We examined the effects of interleukin (IL)-13 and IL-4 on growth of hematopoietic progenitors from 5-fluorouracil (5-FU)-treated mice, using an in vitro culture system. IL-13 or IL-4 alone failed to support colony formation by enriched marrow cells. IL-4 but not IL-13 in combination with IL-11 yielded a significant number of colonies. Neither IL-4 nor IL-13 affected colony formation supported by IL-3. When tested with two-factor combinations, IL-4 but not IL-13 suppressed the formation of colonies including multilineage colonies in the absence of IL-11. The inhibitory effects of IL-4 were not lineage-specific. Delayed addition experiments demonstrated that IL-4 is inhibitory in the early stage of colony formation. Effects of IL-4 on colony formation by pooled blast cells derived from 5-day culture of post-5-FU marrow cells was not evident. These findings indicate that IL-4 but not IL-13 is a negative regulator of hematopoietic progenitors, suggesting distinctive roles for IL-4 and IL-13 in early hematopoiesis.

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