INTERLEUKIN-18 (INTERFERON-GAMMA-INDUCING FACTOR) IS PRODUCED BY OSTEOBLASTS AND ACTS VIA GRANULOCYTE MACROPHAGE COLONY-STIMULATING FACTOR AND NOT VIA INTERFERON-GAMMA TO INHIBIT OSTEOCLAST FORMATION/

We have established by differential display polymerase chain reaction of mRNA that interleukin (IL)-18 is expressed by osteoblastic stromal cells. The stromal cell populations used for comparison differed in th eir ability to promote osteoclast-like multinucleated cell (OCL) forma tion. mRNA for IL-18 was found to be expressed in greater abundance in lines that were unable to support OCL formation than in supportive ce lls. Recombinant IL-18 was found to inhibit OCL formation in coculture s of osteoblasts and hemopoietic cells of spleen or bone marrow origin . IL-18 inhibited OCL formation in the presence of osteoclastogenic ag ents including 1 alpha,25-dihydroxyvitamin D-3, prostaglandin E(2), pa rathyroid hormone, IL-1, and IL-11. The inhibitory effect of IL-18 was limited to the early phase of the cocultures, which coincides with pr oliferation of hemopoietic precursors. IL-18 has been reported to indu ce interferon-gamma (IFN-gamma) and granulocyte/macrophage colony-stim ulating factor (GM-CSF) production in T cells, and both agents also in hibit OCL formation in vitro. Neutralizing antibodies to GM-CSF were a ble to rescue IL-18 inhibition of OCL formation, whereas neutralizing antibodies to IFN-gamma did not. In cocultures with osteoblasts and sp leen cells from IFN-gamma receptor type II-deficient mice, IL-18 was f ound to inhibit OCL formation, indicating that IL-18 acted independent ly of IFN-gamma production: IFN-gamma had no effect in these coculture s. Additionally, in cocultures in which spleen cells were derived from receptor-deficient mice and osteoblasts were from wild-type mice and vice versa, we identified that the target cells for IFN-gamma inhibiti on of OCL formation were the hemopoietic cells. The work provides evid ence that IL-18 is expressed by osteoblasts and inhibits OCL formation via GM-CSF production and not via IFN-gamma production.