Spontaneous in Vitro Death of CLL Cells Can Be Prevented by Mitochondrial Stabilization Via CD160 Signaling.

Abstract 4372

B-cell chronic lymphocytic leukemia (CLL) is an incurable disease, which is at least partly attributable to the majority of cells being in the G0/G1 phase of the cell cycle and expressing high levels of anti-apoptotic Bcl-2 family proteins. Despite their prolonged survival in vivo, CLL cells rapidly undergo spontaneous apoptosis in vitro, suggesting that survival signals in vivo have been lost in in vitro culture conditions. CD160, a glycosylphosphatidylinositol-linked surface antigen, was found to be expressed by CLL cells. In normal NK and T-cells, CD160 mediates cellular growth and activation, but its role in CLL is unclear. Using monoclonal antibodies to CD160 (CL1-R2 or BY55 - non cross blocking) led to increased expression of Bcl-2, Bcl-xL and Mcl-1 anti-apoptotic proteins and protected CLL from spontaneous apoptosis in vitro - mean cell viability increased from 66.8 to 79.4% (n = 17, p = 0.02). These CD160-mediated events were also accompanied by decreased cytochrome C release and prevention of mitochondrial membrane potential collapse, indicating stabilization of both inner and outer mitochondrial membrane integrity. PI3K/AKT signalling is a well known survival pathway in cancer cells and in normal lymphocytes CD160 has been shown to act via PI3K/AKT. Activation of CD160 in CLL led to phosphorylated AKT, while inhibition of PI3K by wortmannin completely blocked AKT phosphorylation and CD160-mediated protection from apoptosis.

In summary, the activation of CD160 protected CLL cells from spontaneous cell death in vitro via a PI3-kinase/AKT pathway. This improved survival was also associated with increased Bcl-2, Bcl-xL and Mcl-1 expression and preservation of mitochondrial function.