SPP1 and rheumatoid arthritis: Integrin alpha-9 (α9) binds to ECM proteins such as tenascin-C [27], protease-cleaved osteopontin [28], and VCAM-1 [29]; contributes to cell adhesion and migration [30, 31]; and is highly expressed in synovial tissue cells, especially FLS, in patients with RA [32, 33].