THE ROLE OF THE B CELL ANTIGEN RECEPTOR IN A MOUSE MODEL OF NON-HODGKIN LYMPHOMA

The B cell antigen receptor (BCR) plays a central role both in early B-cell development and in mature B cells, where it controls survival and allows the organism to mount protective immune responses against foreign antigens. Surface BCR expression is retained in most types of Non-Hodgkin B-cell lymphomas (NHL), where it has been proposed to participate to tumor initiation, maintenance and progression. This study has provided for the first time an in vivo genetic proof for the role of the BCR in B-cell lymphomagenesis. Using the λ-MYC Burkitt lymphoma (BL) mouse model, in which conditional ablation of the BCR can be induced in c-MYC transformed B cells through the Cre/loxP recombination system, I provide evidence for the essential role of the BCR in tumor maintenance. Acute BCR ablation in λ-MYC lymphomas delayed cell cycle progression and augmented apoptosis, causing the rapid disappearance of BCR- tumor cells when these cells were grown both in vitro and in vivo in competition with their BCR+ counterparts. Similar results were obtained using the human Burkitt lymphoma cell line, RAMOS. This study has also elucidated crucial effectors of tonic BCR signalling that influence the survival and growth of BL cells. BCR inactivation led to a reduction in GSK3β phosphorylation, leading to an increase in its enzymatic activity. Importantly, treatment of lymphoma cells with a GSK3β small molecule inhibitor restored the ability of BCR-deficient lymphomas to compete with their BCR+ counterparts. I could also show that c-MYC transformed B cells acquired exquisite sensitivity to the mTOR inhibitor rapamycin upon BCR inactivation. Using a combination of quantitative proteome and whole transcriptome analyses we have started to shed light on the contribution of the BCR to the maintenance of c-MYC transformed B cells. In λ-MYC lymphomas, the BCR regulated a substantial number of MYC targets genes. Notably, the majority of these shared targets were normalized in BCR- tumor cells upon pharmacological inhibition of GSK3β. These result point to the tempting hypothesis that the BCR supports MYC-driven lymphomagenesis, by the inhibition of GSK3β-mediated phosphorylation and hence degradation of c-MYC protein. Finally, our study demonstrates that the failure of c-MYC transformed B cells to grow in vivo is strictly dependent on the concomitant presence of their BCR+ counterparts. This result has important implications for the design of clinical trials limited to the use of inhibitors of the BCR signalling pathway for the treatment of BL and other forms of aggressive NHL. To avoid the outgrowth of BCR- escape variants in response to such treatments, we propose the integration in the therapeutic protocol of the mTOR inhibitor rapamacyin that showed high efficacy in the killing Ig receptor-less lymphoma cells.