SPP1 and rheumatoid arthritis: OPN, an extracellular matrix phosphor-glycoprotein, has long been recognized as a key mediator in Th1-related immunity and is involved in Th1-related diseases, including systemic lupus erythematosus (SLE), multiple sclerosis (MS), inflammatory bowel disease (IBD), and rheumatoid arthritis (RA) [21,22,23,24,25].