Stimulation of human monocytes with bacterial endotoxin, lipopolysaccharide (LPS), induces expression of multiple cytokines, including tumor necrosis factor-alpha (TNF-a), interleukin-1 (IL-1), IL-6 and IL-10. IL-10 expression is delayed relative to that of TNF, IL-1 and IL-6. Furthermore, IL-10 feedback inhibits expression of TNF, IL-1 and IL-6, thus providing an efficient autocrine mechanism for controlling proinflammatory cytokine production in monocytes. We are examining the mechanism by which IL-10 down-regulates production of cytokines such as TNF and IL-1 in endotoxin-stimulated monocytes. We are also evaluating the effects of IL-10 on cell signaling events that are activated by specific cytokines in monocytes. We have found that IL-10 can antagonize activation and gene expression by a variety of cytokines, including IL-4 and IFN-gamma (Dickensheets and Donnelly. 1997. J. Immunol. 159:6226). We have also determined that the ability of IL-10 to inhibit IL-4-induced gene expression is a consequence of decreased tyrosine phosphorylation and nuclear translocation of the IL-4-inducible transcription factor, STAT6. In future studies, we will examine the cell type specificity of these IL-10-induced effects by evaluating the effects of IL-10 on gene expression in a variety of hematopoietic and non-hematopoietic cell types. To further define the actions of IL-10 on monocyte functional activities, we are evaluating the effects of this cytokine on synthesis and release of soluble cytokine receptors, including the type-I and type-II IL-1 receptors (IL-1RI and IL-1RII) and the type-1 and type-2 TNF receptors by monocytes. TNF-R are shed from monocytes after stimulation by LPS, and can function as TNF antagonists by competing with membrane-associated TNF-R for available TNF. We have found that IFN-gamma (IFN-g) down-regulates expression of both membrane TNF-R2 and solubleTNF-R2 (sTNF-R2) by LPS-stimulated monocytes (Dickensheets et al. 1997. Blood 90:4162). The decreased production of sTNF-R2 in cultures of IFN-g-treated monocytes correlates directly with decreased levels of TNF-R2 mRNA and inversely with the levels of TNF-a mRNA. In contrast, IL-10 up-regulates production of sTNF-R2 and markedly inhibits production of TNF-a. IL-10 also reverses the ability of IFN-g to suppress production of sTNF-R2 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-R2 (a TNF-R antagonist) in monocytes. They also provide another example of how IL-10 can antagonize cytokine-induced responses 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 Crohn~s disease. The results of our studies will increase our knowledge of the biological actions of IL-10, and thereby improve our ability to regulate the clinical use of this biologic agent.
