A20 deficiency in B cells enhances B-cell proliferation and results in the development of autoantibodies

Development of a high-resolution melting method for the screening of TNFAIP3 gene mutations

Tumor necrosis factor, α-induced protein 3 (TNFAIP3) which encodes a ubiquitin-modifying enzyme (A20), acts as a negative regulator of the NF-κB pathway, and in lymphoma and autoimmune diseases it is frequently inactivated by mutations and/or deletions. We investigated the prevalence of the inactivation of TNFAIP3 in oral squamous cell carcinoma (OSCC). DNA was extracted from 81 cases of OSCC and 50 peripheral blood samples from normal controls. A high-resolution melting (HRM) analysis was used to characterize TNFAIP3 mutations, and the results were confirmed by direct DNA sequencing. Three mutations and three single-nucleotide polymorphisms (SNPs) were found to be associated with OSCC; the TNFAIP3 mutation occurred in 3.7% (3/81) of the OSCC cases examined. All mutations were in exon 7 [c.1081G>A (p.E361K), c.1398C>G (p.S466R) (rs200878487) and c.1760C>T (p.P587L) (rs150056192)], and p.E361K was identified as a novel mutation. We further used SIFT and PolyPhen-2 software to assess potentially functional mutations. Two SNPs, c.296‑20_296-18delCTC (rs71670547) and c.380T>G (p.F127C) (rs2230926), were located in exon 3, and c.2140C>T (p.P714S) was located in exon 9. A novel SNP, p.P714S differed from the one reported previously (p.P714A) (rs369155845) at that site. We also identified five SNPs in 50 normal Taiwanese individuals, and two of them [c.296‑15C>T (rs377482653) and c.305A>G (p.N102S) (rs146534657)] were not found in our OSCC tissue. HRM facilitated the screening of genetic changes. In addition, our results indicate that the prevalence of the TNFAIP3 mutation is low in OSCC.

NF-κB-inducing kinase is essential for B-cell maintenance in mice

NF-κB-inducing kinase (NIK) is a key mediator of the noncanonical NF-κB signaling pathway, which is critical for normal B-cell development and function. It is well established that the complete deletion of NIK in mice results in defective B cells and impaired secondary lymphoid organogenesis. To address the role of NIK deficiency specifically in B cells, we generated a new mouse strain for the conditional deletion of this kinase. Deletion of NIK during B-cell development results in a drastic reduction of mature B cells from the transitional 2 stage on, while B-1 B cells are less affected. Moreover, deletion of NIK in the germinal centers decreases the numbers of germinal center B cells and impairs the ability of NIK-deficient B cells to develop into class-switched cells in vivo. This new mouse strain will be helpful for studying the role of NIK in different cell types of the body.

TALEN-mediated enhancer knockout influences TNFAIP3 gene expression and mimics a molecular phenotype associated with systemic lupus erythematosus

Linkage disequilibrium poses a major challenge to the functional characterization of specific disease-associated susceptibility variants. Precision genome editing technologies have provided new opportunities to address this challenge. As proof-of-concept, we employed TALEN-mediated genome editing to specifically disrupt the TT>A enhancer region to mimic candidate causal variants identified in the systemic lupus erythematosus-associated susceptibility gene, TNFAIP3, in an isogenic HEK293T cell line devoid of other linkage disequilibrium-associated variants. Targeted disruption of the TT>A enhancer impaired its interaction with the TNFAIP3 promoter by long-range DNA looping, thereby reducing TNFAIP3 gene expression. Loss of TNFAIP3 mRNA and its encoded protein, A20, impaired TNFα-induced receptor-mediated downregulation of NF-κB signaling; a hallmark of autoimmunity. Results demonstrate that the TT>A enhancer variants contribute to causality and function independently of other variants to disrupt TNFAIP3 expression. Further, we believe this approach can be implemented to independently examine other candidate casual variants in the future.

Deconstructing innate immune signaling in myelodysplastic syndromes

Overexpression of immune-related genes is widely reported in myelodysplastic syndromes (MDSs), and chronic immune stimulation increases the risk for developing MDS. Aberrant innate immune activation, such as that caused by increased toll-like receptor (TLR) signaling, in MDS can contribute to systemic effects on hematopoiesis, in addition to cell-intrinsic defects on hematopoietic stem/progenitor cell (HSPC) function. This review will deconstruct aberrant function of TLR signaling mediators within MDS HSPCs that may contribute to cell-intrinsic consequences on hematopoiesis and disease pathogenesis. We will discuss the contribution of chronic TLR signaling to the pathogenesis of MDS based on evidence from patients and mouse genetic models.

Altered TNFAIP3 mRNA expression in peripheral blood mononuclear cells from patients with rheumatoid arthritis

The tumor necrosis factor α-induced protein-3 (TNFAIP3) gene functions in negative immunoregulation and its single-nucleotide polymorphisms (SNPs) are associated with rheumatoid arthritis (RA) disease. However, its expression level in immune cells from RA patients remains unclear. The aim of the present study was to investigate whether the expression of TNFAIP3 is changed in patients with RA. Reverse transcription-quantitative polymerase chain reaction analysis was used to determine TNFAIP3 mRNA expression in peripheral blood mononuclear cells (PBMCs) from patients with RA and healthy controls. TNFAIP3 expression was decreased in RA patients compared with the healthy controls. The expression level of the TNFAIP3 gene negatively correlated with the RA score, anti-cyclic citrullinated peptide (CCP) antibody levels and C-reactive protein levels. Furthermore, RA patients with positive results of anti-CCP antibodies had a lower expression of TNFAIP3 than those without anti-CCP antibodies. In conclusion, the present results suggest that the insufficient expression of the TNFAIP3 gene in PBMCs may correlate with the diagnosis of RA.

The ubiquitin-modifying enzyme A20 restricts ubiquitination of the kinase RIPK3 and protects cells from necroptosis

A20 is an anti-inflammatory protein linked to multiple human diseases; however, the mechanisms by which A20 prevents inflammatory disease are incompletely defined. We found that A20-deficient T cells and fibroblasts were susceptible to caspase-independent and kinase RIPK3-dependent necroptosis. Global deficiency in RIPK3 significantly restored the survival of A20-deficient mice. A20-deficient cells exhibited exaggerated formation of RIPK1-RIPK3 complexes. RIPK3 underwent physiological ubiquitination at Lys5 (K5), and this ubiquitination event supported the formation of RIPK1-RIPK3 complexes. Both the ubiquitination of RIPK3 and formation of the RIPK1-RIPK3 complex required the catalytic cysteine of A20's deubiquitinating motif. Our studies link A20 and the ubiquitination of RIPK3 to necroptotic cell death and suggest additional mechanisms by which A20 might prevent inflammatory disease.

Deubiquitinating enzyme CYLD mediates pressure overload-induced cardiac maladaptive remodeling and dysfunction via downregulating Nrf2

Ubiquitin proteasome system (UPS) consists of ubiquitin, ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), ubiquitin ligases (E3s), proteasomes, and deubiquitinating enzymes (DUBs). Ubiquitin, E1s, several E2s, E3s, and proteasomes play an important role in the regulation of cardiac homeostasis and dysfunction; however, less is known about the role of DUBs in the heart. Here, we uncovered a crucial role of cyclindromatosis (CYLD), a DUB, in mediating cardiac maladaptive remodeling and dysfunction. CYLD expression was dramatically upregulated in the cardiomyocytes of hypertrophic and failing human and murine hearts. Knockout of CYLD improved survival rate and alleviated cardiac hypertrophy, fibrosis, apoptosis, oxidative stress, and dysfunction in mice that were subjected to sustained pressure overload induced by transverse aortic constriction. Deep sequencing and gene array analyses revealed that the most dramatically changed genes are those involving in the free radical scavenging pathway and cardiovascular disease, including fos, jun, myc, and nuclear factor erythroid-2 related factor 2 (Nrf2) in the heart. Moreover, knockdown of CYLD enhanced mitogen-activated protein kinase (MAPK) ERK- and p38-mediated expression of c-jun, c-fos, and c-myc, which govern Nrf2 expression in cardiomyocytes. The CYLD deficiency-induced suppression of reactive oxygen species (ROS) formation, death and hypertrophy in cardiomyocytes was blocked by additional knockdown of Nrf2. Taken together, our findings demonstrate for the first time that CYLD mediates cardiac maladaptive remodeling and dysfunction, most likely via enhancing myocardial oxidative stress in response to pressure overload. At the molecular level, CYLD interrupts the ERK- and p38-/AP-1 and c-Myc pathways to suppress Nrf2-operated antioxidative capacity, thereby enhancing oxidative stress in the heart.

Genetics and pathogenesis of systemic lupus erythematosus and lupus nephritis

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disorder that has a broad spectrum of effects on the majority of organs, including the kidneys. Approximately 40-70% of patients with SLE will develop lupus nephritis. Renal assault during SLE is initiated by genes that breach immune tolerance and promote autoantibody production. These genes might act in concert with other genetic factors that augment innate immune signalling and IFN-I production, which in turn can generate an influx of effector leucocytes, inflammatory mediators and autoantibodies into end organs, such as the kidneys. The presence of cognate antigens in the glomerular matrix, together with intrinsic molecular abnormalities in resident renal cells, might further accentuate disease progression. This Review discusses the genetic insights and molecular mechanisms for key pathogenic contributors in SLE and lupus nephritis. We have categorized the genes identified in human studies of SLE into one of four pathogenic events that lead to lupus nephritis. We selected these categories on the basis of the cell types in which these genes are expressed, and the emerging paradigms of SLE pathogenesis arising from murine models. Deciphering the molecular basis of SLE and/or lupus nephritis in each patient will help physicians to tailor specific therapies.

Signaling mechanisms regulating B-lymphocyte activation and tolerance

It is becoming more and more accepted that, in addition to producing autoantibodies, B lymphocytes have other important functions that influence the development of autoimmunity. For example, autoreactive B cells are able to produce inflammatory cytokines and activate pathogenic T cells. B lymphocytes can react to extracellular signals with a range of responses from anergy to autoreactivity. The final outcome is determined by the relative contribution of signaling events mediated by activating and inhibitory pathways. Besides the B cell antigen receptor (BCR), several costimulatory receptors expressed on B cells can also induce B cell proliferation and survival, or regulate antibody production. These include CD19, CD40, the B cell activating factor receptor, and Toll-like receptors. Hyperactivity of these receptors clearly contributes to breaking B-cell tolerance in several autoimmune diseases. Inhibitors of these activating signals (including protein tyrosine phosphatases, deubiquitinating enzymes and several adaptor proteins) are crucial to control B-cell activation and maintain B-cell tolerance. In this review, we summarize the inhibitory signaling mechanisms that counteract B-cell activation triggered by the BCR and the coreceptors.

The tumor necrosis factor alpha-induced protein 3 (TNFAIP3, A20) imposes a brake on antitumor activity of CD8 T cells

The transcription factor NF-κB is central to inflammatory signaling and activation of innate and adaptive immune responses. Activation of the NF-κB pathway is tightly controlled by several negative feedback mechanisms, including A20, an ubiquitin-modifying enzyme encoded by the tnfaip3 gene. Mice with selective deletion of A20 in myeloid, dendritic, or B cells recapitulate some human inflammatory pathology. As we observed high expression of A20 transcripts in dysfunctional CD8 T cells in an autochthonous melanoma, we analyzed the role of A20 in regulation of CD8 T-cell functions, using mice in which A20 was selectively deleted in mature conventional T cells. These mice developed lymphadenopathy and some organ infiltration by T cells but no splenomegaly and no detectable pathology. A20-deleted CD8 T cells had increased sensitivity to antigen stimulation with production of large amounts of IL-2 and IFNγ, correlated with sustained nuclear expression of NF-κB components reticuloendotheliosis oncogene c-Rel and p65. Overexpression of A20 by retroviral transduction of CD8 T cells dampened their intratumor accumulation and antitumor activity. In contrast, relief from the A20 brake in NF-κB activation in adoptively transferred antitumor CD8 T cells led to improved control of melanoma growth. Tumor-infiltrating A20-deleted CD8 T cells had enhanced production of IFNγ and TNFα and reduced expression of the inhibitory receptor programmed cell death 1. As manipulation of A20 expression in CD8 T cells did not result in pathologic manifestations in the mice, we propose it as a candidate to be targeted to increase antitumor efficiency of adoptive T-cell immunotherapy.

Bladder polypoid cystitis-derived A20 associates with tumorigenesis

Bladder chronic inflammation is associated with the pathogenesis of bladder cancer; the underlying mechanism is unclear. The PT53 gene is an important anticancer gene in the body, which is suppressed in cancer. The ubiquitin E3 ligase A20 (A20) plays a role in regulating the activities of epithelial cells. This study was designed to investigate the correlation between A20 and the pathogenesis of bladder cancer. The biopsy tissues of human bladder cancer, bladder polypoid cystitis, and chronic inflammation were collected; the levels of A20 and p53 were analyzed by quantitative real-time RT-PCR, Western blotting, and immune precipitation. HEK293 cells were employed to test the role of overexpression of A20 in the suppression of the p53 gene in the cells. Fifty-six patients with bladder cancer, 48 patients with bladder polypoid cystitis, and 16 patients with bladder chronic inflammation were recruited into this study. Human bladder cancer tissue and the polypoid tissue showed high levels of A20, which had a positive correlation with the tumorigenesis in the bladder; 12 out of 46 (26.1%) patients with bladder polypoid cystitis were diagnosed as bladder cancer. A20 bound to p53 to form complexes in bladder cancer tissue and bladder polypoid tissue. The overexpression of A20 suppresses p53 protein levels in HEK293 cells. A20 has a positive correlation in the tumorigenesis of bladder polypoid disorders.

Does antigen masking by ubiquitin chains protect from the development of autoimmune diseases?

Autoimmune diseases are characterized by the production of antibodies against self-antigens and generally arise from a failure of central or peripheral tolerance. However, these diseases may develop when newly appearing antigens are not recognized as self by the immune system. The mechanism by which some antigens are “invisible” to the immune system is not completely understood. Apoptotic and complement system defects or autophagy imbalance can generate this antigenic autoreactivity. Under particular circumstances, cellular debris containing autoreactive antigens can be recognized by innate immune receptors or other sensors and can eventually lead to autoimmunity. Ubiquitination may be one of the mechanisms protecting autoreactive antigens from the immune system that, if disrupted, can lead to autoimmunity. Ubiquitination is an essential post-translational modification used by cells to target proteins for degradation or to regulate other intracellular processes. The level of ubiquitination is regulated during T cell tolerance and apoptosis and E3 ligases have emerged as a crucial signaling pathway for the regulation of T cell tolerance toward self-antigens. I propose here that an unrecognized role of ubiquitin and ubiquitin-like proteins could be to render intracellular or foreign antigens (present in cellular debris resulting from apoptosis, complement system, or autophagy defects) invisible to the immune system in order to prevent the development of autoimmunity.

Role of A20 in cIAP-2 protection against tumor necrosis factor α (TNF-α)-mediated apoptosis in endothelial cells

Tumor necrosis factor α (TNF-α) influences endothelial cell viability by altering the regulatory molecules involved in induction or suppression of apoptosis. However, the underlying mechanisms are still not completely understood. In this study, we demonstrated that A20 (also known as TNFAIP3, tumor necrosis factor α-induced protein 3, and an anti-apoptotic protein) regulates the inhibitor of apoptosis protein-2 (cIAP-2) expression upon TNF-α induction in endothelial cells. Inhibition of A20 expression by its siRNA resulted in attenuating expression of TNF-α-induced cIAP-2, yet not cIAP-1 or XIAP. A20-induced cIAP-2 expression can be blocked by the inhibition of phosphatidyl inositol-3 kinase (PI3-K), but not nuclear factor (NF)-κB, while concomitantly increasing the number of endothelial apoptotic cells and caspase 3 activation. Moreover, TNF-α-mediated induction of apoptosis was enhanced by A20 inhibition, which could be rescued by cIAP-2. Taken together, these results identify A20 as a cytoprotective factor involved in cIAP-2 inhibitory pathway of TNF-α-induced apoptosis. This is consistent with the idea that endothelial cell viability is dependent on interactions between inducers and suppressors of apoptosis, susceptible to modulation by TNF-α.

Acquired deficiency of A20 results in rapid apoptosis, systemic inflammation, and abnormal hematopoietic stem cell function

A20 is a negative regulator of NF-κB, and mutational loss of A20 expression is involved in the pathogenesis of autoimmune diseases and B-cell lymphomas. To clarify the role of A20 in adult hematopoiesis, we generated conditional A20 knockout mice (A20^flox/flox) and crossed them with Mx–1Cre (MxCre⁺) and ERT2Cre (ERT2Cre⁺) transgenic mice in which Cre is inducibly activated by endogenous interferon and exogenous tamoxifen, respectively. A20^flox/flox MxCre⁺ (A20Mx) mice spontaneously exhibited myeloid proliferation, B cell apoptosis, and anemia with overproduction of pro-inflammatory cytokines. Bone marrow transplantation demonstrated that these changes were caused by hematopoietic cells. NF-κB was constitutively activated in A20Mx hematopoietic stem cells (HSCs), which caused enhanced cell cycle entry and impaired repopulating ability. Tamoxifen stimulation of A20^flox/flox ERT2Cre⁺ (A20ERT2) mice induced fulminant apoptosis and subsequent myeloproliferation, lymphocytopenia, and progressive anemia with excessive production of pro-inflammatory cytokines, as observed in A20Mx mice. These results demonstrate that A20 plays essential roles in the homeostasis of adult hematopoiesis by preventing apoptosis and inflammation. Our findings provide insights into the mechanism underlying A20 dysfunction and human diseases in which A20 expression is impaired.

A20 in inflammation and autoimmunity

Although known for many years as a nuclear factor (NF)-κB inhibitory and antiapoptotic signaling protein, A20 has recently attracted much attention because of its ubiquitin-regulatory activities and qualification by genome-wide association studies (GWASs) as a susceptibility gene for inflammatory disease. Here, we review new findings that have shed light on the molecular and biochemical mechanisms by which A20 regulates inflammatory signaling cascades, and discuss recent experimental evidence characterizing A20 as a crucial gatekeeper preserving tissue homeostasis.

Astrocytic A20 ameliorates experimental autoimmune encephalomyelitis by inhibiting NF-κB- and STAT1-dependent chemokine production in astrocytes

Single-nucleotide polymorphisms in the tumor necrosis factor, alpha-induced protein 3 gene, which encodes the ubiquitin-modifying protein A20, are linked to susceptibility to multiple sclerosis (MS), a demyelinating autoimmune disease of the central nervous system (CNS). Since it is unresolved how A20 regulates MS pathogenesis, we examined its function in a murine model of MS, namely experimental autoimmune encephalomyelitis (EAE). Deletion of A20 in neuroectodermal cells (astrocytes, neurons, and oligodendrocytes; Nestin-Cre A20^fl/fl mice) or selectively in astrocytes (GFAP-Cre A20^fl/fl mice) resulted in more severe EAE as compared to control animals. In Nestin-Cre A20^fl/fl and GFAP-Cre A20^fl/fl mice demyelination and recruitment of inflammatory leukocytes were increased as compared to A20^fl/fl control mice. Importantly, numbers of encephalitogenic CD4⁺ T cells producing interferon (IFN)-γ, interleukin (IL)-17, and granulocyte-macrophage colony-stimulating factor (GM-CSF), respectively, as well as mRNA production of IFN-γ, IL-17, tumor necrosis factor (TNF), GM-CSF, IL-6, CXCL1, CCL2, and CXCL10 were significantly increased in spinal cords of Nestin-Cre A20^fl/fl and GFAP-Cre A20^fl/fl mice, respectively. Compared to A20-sufficient astrocytes, A20-deficient astrocytes displayed stronger activation of nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB) in response to TNF, IL-17, and GM-CSF, and of signal transducer and activator of transcription 1 (STAT1) upon IFN-γ stimulation. Due to NF-κB and STAT1 hyperactivation, A20-deficient astrocytes produced significantly more chemokines in response to these key encephalitogenic cytokines of autoimmune CD4⁺ T cells resulting in an amplification of CD4⁺ T cell recruitment to the CNS. Thus, astrocytic A20 is an important inhibitor of autoimmune-mediated demyelination in the CNS.

Deubiquitylases from genes to organism

Ubiquitylation is a major posttranslational modification that controls most complex aspects of cell physiology. It is reversed through the action of a large family of deubiquitylating enzymes (DUBs) that are emerging as attractive therapeutic targets for a number of disease conditions. Here, we provide a comprehensive analysis of the complement of human DUBs, indicating structural motifs, typical cellular copy numbers, and tissue expression profiles. We discuss the means by which specificity is achieved and how DUB activity may be regulated. Generically DUB catalytic activity may be used to 1) maintain free ubiquitin levels, 2) rescue proteins from ubiquitin-mediated degradation, and 3) control the dynamics of ubiquitin-mediated signaling events. Functional roles of individual DUBs from each of five subfamilies in specific cellular processes are highlighted with an emphasis on those linked to pathological conditions where the association is supported by whole organism models. We then specifically consider the role of DUBs associated with protein degradative machineries and the influence of specific DUBs upon expression of receptors and channels at the plasma membrane.

An enhancer element harboring variants associated with systemic lupus erythematosus engages the TNFAIP3 promoter to influence A20 expression

Functional characterization of causal variants present on risk haplotypes identified through genome-wide association studies (GWAS) is a primary objective of human genetics. In this report, we evaluate the function of a pair of tandem polymorphic dinucleotides, 42 kb downstream of the promoter of TNFAIP3, (rs148314165, rs200820567, collectively referred to as TT>A) recently nominated as causal variants responsible for genetic association of systemic lupus erythematosus (SLE) with tumor necrosis factor alpha inducible protein 3 (TNFAIP3). TNFAIP3 encodes the ubiquitin-editing enzyme, A20, a key negative regulator of NF-κB signaling. A20 expression is reduced in subjects carrying the TT>A risk alleles; however, the underlying functional mechanism by which this occurs is unclear. We used a combination of electrophoretic mobility shift assays (EMSA), mass spectrometry (MS), reporter assays, chromatin immunoprecipitation-PCR (ChIP-PCR) and chromosome conformation capture (3C) EBV transformed lymphoblastoid cell lines (LCL) from individuals carrying risk and non-risk TNFAIP3 haplotypes to characterize the effect of TT>A on A20 expression. Our results demonstrate that the TT>A variants reside in an enhancer element that binds NF-κB and SATB1 enabling physical interaction of the enhancer with the TNFAIP3 promoter through long-range DNA looping. Impaired binding of NF-κB to the TT>A risk alleles or knockdown of SATB1 expression by shRNA, inhibits the looping interaction resulting in reduced A20 expression. Together, these data reveal a novel mechanism of TNFAIP3 transcriptional regulation and establish the functional basis by which the TT>A risk variants attenuate A20 expression through inefficient delivery of NF-κB to the TNFAIP3 promoter. These results provide critical functional evidence supporting a direct causal role for TT>A in the genetic predisposition to SLE.

A key objective of human genetics is the identification and characterization of variants responsible for association with complex diseases. A pair of single nucleotide polymorphisms (rs148314165, rs200820567) 42 kb downstream from the promoter of TNFAIP3, have been proposed as the variants responsible for association with systemic lupus erythematosus based on comprehensive genetic and bioinformatic analyses. TNFAIP3 encodes for the ubiquitin-editing enzyme, A20, which plays a central role in maintaining immune system homeostasis through restriction of NF-κB signaling. Cells that carry this risk haplotype express low levels of TNFAIP3 compared to cells carrying the nonrisk haplotype. How the risk alleles of rs148314165 and rs200820567 might influence low TNFAIP3 expression is unknown. In this paper, we demonstrate that these variants reside in an enhancer element that binds NF-κB and SATB1 enabling the interaction of the enhancer with the TNFAIP3 promoter through long-range DNA looping. Impaired binding of NF-κB directly to the risk alleles or shRNA-mediated knockdown of SATB1 inhibits interaction of the enhancer with the TNFAIP3 promoter resulting in reduced A20 expression. These results clarify the functional mechanism by which rs148314165 and rs200820567 attenuate A20 expression and support a causal role for these variants in the predisposition to autoimmune disease.

Single-nucleotide Polymorphism and Haplotypes of TNIP1 Associated with Systemic Lupus Erythematosus in a Chinese Han Population

Objective.

To determine the association of systemic lupus erythematosus (SLE) with single-nucleotide polymorphisms (SNP) in the TNIP1 gene and compare the expression of this gene in cases and controls from a Chinese Han population in this replication study.

Methods.

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry was used to genotype 19 SNP in TNIP1 in Chinese Han patients with SLE (n = 341) and controls (n = 356). Genotypes were analyzed by codominant, dominant, and recessive models. Analysis of allele frequencies and linkage disequilibrium was also performed. Western blotting and qRT-PCR were used to measure the expression of these genes in peripheral blood mononuclear cells of SLE cases and controls.

Results.

Seven SNP loci were significantly associated with SLE in our population (p < 0.05 for all comparisons). Two TNIP1 gene haplotypes (ATTGCGC and GTCCTAT) were associated with SLE (p = 0.0246 and p = 0.0024, respectively). Western blotting and qRT-PCR results provide evidence that patients with SLE had significantly reduced expression of TNIP1/ABIN-1 relative to controls.

Conclusion.

Analysis of SNP in the TNIP1 gene and expression of this gene in peripheral blood lymphocytes indicated these SNP were associated with the occurrence of SLE in Han Chinese patients. Future studies should examine the roles of these SNP in the pathogenesis of SLE.

Co-stimulatory molecules as targets for treatment of lupus

Co-stimulatory molecules help to regulate interactions between T cells and antigen-presenting cells and may play an important role in the pathogenesis of lupus. Both work in murine models and some early studies in human lupus support further examination of these molecules as therapeutic targets. Complexities of lupus clinical trial variables may have hampered progress in this area but recent developments in the field may make interventional trials more feasible in the near future. To date biologics which provide direct blockade of interactions between CD40 and CD154, B7RP-1 and ICOS, and CD80 or CD86 with CD28 have been assessed in multicenter clinical trials. These data will be reviewed and critiqued.

Regulation of nuclear factor-κB in autoimmunity

Nuclear factor (NF)-κB transcription factors are pivotal regulators of innate and adaptive immune responses, and perturbations of NF-κB signaling contribute to the pathogenesis of immunological disorders. NF-κB is a well-known proinflammatory mediator, and its deregulated activation is associated with the chronic inflammation of autoimmune diseases. Paradoxically, NF-κB plays a crucial role in the establishment of immune tolerance, including both central tolerance and the peripheral function of regulatory T (Treg) cells. Thus, defective or deregulated activation of NF-κB may contribute to autoimmunity and inflammation, highlighting the importance of tightly controlled NF-κB signaling. This review focuses on recent progress regarding NF-κB regulation and its association with autoimmunity.

Clinicopathologic implication of A20/TNFAIP3 deletion in diffuse large B-cell lymphoma: an analysis according to immunohistochemical subgroups and rituximab treatment

We analyzed the clinicopathologic implication of A20/tumor necrosis factor α-induced protein 3 deletion in diffuse large B-cell lymphoma (DLBCL) using fluorescence in situ hybridization, according to germinal center B-cell (GCB) versus non-GCB/activated B-cell (ABC) phenotypes and rituximab treatment. Excluding primary central nervous system (CNS) and Epstein-Barr virus (EBV)-positive lymphomas, 134 DLBCLs were analyzed. A20 was deleted in 23.1% (31/134) of DLBCLs including 21.6% (29/ 134) of monoallelic and 1.5% (2/134) of biallelic deletion, with no predilection for GCB versus non-GCB/ABC. In univariate analysis, A20 deletion was marginally associated with favorable prognosis in the rituximab-treated subgroup (n = 109; p = 0.0454), non-gastrointestinal lymphoma (n = 108; p = 0.0320) and nodal lymphoma (n = 46; p = 0.0411). In multivariate analysis in rituximab-treated DLBCL, MUM1 and international prognostic index (IPI) were independent prognostic factors (p = 0.021 [IPI]; p = 009 [MUM1]) with a marginally favorable prognostic effect for A20 deletion (p = 0.047). Taken together, A20 deletion was observed in similar frequencies in GCB and non-GCB/ABC, and was not a poor prognostic factor in DLBCL.

A20: linking a complex regulator of ubiquitylation to immunity and human disease

A20 (also known as TNFAIP3) is a potent anti-inflammatory signalling molecule that restricts multiple intracellular signalling cascades. Recent studies in three general areas have converged to highlight the clinical and biological importance of A20. First, human genetic studies have strongly linked polymorphisms and mutations in the gene encoding A20 to inflammatory, autoimmune and malignant diseases. Second, studies in gene-targeted mice have revealed that A20 regulates multiple immune cell functions and prevents experimental diseases that closely mimic human conditions. Third, biochemical studies have unveiled complex mechanisms by which A20 regulates ubiquitin-dependent nuclear factor-κB and cell-survival signals. Taken together, these studies are revealing the importance of A20-mediated regulation of ubiquitin-dependent signalling in human disease.

TNFAIP3 facilitates degradation of microbial antigen SEB in enterocytes

BACKGROUND AND AIMS

The enterocytes have the potential to absorb noxious substances, such as microbial products, from the gut lumen. How the enterocytes process the substances to harmless materials is not fully understood. This study aims to elucidate the role of ubiquitin E3 ligase TNFAIP3 (TNFAIP3) in facilitating the degradation of endocytic microbial products in enterocytes.

METHODS

Human intestinal epithelial cell line, HT-29 cells, was cultured to monolayers using as an in vitro model to observe the endocytosis and degradation of microbial products, Staphylococcal enterotoxin B (SEB) in epithelial cells. The RNA interference was employed to knock down the TNFAIP3 gene in HT-29 cells to observe the role of TNFAIP3 in the degradation of endocytic SEB. The role of TNFAIP3 in facilitating the endosome/lysosome fusion was observed by immunocytochemistry.

RESULTS

Upon the absorption of SEB, the expression of TNFAIP3 was increased in HT-29 cells. Silencing the TNFAIP3 gene in HT-29 cells resulted in a large quantity of SEB to be transported across the HT-29 monolayers to the transwell basal chambers; the transportation was via the intracellular pathway. TNFAIP3 was required in the fusion of SEB-carrying endosomes and lysosomes.

CONCLUSIONS

TNFAIP3 plays a critical role in the degradation of endocytic SEB in enterocytes.

A20 and CYLD do not share significant overlapping functions during B cell development and activation

The ubiquitin-editing enzyme A20 (TNFAIP3) and the deubiquitinase CYLD are central negative regulators of NF-κB signaling. Both can act by removing nonproteolytic K63-linked polyubiquitin chains from an overlapping set of signaling molecules. In B cells, A20 deficiency results in hyperactivity, loss of immune homeostasis, inflammation, and autoimmunity. The reported consequences of CYLD deficiency are controversial, ranging from an absence of effects to dramatic B cell hyperplasia. These differences could be due to varying compensation for the loss of CYLD function by A20. Therefore, to explore potential overlapping physiological functions between A20 and CYLD, we generated and characterized A20/CYLD double-deficient B cells. Interestingly, the lack of both A20 and CYLD did not exacerbate the developmental defects and hyperresponsive activity of A20-deficient B cells. In addition, the extent of B cell activation after in vitro stimulation with anti-CD40, LPS, and CpG was comparable in B cells lacking A20/CYLD and A20 alone. However, in response to BCR cross-linking, we observed small but reproducible additive effects of the lack of A20 and CYLD. Taken together, our results demonstrate that A20 and CYLD do not share significant functions during B cell development and activation.

A20 inhibits LUBAC-mediated NF-κB activation by binding linear polyubiquitin chains via its zinc finger 7

Linear polyubiquitination of proteins has recently been implicated in NF-κB signalling and is mediated by the linear ubiquitin chain assembly complex (LUBAC), consisting of HOIL-1, HOIP and Sharpin. However, the mechanisms that regulate linear ubiquitination are still unknown. Here, we show that A20 is rapidly recruited to NEMO and LUBAC upon TNF stimulation and that A20 inhibits LUBAC-induced NF-κB activation via its C-terminal zinc-finger 7 (ZF7) domain. Expression of a polypeptide corresponding to only ZF7 was sufficient to inhibit TNF-induced NF-κB activation. Both A20 and ZF7 can form a complex with NEMO and LUBAC, and are able to prevent the TNF-induced binding of NEMO to LUBAC. Finally, we show that ZF7 preferentially binds linear polyubiquitin chains in vitro, indicating A20-ZF7 as a novel linear ubiquitin-binding domain (LUBID). We thus propose a model in which A20 inhibits TNF- and LUBAC-induced NF-κB signalling by binding to linear polyubiquitin chains via its seventh zinc finger, which prevents the TNF-induced interaction between LUBAC and NEMO.

Down-regulation of A20 mRNA expression in peripheral blood mononuclear cells from patients with systemic lupus erythematosus

PURPOSE

A20 gene functions in negative immunoregulation and its SNP is related to SLE disease. But its expression level in immune cells from SLE patients is still unclear. The aim of this study is to investigate whether the expression of A20 is associated with pathogenesis of SLE.

METHODS

Real-time transcription-polymerase chain reaction analysis (RT-PCR) was used to determine expression of A20 mRNA in peripheral blood mononuclear cells (PBMC) from 37 patients with SLE and 32 healthy controls.

RESULTS

A20 expression was decreased in SLE patients compared with healthy controls (p = 0.0133). The expression level of A20 gene negatively correlated with the SLE disease activity index (SLEDAI) (r =-0.4661, p = 0.0036) and erythrocyte sedimentation rate (ESR) (r =-0.5222, p = 0.0009). Furthermore, SLE patients with nephritis had a lower expression of A20 than those without nephritis (p = 0.0188).

CONCLUSIONS

Our results suggest that the insufficient expression of A20 gene in PBMC may take part in the pathogenesis of SLE disease.

Regulation of NF-κB by deubiquitinases

The nuclear factor-κB (NF-κB) pathway is a critical regulator of innate and adaptive immunity. Noncanonical K63-linked polyubiquitination plays a key regulatory role in NF-κB signaling pathways by functioning as a scaffold to recruit kinase complexes containing ubiquitin-binding domains. Ubiquitination is balanced by deubiquitinases that cleave polyubiquitin chains and oppose the function of E3 ubiquitin ligases. Deubiquitinases therefore play an important role in the termination of NF-κB signaling and the resolution of inflammation. In this review, we focus on NF-κB regulation by deubiquitinases with an emphasis on A20 and CYLD. Deubiquitinases and the ubiquitin/proteasome components that regulate NF-κB may serve as novel therapeutic targets for inflammatory diseases and cancer.

Genetics of rheumatoid arthritis: an impressionist perspective

Rheumatoid arthritis (RA) is the most common rheumatic disease. The genetic basis of RA is supported through the identification of more than 30 susceptibility genetic variants. Each of these genes individually makes only a slight contribution to the risk of disease. Moreover, there is significant disparity in the genetic variants associated with different RA subgroups and patient ethnicities, which emphasizes the intricate nature of the disease's pathogenesis, and the complexities involved in large-scale genetic studies. This review evaluates critically the recent literature on the genetic contribution to RA and assesses the methodology used to identify these risk alleles.

A20 (Tnfaip3) deficiency in myeloid cells protects against influenza A virus infection

The innate immune response provides the first line of defense against viruses and other pathogens by responding to specific microbial molecules. Influenza A virus (IAV) produces double-stranded RNA as an intermediate during the replication life cycle, which activates the intracellular pathogen recognition receptor RIG-I and induces the production of proinflammatory cytokines and antiviral interferon. Understanding the mechanisms that regulate innate immune responses to IAV and other viruses is of key importance to develop novel therapeutic strategies. Here we used myeloid cell specific A20 knockout mice to examine the role of the ubiquitin-editing protein A20 in the response of myeloid cells to IAV infection. A20 deficient macrophages were hyperresponsive to double stranded RNA and IAV infection, as illustrated by enhanced NF-κB and IRF3 activation, concomitant with increased production of proinflammatory cytokines, chemokines and type I interferon. In vivo this was associated with an increased number of alveolar macrophages and neutrophils in the lungs of IAV infected mice. Surprisingly, myeloid cell specific A20 knockout mice are protected against lethal IAV infection. These results challenge the general belief that an excessive host proinflammatory response is associated with IAV-induced lethality, and suggest that under certain conditions inhibition of A20 might be of interest in the management of IAV infections.

Influenza virus or flu epidemics represent a recurrent threat to the public health, especially for individuals which are part of a high-risk group such as children, elderly or immune-compromised people. Sporadic pandemic flu outbreaks, such as the Spanish flu of 1918, may cause high grades of mortality among healthy persons. A better understanding of how the immune system deals with these pathogens is of key importance. The protein A20 is an important negative regulator of both innate and adaptive immune responses. We show that the specific deletion of A20 in myeloid cells, such as macrophages and neutrophils, improves the resistance against otherwise lethal influenza infections. This protective effect is mediated by an enhanced innate immune response following respiratory challenge with influenza virus. Although exaggerated pulmonary immune responses are believed to be the primary cause of often life threatening influenza virus induced pneumonia, we demonstrate that boosting the innate immune response by selectively targeting the functionality of A20 in myeloid cells is beneficial for the host survival. This finding provides us with a novel valuable approach for treating influenza and potentially other respiratory viral infections.

Regulation of NF-κB signaling by the A20 deubiquitinase

The NF-κB transcription factor is a central mediator of inflammatory and innate immune signaling pathways. Activation of NF-κB is achieved by K63-linked polyubiquitination of key signaling molecules which recruit kinase complexes that in turn activate the IκB kinase (IKK). Ubiquitination is a highly dynamic process and is balanced by deubiquitinases that cleave polyubiquitin chains and terminate downstream signaling events. The A20 deubiquitinase is a critical negative regulator of NF-κB and inflammation, since A20-deficient mice develop uncontrolled and spontaneous multi-organ inflammation. Furthermore, specific polymorphisms in the A20 genomic locus predispose humans to autoimmune disease. Recent studies also indicate that A20 is an important tumor suppressor that is inactivated in B-cell lymphomas. Therefore, targeting A20 may form the basis of novel therapies for autoimmune disease and lymphomas.

Targeting lymphocyte signaling pathways as a therapeutic approach to systemic lupus erythematosus

PURPOSE OF REVIEW

Over the past year several key pathways in systemic lupus erythematosus (SLE) lymphocyte signaling have been identified. Pathways that can be exploited for therapy are discussed in this review.

RECENT FINDINGS

Inhibition of SLE T cell activation by blocking spleen tyrosine kinase (Syk) and SLE T cell migration by blocking CD44 or CXCR4 lead to amelioration of lupus in lupus-prone mice. Similar results can be achieved by boosting CD8+ Treg numbers. Small molecules that block the kinases CaMKIV (calcium and calmodulin dependent kinase IV) and Bruton Tyrosine kinase (Btk) and the phosphatase calcineurin were shown to be effective in treating murine lupus. Finally, gene methylation status determines the expression of several key genes in SLE and strategies to correct it have shown promising results in preclinical studies.

SUMMARY

Molecules that enhance T cell receptor (TCR) signaling or increase lymphocyte migration can be inhibited successfully with significant improvement of disease intensity in lupus-prone mice using small molecules. Manipulation of promoter methylation and histone acetylation represents a novel way to alter gene transcription in SLE.

Genetic relationships between A20/TNFAIP3, chronic inflammation and autoimmune disease

A20 [also known as TNFAIP3 (tumour necrosis factor α-induced protein 3)] restricts and terminates inflammatory responses through modulation of the ubiquitination status of central components in NF-κB (nuclear factor κB), IRF3 (interferon regulatory factor 3) and apoptosis signalling cascades. The phenotype of mice with full or conditional A20 deletion illustrates that A20 expression is essential to prevent chronic inflammation and autoimmune pathology. In addition, polymorphisms within the A20 genomic locus have been associated with multiple inflammatory and autoimmune disorders, including SLE (systemic lupus erythaematosis), RA (rheumatoid arthritis), Crohn's disease and psoriasis. A20 has also been implicated as a tumour suppressor in several subsets of B-cell lymphomas. The present review outlines recent findings that illustrate the effect of A20 defects in disease pathogenesis and summarizes the identified A20 polymorphisms associated with different immunopathologies.