Effect of anti-APO1 on spontaneous apoptosis of B cells in chronic lymphocytic leukaemia: the role of bcl-2 and interleukin 4.

The cell surface protein apolipoprotein 1 (APO1) is expressed on various cell types including malignant lymphoid cells. Triggering of APO1 protein with antibody (Ab) induces apoptosis in APO1-expressing cells. We examined the effect of anti (alpha) APO1 Ab on spontaneous apoptosis (SA) and bcl-2 expression in B cell chronic lymphocytic leukaemia (B-CLL) in vitro. We also investigated the anti-apoptotic activity of interleukin 4 (IL4) on the aAPO1-induced apoptosis in B-CLL cells. Although expression of APO1 on B-CLL cells was not detectable by immunofluorescence, alpha APO1 Ab induced apoptosis in these cells. At 24 hours in culture the number of apoptotic cells was increased by a mean percentage (%) of 27% (range: 21-38) in only half of the cases studied. But in all twelve cases studied, at 48 hours alpha APO1 increased SA by a mean of 72% (range: 26-114) (P < .001) and at 72 hours, the mean % increase was 69% (range: 31-96) in 6/7 cases (P < .001). This effect was alpha APO1 concentration dependent. Interleukin 4 significantly protected B-CLL cells against alpha APO1-induced apoptosis by a mean of 53% (range: 28-76) (P < .001). This protection was specific to IL4 and it was significantly reduced or abolished with alpha IL4 Ab. Expression of bcl-2 protein in untreated cultures was not significantly different from that of the alpha APO1-treated cells; the mean equivalent of soluble fluorochrome (MESF) (range) was 4.9 (3.0-6.8) and 5.2 (3.5-6.0) respectively (P > 0.2). In fresh B-CLL cells the MESF (range) was 4.5 (2.4-6.6). Thus alpha APO1 Ab induced apoptosis in B-CLL cells by a pathway that is independent of bcl-2 expression and partially blocked by IL4.