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Obesity-induced inflammation is a major driving force in the development of insulin resistance, type 2 diabetes (T2D), and related metabolic disorders. During obesity, macrophages accumulate in the visceral adipose tissue, creating a low-grade inflammatory environment. Nuclear factor κB (NF-κB) signaling is a central coordinator of inflammatory responses and is tightly regulated by the anti-inflammatory protein A20. Here, we find that myeloid-specific A20-deficient mice are protected from diet-induced obesity and insulin resistance despite an inflammatory environment in their metabolic tissues. Macrophages lacking A20 show impaired mitochondrial respiratory function and metabolize more palmitate both in vitro and in vivo. We hypothesize that A20-deficient macrophages rely more on palmitate oxidation and metabolize the fat present in the diet, resulting in a lean phenotype and protection from metabolic disease. These findings reveal a role for A20 in regulating macrophage immunometabolism., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2021 The Author(s). Published by Elsevier Inc. All rights reserved.)

Background: A20, a protein encoded by the tumor necrosis factor alpha-induced protein 3 gene (TNFAIP3), plays a vital role in the negative regulation of inflammation and immunity. Loss-of-function mutation in TNFAIP3 leads to a new described autoinflammatory disease-haploinsufficiency of A20 (HA20). Since HA20 was first described in 2016, a number of new cases have been described in this literature, however, the disease and its pathogenesis are poorly understood. This review seeks to improve clinical recognition of this disorder, and promote both earlier diagnosis and initiation of targeted therapies to improve patients' outcomes., Methods: We reviewed 26 papers about A20 and HA20, and we summarized genetic variants and clinical manifestations of a total of 61 reported patients from 26 families identified to have a genetic diagnosis of germline pathogenic variants in TNFAIP3/A20. Additionally, we discussed the pathogenesis and treatment of HA20., Results: A total of 24 pathogenic variants of A20 had been reported. There was significant clinical heterogeneity, even among those with the same variants in TNFAIP3. Prior to receiving a molecular diagnosis of HA20, patients had been diagnosed with Behcet's disease, rheumatoid arthritis, rheumatic fever, juvenile idiopathic arthritis, systemic lupus erythematosus, and even adult-onset Stills' disease. The patients with HA20 that presented with inflammatory signatures in NF-κB signaling were mostly responsive to treatment., Conclusions: HA20 is a monogenic autoinflammatory disease with highly variable clinical manifestations. This extensive heterogeneity makes it difficult to set a clinical diagnostic criteria, and genetic sequencing is necessary for a definitive diagnosis of HA20.

Germline loss-of-function variation in TNFAIP3, encoding A20, has been implicated in a wide variety of autoinflammatory and autoimmune conditions, with acquired somatic missense mutations linked to cancer progression. Furthermore, human sequence data reveals that the A20 locus contains ~ 400 non-synonymous coding variants, which are largely uncharacterised. The growing number of A20 coding variants with unknown function, but potential clinical impact, poses a challenge to traditional mouse-based approaches. Here we report the development of a novel functional genomics approach that utilizes a new A20-deficient zebrafish (Danio rerio) model to investigate the impact of TNFAIP3 genetic variants in vivo. A20-deficient zebrafish are hyper-responsive to microbial immune activation and exhibit spontaneous early lethality. Ectopic addition of human A20 rescued A20-null zebrafish from lethality, while missense mutations at two conserved A20 residues, S381A and C243Y, reversed this protective effect. Ser381 represents a phosphorylation site important for enhancing A20 activity that is abrogated by its mutation to alanine, or by a causal C243Y mutation that triggers human autoimmune disease. These data reveal an evolutionarily conserved role for TNFAIP3 in limiting inflammation in the vertebrate linage and show how this function is controlled by phosphorylation. They also demonstrate how a zebrafish functional genomics pipeline can be utilized to investigate the in vivo significance of medically relevant human TNFAIP3 gene variants.

C-type lectin receptors (CLRs) play key roles in antifungal defense. CLR-induced NF-κB is central to CLR functions in immunity, and thus, molecules that control the amplitude of CLR-induced NF-κB could profoundly influence host defense against fungal pathogens. However, little is known about the mechanisms that negatively regulate CLR-induced NF-κB, and molecules which act on the CLR family broadly and which directly regulate acute CLR-signaling cascades remain unidentified. Here, we identify the ubiquitin-editing enzyme A20 as a negative regulator of acute NF-κB activation downstream of multiple CLR pathways. Absence of A20 suppression results in exaggerated CLR responses in cells which are A20 deficient and also cells which are A20 haplosufficient, including multiple primary immune cells. Loss of a single allele of A20 results in enhanced defense against systemic Candida albicans infection and prolonged host survival. Thus, A20 restricts CLR-induced innate immune responses in vivo and is a suppressor of host defense against systemic fungal infection., (Copyright © 2020 American Society for Microbiology.)

The intracellular-ubiquitin-ending-enzyme tumor necrosis factor alpha-induced protein 3 (TNFAIP3) is a potent inhibitor of the pro-inflammatory nuclear factor kappa-light-chain-enhancer of activated B cell (NF-kB) pathway. Single nucleotide polymorphisms in TNFAIP3 locus have been associated to autoimmune inflammatory disorders, including Multiple Sclerosis (MS). Previously, we reported a TNFAIP3 down-regulated gene expression level in blood and specifically in monocytes obtained from treatment-naïve MS patients compared to healthy controls (HC). Myeloid cells exert a key role in the pathogenesis of MS. Here we evaluated the effect of specific TNFAIP3 deficiency in myeloid cells including monocytes, monocyte-derived cells (M-MDC) and microglia analyzing lymphoid organs and microglia of mice. TNFAIP3 deletion is induced using conditional knock-out mice for myeloid lineage. Flow-cytometry and histological procedures were applied to assess the immune cell populations of spleen, lymph nodes and bone marrow and microglial cell density in the central nervous system (CNS), respectively. We found that TNFAIP3 deletion in myeloid cells induces a reduction in body weight, a decrease in the number of M-MDC and of common monocyte and granulocyte precursor cells (CMGPs). We also reported that the lack of TNFAIP3 in myeloid cells induces an increase in microglial cell density. The results suggest that TNFAIP3 in myeloid cells critically controls the development of M-MDC in lymphoid organ and of microglia in the CNS.

Tumor-necrosis-factor-alpha-induced protein 3 (TNFAIP3), or A20, is a ubiquitin-modifying protein and negative regulator of canonical nuclear factor κB (NF-κB) signaling. Several single-nucleotide polymorphisms in TNFAIP3 are associated with autoimmune diseases, suggesting a role in tissue inflammation. While the role of A20 in peripheral immune cells has been well investigated, less is known about its role in the central nervous system (CNS). Here, we show that microglial A20 is crucial for maintaining brain homeostasis. Without microglial A20, CD8 + T cells spontaneously infiltrate the CNS and acquire a viral response signature. The combination of infiltrating CD8 + T cells and activated A20-deficient microglia leads to an increase in VGLUT1 + terminals and frequency of spontaneous excitatory currents. Ultimately, A20-deficient microglia upregulate genes associated with the antiviral response and neurodegenerative diseases. Together, our data suggest that microglial A20 acts as a sensor for viral infection and a master regulator of CNS homeostasis., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2019 The Author(s). Published by Elsevier Inc. All rights reserved.)

The ubiquitin-editing protein A20 (TNFAIP3) is a known key player in the regulation of immune responses in many organs. Genome-wide associated studies (GWASs) have linked A20 with a number of inflammatory and autoimmune disorders, including psoriasis. Here, we identified a previously unrecognized role of A20 as a pro-apoptotic factor in TNF-induced cell death in keratinocytes. This function of A20 is mediated via the NF-κB-dependent alteration of cIAP1/2 expression. The changes in cIAP1/2 protein levels promote NIK stabilization and subsequent activation of noncanonical NF-κB signaling. Upregulation of TRAF1 expression triggered by the noncanonical NF-κB signaling further enhances the NIK stabilization in an autocrine manner. Finally, stabilized NIK promotes the formation of the ripoptosome and the execution of cell death. Thus, our data demonstrate that A20 controls the execution of TNF-induced cell death on multiple levels in keratinocytes. This signaling mechanism might have important implications for the development of new therapeutic strategies for the treatment of A20-associated skin diseases., Competing Interests: The authors declare no conflict of interest.

Background: The activation of the nuclear factor-κB (NF-κB) signaling pathway gives rise to inflammation in the pathogenesis of lupus nephritis (LN), with A20 serving as a negative feedback regulator and ubiquitin C‑terminal hydrolase L1 (UCH-L1) acting as a downstream target protein. However, their roles in the mechanism of LN remain undetermined., Methods: In the present study, the expression of A20 and UCH-L1, the activity of NF-κB and ubiquitin-proteasome system (UPS) were measured in MRL/lpr mice and A20 gene silenced podocytes. The severity of podocyte injury and immune complex deposits were detected by transmission electron microscopy., Results: The in vivo experiments revealed that A20 failed to terminate the activation of NF-κB, which was accompanied by UCH-L1 overexpression, ubiquitin accumulation, and glomerular injury in LN mice. Immunosuppression therapy did improve LN progression by attenuating A20 deficiency. In vitro experiments confirmed that tumor necrosis factor-α induced NF-κB activation, which led to UCH-L1 overexpression, UPS impairment, the upregulation of desmin and the downregulation of synaptopodin in A20 gene silenced podocytes., Conclusion: Thus, the results of the present study suggest that A20 regulates UCH-L1 expression via the NF-κB signaling pathway and A20 deficiency might play an important role in LN pathogenesis. Therefore, the A20 protein may serve as a promising therapeutic target for LN.

Rationale: The ubiquitin-editing protein A20 in dendritic cells (DCs) suppresses NF-κB (nuclear factor-κB) signaling and constrains DC-mediated T-cell stimulation, but the role of A20 in modulating the hypertensive response requires elucidation., Objective: Here, we tested the hypothesis that A20 in CD11c-expressing myeloid cells mitigates Ang II (angiotensin II)-induced hypertension by limiting renal T-cell activation., Methods and Results: Mice with heterozygous deletion of A20 in CD11c-expressing myeloid cells (DC ACT[ Cd11c-Cre + A20 flox/wt ]) have spontaneous DC activation but have normal baseline blood pressures. In response to low-dose chronic Ang II infusion, DC ACT mice compared with WT (wild type) controls had an exaggerated hypertensive response and augmented proportions of CD62L lo CD44 hi effector memory T lymphocytes in the kidney lymph node. After 10 days of Ang II, DC ACT kidneys had increased numbers of memory effector CD8 + , but not CD4 + T cells, compared with WTs. Moreover, the expressions of TNF-α (tumor necrosis factor-α) and IFN-γ (interferon-γ) were upregulated in the DC ACT renal CD8 + T cells but not CD4 + T cells. Saline challenge testing revealed enhanced renal fluid retention in the DC ACT mice. DC ACT kidneys showed augmented protein expression of γ-epithelial sodium channel and NHE3 (sodium-hydrogen antiporter 3). DC ACT mice also had greater reductions in renal blood flow following acute injections with Ang II and enhanced oxidant stress in the vasculature as evidenced by higher circulating levels of malondialdehyde compared with WT controls. To directly test whether enhanced T-cell activation in the DC ACT cohort was responsible for their exaggerated hypertensive response, we chronically infused Ang II into lymphocyte-deficient DC ACT Rag1 (recombination activating protein 1)-deficient ( Rag1 -/- ) mice and WT ( Cd11c - Cre - A20 flox/wt ) Rag1 -/- controls. The difference in blood pressure elevation accruing from DC activation was abrogated on the Rag1 -/- strain., Conclusions: Following stimulation of the renin-angiotensin system, A20 suppresses DC activation and thereby mitigates T-cell-dependent blood pressure elevation.

The ubiquitin-editing enzyme A20 is a well-known regulator of immune cell function and homeostasis. In addition, A20 protects cells from death in an ill-defined manner. While most studies focus on its role in the TNF-receptor complex, we here identify a novel component in the A20-mediated decision between life and death. Loss of A20 in NK cells led to spontaneous NK cell death and severe NK cell lymphopenia. The few remaining NK cells showed an immature, hyperactivated phenotype, hallmarked by the basal release of cytokines and cytotoxic molecules. NK-A20 -/- cells were hypersensitive to TNF-induced cell death and could be rescued, at least partially, by a combined deficiency with TNF. Unexpectedly, rapamycin, a well-established inhibitor of mTOR, also strongly protected NK-A20 -/- cells from death, and further studies revealed that A20 restricts mTOR activation in NK cells. This study therefore maps A20 as a crucial regulator of mTOR signaling and underscores the need for a tightly balanced mTOR pathway in NK cell homeostasis., (© 2019 Vetters et al.)

Inflammation and inflammatory cytokines have been shown to exert both positive and negative effects on hematopoietic stem cells (HSCs) and hematopoiesis. While the significance of inflammation driven hematopoiesis has begun to unfold, molecular players that regulate this phenomenon remain largely unknown. In the present study, we identified A20 as a critical regulator of inflammation controlled hematopoietic cell fate decisions of HSCs. A20 deficiency in HSCs leads to increased differentiation of myeloid cells and myeloproliferation. Analysis of erythroid lineage cells of A20 deficient mice indicated a striking reduction of erythrocytes in the bone marrow (BM), but elevated numbers in the spleen. Loss of A20 in HSCs causes a severe blockade of B cell differentiation in the BM and absence of peripheral B cells in the spleen, liver and blood. T cell differentiation studies revealed a reduction of both T cell progenitors and differentiated T cells in the thymus and altered T cell numbers in the spleens of A20 mutant mice. Analysis of lineage committed progenitors of the myeloid, erythroid and lymphoid lineages specified an altered composition in the A20 deficient BM. Genetic studies identified that specific loss of A20 in the myeloid lineage cells results in myeloproliferation. Bone marrow transplantation studies and mixed bone marrow chimera studies suggested an involvement of inflammatory cytokines, particularly interferon (IFN)- γ, in the onset of myeloproliferation and lymphopenia of A20 deficient mice. Finally, ablation of IFNγ signals in A20 deficient mice rescued the hematopoietic defects. In essence, these studies highlight a previously unknown role for A20 in the restriction of inflammation driven pathologic hematopoiesis. We believe that our studies based on A20 mutant mice will be helpful in understanding the pathophysiology and in the treatment of patients with A20 (TNFAIP3) mutations.

Inflammatory signals have been shown to play a critical role in controlling the maintenance and functions of hematopoietic stem cells (HSCs). While the significance of inflammation in hematopoiesis has begun to unfold, molecular mechanisms and players that govern this mode of HSC regulation remain largely unknown. The E3 ubiquitin ligase A20 has been considered as a central gatekeeper of inflammation. Here, we have specifically depleted A20 in multi-potent progenitors (MPPs) and studied its impact on hematopoiesis. Our data suggest that lack of A20 in Flt3 + progenitors causes modest alterations in hematopoietic differentiation. Analysis of hematopoietic stem and progenitor cell (HSPC) pool revealed alterations in HSPC subsets including, HSCs, MPP1, MPP2, MPP3 and MPP4. Interestingly, A20 deficiency in MPPs caused loss of HSC quiescence and compromised long-term hematopoietic reconstitution. Mechanistic studies identified that A20 deficiency caused elevated levels of Interferon-γ signaling and downregulation of p57 in HSCs. In essence, these studies identified A20 as a key regulator of HSC quiescence and cell fate decisions., (Published by Elsevier B.V.)

Background: Asthma is a heterogeneous disease of the airways that involves several types of granulocytic inflammation. Recently, we have shown that the activation status of myeloid cells regulated by TNFAIP3/A20 is a crucial determinant of eosinophilic or neutrophilic airway inflammation. However, whether neutrophilic inflammation observed in this model is dependent on IL-17 remains unknown., Objective: In this study, we investigated whether IL-17RA-signalling is essential for eosinophilic or neutrophilic inflammation in house dust mite (HDM)-driven airway inflammation., Methods: Tnfaip3 fl/fl xLyz2 +/cre (Tnfaip3 LysM-KO ) mice were crossed to Il17ra KO mice, generating Tnfaip3 LysM Il17ra KO mice and subjected to an HDM-driven airway inflammation model., Results: Both eosinophilic and neutrophilic inflammation observed in HDM-exposed WT and Tnfaip3 LysM-KO mice respectively were unaltered in the absence of IL-17RA. Production of IL-5, IL-13 and IFN-γ by CD4 + T cells was similar between WT, Tnfaip3 LysM-KO and Il17ra KO mice, whereas mucus-producing cells in Tnfaip3 LysM-KO Il17ra KO mice were reduced compared to controls. Strikingly, spontaneous accumulation of pulmonary Th1, Th17 and γδ-17 T cells was observed in Tnfaip3 LysM-KO Il17ra KO mice, but not in the other genotypes. Th17 cell-associated cytokines such as GM-CSF and IL-22 were increased in the lungs of HDM-exposed Tnfaip3 LysM-KO Il17ra KO mice, compared to IL-17RA-sufficient controls. Moreover, neutrophilic chemo-attractants CXCL1, CXCL2, CXCL12 and Th17-promoting cytokines IL-1β and IL-6 were unaltered between Tnfaip3 LysM-KO and Tnfaip3 LysM-KO Il17ra KO mice., Conclusion and Clinical Relevance: These findings show that neutrophilic airway inflammation induced by activated TNFAIP3/A20-deficient myeloid cells can develop in the absence of IL-17RA-signalling. Neutrophilic inflammation is likely maintained by similar quantities of pro-inflammatory cytokines IL-1β and IL-6 that can, independently of IL-17-signalling, induce the expression of neutrophil chemo-attractants., (© 2018 The Authors. Clinical & Experimental Allergy Published by John Wiley & Sons Ltd.)

MYD88 mutations are one of the most recurrent mutations in hematologic malignancies. However, recent mouse models suggest that MYD88 L265P alone may not be sufficient to induce tumor formation. Interplay between MYD88 L265P and other genetic events is further supported by the fact that TNFAIP3 (A20) inactivation often accompanies MYD88 L265P . However, we are still lacking information about the consequence of MYD88 L265P in combination with TNFAIP3 loss in human B cell lymphoma. Review of our genetic data on diffuse large B cell lymphoma (DLBCL) and Waldenstrom macroglobulinemia (WM), found that a large percentage of DLBCL and WM cases that have a MYD88 mutation also harbor a TNFAIP3 loss, 55% DLBCL and 28% of WM, respectively. To mimic this combination of genetic events, we used genomic editing technology to knock out TNFAIP3 in MYD88 L265P non-Hodgkin's lymphoma (NHL) cell lines. Loss of A20 expression resulted in increased NF-κB and p38 activity leading to upregulation of the NF-κB target genes BCL2 and MYC. Furthermore, we detected the increased production of IL-6 and CXCL10 which led to an upregulation of the JAK/STAT pathway. Overall, these results suggest that MYD88 L265P signaling can be enhanced by a second genetic alteration in TNFAIP3 and highlights a potential opportunity for therapeutic targeting.

The autoimmune disease primary Sjögren's syndrome (pSS) is characterized by hypofunction of the salivary glands (SGs), the cause of which is not correlated to lymphocytic SG infiltration, as prevailing dogma often states. We knocked out the NF-κB proinflammatory pathway inhibitor A20 in keratin14+ epithelial cells, to investigate if immune activated epithelial cells are capable of initiating pSS SG hallmarks. We show that immune activated epithelial cells can cause T cell dominated leukocytic infiltration and immune foci development of the SGs, reflecting the early clinical picture. Infiltrating leukocytes invaded striated ducts, similar to early stage lymphoepithelial lesions observed clinically. Expression of proinflammatory cyto-/chemokines IFNɣ, TNFα, IL-6, CXCL10 and CXCL13 increased in A20-/- SGs, and functionally both volume and mucin 10 content of whole stimulated saliva from A20-/- mice was significantly reduced. Epithelial cells may therefore represent the initial trigger for pSS SG pathologies, as opposed to simple reactionaries to pre-existing stimuli., Competing Interests: The authors have declared that no competing interests exist.

A20 is a negative regulator of NF-κB signaling; it controls inflammatory responses and ensures tissue homeostasis. A20 is thought to restrict NF-κB activation both by its ubiquitin-editing activity as well as by its nonenzymatic activities. Besides its role in NF-κB signaling, A20 also acts as a protective factor inhibiting apoptosis and necroptosis. Because of the ability of A20 to both ubiquitinate and deubiquitinate substrates, and its involvement in many cellular processes, we hypothesized that deletion of A20 might generally impact on protein levels, thereby disrupting cellular signaling. We performed a differential proteomics study on bone marrow-derived macrophages (BMDMs) from control and myeloid-specific A20 knockout mice, both in untreated conditions and after LPS or TNF treatment, and demonstrated A20-dependent changes in protein expression. Several inflammatory proteins were found up-regulated in the absence of A20, even without an inflammatory stimulus, but, depending on the treatment and the treatment time, more proteins were found regulated. Together these protein changes may affect normal signaling events, which may disturb tissue homeostasis and induce (autoimmune) inflammation, in agreement with A20s proposed identity as a susceptibility gene for inflammatory disease. We further verify that immune-responsive gene 1 (IRG1) is up-regulated in the absence of A20 and that its levels are transcriptionally regulated.

Background: Acute T-lymphocyte leukaemia is a form of haematological malignancy with abnormal activation of NF-κB pathway, which results in high expression of A20 and ABIN1, which constitute a negative feedback mechanism for the regulation of NF-κB activation. Clinical studies have found that acute T-lymphocyte leukaemia patients are susceptible to Toxoplasma gondii infection; however, the effect of T. gondii on the proliferation and apoptosis of human leukaemia T-cells remains unclear. Here, we used the T. gondii ME-49 strain to infect human leukaemia T-cell lines Jurkat and Molt-4, to explore the effect of T. gondii on proliferation and apoptosis, which is mediated by NF-κB in human leukaemia T-cells., Methods: The Tunel assay was used to detect cell apoptosis. Cell Counting Kit-8 was used to detect cell proliferation viability. The apoptosis level and the expression level of NF-κB related proteins in human leukaemia T-cells were detected by flow cytometry and Western blotting., Results: Western blotting analyses revealed that the T. gondii ME-49 strain increased the expression of A20 and decreased both ABIN1 expression and NF-κB p65 phosphorylation. By constructing a lentiviral-mediated shRNA to knockdown the A20 gene in Jurkat T-cells and Molt-4 T-cells, the apoptosis levels of the two cell lines decreased after T. gondii ME-49 infection, and levels of NF-κB p65 phosphorylation and ABIN1 were higher than in the non-konckdown group. After knockingdown ABIN1 gene expression by constructing the lentiviral-mediated shRNA and transfecting the recombinant expression plasmid containing the ABIN1 gene into two cell lines, apoptosis levels and cleaved caspase-8 expression increased or decreased in response to T. gondii ME-49 infection, respectively., Conclusions: Our data suggest that ABIN1 protects human leukaemia T-cells by allowing them to resist the apoptosis induced by T. gondii ME-49 and that the T. gondii ME-49 strain induces the apoptosis of human leukaemia T-cells via A20-mediated downregulation of ABIN1 expression.

A20 is a ubiquitin-editing enzyme that is known to regulate inflammatory signaling and cell death. However, A20 mutations are also frequently found in multiple malignancies suggesting a potential role as a tumor suppressor as well. We recently described a novel role for A20 in regulating the wnt-beta-catenin signaling pathway and suppressing colonic tumor development in mice. The underlying mechanisms for this phenomenon are unclear. To study this, we first generated A20 knockout cell lines by genome-editing techniques. Using these cells, we show that loss of A20 causes dysregulation of wnt-dependent gene expression by RNAseq. Mechanistically, A20 interacts with a proximal signaling component of the wnt-signaling pathway, receptor interacting protein kinase 4 (RIPK4), and regulation of wnt-signaling by A20 occurs through RIPK4. Finally, similar to the mechanism by which A20 regulates other members of the receptor interacting protein kinase family, A20 modifies ubiquitin chains on RIPK4 suggesting a possible molecular mechanism for A20's control over the wnt-signaling pathway.

Maintaining immune tolerance requires the production of Foxp3-expressing regulatory T (T reg ) cells in the thymus. Activation of NF-κB transcription factors is critically required for T reg cell development, partly via initiating Foxp3 expression. NF-κB activation is controlled by a negative feedback regulation through the ubiquitin editing enzyme A20, which reduces proinflammatory signaling in myeloid cells and B cells. In naive CD4 + T cells, A20 prevents kinase RIPK3-dependent necroptosis. Using mice deficient for A20 in T lineage cells, we show that thymic and peripheral T reg cell compartments are quantitatively enlarged because of a cell-intrinsic developmental advantage of A20-deficient thymic T reg differentiation. A20-deficient thymic T reg cells exhibit reduced dependence on IL-2 but unchanged rates of proliferation and apoptosis. Activation of the NF-κB transcription factor RelA was enhanced, whereas nuclear translocation of c-Rel was decreased in A20-deficient thymic T reg cells. Furthermore, we found that the increase in T reg cells in T cell-specific A20-deficient mice was already observed in CD4 + single-positive CD25 + GITR + Foxp3 - thymic T reg cell progenitors. T reg cell precursors expressed high levels of the tumor necrosis factor receptor superfamily molecule GITR, whose stimulation is closely linked to thymic T reg cell development. A20-deficient T reg cells efficiently suppressed effector T cell-mediated graft-versus-host disease after allogeneic hematopoietic stem cell transplantation, suggesting normal suppressive function. Holding thymic production of natural T reg cells in check, A20 thus integrates T reg cell activity and increased effector T cell survival into an efficient CD4 + T cell response., (Copyright © 2017 by The American Association of Immunologists, Inc.)

Necroptosis is a caspase-independent form of cell death that is triggered by activation of the receptor interacting serine/threonine kinase 3 (RIPK3) and phosphorylation of its pseudokinase substrate mixed lineage kinase-like (MLKL), which then translocates to membranes and promotes cell lysis. Activation of RIPK3 is regulated by the kinase RIPK1. Here we analyze the contribution of RIPK1, RIPK3, or MLKL to several mouse disease models. Loss of RIPK3 had no effect on lipopolysaccharide-induced sepsis, dextran sodium sulfate-induced colitis, cerulein-induced pancreatitis, hypoxia-induced cerebral edema, or the major cerebral artery occlusion stroke model. However, kidney ischemia-reperfusion injury, myocardial infarction, and systemic inflammation associated with A20 deficiency or high-dose tumor necrosis factor (TNF) were ameliorated by RIPK3 deficiency. Catalytically inactive RIPK1 was also beneficial in the kidney ischemia-reperfusion injury model, the high-dose TNF model, and in A20(-/-) mice. Interestingly, MLKL deficiency offered less protection in the kidney ischemia-reperfusion injury model and no benefit in A20(-/-) mice, consistent with necroptosis-independent functions for RIPK1 and RIPK3. Combined loss of RIPK3 (or MLKL) and caspase-8 largely prevented the cytokine storm, hypothermia, and morbidity induced by TNF, suggesting that the triggering event in this model is a combination of apoptosis and necroptosis. Tissue-specific RIPK3 deletion identified intestinal epithelial cells as the major target organ. Together these data emphasize that MLKL deficiency rather than RIPK1 inactivation or RIPK3 deficiency must be examined to implicate a role for necroptosis in disease., Competing Interests: All authors were employees of Genentech.

An important regulator of inflammatory signalling is the ubiquitin-editing protein A20 that acts as a break on nuclear factor-κB (NF-κB) activation, but also exerts important cytoprotective functions. A20 knockout mice are cachectic and die prematurely due to excessive multi-organ inflammation. To establish the importance of A20 in liver homeostasis and pathology, we developed a novel mouse line lacking A20 specifically in liver parenchymal cells. These mice spontaneously develop chronic liver inflammation but no fibrosis or hepatocellular carcinomas, illustrating an important role for A20 in normal liver tissue homeostasis. Hepatocyte-specific A20 knockout mice show sustained NF-κB-dependent gene expression in the liver upon tumor necrosis factor (TNF) or lipopolysaccharide injection, as well as hepatocyte apoptosis and lethality upon challenge with sublethal doses of TNF, demonstrating an essential role for A20 in the protection of mice against acute liver failure. Finally, chronic liver inflammation and enhanced hepatocyte apoptosis in hepatocyte-specific A20 knockout mice was associated with increased susceptibility to chemically or high fat-diet-induced hepatocellular carcinoma development. Together, these studies establish A20 as a crucial hepatoprotective factor.