From signaling to survival: Dissecting drug resistance and proliferation in CLL

Although the development of novel targeted therapies such as ibrutinib and venetoclax have significantly improved the therapeutic outlook for chronic lymphocytic leukemia (CLL) patients, CLL remains an incurable disease. An important mechanism believed to contribute to relapse and the development of drug resistance is the lymphoid microenvironment, where CLL cells depend on external stimuli and interactions with surrounding healthy cells. Despite recognizing the contribution of the CLL microenvironment, the exact mechanisms of how CLL cells receive and translate microenvironmental signals into an anti-apoptotic and proliferative phenotype are not fully understood. Therefore, in this thesis we investigated the key signaling pathways that are engaged in the CLL microenvironment and how they contribute to drug resistance and proliferation. We studied the expression of Bcl-2 proteins in CLL cells isolated from patients undergoing ibrutinib or venetoclax therapy which allowed us to relate Bcl-2 member expression to clinical response. We also investigated the contribution of different Bcl-2 proteins in resistance to venetoclax where we found a dominant role for Bcl-XL in CD40-mediated drug resistance and subsequently we investigated how Bcl-XL is regulated. We show that NF-κB and JAK-STAT signaling influence the expression of Bcl-XL and we demonstrate two novel inhibitors targeting alternative NF-κB or BCR signaling that can downregulate Bcl-XL and tip the balance of resistant CLL cells towards apoptosis sensitivity. Finally, we assembled multiple facets of the CLL microenvironment by developing a novel 3D culture system to more accurately study CLL drug resistance and proliferation in the context of the CLL microenvironment.