To determine the mechanism(s) by which interleukin-1 (IL-1) promotes granulopoiesis in vivo, we examined the effect of in vivo administration of IL-1 alpha on colony-stimulating factor (CSF) receptor expression on bone marrow cells (BMCs) and whether this directly correlated with progenitor cell responsiveness. Administration of IL-1 alpha to mice induced the upregulation of both granulocyte- macrophage-CSF (GM-CSF) and IL-3 receptors, which reached a maximum 24 hours after IL-1 alpha injection on unfractionated BMCs. This upregulation was more pronounced on the progenitor-enriched cell population (lineage-negative [Lin(-)]). The enhanced GM-CSF and IL-3 receptor expression directly correlated with enhanced IL-3- or GM-CSF- induced growth of colony-forming unit-culture (CFU-c) or CFU-mixture (CFU-Mix; colonies containing macrophages, granulocytes, and erythroid cells). In addition, the absolute number of high proliferative potential-colony-forming cells (HPP-CFC) was increased fivefold. In contrast, granulocyte-CSF (G-CSF)-specific binding on unfractionated BMCs was rapidly (4 hours) reduced after IL-1 alpha administration and returned to control levels by 24 hours. This reduction correlated with IL-1 alpha-induced margination of mature granulocytes (RBC-8C5hi cells), which express high levels of G-CSF receptors. IL-1 alpha treatment did not affect G-CSF receptor expression on Lin- cells. Pretreatment of mice with anti-type I IL-1 receptor antibody blocked the IL-1 alpha-induced upregulation of GM-CSF and IL-3 receptor expression on BMCs. Taken together, as one possible mechanism, IL-1 alpha in vivo may stimulate the expression of functional GM-CSF and IL- 3 receptors on BMCs indirectly, and, in concert with the induction of circulating CSF levels, may account for the ability of IL-1 alpha to stimulate hematopoiesis in vivo.

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