Stimulation of human monocytes with bacterial endotoxin, lipopolysaccharide (LPS), induces expression of multiple cytokines, including tumor necrosis factor (TNF-a), interleukin-1 (IL-1b), IL-6 and IL-10. IL-10 expression is delayed relative to that of TNF-a, IL-1b and IL-6. Furthermore, IL-10 feedback inhibits expression of TNF-a, IL-1b and IL-6, thus providing an efficient autocrine mechanism for controlling proinflammatory cytokine production in monocytes. We have found that the Th1-type lymphokine, IFN-g, markedly up-regulates production of TNF-a in endotoxin-stimulated monocytes, and that this potentiation of cytokine production by IFN-g is due in part to its ability to suppress IL-10 production in monocytes. Nuclear run-on analyses showed that these effects are transcriptionally-mediated. Thus, potentiation of TNF-a production by IFN-g in monocytes is coupled to inhibition of endogenous IL-10 expression (Donnelly et al. 1995. J. Immunol. 155:1420-1427). In contrast to the effects of IFN-g, IL-10 down-regulates production of TNF-a in monocytes. In future experiments, we will attempt to define the mechanism by which IL-10 down-regulates cytokine production, particularly TNF-a, in activated monocytes. In this context, we will also examine the cell type specificity of these IL-10-induced effects by comparing the effects of IL-10 on cytokine production in purified populations of human monocytes and T cells. In preliminary experiments, we have found that although IL-10 markedly inhibits production of TNF-a in monocytes, it does not inhibit expression of TNF-a in activated T cells. To further define the actions of IFN-g and IL-10 on monocyte functional activities, we are evaluating the effects of these two cytokines on synthesis and release of TNF receptors, particularly type-II TNF receptors, by LPS-stimulated monocytes. TNF-R are shed from monocytes after stimulation by LPS, and can function as TNF antagonists by competing with membrane TNF-R (mTNF-R). We have found that IFN-g down-regulates expression of both mTNF-RII and solubleTNF-RII (sTNF-RII) by LPS-stimulated monocytes. The decreased production of sTNF-RII in cultures of IFN-g-treated monocytes correlates directly with decreased levels of TNF-RII mRNA and inversely with the levels of TNF-a mRNA. In contrast, IL-10 up-regulates production of sTNF-RII and markedly inhibits production of TNF-a. IL-10 also blocks the ability of IFN-g to suppress production of sTNF-RII and to potentiate production of TNF-a. These findings demonstrate that IL-10 coordinately down-regulates production of TNF-a (a TNF-R agonist), and up-regulates production of sTNF-RII (a TNF-R antagonist) in monocytes. IL-10 is currently being tested as a potential therapeutic agent for the treatment of a number of inflammatory diseases, including rheumatoid arthritis and Chron's disease. The results of these studies will enhance our knowledge of the biological actions of IL-10, and thereby improve our ability to regulate the clinical use of this biologic agent.
