TNF and rheumatoid arthritis: Stimulation with IL-26 increased RANKL mRNA (Fig. 2a, b) and protein (Fig. 2c) levels in a dose-dependent manner; however, we found that stimulation of RA-FLSs with IL-26 did not change IL-1β, IL-6, and TNF-α levels in the cultured media (Additional file 1: Figure S2).