibited expression of Tn-C, suggesting existence of a positive feedback loop in which the engagement of alpha 9 with Tn-C self-amplifies its own signaling and promotes progression of synovial hyperplasia. Depletion of alpha 9 also suppressed the platelet-derived growth factorinduced hyperplastic response of RA-FLSs and blunted the TNF-alpha-induced expression of matrix metalloproteinases and IL-6. Finally, alpha 9-blocking Ab also suppressed the formation of the condensed cellular lining by RA-FLSs in 3D cultures in a concentration-related manner. This study demonstrates the central role of alpha 9 in pathogenic behaviors of RA-FLSs and highlights the potential of alpha 9-blocking agents as a nonimmunosuppressive treatment for RA-associated synovitis.