CTSK and rheumatoid arthritis: Upon co-culture of isolated monocytes with IL-26-pretreated RA-FLSs in the absence of RANKL, we observed differentiation of TRAP+ multinucleated cells (Fig. 6a) accompanied by significant increases in the osteoclast markers TRAP, cathepsin K, NFATc1, DC-STAMP, OC-STAMP, and ATP6Vod2 (Fig. 6b; **P < 0.01, ***P < 0.001).