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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. [ABSTRACT FROM AUTHOR]

Abstract The MALT1 (Mucosa associated lymphoid tissue lymphoma translocation protein 1) paracaspase couples antigen receptors on lymphocytes to downstream signaling events. Activation of MALT1 is known to involve stimulus-dependent CBM complex formation, that is, the recruitment of BCL10-bound MALT1 to a CARD-Coiled Coil protein. Beyond this canonical, CBM-dependent mechanism of MALT1 activation, recent studies suggest that MALT1 protease activity may be triggered by alternative mechanisms. For instance, the E3-ligase TRAF6 can activate MALT1 proteolytic function and induce MALT1 auto-cleavage. However, the interplay between CBM and TRAF6 with regard to MALT1 activation has remained incompletely elucidated. Here, by generating CRISPR/Cas9-derived knock-out Jurkat T-cells, we show that TRAF6 was dispensable for CARD11/BCL10-dependent MALT1 activation upon T-cell stimulation. However, ectopically-expressed TRAF6 could induce MALT1 activity in Jurkat T-cells devoid of either CARD11 or BCL10. These data provide unequivocal evidence that TRAF6-mediated MALT1 activation does not require the upstream scaffold CARD11 or the interaction between MALT1 and BCL10. Thus, TRAF6 may be part of a previously unidentified non-canonical pathway that triggers MALT1 protease activity independently of canonical CBM signalosomes. Highlights • TRAF6 is not strictly required for MALT1 activation downstream of CARD11/BCL10. • TRAF6 can activate MALT1 independently of CARD11 or BCL10. • MALT1 oligomerization is involved in its activation by TRAF6. • Pathways leading to TRAF6 oligomerization may broaden the range of MALT1 activation mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: Mucosa‐associated lymphoid tissue lymphoma translocation protein 1 (MALT1) is essential for immune responses triggered by antigen receptors but the contribution of its paracaspase activity is not fully understood. Here, we studied how MALT1 proteolytic function regulates T‐cell activation and fate after engagement of the T‐cell receptor pathway. We show that MLT‐827, a potent and selective MALT1 paracaspase inhibitor, does not prevent the initial phase of T‐cell activation, in contrast to the pan‐protein kinase C inhibitor AEB071. However, MLT‐827 strongly impacted cell expansion after activation. We demonstrate this is the consequence of profound inhibition of IL‐2 production as well as reduced expression of the IL‐2 receptor alpha subunit (CD25), resulting from defective canonical NF‐κB activation and accelerated mRNA turnover mechanisms. Accordingly, MLT‐827 revealed a unique transcriptional fingerprint of MALT1 protease activity, providing evidence for broad control of T‐cell signaling pathways. Altogether, this first report with a potent and selective inhibitor elucidates how MALT1 paracaspase activity integrates several T‐cell activation pathways and indirectly controls gamma‐chain receptor dependent survival, to impact on T‐cell expansion. [ABSTRACT FROM AUTHOR]

The paracaspase MALT1 has arginine-directed proteolytic activity triggered by engagement of immune receptors. Recruitment of MALT1 into activation complexes is required for MALT1 proteolytic function. Here, co-expression of MALT1 in HEK293 cells, either with activated CARD11 and BCL10 or with TRAF6, was used to explore the mechanism of MALT1 activation at the molecular level. This work identified a prominent self-cleavage site of MALT1 isoform A (MALT1A) at R781 (R770 in MALT1B) and revealed that TRAF6 can activate MALT1 independently of the CBM. Intramolecular cleavage at R781/R770 removes a C-terminal TRAF6-binding site in both MALT1 isoforms, leaving MALT1B devoid of the two key interaction sites with TRAF6. A previously identified auto-proteolysis site of MALT1 at R149 leads to deletion of the death-domain, thereby abolishing interaction with BCL10. By using MALT1 isoforms and cleaved fragments thereof, as well as TRAF6 WT and mutant forms, this work shows that TRAF6 induces N-terminal auto-proteolytic cleavage of MALT1 at R149 and accelerates MALT1 protein turnover. The MALT1 fragment generated by N-terminal self-cleavage at R149 was labile and displayed enhanced signaling properties that required an intact K644 residue, previously shown to be a site for mono-ubiquitination of MALT1. Conversely, C-terminal self-cleavage at R781/R770 hampered the ability for self-cleavage at R149 and stabilized MALT1 by hindering interaction with TRAF6. C-terminal self-cleavage had limited impact on MALT1A but severely reduced MALT1B proteolytic and signaling functions. It also abrogated NF-κB activation by N-terminally cleaved MALT1A. Altogether, this study provides further insights into mechanisms that regulate the scaffolding and activation cycle of MALT1. It also emphasizes the reduced functional capacity of MALT1B as compared to MALT1A. [ABSTRACT FROM AUTHOR]