Transcriptional regulatory networks direct the development of specialized cell types. The transcription factors signal tranducer and activator of transcription 4 (Stat4) and T-bet are required for the interleukin-12 (IL-12)-stimulated development of T helper 1 (Th1) cells, although the hierarchy of activity by these factors has not been clearly defined. In this report, we show that these factors did not function in a linear pathway and that each factor played a unique role in programming chromatin architecture for Th1 gene expression, with subsets of genes depending on Stat4, T-bet, or both for expression in Th1 cells. T-bet was not able to transactivate expression of Stat4-dependent genes in the absence of endogenous Stat4 expression. Thus, T-bet requires Stat4 to achieve complete IL-12-dependent Th1 cell-fate determination. These data provide a basis for understanding how transiently activated and lineage-specific transcription factors cooperate in promoting cellular differentiation.

The "Bermuda triangle" of genetics, environment and autoimmunity is involved in the pathogenesis of rheumatoid arthritis (RA). Various aspects of genetic contribution to the etiology, pathogenesis and outcome of RA are discussed in this review. The heritability of RA has been estimated to be about 60 %, while the contribution of HLA to heritability has been estimated to be 11-37 %. Apart from known shared epitope (SE) alleles, such as HLA-DRB1*01 and DRB1*04, other HLA alleles, such as HLA-DRB1*13 and DRB1*15 have been linked to RA susceptibility. A novel SE classification divides SE alleles into S1, S2, S3P and S3D groups, where primarily S2 and S3P groups have been associated with predisposition to seropositive RA. The most relevant non-HLA gene single nucleotide polymorphisms (SNPs) associated with RA include PTPN22, IL23R, TRAF1, CTLA4, IRF5, STAT4, CCR6, PADI4. Large genome-wide association studies (GWAS) have identified more than 30 loci involved in RA pathogenesis. HLA and some non-HLA genes may differentiate between anti-citrullinated protein antibody (ACPA) seropositive and seronegative RA. Genetic susceptibility has also been associated with environmental factors, primarily smoking. Some GWAS studies carried out in rodent models of arthritis have confirmed the role of human genes. For example, in the collagen-induced (CIA) and proteoglycan-induced arthritis (PgIA) models, two important loci - Pgia26/Cia5 and Pgia2/Cia2/Cia3, corresponding the human PTPN22/CD2 and TRAF1/C5 loci, respectively - have been identified. Finally, pharmacogenomics identified SNPs or multiple genetic signatures that may be associated with responses to traditional disease-modifying drugs and biologics.

Recent advances in genetics and technology have led to breakthroughs in understanding the genes that predispose individuals to autoimmune diseases. A common haplotype of the signal transducer and activator of transcription 4 (STAT4) gene has been shown to be associated with susceptibility to rheumatoid arthritis, systemic lupus erythematosus, and primary Sjögren's syndrome. STAT4 is a transcription factor that transduces interleukin-12, interleukin-23, and type 1 interferon cytokine signals in T cells and monocytes, leading to T-helper type 1 and T-helper type 17 differentiation, monocyte activation, and interferon-gamma production. Although the evidence for this association is very strong and well replicated, the exact mechanism by which polymorphisms in this gene lead to disease remains unknown. In concert with the identification of other disease-associated loci, elucidating how the variant form of STAT4 modulates immune function should lead to an improved understanding of the pathophysiology of autoimmunity.

T helper 1 (T(H)1) differentiation and IFN-gamma production are crucial in cell-mediated immune responses. IL-12 is an important regulator of this process and mediates its effects through signal transducer and activator of transcription 4 (STAT4). IFN-gamma production is also regulated by the p38 mitogen-activated kinase pathway, although the mechanisms are ill-defined. We show here that GADD45-beta and GADD45-gamma can induce STAT4 S721 phosphorylation via the MKK6/p38 pathway. Thus, STAT4 could be a target that accounts for the defects in cell-mediated immunity associated with perturbations in the p38 pathway. To investigate the biological significance of STAT4 S721 phosphorylation, we reconstituted primary spleen cells from STAT4-deficient mice with wild-type and mutated STAT4, by using a retroviral gene transduction. We demonstrated that expression of wild-type STAT4, but not the S721A mutant, restored normal T(H)1 differentiation and IFN-gamma synthesis. The inability of STAT4 S721 to restore IFN-gamma production was not caused by decreased IL-12R expression because the STAT4 S721 mutant also failed to restore IFN-gamma production in STAT4-deficient IL-12Rbeta2 transgenic cells. Importantly, STAT4 S721A-transduced cells showed normal proliferative response to IL-12, illustrating that serine phosphorylation is not required for IL-12-induced proliferation. Additionally, the results imply the existence of STAT4 serine phosphorylation-dependent and -independent target genes. We conclude that phosphorylation of STAT4 on both tyrosine and serine residues is important in promoting normal T(H)1 differentiation and IFN-gamma secretion.

Type I IFNs (IFN-alpha/beta), in addition to IL-12, have been shown to play an important role in the differentiation of human, but not mouse, Th cells. We show here that IFN-alpha/beta act directly on human T cells to drive Th1 development, bypassing the need for IL-12-induced signaling, whereas IFN-alpha cannot substitute IL-12 for mouse Th1 development. The molecular basis for this species specificity is that IFN-alpha/beta activate Stat4 in differentiating human, but not mouse, Th cells. Unlike IL-12, which acts only on Th1 cells, IFN-alpha/beta can activate Stat4 not only in human Th1, but also in Th2 cells. However, restimulation of human Th2 lines and clones in the presence of IFN-alpha does not induce the production of IFN-gamma. These results suggest that activation of Stat4, which is necessary for the differentiation of naive T cells into polarized Th1 cells, is not sufficient to induce phenotype reversal of human Th2 cells.

The aim of this study was to investigate the possible role of STAT4 gene in the genetic predisposition to systemic sclerosis (SSc) susceptibility or clinical phenotype. A total of 1317 SSc patients [896 with limited cutaneous SSc (lcSSc) and 421 with diffuse cutaneous SSc (dcSSc)] and 3113 healthy controls, from an initial case-control set of Spanish Caucasian ancestry and five independent cohorts of European ancestry (The Netherlands, Germany, Sweden, Italy and USA), were included in the study. The rs7574865 polymorphism was selected as STAT4 genetic marker. We observed that the rs7574865 T allele was significantly associated with susceptibility to lcSSc in the Spanish population [P = 1.9 x 10(-5) odds ratio (OR) 1.61 95% confidence intervals (CI) 1.29-1.99], but not with dcSSc (P = 0.41 OR 0.84 95% CI 0.59-1.21). Additionally, a dosage effect was observed showing individuals with rs7574865 TT genotype higher risk for lcSSc (OR 3.34, P = 1.02 x 10(-7) 95% CI 2.11-5.31). The association of the rs7574865 T allele with lcSSc was confirmed in all the replication cohorts with different effect sizes (OR ranging between 1.15 and 1.86), as well as the lack of association of STAT4 with dcSSc. A meta-analysis to test the overall effect of the rs7574865 polymorphism showed a strong risk effect of the T allele for lcSSc susceptibility (pooled OR 1.54 95% CI 1.36-1.74; P < 0.0001). Our data show a strong and reproducible association of the STAT4 gene with the genetic predisposition to lcSSc suggesting that this gene seems to be one of the genetic markers influencing SSc phenotype.

The goal of this study was to determine the role of the Janus tyrosine kinase (JAK)-signal transducers and activators of transcription (STAT) pathway in the late phase of ischemic preconditioning (PC). A total of 230 mice were used. At 5 min after ischemic PC (induced with six cycles of 4-min coronary occlusion/4-min reperfusion), immunoprecipitation with anti-phosphotyrosine (anti-pTyr) antibodies followed by immunoblotting with anti-JAK antibodies revealed increased tyrosine phosphorylation of JAK1 (+257 +/- 53%) and JAK2 (+238 +/- 35%), indicating rapid activation of these two kinases. Similar results were obtained by immunoblotting with anti-pTyr-JAK1 and anti-pTyr-JAK2 antibodies. Western analysis with anti-pTyr-STAT antibodies demonstrated a marked increase in nuclear pTyr-STAT1 (+301 +/- 61%) and pTyr-STAT3 (+253 +/- 60%) 30 min after ischemic PC, which was associated with redistribution of STAT1 and STAT3 from the cytosolic to the nuclear fraction and with an increase in STAT1 and STAT3 gamma-IFN activation site DNA-binding activity (+606 +/- 64%), indicating activation of STAT1 and STAT3. No nuclear translocation or tyrosine phosphorylation of STAT2, STAT4, STAT5A, STAT5B, or STAT6 was observed. Pretreatment with the JAK inhibitor AG-490 20 min before the six occlusion/reperfusion cycles blocked the enhanced tyrosine phosphorylation of JAK1 and JAK2 and the increased tyrosine phosphorylation, nuclear translocation, and enhanced DNA-binding activity of STAT1 and STAT3. The same dose of AG-490 abrogated the protection against myocardial infarction and the concomitant up-regulation of inducible NO synthase (iNOS) protein and activity observed 24 h after ischemic PC. Taken together, these results demonstrate that ischemic PC induces isoform-selective activation of JAK1, JAK2, STAT1, and STAT3, and that ablation of this response impedes the up-regulation of iNOS and the concurrent acquisition of ischemic tolerance. This study demonstrates that the JAK-STAT pathway plays an essential role in the development of late PC. The results reveal a signaling mechanism that underlies the transcriptional up-regulation of the cardiac iNOS gene and the adaptation of the heart to ischemic stress.

Rheumatoid arthritis (RA) is an archetypal, common, complex autoimmune disease with both genetic and environmental contributions to disease aetiology. Two novel RA susceptibility loci have been reported from recent genome-wide and candidate gene association studies. We, therefore, investigated the evidence for association of the STAT4 and TRAF1/C5 loci with RA using imputed data from the Wellcome Trust Case Control Consortium (WTCCC). No evidence for association of variants mapping to the TRAF1/C5 gene was detected in the 1860 RA cases and 2930 control samples tested in that study. Variants mapping to the STAT4 gene did show evidence for association (rs7574865, P = 0.04). Given the association of the TRAF1/C5 locus in two previous large case-control series from populations of European descent and the evidence for association of the STAT4 locus in the WTCCC study, single nucleotide polymorphisms mapping to these loci were tested for association with RA in an independent UK series comprising DNA from >3000 cases with disease and >3000 controls and a combined analysis including the WTCCC data was undertaken. We confirm association of the STAT4 and the TRAF1/C5 loci with RA bringing to 5 the number of confirmed susceptibility loci. The effect sizes are less than those reported previously but are likely to be a more accurate reflection of the true effect size given the larger size of the cohort investigated in the current study.

IL-17-producing CD8(+) T cells (Tc17) appear to play a role in a range of conditions, such as autoimmunity and cancer. Thus far, Tc17 cells have been only marginally studied, resulting in a paucity of data on their biology and function. We demonstrate that Tc17 and Th17 cells share similar developmental characteristics, including the previously unknown promoting effect of IL-21 on Tc17 cell differentiation and IL-23-dependent expression of IL-22. Both STAT1 and STAT4 are required for optimal development of Tc17 cells and maximal secretion of cytokines. Tc17 cells are cytotoxic, and they can be either pathogenic or nonpathogenic upon adoptive transfer in the model of autoimmune diabetes. Tc17 cells treated with TGF-beta1 plus IL-6 are not diabetogenic, whereas IL-23-treated cells potently induce the disease. IL-17A and IL-17F are necessary but not sufficient for diabetes induction by Tc17 cells. Tc17 cells treated with TGF-beta1 plus IL-6 or IL-23 likely differ in pathogenicity due to their disparate capacity to attract other immune cells and initiate inflammation.

By searching a database of expressed sequences, we identified a member of the signal transducers and activators of transcription (Stat) family of proteins. Human and murine full-length cDNA clones were obtained and sequenced. The sequence of the human cDNA was identical to the recently published sequence for interleukin-4 (IL-4)-Stat (J. Hou, U. Schindler, W.J. Henzel, T.C. Ho, M. Brasseur, and S. L. McKnight, Science 265:1701-1706, 1994), while the murine Stat6 amino acid and nucleotide sequences were 83 and 84% identical to the human sequences, respectively. Using Stat6-specific antiserum, we demonstrated that Stat6 is rapidly tyrosine phosphorylated following stimulation of appropriate cell lines with IL-4 or IL-3 but is not detectably phosphorylated following stimulation with IL-2, IL-12, or erythropoietin. In contrast, IL-2, IL-3, and erythropoietin induce the tyrosine phosphorylation of Stat5 while IL-12 uniquely induces the tyrosine phosphorylation of Stat4. Inducible tyrosine phosphorylation of Stat6 requires the membrane-distal region of the IL-4 receptor alpha chain. This region of the receptor is not required for cell growth, demonstrating that Stat6 tyrosine phosphorylation does not contribute to mitogenesis.

Interferon lambda-1 (IFN-lambda1/IL-29) is a member of the Type-III interferon family, which contains three ligands: IFN-lambda1, 2 and 3. These three ligands use the same unique heterodimeric receptor composed of CRF2-12 (IFN-lambda-R1/IL-28Ralpha) and CRF2-4 (IL10-R-beta) chains. Like their close relatives, the Type-I interferons, IFN-lambda1, 2 and 3, promote the phosphorylation of STAT1 and STAT2, induce the ISRE3 complex, elevate OAS and MxA expression and exhibit antiviral activity in vitro. Their use of the IL10-R-beta chain and their ability to phosphorylate STAT3, STAT4 and STAT5 suggested that they may also exhibit immunomodulatory activity; their antiviral action led us to hypothesize that this activity might be directed toward the Th1/Th2 system. Here, we have demonstrated that IFN-lambda1 altered the activity of Th cells in three separate experimental systems: (i) mitogen stimulation, (ii) mixed-lymphocyte reaction (MLR) and (iii) stimulation of naive T cells by monocyte-derived dendritic cells (mDC). In Con-A stimulation assays, the inclusion of IFN-lambda1 consistently led to markedly diminished levels of secreted interleukin (IL-13) with occasional coincident, modest elevation of secreted IFN-gamma. IL-13 secretion was 100-fold more sensitive to IFN-lambda1 than was IFN-gamma secretion. These observations were also made in the allogeneic two-way MLR. IFN-lambda1 was able to alter cytokine-mediated Th biasing and when naive T cells were exposed to allogeneic mDC that had been matured in the presence of IFN-lambda1, secreted IL-13 was again markedly and consistently reduced, whereas secreted IFN-gamma was largely unaltered. These functions were independent of IL-10. Our data support a hitherto unsuspected role for IFN-lambda1 in modulating the development of Th1 and Th2 cells, with an apparent emphasis on the diminution of IL-13 secretion.

STAT4 and STAT6 are transcription factors that play crucial roles in responding to IL-12 and IL-4, respectively. STAT4 gene knockout (STAT4(-/-)) mice have markedly reduced Th1 responses and enhanced Th2 responses. STAT6(-/-) mice show the inverse phenotype. We compared the ability of bone marrow transplantation (BMT) with the inclusion of spleen cells from STAT6(-/-), STAT4(-/-), and wild-type (WT) mice to produce graft-versus-host disease (GVHD) in lethally irradiated MHC-mismatched recipients. Acute GVHD mortality was more rapid when induced by cells from STAT6(-/-) mice than when induced by STAT4(-/-) cells. However, cells from STAT4(-/-) and STAT6(-/-) donors both induced delayed GVHD mortality compared with WT controls, or compared with combined STAT4(-/-) and STAT6(-/-) cells, indicating a contribution of both Th1 cells and Th2 cells to acute GVHD. Recipients of STAT6(-/-) BMT showed evidence of acute GVHD with severe diarrhea and marked weight loss. Recipients of STAT4(-/-) BMT showed signs of GVHD with only initial transient weight loss and later development of severe skin GVHD. Histopathology showed that Th2 responses were required for the induction of both hepatic and severe skin GVHD. In contrast, both Th1 cells and Th2 cells were capable of causing intestinal pathology of GVHD. Our studies demonstrate an additive role for Th1 and Th2 cells in producing acute GVHD, and suggest a cytokine-directed approach to treating end-organ manifestations of GVHD.

CD4 T(h) are critical for orchestrating adaptive immune responses. The expression of the transcription factor GATA3 (GATA-binding protein 3) is up-regulated or down-regulated during T(h)2 or T(h)1 cell differentiation, respectively. Furthermore, GATA3 is responsible for induction of T(h)2 differentiation and represses T(h)1 differentiation. In this review, we present an updated view on the molecular mechanisms through which GATA3 regulates T(h)1/T(h)2 differentiation. During T(h)2 cell differentiation, GATA3 directly binds to the T(h)2 cytokine gene locus at several regions and regulates expression. On the other hand, GATA3 inhibits T(h)1 cell differentiation by preventing up-regulation of IL-12 receptor β2 and STAT4 (signal transducer and activator of transcription 4) and neutralization of Runx3 (runt-related transcription factor 3) function through protein-protein interaction. GATA3 may also directly act on the Ifng gene. In summary, GATA3 serves as a transcriptional activator or repressor through direct action on transcriptional machinery and/or affecting chromatin remodeling at many critical loci encoding cytokines, cytokine receptors, signaling molecules as well as transcription factors that are involved in the regulation of T(h)1 and T(h)2 differentiation.

OBJECTIVE

To identify susceptibility loci for Behçet's disease (BD) and elucidate their functional role.

METHODS

A genome-wide association study (GWAS) and functional studies were conducted. A total of 149 patients and 951 controls were enrolled in the initial GWAS, and 554 patients and 1,159 controls were enrolled in the replication study. Real-time polymerase chain reaction, luciferase reporter assay, and enzyme-linked immunosorbent assay were performed.

RESULTS

Our GWAS and replication studies identified a susceptibility locus around STAT4 (single-nucleotide polymorphisms [SNPs] rs7574070, rs7572482, and rs897200; P = 3.36 × 10(-7) to 6.20 × 10(-9) ). Increased expression of STAT4 was observed in individuals carrying the rs897200 risk genotype AA. Consistent with the idea that STAT4 regulates the production of interleukin-17 (IL-17) and interferon-γ, IL17 messenger RNA and protein levels were increased in individuals carrying the rs897200 risk genotype AA. Interestingly, the risk allele A of rs897200 creates a putative transcription factor binding site. To test whether it directly affects STAT4 transcription, an in vitro luciferase reporter gene assay was performed. Higher transcription activity was observed in individuals carrying the risk allele A, suggesting that rs897200 is likely to directly affect STAT4 expression. Additionally, 2 SNPs, rs7574070 and rs7572482, which are tightly linked with rs897200, were cis-expression quantitative trait loci (eQTL) SNPs, suggesting that SNP rs897200 is an eQTL SNP. Most importantly, the clinical disease severity score was higher in individuals with the rs897200 risk genotype AA.

CONCLUSIONS

These findings strongly suggest that STAT4 is a novel locus underlying BD. We propose a model in which up-regulation of STAT4 expression and subsequent STAT4-driven production of inflammatory cytokines, such as IL-17, constitute a potential pathway leading to BD.

The activation/inactivation cycle of STAT transcription factors entails their transition between different dimer conformations. Unphosphorylated STATs can dimerize in an antiparallel conformation via extended interfaces of the globular N-domains, whereas STAT activation triggers a parallel dimer conformation with mutual phosphortyrosine:SH2 domain interactions, resulting in DNA-binding and nuclear retention. However, despite the crucial role of STAT tyrosine phosphorylation in cytokine signaling, it has not been determined how this modification affects the stability and the conformational flexibility of STAT dimers. Here, we use analytical ultracentrifugation and electrophoretic mobility shift assay (EMSA) to study the association of STAT1 in solution before and after tyrosine phosphorylation. It is revealed that STAT1 formed high-affinity dimers (K(d) of approximately 50 nM) with estimated half-lives of 20-40 min irrespective of the phosphorylation status. Our results demonstrate that parallel and antiparallel conformations of STAT1 were present simultaneously, supported by mutually exclusive interfaces; and the transition between conformations occurred through affinity-driven dissociation/association reactions. Therefore, tyrosine phosphorylation was dispensable for DNA binding, but the phosphorylation enforced preformed SH2 domain-mediated dimers, thus enhancing the DNA-binding activity of STAT1 >200-fold. Moreover, upon STAT1 activation the N-domains adopted an open conformation and engaged in interdimer interactions, as demonstrated by their participation in tetramerization instead of dimerization. Yet, homotypic N-domain interactions are not conserved in the STAT family, because the N-domain dissociation constants of STAT1, STAT3, and STAT4 differed by more than three orders of magnitude. In conclusion, STAT1 constantly oscillated between different dimer conformations, whereby the abundance of the dimerization interfaces was determined by tyrosine phosphorylation.

Interleukin (IL)-12 plays a critical role in modulating the activities of natural killer (NK) cells and T lymphocytes. In animal models, IL-12 has potent antitumor effects that are likely mediated by its ability to enhance the cytotoxic activity of NK cells and cytotoxic T lymphocytes, and to induce the production of interferon (IFN)-gamma by NK and T cells. In addition to IL-12, NK cells are responsive to IL-2, and may mediate some of the antitumor effects of IL-2. In this study, we examine the interaction between IL-2 and the signaling events induced by IL-12 in NK cells. We find that IL-2 not only up-regulates the expression of IL-12Rbeta1 and IL-12Rbeta2, it also plays an important role in up-regulating and maintaining the expression of STAT4, a critical STAT protein involved in IL-12 signaling in NK cells. In contrast to the effects of IL-2 alone, expression of IL-12 receptors and STAT4 are unaffected or decreased by IL-12 or the combination of IL-2 and IL-12. Through expression of high levels of IL-12 receptors and STAT4, IL-2-primed NK cells show enhanced functional responses to IL-12 as measured by IFN-gamma production and the killing of target cells. NK cells from cancer patients who received low-dose IL-2 treatment also exhibited increased expression of IL-12 receptor chains, suggesting that IL-2 may enhance the response to IL-12 in vivo. These findings provide a molecular framework to understand the interaction between IL-2 and IL-12 in NK cells, and suggest strategies for improving the effectiveness of these cytokines in the immunotherapy of cancer.

IL-21 is a recently characterized T cell-derived cytokine that regulates NK and T cell function. IL-21R shares the common gamma-chain (gamma(c)) with the receptors for IL-2, IL-4, IL-7, IL-9, and IL-15. Despite the same gamma(c), these cytokines have different effects on diverse cells. In this study, we have studied IL-15- and IL-21-induced gene expression in human primary NK and T cells and the NK-92 cell line. Both IL-15 and IL-21 rapidly induced mRNA synthesis for IFN-gamma, T-bet, IL-2Ralpha, IL-12Rbeta2, IL-18R, and myeloid differentiation factor 88 (MyD88), the genes that are important in activating innate immunity and Th1 response. IL-15 induced STAT5 DNA binding to the IL-2Ralpha IFN-gamma-activated sequence (GAS), MyD88 GAS, and c-cis-inducible elements, whereas IL-21 induced STAT3 DNA binding to MyD88 GAS and c-sis-inducible elements. IL-21-induced STAT3 activation was verified by immunoprecipitation and Western blotting with anti-phosphotyrosine Ab. In addition, pretreatment of NK-92 cells with IL-15 or IL-21 strongly enhanced IL-12-induced STAT4 DNA binding to IL-2Ralpha GAS. The induction of IFN-gamma, T-bet, IL-12Rbeta2, and IL-18R gene expression in NK cells, along with STAT3 activation, suggests that IL-21 is involved in the activation of innate immune responses. Moreover, the enhanced transcription of these genes in T cells establishes a significant role for IL-21 also in the Th1 response.

Increased IFN-α signaling is a heritable risk factor for systemic lupus erythematosus (SLE). IFN induced with helicase C domain 1 (IFIH1) is a cytoplasmic dsRNA sensor that activates IFN-α pathway signaling. We studied the impact of the autoimmune-disease-associated IFIH1 rs1990760 (A946T) single nucleotide polymorphism upon IFN-α signaling in SLE patients in vivo. We studied 563 SLE patients (278 African-American, 179 European-American, and 106 Hispanic-American). Logistic regression models were used to detect genetic associations with autoantibody traits, and multiple linear regression was used to analyze IFN-α-induced gene expression in PBMCs in the context of serum IFN-α in the same blood sample. We found that the rs1990760 T allele was associated with anti-dsDNA Abs across all of the studied ancestral backgrounds (meta-analysis odds ratio = 1.34, p = 0.026). This allele also was associated with lower serum IFN-α levels in subjects who had anti-dsDNA Abs (p = 0.0026). When we studied simultaneous serum and PBMC samples from SLE patients, we found that the IFIH1 rs1990760 T allele was associated with increased IFN-induced gene expression in PBMCs in response to a given amount of serum IFN-α in anti-dsDNA-positive patients. This effect was independent of the STAT4 genotype, which modulates sensitivity to IFN-α in a similar way. Thus, the IFIH1 rs1990760 T allele was associated with dsDNA Abs, and in patients with anti-dsDNA Abs this risk allele increased sensitivity to IFN-α signaling. These studies suggest a role for the IFIH1 risk allele in SLE in vivo.

Rheumatoid arthritis is a chronic inflammatory polyarthritis whose etiology remains uncertain. Recently we have learned that autoimmunity to citrullinated protein antigens has specificity for rheumatoid arthritis and defines a clinically and genetically distinct form of the disease. Multiple genes contribute to disease susceptibility, with the HLA locus accounting for 30% to 50% of overall genetic risk. Five risk loci have been identified and validated: HLA-DRB1, PTPN22, STAT4, a region in 6q23, and the TRAF1/C5 locus. Also, there is renewed interest in the contribution of T cells to ongoing inflammation in rheumatoid arthritis. Autoantibodies to citrullinated protein epitopes are specific for rheumatoid arthritis, are associated with a more aggressive disease course, and are pathogenic in an animal model of autoimmune arthritis. There is a strong association between shared-epitope-expressing HLA-DRB1 alleles and the development of rheumatoid arthritis associated with autoimmunity to citrullinated protein antigens.

Differentiation of naive CD4+ T cells into T helper type 1 (Th1) effector cells requires both T cell receptor (TCR) signaling and cytokines such as interleukin-12 and interferon gamma (IFN-gamma). Here, we report that a third cytokine signal, mediated by the Janus family tyrosine kinase 3 (Jak3) and signal transducer and activator of transcription 5 (STAT5) pathway, is also required for Th1 cell differentiation. In the absence of Jak3-dependent signals, naive CD4+ T cells proliferate robustly but produce little IFN-gamma after Th1 cell polarization in vitro. This defect is not due to reduced activation of STAT1 or STAT4 or to impaired upregulation of the transcription factor T-bet. Instead, we find that T-bet binding to the Ifng promoter is greatly diminished in the absence of Jak3-dependent signals, correlating with a decrease in Ifng promoter accessibility and histone acetylation. These data indicate that Jak3 regulates epigenetic modification and chromatin remodeling of the Ifng locus during Th1 cell differentiation.

During the development of experimental autoimmune encephalomyelitis (EAE), the proportion of pathogenic and myelin-specific cells within CNS-infiltrating cytokine-producing Th cells is unknown. Using an IL-17A/IFN-γ double reporter mouse and I-A(b)/myelin oligodendrocyte glycoprotein 38-49 tetramer, we show in this study that IL-17(+)IFN-γ(+) Th cells, which are expanded in the CNS during EAE, are highly enriched in myelin oligodendrocyte glycoprotein-specific T cells. We further demonstrate that IL-23 is essential for the generation and expansion of IFN-γ-producing Th17 cells independently of the Th1-associated transcription factors T-bet, STAT1, and STAT4. Furthermore, Th17 and IL-17(+)IFN-γ(+) Th cells can induce CNS autoimmunity independently of T-bet. Whereas T-bet is crucial for Th1-mediated EAE, it is dispensable for Th17 cell-mediated autoimmunity. Our results suggest the existence of different epigenetic programs that regulate IFN-γ expression in Th1 and Th17 cells.

Human NK cells cultured in the presence of IL-12 or IL-4 differentiate into cell populations with distinct patterns of cytokine secretion similar to Th1 and Th2 cells. NK cells grown in IL-12 (NK1) produce IL-10 and IFN-gamma, whereas NK cells grown in IL-4 (NK2) produce IL-5 and IL-13. Although these NK cell subsets do not differ in cytotoxic activity, NK1 cells express higher levels of cell surface CD95 (Fas) Ag than NK2 cells and are more sensitive to Ab or chemically induced apoptosis. Like Th1 cells, NK1 cells accumulate much higher levels of the IL-12Rbeta2-chain mRNA and are significantly more responsive to IL-12 than NK2 cells at the level of activation of STAT4 transcription factor. The identification of NK cell subsets that are analogous to T cell subsets suggests a new role for NK cells in innate inflammatory responses and in their effect on adaptive immunity.

Recent discoveries of interleukin (IL)-23, its receptor, and its signal-transduction pathway add to our understanding of cellular immunity. IL-23 is a heterodimer, comprising IL-12 p40 and the recently cloned IL-23-specific p19 subunit. IL-23 uses many of the same signal-transduction components as IL-12, including IL-12Rbeta1, Janus kinase 2, Tyk2, signal transducer and activator of transcription (Stat)1, Stat3, Stat4, and Stat5. This may explain the similar actions of IL-12 and IL-23 in promoting cellular immunity by inducing interferon-gamma production and proliferative responses in target cells. Additionally, both cytokines promote the T helper cell type 1 costimulatory function of antigen-presenting cells. IL-23 does differ from IL-12 in the T cell subsets that it targets. Whereas IL-12 acts on naïve CD4+ T cells, IL-23 preferentially acts on memory CD4+ T cells. This review summarizes recent advances regarding IL-23, providing a functional and mechanistic basis for the unique niche that IL-23 occupies in cellular immunity.