Elevated Bcl-2/Bax are a consistent feature of apoptosis resistance in B-cell chronic lymphocytic leukaemia and are correlated with in vivo chemoresistance.

We investigated the relationship between drug resistance and Bcl-2/Bax in B-cell chronic lymphocytic leukaemia (B-CLL). Apoptosis was induced in vitro with chlorambucil and cell death was monitored by dual-labelled FACS analysis using Annexin V and propidium iodide. Bcl-2 and Bax protein expression was quantified using FACS and a correlation between drug-induced apoptosis and Bcl-2/Bax was established. Cells were then sorted into viable and nonviable populations according to their forward and side-scatter characteristics and re-analysed for Bcl-2/Bax. The most resistant cells had elevated Bcl-2 levels and low Bax expression. Furthermore, those cells which were undergoing apoptosis showed only a marginal reduction in Bcl-2 expression, but significantly elevated Bax expression following exposure to chlorambucil. The Bcl-2/Bax was significantly greater in the cell fractions resistant to chlorambucil-induced apoptosis. This observation further supports the suggestion that Bax is the pivotal protein in determining the fate of cells following apoptotic signals.