MALT1 Protease Activity Is Required for Innate and Adaptive Immune Responses

MALT1 protease inhibition restrains glioblastoma progression by reversing tumor-associated macrophage-dependent immunosuppression

MALT1 protease is an intracellular signaling molecule that promotes tumor progression via cancer cell-intrinsic and cancer cell-extrinsic mechanisms. MALT1 has been mostly studied in lymphocytes, and little is known about its role in tumor-associated macrophages. Here, we show that MALT1 plays a key role in glioblastoma (GBM)-associated macrophages. Mechanistically, GBM tumor cells induce a MALT1-NF-κB signaling axis within macrophages, leading to macrophage migration and polarization toward an immunosuppressive phenotype. Inactivation of MALT1 protease promotes transcriptional reprogramming that reduces migration and restores a macrophage "M1-like" phenotype. Preclinical in vivo analysis shows that MALT1 inhibitor treatment results in increased immuno-reactivity of GBM-associated macrophages and reduced GBM tumor growth. Further, the addition of MALT1 inhibitor to temozolomide reduces immunosuppression in the tumor microenvironment, which may enhance the efficacy of this standard-of-care chemotherapeutic. Together, our findings suggest that MALT1 protease inhibition represents a promising macrophage-targeted immunotherapeutic strategy for the treatment of GBM.

Graphical abstract

MALT1 inhibition suppresses antigen-specific T cell responses

The aim of this study was to assess the potential use of a selective small molecule MALT1 inhibitor in solid tumor treatment as an immunotherapy targeting regulatory T-cells (Tregs). In vitro, MALT1 inhibition suppressed the proteolytic cleavage of the MALT1-substrate HOIL1 and blocked IL-2 secretion in Jurkat cells. It selectively suppressed the proliferation of PBMC-derived Tregs, with no effect on conventional CD4+T-cells. In vivo, however, no evident anti-tumor effect was achieved by MALT1 inhibition monotherapy or in combination with anti-CTLA4 in the MB49 cancer model. Despite decreased Treg-frequencies in lymph nodes of tumor-bearing animals, intratumoral Treg depletion was not observed. We also showed that MALT1-inhibition caused a reduction of antigen-specific CD8+T-cells in an adoptive T-cell transfer model. Thus, selective targeting of Tregs would be required to improve the immunotherapeutic effect of MALT1-inhibition. Also, various dosing schedules and combination therapy strategies should be carefully designed and evaluated further.

Identification of Tensin-3 as a MALT1 substrate that controls B cell adhesion and lymphoma dissemination

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.

Chemical Probes for Profiling of MALT1 Protease Activity

The paracaspase MALT1 is a key regulator of the human immune response. It is implicated in a variety of human diseases. For example, deregulated protease activity drives the survival of malignant lymphomas and is involved in the pathophysiology of autoimmune/inflammatory diseases. Thus, MALT1 has attracted attention as promising drug target. Although many MALT1 inhibitors have been identified, molecular tools to study MALT1 activity, target engagement and inhibition in complex biological samples, such as living cells and patient material, are still scarce. Such tools are valuable to validate MALT1 as a drug target in vivo and to assess yet unknown biological roles of MALT1. In this review, we discuss the recent literature on the development and biological application of molecular tools to study MALT1 activity and inhibition.

Function and targeting of MALT1 paracaspase in cancer

The paracaspase MALT1 has emerged as a key regulator of immune signaling, which also promotes tumor development by both cancer cell-intrinsic and -extrinsic mechanisms. As an integral subunit of the CARD11-BCL10-MALT1 (CBM) signaling complex, MALT1 has an intriguing dual function in lymphocytes. MALT1 acts as a scaffolding protein to drive activation of NF-κB transcription factors and as a protease to modulate signaling and immune activation by cleavage of distinct substrates. Aberrant MALT1 activity is critical for NF-κB-dependent survival and proliferation of malignant cancer cells, which is fostered by paracaspase-catalyzed inactivation of negative regulators of the canonical NF-κB pathway like A20, CYLD and RelB. Specifically, B cell receptor-addicted lymphomas rely strongly on this cancer cell-intrinsic MALT1 protease function, but also survival, proliferation and metastasis of certain solid cancers is sensitive to MALT1 inhibition. Beyond this, MALT1 protease exercises a cancer cell-extrinsic role by maintaining the immune-suppressive function of regulatory T (Treg) cells in the tumor microenvironment (TME). MALT1 inhibition is able to convert immune-suppressive to pro-inflammatory Treg cells in the TME of solid cancers, thereby eliciting a robust anti-tumor immunity that can augment the effects of checkpoint inhibitors. Therefore, the cancer cell-intrinsic and -extrinsic tumor promoting MALT1 protease functions offer unique therapeutic opportunities, which has motivated the development of potent and selective MALT1 inhibitors currently under pre-clinical and clinical evaluation.

TRAF6 controls T cell homeostasis by maintaining the equilibrium of MALT1 scaffolding and protease functions

MALT1 is a core component of the CARD11-BCL10-MALT1 (CBM) signalosome, in which it acts as a scaffold and a protease to bridge T cell receptor (TCR) ligation to immune activation. As a scaffold, MALT1 binds to TRAF6, and T cell-specific TRAF6 ablation or destruction of MALT1-TRAF6 interaction provokes activation of conventional T (Tconv) effector cells. In contrast, MALT1 protease activity controls the development and suppressive function of regulatory T (Treg) cells in a T cell-intrinsic manner. Thus, complete loss of TRAF6 or selective inactivation of MALT1 catalytic function in mice skews the immune system towards autoimmune inflammation, but distinct mechanisms are responsible for these immune disorders. Here we demonstrate that TRAF6 deletion or MALT1 paracaspase inactivation are highly interdependent in causing the distinct immune pathologies. We crossed mice with T cell-specific TRAF6 ablation (Traf6-ΔT) and mice with a mutation rendering the MALT1 paracaspase dead in T cells (Malt1 PD-T) to yield Traf6-ΔT;Malt1 PD-T double mutant mice. These mice reveal that the autoimmune inflammation caused by TRAF6-ablation relies strictly on the function of the MALT1 protease to drive the activation of Tconv cells. Vice versa, despite the complete loss of Treg cells in Traf6-ΔT;Malt1 PD-T double mutant mice, inactivation of the MALT1 protease is unable to cause autoinflammation, because the Tconv effector cells are not activated in the absence of TRAF6. Consequentially, combined MALT1 paracaspase inactivation and TRAF6 deficiency in T cells mirrors the immunodeficiency seen upon T cell-specific MALT1 ablation.

The dynamic dysregulated network identifies stage-specific markers during lung adenocarcinoma malignant progression and metastasis

Brain metastasis occurs in approximately 30% of patients with lung adenocarcinoma (LUAD) and is closely associated with poor prognosis, recurrence, and death. However, dynamic gene regulation and molecular mechanism driving LUAD progression remain poorly understood. In this study, we performed a comprehensive single-cell transcriptome analysis using data from normal, early stage, advanced stage, and brain metastasis LUAD. Our single-cell-level analysis reveals the cellular composition heterogeneity at different stages during LUAD progression. We identified stage-specific risk genes that could contribute to LUAD progression and metastasis by reprogramming immune-related and metabolic-related functions. We constructed an early advanced metastatic dysregulated network and revealed the dynamic changes in gene regulations during LUAD progression. We identified 6 early advanced (HLA-DRB1, HLA-DQB1, SFTPB, SFTPC, PLA2G1B, and FOLR1), 8 advanced metastasis (RPS15, RPS11, RPL13A, RPS24, HLA-DRB5, LYPLA1, KCNJ15, and PSMA3), and 2 common risk genes in different stages (SFTPD and HLA-DRA) as prognostic markers in LUAD. Particularly, decreased expression of HLA-DRA, HLA-DRB1, HLA-DQB1, and HLA-DRB5 refer poor prognosis in LUAD by controlling antigen processing and presentation and T cell activation. Increased expression of PSMA3 and LYPLA1 refer poor prognosis by reprogramming fatty acid metabolism and RNA catabolic process. Our findings will help further understanding the pathobiology of brain metastases in LUAD.

Combining precision oncology and immunotherapy by targeting the MALT1 protease

An innovative strategy for cancer therapy is to combine the inhibition of cancer cell-intrinsic oncogenic signaling with cancer cell-extrinsic immunological activation of the tumor microenvironment (TME). In general, such approaches will focus on two or more distinct molecular targets in the malignant cells and in cells of the surrounding TME. In contrast, the protease Mucosa-associated lymphoid tissue protein 1 (MALT1) represents a candidate to enable such a dual approach by engaging only a single target. Originally identified and now in clinical trials as a lymphoma drug target based on its role in the survival and proliferation of malignant lymphomas addicted to chronic B cell receptor signaling, MALT1 proteolytic activity has recently gained additional attention through reports describing its tumor-promoting roles in several types of non-hematological solid cancer, such as breast cancer and glioblastoma. Besides cancer cells, regulatory T (Treg) cells in the TME are particularly dependent on MALT1 to sustain their immune-suppressive functions, and MALT1 inhibition can selectively reprogram tumor-infiltrating Treg cells into Foxp3-expressing proinflammatory antitumor effector cells. Thereby, MALT1 inhibition induces local inflammation in the TME and synergizes with anti-PD-1 checkpoint blockade to induce antitumor immunity and facilitate tumor control or rejection. This new concept of boosting tumor immunotherapy in solid cancer by MALT1 precision targeting in the TME has now entered clinical evaluation. The dual effects of MALT1 inhibitors on cancer cells and immune cells therefore offer a unique opportunity for combining precision oncology and immunotherapy to simultaneously impair cancer cell growth and neutralize immunosuppression in the TME. Further, MALT1 targeting may provide a proof of concept that modulation of Treg cell function in the TME represents a feasible strategy to augment the efficacy of cancer immunotherapy. Here, we review the role of MALT1 protease in physiological and oncogenic signaling, summarize the landscape of tumor indications for which MALT1 is emerging as a therapeutic target, and consider strategies to increase the chances for safe and successful use of MALT1 inhibitors in cancer therapy.

Porcine Reproductive and Respiratory Syndrome Virus Adapts Antiviral Innate Immunity via Manipulating MALT1

To fulfill virus replication and persistent infection in hosts, viruses have to find ways to compromise innate immunity, including timely impedance on antiviral RNases and inflammatory responses. Porcine reproductive and respiratory syndrome virus (PRRSV) is a major swine pathogen causing immune suppression. MALT1 is a central immune regulator in both innate and adaptive immunity. In this study, MALT1 was confirmed to be induced rapidly upon PRRSV infection and mediate the degradation of two anti-PRRSV RNases, MCPIP1 and N4BP1, relying on its proteolytic activity, consequently facilitating PRRSV replication. Multiple PRRSV nsps, including nsp11, nsp7β, and nsp4, contributed to MALT1 elicitation. Interestingly, the elevated expression of MALT1 began to decrease once intracellular viral expression reached a high enough level. Higher infection dose brought earlier MALT1 inflection. Further, PRRSV nsp6 mediated significant MALT1 degradation via ubiquitination-proteasome pathway. Downregulation of MALT1 suppressed NF-κB signals, leading to the decrease in proinflammatory cytokine expression. In conclusion, MALT1 expression was manipulated by PRRSV in an elaborate manner to antagonize precisely the antiviral effects of host RNases without excessive and continuous activation of inflammatory responses. These findings throw light on the machinery of PRRSV to build homeostasis in infected immune system for viral settlement. IMPORTANCE PRRSV is a major swine pathogen, suppresses innate immunity, and causes persistent infection and coinfection with other pathogens. As a central immune mediator, MALT1 plays essential roles in regulating immunity and inflammation. Here, PRRSV was confirmed to manipulate MALT1 expression in an accurate way to moderate the antiviral immunity. Briefly, multiple PRRSV nsps induced MALT1 protease to antagonize anti-PRRSV RNases N4BP1 and MCPIP1 upon infection, thereby facilitating viral replication. In contrast, PRRSV nsp6 downregulated MALT1 expression via ubiquitination-proteasome pathway to suppress the inflammatory responses upon infection aggravation, contributing to immune defense alleviation and virus survival. These findings revealed the precise expression control on MALT1 by PRRSV for antagonizing antiviral RNases, along with recovering immune homeostasis. For the first time, this study enlightens a new mechanism of PRRSV adapting antiviral innate immunity by modulating MALT1 expression.

CARD11 signaling in regulatory T cell development and function

Regulatory T cells (Tregs) are a critical subset of CD4 T cells that modulate the immune response to prevent autoimmunity and chronic inflammation. CARD11, a signaling hub and scaffold protein that links antigen receptor engagement to activation of NF-κB and other downstream signaling pathways, is essential for the development and function of thymic Tregs. Mouse models with deficiencies in CARD11 and CARD11-associated signaling components generally have Treg defects, but some mouse models develop overt autoimmunity and inflammatory disease whereas others do not. Inhibition of CARD11 signaling in Tregs within the tumor microenvironment can potentially promote anti-tumor immunity. In this review, we summarize evidence for the involvement of CARD11 signaling in Treg development and function and discuss key unanswered questions and future research opportunities.

TRAF6 prevents fatal inflammation by homeostatic suppression of MALT1 protease

[Figure: see text].

MALT1 protease function in regulatory T cells induces MYC activity to promote mitochondrial function and cellular expansion

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.

Cyclin D2 overexpression drives B1a-derived MCL-like lymphoma in mice

Mantle cell lymphoma (MCL) is an aggressive B cell lymphoma with poor long-term overall survival. Currently, MCL research and development of potential cures is hampered by the lack of good in vivo models. MCL is characterized by recurrent translocations of CCND1 or CCND2, resulting in overexpression of the cell cycle regulators cyclin D1 or D2, respectively. Here, we show, for the first time, that hematopoiesis-specific activation of cyclin D2 is sufficient to drive murine MCL-like lymphoma development. Furthermore, we demonstrate that cyclin D2 overexpression can synergize with loss of p53 to form aggressive and transplantable MCL-like lymphomas. Strikingly, cyclin D2-driven lymphomas display transcriptional, immunophenotypic, and functional similarities with B1a B cells. These MCL-like lymphomas have B1a-specific B cell receptors (BCRs), show elevated BCR and NF-κB pathway activation, and display increased MALT1 protease activity. Finally, we provide preclinical evidence that inhibition of MALT1 protease activity, which is essential for the development of early life-derived B1a cells, can be an effective therapeutic strategy to treat MCL.

MALT1 positively correlates with Th1 cells, Th17 cells, and their secreted cytokines and also relates to disease risk, severity, and prognosis of acute ischemic stroke

Background

This study aimed to explore the association of mucosa‐associated lymphoid tissue lymphoma translocation protein 1 (MALT1) with acute ischemic stroke (AIS) risk and also to explore its association with T helper type 1 (Th1) cells, Th17 cells, disease severity, and prognosis in AIS patients.

Methods

One hundred twenty first‐episode AIS patients and 120 non‐AIS patients with high‐stroke‐risk factors (as controls) were recruited. Besides, in the cluster of differentiation 4‐positive (CD4⁺) T cells, the MALT1 gene expression was detected by reverse transcription quantitative polymerase chain reaction; meanwhile, Th1 and Th17 were detected by flow cytometry. Moreover, serum interferon (IFN)‐γ and interleukin (IL)‐17 were determined by enzyme‐linked immunosorbent assay.

Results

MALT1 expression was increased in AIS patients compared with controls and also it could differentiate AIS patients from controls, with an area under curve of 0.905 (95% confidence interval: 0.869–0.941). In AIS patients, MALT1 positively correlated with Th1 cells, Th17 cells, IFN‐γ, and IL‐17. Besides, MALT1 positively correlated with the National Institutes of Health Stroke Scale score. Furthermore, the Kaplan‐Meier curve and univariate Cox's regression analyses showed no correlation of MALT1 high expression with recurrence‐free survival (RFS) in AIS patients, although after adjustment using multivariant Cox's regression, high MALT1 expression independently correlated with worse RFS in AIS patients.

Conclusion

MALT1 expression is increased and positively correlates with disease severity, Th1 cells, and Th17 cells, whose high expression severs as an independent risk factor for worse RFS in AIS patients.

Candida Administration in Bilateral Nephrectomy Mice Elevates Serum (1→3)-β-D-glucan That Enhances Systemic Inflammation Through Energy Augmentation in Macrophages

Systemic inflammation, from gut translocation of organismal molecules, might worsen uremic complications in acute kidney injury (AKI). The monitoring of gut permeability integrity and/or organismal molecules in AKI might be clinically beneficial. Due to the less prominence of Candida albicans in human intestine compared with mouse gut, C. albicans were orally administered in bilateral nephrectomy (BiN) mice. Gut dysbiosis, using microbiome analysis, and gut permeability defect (gut leakage), which was determined by fluorescein isothiocyanate-dextran and intestinal tight-junction immunofluorescent staining, in mice with BiN-Candida was more severe than BiN without Candida. Additionally, profound gut leakage in BiN-Candida also resulted in gut translocation of lipopolysaccharide (LPS) and (1→3)-β-D-glucan (BG), the organismal components from gut contents, that induced more severe systemic inflammation than BiN without Candida. The co-presentation of LPS and BG in mouse serum enhanced inflammatory responses. As such, LPS with Whole Glucan Particle (WGP, a representative BG) induced more severe macrophage responses than LPS alone as determined by supernatant cytokines and gene expression of downstream signals (NFκB, Malt-1 and Syk). Meanwhile, WGP alone did not induced the responses. In parallel, WGP (with or without LPS), but not LPS alone, accelerated macrophage ATP production (extracellular flux analysis) through the upregulation of genes in mitochondria and glycolysis pathway (using RNA sequencing analysis), without the induction of cell activities. These data indicated a WGP pre-conditioning effect on cell energy augmentation. In conclusion, Candida in BiN mice accelerated gut translocation of BG that augmented cell energy status and enhanced pro-inflammatory macrophage responses. Hence, gut fungi and BG were associated with the enhanced systemic inflammation in acute uremia.

Identification of MALT1 feedback mechanisms enables rational design of potent antilymphoma regimens for ABC-DLBCL

MALT1 inhibitors are promising therapeutic agents for B-cell lymphomas that are dependent on constitutive or aberrant signaling pathways. However, a potential limitation for signal transduction-targeted therapies is the occurrence of feedback mechanisms that enable escape from the full impact of such drugs. Here, we used a functional genomics screen in activated B-cell-like (ABC) diffuse large B-cell lymphoma (DLBCL) cells treated with a small molecule irreversible inhibitor of MALT1 to identify genes that might confer resistance or enhance the activity of MALT1 inhibition (MALT1i). We find that loss of B-cell receptor (BCR)- and phosphatidylinositol 3-kinase (PI3K)-activating proteins enhanced sensitivity, whereas loss of negative regulators of these pathways (eg, TRAF2, TNFAIP3) promoted resistance. These findings were validated by knockdown of individual genes and a combinatorial drug screen focused on BCR and PI3K pathway-targeting drugs. Among these, the most potent combinatorial effect was observed with PI3Kδ inhibitors against ABC-DLBCLs in vitro and in vivo, but that led to an adaptive increase in phosphorylated S6 and eventual disease progression. Along these lines, MALT1i promoted increased MTORC1 activity and phosphorylation of S6K1-T389 and S6-S235/6, an effect that was only partially blocked by PI3Kδ inhibition in vitro and in vivo. In contrast, simultaneous inhibition of MALT1 and MTORC1 prevented S6 phosphorylation, yielded potent activity against DLBCL cell lines and primary patient specimens, and resulted in more profound tumor regression and significantly improved survival of ABC-DLBCLs in vivo compared with PI3K inhibitors. These findings provide a basis for maximal therapeutic impact of MALT1 inhibitors in the clinic, by disrupting feedback mechanisms that might otherwise limit their efficacy.

Mucosa-Associated Lymphoid Tissue Lymphoma Translocation 1 Inhibitor as a Novel Therapeutic Tool for Lung Injury

Pulmonary fibrosis is a progressive disease characterized by lung remodeling due to excessive deposition of extracellular matrix. In this study, the bleomycin experimental model of pulmonary fibrosis was employed to investigate the anti-fibrotic and immunomodulatory activity of the inhibition of MALT1 protease activity. Mice received a single intra-tracheal administration of bleomycin (1 mg/kg) in the presence or absence of MI-2, a selective MALT1 inhibitor, (a dose of 30 mg/kg administered intra-peritoneally 1 h after bleomycin and daily until the end of the experiment). Seven days after bleomycin instillation mice were sacrificed and bronchoalveolar lavage fluid analysis, measurement of collagen content in the lung, histology, molecular analysis and immunohistochemistry were performed. To evaluate mortality and body weight gain a subset of mice was administered daily with MI-2 for 21 days. Mice that received MI-2 showed decreased weight loss and mortality, inflammatory cells infiltration, cytokines overexpression and tissue injury. Moreover, biochemical and immunohistochemical analysis displayed that MI-2 was able to modulate the excessive production of reactive oxygen species and the inflammatory mediator upregulation induced by bleomycin instillation. Additionally, MI-2 demonstrated anti-fibrotic activity by reducing transforming growth factor-β (TGF-β), α-smooth muscle actin (α-SMA) and receptor associated factor 6 (TRAF6) expression. The underlying mechanisms for the protective effect of MI-2 bleomycin induced pulmonary fibrosis may be attributed to its inhibition on NF-κB pathway. This is the first report showing the therapeutic role of MALT1 inhibition in a bleomycin model of pulmonary fibrosis, thus supporting further preclinical and clinical studies.

Therapeutic Potential of Targeting Malt1-Dependent TCR Downstream Signaling to Promote the Survival of MHC-Mismatched Allografts

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.

Long-Term MALT1 Inhibition in Adult Mice Without Severe Systemic Autoimmunity

The protease MALT1 is a key regulator of NF-κB signaling and a novel therapeutic target in autoimmunity and cancer. Initial enthusiasm supported by preclinical results with MALT1 inhibitors was tempered by studies showing that germline MALT1 protease inactivation in mice results in reduced regulatory T cells and lethal multi-organ inflammation due to expansion of IFN-γ-producing T cells. However, we show that long-term MALT1 inactivation, starting in adulthood, is not associated with severe systemic inflammation, despite reduced regulatory T cells. In contrast, IL-2-, TNF-, and IFN-γ-producing CD4⁺ T cells were strongly reduced. Limited formation of tertiary lymphoid structures was detectable in lungs and stomach, which did not affect overall health. Our data illustrate that MALT1 inhibition in prenatal or adult life has a different outcome and that long-term MALT1 inhibition in adulthood is not associated with severe side effects.

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Inducible MALT1 inactivation for up to 6 months in the absence of severe toxicity

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MALT1 inactivation in adult mice decreases Tregs without effector T cell activation

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Long-term MALT1 inactivation results in tertiary lymphoid structure formation

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MALT1 inhibition in prenatal or adult life has a different outcome

A MALT1 inhibitor suppresses human myeloid DC, effector T-cell and B-cell responses and retains Th1/regulatory T-cell homeostasis

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.

RNA Recognition and Immunity-Innate Immune Sensing and Its Posttranscriptional Regulation Mechanisms

RNA acts as an immunostimulatory molecule in the innate immune system to activate nucleic acid sensors. It functions as an intermediate, conveying genetic information to control inflammatory responses. A key mechanism for RNA sensing is discriminating self from non-self nucleic acids to initiate antiviral responses reliably, including the expression of type I interferon (IFN) and IFN-stimulated genes. Another important aspect of the RNA-mediated inflammatory response is posttranscriptional regulation of gene expression, where RNA-binding proteins (RBPs) have essential roles in various RNA metabolisms, including splicing, nuclear export, modification, and translation and mRNA degradation. Recent evidence suggests that the control of mRNA stability is closely involved in signal transduction and orchestrates immune responses. In this study, we review the current understanding of how RNA is sensed by host RNA sensing machinery and discuss self/non-self-discrimination in innate immunity focusing on mammalian species. Finally, we discuss how posttranscriptional regulation by RBPs shape immune reactions.

MALT1 Protease Plays a Dual Role in the Allergic Response by Acting in Both Mast Cells and Endothelial Cells

The signaling protein MALT1 plays a key role in promoting NF-κB activation in Ag-stimulated lymphocytes. In this capacity, MALT1 has two functions, acting as a scaffolding protein and as a substrate-specific protease. MALT1 is also required for NF-κB-dependent induction of proinflammatory cytokines after FcεR1 stimulation in mast cells, implicating a role in allergy. Because MALT1 remains understudied in this context, we sought to investigate how MALT1 proteolytic activity contributes to the overall allergic response. We compared bone marrow-derived mast cells from MALT1 knockout (MALT1^-/-) and MALT1 protease-deficient (MALT^PD/PD) mice to wild-type cells. We found that MALT1^-/- and MALT1^PD/PD mast cells are equally impaired in cytokine production following FcεRI stimulation, indicating that MALT1 scaffolding activity is insufficient to drive the cytokine response and that MALT1 protease activity is essential. In addition to cytokine production, acute mast cell degranulation is a critical component of allergic response. Intriguingly, whereas degranulation is MALT1-independent, MALT1^PD/PD mice are protected from vascular edema induced by either passive cutaneous anaphylaxis or direct challenge with histamine, a major granule component. This suggests a role for MALT1 protease activity in endothelial cells targeted by mast cell-derived vasoactive substances. Indeed, we find that in human endothelial cells, MALT1 protease is activated following histamine treatment and is required for histamine-induced permeability. We thus propose a dual role for MALT1 protease in allergic response, mediating 1) IgE-dependent mast cell cytokine production, and 2) histamine-induced endothelial permeability. This dual role indicates that therapeutic inhibitors of MALT1 protease could work synergistically to control IgE-mediated allergic disease.

Pharmacological Inhibition of MALT1 Protease Leads to a Progressive IPEX-Like Pathology

Genetic disruption or short-term pharmacological inhibition of MALT1 protease is effective in several preclinical models of autoimmunity and B cell malignancies. Despite these protective effects, the severe reduction in regulatory T cells (Tregs) and the associated IPEX-like pathology occurring upon congenital disruption of the MALT1 protease in mice has raised concerns about the long-term safety of MALT1 inhibition. Here we describe the results of a series of toxicology studies in rat and dog species using MLT-943, a novel potent and selective MALT1 protease inhibitor. While MLT-943 effectively prevented T cell-dependent B cell immune responses and reduced joint inflammation in the collagen-induced arthritis rat pharmacology model, in both preclinical species, pharmacological inhibition of MALT1 was associated with a rapid and dose-dependent reduction in Tregs and resulted in the progressive appearance of immune abnormalities and clinical signs of an IPEX-like pathology. At the 13-week time point, rats displayed severe intestinal inflammation associated with mast cell activation, high serum IgE levels, systemic T cell activation and mononuclear cell infiltration in multiple tissues. Importantly, using thymectomized rats we demonstrated that MALT1 protease inhibition affects peripheral Treg frequency independently of effects on thymic Treg output and development. Our data confirm the therapeutic potential of MALT1 protease inhibitors but highlight the safety risks and challenges to consider before potential application of such inhibitors into the clinic.

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 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.

Requirement of Mucosa-Associated Lymphoid Tissue Lymphoma Translocation Protein 1 Protease Activity for Fcγ Receptor-Induced Arthritis, but Not Fcγ Receptor-Mediated Platelet Elimination, in Mice

OBJECTIVE

Fcγ receptors (FcγR) play important roles in both protective and pathogenic immune responses. The assembly of the CBM signalosome encompassing caspase recruitment domain-containing protein 9, B cell CLL/lymphoma 10, and mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT-1) is required for optimal FcγR-induced canonical NF-κB activation and proinflammatory cytokine release. This study was undertaken to clarify the relevance of MALT-1 protease activity in FcγR-driven events and evaluate the therapeutic potential of selective MALT-1 protease inhibitors in FcγR-mediated diseases.

METHODS

Using genetic and pharmacologic disruption of MALT-1 scaffolding and enzymatic activity, we assessed the relevance of MALT-1 function in murine and human primary myeloid cells upon stimulation with immune complexes (ICs) and in murine models of autoantibody-driven arthritis and immune thrombocytopenic purpura (ITP).

RESULTS

MALT-1 protease function is essential for optimal FcγR-induced production of proinflammatory cytokines by various murine and human myeloid cells stimulated with ICs. In contrast, MALT-1 protease inhibition did not affect the Syk-dependent, FcγR-mediated production of reactive oxygen species or leukotriene B₄ . Notably, pharmacologic MALT-1 protease inhibition in vivo reduced joint inflammation in the murine K/BxN serum-induced arthritis model (mean area under the curve for paw swelling of 45.42% versus 100% in control mice; P = 0.0007) but did not affect platelet depletion in a passive model of ITP.

CONCLUSION

Our findings indicate a specific contribution of MALT-1 protease activity to FcγR-mediated events and suggest that MALT-1 protease inhibitors have therapeutic potential in a subset of FcγR-driven inflammatory disorders.

Allosteric activation of MALT1 by its ubiquitin-binding Ig3 domain

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.

Physiological and Pathological Functions of CARD9 Signaling in the Innate Immune System

Caspase recruitment domain protein 9 (CARD9) forms essential signaling complexes in the innate immune system that integrate cues from C-type lectin receptors and specific intracellular pattern recognition receptors. These CARD9-mediated signals are pivotal for host defense against fungi, and they mediate immunity against certain bacteria, viruses and parasites. Furthermore, CARD9-regulated pathways are involved in sterile inflammatory responses critical for immune homeostasis and can control pro- and antitumor immunity in cancer microenvironments. Consequently, multiple genetic alterations of human CARD9 are connected to primary immunodeficiencies or prevalent inflammatory disorders in patients. This review will summarize our current understanding of CARD9 signaling in the innate immune system, its physiological and pathological functions and their implications for human immune-mediated diseases.

Malt1 Protease Deficiency in Mice Disrupts Immune Homeostasis at Environmental Barriers and Drives Systemic T Cell-Mediated Autoimmunity

The paracaspase Malt1 is a key regulator of canonical NF-κB activation downstream of multiple receptors in both immune and nonimmune cells. Genetic disruption of Malt1 protease function in mice and MALT1 mutations in humans results in reduced regulatory T cells and a progressive multiorgan inflammatory pathology. In this study, we evaluated the altered immune homeostasis and autoimmune disease in Malt1 protease-deficient (Malt1PD) mice and the Ags driving disease manifestations. Our data indicate that B cell activation and IgG1/IgE production is triggered by microbial and dietary Ags preferentially in lymphoid organs draining mucosal barriers, likely as a result of dysregulated mucosal immune homeostasis. Conversely, the disease was driven by a polyclonal T cell population directed against self-antigens. Characterization of the Malt1PD T cell compartment revealed expansion of T effector memory cells and concomitant loss of a CD4⁺ T cell population that phenotypically resembles anergic T cells. Therefore, we propose that the compromised regulatory T cell compartment in Malt1PD animals prevents the efficient maintenance of anergy and supports the progressive expansion of pathogenic, IFN-γ-producing T cells. Overall, our data revealed a crucial role of the Malt1 protease for the maintenance of intestinal and systemic immune homeostasis, which might provide insights into the mechanisms underlying IPEX-related diseases associated with mutations in MALT1.

MALT1-Deficient Mice Develop Atopic-Like Dermatitis Upon Aging

MALT1 plays an important role in innate and adaptive immune signaling by acting as a scaffold protein that mediates NF-κB signaling. In addition, MALT1 is a cysteine protease that further fine tunes proinflammatory signaling by cleaving specific substrates. Deregulated MALT1 activity has been associated with immunodeficiency, autoimmunity, and cancer in mice and humans. Genetically engineered mice expressing catalytically inactive MALT1, still exerting its scaffold function, were previously shown to spontaneously develop autoimmunity due to a decrease in Tregs associated with increased effector T cell activation. In contrast, complete absence of MALT1 does not lead to autoimmunity, which has been explained by the impaired effector T cell activation due to the absence of MALT1-mediated signaling. However, here we report that MALT1-deficient mice develop atopic-like dermatitis upon aging, which is preceded by Th2 skewing, an increase in serum IgE, and a decrease in Treg frequency and surface expression of the Treg functionality marker CTLA-4.

CD16+ monocytes give rise to CD103+RALDH2+TCF4+ dendritic cells with unique transcriptional and immunological features

Classical CD16^- vs intermediate/nonclassical CD16⁺ monocytes differ in their homing potential and biological functions, but whether they differentiate into dendritic cells (DCs) with distinct contributions to immunity against bacterial/viral pathogens remains poorly investigated. Here, we employed a systems biology approach to identify clinically relevant differences between CD16⁺ and CD16^- monocyte-derived DCs (MDDCs). Although both CD16⁺ and CD16^- MDDCs acquire classical immature/mature DC markers in vitro, genome-wide transcriptional profiling revealed unique molecular signatures for CD16⁺ MDDCs, including adhesion molecules (ITGAE/CD103), transcription factors (TCF7L2/TCF4), and enzymes (ALDH1A2/RALDH2), whereas CD16^- MDDCs exhibit a CDH1/E-cadherin⁺ phenotype. Of note, lipopolysaccharides (LPS) upregulated distinct transcripts in CD16⁺ (eg, CCL8, SIGLEC1, MIR4439, SCIN, interleukin [IL]-7R, PLTP, tumor necrosis factor [TNF]) and CD16^- MDDCs (eg, MMP10, MMP1, TGM2, IL-1A, TNFRSF11A, lysosomal-associated membrane protein 1, MMP8). Also, unique sets of HIV-modulated genes were identified in the 2 subsets. Further gene set enrichment analysis identified canonical pathways that pointed to "inflammation" as the major feature of CD16⁺ MDDCs at immature stage and on LPS/HIV exposure. Finally, functional validations and meta-analysis comparing the transcriptome of monocyte and MDDC subsets revealed that CD16⁺ vs CD16^- monocytes preserved their superior ability to produce TNF-α and CCL22, as well as other sets of transcripts (eg, TCF4), during differentiation into DC. These results provide evidence that monocyte subsets are transcriptionally imprinted/programmed with specific differentiation fates, with intermediate/nonclassical CD16⁺ monocytes being precursors for pro-inflammatory CD103⁺RALDH2⁺TCF4⁺ DCs that may play key roles in mucosal immunity homeostasis/pathogenesis. Thus, alterations in the CD16⁺ /CD16^- monocyte ratios during pathological conditions may dramatically influence the quality of MDDC-mediated immunity.

MALT1 Proteolytic Activity Suppresses Autoimmunity in a T Cell Intrinsic Manner

MALT1 is a central signaling component in innate and adaptive immunity by regulating NF-κB and other key signaling pathways in different cell types. Activities of MALT1 are mediated by its scaffold and protease functions. Because of its role in lymphocyte activation and proliferation, inhibition of MALT1 proteolytic activity is of high interest for therapeutic targeting in autoimmunity and certain lymphomas. However, recent studies showing that Malt1 protease-dead knock-in (Malt1-PD) mice suffer from autoimmune disease have somewhat tempered the initial enthusiasm. Although it has been proposed that an imbalance between immune suppressive regulatory T cells (Tregs) and activated effector CD4+ T cells plays a key role in the autoimmune phenotype of Malt1-PD mice, the specific contribution of MALT1 proteolytic activity in T cells remains unclear. Using T cell-conditional Malt1 protease-dead knock-in (Malt1-PDT) mice, we here demonstrate that MALT1 has a T cell-intrinsic role in regulating the homeostasis and function of thymic and peripheral T cells. T cell-specific ablation of MALT1 proteolytic activity phenocopies mice in which MALT1 proteolytic activity has been genetically inactivated in all cell types. The Malt1-PDT mice have a reduced number of Tregs in the thymus and periphery, although the effect in the periphery is less pronounced compared to full-body Malt1-PD mice, indicating that also other cell types may promote Treg induction in a MALT1 protease-dependent manner. Despite the difference in peripheral Treg number, both T cell-specific and full-body Malt1-PD mice develop ataxia and multi-organ inflammation to a similar extent. Furthermore, reconstitution of the full-body Malt1-PD mice with T cell-specific expression of wild-type human MALT1 eliminated all signs of autoimmunity. Together, these findings establish an important T cell-intrinsic role of MALT1 proteolytic activity in the suppression of autoimmune responses.

Bcl10-controlled Malt1 paracaspase activity is key for the immune suppressive function of regulatory T cells

Regulatory T cells (Tregs) have crucial functions in the inhibition of immune responses. Their development and suppressive functions are controlled by the T cell receptor (TCR), but the TCR signaling mechanisms that mediate these effects remain ill-defined. Here we show that CARD11-BCL10-MALT1 (CBM) signaling mediates TCR-induced NF-κB activation in Tregs and controls the conversion of resting Tregs to effector Tregs under homeostatic conditions. However, in inflammatory milieus, cytokines can bypass the CBM requirement for this differentiation step. By contrast, CBM signaling, in a MALT1 protease-dependent manner, is essential for mediating the suppressive function of Tregs. In malignant melanoma models, acute genetic blockade of BCL10 signaling selectively in Tregs or pharmacological MALT1 inhibition enhances anti-tumor immune responses. Together, our data uncover a segregation of Treg differentiation and suppressive function at the CBM complex level, and provide a rationale to explore MALT1 inhibitors for cancer immunotherapy.

An allosteric MALT1 inhibitor is a molecular corrector rescuing function in an immunodeficient patient

MALT1 paracaspase is central for lymphocyte antigen-dependent responses including NF-κB activation. We discovered nanomolar, selective allosteric inhibitors of MALT1 that bind by displacing the side chain of Trp580, locking the protease in an inactive conformation. Interestingly, we had previously identified a patient homozygous for a MALT1 Trp580-to-serine mutation who suffered from combined immunodeficiency. We show that the loss of tryptophan weakened interactions between the paracaspase and C-terminal immunoglobulin MALT1 domains resulting in protein instability, reduced protein levels and functions. Upon binding of allosteric inhibitors of increasing potency, we found proportionate increased stabilization of MALT1-W580S to reach that of wild-type MALT1. With restored levels of stable MALT1 protein, the most potent of the allosteric inhibitors rescued NF-κB and JNK signaling in patient lymphocytes. Following compound washout, MALT1 substrate cleavage was partly recovered. Thus, a molecular corrector rescues an enzyme deficiency by substituting for the mutated residue, inspiring new potential precision therapies to increase mutant enzyme activity in other deficiencies.

Treatment of Dextran Sulfate Sodium-Induced Colitis with Mucosa-Associated Lymphoid Tissue Lymphoma Translocation 1 Inhibitor MI-2 Is Associated with Restoration of Gut Immune Function and the Microbiota

Disruption of the healthy intestinal microbiome and homeostasis of the intestinal immune system, which are closely interactive, are two key factors for ulcerative colitis. Here, we show that MI-2, a selective inhibitor of mucosa-associated lymphoid tissue lymphoma translocation-1 (MALT1), alleviated excessive inflammatory responses and was associated with restoration of healthy intestinal microbiome in mice suffering from dextran sulfate sodium (DSS)-induced colitis. We found that the diversity of intestinal microbiome of mice with DSS-induced colitis was significantly lower than that of healthy mice. However, MI-2 treatment in mice with DSS-induced colitis resulted in restored microbially diverse populations. To understand the possibility of the beneficial effect of the restored microbially diverse populations of MI-2-treated mice with DSS-induced colitis, we showed that inserting fecal microbiota from MI-2-treated mice with DSS-induced colitis and healthy control mice into mice with DSS-induced colitis could alleviate symptoms of colitis. The possibility of MI-2 treatment in DSS-induced colitis, associated with restoration of healthy microbially diverse populations in addition to reshaping host immune modulating capacity by reducing inflammatory cytokines (tumor necrosis factor alpha, interleukin-1β [IL-1β], IL-17α, and IL-22), may be considered therapeutic for ulcerative colitis.

Cellular metabolism constrains innate immune responses in early human ontogeny

Pathogen immune responses are profoundly attenuated in fetuses and premature infants, yet the mechanisms underlying this developmental immaturity remain unclear. Here we show transcriptomic, metabolic and polysome profiling and find that monocytes isolated from infants born early in gestation display perturbations in PPAR-γ-regulated metabolic pathways, limited glycolytic capacity and reduced ribosomal activity. These metabolic changes are linked to a lack of translation of most cytokines and of MALT1 signalosome genes essential to respond to the neonatal pathogen Candida. In contrast, they have little impact on house-keeping phagocytosis functions. Transcriptome analyses further indicate a role for mTOR and its putative negative regulator DNA Damage Inducible Transcript 4-Like in regulating these metabolic constraints. Our results provide a molecular basis for the broad susceptibility to multiple pathogens in these infants, and suggest that the fetal immune system is metabolically programmed to avoid energetically costly, dispensable and potentially harmful immune responses during ontogeny.

Little is known about developmental set points of immune responses, especially in humans. Here the authors show that the metabolic state of monocytes isolated from prematurely born infants underlies attenuated responsiveness to fungal infection via selective control of protein translation.

Post-translational Modifications of the CARMA1-BCL10-MALT1 Complex in Lymphocytes and Activated B-Cell Like Subtype of Diffuse Large B-Cell Lymphoma

Piracy of the NF-κB transcription factors signaling pathway, to sustain its activity, is a mechanism often deployed in B-cell lymphoma to promote unlimited growth and survival. The aggressive activated B-cell like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) exploits a multi-protein complex of CARMA1, BCL10, and MALT1 (CBM complex), which normally conveys NF-κB signaling upon antigen receptors engagement. Once assembled, the CBM also unleashes MALT1 protease activity to finely tune the immune response. As a result, ABC DLBCL tumors develop a profound addiction to NF-κB and to MALT1 enzyme, leaving open a breach for therapeutics. However, the pleiotropic nature of NF-κB jeopardizes the success of its targeting and urges us to develop new strategies. In this review, we discuss how post-translational modifications, such as phosphorylation and ubiquitination of the CBM components, as well as, MALT1 proteolytic activity, shape the CBM activity in lymphocytes and ABC DLBCL, and may provide new avenues to restore vulnerability in lymphoma.

The CBM-opathies-A Rapidly Expanding Spectrum of Human Inborn Errors of Immunity Caused by Mutations in the CARD11-BCL10-MALT1 Complex

The caspase recruitment domain family member 11 (CARD11 or CARMA1)-B cell CLL/lymphoma 10 (BCL10)-MALT1 paracaspase (MALT1) [CBM] signalosome complex serves as a molecular bridge between cell surface antigen receptor signaling and the activation of the NF-κB, JNK, and mTORC1 signaling axes. This positions the CBM complex as a critical regulator of lymphocyte activation, proliferation, survival, and metabolism. Inborn errors in each of the CBM components have now been linked to a diverse group of human primary immunodeficiency diseases termed "CBM-opathies." Clinical manifestations range from severe combined immunodeficiency to selective B cell lymphocytosis, atopic disease, and specific humoral defects. This surprisingly broad spectrum of phenotypes underscores the importance of "tuning" CBM signaling to preserve immune homeostasis. Here, we review the distinct clinical and immunological phenotypes associated with human CBM complex mutations and introduce new avenues for targeted therapeutic intervention.

Holding All the CARDs: How MALT1 Controls CARMA/CARD-Dependent Signaling

The scaffold proteins CARMA1-3 (encoded by the genes CARD11, -14 and -10) and CARD9 play major roles in signaling downstream of receptors with immunoreceptor tyrosine activation motifs (ITAMs), G-protein coupled receptors (GPCR) and receptor tyrosine kinases (RTK). These receptors trigger the formation of oligomeric CARMA/CARD-BCL10-MALT1 (CBM) complexes via kinases of the PKC family. The CBM in turn regulates gene expression by the activation of NF-κB and AP-1 transcription factors and controls transcript stability. The paracaspase MALT1 is the only CBM component having an enzymatic (proteolytic) activity and has therefore recently gained attention as a potential drug target. Here we review recent advances in the understanding of the molecular function of the protease MALT1 and summarize how MALT1 scaffold and protease function contribute to the transmission of CBM signals. Finally, we will highlight how dysregulation of MALT1 function can cause pathologies such as immunodeficiency, autoimmunity, psoriasis, and cancer.

The Diverse Roles of microRNAs at the Host⁻Virus Interface

MicroRNAs (miRNAs) are small, non-coding RNAs that regulate gene expression at the post-transcriptional level. Through this activity, they are implicated in almost every cellular process investigated to date. Hence, it is not surprising that miRNAs play diverse roles in regulation of viral infections and antiviral responses. Diverse families of DNA and RNA viruses have been shown to take advantage of cellular miRNAs or produce virally encoded miRNAs that alter host or viral gene expression. MiRNA-mediated changes in gene expression have been demonstrated to modulate viral replication, antiviral immune responses, viral latency, and pathogenesis. Interestingly, viruses mediate both canonical and non-canonical interactions with miRNAs to downregulate specific targets or to promote viral genome stability, translation, and/or RNA accumulation. In this review, we focus on recent findings elucidating several key mechanisms employed by diverse virus families, with a focus on miRNAs at the host–virus interface during herpesvirus, polyomavirus, retroviruses, pestivirus, and hepacivirus infections.

Specific covalent inhibition of MALT1 paracaspase suppresses B cell lymphoma growth

The paracaspase MALT1 plays an essential role in activated B cell-like diffuse large B cell lymphoma (ABC DLBCL) downstream of B cell and TLR pathway genes mutated in these tumors. Although MALT1 is considered a compelling therapeutic target, the development of tractable and specific MALT1 protease inhibitors has thus far been elusive. Here, we developed a target engagement assay that provides a quantitative readout for specific MALT1-inhibitory effects in living cells. This enabled a structure-guided medicinal chemistry effort culminating in the discovery of pharmacologically tractable, irreversible substrate-mimetic compounds that bind the MALT1 active site. We confirmed that MALT1 targeting with compound 3 is effective at suppressing ABC DLBCL cells in vitro and in vivo. We show that a reduction in serum IL-10 levels exquisitely correlates with the drug pharmacokinetics and degree of MALT1 inhibition in vitro and in vivo and could constitute a useful pharmacodynamic biomarker to evaluate these compounds in clinical trials. Compound 3 revealed insights into the biology of MALT1 in ABC DLBCL, such as the role of MALT1 in driving JAK/STAT signaling and suppressing the type I IFN response and MHC class II expression, suggesting that MALT1 inhibition could prime lymphomas for immune recognition by cytotoxic immune cells.

Autocleavage of the paracaspase MALT1 at Arg-781 attenuates NF-κB signaling and regulates the growth of activated B-cell like diffuse large B-cell lymphoma cells

MALT1 controls several receptors-mediated signaling to nuclear factor κB (NF-κB) through both its scaffold and protease function. MALT1 protease activity is shown to inactivate several negative regulators of NF-κB signaling and augment NF-κB activation ability. In this study, MALT1 was demonstrated to autoprocess itself in the presence of oligomerization-competent BCL10. Cleavage occurred after Arginine 781 located in the C-terminus of MALT1. Shortened MALT1 cleavage products showed attenuated binding ability with TRAF6. Its NF-κB activation ability was also weakened. Various MALT1 constructs including wild type, catalytically-inactive (MALT1_C464A), cleavage-defective (MALT1_R781L), or truncated (MALT1_1–781) form of MALT1 was introduced into MALT1-knocked-down-Jurkat T cells. Cleavage-defective MALT1_R781L retained its proteolytic and initial IκBα phosphorylation activity as MALT1. Truncated MALT1_1–781 mutant showed weakness in IκBα phosphorylation and the expression of NF-κB targets IL-2 and IFN-γ. Cleavage at R781 was detectable but marginal after activation with TPA/ionomycin or anti-CD3 antibody in lymphocytes. However, cleavage at R781 was evident in ABC-DLBCL cells such as OCI-Ly3, HBL-1. HBL-1 cells with induced expression of catalytically-inactive MALT1_C464A or cleavage-defective MALT1_R781L exhibited characteristic of retarded-growth. These findings suggested that cleavage at R781 of MALT1 played a role in the survival of ABC-DLBCL cells.

Central role of myeloid MCPIP1 in protecting against LPS-induced inflammation and lung injury

Although systemic inflammatory responses attributable to infection may lead to significant lung injury, the precise molecular mechanisms leading to lung damage are poorly understood and therapeutic options remain limited. Here, we show that myeloid monocyte chemotactic protein-inducible protein 1 (MCPIP1) plays a central role in protecting against LPS-induced inflammation and lung injury. Myeloid-specific MCPIP1 knockout mice developed spontaneous inflammatory syndromes, but at a late age compared to global MCPIP1 knockout mice. Moreover, mice with a myeloid-specific deletion of MCPIP1 were extremely sensitive to LPS-induced lung injury due to overproduction of proinflammatory cytokines and chemokines. We identified C/EBPβ and C/EBPδ, two critical transcriptional factors that drive cytokine production and lung injury, as targets of MCPIP1 RNase. LPS administration caused MCPIP1 protein degradation in the lungs. Pharmacological inhibition of MALT1, a paracaspase that cleaves MCPIP1, by MI-2 selectively increased the MCPIP1 protein levels in macrophages and in the lungs. Meanwhile, administration of MI-2 protected mice from LPS-induced inflammation, lung injury and death. Collectively, these results indicate that myeloid MCPIP1 is central in controlling LPS-induced inflammation and lung injury. Pharmacological inhibition of MALT1 protease activity may be a good strategy to treat inflammatory diseases by enhancing MCPIP1 expression in myeloid cells.

MALT1 Protease Activation Triggers Acute Disruption of Endothelial Barrier Integrity via CYLD Cleavage

Microvascular endothelial cells maintain a tight barrier to prevent passage of plasma and circulating immune cells into the extravascular tissue compartment, yet endothelial cells respond rapidly to vasoactive substances, including thrombin, allowing transient paracellular permeability. This response is a cornerstone of acute inflammation, but the mechanisms responsible are still incompletely understood. Here, we demonstrate that thrombin triggers MALT1 to proteolytically cleave cylindromatosis (CYLD). Fragmentation of CYLD results in microtubule disruption and a cascade of events leading to endothelial cell retraction and an acute permeability response. This finding reveals an unexpected role for the MALT1 protease, which previously has been viewed mostly as a driver of pro-inflammatory NF-κB signaling in lymphocytes. Thus, MALT1 not only promotes immune cell activation but also acutely regulates endothelial cell biology, actions that together facilitate tissue inflammation. Pharmacologic inhibition of MALT1 may therefore have synergistic impact by targeting multiple disparate steps in the overall inflammatory response.

Mucosa-associated lymphoid tissue lymphoma translocation 1 as a novel therapeutic target for rheumatoid arthritis

Emerging evidence suggests that mucosa-associated lymphoid tissue lymphoma translocation 1 (MALT1) is a key regulator of inflammatory diseases; however, the pathological role of MALT1 in rheumatoid arthritis (RA) is not well understood. Consequently, this protein has not been therapeutically targeted for the treatment of RA. MALT1 plays a role in the paracaspase pathway, has proteolytic activity and is involved in the regulation of inflammatory responses. In this study, we found that the MALT1-targeting inhibitory small molecule, MALT1 selective inhibitor 2-chloro-N-[4-[5-(3,4-dichlorophenyl)-3-(2-methoxyethoxy)-1H-1,2,4-triazol-1-yl]phenylacetamide (MI-2) strongly suppresses the differentiation of monocytes into osteoclasts in the absence or presence of the inflammatory cytokine tumour necrosis factor α. Furthermore, MI-2 ameliorates pathologic bone erosion and synovitis in an in vivo mouse model of collagen-induced arthritis. Mechanistically, MI-2 blocked expression of the master osteoclast regulator - nuclear factor of activated T cells 1 (NFATc1) - by inhibiting nuclear factor κB (NF-κB), which is a critical regulator of NFATc1. These findings highlight the important regulatory role of MALT1 in the NF-κB-NFATc1-signalling axis during osteoclastogenesis and suggest that targeting MALT1 is a promising treatment option for rheumatoid arthritis.

Lymphocyte signaling and activation by the CARMA1-BCL10-MALT1 signalosome

The CARMA1-BCL10-MALT1 (CBM) signalosome triggers canonical NF-κB signaling and lymphocyte activation upon antigen-receptor stimulation. Genetic studies in mice and the analysis of human immune pathologies unveiled a critical role of the CBM complex in adaptive immune responses. Great progress has been made in elucidating the fundamental mechanisms that dictate CBM assembly and disassembly. By bridging proximal antigen-receptor signaling to downstream signaling pathways, the CBM complex exerts a crucial scaffolding function. Moreover, the MALT1 subunit confers a unique proteolytic activity that is key for lymphocyte activation. Deregulated 'chronic' CBM signaling drives constitutive NF-κB signaling and MALT1 activation, which contribute to the development of autoimmune and inflammatory diseases as well as lymphomagenesis. Thus, the processes that govern CBM activation and function are promising targets for the treatment of immune disorders. Here, we summarize the current knowledge on the functions and mechanisms of CBM signaling in lymphocytes and how CBM deregulations contribute to aberrant signaling in malignant lymphomas.