Two distinct ubiquitin-binding motifs in A20 mediate its anti-inflammatory and cell-protective activities.

Protein ubiquitination regulates protein stability and modulates the composition of signaling complexes. A20 is a negative regulator of inflammatory signaling, but the molecular mechanisms involved are ill understood. Here, we generated Tnfaip3 gene-targeted A20 mutant mice bearing inactivating mutations in the zinc finger 7 (ZnF7) and ZnF4 ubiquitin-binding domains, revealing that binding to polyubiquitin is essential for A20 to suppress inflammatory disease. We demonstrate that a functional ZnF7 domain was required for recruiting A20 to the tumor necrosis factor receptor 1 (TNFR1) signaling complex and to suppress inflammatory signaling and cell death. The combined inactivation of ZnF4 and ZnF7 phenocopied the postnatal lethality and severe multiorgan inflammation of A20-deficient mice. Conditional tissue-specific expression of mutant A20 further revealed the key role of ubiquitin-binding in myeloid and intestinal epithelial cells. Collectively, these results demonstrate that the anti-inflammatory and cytoprotective functions of A20 are largely dependent on its ubiquitin-binding properties.

Limiting inflammation-the negative regulation of NF-κB and the NLRP3 inflammasome.

A properly mounted immune response is indispensable for recognizing and eliminating danger arising from foreign invaders and tissue trauma. However, the 'inflammatory fire' kindled by the host response must be tightly controlled to prevent it from spreading and causing irreparable damage. Accordingly, acute inflammation is self-limiting and is normally attenuated after elimination of noxious stimuli, restoration of homeostasis and initiation of tissue repair. However, unresolved inflammation may lead to the development of chronic autoimmune and degenerative diseases and cancer. Here, we discuss the key molecular mechanisms that contribute to the self-limiting nature of inflammatory signaling, with emphasis on the negative regulation of the NF-κB pathway and the NLRP3 inflammasome. Understanding these negative regulatory mechanisms should facilitate the development of much-needed therapeutic strategies for treatment of inflammatory and autoimmune pathologies.

Perinatal Activation of the Interleukin-33 Pathway Promotes Type 2 Immunity in the Developing Lung.

Allergic disease originates in early life and polymorphisms in interleukin-33 gene (IL33) and IL1RL1, coding for IL-33R and decoy receptor sST2, confer allergy risk. Early life T helper 2 (Th2) cell skewing and allergy susceptibility are often seen as remnants of feto-maternal symbiosis. Here we report that shortly after birth, innate lymphoid type 2 cells (ILC2s), eosinophils, basophils, and mast cells spontaneously accumulated in developing lungs in an IL-33-dependent manner. During the phase of postnatal lung alveolarization, house dust mite exposure further increased IL-33, which boosted cytokine production in ILC2s and activated CD11b+ dendritic cells (DCs). IL-33 suppressed IL-12p35 and induced OX40L in neonatal DCs, thus promoting Th2 cell skewing. Decoy sST2 had a strong preventive effect on asthma in the neonatal period, less so in adulthood. Thus, enhanced neonatal Th2 cell skewing to inhaled allergens results from postnatal hyperactivity of the IL-33 axis during a period of maximal lung remodeling.

Yolk Sac Macrophages, Fetal Liver, and Adult Monocytes Can Colonize an Empty Niche and Develop into Functional Tissue-Resident Macrophages.

Tissue-resident macrophages can derive from yolk sac macrophages (YS-Macs), fetal liver monocytes (FL-MOs), or adult bone-marrow monocytes (BM-MOs). The relative capacity of these precursors to colonize a niche, self-maintain, and perform tissue-specific functions is unknown. We simultaneously transferred traceable YS-Macs, FL-MOs, and BM-MOs into the empty alveolar macrophage (AM) niche of neonatal Csf2rb(-/-) mice. All subsets produced AMs, but in competition preferential outgrowth of FL-MOs was observed, correlating with their superior granulocyte macrophage-colony stimulating factor (GM-CSF) reactivity and proliferation capacity. When transferred separately, however, all precursors efficiently colonized the alveolar niche and generated AMs that were transcriptionally almost identical, self-maintained, and durably prevented alveolar proteinosis. Mature liver, peritoneal, or colon macrophages could not efficiently colonize the empty AM niche, whereas mature AMs could. Thus, precursor origin does not affect the development of functional self-maintaining tissue-resident macrophages and the plasticity of the mononuclear phagocyte system is largest at the precursor stage.

CARD14 SIGNALOSOME FORMATION IS ASSOCIATED WITH ITS ENDOSOMAL RELOCATION AND mTORC1-INDUCED KERATINOCYTE PROLIFERATION.

Rare mutations in CARD14 promote psoriasis by inducing CARD14-BCL10-MALT1 complexes that activate NF-kB and MAP kinases. Here, the downstream signalling mechanism of the highly penetrant CARD14E138Aalteration is described. In addition to BCL10 and MALT1, CARD14E138A associated with several proteins important in innate immune signalling. Interactions with M1-specific ubiquitin E3 ligase HOIP, and K63-specific ubiquitin E3 ligase TRAF6 promoted BCL10 ubiquitination and were essential for NF-kB and MAP kinase activation. In contrast, the ubiquitin binding proteins A20 and ABIN1, both genetically associated with psoriasis development, negatively regulated signalling by inducing CARD14E138A turnover. CARD14E138A localized to early endosomes and was associated with the AP2 adaptor complex. AP2 function was required for CARD14E138A activation of mTOR complex 1, which stimulated keratinocyte metabolism, but not for NF-kB nor MAP kinase activation. Furthermore, rapamycin ameliorated CARD14E138A-induced keratinocyte proliferation and epidermal acanthosis in mice, suggesting that blocking mTORC1 may be therapeutically beneficial in CARD14-dependent psoriasis.

The p110δ isoform of the kinase PI(3)K controls the subcellular compartmentalization of TLR4 signaling and protects from endotoxic shock.

Lipopolysaccharide activates plasma-membrane signaling and endosomal signaling by Toll-like receptor 4 (TLR4) through the TIRAP-MyD88 and TRAM-TRIF adaptor complexes, respectively, but it is unclear how the signaling switch between these cell compartments is coordinated. In dendritic cells, we found that the p110δ isoform of phosphatidylinositol-3-OH kinase (PI(3)K) induced internalization of TLR4 and dissociation of TIRAP from the plasma membrane, followed by calpain-mediated degradation of TIRAP. Accordingly, inactivation of p110δ prolonged TIRAP-mediated signaling from the plasma membrane, which augmented proinflammatory cytokine production while decreasing TRAM-dependent endosomal signaling that generated anti-inflammatory cytokines (interleukin 10 and interferon-ß). In line with that altered signaling output, p110δ-deficient mice showed enhanced endotoxin-induced death. Thus, by controlling the 'topology' of TLR4 signaling complexes, p110δ balances overall homeostasis in the TLR4 pathway.

Proteolytic Processing of Interleukin-1 Family Cytokines: Variations on a Common Theme.

Members of the extended interleukin-1 (IL-1) cytokine family, such as IL-1, IL-18, IL-33, and IL-36, play a pivotal role in the initiation and amplification of immune responses. However, deregulated production and/or activation of these cytokines can lead to the development of multiple inflammatory disorders. IL-1 family members share a broadly similar domain organization and receptor signaling pathways. Another striking similarity between IL-1 family members is the requirement for proteolytic processing in order to unlock their full biological potential. Although much emphasis has been put on the role of caspase-1, another emerging theme is the involvement of neutrophil- and mast cell-derived proteases in IL-1 family cytokine processing. Elucidating the regulation of IL-1 family members by proteolytic processing is of great interest for understanding inflammation and immunity. Here, we review the identity of the proteases involved in the proteolytic processing of IL-1 family cytokines and the therapeutic implications in inflammatory disease.

Interleukin-21-Producing CD4(+) T Cells Promote Type 2 Immunity to House Dust Mites.

Asthma is a T helper 2 (Th2)-cell-mediated disease; however, recent findings implicate Th17 and innate lymphoid cells also in regulating airway inflammation. Herein, we have demonstrated profound interleukin-21 (IL-21) production after house dust mite (HDM)-driven asthma by using T cell receptor (TCR) transgenic mice reactive to Dermatophagoides pteronyssinus 1 and an IL-21GFP reporter mouse. IL-21-producing cells in the mediastinal lymph node (mLN) bore characteristics of T follicular helper (Tfh) cells, whereas IL-21(+) cells in the lung did not express CXCR5 (a chemokine receptor expressed by Tfh cells) and were distinct from effector Th2 or Th17 cells. Il21r(-/-) mice developed reduced type 2 responses and the IL-21 receptor (IL-21R) enhanced Th2 cell function in a cell-intrinsic manner. Finally, administration of recombinant IL-21 and IL-25 synergistically promoted airway eosinophilia primarily via effects on CD4(+) lymphocytes. This highlights an important Th2-cell-amplifying function of IL-21-producing CD4(+) T cells in allergic airway inflammation.

Toll-like Receptor 4 Engagement on Dendritic Cells Restrains Phago-Lysosome Fusion and Promotes Cross-Presentation of Antigens.

The initiation of cytotoxic immune responses by dendritic cells (DCs) requires the presentation of antigenic peptides derived from phagocytosed microbes and infected or dead cells to CD8(+) T cells, a process called cross-presentation. Antigen cross-presentation by non-activated DCs, however, is not sufficient for the effective induction of immune responses. Additionally, DCs need to be activated through innate receptors, like Toll-like receptors (TLRs). During DC maturation, cross-presentation efficiency is first upregulated and then turned off. Here we show that during this transient phase of enhanced cross-presentation, phago-lysosome fusion was blocked by the topological re-organization of lysosomes into perinuclear clusters. LPS-induced lysosomal clustering, inhibition of phago-lysosome fusion and enhanced cross-presentation, all required expression of the GTPase Rab34. We conclude that TLR4 engagement induces a Rab34-dependent re-organization of lysosomal distribution that delays antigen degradation to transiently enhance cross-presentation, thereby optimizing the priming of CD8(+) T cell responses against pathogens.

Reprogramming of glucocorticoid receptor function by hypoxia.

Here, we investigate the impact of hypoxia on the hepatic response of glucocorticoid receptor (GR) to dexamethasone (DEX) in mice via RNA-sequencing. Hypoxia causes three types of reprogramming of GR: (i) much weaker induction of classical GR-responsive genes by DEX in hypoxia, (ii) a number of genes is induced by DEX specifically in hypoxia, and (iii) hypoxia induces a group of genes via activation of the hypothalamic-pituitary-adrenal (HPA) axis. Transcriptional profiles are reflected by changed GR DNA-binding as measured by ChIP sequencing. The HPA axis is induced by hypothalamic HIF1α and HIF2α activation and leads to GR-dependent lipolysis and ketogenesis. Acute inflammation, induced by lipopolysaccharide, is prevented by DEX in normoxia but not during hypoxia, and this is attributed to HPA axis activation by hypoxia. We unfold new physiological pathways that have consequences for patients suffering from GC resistance.

A20 is a master switch of IL-33 signaling in macrophages and determines IL-33-induced lung immunity.

BACKGROUND: IL-33 plays a major role in the pathogenesis of allergic diseases such as asthma and atopic dermatitis. On its release from lung epithelial cells, IL-33 primarily drives type 2 immune responses, accompanied by eosinophilia and robust production of IL-4, IL-5, and IL-13. However, several studies show that IL-33 can also drive a type 1 immune response. OBJECTIVE: We sought to determine the role of A20 in the regulation of IL-33 signaling in macrophages and IL-33-induced lung immunity. METHODS: We studied the immunologic response in lungs of IL-33-treated mice that specifically lack A20 in myeloid cells. We also analyzed IL-33 signaling in A20-deficient bone marrow-derived macrophages. RESULTS: IL-33-induced lung innate lymphoid cell type 2 expansion, type 2 cytokine production, and eosinophilia were drastically reduced in the absence of macrophage A20 expression, whereas neutrophils and interstitial macrophages in lungs were increased. In vitro, IL-33-mediated nuclear factor kappa B activation was only weakly affected in A20-deficient macrophages. However, in the absence of A20, IL-33 gained the ability to activate signal transducer and activator of transcription 1 (STAT1) signaling and STAT1-dependent gene expression. Surprisingly, A20-deficient macrophages produced IFN-γ in response to IL-33, which was fully STAT1-dependent. Furthermore, STAT1 deficiency partially restored the ability of IL-33 to induce ILC2 expansion and eosinophilia in myeloid cell-specific A20 knockout mice. CONCLUSIONS: We reveal a novel role for A20 as a negative regulator of IL-33-induced STAT1 signaling and IFN-γ production in macrophages, which determines lung immune responses.

Proteolytic Activity of the Paracaspase MALT1 Is Involved in Epithelial Restitution and Mucosal Healing.

The paracaspase MALT1 is a crucial regulator of immune responses in various cellular contexts. Recently, there is increasing evidence suggesting that MALT1 might represent a novel key player in mucosal inflammation. However, the molecular mechanisms underlying this process and the targeted cell population remain unclear. In this study, we investigate the role of MALT1 proteolytic activity in the context of mucosal inflammation. We demonstrate a significant enrichment of MALT1 gene and protein expression in colonic epithelial cells of UC patients, as well as in the context of experimental colitis. Mechanistically we demonstrate that MALT1 protease function inhibits ferroptosis, a form of iron-dependent cell death, upstream of NF-κB signaling, which can promote inflammation and tissue damage in IBD. We further show that MALT1 activity contributes to STAT3 signaling, which is essential for the regeneration of the intestinal epithelium after injury. In summary, our data strongly suggests that the protease function of MALT1 plays a critical role in the regulation of immune and inflammatory responses, as well as mucosal healing. Understanding the mechanisms by which MALT1 protease function regulates these processes may offer novel therapeutic targets for the treatment of IBD and other inflammatory diseases.

Exploring Species-Specificity in TLR4/MD-2 Inhibition with Amphiphilic Lipid A Mimicking Glycolipids.

The Toll-like receptor 4 (TLR4)/myeloid differentiation factor 2 (MD-2) complex is a key receptor of the innate immune system and a major driver of inflammation that is responsible for the multifaceted defense response to Gram-negative infections. However, dysfunction in the tightly regulated mechanisms of TLR4-mediated signaling leads to the uncontrolled upregulation of local and systemic inflammation, often resulting in acute or chronic disease. Therefore, the TLR4/MD-2 receptor complex is an attractive target for the design and development of anti-inflammatory therapies which aim to control the unrestrained activation of TLR4-mediated signaling. Complex structure-activity relationships and species-specificity behind ligand recognition by the TLR4/MD-2 complex complicate the development of MD-2-specific TLR4 antagonists. The restriction of the conformational flexibility of the disaccharide polar head group is one of the key structural features of the newly developed lipid A-mimicking glycophospholipids, which are potential inhibitors of TLR4-mediated inflammation. Since phosphorylation has a crucial influence on MD-2-ligand interaction, glycolipids with variable numbers and positioning of phosphate groups were synthesized and evaluated for their ability to inhibit TLR4-mediated pro-inflammatory signaling in human and murine immune cells. A bis-phosphorylated glycolipid was found to have nanomolar antagonist activity on human TLR4 while acting as a partial agonist on murine TLR4. The glycolipid inhibited mTLR4/MD-2-mediated cytokine release, acting as an antagonist in the presence of lipopolysaccharide (LPS), but at the same time induced low-level cytokine production.

Inflammatory cell-derived CXCL3 promotes pancreatic cancer metastasis through a novel myofibroblast-hijacked cancer escape mechanism.

OBJECTIVE: Pancreatic ductal adenocarcinoma (PDAC) is the most lethal malignancy and lacks effective treatment. We aimed to understand molecular mechanisms of the intertwined interactions between tumour stromal components in metastasis and to provide a new paradigm for PDAC therapy. DESIGN: Two unselected cohorts of 154 and 20 patients with PDAC were subjected to correlation between interleukin (IL)-33 and CXCL3 levels and survivals. Unbiased expression profiling, and genetic and pharmacological gain-of-function and loss-of-function approaches were employed to identify molecular signalling in tumour-associated macrophages (TAMs) and myofibroblastic cancer-associated fibroblasts (myoCAFs). The role of the IL-33-ST2-CXCL3-CXCR2 axis in PDAC metastasis was evaluated in three clinically relevant mouse PDAC models. RESULTS: IL-33 was specifically elevated in human PDACs and positively correlated with tumour inflammation in human patients with PDAC. CXCL3 was highly upregulated in IL-33-stimulated macrophages that were the primary source of CXCL3. CXCL3 was correlated with poor survival in human patients with PDAC. Mechanistically, activation of the IL-33-ST2-MYC pathway attributed to high CXCL3 production. The highest level of CXCL3 was found in PDAC relative to other cancer types and its receptor CXCR2 was almost exclusively expressed in CAFs. Activation of CXCR2 by CXCL3 induced a CAF-to-myoCAF transition and α-smooth muscle actin (α-SMA) was uniquely upregulated by the CXCL3-CXCR2 signalling. Type III collagen was identified as the CXCL3-CXCR2-targeted adhesive molecule responsible for myoCAF-driven PDAC metastasis. CONCLUSIONS: Our work provides novel mechanistic insights into understanding PDAC metastasis by the TAM-CAF interaction and targeting each of these signalling components would provide an attractive and new paradigm for treating pancreatic cancer.

Mutations in RNU7-1 Weaken Secondary RNA Structure, Induce MCP-1 and CXCL10 in CSF, and Result in Aicardi-Goutières Syndrome with Severe End-Organ Involvement.

BACKGROUND: Aicardi-Goutières syndrome (AGS) is a type I interferonopathy usually characterized by early-onset neurologic regression. Biallelic mutations in LSM11 and RNU7-1, components of the U7 small nuclear ribonucleoprotein (snRNP) complex, have been identified in a limited number of genetically unexplained AGS cases. Impairment of U7 snRNP function results in misprocessing of replication-dependent histone (RDH) pre-mRNA and disturbance of histone occupancy of nuclear DNA, ultimately driving cGAS-dependent type I interferon (IFN-I) release. OBJECTIVE: We performed a clinical, genetic, and immunological workup of 3 unrelated patients with uncharacterized AGS. METHODS: Whole exome sequencing (WES) and targeted Sanger sequencing of RNU7-1 were performed. Primary fibroblasts were used for mechanistic studies. IFN-I signature and STAT1/2 phosphorylation were assessed in peripheral blood. Cytokines were profiled on serum and cerebrospinal fluid (CSF). Histopathology was examined on brain and kidney tissue. RESULTS: Sequencing revealed compound heterozygous RNU7-1 mutations, resulting in impaired RDH pre-mRNA processing. The 3' stem-loop mutations reduced stability of the secondary U7 snRNA structure. A discrete IFN-I signature in peripheral blood was paralleled by MCP-1 (CCL2) and CXCL10 upregulation in CSF. Histopathological analysis of the kidney showed thrombotic microangiopathy. We observed dysregulated STAT phosphorylation upon cytokine stimulation. Clinical overview of all reported patients with RNU7-1-related disease revealed high mortality and high incidence of organ involvement compared to other AGS genotypes. CONCLUSIONS: Targeted RNU7-1 sequencing is recommended in genetically unexplained AGS cases. CSF cytokine profiling represents an additional diagnostic tool to identify aberrant IFN-I signaling. Clinical follow-up of RNU7-1-mutated patients should include screening for severe end-organ involvement including liver disease and nephropathy.

MALT1 targeting suppresses CARD14-induced psoriatic dermatitis in mice.

CARD14 gain-of-function mutations cause psoriasis in humans and mice. Together with BCL10 and the protease MALT1, mutant CARD14 forms a signaling node that mediates increased NF-κB signaling and proinflammatory gene expression in keratinocytes. However, it remains unclear whether psoriasis in response to CARD14 hyperactivation is keratinocyte-intrinsic or requires CARD14 signaling in other cells. Moreover, the in vivo effect of MALT1 targeting on mutant CARD14-induced psoriasis has not yet been documented. Here, we show that inducible keratinocyte-specific expression of CARD14E138A in mice rapidly induces epidermal thickening and inflammation as well as increased expression of several genes associated with psoriasis in humans. Keratinocyte-specific MALT1 deletion as well as oral treatment of mice with a specific MALT1 protease inhibitor strongly reduces psoriatic skin disease in CARD14E138A mice. Together, these data illustrate a keratinocyte-intrinsic causal role of enhanced CARD14/MALT1 signaling in the pathogenesis of psoriasis and show the potential of MALT1 inhibition for the treatment of psoriasis.

Immune responses and therapeutic options in psoriasis.

Psoriasis is a chronic inflammatory disease of the skin that affects about 2-3% of the population and greatly impairs the quality of life of affected individuals. Psoriatic skin is characterized by excessive proliferation and aberrant differentiation of keratinocytes, as well as redness caused by increased dilation of the dermal blood vessels and infiltration of immune cells. Although the pathogenesis of psoriasis has not yet been completely elucidated, it is generally believed to arise from a complex interplay between hyperproliferating keratinocytes and infiltrating, activated immune cells. So far, the exact triggers that elicit this disease are still enigmatic, yet, it is clear that genetic predisposition significantly contributes to the development of psoriasis. In this review, we summarize current knowledge of important cellular and molecular mechanisms driving the initiation and amplification stages of psoriasis development, with a particular focus on cytokines and emerging evidence illustrating keratinocyte-intrinsic defects as key drivers of inflammation. We also discuss mouse models that have contributed to a better understanding of psoriasis pathogenesis and the preclinical development of novel therapeutics, including monoclonal antibodies against specific cytokines or cytokine receptors that have revolutionized the treatment of psoriasis. Future perspectives that may have the potential to push basic research and open up new avenues for therapeutic intervention are provided.