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IL-10, originally described as a cytokine synthesis inhibitory factor, is secreted by a number of cells of the immune system, including monocytes and T cells. Although IL-10 is being assigned as an immunosuppressive cytokine, our study showed that FMLP-R mRNA was rapidly up-regulated by exposure of monocytes to graded concentrations of this cytokine, with maximal (three- to fourfold) stimulation with 10 ng/ml. The effect was rapid, being observable as early as 1 h of treatment with IL-10, maximal between 2 and 4 h, and still evident after 24 h and was associated with an increase of receptor expression on the cell surface as assessed by flow cytometry analysis. Pretreatment of monocytes with actinomycin D completely abrogated the effect of IL-10, suggesting a transcriptional regulation. Moreover, IL-10-treated monocytes showed a significantly enhanced functional responsiveness to FMLP with enhanced (three- to fourfold) chemotaxis and augmented (twofold) intracellular calcium mobilization. In polymorphonuclear neutrophils (PMN), IL-10 also mediated a twofold augmentation of FMLP-R expression. In parallel experiments, we observed that IL-10 could differentially modulate other chemotactic receptors. Hence, we observed that IL-10 augmented two-to threefold platelet-activating factor receptor (PAF-R) expression, whereas it had no significant effect on the fifth component of complement (C5a) receptor (C5a-R) expression. Collectively, our results demonstrate that IL-10 may play an important role in inflammatory process through modulation of chemotactic receptor expression.

The effect of PMA on early and late regulation of platelet-activating factor receptor (PAF-R) expression was examined in human monocytes. Treatment with 16 nM PMA for 5 min initiated a rapid reduction (50-75%) in [3H]WEB 2086 binding, which was maximal between 5 and 15 min. Scatchard analysis revealed that PMA treatment reduced the number of binding sites to 50% of control cells without an appreciable change in their affinity. In parallel cultures, flow cytometry analysis using anti-PAF-R Abs failed to reveal any significant decrease in the level of PAF-R expression until after 4 h of treatment with PMA. By 24 h, PAF-R expression had declined by 80 to 90%. This PMA-induced down-regulation of surface PAF-R expression was preceded by a rapid down-regulation of PAF-R mRNA expression. PMA-mediated down-regulation could be blocked by the protein kinase C (PKC) inhibitors H-7 and calphostin C. Activation of PKC by the diacylglycerol analogue 1-oleoyl-2-acetylglycerol also resulted in down-regulation of PAF-R mRNA accumulation, whereas the inactive phorbol diester 4alpha-PMA was ineffective. The rapid disappearance of the PAF-R transcripts was associated with decreased stability of receptor mRNA and not with a change in the nuclear transcription rate of the PAF-R gene. These findings indicate that PAF-R gene expression in human leukocytes can be regulated through a PKC-dependent pathway and involves post-transcriptional destabilization of receptor mRNA.