Human interleukin-9 (IL-9) stimulates the proliferation of primitive hematopoietic erythroid and pluripotent progenitor cells, as well as the growth of selected colony-stimulating factor (CSF)-dependent myeloid cell lines. To further address the role of IL-9 in the development of acute leukemia, we evaluated the proliferative response of three leukemic cell lines and 32 primary samples from acute myeloblastic leukemia (AML) patients to recombinant human (rh)-IL-9 alone and combined with rh-IL-3, granulocyte-macrophage CSF (GM-CSF), and stem cell factor ([SCF] c-kit ligand). The colony-forming ability of HL60, K562, and KG1 cells and fresh AML cell populations upon IL-9 stimulation was assessed by a clonogenic assay in methylcellulose, whereas the cell-cycle characteristics of leukemic samples were determined by the acridine-orange flow cytometric technique and the bromodeoxyuridine (BRDU) incorporation assay. In addition, the terminal deoxynucleotidyl transferase assay (TDTA) and standard analysis of DNA cleavage by gel electrophoresis were used to evaluate induction of prevention of apoptosis by IL-9. Il-9, as a single cytokine, at various concentrations stimulated the colony formation of the three myeloid cell lines under serum-containing and serum-free conditions, and this effect was completely abrogated by anti-IL-9 monoclonal antibodies (MoAbs). When tested on fresh AML samples, optimal concentrations of IL-9 resulted in an increase of blast colony formation in all the cases studied (mean +/- SEM: 19 +/- 10 colony-forming unit-leukemic [CFU-L]/10(5) cells plated in control cultures v 107 +/- 32 in IL-9-supplemented dishes, P < .02). IL-9 stimulated 36.8% of CFU-L induced by phytohemagglutinin-lymphocyte-conditioned medium (PHA-LCM), and it was the most effective CSF for promoting leukemic cell growth among those tested in this study (i.e., SCF, IL-3, and GM-CSF). The proliferative activity of IL-9 was also observed when T-cell-depleted AML specimens were incubated with increasing concentrations of the cytokine. Addition of SCF to IL-9 had an additive or synergistic effect of the two cytokines in five of eight AML cases tested for CFU-L growth (187 +/- 79 colonies v 107 +/- 32 CFU-L, P = .05). Positive interaction was also observed when IL-9 was combined with IL-3 and GM-CSF. Studies of cell-cycle distribution of AML samples demonstrated that IL-9 alone significantly augmented the number of leukemic cells in S-phase in the majority of cases evaluated. IL-9 and SCF in combination resulted in a remarkable decrease of the G0 cell fraction (38.2% +/- 24% v 58.6% +/- 22% of control cultures, P < .05) and induced an increase of G1- and S-phase cells. Conversely, neither IL-9 alone nor the combination of IL-9 and SCF had any effect on induction or prevention of apoptosis of leukemic cells. In summary, our results indicate that IL-9 may play a role in the development of AML by stimulating leukemic cells to enter the S-phase rather than preventing cell death. Moreover, IL-9 acts synergistically with SCF for recruiting quiescent leukemic cells in cell cycle.