Background
The B cell receptor is activated together with co-receptors to regulate crucial signalling and metabolic pathways that mediate B lymphocyte growth, survival and expansion. Central to these pathways is the CBM complex, composed of the molecules CARD11, BCL10 and MALT1. Importantly, alongside serving as a scaffold protein, MALT1 is a paracaspase which regulates gene expression through its activity on RNases. Gain-of-function CARD11 mutations, for example CARD11L225LI, drive aberrant CBM complex activation and are recurrently detected in human B cell lymphomas, including diffuse large B cell lymphoma.
Materials and Methods
Here we have generated the novel mouse model CARD11L225LI;mb1-CreERT2 which enables controlled GOF CARD11 expression only in the B cell lineage using Tamoxifen mediated Cre activity. We determined the role of MALT1 activity in these mice by generating mouse models that specifically lack MALT1 function. We performed transcriptomic analyses of splenic B cells of the different genetic models.
Results
Acute B cell intrinsic expression of GOF CARD11 in CARD11L225LI;mb1-CreERT2 mice resulted in a pathological expansion of transgenic B cells leading to B cell lymphoproliferative disease, increased immunoglobulins and high levels of inflammatory cytokines. These CARD11L225LI expressing B cells also had increased survival and proliferation. Next, we determined the role of MALT1 paracaspase action on this pathology and observed that the absence of MALT1 activity rescued B cell lymphoproliferative disease upon CARD11L225LI expression. To functionally understand the role of MALT1 in GOF CARD11 signalling, we completed RNAseq analysis and found upregulation of gene sets that correspond to plasma cells and lymphoid tumours, as well as NF-B and Myc activation upon CARD11L225LI expression, independent of MALT1 function. Therefore, we next completed differential expression analysis of individual genes and observed that genes known to be critically involved in B cell survival and proliferation were upregulated in cells with MALT1 activity, but not without MALT1 activity. Protein analysis validated these findings, demonstrating increased BCL-xL, Cyclin D2 and Cyclin E expression following acute CBM complex activation in a MALT1 dependent manner.
Conclusions
Through our novel genetic system, we show that GOF CARD11L225LI signalling induces a pathological B cell lymphoproliferative disease which is strictly dependent on MALT1 activity. We also show that MALT1 selectively affects the expression of a series of proliferation and survival factors that are known to mediate physiological and pathological B cell expansion. Lastly, these data highlight the role of paracaspase-mediated regulation of immune signalling events in B cells.
A. Kratzert: None. E. Morrish: None. S.J. Keppler: None. J. Ruland: None.