Consequences of oncogenic CARD11 signalling for B cells
Creators: name: Jeelall, Yogesh
Publisher: name: The Australian National University
Year: 2015
Subjects: subject: CARD11
subject: NFkB
subject: Lymphoma
subject: Tolerance
subject: Immunology
subject: FOS: Clinical medicine
subject: Cancer

description: Diffuse large B cell lymphoma is the most common form of Non Hodgkin’s lymphoma, accounting for forty percent of cases. Gain of function CARD11 mutations have been found in approximately thirteen percent of DLBCL patients. CARD11 functions as a scaffold protein and relays signals from the antigen receptor to the JNK and NFkB signaling pathways, which contribute to the survival, proliferation and differentiation of lymphocytes. The consequences of acquiring gain of function CARD11 mutations in normal B lymphocytes, in isolation from the other approximately 30 mutations that accumulate on average in lymphoma cells, had not been previously addressed. By introducing lymphoma CARD11 mutations into antigen activated B lymphocytes, we found that gain of function CARD11 mutations result in proliferation, growth and survival in vitro. The CARD11 mutants also increased signalling to NFkB and JNK pathways to varying degrees. Moreover, when the genetically modified B cells were transferred in vivo to investigate the consequences of the acquisition of the CARD11Mut10 version of CARD11, we observed that the B cells proliferate rapidly and differentiate into plasmablasts. CARD11Mut10 induced BLIMP1 mediated plasmablast differentiation and autocrine production of IL6, which contributed to differentiation into plasmablasts. However, this initial rapid population expansion was self-limiting: it peaked at day 10, and by 20 days the numbers of CARD11Mut10 expressing B cells decreased drastically. The induction of the NFkB negative regulator A20 contributed to limiting the expansion and accumulation of CARD11Mut10 expressing B cells. By introducing CARD11 mutations from human lymphomas into antigen activated mature B lymphocytes in mice, we tested the hypothesis that lymphoma somatic mutations corrupt the normal response to self-antigens. Here, we show that all lymphoma derived CARD11 mutations tested, but not NFkB activating mutations in Ikbkb, block self-antigen induced death and cooperate with chronic antigen receptor stimulation to induce extensive T cell independent proliferation, BLIMP1 mediated plasmablast differentiation and autoantibody secretion. Our findings demonstrate that regulation of CARD11 signalling is a critical switch governing the decision between death and proliferation in antigen-stimulated mature B cells, and that mutations in this switch represent a powerful initiator for aberrant B cell responses in vivo. Recently, patients with germline activating mutations in CARD11 were reported. These patients are characterised by a B cell lymphoproliferative disorder, with B cell expansion with NFkB and T cell anergy. We show that E127G CARD11, discovered in patients with germline CARD11 mutations, increased NFkB signalling and, akin to lymphoma derived CARD11 mutants, E127G CARD11 protected the B cells from death and induced accumulation of self reactive B cells. However, there was no apparent expansion of B cells in mice expressing an ENU induced germline CARD11M365K mutation. In vitro assays with B cells showed that CARD11M365K has stronger NFkB and growth inducing effects than CARD11E127G. Together, these results reveal an intriguing difference between the effects of overactive CARD11 signalling in humans and mice. The availability of an ENU induced CARD11M365K mutant mouse strain presents opportunities to further study the effects of overactive CARD11 in lymphocytes.