A number of in vitro studies carried out in our laboratory over the past ten years have led to some clarification of the role of mononuclear phagocytes in hematopoietic regulation. The results of these studies have demonstrated that mononuclear phagocytes produce proteins, notably interleukin-1 (IL-1), that induce the expression of multilineage hematopoietic growth factors by human vascular endothelial cells, fibroblasts, T-lymphocytes, and thymic epithelial cells. More recently we and others have identified these induced factors as G-CSF, GM-CSF, IL-6, and IL-1. Although IL-1 seems to stimulate expression of these genes by inducing the accumulation of gene transcripts, interestingly the accumulation results from prolongation of mRNA half-life. We propose that the inductive capacity of IL-1 results from its activation of ribonuclease inhibitory activity in the cytoplasm of IL-1 induced cells and hypothesize that this may be a general mechanism by which IL-1 induces gene expression.
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