The genetics and biology of Irf5-mediated signaling in lupus

Recently much attention was attracted to the importance of the type I interferon pathway in the initiation and development of the autoimmune disease systemic lupus erythematosus (SLE). Many SLE patients have increased serum levels of IFN-alpha and display an IFN gene expression "signature" characterized by strong overexpression of IFN-responsive genes in leukocytes and target tissues. Moreover, about 20% of cancer patients treated with IFN-alpha therapy manifest symptoms resembling SLE and some later develop the disease. One of the key genes of the IFN-alpha pathway, IRF5, was found to be strongly associated with SLE. Two functional SNPs lead to alternative splicing and altered steady-state level of IRF5 gene expression. Besides, the gene has a polymorphic inserion/deletion in exon 6, which contributes to the diversity in the isoform pattern of IRF5. Interestingly, recent studies have not found association of IRF5 with the other autoimmune diseases, such as rheumatoid arthritis or psoriasis, suggesting the unique role for IRF5 in the development of lupus. Here, we present the current knowledge on IRF5 genetics and its biological function and discuss the possible ways in which IRF5 contributes to susceptibility to SLE.     

Emerging roles for the interferon-inducible p200-family proteins in sex bias in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a complex autoimmune disease involving multiple organs. The disease is characterized by the production of pathogenic autoantibodies to DNA and certain nuclear antigens, chronic inflammation, and immune dysregulation. Genetic studies involving SLE patients and mouse models have indicated that multiple lupus susceptible genes contribute to the disease phenotype. Notably, the development of SLE in patients and in certain mouse models exhibits a strong sex bias. In addition, several lines of evidence indicates that activation of interferon-α (IFN-α) signaling in immune cells and alterations in the expression of certain immunomodulatory cytokines contribute to lupus pathogenesis. Studies have implicated factors, such as the X chromosomal gene dosage effect and the sex hormones, in gender bias in SLE. However, the molecular mechanisms remain unclear. Additionally, it remains unclear whether these factors influence the "IFN-signature," which is associated with SLE. In this regard, a mutually positive regulatory feedback loop between IFNs and estrogen receptor-α (ERα) has been identified in immune cells. Moreover, studies indicate that the expression of certain IFN-inducible p200-family proteins that act as innate immune sensors for cytosolic DNA is differentially regulated by sex hormones. In this review, we discuss how the modulation of the expression of the p200-family proteins in immune cells by sex hormones and IFNs contributes to sex bias in SLE. An improved understanding of the regulation and roles of the p200-family proteins in immune cells is critical to understand lupus pathogenesis as well as response (or the lack of it) to various therapies.     

I型干扰素信号通路与系统性红斑狼疮发病机制的研究进展

系统性红斑狼疮（SLE）是一种常见的自身免疫性疾病,多数患者血清中干扰素-α（IFN-α）水平升高,并且在淋巴细胞和组织中出现大量IFN诱导应答基因。含有核酸的免疫复合物作为IFN—α的诱导物,结合外源性因素的作用,可激活体内依赖Toll样受体（TLR）和不依赖TLR等多条信号通路,刺激产生更多的IFN-α,形成恶性循环,启动和维持SLE的自身免疫过程。     

Role of Natural Interferon-α Producing Cells (Plasmacytoid Dendritic Cells) in Autoimmunity

The type I interferons (IFNs) have antiviral, cytostatic and prominent immunomodulatory effects, which all are of great importance during viral infections. However, prolonged exposure of the immune system to type I IFN can break tolerance and initiate an autoimmune reaction, eventually leading to autoimmune disease. Recent observations in patients with systemic lupus erythematosus (SLE) have revealed that such individuals have endogenous IFN-α inducers, causing an ongoing IFN-α production and consequently a continuous stimulation of the immune system. These IFN-α inducers consist of small immune complexes (IC) containing DNA or RNA and act on the principal IFN-α producing cell, the natural IFN-α producing cell (NIPC), also termed the plasmacytoid dendritic cell (PDC). The NIPC/PDC is a key cell in both the innate and adaptive immune response but can also, either directly or via produced IFN-α, have a pivotal role in autoimmunity. In this review we summarize recent data concerning NIPC/PDC, including their activation, regulation, function and possible role in autoimmune diseases, especially SLE.     

Comprehensive evaluation of the genetic variants of interferon regulatory factor 5 (IRF5) reveals a novel 5 bp length polymorphism as strong risk factor for systemic lupus erythematosus

We analyzed a comprehensive set of single-nucleotide polymorphisms (SNPs) and length polymorphisms in the interferon regulatory factor 5 (IRF5) gene for their association with the autoimmune disease systemic lupus erythematosus (SLE) in 485 Swedish patients and 563 controls. We found 16 SNPs and two length polymorphisms that display association with SLE (P < 0.0005, OR > 1.4). Using a Bayesian model selection and averaging approach we identified parsimonious models with exactly two variants of IRF5 that are independently associated with SLE. The variants of IRF5 with the highest posterior probabilities (1.00 and 0.71, respectively) of being causal in SLE are a SNP (rs10488631) located 3' of IRF5, and a novel CGGGG insertion-deletion (indel) polymorphism located 64 bp upstream of the first untranslated exon (exon 1A) of IRF5. The CGGGG indel explains the association signal from multiple SNPs in the IRF5 gene, including rs2004640, rs10954213 and rs729302 previously considered to be causal variants in SLE. The CGGGG indel contains three or four repeats of the sequence CGGGG with the longer allele containing an additional SP1 binding site as the risk allele for SLE. Using electrophoretic mobility shift assays we show increased binding of protein to the risk allele of the CGGGG indel and using a minigene reporter assay we show increased expression of IRF5 mRNA from a promoter containing this allele. Increased expression of IRF5 protein was observed in peripheral blood mononuclear cells from SLE patients carrying the risk allele of the CGGGG indel. We have found that the same IRF5 allele also confers risk for inflammatory bowel diseases and multiple sclerosis, suggesting a general role for IRF5 in autoimmune diseases.     

Toll样受体7及I型干扰素通路在系统性红斑狼疮中的作用研究

探讨Toll样受体7(TLR7)及I型干扰素(IFN-α)通路在系统性红斑狼疮(SLE)发病中的作用。采用实时荧光定量PCR方法检测42例SLE患者和34例正常人外周血TLR7mRNA以及4个干扰素调节基因mRNA的表达水平,同时观察TLR7mRNA的表达量与SLE疾病活动相关指标和干扰素积分(IFN score)的关系。结果,SLE患者外周血TLR7mRNA的表达水平显著增高;TLR7mRNA的表达水平与SLEDAI积分、肾脏损伤指数、抗双链DNA(dsDNA)抗体、抗RNA相关抗体水平及干扰素积分呈正相关;与补体C3、C4、白细胞数呈负相关。TLR7—IFN-α通路可能参与了SLE的病理过程。     

Interferon alpha as a primary pathogenic factor in human lupus

Interferon alpha (IFN-α) is a critical mediator of human systemic lupus erythematosus (SLE). This review will summarize evidence supporting the role for IFN-α in the initiation of human SLE. IFN-α functions in viral immunity at the interface of innate and adaptive immunity, a position well suited to setting thresholds for autoimmunity. Some individuals treated with IFN-α for chronic viral infections develop de novo SLE, which frequently resolves when IFN-α is withdrawn, supporting the idea that IFN-α was causal. Abnormally high IFN-α levels are clustered within SLE families, suggesting that high serum IFN-α is a heritable risk factor for SLE. Additionally, SLE-risk genetic variants in the IFN-α pathway are gain of function in nature, resulting in either higher circulating IFN-α levels or greater sensitivity to IFN-α signaling in SLE patients. A recent genome-wide association study has identified additional novel genetic loci associated with high serum IFN-α in SLE patients. These data support the idea that genetically determined endogenous elevations in IFN-α predispose to human SLE. It is possible that some of these gain-of-function polymorphisms in the IFN-α pathway are useful in viral defense, and that risk of SLE is a burden we have taken on in the fight to defend ourselves against viral infection.     

Interferon-inducible Ifi200-family genes in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is the prototype of complex autoimmune diseases. Studies have suggested that genetic, hormonal, and environmental factors contribute to the development of the disease. Interestingly, several recent studies involving SLE patients and mouse models of the disease have suggested a role for interferon (IFN)-stimulated genes (ISGs) in the development of SLE. One family of ISGs is the Ifi200-family, which includes mouse (Ifi202a, Ifi202b, Ifi203, Ifi204, and Ifi205) and human (IFI16, MNDA, AIM2, and IFIX) genes. The mouse genes cluster between serum amyloid P-component (Apcs) and alpha-spectrin (Spna-1) genes on chromosome 1 and the human genes cluster in syntenic region 1q23. The Ifi200-family genes encode structurally and functionally related proteins (the p200-family proteins). Increased expression of certain p200-family proteins in cells is associated with inhibition of cell proliferation, modulation of apoptosis, and cell differentiation. Our studies involving generation of B6.Nba2 congenic mice, coupled with gene expression analyses, identified the Ifi202 as a candidate lupus-susceptibility gene. Importantly, recent studies using different mouse models of SLE have suggested that increased expression of Ifi202 gene (encoding p202 protein) in immune cells contributes to lupus susceptibility. Consistent with a functional role for the p202 protein in lupus susceptibility, increased levels of IFI16 protein in human SLE patients are associated with the diseases. This review summarizes recent findings concerning the regulation and role of p200-family proteins in the development of SLE.     

干扰素治疗相关的自身免疫性副作用研究进展

干扰素(IFNs)是一种广泛存在的具有强烈抗病毒作用的细胞因子,IFN-α被认为是目前慢性乙型肝炎和丙型肝炎的标准治疗方法.病毒感染时,尤其是丙型肝炎病毒(HCV)感染,包含核酸的免疫复合物诱导的IFN-α产生增加,持续性的免疫系统激活诱导自身免疫性损伤.IFN为基础的治疗可以加重这种自身免疫损伤.IFNs可以影响多种类型的细胞,使多个系统受到影响.因此,用IFN-α治疗的患者可以出现广谱的自身免疫性疾病,如自身免疫性甲状腺、风湿性关节炎、冷球蛋白血症、结节病、系统性红斑狼疮、1型糖尿病和重症肌无力等.     

The IRF5 polymorphism in type 1 diabetes

The interferon regulatory factor 5 gene (IRF5) has been shown to play a crucial role in harmful immune responses by induction of proinflammatory cytokines. Functional genetic variants associated with increased IRF5 expression of specific isoforms are associated with systemic lupus erythematosus (SLE) and it is possible that they may also predispose to other autoimmune disorders. We tested the association of two IRF5 SNPs, correlated with IRF5 expression and SLE risk, in 947 nuclear family trios type 1 diabetes (T1D) using the transmission disequilibrium test. Our results suggest that the functional IRF5 variations do not confer an obvious risk for T1D.     

Interferon regulatory factor 3 as key element of the interferon signature in plasmacytoid dendritic cells from systemic lupus erythematosus patients: novel genetic associations in the Mexican mestizo population

Many genetic studies have found an association between interferon regulatory factors (IRF) single nucleotide polymorphisms (SNPs) and systemic lupus erythematosus (SLE); however, specific dendritic cell (DC) alterations have not been assessed. The aim of the present study was to address the expression of IRF3 and IRF5 on different DC subsets from SLE patients, as well as their association with interferon (IFN)-α production and novel SNPs. For the genetic association analyses, 156 SLE patients and 272 healthy controls from the Mexican mestizo population were included. From these, 36 patients and 36 controls were included for functional analysis. Two IRF3 SNPs − rs2304206 and rs2304204 – were determined. We found an increased percentage of circulating pDC in SLE patients in comparison to controls (8·04 ± 1·48 versus 3·35 ± 0·8, P = 0·032). We also observed enhanced expression of IRF3 (64 ± 6·36 versus 36·1 ± 5·57, P = 0·004) and IRF5 (40 ± 5·25 versus 22·5 ± 2·6%, P = 0·010) restricted to this circulating pDC subset from SLE patients versus healthy controls. This finding was associated with higher IFN-α serum levels in SLE (160·2 ± 21 versus 106·1 ± 14 pg/ml, P = 0·036). Moreover, the IRF3 rs2304206 polymorphism was associated with increased susceptibility to SLE [odds ratio (OR), 95% confidence interval (CI) = 2·401 (1·187–4·858), P = 0·021] as well as enhanced levels of serum type I IFN in SLE patients who were positive for dsDNA autoantibodies. The IRF3 rs2304204 GG and AG genotypes conferred decreased risk for SLE. Our findings suggest that the predominant IRF3 expression on circulating pDC is a key element for the increased IFN-α activation based on the interplay between the rs2304206 gene variant and the presence of dsDNA autoantibodies in Mexican mestizo SLE patients.     

Interferon-inducible Ifi200-family genes as modifiers of lupus susceptibility

Both genetic and environmental factors contribute to the development and progression of systemic lupus erythematosus (SLE), a complex autoimmune disease. The disease exhibits a strong gender bias and develops predominantly in females. Additionally, most SLE patients exhibit increased serum levels of interferon-α (IFN-α) and the "IFN signature". Studies using the mouse models of lupus have identified several lupus susceptibility loci, including the New Zealand Black (NZB)-derived autoimmunity 2 (Nba2) interval on the chromosome 1. The interval, which is syntenic to the human chromosome 1q region, harbors the FcγR family, SLAM/CD2-family, and the IFN-inducible Ifi200-family genes (encoding for the p200-family proteins). Studies involving the B6.Nba2 congenic mice revealed that the development of antinuclear autoantibodies (ANAs) depends on the age, gender, and activation of type I IFN-signaling. Interestingly, recent studies involving the generation of Nba2 subcongenic mouse lines and generation of mice deficient for the Fcgr2b or Aim2 gene within the interval have provided evidence that epistatic interactions among the Nba2 genes contribute to increased lupus susceptibility. Given that the expression of some of the p200-family proteins is differentially regulated by sex hormones and these proteins differentially regulate cytosolic DNA-induced production of type I IFN and proinflammatory cytokines (IL-1β and IL-18), the major known contributors of SLE-associated inflammation, we discuss the recent advancements in our understanding of the role of p200-family proteins in lupus susceptibility modification. An improved understanding of the role of p200-family proteins in the development of autoimmunity is likely to identify new approaches to treat SLE patients.     

Patients with systemic lupus erythematosus (SLE) have a circulating inducer of interferon-alpha (IFN-α) production acting on leucocytes resembling immature dendritic cells

Patients with active SLE often have an ongoing production of IFN-α. We therefore searched for an endogenous IFN-α-inducing factor (IIF) in SLE patients and found that their sera frequently induced production of IFN-α in cultures of peripheral blood mononuclear cells (PBMC) from healthy blood donors, especially when the PBMC were costimulated with the cytokines IFN-α2b and granulocyte-macrophage colony-stimulating factor (GM-CSF). The phenotype of the IFN-α-producing cells (IPC) as determined by flow cytometry corresponded to that of the natural IPC, resembling immature dendritic cells. The IIF activity in SLE sera was sometimes as high as that of a virus and was present especially in patients with active disease and with measurable IFN-α levels in serum. The IIF had an apparent molecular weight of 300–1000 kD and appeared to consist of both immunoglobulin and DNA, possibly being immune complexes. This endogenous IFN-α inducer may be of pathogenic significance, since a reported occasional adverse effect of IFN-α therapy in patients with non-autoimmune disorders is development of anti-dsDNA antibodies and SLE.