Human myeloma cells promote the production of interleukin 6 by primary human osteoblasts

Interleukin-6 (IL-6) is an important growth and survival factor for myeloma cells. However, the identity of the cells producing IL-6 in vivo remains u nclear. Myeloma cells are found closely associated with sites of active bon e turnover, and cells of the osteogenic lineage, including bone marrow oste oprogenitors, osteoblasts and bone lining cells, may therefore be ideally p laced to synthesize IL-6. We have examined the possibility that human osteo genic cells may produce IL-6 in response to stimulation by myeloma cells. P rimary human osteoblasts (hOBs) were isolated from normal donors, co-cultur ed with the human myeloma cell lines, JJN-3, RPMI-8226 and NCI-H929, and th e amount of IL-6 released was determined by enzyme-linked immunosorbent ass ay (ELISA). All myeloma cells stimulated a significant increase in the prod uction of IL-6 when cultured with hOBs (P < 0.05). Prior fixation of hOBs c ompletely abrogated release of IL-6 in the co-cultures. In contrast, fixed myeloma cells retained the ability to induce IL-6 production, suggesting th at hOBs were the principal source of IL-6. Physical separation of myeloma c ells from hOBs using transwell inserts caused a partial inhibition of IL-6 release (P < 0.05), whereas the addition of media conditioned by myeloma ce lls to cultures of hOBs stimulated a significant increase in IL-6 productio n (P < 0.05). hOBs secreted greater amounts of IL-6 than human bone marrow stromal cells (hBMSCs) (2.2- to 3.5-fold, P < 0.05), but incubating hBMSCs with dexamethasone to stimulate osteoblastic differentiation resulted in an increase in their ability to produce IL-6 (1.7- to 4.8-fold, P < 0.05) and to respond to myeloma cells (P < 0.05). These data clearly indicate that c ells of the osteoblast lineage release significant amounts of IL-6 in respo nse to stimulation by myeloma cells and may contribute to the IL-6 that pro motes the proliferation and survival of myeloma cells in vivo.