Thome Miazza Margot, Associate Professor

Lymphocytes play a crucial role in the defense against pathogens and tumor cells. One focus of our research is to understand the molecular mechanisms that control the activation of T-lymphocytes, initiated upon triggering of the T-cell antigen receptor by MHC-bound antigen. This leads to the initiation of multiple signaling pathways that regulate changes in cell shape and gene expression that are critical for efficient T-cell activation, proliferation and survival. Another focus of our research is to understand the molecular mechanisms underlying aberrant lymphocyte proliferation and survival that occurs in certain lymphoid tumors (lymphomas).

By uncovering new molecular players and enzymatic activities relevant to these pathways, we aim at identifying possible targets for therapeutic immuno-modulation or treatment of lymphomas. 

Signaling pathways relevant for lymphocyte activation and survival

The initiation of the adaptive immune response depends on the recognition of pathogenic substances or tumor-specific molecules (microbial or tumor antigens) by antigen receptors on the lymphocyte cell surface. Antigen recognition leads to a variety of signaling events that result in dramatic changes in the expression of genes that control the activation, clonal proliferation and survival of the stimulated lymphocytes.

We are studying a complex of proteins comprising CARMA1, BCL10 and MALT1 (CBM proteins) that play key roles in the initiation of the adaptive immune response. Mutation of the genes encoding CARMA1, BCL10 or MALT1 or their upstream regulators is associated with constitutive CBM-dependent signaling, and the development of particular subtypes of human B-cell lymphomas, such as lymphomas of the mucosa-associated lymphoid tissue (MALT lymphomas) and diffuse large B-cell lymphomas (DLBCL).

An important role of CBM proteins is the activation of the transcription factor NF-κB, which in turn controls the expression of genes that are essential for lymphocyte proliferation and survival (for review, see: Thome, 2004; Thome, 2008; Hailfinger et al., 2009b). The main focus of our recent work has been the identification and characterization of a protease activity of MALT1 (Rebeaud, Hailfinger et al., 2008). We could show that this protease activity is essential for optimal NF-κB-dependent T-cell activation (Rebeaud, Hailfinger et al., 2008; Thome, 2008) and for the proliferation and survival of cell lines derived from the activated B-cell (ABC) subtype of diffuse large B-cell lymphoma (Hailfinger et al., 2009a).

The proteolytic activity of MALT1 is key to T-cell activation

The transcription factor NF-κB is pivotal to the expression of genes that control lymphocyte activation and the generation of the immune response. In resting lymphocytes, NF-κB family members are present in the cytoplasm in an inactive form, bound to inhibitory κ-B (IκB) proteins. Triggering of the T-cell antigen receptor leads to activation of the IκB kinase (IKK) complex that induces phosphorylation and subsequent degradation of IκB proteins. This allows NF-κB to translocate into the nucleus and to initiate the transcription of genes that control lymphocyte proliferation and survival.

Fig. 1: MALT1 controls antigen receptor-mediated lymphocyte activation by its scaffold and protease functions. (A) Antigen receptor triggering leads to formation of the CARMA1-BCL10-MALT1 complex. MALT1 recruits the ubiquitin ligase TRAF6, which promotes IKK activation via the Ser/Thr kinase TAK1. MALT1 also contributes to T-cell activation via its protease function. Cleavage of BCL10 promotes lymphocyte adhesion (Rebeaud, Hailfinger et al. Nature Immunol. 2008), while cleavage of the IKK inhibitor A20 is thought to further increase IKK activation (Coornaert et al. Nature Immunol. 2008). MALT1 also promotes NF-kB activation in an IKK-independent manner, via cleavage of an unknown substrate (???). (B) Silencing of MALT1 expression prevents BCL10 cleavage in Jurkat T cells stimulated for the indicated times. Migration of full length and cleaved BCL10 are indicated by black and white arrowheads, respectively.

A major issue in the field of T-cell activation has been to understand the molecular mechanisms linking T-cell receptor engagement to the activation of the IKK complex (Fig. 1A). One of the earliest events following T-cell receptor (TCR) engagement is the activation of tyrosine kinases and the tyrosine phosphorylation of a restricted set of substrates. These in turn control the activation of Ser/Thr kinases of the protein kinase C family. PKCtheta and PKCbeta are T- and B-cell specific PKC family members essential for antigen receptor-induced NF-κB activation, by phosphorylation of CARMA1. We and others have previously identified CARMA1 as an essential signaling component in the antigen-recptor-induced NF-κB pathway, and shown that a caspase-recruitment domain (CARD)-mediated interaction between CARMA1 and BCL10 plays a pivotal role in signal transmission to the IKK complex (Gaide et al., 2002; Egawa et al., 2003; Thome, 2004).

MALT1 is a BCL10-binding protein that contributes to NF-κB activation by binding to multiple proteins. MALT1 contains binding sites for the ubiquitin ligase TRAF6, which in turn controls the recruitment and activation of the IKK complex (Fig. 1A). MALT1 also has a protein domain that shares homology with proteases of the caspase family, but despite intensive efforts it had remained enigmatic whether this domain has proteolytic activity and if so, whether this contributes to NF-κB activation. Through the identification of a BCL10 cleavage product that is present exclusively in activated T- and B-cells, we could demonstrate that MALT1 has an unexpected, Arginine-directed proteolytic activity that is transiently induced upon T-cell stimulation and cleaves the C-terminal part of BCL10 (Fig. 1, A and B, Fig. 2A). Moreover, the development of a peptide-based inhibitor of MALT1 (in collaboration with the group of Prof. Nicolas Fasel), has allowed us to show that the proteolytic activity of MALT1 (but not the cleavage of BCL10) is required for optimal NF-κB activation and cytokine production in human T cells (Fig. 2, B and C). Collectively, these data suggest a central role for the MALT1 protease activity in the adaptive immune response, and identify MALT1 as an interesting drug target for immunosuppression.

Fig. 2: MALT1 activity, but not BCL10 cleavage, is required for NF-kB activation. (A) Analysis of MALT1 activity and BCL10 cleavage in T cells activated for the indicated times with PMA and ionomycin to mimic natural T-cell activation. T-cell activation leads to a transient increase in MALT1 activation (but not caspase-3 activation) that peaks at 30 min after stimulation. MALT1 activation correlates with the generation of a BCL10 cleavage product that is stable for many hours (data not shown). (B) Treatment of human Jurkat T cells with a cell permeable MALT1 inhibitor, z-VRPR-fmk, leads to impaired NF-κB activation upon stimulation with agonistic anti-CD3 and anti-CD28 antibodies. (C) Silencing of BCL10 leads to impaired NF-kB activation that can be restored by either wt or uncleavable BCL10 (R228G mutant). Original data are from Rebeaud, Hailfinger et al., Nature Immunol. 2008).

The MALT1 protease activity is essential for the growth and survival of cell lines derived from diffuse large B-cell lymphoma of the activated B-cell subtype

A key element for the development of suitable anti-cancer drugs is the identification of cancer-specific enzymatic activities that can be therapeutically targeted. MALT1 is a proto-oncogene that contributes to tumorigenesis in lymphomas of the mucosa-associated tissue (MALT lymphomas) and certain forms of diffuse large B-cell lymphomas (DLBCL). Different subtypes of DLBCL have been identified depending on their gene expression pattern. We are particularly interested in DLBCL of the activated B-cell (ABC) subtype, the least curable subtype of DLBCL. ABC DLBCL are characterized by constitutive activation of the NF-κB pathway that has been attributed to oncogenic mutations in the B-cell receptor-associated proteins CD79A and CD79B and in CARMA1 (Fig. 3A). Consistent with these findings, cell lines derived from ABC DLBCL depend on the expression of CARMA1, BCL10 and MALT1 for survival.

Our recent data suggested that MALT1 has protease activity but it was unknown whether this activity is relevant for tumor growth. We have assessed the relevance of MALT1 protease activity and discovered that MALT1 is constitutively active in DLBCL lines of the ABC but not the germinal center B-cell (GCB) subtype (Hailfinger et al., 2009a) (Fig. 3B). Inhibition of the MALT1 protease activity led to reduced expression of growth factors and apoptosis inhibitors, and specifically affected the growth and survival of ABC DLBCL lines (Fig. 3C). Moreover, gene arrays (performed in collaboration with the group of Louis Staudt) revealed that MALT1 inhibition led to reduced expression of a previously defined NF-κB gene signature (Hailfinger et al., 2009a). These results demonstrate a key role for the protease activity of MALT1 in DLBCL of the ABC subtype, and provide a rationale for the development of pharmacological inhibitors of MALT1 in DLBCL therapy. Further studies are now targeted at the identification of the NF-κB-relevant MALT1 substrate(s).

Fig. 3: Role of MALT1 protease activity in human B-cell lymphomas. (A) Human B-cell lymphomas of the activated B-cell (ABC) subtype of diffuse large B-cell lymphomas have oncogneic mutations in the BCR-associated CD79 chains and in CARMA1 that drive constitutive NF-kB activation (Lenz et al., Science 2008; Compagno, Nature 2009; Davis et al., Nature 2010). (B, C) Using an antibody specifically recognizing cleaved BCL10, we could show that MALT1 is constitutively active in ABC, but not germinal center B-cell (GCB) DLBCL (B), and that MALT1 inhibition specifically impairs viability of cell lines derived from ABC DLBCL (C).

Collaborations

Parts of this work were done in collaboration with the laboratories of Nicolas Fasel (Department of Biochemistry of the University of Lausanne), Nathalie Rufer (Multidisciplinary Oncology Center, University Hospital of Lausanne), Rüdiger Arnold and Peter Krammer (DKFZ, Heidelberg, Germany) and Louis Staudt (National Cancer Institute, Bethesda, MD, USA).

Prix Leenaards 2009 pour la promotion de la recherche scientifique

Eclosion Prize 2009

Recent publications

• Mühlethaler-Mottet A, Flahaut M, Bourloud KB, Nardou K, Coulon A, Liberman J, Thome M, Gross N. (2011). Individual caspase-10 isoforms play distinct and opposing roles in the initiation of death receptor-mediated tumour cell apoptosis.
  Cell Death Dis. Mar 3;2:e125. PubMed
• Hailfinger S, Nogai H, Pelzer C, Jaworski M, Cabalzar K, Charton JE, Guzzardi M, Décaillet C, Grau M, Dörken B, Lenz P, Lenz G, Thome M. (2011). Malt1- dependent RelB cleavage promotes canonical NF-κB activation in lymphocytes and lymphoma cell lines. Proc Natl Acad Sci USA. 108, 14596-14601. PubMed
• Jevnikar Z, Obermajer N, Doljak B, Turk S, Gobec S, Svajger U, Hailfinger S, Thome M, Kos J. (2011). Cathepsin X cleavage of the beta2 integrin regulates talin-binding and LFA-1 affinity in T cells. J Leukoc Biol. 90, 99-109. PubMed
• Schmid DA, Irving MB, Posevitz V, Hebeisen M, Posevitz-Fejfar A, Sarria JC, Gomez-Eerland R, Thome M, Schumacher TN, Romero P, Speiser DE, Zoete V, Michielin O, Rufe, N. (2010). Evidence for a TCR affinity threshold delimiting maximal CD8 T cell function. J Immunol. 184, 4936-4946. PubMed
• Hailfinger S, Lenz G, Ngo V, Posevitz-Fejfar A, Rebeaud F, Guzzardi M, Murga Penas E, Dierlamm J, Chan WC, Staudt LM, Thome M. (2009). Essential role of MALT1 protease activity in activated B cell-like diffuse large B-cell lymphoma. Proc Natl Acad Sci USA. 106(47), 19946-51. PubMed
• Thurau M, Marquardt G, Gonin-Laurent N, Weinländer K, Naschberger E, Jochmann R, Alkharsah KR, Schulz TF, Thome M, Neipel F, Stürzl M. (2009). The Viral inhibitor of apoptosis vFLIP/K13 protects endothelial cells against superoxide-induced cell death. J Virol. 83, 598-611. PubMed
• Brenner D, Brechmann M, Rohling S, Tapernoux M, Mock T, Winter D, Lehmann WD, Kiefer F, Thome M, Krammer PH, Arnold R. (2009). Phosphorylation of CARMA1 by HPK1 is critical for NF-κB activation in T cells. Proc Natl Acad Sci USA. 106, 14508-14513. PubMed
• Rebeaud F, Hailfinger S, Posevitz-Fejfar A, Tapernoux M, Moser R, Rueda D, Gaide O, Guzzardi M, Iancu E, Rufer N, Fasel N, Thome M. (2008a). The proteolytic activity of the paracaspase MALT1 is key in T cell activation. Nat Immunol. 9, 272-81. PubMed
• Torgler R, Bongfen SE, Romero JC, Tardivel A, Thome M, Corradin G. (2008). Sporozoite-Mediated Hepatocyte Wounding Limits Plasmodium Parasite Development via MyD88-Mediated NF-kappaB Activation and Inducible NO Synthase Expression. J Immunol. 180, 3990-3999. PubMed
• Misra RS, Russell JQ, Koenig A, Hinshaw-Makepeace JA, Wen R, Wang D, Huo H, Littman DR, Ferch U, Ruland J, Thome M, Budd RC. (2007). Caspase-8 and c-FLIPL associate in lipid rafts with NF-κB adaptors during T cell activation. J Biol Chem. 282, 19365-19374. PubMed
• Rueda D, Gaide O, Ho L, Lewkowicz E, Niedergang F, Hailfinger S, Rebeaud F, Guzzardi M, Conne B, Thelen M, Delon J, Ferch U, Ruland J, Mak T, Schwaller J, Thome M. (2007). Bcl10 controls T-cell receptor- and FcgR-induced actin polymerization. J Immunol. 178, 4373-4384. PubMed

Reviews

• Pelzer C, Thome M. (2011). IKKα takes control of canonical NF-κB activation. Nature Immunol. 12, 815-816. PubMed
• Thome M, Charton J, Pelzer C, Hailfinger S. (2010). Antigen receptor signaling to NF-κB via CARMA1, BCL10 and MALT1. Cold Spring Harb Perspect Biol. 2, a003004. PubMed
  Thome, M., Rebeaud, F., Hailfinger, S. (2009). Adapter and enzymatic functions of proteases in T-cell activation. Immunol. Rev. 232, 334-347. PubMed
• Thome M. (2008). Multifunctional roles for MALT1 in T cell activation. Nat. Rev. Immunol. 8, 495-500. PubMed
• Thome M, Weil R. (2007). Posttranslational modifications regulate distinct functions of Carma1 and Bcl10. Trends Immunol. 28, 281-288. PubMed
• Rebeaud F, Hailfinger S, Thome M. (2007). Dlgh1 and Carma1 MAGUK proteins contribute to signal specificity downstream of TCR activation. Trends Immunol. 28, 196-200. PubMed