The Bcl10-Malt1 complex segregates Fc epsilon RI-mediated nuclear factor kappa B activation and cytokine production from mast cell degranulation

Fcε- and Fcγ-receptor signaling in diseases

It has become increasingly clear that receptors for the immunoglobulin Fc region play pivotal roles in immune homeostasis and disease. This review describes the fine regulation of the high-affinity IgE-receptor (FcepsilonRI) signaling, especially focusing on the early events that are coordinately regulated by Src family protein tyrosine kinases (PTKs), FcepsilonRI beta-subunit, and membrane lipid rafts. Because allergen-mediated FcepsilonRI cross-linking leads to the synthesis and release of a variety of proinflammatory mediators and cytokines, the duration and amplitude of the signal need to be strictly controlled, and the counterbalancing signaling is provided by specialized inhibitory receptors and molecules. However, recent work have revealed that Src family PTKs and FcepsilonRI beta-subunit transduce both positive and negative signaling with unexpectedly complex mechanisms. FcgammaRIIB exerts a unique inhibitory function on cell activation processes after the engagement of Fcgamma, FcepsilonRI and B cell receptors. Recent work has shown that FcgammaRIIB polymorphisms are associated with systemic lupus erythematosus, and that a transmembrane polymorphism in FcgammaRIIB results in an impaired distribution to lipid rafts and a reduced inhibitory function. Studies addressing the functions of disease-associated polymorphisms in the FcepsilonRI beta-subunit and low-affinity FcgammaRs are also considered.

Bcl10 and Malt1 control lysophosphatidic acid-induced NF-kappaB activation and cytokine production

Lysophosphatidic acid (LPA) is a potent bioactive phospholipid that stimulates a variety of cellular responses by acting on cognate G protein-coupled receptors (GPCRs). There is increasing evidence that LPA signaling reprograms gene expression, but the GPCR-induced pathways connecting LPA receptor stimulation to downstream transcription factors are not well characterized. Here, we identify the adapter proteins Bcl10 and Malt1 as essential mediators of LPA-induced NF-kappaB activation. Both proteins were previously known to activate NF-kappaB in response to antigen receptor ligation on lymphocytes, but their functions in nonimmune cells are still largely undefined. By using murine embryonic fibroblasts from Bcl10- or Malt1-deficient mice as a genetic model, we report that Bcl10 and Malt1 are critically required for the degradation of IkappaB-alpha and the subsequent NF-kappaB induction in response to LPA stimulation. Bcl10 and Malt1 cooperate with PKCs selectively for LPA-induced NF-kappaB activation but are dispensable for the activation of the Jnk, p38, Erk MAP kinase, and Akt signaling pathways. In a biological readout, we demonstrate that LPA-induced IL-6 production is abolished in the absence of Bcl10. Thus, our results identify a NF-kappaB-inducing signaling pathway downstream of GPCRs and reveal previously unrecognized functions for Bcl10/Malt1 signaling in nonimmune cells.

Highlights in cellular and molecular mechanisms of allergic diseases. XXVth Congress of the European Academy of Allergology and Clinical Immunology in Vienna

This year, the annual congress of the European Academy of Allergology and Clinical Immunology was held on 10-14 June in Vienna. More than 6,000 delegates, practicing bench or bedside work or both, gathered from over 50 countries throughout the world. Health professionals, basic scientists and fellows in training could choose between a variety of activities in plenary, main, educational and workshop sessions, highlights of the past year, pros and cons, and oral abstract and poster sessions, and met with experts. A total of 1,713 abstracts, 31 symposia, and 54 workshops were presented, ranging from basic science to clinical trials and modern treatment of allergic diseases. Here, we summarize the highlights of cellular and molecular mechanisms of allergic disease.

Card9 controls a non-TLR signalling pathway for innate anti-fungal immunity

Fungal infections are increasing worldwide due to the marked rise in immunodeficiencies including AIDS; however, immune responses to fungi are poorly understood. Dectin-1 is the major mammalian pattern recognition receptor for the fungal component zymosan. Dectin-1 represents the prototype of innate non-Toll-like receptors (TLRs) containing immunoreceptor tyrosine-based activation motifs (ITAMs) related to those of adaptive antigen receptors. Here we identify Card9 as a key transducer of Dectin-1 signalling. Although being dispensable for TLR/MyD88-induced responses, Card9 controls Dectin-1-mediated myeloid cell activation, cytokine production and innate anti-fungal immunity. Card9 couples to Bcl10 and regulates Bcl10-Malt1-mediated NF-kappaB activation induced by zymosan. Yet, Card9 is dispensable for antigen receptor signalling that uses Carma1 as a link to Bcl10-Malt1. Thus, our results define a novel innate immune pathway and indicate that evolutionarily distinct ITAM receptors in innate and adaptive immune cells use diverse adaptor proteins to engage selectively the conserved Bcl10-Malt1 module.

Essential Role for IκB Kinase β in Remodeling Carma1-Bcl10-Malt1 Complexes upon T Cell Activation

T cell receptor (TCR) signaling to IkappaB kinase (IKK)/NF-kappaB is controlled by PKCtheta-dependent activation of the Carma1, Bcl10, and Malt1 (CBM) complex. Antigen-induced phosphorylation of Bcl10 has been reported, but its physiological function is unknown. Here we show that the putative downstream kinase IKKbeta is required for initial CBM complex formation. Further, upon engagement of IKKbeta/Malt1/Bcl10 with Carma1, IKKbeta phosphorylates Bcl10 in the C terminus and thereby interferes with Bcl10/Malt1 association and Bcl10-mediated IKKgamma ubiquitination. Mutation of the IKKbeta phosphorylation sites on Bcl10 enhances expression of NF-kappaB target genes IL-2 and TNFalpha after activation of primary T cells. Thus, our data provide evidence that IKKbeta serves a dual role upstream of its classical substrates, the IkappaB proteins. While being essential for triggering initial CBM complex formation, IKKbeta-dependent phosphorylation of Bcl10 exhibits a negative regulatory role in T cell activation.

Adaptors discriminate mast-cell cytokine production from eicosanoid production and degranulation

The Fc epsilonRI-dependent activation of nuclear factor (NF)kappaB is key for mast-cell cytokine production. The CARMA1-Bcl10-Malt1 adaptor complex regulates NFkappaB activation by antigen receptors in lymphocytes. A recent study reveals that the Bcl10-Malt1 complex promotes mast-cell interleukin-6 and tumor necrosis factor production, independent of degranulation, eicosanoid secretion or survival. The new findings place this complex at the forefront in discriminating between signals required for cytokine production and those required for mast-cell degranulation and eicosanoid production.