Indeed, TRAP+ multinucleated OCs were significantly increased in co-culture monocytes with RA SF CD19+CD24hiCD27+ B cells compared with RA PB CD19+CD24hiCD27+ B cells, co-culture with OA SF CD19+CD24hiCD27+ B cells and monocytes alone (Fig. 5b–e, g). Here, ACP5 is linked to rheumatoid arthritis.