JAK/STAT-deficient cell lines

Molecular mechanism and structural basis of gain-of-function of STAT1 caused by pathogenic R274Q mutation

Gain-of-function (GOF) mutations in the STAT1 gene are critical for the onset of chronic mucocutaneous candidiasis (CMC) disease. However, the molecular basis for the gain of STAT1 function remains largely unclear. Here, we investigated the structural features of STAT1 GOF residues to better understand the impact of these pathogenic mutations. We constructed STAT1 alanine mutants of the α3 helix residues of the coiled-coil domain, which are frequently found in CMC pathogenic mutations, and measured their transcriptional activities. Most of the identified GOF residues were located inside the coiled-coil domain stem structure or at the protein surface of the anti-parallel dimer interface. Unlike those, Arg-274 was adjacent to the DNA-binding domain. In addition, Arg-274 was found to functionally interact with Gln-441 in the DNA-binding domain. Because Gln-441 is located at the anti-parallel dimer contact site, Gln-441 reorientation by Arg-274 mutation probably impedes formation of the dimer. Further, the statistical analysis of RNA-seq data with STAT1-deficient epithelial cells and primary T cells from a CMC patient revealed that the R274Q mutation affected gene expression levels of 66 and 76 non-overlapping RefSeq genes, respectively. Because their transcription levels were only slightly modulated by wild-type STAT1, we concluded that the R274Q mutation increased transcriptional activity but did not change dramatically the repertoire of STAT1 targets. Hence, we provide a novel mechanism of STAT1 GOF triggered by a CMC pathogenic mutation.

STAT2/IRF9 directs a prolonged ISGF3-like transcriptional response and antiviral activity in the absence of STAT1

Evidence is accumulating for the existence of a signal transducer and activator of transcription 2 (STAT2)/interferon regulatory factor 9 (IRF9)-dependent, STAT1-independent interferon alpha (IFNα) signalling pathway. However, no detailed insight exists into the genome-wide transcriptional regulation and the biological implications of STAT2/IRF9-dependent IFNα signalling as compared with interferon-stimulated gene factor 3 (ISGF3). In STAT1-defeicient U3C cells stably overexpressing human STAT2 (hST2-U3C) and STAT1-deficient murine embryonic fibroblast cells stably overexpressing mouse STAT2 (mST2-MS1KO) we observed that the IFNα-induced expression of 2'-5'-oligoadenylate synthase 2 (OAS2) and interferon-induced protein with tetratricopeptide repeats 1 (Ifit1) correlated with the kinetics of STAT2 phosphorylation, and the presence of a STAT2/IRF9 complex requiring STAT2 phosphorylation and the STAT2 transactivation domain. Subsequent microarray analysis of IFNα-treated wild-type (WT) and STAT1 KO cells overexpressing STAT2 extended our observations and identified ∼120 known antiviral ISRE-containing interferon-stimulated genes (ISGs) commonly up-regulated by STAT2/IRF9 and ISGF3. The STAT2/IRF9-directed expression profile of these IFN-stimulated genes (ISGs) was prolonged as compared with the early and transient response mediated by ISGF3. In addition, we identified a group of 'STAT2/IRF9-specific' ISGs, whose response to IFNα was ISGF3-independent. Finally, STAT2/IRF9 was able to trigger an antiviral response upon encephalomyocarditis virus (EMCV) and vesicular stomatitis Indiana virus (VSV). Our results further prove that IFNα-activated STAT2/IRF9 induces a prolonged ISGF3-like transcriptome and generates an antiviral response in the absence of STAT1. Moreover, the existence of 'STAT2/IRF9-specific' target genes predicts a novel role of STAT2 in IFNα signalling.

Novel STAT1 alleles in otherwise healthy patients with mycobacterial disease

The transcription factor signal transducer and activator of transcription-1 (STAT1) plays a key role in immunity against mycobacterial and viral infections. Here, we characterize three human STAT1 germline alleles from otherwise healthy patients with mycobacterial disease. The previously reported L706S, like the novel Q463H and E320Q alleles, are intrinsically deleterious for both interferon gamma (IFNG)–induced gamma-activating factor–mediated immunity and interferon alpha (IFNA)–induced interferon-stimulated genes factor 3–mediated immunity, as shown in STAT1-deficient cells transfected with the corresponding alleles. Their phenotypic effects are however mediated by different molecular mechanisms, L706S affecting STAT1 phosphorylation and Q463H and E320Q affecting STAT1 DNA-binding activity. Heterozygous patients display specifically impaired IFNG-induced gamma-activating factor–mediated immunity, resulting in susceptibility to mycobacteria. Indeed, IFNA-induced interferon-stimulated genes factor 3–mediated immunity is not affected, and these patients are not particularly susceptible to viral disease, unlike patients homozygous for other, equally deleterious STAT1 mutations recessive for both phenotypes. The three STAT1 alleles are therefore dominant for IFNG-mediated antimycobacterial immunity but recessive for IFNA-mediated antiviral immunity at the cellular and clinical levels. These STAT1 alleles define two forms of dominant STAT1 deficiency, depending on whether the mutations impair STAT1 phosphorylation or DNA binding.

Mendelian susceptibility to mycobacterial disease is a rare syndrome. It is defined by the occurrence of severe disease caused by low virulence mycobacteria in otherwise healthy individuals, in whom antiviral immune response is not affected. Eleven known genetic defects, affecting five genes, have been involved in this type of deficient response to infection, involving immune-mediator molecules IL12 and interferon gamma: IL12B, IL12RB1, IFNGR1, IFNGR2, and STAT1. The signal transducer and activator of transcription-1 (STAT1) amino acid change L706S was previously shown to cause disease by impairing STAT1 phosphorylation. Here, we report two new STAT1 mutations that impair STAT1 DNA-binding activity. We show, by functional analysis of the three STAT1 mutant alleles, that they are intrinsically deleterious for both interferon gamma–induced antimycobacterial immunity, which is mediated through gamma-activated factor and for interferon alpha–induced antiviral immunity, which is mediated through interferon-stimulated genes factor 3. Interestingly, the three alleles are dominant for interferon gamma–induced gamma-activated factor–mediated antimycobacterial immunity, but recessive for interferon alpha–induced interferon-stimulated genes factor 3–mediated antiviral immunity at the cellular and clinical levels. These two new STAT1 alleles, which affect the binding of STAT1 to DNA, define distinct novel genetic causes of Mendelian susceptibility to mycobacterial disease and provide further insight into the molecular mechanism of disease.

Interferons (IFNs) are key cytokines in both innate and adaptive antiviral immune responses--and viruses counteract IFN action

Interferons (IFNs) are transcriptionally regulated cytokines and key players in the innate antiviral immune response. Upon recognition of a virus, or its molecular patterns, by the Toll-like receptors of dendritic cells (DCs), high levels of IFNs are expressed by these cells. This in turn stimulates DC maturation and the subsequent expression of proinflammatory cytokines and costimulatory molecules, leading to the transition to an adaptive antiviral immunity. Conversely, viruses have developed diverse strategies to counterattack host defenses in order to generate their progenies.

Apoptosis induced by butyrate is independent of Jak/STAT signaling in a fibrosarcoma cell line

The aim of this study was to evaluate the participation of the Jak-1 and STAT-1 proteins in sodium butyrate-induced apoptosis in 2C4 cells derived from human fibrosarcoma. Making use of Jak-1 or STAT-1 deficient cell lines, we demonstrated that the apoptotic process induced by butyrate is independent of the presence of these proteins. In addition, this work showed that, although the constitutive expression of pro-caspases-2 and -3 is reduced in STAT-1 cells, the activity of caspase-3 is preserved in both Jak-1 and STAT-1 deficient cells and is similar to that seen in 2C4 parental cells. In conclusion, we demonstrated that the absence of functionally active Jak-1 or STAT-1 protein directly affects the TNF-alpha-induced apoptosis, but does not alter the sodium butyrate-induced apoptosis in cells derived from human fibrosarcoma.

A unique ISRE, in the TATA-less human Isg20 promoter, confers IRF-1-mediated responsiveness to both interferon type I and type II

Interferons (IFNs) encode a family of secreted proteins involved in a number of regulatory functions such as control of cell proliferation, differentiation and regulation of the immune system. Their diverse biological actions are thought to be mediated by the products of specific but usually overlapping sets of cellular genes induced in the target cells. We have recently isolated a human cDNA encoding a new nuclear bodies-associated protein (PML-NBs), which we have termed Isg20. In this report, we describe the cloning and functional characterization of the Isg20 promoter region and the identification of sequence elements and trans-acting factors implicated in its regulation. In the absence of any recognizable TATA or CAAT elements, Isg20 promoter basal activity is dependent upon the positive transcription factors Sp-1 and USF-1. Interestingly, we demonstrate that a unique interferon stimulated response element (ISRE) mediates both IFN type I and type II Isg20 induction in the absence of functional gamma-activated sequence. These inductions are strictly dependent upon of the IFN regulatory factor 1 (IRF-1). In addition, we show that the ISRE is also implicated in the constitutive transcriptional activity of Isg20 gene.

Replication of Toxoplasma gondii, but not Trypanosoma cruzi, is regulated in human fibroblasts activated with gamma interferon: requirement of a functional JAK/STAT pathway

To study the role of tryptophan degradation by indoleamine 2, 3-dioxygenase (INDO) in the control of Trypanosoma cruzi or Toxoplasma gondii replication, we used human fibroblasts and a fibrosarcoma cell line (2C4). The cells were cultured in the presence or absence of recombinant gamma interferon (rIFN-gamma) and/or recombinant tumor necrosis factor alpha (rTNF-alpha) for 24 h and were then infected with either T. cruzi or T. gondii. Intracellular parasite replication was evaluated 24 or 48 h after infection. Treatment with rIFN-gamma and/or rTNF-alpha had no inhibitory effect on T. cruzi replication. In contrast, 54, 73, or 30% inhibition of T. gondii replication was observed in the cells treated with rIFN-gamma alone, rIFN-gamma plus rTNF-alpha, or TNF-alpha alone, respectively. The replication of T. gondii tachyzoites in cytokine-activated cells was restored by the addition of extra tryptophan to the culture medium. Similarly, T. gondii tachyzoites transfected with bacterial tryptophan synthase were not sensitive to the microbiostatic effect of rIFN-gamma. We also investigated the basis of the cytokine effect on parasite replication by using the three mutant cell lines B3, B9, and B10 derived from 2C4 and expressing defective STAT1alpha (signal transducer and activator of transcription), JAK2 (Janus family of cytoplasmic tyrosine kinases), or JAK1, respectively, three important elements of a signaling pathway triggered by rIFN-gamma. We found that rTNF-alpha was able to induce low levels expression of INDO mRNA in the parental cell line, as well as the cell line lacking functional JAK2. In contrast to the parental cell line (2C4), rIFN-gamma was not able to induce the expression of INDO mRNA or microbiostatic activity in any of the mutant cell lines. These findings indicate the essential requirement of the JAK/STAT pathway for the induction of high levels of INDO mRNA, tryptophan degradation, and the anti-Toxoplasma activity inside human nonprofessional phagocytic cells.