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.

Deficiency of MALT1 paracaspase activity results in unbalanced regulatory and effector T and B cell responses leading to multiorgan inflammation.

The paracaspase MALT1 plays an important role in immune receptor-driven signaling pathways leading to NF-κB activation. MALT1 promotes signaling by acting as a scaffold, recruiting downstream signaling proteins, as well as by proteolytic cleavage of multiple substrates. However, the relative contributions of these two different activities to T and B cell function are not well understood. To investigate how MALT1 proteolytic activity contributes to overall immune cell regulation, we generated MALT1 protease-deficient mice (Malt1(PD/PD)) and compared their phenotype with that of MALT1 knockout animals (Malt1(-/-)). Malt1(PD/PD) mice displayed defects in multiple cell types including marginal zone B cells, B1 B cells, IL-10-producing B cells, regulatory T cells, and mature T and B cells. In general, immune defects were more pronounced in Malt1(-/-) animals. Both mouse lines showed abrogated B cell responses upon immunization with T-dependent and T-independent Ags. In vitro, inactivation of MALT1 protease activity caused reduced stimulation-induced T cell proliferation, impaired IL-2 and TNF-α production, as well as defective Th17 differentiation. Consequently, Malt1(PD/PD) mice were protected in a Th17-dependent experimental autoimmune encephalomyelitis model. Surprisingly, Malt1(PD/PD) animals developed a multiorgan inflammatory pathology, characterized by Th1 and Th2/0 responses and enhanced IgG1 and IgE levels, which was delayed by wild-type regulatory T cell reconstitution. We therefore propose that the pathology characterizing Malt1(PD/PD) animals arises from an immune imbalance featuring pathogenic Th1- and Th2/0-skewed effector responses and reduced immunosuppressive compartments. These data uncover a previously unappreciated key function of MALT1 protease activity in immune homeostasis and underline its relevance in human health and disease.

Different adaptations of IgG effector function in human and nonhuman primates and implications for therapeutic antibody treatment.

Safety of human therapeutic Abs is generally assessed in nonhuman primates. Whereas IgG1 shows identical FcγR interaction and effector function profile in both species, fundamental differences in the IgG2 and IgG4 Ab subclasses were found between the two species. Granulocytes, the main effector cells against IgG2- and IgG4-opsonized bacteria and parasites, do not express FcγRIIIb, but show higher levels of FcγRII in cynomolgus monkey. In humans, IgG2 and IgG4 adapted a silent Fc region with weak binding to FcγR and effector functions, whereas, in contrast, cynomolgus monkey IgG2 and IgG4 display strong effector function as well as differences in IgG4 Fab arm exchange. To balance this shift toward activation, the cynomolgus inhibitory FcγRIIb shows strongly increased affinity for IgG2. In view of these findings, in vitro and in vivo results for human IgG2 and IgG4 obtained in the cynomolgus monkey have to be cautiously interpreted, whereas effector function-related effects of human IgG1 Abs are expected to be predictable for humans.

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.

Myeloid Innate Signaling Pathway Regulation by MALT1 Paracaspase Activity.

Besides its function in lymphoid cells, which has been addressed by numerous studies, the paracaspase MALT1 also plays an important role in innate cells downstream of pattern recognition receptors. Best studied are the Dectin-1 and Dectin-2 members of the C-type lectin-like receptor family that induce a SYK- and CARD9-dependent signaling cascade leading to NF-κB activation, in a MALT1-dependent manner. By contrast, Toll-like receptors (TLR), such as TLR-4, propagate NF-κB activation but signal via an MYD88/IRAK-dependent cascade. Nonetheless, whether MALT1 might contribute to TLR-4 signaling has remained unclear. Recent evidence with MLT-827, a potent and selective inhibitor of MALT1 paracaspase activity, indicates that TNF- production downstream of TLR-4 in human myeloid cells is independent of MALT1, as opposed to TNF- production downstream of Dectin-1, which is MALT1 dependent. Here, we addressed the selective involvement of MALT1 in pattern recognition sensing further, using a variety of human and mouse cellular preparations, and stimulation of Dectin-1, MINCLE or TLR-4 pathways. We also provided additional insights by exploring cytokines beyond TNF-, and by comparing MLT-827 to a SYK inhibitor (Cpd11) and to an IKK inhibitor (AFN700). Collectively, the data provided further evidence for the MALT1-dependency of C-type lectin-like receptor -signaling by contrast to TLR-signaling.