Tumor necrosis factor-alpha mediates RANK ligand stimulation of osteoclastdifferentiation by an autocrine mechanism

Osteoblasts or bone marrow stromal cells are required as supporting cells f or the in vitro differentiation of osteoclasts from their progenitor cells. Soluble receptor activator of nuclear factor-kappaB ligand (RANKL) in the presence of macrophage colony-stimulating factor (M-CSF) is capable of repl acing the supporting cells in promoting osteoclastogenesis. In the present study, using Balb/c-derived cultures, osteoclast formation in both systems - osteoblast/bone-marrow cell co-cultures and in RANKL-induced osteoclastog enesis - was inhibited by antibody to tumor necrosis factor-alpha (TNF-alph a), and was enhanced by the addition of this cytokine. TNF-alpha itself pro moted osteoclastogenesis in the presence of M-CSF. However, even at high co ncentrations of TNF-a the efficiency of this activity was much lower than t he osteoclastogenic activity of RANKL. RANKL increased the level of TNF-alp ha mRNA and induced TNF-alpha release from osteoclast progenitors. Furtherm ore, antibody to p55 TNF-alpha receptors (TNF receptors-1) (but not to p75 TNF-alpha receptors (TNF receptors-2) inhibited effectively RANKL- (and TNF -alpha() induced osteoclastogenesis. Anti-TNF receptors-1 antibody failed t o inhibit osteoclastogenesis in C57BL/6-derived cultures. Taken together, o ur data support the hypothesis that in Balb/c, but not in C57BL/6 (strains known to differ in inflammatory responses and cytokine modulation), TNF-alp ha is an autocrine factor in osteoclasts, promoting their differentiation, and mediates, at least in part, RANKL's induction of osteoclastogenesis. J. Cell. Biochem. 83: 70-83, 2001. (C) 2001 Wiley-Liss, Inc.