Mantle-Cell Lymphoma (MCL) Cells Are Highly Sensitive To ABT-199 But Their Sensitivity May Be Altered By The Microenvironment Via The Up-Regulation Of Bcl-Xl and Bcl2A1

Despite improvement in the treatment of Mantle-cell lymphoma (MCL), relapse invariably occurs and innovative strategies are needed. Bcl-2 inhibitors such as ABT-737 and ABT-263 (navitoclax), which target both Bcl-2 and Bcl-x_L, demonstrated antitumor activity in B-cell malignancies. However, the clinical development of navitoclax is limited by a deep thrombocytopenia, which is induced by inhibition of Bcl-x_L in platelets. To overcome this toxicity, ABT-199, the first-in-class orally bioavailable Bcl-2-selective BH3 mimetic, has been developed and showed promising antitumor activity in B-cell lymphoma while sparing platelets. In the present study, the apoptotic efficiency of ABT-199 in comparison with that of ABT-737 was evaluated in seven MCL cell lines. We found two MCL cell lines sensitive to ABT-199 (LD50 of 100 and 200 nM), one intermediate (LD50 of 1000nM) and 4 resistant (LD50 from 5000 to 10000 nM). Surprisingly, LD50 values of the 2 sensitive cell lines (MINO, GRANTA-519) were slightly higher for ABT-199 than for ABT-737. We further demonstrated that the Bcl-2/Mcl-1 ratio determined by RT-PCR is a predictive biomarker for ABT-199 sensitivity. To further determine the role of Mcl-1 in ABT-199 resistance, Mcl-1 siRNA were transfected in Z-138 and JEKO-1 cells. Mcl-1 silencing sensitized these 2 cell lines to low dose ABT-199 confirming the importance of Mcl-1 in ABT-199 resistance as previously shown for ABT-737. Moreover, in Z-138 cells, which highly express Bcl-x_L, we showed that Bcl-x_L silencing sensitized them to ABT-199. These results show that in addition to Mcl-1, Bcl-x_L might also confer resistance to ABT-199-induced apoptosis in MCL. This could explain the slight difference of sensitivity of MCL cells between ABT-199 and ABT-737. In contrast to MCL cell lines, we found so far that ABT-199 efficiency killed all tested circulating primary cells from MCL patients (n=7) with LD50 values inferior to 10 nM. Because MCL cells reside mainly in lymph nodes, we wondered whether mimicking the microenvironment could impact the sensitivity of MCL cells to BH3 mimetics like it was previously demonstrated for chronic lymphoid leukemia cells. Thus, the ABT-199 sensitive MINO and GRANTA-519 cells were cultured on CD40L-expressing fibroblasts L in order to mimic the lymph node microenvironment. Both cell lines and primary cells became resistant to ABT-199 within 24h. Investigation of the underlying mechanism revealed a strong up-regulation of both Bcl-x_L and Bcl2A1 protein expression. By contrast, culture of MCL cells with parental CD40L- fibroblasts or in conditioned medium from CD40L+ L fibroblasts culture failed to induce ABT-199 resistance. These results highlight the implication of the CD40L pathway in ABT-199 resistance through the up-regulation of Bcl-x_L and Bcl2A1 in MCL. In conclusion, while circulating primary MCL cells are highly sensitive to ABT-199, it would be important to address the impact of microenvironment on long-term survival of MCL cells within lymph nodes under ABT-199 treatment.