REVIEWS REGULATION OF JAK–STAT SIGNALLING IN THE IMMUNE SYSTEM Ke Shuai*‡ and Bin Liu* The cytokine-activated Janus kinase (JAK)–signal transducer and activator of transcription (STAT) pathway has an important role in the control of immune responses. Dysregulation of JAK–STAT signalling is associated with various immune disorders. The signalling strength, kinetics and specificity of the JAK–STAT pathway are modulated at many levels by distinct regulatory proteins. Here, we review recent studies on the regulation of the JAK–STAT pathway that will enhance our ability to design rational therapeutic strategies for immune diseases. Cytokines have essential roles in the control of immune which is located at the carboxyl terminus8,9.The amino- responses. The biological functions of cytokines mainly terminal region of STATs is involved in regulating STAT depend on cytokine-mediated gene activation or repres- activity, such as the formation of STAT tetramers10 and sion. Studies on gene induction by interferons (IFNs) led tyrosine dephosphorylation11. to the discovery of the Janus kinase (JAK)–signal trans- Genetic knockout studies have shown that JAKs ducer and activator of transcription (STAT) pathway1, and STATs have highly specific functions in the con- which has since been shown to be a common signalling trol of various immune responses12–14 (TABLE 1). pathway used by many cytokines. The binding of a Recent studies have shown that JAK–STAT signalling cytokine to its cell-surface receptor results in receptor can be regulated at many steps through distinct dimerization and the subsequent activation of JAK tyro- mechanisms. Key regulators include the suppressor sine kinases, which are constitutively associated with the of cytokine signalling (SOCS) proteins and the receptor. Specific tyrosine residues on the receptor are recently discovered protein inhibitor of activated then phosphorylated by activated JAKs and serve as STAT (PIAS) family, as well as various protein tyro- docking sites for a family of latent cytoplasmic transcrip- sine phosphatases (PTPs). The modulation of JAKs tion factors known as STATs.STATs are phosphorylated and STATs by various protein modifications and the by JAKs, then dimerize and subsequently leave the recep- cross-talk between different JAK–STAT pathways and tor and translocate to the nucleus, where they activate other cellular signalling pathways provide additional gene transcription2,3 (FIG. 1a). levels of regulation that might be crucial. This review The mammalian JAK family has four members: discusses the latest insights into how these various JAK1, JAK2, JAK3 and tyrosine kinase 2 (TYK2)4.Each mechanisms regulate JAK–STAT signalling in the contains a conserved kinase domain and a related, but immune system. *Division of Hematology- catalytically inactivate, pseudo-kinase domain at the Oncology, Department of carboxyl terminus (FIG. 1b).The pseudo-kinase domain The regulation of JAKs Medicine and ‡Department of Biological Chemistry, might regulate the kinase activity of JAKs. JAKs are mainly regulated at the post-translational University of California, There are seven mammalian STATs: STAT1, STAT2, level through various mechanisms (FIG. 2). Los Angeles, California STAT3, STAT4, STAT5A, STAT5B and STAT6 (REF. 2), 90095, USA. which are highly homologous in several regions (FIG. 1c), Regulation of JAKs by the SOCS proteins. SOCS pro- Correspondence to K. S. including a SRC homology 2 (SH2) domain, which is teins are the most thoroughly studied regulators of e-mail: kshuai@mednet. 5,6 ucla.edu involved in the activation and dimerization of STATs ,a JAK–STAT signalling. Biochemical and genetic studies doi:10.1038/nri1226 DNA-binding domain7, and a transacivation domain , have clearly shown a crucial role for SOCS proteins in 900 | NOVEMBER 2003 | VOLUME 3 www.nature.com/reviews/immunol REVIEWS a involvement of SOCS proteins in the degradation of sig- Cytokine nalling proteins through the ubiquitin-proteasome Cytokine receptor pathway (BOX 1) has also been suggested. The SOCS box can bind to elongins B and C, which are known compo- nents of a ubiquitin E3 ligase complex26,27.So,SOCS JAK JAK proteins might target signalling proteins for degradation. The regulation of SOCS3 by cytokines can also occur at the post-translational level. It has been docu- mented that SOCS3 becomes rapidly phosphorylated at residues Tyr204 and Tyr221, which are present in the STAT Y STAT Y P conserved SOCS box, by JAKs and receptor tyrosine Y P STAT kinases in response to certain cytokine and growth- factor stimulation28,29.Mutational studies indicate that Nucleus tyrosine phosphorylation of SOCS3 accelerates the PY STAT degradation of SOCS3 protein, thereby regulating 30 STAT Y P feedback inhibition of JAK–STAT signalling . Gene-targeting studies indicate that SOCS1 and Transcription SOCS3 have essential roles in the regulation of immune functions (TABLE 1). Socs1–/– mice die within three weeks b Pseudo-kinase Kinase γ 31–34 domain domain of birth, mainly due to dysregulated IFN- responses . JH7 JH6 JH5 JH4 JH3 JH2 JH1 Socs1–/– mice show lymphopaenia, increased apoptosis JAK N YY C in lymphoid organs and increased levels of IFN-γ in the P P serum. Socs3 knockout is embryonic lethal, probably due to placental insufficiency and erythrocytosis35–37. c Amino-terminal Coiled-coil DNA-binding Linker SH2 Transactivation Conditional Socs3 gene-targeting studies have shown an domain domain domain domain domain domain STAT N YSC important role for SOCS3 in the negative regulation of 38–40 ?? P P interleukin-6 (IL-6) signalling in macrophages . Met ISG15 Ace SUMO Earlier studies indicated that both SOCS1 and SOCS3 are induced by IFΝ-γ and IL-6, and both can inhibit Figure 1 | The JAK–STAT pathway. a | A schematic representation of the Janus kinase IFN-γ and IL-6 responses. However, recent genetic stud- (JAK)–signal transducer and activator of transcription (STAT) pathway. The activation of JAKs ies have shown an unexpected in vivo specificity of after cytokine stimulation results in the phosphorylation of STATs, which then dimerize and SOCS1 and SOCS3 in the regulation of cytokine sig- translocate to the nucleus to activate gene transcription. b | The domain structure of JAKs. nalling. In Socs3–/– macrophages, the activation of STAT1 The domains JH1–JH7 are based on sequence similarity of four known JAKs. JH1 is the kinase by IL-6 is prolonged, whereas the activation of STAT1 by domain, which contains two tyrosines that can be phosphorylated after ligand stimulation. JH2 γ 38–40 –/– is the pseudo-kinase domain. The JH6 and JH7 domains mediate the binding of JAKs to IFN- is normal .By contrast, in Socs1 macro- receptors. c | The domain structure of STATs. The activity of STATs can be regulated by protein phages, the opposite is true. In addition, although STAT3 modification, including tyrosine and serine phosphorylation, methylation (Met), sumoylation is activated by both IL-6 and IL-10, the tyrosine phos- (SUMO), ISGylation (ISG15) and acetylation (Ace). The modification sites of ISGylation and phorylation of STAT3 induced by IL-6, but not IL-10, is acetylation have not been identified. prolonged in Socs3–/– macrophages. So, SOCS proteins have specificity for cytokines, but not JAKs or STATs. An interesting property of SOCS proteins is that they the immune system. The SOCS family of proteins has specifically affect the kinetics of JAK–STAT-signalling ter- eight members: CIS (cytokine-inducible SH2 domain mination, but have no effect on the initial activation protein) and SOCS1–SOCS7 (REFS 15,16).SOCS proteins process or signalling strength. As a result, prolonged STAT contain an SH2 domain, which is flanked by a variable activation induced by cytokines is observed in the absence amino-terminal domain and a carboxy-terminal SOCS of SOCS proteins. As discussed later, prolonged STAT box17 (FIG. 3a).Among the SOCS family, the roles of activation might lead to altered cytokine responses. In SOCS1, SOCS2, SOCS3 and CIS have been well studied fact, microarray analysis shows that the gene-activation in the regulation of JAK–STAT signalling18.These SOCS profile induced by IL-6 mimics that induced by IFN-γ in proteins are generally expressed at low levels in unstimu- Socs3–/– cells, indicating a role for SOCS3 in the control lated cells and become rapidly induced by cytokines, of cytokine-response specificity38,39.A similar alteration thereby inhibiting JAK–STAT signalling and forming a of the IL-6 response has also been observed in Stat3–/– classic negative-feedback loop19.SOCS proteins inhibit cells, in which STAT1 activation is prolonged41. cytokine signalling through distinct mechanisms (FIG. 3b). SOCS proteins have also been implicated in cross- The SH2 domain of SOCS1 binds directly to tyrosine talk between different cytokine-signalling pathways, as phosphorylated JAKs, resulting in the direct inhibition of discussed later. JAK activity20–22.However, the inhibition of JAKs by SOCS3 requires binding of SOCS3 to the activated Regulation of JAKs by protein tyrosine phosphotases. receptor23,24.Instead of acting on JAKs, CIS seems to Several PTPs have been indicated to regulate JAKs, inhibit STAT activation by competing with STATs for including SHP1, SHP2, CD45, PTP1B and T-cell PTP binding to the receptor docking sites25. Finally, the (TCPTP). SHP1 and SHP2 are SH2-domain-containing NATURE REVIEWS | IMMUNOLOGY VOLUME 3 | NOVEMBER 2003 | 901 REVIEWS PTPs42. SHP1 is mainly expressed by haematopoietic JAK2 and TYK2, but not JAK1, can serve as substrates of cells and has been shown to be physically associated PTP1B49, and increased JAK2 phosphorylation has with the IL-3 receptor β chain, the c-KIT receptor, and been observed in Ptp1b–/– mouse embryo fibroblasts. the erythropoietin receptor (EPOR). Cells expressing In addition, PTP1B has been implicated in the negative a mutant EPOR that is defective in SHP1 binding regulation of LEPTIN signalling, possibly by targeting are hypersensitive to EPO and have prolonged EPO- JAK2 (REFS 50,51).However, no immunological pheno- induced autophosphorylation of JAK2, indicating a role type of PTP1B has been reported.