IL-6 MEDIATES THE EFFECTS OF IL-1 OR TNF, BUT NOT PTHRP OR 1,25(OH)(2)D-3, ON OSTEOCLAST-LIKE CELL-FORMATION IN NORMAL HUMAN BONE-MARROW CULTURES

A potent interleukin-6 (IL-6) antagonist (Sant 5), which binds tightly to the IL-6 alpha receptor but has impaired gp130 heterodimerization, has been developed recently by site-directed mutagenesis of human IL- 6. We report here that Sant 5 inhibits IL-6-stimulated osteoclast-like multinucleated cell (MNC) formation in human marrow cultures but also inhibits the stimulatory effects of IL-1 or tumor necrosis factor alp ha (TNF-alpha in these cultures, We further show that a neutralizing a ntibody to IL-6 also inhibits the stimulatory effects of IL-1 or TNF-a lpha in these cultures. In contrast, Sant 5 had no effect on parathyro id hormone related protein (PTHrP) or 1,25-dihydroxyvitamin D-3 stimul ated MNC formation in human marrow cultures, Transfection of a human m arrow stromal cell line, which normally induces osteoclast formation t hrough production of IL-6, with the Sant 5 cDNA driven by a cytomegalo virus (CMV) promoter blocked the capacity of these cells to stimulate osteoclast-like cell formation, These Sant 5 transfected cells and con ditioned media from these cells also inhibited the stimulatory effects of the parent cell line on MNC formation, These data suggest that IL- 6 mediates the effects of IL-1 and TNF on human osteoclast formation, but in contrast to murine systems, does not mediate the effects of PTH rP, These data further demonstrate that stromal cells transfected with the Sant 5 cDNA can constitutively produce high levels of the IL-6 an tagonist and inhibit osteoclast formation in vitro.