Stromal cell-derived cytokines such as IL-11, which utilize the gp130 signal transducer may be pivotal in osteoclast development. To study the regulation of IL-11 production by primary murine osteoblasts, we used a new bioassay employing transfected Ba/F3 cells which expressed high affinity IL-11 receptors. Recombinant human IL-11 specifically stimulated proliferation of these cells in a dose dependent fashion. In order to define the role of gp130-mediated signals in osteoclastogenesis induced by various osteotropic factors, we studied the effect of neutralizing mouse monoclonal gp130 antibodies (mAb) on osteoclast formation induced by each factor in co-cultures of bone marrow and primary osteoblastic cells. Unstimulated primary osteoblasts did not secrete detectable IL-11 into the media. However, both IL-1α and TNFα (EC 5 0 0.1-1.0 ng/mL) induced IL-11 production in a dose and time dependent manner. This effect was largely prostaglandin dependent, and exogenous PGE2 itself potently induced IL-11 secretion (EC 5 0 <10nM). Other osteotropic cytokines such as IL-6, OSM and IL-4 did not individually affect IL-11 production. However, 1,25(OH)2D3 and PTH similarly induced IL-11. Further, the bioactivity of IL-11 accumulated in a time dependent manner in the supernatants of co-cultures of bone marrow and osteoblastic cells which were maintained in the presence of 10nM 1,25(OH)2D3 for 6 days. In contrast, unstimulated cultures did not generate IL-11, and osteoclasts were not formed. Primary osteoblastic cells were found to constitutively express mRNAs for both the IL-11 receptor α-subunit (IL-11R) and gp130. Interestingly, 1,25(OH)2D3 did not change the level of IL-11R mRNA, but the abundance of the steady-state gp130 mRNA was up-regulated 3-4 fold. In co-cultures, a neutralising mouse gp130 mAb consistently and completely suppressed the osteoclastogenic activity not only of IL-11 and IL-6-sIL-6 receptor complex but also of IL-1α. The effects of PGE2, 1,25(OH)2D3, and PTH were suppressed partially (50-90%). In conclusion, IL-1, TNF, PGE2, 1,25(OH)2D3 and PTH each induce IL-11 production by osteoblastic-stromal cells. These cells also express the functional subunits of the IL-11 receptor, implying an autocrine-paracrine action of IL-11. Moreover, our data indicate that the signals mediated by IL-11 and other cytokines which utilize the gp130 signal transducer are critical for osteoclast development induced by both local and systemic factors.