Human interleukin (IL)-9 specifically stimulates proliferation of CD34+++DR+CD33- erythroid progenitors in normal human bone marrow in the absence of serum.

The cDNA encoding human interleukin (IL)-9 has recently been cloned and the recombinant molecule found to enhance erythroid colony formation in vitro by bone marrow, peripheral blood, and cord blood cells. In our present report, recombinant human (rhu) IL-9 was evaluated, alone and in combination with other cytokines, for its effect on colony formation by erythroid progenitor (erythroid burst-forming units, BFU-E) and precursor (erythroid colony-forming units, CFU-E) cells in low density (LD), nonadherent LD density T-lymphocyte-depleted (NALT-), and immunofluorescence-sorted CD34+++DR+ and CD34+++DR+CD33- cells from normal human bone marrow. When highly enriched CD34+++DR+ and CD34+++DR+CD33- cells were plated at 200 and 100 cells/ml in the presence of 5% (vol/vol) 5637-cell-conditioned medium and erythropoietin (Epo) under serum-containing conditions, 46 and 51 day-14 BFU-E were observed, respectively. The enhancing effect of rhuIL-9 was similar to that of 5637 CM on colony formation by Epo-dependent BFU-E and CFU-E in these enriched sorted CD34+++DR+ and CD34+++DR+CD33- cells under serum-containing and serum-depleted culture conditions. No significant synergistic or additive effect of rhuIL-9 was noted when used in conjunction with rhu interleukin 3 (rhuIL-3), rhu interleukin 6 (rhuIL-6), and/or rhu granulocyte-macrophage colony-stimulating factor (rhuGM-CSF) under the same culture conditions. The cloning enhancing effect elicited by human IL-9 is Epo dependent, although IL-9 alone sustains the survival of erythroid progenitor cells in vitro, as assessed by delayed additions of Epo to the cultures. The ability of human IL-9 to stimulate BFU-E and CFU-E colony formation using low numbers of highly enriched progenitor cells in serum-depleted conditions demonstrates the direct effect of IL-9 on erythroid progenitors and implicates its potential role in the enhancement of erythropoiesis.