Recombinant murine interleukin 9 enhances the erythropoietin-dependent colony formation of human BFU-E

Murine interleukin 9 (mIL-9) is a novel T-cell-derived lymphokine previously described as a T-cell growth factor (P40/TCGFIII) and as a mast cell growth-enhancing activity (MEA). In the present study we examined the potency of recombinant (r)mIL-9 to exhibit hemopoietic growth factor activity in the human system. In semisolid cultures of normal human bone marrow-derived mononuclear cells, rmIL-9 alone at a concentration range from 25 to 200 U/ml did not reveal any colony-stimulating activity on human granulocyte-macrophage colony-forming cells (GM-CFC), erythroid colony-forming units (CFU-E), and erythroid burst-forming units (BFU-E). Furthermore, we did not observe synergistic effects of rmIL-9 on the number, size, and morphological composition of human granulocyte-macrophage colonies in cultures stimulated with giant cell tumor-conditioned medium. However, a synergistic effect of rmIL-9 in the human erythropoietic culture system was clearly demonstrated in the presence of recombinant human erythropoietin (rhEpo). Recombinant murine IL-9 at a concentration of 200 U/ml enhanced the number of BFU-E-derived day-14 colonies about 3.6-fold as compared to control cultures stimulated with Epo alone. The formation of CFU-E-derived day-7 colonies was not significantly altered under the same conditions. Our results demonstrate that in the presence of rhEpo, rmIL-9 is synergistically active in human bone marrow cultures as an erythroid burst-promoting factor. The development of granulocyte-macrophage colonies obviously is not affected. This finding strongly suggests that mIL-9 can mediate signals via human IL-9 receptors and further extends the range of biological activities hitherto ascribed to mIL-9.