Interleukin-10 (IL-10) activates a diverse array of functional responses in mononuclear phagocytes. Functional IL-10 receptor (IL-10R) complexes are tetramers consisting of two IL-10R1 polypeptide chains and two IL-10R2 chains. Binding of IL-10 to the extracellular domain of IL-10R1 activates phosphorylation of the receptor-associated Janus tyrosine kinases, JAK1 and Tyk2. These kinases then phosphorylate specific tyrosine residues (Y446 and Y496) on the intracellular domain of the IL-10R1 chain. Once phosphorylated, these tyrosine residues (and their flanking peptide sequences) serve as temporary docking sites for the latent transcription factor, STAT3 (signal transducer and activator of transcription-3). STAT3 binds to these sites via its SH2 (Src homology 2) domain, and is, in turn, tyrosine-phosphorylate d by the receptor-associated JAKs. It then homodimerizes and translocates to the nucleus where it binds with high affinity to STAT-binding elements (SBE) in the promoters of various IL-10-responsive genes. One of these genes, SOCS-3 (Suppressor of Cytokine Signaling-3) is a member of a newly identified family of genes that inhibit JAK/STAT-dependent signaling. Moreover, the ability of IL-10 to induce de novo synthesis of SOCS-3 in monocytes correlates with its ability to inhibit expression of many genes in these cells, including endotoxin-inducible cytokines such as tumor necrosis factor-a (TNF-a ) and IL-1. Thus, the ability of IL-10 to inhibit gene expression in monocytes is associated with its ability to rapidly induce synthesis of SOCS-3. 563

The cytokines interleukin 2 (IL-2) and IL-15 have similar biological effects on T cells and bind common hematopoietin receptor subunits. Pathways that involve Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) have been shown to be important for hematopoietin receptor signaling. In this study we identify the STAT proteins activated by in human T cells. IL-2 and IL-15 rapidly induced the tyrosine phosphorylation of STAT3 and STAT5, and DNA-binding complexes containing STAT3 and STAT5 were rapidly activated by these cytokines in T cells. IL-4 induced tyrosine phosphorylation and activation of STAT3 but not STAT5. JAK1 and JAK3 were tyrosinephosphorylated in response to IL-2 and IL-S15. Hence, the JAK and STAT molecules that are activated in response to IL-2 and IL-15 are similar but differ from those induced by IL-4. These observations identify the STAT proteins activated by IL-2 and IL-15 and therefore define signaling pathways by which these T-cell growth factors may regulate gene transcription.Interleukin 2 (IL-2) is a key growth factor that induces the proliferation and functional differentiation of T lymphocytes and natural killer cells. IL-15 shares many characteristics with IL-2, such as the generation of cytotoxic T cells and lymphokine-activated killer cells (1-3). Two IL-2 receptor subunits, the , chain (p75) and the common y chain (-yc), are used by IL-15, and Yc is also shared by other cytokine receptors, including the IL-4 receptor (4, 5). Studies also suggest the existence of an IL-15 receptor subunit distinct from the IL-2 receptor subunits (3). The finding that IL-15 and IL-2 share receptor subunits and exhibit overlapping biological effects implies that they may activate common intracellular substrates but, as yet, this has not been explored.Recently, a signal transduction pathway that involves Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) has been found to be utilized by a number of growth factors and cytokines that bind members of the hematopoietin receptor family (6, 7). We and others (8-11) have cloned a member of the Janus family (JAK3) and have shown that JAK3 and JAK1 are functionally coupled to the IL-2 receptor, as well as other receptors that use 'yc (receptors for IL-4, -7, and -9). The requisite role of yc in signaling is perhaps best illustrated by the discovery that mutations of this subunit result in X chromosome-linked severe combined immunodeficiency (12). Many of these mutations disrupt JAK3-,yc interactions, suggesting that this disruption might be important in the pathogenesis of this immunodeficiency (13). Given that IL-15 shares this common receptor subunit, it was important to examine whether IL-15 might also activate JAK3 and JAK1. Studies of the mechanism by which hematopoietin receptors activate gene transcription indicate that STAT proteins become rapidly tyrosine-phosphorylated in the receptor complex and directly translocate to the nucleus, where they bind DNA and activate transcription (14...

Stat5a was identified as a prolactin-induced transcription factor but also is activated by other cytokines, including interleukin-2 (IL-2) and IL-7. We have now analyzed the immune system of Stat5a-deficient mice. Stat5a-/- splenocytes exhibited defective IL-2-induced expression of the IL-2 receptor alpha chain (IL-2R alpha), a protein that together with IL-2R beta and the common cytokine receptor gamma chain (gamma(c)) mediates high-affinity IL-2 binding. Correspondingly, Stat5a-/- splenocytes exhibited markedly decreased proliferation to IL-2, although maximal proliferation was still achieved at IL-2 concentrations high enough to titrate intermediate-affinity IL-2R beta/gamma(c) receptors. Thus, defective Stat5a expression results in diminished proliferation by an indirect mechanism, resulting from defective receptor expression. Correspondingly, we show that Stat5a is essential for maximal responsiveness to antigenic stimuli in vivo, underscoring the physiological importance of IL-2-induced IL-2R alpha expression.

Interferon-alpha (IFN-alpha) and IFN-gamma regulate gene expression by tyrosine phosphorylation of several transcription factors that have the 91-kilodalton (p91) protein of interferon-stimulated gene factor-3 (ISGF-3) as a common component. Interferon-activated protein complexes bind enhancers present in the promoters of early response genes such as the high-affinity Fc gamma receptor gene (Fc gamma RI). Treatment of human peripheral blood monocytes or basophils with interleukin-3 (IL-3), IL-5, IL-10, or granulocyte-macrophage colony-stimulating factor (GM-CSF) activated DNA binding proteins that recognized the IFN-gamma response region (GRR) located in the promoter of the Fc gamma RI gene. Although tyrosine phosphorylation was required for the assembly of each of these GRR binding complexes, only those formed as a result of treatment with IFN-gamma or IL-10 contained p91. Instead, complexes activated by IL-3 or GM-CSF contained a tyrosine-phosphorylated protein of 80 kilodaltons. Induction of Fc gamma RI RNA occurred only with IFN-gamma and IL-10, whereas pretreatment of cells with GM-CSF or IL-3 inhibited IFN-gamma induction of Fc gamma RI RNA. Thus, several cytokines other than interferons can activate putative transcription factors by tyrosine phosphorylation.