EBI3 and rheumatoid arthritis: To examine the expression of EBI3 and its relevance to ER stress in MSCs under inflammatory conditions in vivo, we firstly stained synovial tissue from RA and osteoarthritis (OA) patients with antibodies to detect CD271 (a promising marker for MSC), CD105 (a marker used for MSC isolation), and EBI3 (S6 Fig).