Activation of human monocytes by bacterial endotoxin, lipopolysaccharide (LPS), induces expression of many 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. We have found that IL-10 inhibits activation and gene expression induced by IL-4 and IFN-gamma. 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. We are now examining the role of a novel family of JAK/STAT inhibitory genes, the Suppressors of Cytokine Signaling (SOCS) genes, in mediating these IL-10-inducible inhibitory effects. We have found that IL-10 selectively induces expression of one member of this newly identified gene family, SOCS-3. Forced expression of SOCS-3 in a macrophage cell line markedly inhibits induction of STAT activity by several cytokines, including IFN-gamma, GM-CSF and IL-4. Therefore, the ability of IL-10 to antagonize cytokine-inducible gene expression appears to be associated with its ability to induce rapid expression of the SOCS-3 gene. In related studies, we are also examining the biological activities of several newly identified IL-10 homologues. One of these, IL-19, shares approximately 21% amino acid homology with IL-10, and may utilize at least one of the two receptor chains that form the functional IL-10 receptor complex. We have found that expression of the IL-19 gene is upregulated by many of the same agents that induce IL-10 gene expression in monocytes. However, unlike the IL-10 gene, IL-19 is not expressed in activated T cells. Another IL-10 homologue, IL-TIF (IL-10-related T cell-derived inducible factor), has also recently been described. Together with scientists at the University of Medicine and Dentistry of New Jersey (UMDNJ), we have identified and characterized the gene encoding the ligand-binding chain of the receptor for this novel IL-10-related cytokine, IL-TIF. This receptor chain heterodimerizes with the IL-10R-beta chain (IL-10R2) to form a functional IL-TIF receptor complex. Therefore, IL-10R2 is component of both the IL-10 and IL-TIF receptors. We have recently found that the IL-TIF receptor is expressed at high levels in liver and kidney, but not in hematopoietic tissues such as the spleen and thymus. This suggests a possible functional role for this receptor in regulating gene expression in these tissues.
