TNFSF11 and periodontitis: RANKL stimulates bone resorption and is upregulated by IL-1β and IL-6 among other cytokines; thus the ratio of RANKL and its natural antagonist osteoprotegerin (OPG) is a particularly important factor in determining bone cell resorption and turnover [52, 54] and this ratio is elevated in periodontitis [24, 36].