TNFSF11 and rheumatoid arthritis: An in vitro study demonstrated that the exposure of RA fibroblast-like synviocytes to 1,25(OH)2D3 (in doses ranging from 0.1 to 100 nM, therefore equivalent to sub-physiological and optimal vitamin D serum concentrations) decreased TNF-α and IL-6 expression and at the same time, reduced osteoclastogenic RANKL (receptor activator of nuclear factor kappa-B ligand) levels relative to OPG (osteoprotegerin), an inhibitor of RANKL activity (59).