MOLECULAR MECHANISMS OF THE INDUCTION OF IL-12 AND ITS INHIBITION BY IL-10

Exogenously added IL-10 rapidly inhibited Staphylococcus aureus- or LP S-induced cytokine mRNA expression In human PBMCs and monocytes, with a maximal effect observed when IL-10 was added from 20 h before until 1 h after the addition of the inducers. Nuclear run-on assays revealed that the inhibition of IL-12 p40, IL-12 p35, and TNF-alpha was at the gene transcriptional level and that the addition of IL-10 to S, aureu s- or LPS-treated PBMCs did not affect mRNA stability, The inhibitory activity of IL-10 was abrogated by cycloheximide (CHX), suggesting the involvement of a newly synthesized protein(s), The addition of CHX at 2 h before S, aureus or LPS also inhibited the accumulation of IL-12 p40 mRNA, but did not inhibit IL-12,p35 and TNF-alpha mRNA, This findi ng suggests that p40 transcription is regulated through a de novo synt hesized protein factor(s), whereas the addition of CHX at 2 h after S, aureus activation caused superinduction of the IL-12 p40, IL-12 p35, and TNF-alpha genes. These results indicate that in human monocytes, t he mechanism(s) of IL-10 suppression of both IL-12 p40 and IL-12 p35 g enes is primarily seen at the transcriptional level, and that the indu ction of the IL-12 p40 and p35 genes have different requirements for d e novo protein synthesis.