Activated STAT4 has an essential role in Th1 differentiation and proliferation that is independent of its role in the maintenance of IL-12R beta 2 chain expression and signaling

The genetics of SLE: an update in the light of genome-wide association studies

Understanding the pathogenesis of SLE remains a considerable challenge. Multiple abnormalities of both the innate and adaptive immune system have been described and, furthermore, immunological dysfunction precedes clinical presentation by many years. There is a strong genetic basis to SLE, which means that genetic studies can play a key role in furthering our understanding of this disease. Since susceptibility variants are present from birth and are unaffected by the course of the disease, or by its treatment, genetic analysis is, perhaps uniquely, capable of identifying fundamental, causative, disease mechanisms. Over the last 12 months, there has been a staggering increase in our understanding of SLE genetics. We have seen the identification of new and important SLE susceptibility genes through candidate gene studies, and we have seen the publication of two whole-genome association analyses. The 'hypothesis free' whole-genome studies have provided additional evidence in support of a number of existing susceptibility genes and have identified novel gene candidates. In this article, we review the current SLE genetics literature in the light of these recent advances and we discuss our current understanding of the functional role of the key susceptibility genes. By considering how these genes fall into clusters with shared function we can begin to understand how dysregulation at a number of key immunological steps may predispose to the development of SLE.

IL-12 is required for anti-OX40-mediated CD4 T cell survival

Engagement of OX40 greatly improves CD4 T cell function and survival. Previously, we showed that both OX40 engagement and CTLA-4 blockade led to enhanced CD4 T cell expansion, but only OX40 signaling increased survival. To identify pathways associated with OX40-mediated survival, the gene expression of Ag-activated CD4 T cells isolated from mice treated with anti-OX40 and -CTLA-4 was compared. This comparison revealed a potential role for IL-12 through increased expression of the IL-12R-signaling subunit (IL-12Rbeta2) on T cells activated 3 days previously with Ag and anti-OX40. The temporal expression of IL-12Rbeta2 on OX40-stimulated CD4 T cells was tightly regulated and peaked approximately 4-6 days after initial activation/expansion, but before the beginning of T cell contraction. IL-12 signaling, during this window of IL-12Rbeta2 expression, was required for enhanced T cell survival and survival was associated with STAT4-specific signaling. The findings from these observations were exploited in several different mouse tumor models where we found that the combination of anti-OX40 and IL-12 showed synergistic therapeutic efficacy. These results may lead to the elucidation of the molecular pathways involved with CD4 T cell survival that contribute to improved memory, and understanding of these pathways could lead to greater efficacy of immune stimulatory Abs in tumor-bearing individuals.

Pro-inflammatory cytokine-induced SAPK/MAPK and JAK/STAT in rheumatoid arthritis and the new anti-depression drugs

BACKGROUND

Adult rheumatoid arthritis (RA) patients are frequently clinically depressed. Peripheral inflammation in RA may influence neurotransmitter metabolism, neuroendocrine function, synaptic plasticity, as well as growth factor production, which can modify neural circuitry and contribute to depression.

OBJECTIVE

A convergence between pro-inflammatory cytokine-induced synovial joint inflammation in RA and the effects of pro-inflammatory cytokines on the brain may occur through activation of the stress-activated/mitogen-activated protein kinases (SAPK/MAPK) and/or Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

METHODS

The PubMed and Medlines databases were critically evaluated for evidence of SAPK/MAPK and/or JAK/STAT pathway activation in RA and depression.

RESULTS/CONCLUSION

Some novel anti-depression drugs that were employed in animal models of 'sickness behavior' and in human depression clinical trials suppressed clinical markers of inflammation, as well as SAPK/MAPK and/or JAK/STAT signaling in vitro. Modifying pro-inflammatory cytokine signaling pathways in the brain with antidepressants may also be useful in ameliorating peripheral inflammation in RA.

STAT4 and the risk of rheumatoid arthritis and systemic lupus erythematosus

BACKGROUND

Rheumatoid arthritis is a chronic inflammatory disease with a substantial genetic component. Susceptibility to disease has been linked with a region on chromosome 2q.

METHODS

We tested single-nucleotide polymorphisms (SNPs) in and around 13 candidate genes within the previously linked chromosome 2q region for association with rheumatoid arthritis. We then performed fine mapping of the STAT1-STAT4 region in a total of 1620 case patients with established rheumatoid arthritis and 2635 controls, all from North America. Implicated SNPs were further tested in an independent case-control series of 1529 patients with early rheumatoid arthritis and 881 controls, all from Sweden, and in a total of 1039 case patients and 1248 controls from three series of patients with systemic lupus erythematosus.

RESULTS

A SNP haplotype in the third intron of STAT4 was associated with susceptibility to both rheumatoid arthritis and systemic lupus erythematosus. The minor alleles of the haplotype-defining SNPs were present in 27% of chromosomes of patients with established rheumatoid arthritis, as compared with 22% of those of controls (for the SNP rs7574865, P=2.81x10(-7); odds ratio for having the risk allele in chromosomes of patients vs. those of controls, 1.32). The association was replicated in Swedish patients with recent-onset rheumatoid arthritis (P=0.02) and matched controls. The haplotype marked by rs7574865 was strongly associated with lupus, being present on 31% of chromosomes of case patients and 22% of those of controls (P=1.87x10(-9); odds ratio for having the risk allele in chromosomes of patients vs. those of controls, 1.55). Homozygosity of the risk allele, as compared with absence of the allele, was associated with a more than doubled risk for lupus and a 60% increased risk for rheumatoid arthritis.

CONCLUSIONS

A haplotype of STAT4 is associated with increased risk for both rheumatoid arthritis and systemic lupus erythematosus, suggesting a shared pathway for these illnesses.

A novel VIP signaling pathway in T cells cAMP-->protein tyrosine phosphatase (SHP-2?)-->JAK2/STAT4-->Th1 differentiation

Vasoactive intestinal peptide (VIP) is a potent anti-inflammatory agent. In addition to the deactivation of macrophages, dendritic cells, and microglia, VIP shifts the Th1/Th2 balance, promoting the preferential differentiation and survival of Th2 cells, to the detriment of the proinflammatory Th1 effectors. Several mechanisms operate in the Th1/Th2 shift induced by VIP. Here we report on a novel mechanism for the effect of VIP on T cell differentiation, and show that VIP inhibits Th1 differentiation by interfering directly with the IL-12Jak2/STAT4 signaling pathway in T cells. The effect of VIP is cAMP-dependent, and appears to be mediated through the activation of protein tyrosine phosphatases (PTP), with SHP-2 as a potential target. The activation of PTPs represents a novel cAMP-downstream target for the immunomodulatory effects of VIP.

Current status and advance on T helper cells

T helper cells play an important role in the immune modulation of human bodies. According to the different cytokines secreted by them, T helper cells can be divided into two subgroups, named Th1 and Th2, and they participate in the modulation of cell-mediated and humoral immune response respectively. Th1 and Th2 can affect immune patterns in bodies by co-adjustment. Coming from the common precursor cells (Th0), Th1 and Th2 can commutate under the influence of different cytokines and antigens. In normal physiological condition, the immune function of Th1/Th2 is in dynamic balance. Once the balance is broken, organisms will be in morbid state.

Microenvironment-dependent requirement of STAT4 for the induction of P-selectin ligands and effector cytokines on CD4+ T cells in healthy and parasite-infected mice

T effector cells require selectin ligands to migrate into inflamed regions. In vitro, IL-12 promotes induction of these ligands as well as differentiation of CD4+ T cells into IFN-gamma-producing Th1 but not Th2 cells. STAT4 is strongly involved in these processes. However, the presence of selectin ligands on various T effector cell subsets in vivo points to more complex regulatory pathways. To clarify the role of the IL-12/STAT4 signaling pathway, we analyzed the impact of STAT4 deficiency on the expression of P-selectin ligands (P-lig) on CD4+ T cells in vitro and in vivo, including conditions of infection. In vitro, we found significant expression of P-lig upon activation not only in the presence, but also in the absence, of IL-12, which was independent of STAT4. TGF-beta, an alternative inducer of selectin ligands in human T cells, was not effective in murine CD4+ T cells, suggesting a role of additional signaling pathways. In vivo, a significant impact of STAT4 for the generation of P-lig+CD4+ T cells was observed for cells from peripheral lymph nodes, but not for those from spleen or lung. However, upon infection with the Th2-inducing parasite Nippostrongylus brasiliensis, P-lig expression became dependent on STAT4 signaling. Interestingly, also the frequency of IL-4-producing cells was greatly diminished in absence of STAT4. These data reveal a hitherto unknown contribution of STAT4 to the generation of Th2 cells in parasite infection and suggest that signals inducing inflammation-seeking properties in vivo vary depending on environmental conditions, such as type of organ and infection.

Interleukin-12 drives the Th1 signaling pathway in Helicobacter pylori-infected human gastric mucosa

In this study we examined mechanisms that regulate T-helper lymphocyte 1 (Th1) commitment in Helicobacter pylori-infected human gastric mucosa. The levels of gamma interferon (IFN-gamma), interleukin-4 (IL-4), and IL-12 in total extracts of gastric biopsies taken from H. pylori-infected and uninfected patients were determined by an enzyme-linked immunosorbent assay. The levels of signal transducer and activator of transcription 4 (STAT4), STAT6, and T-box expressed in T cells (T-bet) in total proteins extracted from gastric biopsies were determined by Western blotting. Finally, the effect of a neutralizing IL-12 antibody on expression of Th1 transcription factors and the levels of IFN-gamma in organ cultures of H. pylori-infected biopsies was examined. Increased levels of IFN-gamma and IL-12 were found in gastric biopsy samples of H. pylori-infected patients compared to the levels in uninfected patients. In addition, H. pylori-infected biopsies exhibited high levels of expression of phosphorylated STAT4 and T-bet. Higher levels of IFN-gamma and expression of Th1 transcription factors were associated with greater infiltration of mononuclear cells in the gastric mucosa. By contrast, production of IL-4 and expression of phosphorylated STAT6 were not associated with the intensity of mononuclear cell infiltration. In ex vivo organ cultures of H. pylori-infected biopsies, neutralization of endogenous IL-12 down-regulated the expression of phosphorylated STAT4 and T-bet and reduced IFN-gamma production. Our data indicated that IL-12 contributes to the Th1 cell commitment in H. pylori-infected human gastric mucosa.

T-bet regulates Th1 responses through essential effects on GATA-3 function rather than on IFNG gene acetylation and transcription

T helper type 1 (Th1) development is facilitated by interrelated changes in key intracellular factors, particularly signal transducer and activator of transcription (STAT)4, T-bet, and GATA-3. Here we show that CD4⁺ cells from T-bet^−/− mice are skewed toward Th2 differentiation by high endogenous GATA-3 levels but exhibit virtually normal Th1 differentiation provided that GATA-3 levels are regulated at an early stage by anti–interleukin (IL)-4 blockade of IL-4 receptor (R) signaling. In addition, under these conditions, Th1 cells from T-bet^−/− mice manifest IFNG promotor accessibility as detected by histone acetylation and deoxyribonuclease I hypersensitivity. In related studies, we show that the negative effect of GATA-3 on Th1 differentiation in T-bet^−/− cells arises from its ability to suppress STAT4 levels, because if this is prevented by a STAT4-expressing retrovirus, normal Th1 differentiation is observed. Finally, we show that retroviral T-bet expression in developing and established Th2 cells leads to down-regulation of GATA-3 levels. These findings lead to a model of T cell differentiation that holds that naive T cells tend toward Th2 differentiation through induction of GATA-3 and subsequent down-regulation of STAT4/IL-12Rβ2 chain unless GATA-3 levels or function is regulated by T-bet. Thus, the principal function of T-bet in developing Th1 cells is to negatively regulate GATA-3 rather than to positively regulate the IFNG gene.

Quantitative PCR on 5 genes reliably identifies CTCL patients with 5% to 99% circulating tumor cells with 90% accuracy

We previously identified a small number of genes using cDNA arrays that accurately diagnosed patients with Sézary Syndrome (SS), the erythrodermic and leukemic form of cutaneous T-cell lymphoma (CTCL). We now report the development of a quantitative real-time polymerase chain reaction (qRT-PCR) assay that uses expression values for just 5 of those genes: STAT4, GATA-3, PLS3, CD1D, and TRAIL. qRT-PCR data from peripheral blood mononuclear cells (PBMCs) accurately classified 88% of 17 patients with high blood tumor burden and 100% of 12 healthy controls in the training set using Fisher linear discriminant analysis (FLDA). The same 5 genes were then assayed on 56 new samples from 49 SS patients with blood tumor burdens of 5% to 99% and 69 samples from 65 new healthy controls. The average accuracy over 1000 resamplings was 90% using FLDA and 88% using support vector machine (SVM). We also tested the classifier on 14 samples from patients with CTCL with no detectable peripheral involvement and 3 patients with atopic dermatitis with severe erythroderma. The accuracy was 100% in identifying these samples as non-SS patients. These results are the first to demonstrate that gene expression profiling by quantitative PCR on a selected number of critical genes can be employed to molecularly diagnosis SS.

Opposing Roles for RelB and Bcl-3 in Regulation of T-Box Expressed in T Cells, GATA-3, and Th Effector Differentiation

CD4+ T cells with a block in the NF-kappaB signaling pathway exhibit decreases in Th1 responses and diminished nuclear levels of multiple transactivating NF-kappaB/Rel/IkappaB proteins. To determine the lineage-intrinsic contributions of these transactivators to Th differentiation, T cells from mice deficient in specific subunits were cultured in exogenous cytokines promoting either Th1 or Th2 differentiation. RelB-deficient cells exhibited dramatic defects in Th1 differentiation and IFN-gamma production, whereas no consistent defect in either Th1 or Th2 responses was observed with c-Rel-deficient cells. In sharp contrast, Bcl-3-null T cells displayed no defect in IFN-gamma production, but their Th2 differentiation and IL-4, IL-5, and IL-13 production were significantly impaired. The absence of RelB led to a dramatic decrease in the expression of T-box expressed in T cells and Stat4. In contrast, Bcl-3-deficient cells exhibited decreased GATA-3, consistent with evidence that Bcl-3 can transactivate a gata3 promoter. These data indicate that Bcl-3 and RelB exert distinct and opposing effects on the expression of subset-determining transcription factors, suggesting that the characteristics of Th cell responses may be regulated by titrating the stoichiometry of transactivating NF-kappaB/Rel/IkappaB complexes in the nuclei of developing helper effector cells.

Issues in T-helper 1 development--resolved and unresolved

T-helper 1 cell (Th1) development participates in immunity to many pathogens in part by providing a source of interferon (IFN)-gamma that contributes numerous protective effects. The process of Th1 development involves signals provided by antigen-presenting cells and cytokines produced in response to pathogens, with IFN-gamma itself, interleukin (IL)-12, and IL-18 each promoting the process in some way. Despite the rapid progress into mechanisms of Th1 development in recent years, there are still a number of important unresolved issues in this area. The precise sequence of effector and cellular mechanisms represents a relatively recent avenue of research but is still the subject of current debate, as is the basis of mechanisms that may stabilize a Th1 response. Another unresolved issue is the role of type I IFNs in substituting for IL-12-mediated activation of signal transducer and activator of transcription 4 (Stat4) and induction of IFN-gamma in either murine or human T cells. It is now clear that Th1 cells acquire the property of being capable of nonantigen-dependent activation through the coordinate signaling of IL-12 and IL-18, but the precise order of intracellular signaling events and the uniqueness of this pathway's reliance on the p38 mitogen-activated protein kinase (MAPK) pathway are still issues in need of resolution. Finally, the process of verifying the effects of Stat4 mutations on functional responses has led to the recognition of an unexpected action of the STAT N-domain that may apply generally to other STAT proteins as well. None of these areas is static or resolved fully, and they likely will remain topics of rapid progress.

Signaling by IL-12 and IL-23 and the immunoregulatory roles of STAT4

Produced in response to a variety of pathogenic organisms, interleukin (IL)-12 and IL-23 are key immunoregulatory cytokines that coordinate innate and adaptive immune responses. These dimeric cytokines share a subunit, designated p40, and bind to a common receptor chain, IL-12R beta 1. The receptor for IL-12 is composed of IL-12R beta 1 and IL-12R beta 2, whereas IL-23 binds to a receptor composed of IL-12R beta 1 and IL-23R. Both cytokines activate the Janus kinases Tyk2 and Jak2, the transcription factor signal transducer and activator of transcription 4 (STAT4), as well as other STATs. A major action of IL-12 is to promote the differentiation of naive CD4+ T cells into T-helper (Th) 1 cells, which produce interferon (IFN)-gamma, and deficiency of IL-12, IL-12R subunits or STAT4 is similar in many respects. In contrast, IL-23 promotes end-stage inflammation. Targeting IL-12, IL-23, and their downstream signaling elements would therefore be logical strategies for the treatment of immune-mediated diseases.

Association analysis of signal transducer and activator of transcription 4 (STAT4) polymorphisms with asthma

The signal transducer and activator of transcription 4 (STAT4) on chromosome 2q32.2-q32.3 is known to be essential for mediating responses to interleukin 12 in lymphocytes and regulating the differentiation of T helper cells. In an effort to discover additional polymorphism(s) in genes in which variant(s) have been implicated in asthma, we investigated the genetic polymorphisms in STAT4 to evaluate it as a potential candidate gene for a host genetic study of asthma. By direct DNA sequencing in 24 individuals, we identified 12 sequence variants within introns and their flanking regions, including the 1.5 kb promoter region of STAT4. Among them, seven common polymorphic sites were selected for genotyping in our asthma cohort (502 asthmatic patients, 164 normal controls). Using logistic regression analysis for association with the risk of asthma, while controlling for age, gender, and smoking status as covariates, no significant associations with the risk of asthma were detected. However, one single nucleotide polymorphism (SNP) in intron 11 (+90089T--> C) and two haplotypes showed positive association (P= 0.03, 0.03 and 0.03, respectively) with production of specific IgE to Dermatophagoides farinae (D.f.) or Dermatophagoides pteronyssinus (D.p.) among asthmatic patients. The minor allele frequencies of +90089T--> C and BLOCK2-ht1 were higher (0.54 and 0.47, respectively) among individuals who produced specific IgE to D.f. or D.p. than frequencies (0.47 and 0.39, respectively) among individuals who did not produce specific IgE (OR=1.38 and 1.40, respectively). Our findings suggest that polymorphisms in STAT4 might be one of the genetic factors for the risk of production of specific IgE to mite allergens.

Different Competitive Capacities of Stat4- and Stat6-Deficient CD4+ T Cells during Lymphophenia-Driven Proliferation

The outcome of an immune response relies on the competitive capacities acquired through differentiation of CD4(+) T cells into Th1 or Th2 effector cells. Because Stat4 and Stat6 proteins are implicated in the Th1 vs Th2 generation and maintenance, respectively, we compare in this study the kinetics of Stat4(-/-) and Stat6(-/-) CD4(+) T cells during competitive bone marrow reconstitution and lymphopenia-driven proliferation. After bone marrow transplantation, both populations reconstitute the peripheral T cell pools equally well. After transfer into lymphopenic hosts, wild-type and Stat6(-/-) CD4(+) T cells show a proliferation advantage, which is early associated with the expression of an active phospho-Stat4 and the down-regulation of Stat6. Despite these differences, Stat4- and Stat6-deficient T cells reach similar steady state numbers. However, when both Stat4(-/-) and Stat6(-/-) CD4(+) T cells are coinjected into the same hosts, the Stat6(-/-) cells become dominant and out-compete Stat4(-/-) cells. These findings suggest that cell activation, through the Stat4 pathway and the down-regulation of Stat6, confers to pro-Th1 T cells a slight proliferation advantage that in a competitive situation has major late repercussions, because it modifies the final homeostatic equilibrium of the populations and favors the establishment of Th1 CD4(+) T cell dominance.

Activation pattern of signal transducers and activators of transcription (STAT) factors in inflammatory bowel diseases

OBJECTIVES

Cytokine signaling pathways involving transcription factors of the signal transducers and activators of transcription (STAT) family play a key role in the pathogenesis of inflammatory bowel diseases (IBD). STAT proteins are latent cytoplasmic transcription factors that induce transcription upon phosphorylation, dimerization, and nuclear translocation. However, their activation pattern in IBD is poorly understood. The aim of our study was to characterize STAT-expression in IBD.

METHODS

Mononuclear cells were isolated from 36 colonic specimens of Crohn's disease, ulcerative colitis, or from control patients. Cells were stimulated overnight with antibodies against human CD2 and CD28 and mononuclear cells were analyzed by flow cytometry. Alternatively, CD4(+) T cells were immunomagnetically separated and then assessed by flow cytometry. Intracellular stainings of the following transcription factors were performed: STAT-1, STAT-2, STAT-3, STAT-4, and STAT-6. In addition, immunofluorescence staining on cryosections for phosphorylated STAT-1 and STAT-3 was performed.

RESULTS

Average expression of the IFN-gamma inducible transcription factor STAT-1 was increased in Crohn's disease as compared to patients with ulcerative colitis and control patients. However, levels of phospho-STAT-1 were surprisingly not markedly upregulated in IBD as compared to controls. In contrast, STAT-3 and phospho-STAT-3 levels were significantly increased in IBD patients as compared to controls (p < 0.01). No differences could be detected in STAT-6 levels. Finally, average expression of STAT-2, which is involved in type I interferon signalling, was downregulated in IBD as compared to control patients.

CONCLUSIONS

The analysis of STAT activation patterns could serve as a helpful tool to characterize intestinal inflammation. Furthermore, the IL-6/STAT-3 rather than the IFN-gamma/STAT-1 signaling pathway emerges as a key target for the development of future therapeutic concepts in IBD.

RNA interference: a potent tool for gene-specific therapeutics

RNA interference (RNAi) is a process through which double-stranded RNA induces the activation of cellular pathways, leading to potent and selective silencing of genes with homology to the double strand. Much excitement surrounding small interfering RNA (siRNA)-mediated therapeutics arises from the fact that this approach overcomes many of the shortcomings previously experienced with approaches such as antibodies, antisense oligonucleotides and pharmacological inhibitors. Induction of RNAi through administration of siRNA has been successfully used in treatment of hepatitis, viral infections, and cancer. In this review we will present a brief history of RNAi, methods of inducing RNAi, application of RNAi in the therapeutic setting, and the possibilities of using this highly promising approach in the context of transplantation.

Impairment of IL-12-Dependent STAT4 Nuclear Translocation in a Patient with RecurrentMycobacterium aviumInfection

We examined the immunological abnormality in a patient with recurrent Mycobacterium avium infection. T cells from the patient showed decreased ability both to produce IFN-gamma and to proliferate in response to IL-12. Despite decreased expression of IL-12R beta1 and beta2 chains in the patient's PHA-activated T cells, there was no difference in IL-12-induced tyrosine and serine phosphorylation of STAT4 in PHA-activated T cells between the patient and healthy subjects, suggesting that IL-12R signals are transmitted to STAT4 in the patient's PHA-activated T cells. Using EMSA, confocal laser microscopy, and Western blotting, we demonstrated that the nuclear translocation of STAT4 in response to IL-12 is reduced in PHA-activated T cells from the patient when compared with those from healthy subjects. Leptomycin B was used to examine whether nuclear export of STAT4 is increased in the patient's T cells. However, leptomycin B treatment did not reverse impaired IL-12-induced nuclear accumulation of STAT4. Although the exact mechanism responsible for the impaired STAT4 nuclear translocation in this patient remains unclear, the absence of mutation in the IL-12Rbeta1, IL-12Rbeta2, STAT4, and STAT4-binding sequence of the IFN-gamma gene and preservation of STAT4 tyrosine and serine phosphorylation suggest the existence of a defective STAT4 nuclear translocation. This defect is likely responsible for the impaired STAT4 nuclear translocation in IL-12-stimulated T cells, leading to impairment of both IFN-gamma production and cell proliferation. To the best of our knowledge, this is the first report of a patient with atypical mycobacterial infection associated with impairment of STAT4 nuclear translocation.

Cytokines and transcription factors that regulate T helper cell differentiation: new players and new insights

The differentiation of naive CD4+ T cells into subsets of T helper cells is a pivotal process with major implications for host defense and the pathogenesis of immune-mediated diseases. Though the basic paradigm was discovered more than 15 years ago, new discoveries continue to be made that offer fresh insights into the regulation of this process. T helper (TH)1 cells produce interferon (IFN)-gamma, promoting cell-mediated immunity and control of intracellular pathogens. We now know that TH1 differentiation is regulated by transcription factors such as T-bet, Stat1, and Stat4, as well as cytokines such as IL-12, IL-23, IL-27, type I IFNs, and IFN-gamma. In contrast, TH2 cells produce IL-4, which promotes allergic responses and is important in host defense against helminths. The transcription factors Stat6, GATA-3, c-Maf, NFATs, and the cytokine IL-4 promote TH2 differentiation. These key regulators of TH differentiation are the subject of this review.

SOCS-3 inhibits IL-12-induced STAT4 activation by binding through its SH2 domain to the STAT4 docking site in the IL-12 receptor beta2 subunit

IL-12 promotes the proliferation of T cells as well as NK cells and plays a critical role in induction of the Th1 differentiation. IL-12 mediates its biological activities through activation of the receptor-associated JAK family kinases and STAT4, which is recruited to phosphorylated Tyr-800 in the human IL-12 receptor beta2 subunit (IL-12Rbeta2). Here we demonstrate that suppressor of cytokine signaling-3 (SOCS-3) is also recruited to IL-12Rbeta2 by the interaction involving the SOCS-3 SH2 domain and phosphorylated Tyr-800 in IL-12Rbeta2. Furthermore, SOCS-3, but not its SH2 domain-defective mutant, inhibited the IL-12-induced activation of DNA-binding and transcriptional activities of STAT4. These results suggest that SOCS-3, expressed at high levels in Th2 cells, plays an inhibitory role in STAT4-mediated IL-12 signaling by binding to the STAT4 docking site in IL-12Rbeta2, thus raising a possibility that SOCS-3 may play a role in regulation of Th differentiation.

STAT4 is required for interleukin-12-induced chromatin remodeling of the CD25 locus

Signal transducer and activator of transcription 4 (STAT4) is a critical mediator of interleukin-12 (IL-12)-stimulated inflammatory immune responses. Despite extensive analysis of the immune responses of STAT4-deficient mice, there is still very little understood about STAT4-dependent gene induction. IL-12 stimulated increases in IL-2 receptor alpha chain gene (CD25) mRNA levels and surface expression require STAT4. In this report, we utilize chromatin immunoprecipitation assays to analyze IL-12-stimulated and STAT4-dependent changes in chromatin remodeling of the CD25 gene. Gene activation requires binding of STAT4 to the PRRIII upstream regulatory element, the recruitment of the CREB-binding protein (CBP), and chromatin remodeling including increased acetylation and decreased methylation of histones within the CD25 promoter. Evidence suggests that STAT4 also facilitates binding of other factors to the CD25 promoter including c-Jun. Thus, these results provide a model for STAT4-dependent gene induction and a mechanism for cytokine-induced expression of the CD25 gene.

The biology of IL-12: coordinating innate and adaptive immune responses

Cytokines play critical roles in regulating all aspects of immune responses, including lymphoid development, homeostasis, differentiation, tolerance and memory. Interleukin (IL)-12 is especially important because its expression during infection regulates innate responses and determines the type and duration of adaptive immune response. IL-12 induces interferon-gamma (IFN-gamma) production by NK, T cells, dendritic cells (DC), and macrophages. IL-12 also promotes the differentiation of naïve CD4+ T cells into T helper 1 (Th1) cells that produce IFN-gamma and aid in cell-mediated immunity. As IL-12 is induced by microbial products and regulates the development of adaptive immune cells, IL-12 plays a central role in coordinating innate and adaptive immunity. IL-12 and the recently identified cytokines, IL-23 and IL-27, define a family of related cytokines that induce IFN-gamma production and promote T cell expansion and proliferation.

The role of signal transducers and activators of transcription in T inflammatory bowel diseases

Signal transducers and activators of transcription (STAT) proteins are intracellular effector molecules of cytokine-modulated signaling. On the one hand, they play an important role in hematopoiesis and the development of the human immune system. STAT transcription factors are necessary for embryogenesis and the maintenance of the mammalian immune response. In the adult, STAT signaling is responsible for T-cell polarization toward interferon gamma-secreting Th1 T cells or interleukin 4-producing Th2 cells. On the other hand, these proteins are involved in the regulation of T-cell survival. STAT activation is strongly associated with tyrosine phosphorylation by tyrosine kinases, namely Jak1, Jak2, Jak3, and Tyk2. Counterregulatory mechanisms protecting from overwhelming STAT activation are represented by protein inhibitors of activated STATs and the SOCS family proteins. Because STAT proteins are key response elements of cytokine-induced T-cell activation, the characterization of STAT proteins is one step to elucidate disturbed T-cell function in inflammatory bowel disease. In particular, an activation of STAT-4 and STAT-3 in T cells seems to play a key pathogenic role in Crohn's disease.

STAT4 requires the N-terminal domain for efficient phosphorylation

STAT4 (signal transducer and activator of transcription-4) mediates biological effects in response to interleukin-12 (IL-12). STAT4 has multiple domains that have distinct functions in signaling and gene activation. To characterize the role of the STAT4 N-terminal domain in mediating STAT4 biological function, we have generated STAT4-deficient transgenic mice that express human full-length STAT4 or an N-terminal deletion mutant (Delta N-STAT4) lacking the N-terminal 51 amino acids. Whereas full-length STAT4 rescued IL-12 responsiveness, T lymphocytes expressing the STAT4 N-terminal mutant failed to proliferate in response to IL-12 and had limited Th1 cell development as evidenced by minimal interferon-gamma production. Deletion of the N-terminal domain resulted in failure of STAT4 to be phosphorylated following IL-12 stimulation despite similar phosphorylation of JAK2 and TYK2 in full-length STAT4 and Delta N-STAT4 transgenic T cells. We demonstrate that the lack of phosphorylation in cultured cells is due to reduced efficiency of phosphorylation of Delta N-STAT4 by Janus kinases. These data support a new model wherein the N-terminal domain is required to mediate the phosphorylation of STAT4 in addition to the previously documented role in gene transactivation.

Mucosal T cells: mediators or guardians of inflammatory bowel disease?

Because the mucosal immune system is continuously exposed to a myriad of potentially harmful environmental antigens, it frequently reacts with antiinflammatory/regulatory T cell responses driven by TGF-beta-producing TH3 cells and IL-10-producing regulatory T cells. Intestinal inflammation in patients with inflammatory bowel diseases is thought to result from an overwhelming uncontrolled activation of the mucosal immune system induced by antigens of the normal luminal flora in genetically susceptible individuals. Inflammatory bowel disease appears to be mediated by subsets of CD4 T lymphocytes or NK T cells secreting high levels of proinflammatory cytokines such as TNF-alpha. The increased expression of integrins/addressins in the inflamed gut and the increased expression of adhesion molecules on T cells facilitate migration of these pathogenic T cell subsets into the lamina propria. Additionally, the local activation of antiapoptotic pathways in pathogenic T lymphocytes leads to a further accumulation of these cells in the lamina propria, causing perpetuation and chronicity of inflammatory bowel disease. This concept is underlined by the finding that most potent immunosuppressive drugs used in treatment of inflammatory bowel disease seem to work by inducing T cell apoptosis via inhibition of STAT-3 and NFkappaB-dependent antiapoptotic pathways. Taken together, distinct T cell subsets appear to act as mediators or guardians of inflammatory bowel disease, and thus they play a central role in controlling the delicate balance between proinflammatory and antiinflammatory immune responses in the gut.

New interleukins: are there any more?

PURPOSE OF REVIEW

It is now well established that cytokines play a critical role in the regulation of the immune system. Processes such as lymphoid development, differentiation, homeostasis, tolerance and memory are all regulated by cytokines that bind the type I family of cytokine receptors. Like the interferons, which bind receptors designated as the type II cytokine receptor family, type I cytokines also have essential functions in host defence.

RECENT FINDINGS

Recently, a number of new interleukins and their receptors have been discovered and their role in mounting an appropriate immune response is currently being studied. In this review we will describe the new interleukin-12 family of cytokines, which now includes two other members: interleukins 23 and 27. We will also review the newly discovered interleukins 28 and 29, also known as interferon-lambdas, which bind to the type II family of cytokine receptors, their structure and the structure of their receptors, their biological activities and modes of signalling.

SUMMARY

These new molecules will certainly be the focus of active research in the immediate future. Their discovery opens the door to new therapeutic approaches to the treatment of various diseases ranging from infections from intracellular pathogens to viral infections.

Cytokines and transcription factors that regulate T helper cell differentiation: new players and new insights

The differentiation of naive CD4+ T cells into subsets of T helper cells is a pivotal process with major implications for host defense and the pathogenesis of immune-mediated diseases. Though the basic paradigm was discovered more than 15 years ago, new discoveries continue to be made that offer fresh insights into the regulation of this process. T helper (TH)1 cells produce interferon (IFN)-gamma, promoting cell-mediated immunity and control of intracellular pathogens. We now know that TH1 differentiation is regulated by transcription factors such as T-bet, Stat1, and Stat4, as well as cytokines such as IL-12, IL-23, IL-27, type I IFNs, and IFN-gamma. In contrast, TH2 cells produce IL-4, which promotes allergic responses and is important in host defense against helminths. The transcription factors Stat6, GATA-3, c-Maf, NFATs, and the cytokine IL-4 promote TH2 differentiation. These key regulators of TH differentiation are the subject of this review.

The lineage decisions of helper T cells

After encountering antigen, helper T (T(H)) cells undergo differentiation to effector cells, which can secrete high levels of interferon-gamma, interleukin-4 (IL-4), IL-10 and other immunomodulators. How T(H) cells acquire, and remember, new patterns of gene expression is an area of intensive investigation. The process is remarkably plastic, with cytokines being key regulators. Extrinsic signals seem to be integrated into cell-intrinsic programming, in what is becoming an intriguing story of regulated development. We summarize the latest insights into mechanisms that govern the lineage choices that are made during T(H)-cell responses to foreign pathogens.