The role and regulation of AKT in CD40-mediated survival and proliferation of CLL cells

Chronic lymphocytic leukaemia (CLL) is a proliferative disease in which malignant B lymphocytes gradually accumulate in the peripheral blood, lymph nodes and bone marrow. It is the most common leukaemia in adults and currently incurable. Novel therapeutic strategies are thus required. The serine/threonine kinase AKT has been shown to contribute to the survival and expansion of CLL cells. However, the exact role of AKT in CD40 stimulation-induced survival and proliferation of CLL cells is not well understood. The aim of this study was to investigate how AKT mediates CLL-cell survival and proliferation, in particular under the conditions that mimic the protective tissue microenvironment in vivo. I therefore used a co-culture system in which primary CLL cells were cultured with transfected mouse fibroblasts expressing human CD154 to model the lymph node environment where CLL cells interact with T cells through the CD40-CD154 signalling pathway, resulting in their enhanced survival and proliferation. I showed that AKT was activated in CD40-stimulated CLL cells. AKT mediated a protective effect of CD40 stimulation against cytotoxic drug (bendamustine)-induced cell death, as inhibiting AKT activity by a selective, small molecule inhibitor potentiated drug-induced killing in the stimulated cells. I also showed that AKT was required for CD40 stimulation-induced cell growth (increase in size) in CLL cells. In addition, I uncovered a selective requirement of AKT for proliferation induced by CD40 + IL-4 in CLL cells, since pharmacological inhibition of AKT significantly inhibited such proliferation in the leukaemic, but not normal B cells. In contrast, the requirement of AKT in proliferation induced by CD40 + IL-21 in CLL cells appears to be case-dependent. Further studies are thus required to understand why inhibition of AKT activity reduces CD40 + IL-21-induced proliferation in CLL cells from some but not other cases. Finally, I examined the regulation of AKT activation by the class I PI3K p110 isoforms and showed for the first time that expression of both PI3K p110δ and p110γ was upregulated in CD40-stimulated CLL cells. This novel observation has provided a rational basis for a future study on the function of the upregulated isoforms, in particular PI3K p110γ isoform in stimulated CLL cells in order to establish its role in the pathogenesis of CLL. In conclusion, I have demonstrated that the PI3K-AKT pathway plays an important role in CLL-cell survival, growth and proliferation in response to CD40 stimulation, a stimulus that is highly relevant to the lymph node microenvironment in CLL