Recombinant gibbon interleukin-3 (IL-3) is a multilineage hematopoietic colony-stimulating factor (CSF) that recently was cloned and found to be highly homologous with human IL-3. Gibbon IL-3, as well as human granulocyte-CSF (G-CSF) and human granulocyte-macrophage CSF (GM-CSF), stimulated normal human bone marrow cells to form myeloid colonies in soft agar in a sigmoidal dose-response manner. When IL-3 was added to increasing concentrations of G-CSF or GM-CSF, synergistic colony formation occurred as compared with the effects of each CSF alone. Synergism was also noted when G-CSF was added with GM-CSF and when all the CSFs were added simultaneously. The combination of IL-3 and GM-CSF was less stimulatory than all the other CSF combinations. At day 11 of culture, IL-3 induced granulocyte-macrophage (38%), eosinophil (30%), granulocyte (18%), and macrophage (14%) colony formation. In summary, gibbon IL-3 is a growth factor that can synergize with other CSFs to enhance proliferation of myeloid-committed progenitors, suggesting that combinations of CSFs may have clinical utility in patients with neutropenia of various etiologies.

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