The anti-inflammatory cytokines IL-4 and IL-10 are well recognized as important negative regulators of proinflammatory gene expression in mononuclear phagocytes. The intracellular mechanisms which mediate these responses appear to be multifactorial. IL-4 is able to markedly suppress transcriptional activation of IFNγ-responsive genes and the promoter sequences required for both IFNγ and IL-4 sensitivity are identical. IFNγ-activated STAT1 and IL-4-activated STAT6 can both form complexes on the same regulatory sequence element; while STAT1 functions to promote transcription, STAT6 is inactive. STAT6 is, however, required for the suppressive activity of IL-4. In this model, IL-4 appears to suppress IFNγ-inducible proinflammatory gene expression through the ability of STAT6 to compete with STAT1 for occupancy of promoter sites necessary for IFNγ-induced transcriptional initiation. In a second model, IL-10 suppresses the expression of genes induced in LPS-stimulated macrophages through a pathway involving destabilization of specific mRNAs. We have demonstrated that nucleotide sequences in the 3′-untranslated region of an IL-10-sensitive gene can both destabilize a stable reporter mRNA (CAT) and confer sensitivity to IL-10-mediated destabilization. Deletion and site-specific mutagenesis have mapped this to an AU-rich sequence motif similar to that found in many cytokine and growth factor mRNAs. IL-10 is able to modulate the activity of proteins capable of binding to this sequence and one or more of these may regulate the rate of mRNA degradation. Thus mechanisms through which IL-10 and IL-4 act to dampen inflammatory responses are mechanistically distinct and involve diverse intracellular signaling pathways.

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