Analysis of apoptosis after anti-CD44 ligation. (A) Apoptosis was analyzed after incubation of cells for 24 h with anti-CD44 mAb (20 μg/ml) or isotope control antibody (20 μg/ml) by annexin-V/PI staining and measured by flow cytometry as shown for cell lines EB-1 (CD44-) and REC-1 (CD44+). Data are representative of three independent experiments with similar results. (B) Mean of early apoptotic cells (annexin-V+, PI-) in two CD44+ (REC-1 and KM-H2) and two CD44- (EB-1 and KARPAS-299) lymphoma cell lines after treatment with anti-CD44 in different concentrations for 24 h (n = 3). The number of early apoptotic cells in the corresponding isotope controls were set to 1. Note that anti-CD44 induced apoptosis occurred only in the CD44+ cell lines.