Humoral autoimmunity paralleled by the accumulation of follicular helper T cells (T(FH) cells) is linked to mutation of the gene encoding the RNA-binding protein roquin-1. Here we found that T cells lacking roquin caused pathology in the lung and accumulated as cells of the T(H)17 subset of helper T cells in the lungs. Roquin inhibited T(H)17 cell differentiation and acted together with the endoribonuclease regnase-1 to repress target mRNA encoding the T(H)17 cell-promoting factors IL-6, ICOS, c-Rel, IRF4, IκBNS and IκBζ. This cooperation required binding of RNA by roquin and the nuclease activity of regnase-1. Upon recognition of antigen by the T cell antigen receptor (TCR), roquin and regnase-1 proteins were cleaved by the paracaspase MALT1. Thus, this pathway acts as a 'rheostat' by translating TCR signal strength via graded inactivation of post-transcriptional repressors and differential derepression of targets to enhance T(H)17 differentiation.

Proteolytic activity of the mucosa-associated lymphoid tissue lymphoma translocation protein-1 (MALT1) paracaspase is required for survival of the activated B cell subtype of diffuse large B cell lymphoma (ABC-DLBCL). We have identified distinct derivatives of medicinal active phenothiazines, namely mepazine, thioridazine, and promazine, as small molecule inhibitors of the MALT1 protease. These phenothiazines selectively inhibit cleavage activity of recombinant and cellular MALT1 by a noncompetitive mechanism. Consequently, the compounds inhibit anti-apoptotic NF-κB signaling and elicit toxic effects selectively on MALT1-dependent ABC-DLBCL cells in vitro and in vivo. Our data provide a conceptual proof for a clinical application of distinct phenothiazines in the treatment of ABC-DLBCL.

The activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) represents a very aggressive human lymphoma entity. Constitutive NF-κB activation caused by chronic active B-cell receptor (BCR) signaling is common feature of many ABC DLBCL cells; however, the pathways linking BCR signaling to the NF-κB prosurvival network are largely unknown. Here we report that constitutive activity of PI3K and the downstream kinase PDK1 are essential for the viability of two ABC DLBCL cell lines that carry mutations in the BCR proximal signaling adaptor CD79B. In these cells, PI3K inhibition reduces NF-κB activity and decreases the expression of NF-κB target genes. Furthermore, PI3K and PDK1 are required for maintaining MALT1 protease activity, which promotes survival of the affected ABC DLBCL cells. These results demonstrate a critical function of PI3K-PDK1 signaling upstream of MALT1 protease and NF-κB in distinct ABC DLBCL cells and provide a rationale for the pharmacologic use of PI3K inhibitors in DLBCL therapy.

The discovery of constitutive nuclear factor-κB (NF-κB) activation in Hodgkin's lymphoma tumor cells almost two decades ago was one of the first reports that directly connected deregulated NF-κB signaling to human cancer. Subsequent studies demonstrated that enhanced NF-κB signaling is a common hallmark of many lymphoid malignancies, including Hodgkin lymphoma, mucosa-associated lymphoid tissue lymphoma, diffuse large B-cell lymphoma and multiple myeloma. By inducing an anti-apoptotic and pro-proliferative gene program, NF-κB is involved in lymphoma survival and growth. Identification of somatic mutations that led to activation of oncogenes and inactivation of tumor suppressor genes in the pathway revealed that specific pathogenic mechanisms are responsible for constitutive NF-κB activation in different lymphoma entities. Thus, the identification of distinct oncogenic events is reflecting the diverse cellular origins of the different lymphomas. Further, elucidation of the mechanisms that drive NF-κB in lymphoma is of high clinical relevance as it will allow the design of target-directed precision therapy. Indeed, a number of drugs that impair constitutive NF-κB activation in lymphoid malignancies are currently in preclinical or clinical development.

The IB kinase (IKK) complex acts as a gatekeeper of canonical NF-B signaling in response to upstream stimulation. IKK activation requires sensing of ubiquitin chains by the essential IKK regulatory subunit IKK␥/NEMO. However, it has remained enigmatic whether NEMO binding to Lys-63-linked or linear ubiquitin chains is critical for triggering IKK activation. We show here that the NEMO C terminus, comprising the ubiquitin binding region and a zinc finger, has a high preference for binding to linear ubiquitin chains. However, immobilization of NEMO, which may be reminiscent of cellular oligomerization, facilitates the interaction with Lys-63 ubiquitin chains. Moreover, selective mutations in NEMO that abolish association with linear ubiquitin but do not affect binding to Lys-63 ubiquitin are only partially compromising NF-B signaling in response to TNF␣ stimulation in fibroblasts and T cells. In line with this, TNF␣-triggered expression of NF-B target genes and induction of apoptosis was partially compromised by NEMO mutations that selectively impair the binding to linear ubiquitin chains. Thus, in vivo NEMO interaction with linear and Lys-63 ubiquitin chains is required for optimal IKK activation, suggesting that both type of chains are cooperating in triggering canonical NF-B signaling.Induction of gene expression by the transcription factor NF-B controls many physiological processes, including immunity, differentiation, or apoptosis. The IB kinase (IKK) 3 complex acts as the gatekeeper of canonical NF-B signaling. In response to extracellular stimuli, IKKs become activated and catalyze the phosphorylation of NF-B inhibitors (IBs), leading to their ubiquitination and proteasomal degradation (1, 2). The IKK complex consists of the two catalytic domains IKK␣ and IKK␤ and the regulatory subunit IKK␥ or NF-B essential modulator (NEMO). NEMO mutations scattered throughout the entire gene are causing severe pathologies such as anhidrotic ectodermal dysplasia with immunodeficiency and incontinentia pigmenti (3-6).NEMO serves as a critical integrating platform coupling upstream receptor signaling to the canonical NF-B pathway. Biochemical and genetic studies have highlighted a pivotal function of polyubiquitination for IKK/NF-B activation. Upon TNF␣ or T cell receptor/CD28 stimulation, signaling adaptors RIP1 or MALT1 are modified by covalent attachment of Lys-63-linked ubiquitin (Ub) chains to recruit NEMO and thereby promote IKK activation (7-9). Recently, the assembly of linear Ub chains by the linear Ub chain assembly complex was shown to promote cytokine-triggered IKK activation (10). The Ub binding surface in NEMO called UBAN (Ub binding in ABIN and NEMO) or NOA (NEMO-optineurin-ABIN) is required for signal induced IKK activation. NEMO UBAN has been cocrystallized with linear as well as with Lys-63 Ub chains (11,12). The crystal structures reveal a bipartite Ub binding region from amino acids 290 to 330, distinguishing between a proximal and a distal moiety of bound di-Ub.In solution, linear di-Ub was found to bind...