Stimulation of human monocytes with bacterial endotoxin, lipopolysaccharide (LPS), induces expression of multiple cytokines, including tumor necrosis factor (TNF), 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. In this project, 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 signal transduction events that are activated by cytokines such as IFN-gamma (IFN-g) and IL-4 in monocytes. We have found that IL-10 inhibits activation and gene expression induced by IL-4 and IFN-gamma (Dickensheets & Donnelly (1997) J. Immunol. 159:6226). We have also determined that the ability of IL-10 to inhibit IL-4-inducible gene expression is a consequence of decreased tyrosine phosphorylation and nuclear translocation of the IL-4-inducible transcription factor, STAT6 (Dickensheets & Donnelly (1999) J. Leukoc. Biol. 65:307). We are now examining the role of a novel family of JAK/STAT inhibitory genes (the SOCS genes) in mediating these IL-10-inducible inhibitory effects. To further define the actions of IL-10 on monocyte functional activities, we also examined the effects of this cytokine on synthesis and release of certain soluble cytokine receptors, particularly the type-I and type-II IL-1 receptors (IL-1RI and IL-1RII) and the type-1 and type-2 TNF receptors. TNF-Rs 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-g down-regulates expression of both membrane TNF-R2 and soluble TNF-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 correlated directly with decreased levels of TNF-R2 mRNA and inversely with the levels of TNF-a mRNA. In contrast, IL-10 up-regulated production of sTNF-R2 and markedly inhibited production of TNF-a. IL-10 also antagonized 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. Recombinant human IL-10 is currently being tested as a potential therapeutic agent for the treatment of certain 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 the agency's ability to regulate the clinical use of this biologic agent.
