Tissue-Specific Features of Innate Lymphoid Cells

Although the function of the circulating immune cell compartment has been studied in detail for decades, limitations in terms of access and cell yields from peripheral tissues have restricted our understanding of tissue-based immunity, particularly in humans. Recent advances in high-throughput protein analyses, transcriptional profiling, and epigenetics have partially overcome these obstacles. Innate lymphoid cells (ILCs) are predominantly tissue-resident, and accumulating data indicate that they have significant tissue-specific functions. We summarize current knowledge of ILC phenotypes in various tissues in mice and humans, aiming to clarify ILC immunity in distinct anatomical locations.

Targeting TRAF6 E3 ligase activity with a small-molecule inhibitor combats autoimmunity

Constitutive NF-κB signaling represents a hallmark of chronic inflammation and autoimmune diseases. The E3 ligase TNF receptor-associated factor 6 (TRAF6) acts as a key regulator bridging innate immunity, pro-inflammatory cytokines, and antigen receptors to the canonical NF-κB pathway. Structural analysis and point mutations have unraveled the essential role of TRAF6 binding to the E2-conjugating enzyme ubiquitin-conjugating enzyme E2 N (Ubc13 or UBE2N) to generate Lys⁶³-linked ubiquitin chains for inflammatory and immune signal propagation. Genetic mutations disrupting TRAF6-Ubc13 binding have been shown to reduce TRAF6 activity and, consequently, NF-κB activation. However, to date, no small-molecule modulator is available to inhibit the TRAF6-Ubc13 interaction and thereby counteract NF-κB signaling and associated diseases. Here, using a high-throughput small-molecule screening approach, we discovered an inhibitor of the TRAF6-Ubc13 interaction that reduces TRAF6-Ubc13 activity both in vitro and in cells. We found that this compound, C25-140, impedes NF-κB activation in various immune and inflammatory signaling pathways also in primary human and murine cells. Importantly, C25-140 ameliorated inflammation and improved disease outcomes of autoimmune psoriasis and rheumatoid arthritis in preclinical in vivo mouse models. Hence, the first-in-class TRAF6-Ubc13 inhibitor C25-140 expands the toolbox for studying the impact of the ubiquitin system on immune signaling and underscores the importance of TRAF6 E3 ligase activity in psoriasis and rheumatoid arthritis. We propose that inhibition of TRAF6 activity by small molecules represents a promising novel strategy for targeting autoimmune and chronic inflammatory diseases.

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.

MK2/3 Are Pivotal for IL-33-Induced and Mast Cell-Dependent Leukocyte Recruitment and the Resulting Skin Inflammation

The IL-1R family member IL-33R mediates Fcε-receptor-I (FcεRI)-independent activation of mast cells leading to NF-κB activation and consequently the production of cytokines. IL-33 also induces the activation of MAPKs, such as p38. We aimed to define the relevance of the p38-targets, the MAPK-activated protein kinases 2 and 3 (MK2 and MK3) in IL-33-induced signaling and the resulting mast cell effector functions in vitro and in vivo. We demonstrate that the IL-33-induced IL-6 and IL-13 production strongly depends on the MK2/3-mediated activation of ERK1/2 and PI3K signaling. Furthermore, in the presence of the stem cell factors, IL-33 did induce an MK2/3-, ERK1/2- and PI3K-dependent production of TNF-α. In vivo, the loss of MK2/3 in mast cells decreased the IL-33-induced leukocyte recruitment and the resulting skin inflammation. Therefore, the MK2/3-dependent signaling in mast cells is essential to mediate IL-33-induced inflammatory responses. Thus, MK2/3 are potential therapeutic targets for suppression of IL-33-induced inflammation skin diseases such as psoriasis.

Modulation of pre-mRNA structure by hnRNP proteins regulates alternative splicing of MALT1

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.

MALT1 Phosphorylation Controls Activation of T Lymphocytes and Survival of ABC-DLBCL Tumor Cells

The CARMA1/CARD11-BCL10-MALT1 (CBM) complex bridges T and B cell antigen receptor (TCR/BCR) ligation to MALT1 protease activation and canonical nuclear factor κB (NF-κB) signaling. Using unbiased mass spectrometry, we discover multiple serine phosphorylation sites in the MALT1 C terminus after T cell activation. Phospho-specific antibodies reveal that CBM-associated MALT1 is transiently hyper-phosphorylated upon TCR/CD28 co-stimulation. We identify a dual role for CK1α as a kinase that is essential for CBM signalosome assembly as well as MALT1 phosphorylation. Although MALT1 phosphorylation is largely dispensable for protease activity, it fosters canonical NF-κB signaling in Jurkat and murine CD4 T cells. Moreover, constitutive MALT1 phosphorylation promotes survival of activated B cell-type diffuse large B cell lymphoma (ABC-DLBCL) cells addicted to chronic BCR signaling. Thus, MALT1 phosphorylation triggers optimal NF-κB activation in lymphocytes and survival of lymphoma cells.

Subthreshold IKK activation modulates the effector functions of primary mast cells and allows specific targeting of transformed mast cells

Mast cell differentiation and proliferation depends on IL-3. IL-3 induces the activation of MAP-kinases and STATs and consequently induces proliferation and survival. Dysregulation of IL-3 signaling pathways also contribute to inflammation and tumorigenesis. We show here that IL-3 induces a SFK- and Ca²⁺-dependent activation of the inhibitor of κB kinases 2 (IKK2) which results in mast cell proliferation and survival but does not induce IκBα-degradation and NFκB activation. Therefore we propose the term "subthreshold IKK activation".This subthreshold IKK activation also primes mast cells for enhanced responsiveness to IL-33R signaling. Consequently, co-stimulation with IL-3 and IL-33 increases IKK activation and massively enhances cytokine production induced by IL-33.We further reveal that in neoplastic mast cells expressing constitutively active Ras, subthreshold IKK activation is associated with uncontrolled proliferation. Consequently, pharmacological IKK inhibition reduces tumor growth selectively by inducing apoptosis in vivo.Together, subthreshold IKK activation is crucial to mediate the full IL-33-induced effector functions in primary mast cells and to mediate uncontrolled proliferation of neoplastic mast cells. Thus, IKK2 is a new molecularly defined target structure.

Synthesis and Evaluation of Macrocyclic Peptide Aldehydes as Potent and Selective Inhibitors of the 20S Proteasome

This research explores the first design and synthesis of macrocyclic peptide aldehydes as potent inhibitors of the 20S proteasome. Two novel macrocyclic peptide aldehydes based on the ring-size of the macrocyclic natural product TMC-95 were prepared and evaluated as inhibitors of the 20S proteasome. Both compounds inhibited in the low nanomolar range and proved to be selective for the proteasome over other serine and cysteine proteases, particularly when compared to linear analogues with similar amino acid sequences. In HeLa cells, both macrocycles efficiently inhibited activation of nuclear factor-κB (NF-κB) transcription factor by blocking proteasomal degradation of the inhibitor protein IκBα after cytokine stimulation. Due to their covalent mechanism of binding these compounds represent a 1000-fold increase in inhibitory potency over previously reported noncovalently binding TMC-95 analogues. Molecular modeling of the macrocyclic peptides confirms the preference of the large S3 pocket for large, hydrophobic residues and the ability to exploit this to improve selectivity of proteasome inhibitors.