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Mucosa-associated lymphoid tissue lymphoma-translocation protein 1 (MALT1) is an attractive target for the development of modulatory compounds in the treatment of lymphoma and other cancers. While the three-dimensional structure of MALT1 has been previously determined through X-ray analysis, its dynamic behaviour in solution has remained unexplored. We present here dynamic analyses of the apo MALT1 form along with the E549A mutation. This investigation used NMR 15 N relaxation and NOE measurements between side-chain methyl groups. Our findings confirm that MALT1 exists as a monomer in solution, and demonstrate that the domains display semi-independent movements in relation to each other. Our dynamic study, covering multiple time scales, along with the assessment of conformational populations by Molecular Dynamic simulations, Alpha Fold modelling and PCA analysis, put the side chain of residue W580 in an inward position, shedding light at potential mechanisms underlying the allosteric regulation of this enzyme., (© 2024. The Author(s).)

The CARMA-BCL10-MALT1 (CBM) signalosome functions as a pivotal supramolecular module, integrating diverse receptor-induced signaling pathways to regulate BCL10-dependent NF-kB activation in innate and adaptive immunity. Conversely, the API2-MALT1 fusion protein in t(11; 18)(q21; q21) MALT lymphoma constitutively induces BCL10-independent NF-kB activation. MALT1 dimer formation is indispensable for the requisite proteolytic activity and is critical for NF-kB activation regulation in both scenarios. However, the molecular assembly of MALT1 individual domains in CBM activation remains elusive. Here we report the crystal structure of the MALT1 death domain (DD) at a resolution of 2.1 Å, incorporating reconstructed residues in previously disordered loops 1 and 2. Additionally, we observe a conformational regulation element (CRE) regulating stem-helix formation in NLRPs pyrin (PYD) within the MALT1 DD structure. The structure reveals a stem-helix-mediated dimer further corroborated in solution. To elucidate how the BCL10 filament facilitates MALT1 dimerization, we reconstitute a BCL10-CARD-MALT1-DD-IG1-IG2 complex model. We propose a N+7 rule for BCL10-dependent MALT1 dimerization via the IG1-IG2 domain and for MALT1-dependent cleavage in trans. Biochemical data further indicates concentration-dependent dimerization of the MALT1 IG1-IG2 domain, facilitating MALT1 dimerization in BCL10-independent manner. Our findings provide a structural and biochemical foundation for understanding MALT1 dimeric mechanisms, shedding light on potential BCL10-independent MALT1 dimer formation and high-order BCL10-MALT1 assembly., Competing Interests: Declaration of competing interest ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Inc. All rights reserved.)

MALT1 has been implicated as an upstream regulator of NF-κB signaling in immune cells and tumors. This study determined the regulatory mechanisms and biological functions of MALT1 in non-small cell lung cancer (NSCLC). In cell culture and orthotopic xenograft models, MALT1 suppression via gene expression interference or protein activity inhibition significantly impaired malignant phenotypes and enhanced radiation sensitivity of NSCLC cells. CSN5, the core subunit of COP9 signalosome, was firstly verified to stabilize MALT1 via disturbing the interaction with E3 ligase FBXO3. Loss of FBXO3 in NSCLC cells reduced MALT1 ubiquitination and promoted its accumulation, which was reversed by CSN5 interference. An association between CSN5/FBXO3/MALT1 regulatory axis and poor prognosis in NSCLC patients was identified. Our findings revealed the detail mechanism of continuous MALT1 activation in NF-κB signaling, highlighting its significance as predictor and potential therapeutic target in NSCLC., (© 2024. The Author(s).)

CARD-BCL10-MALT1 (CBM) signalosomes connect distal signaling of innate and adaptive immune receptors to proximal signaling pathways and immune activation. Four CARD scaffold proteins (CARD9, 10, 11, 14) can form seeds that nucleate the assembly of BCL10-MALT1 filaments in a cell- and stimulus-specific manner. MALT1 (also known as PCASP1) serves a dual function within the assembled CBM complexes. By recruiting TRAF6, MALT1 acts as a molecular scaffold that initiates IκB kinase (IKK)/NF-κB and c-Jun N-terminal kinase (JNK)/AP-1 signaling. In parallel, proximity-induced dimerization of the paracaspase domain activates the MALT1 protease which exerts its function by cleaving a set of specific substrates. While complete MALT1 ablation leads to immune deficiency, selective destruction of either scaffolding or protease function provokes autoimmune inflammation. Thus, balanced MALT1-TRAF6 recruitment and MALT1 substrate cleavage are critical to maintain immune homeostasis and to promote optimal immune activation. Further, MALT1 protease activity drives the survival of aggressive lymphomas and other non-hematologic solid cancers. However, little is known about the relevance of the cleavage of individual substrates for the pathophysiological functions of MALT1. Unbiased serendipity, screening and computational predictions have identified and validated ~20 substrates, indicating that MALT1 targets a quite distinct set of proteins. Known substrates are involved in CBM auto-regulation (MALT1, BCL10 and CARD10), regulation of signaling and adhesion (A20, CYLD, HOIL-1 and Tensin-3), or transcription (RelB) and mRNA stability/translation (Regnase-1, Roquin-1/2 and N4BP1), indicating that MALT1 often targets multiple proteins involved in similar cellular processes. Here, we will summarize what is known about the fate and functions of individual MALT1 substrates and how their cleavage contributes to the biological functions of the MALT1 protease. We will outline what is needed to better connect critical pathophysiological roles of the MALT1 protease with the cleavage of distinct substrates., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Moud, Ober, O’Neill and Krappmann.)

B-cell acute lymphoblastic leukemia (B-ALL) remains a hard-to-treat disease with a poor prognosis in adults. Mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) is a para-caspase required for B-cell receptor (BCR)-mediated NF-κB activation. Inhibition of MALT1 in preclinical models has proven efficacious in many B-cell malignancies including chronic lymphocytic leukemia, mantle cell lymphoma and diffuse large B-cell lymphoma. We sought to examine the role of MALT1 in B-ALL and determine the biological consequences of its inhibition. Targeting MALT1 with both Z-VRPR-fmk and MI-2 efficiently kills B-ALL cells independent of the cell-of-origin (pro, pre, mature) or the presence of the Philadelphia chromosome, and spares normal B cells. The mechanism of cell death was through apoptotic induction, mostly in cycling cells. The proteolytic activity of MALT1 can be studied by measuring its ability to cleave its substrates. Surprisingly, with the exception of mature B-ALL, we did not detect cleavage of MALT1 substrates at baseline, nor after proteasomal inhibition or following activation of pre-BCR. To explore the possibility of a distinct role for MALT1 in B-ALL, independent of signaling through BCR, we studied the changes in gene expression profiling following a 24-hour treatment with MI-2 in 12 B-ALL cell lines. Our transcriptome analysis revealed a strong inhibitory effect on MYC-regulated gene signatures, further confirmed by Myc protein downregulation, concomitant with an increase in the Myc degrader FBXW7. In conclusion, our evidence suggests a novel role for MALT1 in B-ALL through Myc regulation and provides support for clinical testing of MALT1 inhibitors in B-ALL.

The objective of this study was to clarify the long-term prognosis of patients with gastric mucosa-associated lymphoid tissue (MALT) lymphoma with additional copies of MALT1. In this multicenter retrospective study, we enrolled 145 patients with gastric MALT lymphoma who underwent fluorescence in situ hybridization (FISH) analysis to detect t(11;18) translocation. The patient cohort was divided into three groups: Group A (n = 87), comprising individuals devoid of the t(11;18) translocation or extra MALT1 copies; Group B (n = 27), encompassing patients characterized by the presence of the t(11;18) translocation; and Group C (n = 31), including patients with extra MALT1 copies. The clinical outcomes in each cohort were collected. Over the course of a mean follow-up of 8.5 ± 4.2 years, one patient died of progressive MALT lymphoma, while 15 patients died due to etiologies unrelated to lymphoma. The progression or relapse of MALT lymphoma was observed in 11 patients: three in Group A, two in Group B, and six in Group C. In Groups A, B, and C, the 10-year overall survival rates were 82.5%, 93.8%, and 86.4%, respectively, and the 10-year event-free survival rates were 96.1%, 96.0%, and 82.9%, respectively. The event-free survival rate in Group C was significantly lower than that in Group A. However, no differences were observed in the 10-year event-free survival rates among individuals limited to stage I or II 1 disease (equivalent to excluding patients with stage IV disease in this study, as there were no patients with stage II 2 ), with rates of 98.6%, 95.8%, and 92.3% for Groups A, B, and C, respectively. In conclusion, the presence of extra copies of MALT1 was identified as an inferior prognostic determinant of event-free survival. Consequently, trisomy/tetrasomy 18 may serve as an indicator of progression and refractoriness to therapeutic intervention in patients with gastric MALT lymphoma, particularly stage IV gastric MALT lymphoma., (© 2024. The Author(s).)

The protease MALT1 promotes lymphocyte activation and lymphomagenesis by cleaving a limited set of cellular substrates, most of which control gene expression. Here, we identified the integrin-binding scaffold protein Tensin-3 as a MALT1 substrate in activated human B cells. Activated B cells lacking Tensin-3 showed decreased integrin-dependent adhesion but exhibited comparable NF-κB1 and Jun N-terminal kinase transcriptional responses. Cells expressing a noncleavable form of Tensin-3, on the other hand, showed increased adhesion. To test the role of Tensin-3 cleavage in vivo, mice expressing a noncleavable version of Tensin-3 were generated, which showed a partial reduction in the T cell-dependent B cell response. Interestingly, human diffuse large B cell lymphomas and mantle cell lymphomas with constitutive MALT1 activity showed strong constitutive Tensin-3 cleavage and a decrease in uncleaved Tensin-3 levels. Moreover, silencing of Tensin-3 expression in MALT1-driven lymphoma promoted dissemination of xenografted lymphoma cells to the bone marrow and spleen. Thus, MALT1-dependent Tensin-3 cleavage reveals a unique aspect of the function of MALT1, which negatively regulates integrin-dependent B cell adhesion and facilitates metastatic spread of B cell lymphomas., Competing Interests: Competing interests statement:The authors declare no competing interest.

Constitutive mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) activity drives survival of malignant lymphomas addicted to chronic B-cell receptor signaling, oncogenic CARD11, or the API2-MALT1 (also BIRC3::MALT1) fusion oncoprotein. Although MALT1 scaffolding induces NF-κB-dependent survival signaling, MALT1 protease function is thought to augment NF-κB activation by cleaving signaling mediators and transcriptional regulators in B-cell lymphomas. However, the pathological role of MALT1 protease function in lymphomagenesis is not well understood. Here, we show that TRAF6 controls MALT1-dependent activation of NF-κB transcriptional responses but is dispensable for MALT1 protease activation driven by oncogenic CARD11. To uncouple enzymatic and nonenzymatic functions of MALT1, we analyzed TRAF6-dependent and -independent as well as MALT1 protease-dependent gene expression profiles downstream of oncogenic CARD11 and API2-MALT1. The data suggest that by cleaving and inactivating the RNA binding proteins Regnase-1 and Roquin-1/2, MALT1 protease induces posttranscriptional upregulation of many genes including NFKBIZ/IκBζ, NFKBID/IκBNS, and ZC3H12A/Regnase-1 in activated B-cell-like diffuse large B-cell lymphoma (ABC DLBCL). We demonstrate that oncogene-driven MALT1 activity in ABC DLBCL cells regulates NFKBIZ and NFKBID induction on an mRNA level via releasing a brake imposed by Regnase-1 and Roquin-1/2. Furthermore, MALT1 protease drives posttranscriptional gene induction in the context of the API2-MALT1 fusion created by the recurrent t(11;18)(q21;q21) translocation in MALT lymphoma. Thus, MALT1 paracaspase acts as a bifurcation point for enhancing transcriptional and posttranscriptional gene expression in malignant lymphomas. Moreover, the identification of MALT1 protease-selective target genes provides specific biomarkers for the clinical evaluation of MALT1 inhibitors., (© 2023 by The American Society of Hematology. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)

It is widely accepted that pancreatic islet β-cell failure and the onset of type 2 diabetes (T2DM) constitute an intricate interplay between the genetic expression of the disease and a host of intracellular events including increased metabolic (oxidative, endoplasmic reticulum) stress under the duress of glucolipotoxicity. Emerging evidence implicates unique roles for Caspase Recruitment Domain containing protein 9 (CARD9) in the onset of metabolic diseases, including obesity and insulin resistance. Mechanistically, CARD9 has been implicated in the regulation of p38MAPK and NFkB signaling pathways culminating in cellular dysfunction. Several regulatory factors, including B-cell lymphoma/leukemia 10 (BCL10) have been identified as modulators of CARD9 function in multiple cell types. Despite this evidence on regulatory roles of CARD9-BCL10 signalome in the onset of various pathological states, putative roles of this signaling module in islet β-cell dysfunction in metabolic stress remain less understood. This brief review is aimed at highlighting roles for CARD9 in islet β-cell function under acute (physiological insulin secretion) and long-term (cell dysfunction) exposure to glucose. Emerging roles of other signaling proteins, such as Rac1, BCL10 and MALT1 as contributors to CARD9 signaling in the islet β-cells are also reviewed. Potential avenues for future research toward the development of novel therapeutics for the prevention CARD9-BCL10-Rac1 (CBR) signalome-induced β-cell defects under metabolic stress are discussed., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Published by Elsevier Inc.)

MALT1 (mucosa-associated lymphoid tissue lymphoma translocation protein 1) serves as a pivotal mediator for NF-κB activation in response to a wide spectrum of transmembrane receptor stimuli. In the present study, a homolog of MALT1, named LvMALT1, is cloned from the Pacific white shrimp ( Litopenaeus vannamei ) and its potential function in shrimp innate immunity is explored. The open reading frame of LvMALT1 is 2364 bp that encodes 787 amino acids. The predicted LvMALT1 protein structure comprises a death domain, three immunoglobulin domains, and a caspase-like domain, exhibiting remarkable similarity to other homologs. LvMALT1 is a cytoplasmic-localized protein and could interact with LvTRAF6. Overexpression of LvMALT1 induces the activation of promoter elements governing the expression of several key antimicrobial peptides (AMPs), including penaeidins (PENs) and crustins (CRUs). Conversely, silencing of LvMALT1 leads to a reduction in the phosphorylation levels of Dorsal and Relish, along with a concomitant decline in the in vivo expression levels of multiple AMPs. Furthermore, LvMALT1 is prominently upregulated in response to a challenge by the white spot syndrome virus (WSSV), facilitating the NF-κB-mediated expression of AMPs as a defense against viral infection. Taken together, we identified a MALT1 homolog from the shrimp L. vannamei , which plays a positive role in the TRAF6/NF-κB/AMPs axis-mediated innate immunity.

Constitutive activation of the MALT1 paracaspase in conventional T cells of Malt1 TBM/TBM (TRAF6 Binding Mutant = TBM) mice causes fatal inflammation and autoimmunity, but the involved targets and underlying molecular mechanisms are unknown. We genetically rendered a single MALT1 substrate, the RNA-binding protein (RBP) Roquin-1, insensitive to MALT1 cleavage. These Rc3h1 Mins/Mins mice showed normal immune homeostasis. Combining Rc3h1 Mins/Mins alleles with those encoding for constitutively active MALT1 (TBM) prevented spontaneous T cell activation and restored viability of Malt1 TBM/TBM mice. Mechanistically, we show how antigen/MHC recognition is translated by MALT1 into Roquin cleavage and derepression of Roquin targets. Increasing T cell receptor (TCR) signals inactivated Roquin more effectively, and only high TCR strength enabled derepression of high-affinity targets to promote Th17 differentiation. Induction of experimental autoimmune encephalomyelitis (EAE) revealed increased cleavage of Roquin-1 in disease-associated Th17 compared to Th1 cells in the CNS. T cells from Rc3h1 Mins/Mins mice did not efficiently induce the high-affinity Roquin-1 target IκB NS in response to TCR stimulation, showed reduced Th17 differentiation, and Rc3h1 Mins/Mins mice were protected from EAE. These data demonstrate how TCR signaling and MALT1 activation utilize graded cleavage of Roquin to differentially regulate target mRNAs that control T cell activation and differentiation as well as the development of autoimmunity., Competing Interests: Competing interests statement:The authors declare no competing interest.

Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer and the third leading cause of cancer-related deaths worldwide. Despite recent advances in treatment options, therapeutic management of HCC remains a challenge, emphasizing the importance of exploring novel targets. MALT1 paracaspase is a druggable signaling molecule whose dysregulation has been linked to hematological and solid tumors. However, the role of MALT1 in HCC remains poorly understood, leaving its molecular functions and oncogenic implications unclear. Here we provide evidence that MALT1 expression is elevated in human HCC tumors and cell lines, and that correlates with tumor grade and differentiation state, respectively. Our results indicate that ectopic expression of MALT1 confers increased cell proliferation, 2D clonogenic growth, and 3D spheroid formation in well differentiated HCC cell lines with relatively low MALT1 levels. In contrast, stable silencing of endogenous MALT1 through RNA interference attenuates these aggressive cancer cell phenotypes, as well as migration, invasion, and tumor-forming ability, in poorly differentiated HCC cell lines with higher paracaspase expression. Consistently, we find that pharmacological inhibition of MALT1 proteolytic activity with MI-2 recapitulates MALT1 depletion phenotypes. Finally, we show that MALT1 expression is positively correlated with NF-kB activation in human HCC tissues and cell lines, suggesting that its tumor promoting functions may involve functional interaction with the NF-kB signaling pathway. This work unveils new insights into the molecular implications of MALT1 in hepatocarcinogenesis and places this paracaspase as a potential marker and druggable liability in HCC., Competing Interests: Declaration of competing interest The authors declare that they have no conflicts of interest with the contents of this article., (Copyright © 2023 Elsevier Inc. All rights reserved.)

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., Competing Interests: Authors declare no conflict of interest.

The uniqueness of MALT1 protease activity in controlling several aspects of immunity in humans has made it a very attractive therapeutic target for multiple autoimmune diseases and lymphoid malignancies. Despite several encouraging preclinical studies with MALT1 inhibitors, severe reduction in regulatory T cells and immune-mediated pathology seen in MALT1 protease-dead (MALT1-PD) mice and some, but not all, studies analysing the effect of prolonged pharmacological MALT1 protease inhibition, indicates the need to further unravel the mechanism of MALT1 protease function. Notably, the contribution of individual MALT1 substrates to the immune defects seen in MALT1-PD mice is still unclear. Previous in vitro studies indicated a role for MALT1-mediated cleavage of the E3 ubiquitin ligase HOIL-1 in the modulation of nuclear factor-κB (NF-κB) signalling and inflammatory gene expression in lymphocytes. Here, we addressed the immunological consequences of inhibition of HOIL-1 cleavage by generating and immunophenotyping MALT1 cleavage-resistant HOIL-1 knock-in (KI) mice. HOIL-1 KI mice appear healthy and have no overt phenotype. NF-κB activation in T or B cells, as well as IL-2 production and in vitro T-cell proliferation, is comparable between control and HOIL-1 KI cells. Inhibition of HOIL-1 cleavage in mice has no effect on thymic T-cell development and conventional T-cell homeostasis. Likewise, B-cell development and humoral immune responses are not affected. Together, these data exclude an important role of MALT1-mediated HOIL-1 cleavage in T- and B-cell development and function in mice., (© 2022 Federation of European Biochemical Societies.)

The paracaspase mucosa-associated lymphoid tissue 1 (MALT1) is a protease and scaffold protein essential in propagating B-cell receptor (BCR) signaling to NF-κB. The deubiquitinating enzyme cylindromatosis (CYLD) is a recently discovered MALT1 target that can negatively regulate NF-κB activation. Here, we show that low expression of CYLD is associated with inferior prognosis of diffuse large B-cell lymphoma (DLBCL) and mantle cell lymphoma (MCL) patients, and that chronic BCR signaling propagates MALT1-mediated cleavage and, consequently, inactivation and rapid proteasomal degradation of CYLD. Ectopic overexpression of WT CYLD or a MALT1-cleavage resistant mutant of CYLD reduced phosphorylation of IκBα, repressed transcription of canonical NF-κB target genes and impaired growth of BCR-dependent lymphoma cell lines. Furthermore, silencing of CYLD expression rendered BCR-dependent lymphoma cell lines less sensitive to inhibition of NF-κΒ signaling and cell proliferation by BCR pathway inhibitors, e.g., the BTK inhibitor ibrutinib, indicating that these effects are partially mediated by CYLD. Taken together, our findings identify an important role for MALT1-mediated CYLD cleavage in BCR signaling, NF-κB activation and cell proliferation, which provides novel insights into the underlying molecular mechanisms and clinical potential of inhibitors of MALT1 and ubiquitination enzymes as promising therapeutics for DLBCL, MCL and potentially other B-cell malignancies., (© 2023. The Author(s).)

Bruton's tyrosine kinase (BTK) is a proven target in mantle cell lymphoma (MCL), an aggressive subtype of non-Hodgkin lymphoma. However, resistance to BTK inhibitors is a major clinical challenge. We here report that MALT1 is one of the top overexpressed genes in ibrutinib-resistant MCL cells, while expression of CARD11, which is upstream of MALT1, is decreased. MALT1 genetic knockout or inhibition produced dramatic defects in MCL cell growth regardless of ibrutinib sensitivity. Conversely, CARD11-knockout cells showed antitumor effects only in ibrutinib-sensitive cells, suggesting that MALT1 overexpression could drive ibrutinib resistance via bypassing BTK/CARD11 signaling. Additionally, BTK knockdown and MALT1 knockout markedly impaired MCL tumor migration and dissemination, and MALT1 pharmacological inhibition decreased MCL cell viability, adhesion, and migration by suppressing NF-κB, PI3K/AKT/mTOR, and integrin signaling. Importantly, cotargeting MALT1 with safimaltib and BTK with pirtobrutinib induced potent anti-MCL activity in ibrutinib-resistant MCL cell lines and patient-derived xenografts. Therefore, we conclude that MALT1 overexpression associates with resistance to BTK inhibitors in MCL, targeting abnormal MALT1 activity could be a promising therapeutic strategy to overcome BTK inhibitor resistance, and cotargeting of MALT1 and BTK should improve MCL treatment efficacy and durability as well as patient outcomes.

A 68-year-old man was admitted because of a left shoulder mass and swollen right testis. Pathological examinations indicated a diagnosis of diffuse large B-cell lymphoma (DLBCL) with the CD20+BCL6+MUM1+BCL2+CD10-MYC- phenotype in both lesions. G-banding of soft tissue showed 47,XY,+18, whereas testicular cells showed 47,X,+X,-Y,der (4) t (4;18) (p15;?),del (5) (q?),+13. Fluorescence in situ hybridization detected additional MALT1 and BCL2 signals in both lesions. Southern blot demonstrated different IGH rearrangements between the soft tissue and testis. The patient was diagnosed with biclonal DLBCL with different karyotypes but similar immunophenotypes. Partial trisomy 18q involving MALT1 and BCL2 may be commonly involved in the pathogenesis of this biclonal DLBCL.

Deep vein thrombosis (DVT) is the third most common cause of cardiovascular mortality. Several studies suggest that DVT occurs at the intersection of dysregulated inflammation and coagulation upon activation of inflammasome and secretion of interleukin 1β (IL-1β) in restricted venous flow conditions. Our recent studies showed a signaling adapter protein, Gab2 (Grb2-associated binder 2), plays a crucial role in propagating inflammatory signaling triggered by IL-1β and other inflammatory mediators in endothelial cells. The present study shows that Gab2 facilitates the assembly of the CBM (CARMA3 [CARD recruited membrane-associated guanylate kinase protein 3]-BCL-10 [B-cell lymphoma 10]-MALT1 [mucosa-associated lymphoid tissue lymphoma translocation protein 1]) signalosome, which mediates the activation of Rho and NF-κB in endothelial cells. Gene silencing of Gab2 or MALT1, the effector signaling molecule in the CBM signalosome, or pharmacological inhibition of MALT1 with a specific inhibitor, mepazine, significantly reduced IL-1β-induced Rho-dependent exocytosis of P-selectin and von Willebrand factor (VWF) and the subsequent adhesion of neutrophils to endothelial cells. MALT1 inhibition also reduced IL-1β-induced NF-κB-dependent expression of tissue factor and vascular cell adhesion molecule 1. Consistent with the in vitro data, Gab2 deficiency or pharmacological inhibition of MALT1 suppressed the accumulation of monocytes and neutrophils at the injury site and attenuated venous thrombosis induced by the inferior vena cava ligation-induced stenosis or stasis in mice. Overall, our data reveal a previously unrecognized role of the Gab2-MALT1 axis in thromboinflammation. Targeting the Gab2-MALT1 axis with MALT1 inhibitors may become an effective strategy to treat DVT by suppressing thromboinflammation without inducing bleeding complications., (© 2022 by The American Society of Hematology.)

Fungal peritonitis is an inflammatory condition of the peritoneum which occurs secondary to peritoneal dialysis. Most cases of peritonitis are caused by microbial invasion into the peritoneal cavity, resulting in high morbidity and mortality. Unlike bacterial peritonitis, little is known on fungal peritonitis. Card9, an adapter protein, plays a critical role in anti-fungal immunity. In this study, by using zymosan-induced peritonitis and C. albicans-induced peritonitis mouse model, we demonstrated that fungal peritonitis was exacerbated in Card9 -/- mice, compared with WT mice. Next, we found the autophagy activation of peritonealmacrophages was impaired in Card9 -/- peritonitis mice. The autophagy agonist, MG132, ameliorated peritonitis in Card9 -/- mice. The result of microarray analysis indicates Malt1 was significantly decreased in Card9 -/- peritonitis mice. Furthermore, we demonstrated that Malt1 interacts with P62 and mediates the function of P62 to clear ubiquitinated proteins. After overexpression of Malt1, impaired autophagy activation caused by Card9 deficient was significantly rescued. Together, our results indicate that Card9 protects fungal peritonitis by regulating Malt1-mediated autophagy in macrophages. Our research provides a new idea for the pathogenesis of fungal peritonitis, which is of great significance for the clinical treatment of fungal peritonitis., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Alternative splicing plays key roles for cell type-specific regulation of protein function. It is controlled by cis-regulatory RNA elements that are recognized by RNA binding proteins (RBPs). The MALT1 paracaspase is a key factor of signaling pathways that mediate innate and adaptive immune responses. Alternative splicing of MALT1 is critical for controlling optimal T cell activation. We demonstrate that MALT1 splicing depends on RNA structural elements that sequester the splice sites of the alternatively spliced exon7. The RBPs hnRNP U and hnRNP L bind competitively to stem-loop RNA structures that involve the 5' and 3' splice sites flanking exon7. While hnRNP U stabilizes RNA stem-loop conformations that maintain exon7 skipping, hnRNP L disrupts these RNA elements to facilitate recruitment of the essential splicing factor U2AF2, thereby promoting exon7 inclusion. Our data represent a paradigm for the control of splice site selection by differential RBP binding and modulation of pre-mRNA structure.

Activated B cell-like diffuse large B-cell lymphomas (ABC-DLBCL) have unfavorable outcomes and chronic activation of CARD11-BCL10-MALT1 (CBM) signal amplification complexes that form due to polymerization of BCL10 subunits, which is affected by recurrent somatic mutations in ABC-DLBCLs. Herein, we show that BCL10 mutants fall into at least two functionally distinct classes: missense mutations of the BCL10 CARD domain and truncation of its C-terminal tail. Truncating mutations abrogated a motif through which MALT1 inhibits BCL10 polymerization, trapping MALT1 in its activated filament-bound state. CARD missense mutations enhanced BCL10 filament formation, forming glutamine network structures that stabilize BCL10 filaments. Mutant forms of BCL10 were less dependent on upstream CARD11 activation and thus manifested resistance to BTK inhibitors, whereas BCL10 truncating but not CARD mutants were hypersensitive to MALT1 inhibitors. Therefore, BCL10 mutations are potential biomarkers for BTK inhibitor resistance in ABC-DLBCL, and further precision can be achieved by selecting therapy based on specific biochemical effects of distinct mutation classes., Significance: ABC-DLBCLs feature frequent mutations of signaling mediators that converge on the CBM complex. We use structure-function approaches to reveal that BCL10 mutations fall into two distinct biochemical classes. Both classes confer resistance to BTK inhibitors, whereas BCL10 truncations confer hyperresponsiveness to MALT1 inhibitors, providing a road map for precision therapies in ABC-DLBCLs. See related commentary by Phelan and Oellerich, p. 1844. This article is highlighted in the In This Issue feature, p. 1825., (©2022 The Authors; Published by the American Association for Cancer Research.)

Objective: MALT1 regulates immunity and inflammation in multiple ways, while its role in rheumatoid arthritis (RA) is obscure. This study aimed to investigate the relationship of MALT1 with disease features, treatment outcome, as well as its effect on Th1/2/17 cell differentiation and underlying molecule mechanism in RA., Methods: Totally 147 RA patients were enrolled. Then their blood Th1, Th2, and Th17 cells were detected by flow cytometry. Besides, PBMC MALT1 expression was detected before treatment (baseline), at week (W) 6, W12, and W24. PBMC MALT1 in 30 osteoarthritis patients and 30 health controls were also detected. Then, blood CD4 + T cells were isolated from RA patients, followed by MALT1 overexpression or knockdown lentivirus transfection and Th1/2/17 polarization assay. In addition, IMD 0354 (NF-κB antagonist) and SP600125 (JNK antagonist) were also added to treat CD4 + T cells., Results: MALT1 was increased in RA patients compared to osteoarthritis patients and healthy controls. Meanwhile, MALT1 positively related to CRP, ESR, DAS28 score, Th17 cells, negatively linked with Th2 cells, but did not link with other features or Th1 cells in RA patients. Notably, MALT1 decreased longitudinally during treatment, whose decrement correlated with RA treatment outcome (treatment response, low disease activity, or disease remission). In addition, MALT1 overexpression promoted Th17 differentiation, inhibited Th2 differentiation, less affected Th1 differentiation, activated NF-κB and JNK pathways in RA CD4 + T cells; while MALT1 knockdown exhibited the opposite effect. Besides, IMD 0354 and SP600125 addition attenuated MALT1's effect on Th2 and Th17 differentiation., Conclusion: MALT1 regulates Th2 and Th17 differentiation via NF-κB and JNK pathways, as well as correlates with disease activity and treatment outcome in RA., Competing Interests: Authors JZ, KH, RL, YF, DZ and RD were employed by Shanghai QeeJen Bio-tech Institution. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Wang, Wang, Liu, Chu, Mi, Jiang, Zhao, Hu, Luo, Feng, Lee, Zhou, Mi and Deng.)

Purpose: MALT1 deficiency is a combined immune deficiency characterized by recurrent infections, eczema, chronic diarrhea, and failure to thrive. Clinical and immunological characterizations of the disease have not been previously reported in large cohorts. We sought to determine the clinical, immunological, genetic features, and the natural history of MALT-1 deficiency., Methods: The clinical findings and treatment outcomes were evaluated in nine new MALT1-deficient patients. Peripheral lymphocyte subset analyses, cytokine secretion, and proliferation assays were performed. We also analyzed ten previously reported patients to comprehensively evaluate genotype/phenotype correlation., Results: The mean age of patients and disease onset were 33 ± 17 and 1.6 ± 0.7 months, respectively. The main clinical findings of the disease were recurrent infections (100%), skin involvement (100%), failure to thrive (100%), oral lesions (67%), chronic diarrhea (56%), and autoimmunity (44%). Eosinophilia and high IgE were observed in six (67%) and two (22%) patients, respectively. The majority of patients had normal T and NK cells, while eight (89%) exhibited reduced B cells. Immunoglobulin replacement and antibiotics prophylaxis were mostly ineffective in reducing the frequency of infections and other complications. One patient received hematopoietic stem cell transplantation (HSCT) and five patients died as a complication of life-threatening infections. Analyzing this cohort with reported patients revealed overall survival in 58% (11/19), which was higher in patients who underwent HSCT (P = 0.03)., Conclusion: This cohort provides the largest analysis for clinical and immunological features of MALT1 deficiency. HSCT should be offered as a curative therapeutic option for all patients at the early stage of life., (© 2021. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)

Mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) is a multifunctional protein. MALT1 functions as an adaptor protein to assemble and recruit proteins such as B-cell lymphoma 10 (BCL10) and caspase-recruitment domain (CARD)-containing coiled-coil protein 11 (CARD11). Conversely it also acts as a paracaspase to cleave specified substrates. Because of its involvement in immunity, inflammation and cancer through its dual functions of scaffolding and catalytic activity, MALT1 is becoming a promising therapeutic target in B cell- and T cell-related diseases. There is growing evidence that the function of MALT1 is subtly modulated via post-translational modifications. This review summarized recent progress in relevant studies regarding the physiological and pathophysiological functions of MALT1, post-translational modifications of MALT1 and its role in B cell- and T cell- related diseases. In addition, the current available MALT1 inhibitors were also discussed., (Copyright © 2022 Elsevier Inc. All rights reserved.)

CARD11 acts as a gatekeeper for adaptive immune responses after T cell or B cell antigen receptor (TCR/BCR) ligation on lymphocytes. PKCθ/β-catalyzed phosphorylation of CARD11 promotes the assembly of the CARD11-BCL10-MALT1 (CBM) complex and lymphocyte activation. Here, we demonstrated that PKCθ/β-dependent CARD11 phosphorylation also suppressed CARD11 functions in T or B cells. Through mass spectrometry-based proteomics analysis, we identified multiple constitutive and inducible CARD11 phosphorylation sites in T cells. We demonstrated that a single TCR- or BCR-inducible phosphorylation on Ser 893 in the carboxyl terminus of CARD11 prevented the activation of the transcription factor NF-κB, the kinase JNK, and the protease MALT1. Moreover, CARD11 Ser 893 phosphorylation sensitized BCR-addicted lymphoma cells to toxicity induced by Bruton's tyrosine kinase (BTK) inhibitors. Phosphorylation of Ser 893 in CARD11 by PKCθ controlled the strength of CARD11 scaffolding by impairing the formation of the CBM complex. Thus, PKCθ simultaneously catalyzes both stimulatory and inhibitory CARD11 phosphorylation events, which shape the strength of CARD11 signaling in lymphocytes.

MALT1 is the effector protein of the CARMA/Bcl10/MALT1 (CBM) signalosome, a multiprotein complex that drives pro-inflammatory signaling pathways downstream of a diverse set of receptors. Although CBM activity is best known for its role in immune cells, emerging evidence suggests that it plays a key role in the pathogenesis of solid tumors, where it can be activated by selected G protein-coupled receptors (GPCR). Here, we demonstrated that overexpression of GPCRs implicated in breast cancer pathogenesis, specifically the receptors for Angiotensin II and thrombin (AT1R and PAR1), drove a strong epithelial-to-mesenchymal transition (EMT) program in breast cancer cells that is characteristic of claudin-low, triple-negative breast cancer (TNBC). In concert, MALT1 was activated in these cells and contributed to the dramatic EMT phenotypic changes through regulation of master EMT transcription factors including Snail and ZEB1. Importantly, blocking MALT1 signaling, through either siRNA-mediated depletion of MALT1 protein or pharmacologic inhibition of its activity, was effective at partially reversing the molecular and phenotypic indicators of EMT. Treatment of mice with mepazine, a pharmacologic MALT1 inhibitor, reduced growth of PAR1+, MDA-MB-231 xenografts and had an even more dramatic effect in reducing the burden of metastatic disease. These findings highlight MALT1 as an attractive therapeutic target for claudin-low TNBCs harboring overexpression of one or more selected GPCRs., Implications: This study nominates a GPCR/MALT1 signaling axis as a pathway that can be pharmaceutically targeted to abrogate EMT and metastatic progression in TNBC, an aggressive form of breast cancer that currently lacks targeted therapies., (©2021 American Association for Cancer Research.)

Regulatory T cells (Tregs) are essential for the inhibition of immunity and the maintenance of tissue homeostasis. Signals from the T-cell antigen receptor (TCR) are critical for early Treg development, their expansion, and inhibitory activity. Although TCR-engaged activation of the paracaspase MALT1 is important for these Treg activities, the MALT1 effector pathways in Tregs remain ill-defined. Here, we demonstrate that MALT1 protease activity controls the TCR-induced upregulation of the transcription factor MYC and the subsequent expression of MYC target genes in Tregs. These mechanisms are important for Treg-intrinsic mitochondrial function, optimal respiratory capacity, and homeostatic Treg proliferation. Consistently, conditional deletion of Myc in Tregs results similar to MALT1 inactivation in a lethal autoimmune inflammatory syndrome. Together, these results identify a MALT1 protease-mediated link between TCR signaling in Tregs and MYC control that coordinates metabolism and Treg expansion for the maintenance of immune homeostasis., (© 2021 The Authors. European Journal of Immunology published by Wiley-VCH GmbH.)

Mucosa-associated lymphoid tissue lymphoma-translocation gene 1 (MALT1)-deficiency is a rare combined immunodeficiency characterized by recurrent infections, dermatitis and enteropathy. We herein investigate the immunological profiles of our patient and previously reported children with MALT1-deficiency. A mutation analysis was performed by targeted panel sequencing for primary immunodeficiency. Lymphocyte subset, activation and B cell differentiation were analyzed by flow cytometry and t-distributed stochastic neighbor embedding. Pneumocystis pneumonia developed in a 6-month-old Japanese infant with atopic dermatitis, enteritis and growth restriction. This infant showed agammaglobulinemia without lymphopenia. At 8 years of age, the genetic diagnosis of MALT1-deficiency was confirmed on a novel homozygous mutation of c.1102G>T, p.E368X. T cell stimulation tests showed impairments in the production of interleukin-2, phosphorylation of nuclear factor kappa B (NF-κB) p65 and differentiation of B cells. In combination with the literature data, we found that the number of circulatory B cells, but not T cells, were inversely correlated with the age of patients. The hematopoietic cell transplantation (HCT) successfully reconstituted the differentiation of mature B cells and T cells. These data conceptualize that patients with complete MALT1-deficiency show aberrant differentiation and depletion of B cells. The early diagnosis and HCT lead to a cure of the disease phenotype associated with the loss-of-function mutations in human CARD11., (© 2021 British Society for Immunology.)

Psoriasis is a chronic inflammatory skin disease arising from poorly defined pathological cross-talk between keratinocytes and the immune system. BCL10 (B cell lymphoma/leukemia 10) and MALT1 (mucosa-associated lymphoid tissue lymphoma translocation protein 1) are ubiquitously expressed inflammatory signaling proteins that can interact with the psoriasis susceptibility factor CARD14, but their functions in psoriasis are insufficiently understood. We report that although keratinocyte-intrinsic BCL10/MALT1 deletions completely rescue inflammatory skin pathology triggered by germline Card14 gain-of-function mutation in mice, the BCL10/MALT1 signalosome is unexpectedly not involved in the CARD14-dependent interleukin-17 receptor (IL-17R) proximal pathway. Instead, it plays a more pleiotropic role by amplifying keratinocyte responses to a series of inflammatory cytokines, including IL-17A, IL-1β, and TNF. Moreover, selective keratinocyte-intrinsic activation of BCL10/MALT1 signaling with an artificial engager molecule is sufficient to initiate lymphocyte-mediated psoriasiform skin inflammation, and aberrant BCL10/MALT1 activity is frequently detected in the skin of human sporadic psoriasis. Together, these results establish that BCL10/MALT1 signalosomes can act as initiators and crucial amplifiers of psoriatic skin inflammation and indicate a critical function for this complex in sporadic psoriasis.

Balanced control of T cell signaling is critical for adaptive immunity and protection from autoimmunity. By combining genetically engineered mouse models, biochemical analyses and pharmacological interventions, we describe an unexpected dual role of the tumor necrosis factor receptor–associated factor 6 (TRAF6) E3 ligase as both a positive and negative regulator of mucosa-associated lymphoid tissue 1 (MALT1) paracaspase. Although MALT1-TRAF6 recruitment is indispensable for nuclear factor κB signaling in activated T cells, TRAF6 counteracts basal MALT1 protease activity in resting T cells. In mice, loss of TRAF6-mediated homeostatic suppression of MALT1 protease leads to severe autoimmune inflammation, which is completely reverted by genetic or therapeutic inactivation of MALT1 protease function. Thus, TRAF6 functions as a molecular brake for MALT1 protease in resting T cells and a signaling accelerator for MALT1 scaffolding in activated T cells, revealing that TRAF6 controls T cell activation in a switch-like manner. Our findings have important implications for development and treatment of autoimmune diseases.

Human germline MALT1 deficiency is an inborn error of immunity characterized by recurrent bacterial, viral, and fungal infections, periodontal disease, enteropathy, dermatitis, and failure to thrive. The number of identified MALT1-deficient patients have greatly increased in the past two years, which has significantly improved our understanding of the clinical features of this disorder. Patients frequently experience infections affecting the respiratory, skin, gastrointestinal, and blood systems. The most frequently detected pathogens are Staphylococcus aureus, Candida albicans, and cytomegalovirus. Enhanced susceptibility to S. aureus and C. albicans is likely due to impaired Th17 immunity, similar to STAT3 and IL-17 pathway deficiencies., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Psoriasis is a frequent autoimmune-related skin disease, which involves various cell types such as T cells, keratinocytes and dendritic cells. Genetic variations, such as mutations of CARD14 , can promote the development of the disease. CARD14 mutations as well as the stimulation of immune and cytokine receptors activate the paracaspase MALT1, a potent activator of the transcription factors NF-κB and AP-1. The disease-promoting role of MALT1 for psoriasis is mediated by both its protease activity as well as its molecular scaffold function. Here, we review the importance of MALT1-mediated signaling and its therapeutic implications in psoriasis., (© 2021 Walter de Gruyter GmbH, Berlin/Boston.)

Signal transduction typically displays a so-called bow-tie topology: Multiple receptors lead to multiple cellular responses but the signals all pass through a narrow waist of central signaling nodes. One such signaling node for several inflammatory and oncogenic signaling pathways is the CARD-CC/BCL10/MALT1 (CBM) complexes, which get activated by protein kinase C (PKC)-mediated phosphorylation of the caspase activation and recruitment domain (CARD)-coiled-coil domain (CC) component. In humans, there are four CARD-CC family proteins (CARD9, CARD10, CARD11, and CARD14) and 9 true PKC isozymes (α to ι). At this moment, less than a handful of PKC::CARD-CC relationships are known. In order to explore the biologically relevant combinatorial space out of all 36 potential permutations in this two-component signaling event, we made use of CARD10-deficient human embryonic kidney 293T cells for subsequent pairwise cotransfections of all CARD-CC family members and all activated PKCs. Upon analysis of NF-κB-dependent reporter gene expression, we could define specific PKC::CARD-CC relationships. Surprisingly, as many as 21 PKC::CARD-CC functional combinations were identified. CARD10 was responsive to most PKCs, while CARD14 was mainly activated by PKCδ. The CARD11 activation profile was most similar to that of CARD9. We also discovered the existence of mixed protein complexes between different CARD-CC proteins, which was shown to influence their PKC response profile. Finally, multiple PKCs were found to use a common phosphorylation site to activate CARD9, while additional phosphorylation sites contribute to CARD14 activation. Together, these data reveal the combinatorial space of PKC::CARD-CC signal transduction nodes, which will be valuable for future studies on the regulation of CBM signaling., (© 2020 Federation of European Biochemical Societies.)

Sensing of pathogenic nucleic acids by pattern recognition receptors (PRR) not only initiates anti-microbe defense but causes inflammatory and autoimmune diseases. E3 ubiquitin ligase(s) critical in innate response need to be further identified. Here we report that the tripartite motif-containing E3 ubiquitin ligase TRIM41 is required to innate antiviral response through facilitating pathogenic nucleic acids-triggered signaling pathway. TRIM41 deficiency impairs the production of inflammatory cytokines and type I interferons in macrophages after transfection with nucleic acid-mimics and infection with both DNA and RNA viruses. In vivo, TRIM41 deficiency leads to impaired innate response against viruses. Mechanistically, TRIM41 directly interacts with BCL10 (B cell lymphoma 10), a core component of CARD proteins-BCL10 - MALT1 (CBM) complex, and modifies the Lys63-linked polyubiquitylation of BCL10, which, in turn, hubs NEMO for activation of NF-κB and TANK-binding kinase 1 (TBK1) - interferon regulatory factor 3 (IRF3) pathways. Our study suggests that TRIM41 is the potential universal E3 ubiquitin ligase responsible for Lys63 linkage of BCL10 during innate antiviral response, adding new insight into the molecular mechanism for the control of innate antiviral response.

Increasing evidence has supported the crucial role of CARD14 in the pathogenesis of psoriasis, whereas the precise cellular signaling involved in skin physiopathology remains poorly understood. In this article, we show that neither genetic ablation of Il17a nor elimination of T cells was sufficient to restrain the skin inflammation in a CARD14-E138A-mutation-induced psoriasis-like mouse model, whereas depletion of Il23 , which extremely blocked the IL-23/T17 axis, was more effective. Targeting CBM complex by conditional deletion of MALT1 or BCL10 in keratinocytes abrogated both the cutaneous and systemic inflammation of heterozygous Card14 E138A/+ mice. Selective inactivation of keratinocyte-specific MALT1 proteolytic activity strongly ameliorated the Card14 E138A/+ - and Card14 ΔQ136/+ -induced skin disease, which was reproduced by using the imiquimod-induced mouse model. Together, our results suggest a sequence of events under CARD14-mutation-induced psoriasis condition that keratinocyte-intrinsic activation of CBM complex initiates the skin inflammation depending on the IL-23/T17 axis. Targeting keratinocytes by inactivation of MALT1 paracaspase activity might be a promising therapeutic target for early psoriasis treatment., (Copyright © 2021 by The American Association of Immunologists, Inc.)

Jurkat T cells have been of central importance for the discovery of signalling mediators driving NF-κB activation in response to T cell antigen receptor (TCR)/CD28 co-stimulation. The critical function of the key regulators identified in Jurkat T cells has subsequently been verified in primary murine and human T cells. CRISPR/Cas9-mediated genomic editing techniques in combination with viral reconstitution are powerful tools that now enable the investigation of the exact molecular mechanisms that govern T cell signalling, especially the impact of protein-protein interactions, protein modifications, or cancer-associated gain- or loss-of-function mutations. As exemplified by the CARD11 gene encoding a key regulator of NF-κB signalling in T cells, we describe here the detailed workflow for the generation of CRISPR/Cas9 knockout (KO) Jurkat T cells and the subsequent reconstitution using a lentiviral transduction protocol. In addition, we explain the use of a stable NF-κB-dependent EGFP reporter system that enables a reliable quantification of NF-κB transcriptional activation in the reconstituted KO Jurkat T cells.

Strategies targeting T cells are the cornerstone of immunosuppression after solid organ transplantation. The transcription factor NF-κB is a key regulator of downstream T-cell activation and induction of inflammatory mediators; its full activation via antigen receptor engagement requires both the scaffold and the protease activity of the paracaspase Malt1. Experimental studies have highlighted that Malt1-deficient mice were resistant to experimental autoimmune encephalomyelitis, although they lacked peripheral regulatory T cells (Treg). Here, we compared targeting Malt1 versus using calcineurin inhibitors as immunosuppression in a stringent experimental transplantation model. We found that Malt1-deficiency impaired Th1-mediated alloresponses in vitro and in vivo and significantly prolonged MHC-mismatched skin allograft survival, compared to cyclosporine. However, it paradoxically enhanced Th17 differentiation in the transplantation setting. Interestingly, more selective inhibition of Malt1 protease activity in wild-type mouse and human peripheral T cells in vitro led to attenuation of alloreactive Th1 cells, while preserving preexisting Treg in the peripheral T-cell pool, and without promoting Th17 differentiation. Thus, there is a place for further investigation of the role of Malt1 signaling in the setting of transplantation., (Copyright © 2020 Govender, Mikulic, Wyss, Gaide, Thome and Golshayan.)

The paracaspase mucosa-associated lymphoid tissue lymphoma translocation protein-1 (MALT1) regulates nuclear-factor-kappa-B (NF-κB) activation downstream of surface receptors with immunoreceptor tyrosine-based activation motifs (ITAMs), such as the B-cell or T-cell receptor and has thus emerged as a therapeutic target for autoimmune diseases. However, recent reports demonstrate the development of lethal autoimmune inflammation due to the excessive production of interferon gamma (IFN-ɣ) and defective differentiation of regulatory T-cells in genetically modified mice deficient in MALT1 paracaspase activity. To address this issue, we explored the effects of pharmacological MALT1 inhibition on the balance between T-effector and regulatory T-cells. Here we demonstrate that allosteric inhibition of MALT1 suppressed Th1, Th17 and Th1/Th17 effector responses, and inhibited T-cell dependent B-cell proliferation and antibody production. Allosteric MALT1 inhibition did not interfere with the suppressive function of human T-regulatory cells, although it impaired de novo differentiation of regulatory T-cells from naïve T-cells. Treatment with an allosteric MALT1 inhibitor alleviated the cytokine storm, including IFN-ɣ, in a mouse model of acute T-cell activation, and long-term treatment did not lead to an increase in IFN-ɣ producing CD4 cells or tissue inflammation. Together, our data demonstrate that the effects of allosteric inhibition of MALT1 differ from those seen in mice with proteolytically inactive MALT1, and thus we believe that MALT1 is a viable target for B and T-cell driven autoimmune diseases., Competing Interests: The authors were all employed by AstraZeneca AB during the course of the work described in the manuscript. AstraZeneca provided support in the form of salaries for authors [C. D., U.S., T.A., L.O., J.M., A, DeM, K.P., G.J., L.Y., R.J. C., F.S., M.H.L., U.G., A.M.D., O.V.], but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section. This does not alter our adherence to PLOS ONE policies on sharing data and materials. There are no patents, products in development or marketed products associated with this research to declare.

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 CARD14 E138A 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 CARD14 E138A 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., (© 2020 The Authors.)

Purpose of Review: To present and discuss key design concepts for optimizing the impact of observational studies in the field of allergy and to highlight recent findings from NIAID-funded research networks., Recent Findings: We discuss three concepts. First, the benefit of prospective, longitudinal observational studies exemplified by recent findings on the seasonal nature of all rhinitis phenotypes in children with asthma and the protective effects of high house dust allergen content during the first year of life on the development of asthma at age 7 years. Second, the benefit of detailed (deep) phenotyping exemplified by the identification of a MALT1 gene variant as a strong genetic link to peanut allergy and the determination that atopic dermatitis with food allergy constitutes a distinct cutaneous endotype, compared with atopic dermatitis alone. Third, the benefit of hypothesis-generating research combined with prospective design and deep phenotyping as exemplified by the unveiling of potential pathophysiologic pathways leading to asthma exacerbations in children, after a 'cold'., Summary: Observational studies can be highly impactful if designed well. Longitudinal study design, deep phenotyping, and hypothesis-generating research are three major design concepts that should be considered in the development of these studies.

As one family of pattern recognition receptors (PRRs), The C-type lectin receptors (CLRs) play a key role in the anti-fungal infection. The CLR pathway signaling is relayed by adaptor complex which comprises CARD9, BCL10 and MALT1. However, the relationship between these three adaptors has not been investigated. In this study, we isolated porcine CARD9, BCL10 and MALT1 and examined their signaling functions. The three ectopic adaptors were similarly and uniformly expressed in cytoplasm, with CARD9 inactive, BCL10 significant active, and MALT1 slightly active for downstream NF-κB signaling and gene expressions. With the three adaptors together, NF-κB signaling and gene expressions were strongly activated, however, no IFN signal was activated in any case. The signaling relationship between the adaptors were dissected, the NF-κB signaling results showed that CARD9 could inhibit both BCL10 and MALT1 activities, while BCL10 and MALT1 synergized each other particularly when moderate amount of BCL10 plus low amount of MALT1 were considered. Low amount of CARD9 could further synergized with BCL10 and MALT1, maximizing signaling activity of the adaptor complex. This study revealed the porcine CLR pathway adaptor signaling functions and their optimal collective activity, thus providing a unique insight into the porcine innate immunity., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The catalytic activity of the protease MALT1 is required for adaptive immune responses and regulatory T (Treg)-cell development, while dysregulated MALT1 activity can lead to lymphoma. MALT1 activation requires its monoubiquitination on lysine 644 (K644) within the Ig3 domain, localized adjacent to the protease domain. The molecular requirements for MALT1 monoubiquitination and the mechanism by which monoubiquitination activates MALT1 had remained elusive. Here, we show that the Ig3 domain interacts directly with ubiquitin and that an intact Ig3-ubiquitin interaction surface is required for the conjugation of ubiquitin to K644. Moreover, by generating constitutively active MALT1 mutants that overcome the need for monoubiquitination, we reveal an allosteric communication between the ubiquitination site K644, the Ig3-protease interaction surface, and the active site of the protease domain. Finally, we show that MALT1 mutants that alter the Ig3-ubiquitin interface impact the biological response of T cells. Thus, ubiquitin binding by the Ig3 domain promotes MALT1 activation by an allosteric mechanism that is essential for its biological function.

Antigen receptor-dependent (AgR-dependent) stimulation of the NF-κB transcription factor in lymphocytes is a required event during adaptive immune response, but dysregulated activation of this signaling pathway can lead to lymphoma. AgR stimulation promotes assembly of the CARMA1-BCL10-MALT1 complex, wherein MALT1 acts as (a) a scaffold to recruit components of the canonical NF-κB machinery and (b) a protease to cleave and inactivate specific substrates, including negative regulators of NF-κB. In multiple lymphoma subtypes, malignant B cells hijack AgR signaling pathways to promote their own growth and survival, and inhibiting MALT1 reduces the viability and growth of these tumors. As such, MALT1 has emerged as a potential pharmaceutical target. Here, we identified G protein-coupled receptor kinase 2 (GRK2) as a new MALT1-interacting protein. We demonstrated that GRK2 binds the death domain of MALT1 and inhibits MALT1 scaffolding and proteolytic activities. We found that lower GRK2 levels in activated B cell-type diffuse large B cell lymphoma (ABC-DLBCL) are associated with reduced survival, and that GRK2 knockdown enhances ABC-DLBCL tumor growth in vitro and in vivo. Together, our findings suggest that GRK2 can function as a tumor suppressor by inhibiting MALT1 and provide a roadmap for developing new strategies to inhibit MALT1-dependent lymphomagenesis.

Glioblastoma is one of the most lethal forms of adult cancer with a median survival of around 15 months. A potential treatment strategy involves targeting glioblastoma stem-like cells (GSC), which constitute a cell autonomous reservoir of aberrant cells able to initiate, maintain, and repopulate the tumor mass. Here, we report that the expression of the paracaspase mucosa-associated lymphoid tissue l (MALT1), a protease previously linked to antigen receptor-mediated NF-κB activation and B-cell lymphoma survival, inversely correlates with patient probability of survival. The knockdown of MALT1 largely impaired the expansion of patient-derived stem-like cells in vitro, and this could be recapitulated with pharmacological inhibitors, in vitro and in vivo. Blocking MALT1 protease activity increases the endo-lysosome abundance, impairs autophagic flux, and culminates in lysosomal-mediated cell death, concomitantly with mTOR inactivation and dispersion from endo-lysosomes. These findings place MALT1 as a new druggable target involved in glioblastoma and unveil ways to modulate the homeostasis of endo-lysosomes., (© 2019 The Authors.)

The proteinase mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1), which forms part of the caspase recruitment domain-containing protein 11-B-cell lymphoma 10-MALT1 signalosome complex, plays a direct role in nuclear factor kappa B activation. Here, we describe the case of a female infant with severe immune dysregulation leading to recurrent systemic infections, failure to thrive and severe crises of ichthyosiform erythroderma with high levels of serum IgE. Hence, initial symptoms indicated Netherton syndrome or Omenn syndrome. Surprisingly, sequence analyses of SPINK5 and RAG1/RAG2, respectively, excluded these diseases. During the hospital stay the patient's health deteriorated, despite intensive care therapy, and she died. In order to delineate the diagnosis, whole-exome sequencing was performed. Two compound heterozygous mutations in MALT1 were found and verified by Sanger sequencing (exon 2 c.245T>C, exon 2 c.310dup), which led to a MALT1 deficiency at the protein level. Based on these results, an immunological analysis was performed, as was immunofluorescence staining of key skin proteins, to confirm a diagnosis of MALT1 deficiency. This case report provides a closer description of the clinical and histological skin phenotype of MALT1 deficiency, and we conclude that MALT1 deficiency must be considered a possible differential diagnosis of Netherton and Omenn syndromes. What's already known about this topic? Mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) deficiency is a combined immunodeficiency. MALT1 is part of the caspase recruitment domain-containing protein 11-B-cell lymphoma 10-MALT1 signalosome complex, which is essential for nuclear factor kappa B activation. Current publications describe a phenotype of recurrent systemic infections; only in a few cases has an inflammatory involvement of the integument been described. What does this study add? A closer description of the cutaneous phenotype of MALT1 deficiency in a patient with two novel MALT1 mutations. Immune mapping of follicular epidermis shows lympho-epithelial Kazal-type-related inhibitor is reduced in MALT1 deficiency and absent on interfollicular staining. Clinically, MALT1 deficiency mimics Netherton syndrome and Omenn syndrome, and should be considered a differential diagnosis., (© 2019 British Association of Dermatologists.)