The recently cloned interleukin 13 (IL-13) shares most investigated biological activities on B lymphocytes and monocytes with IL-4. In this study we investigated for the first time the potential role of IL-13 in the regulation of the growth of hematopoietic progenitor cells. IL-13 enhanced stem cell factor (SCF)-induced proliferation of Lin-Sca-1+ bone marrow progenitor cells more potently than IL-4. The effect of IL-13 was purely synergistic, since IL-13 alone stimulated no colony formation. Single cell experiments suggested that the synergistic effect of IL-13 on Lin-Sca-1+ progenitors was directly mediated. In contrast, IL-13 had no synergistic activity on SCF-induced proliferation of the more mature Lin-Sca-1- progenitor cells. Thus, the cloning frequency in response to SCF + IL-13 was at least 20-fold higher in the Lin-Sca-1+ than the Lin-Sca-1- progenitor cell population. Furthermore, IL-13 but not IL-4 synergistically enhanced colony formation of Lin-Sca-1+ progenitors in response to granulocyte/macrophage colony-stimulating factor (GM-CSF) (threefold), whereas both IL-4 and IL-13 enhanced G-CSF-induced colony formation (threefold), and neither of the two significantly affected CSF-1 and IL-3-induced proliferation. Finally, whereas stimulation of Lin-Sca-1+ progenitors by SCF + G-CSF resulted in the formation of 90% granulocytes, the addition of IL-13 resulted in the production of macrophages exclusively. This novel effect on differentiation was directly mediated, shared with IL-4, and could not be observed on Lin-Sca-1- progenitor cells. Collectively, these findings indicate a novel role of IL-13 in early myelopoiesis, partially overlapping but also different from that of IL-4.

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