BCR engagement protects CLL cells from undergoing apoptosis. Presented are contour maps of CLL B cells from one patient defining the relative green (DiOC₆) and red (PI) fluorescence intensities of CLL cells on the horizontal and vertical axes, respectively. The viability was determined after 24 hours of culture in medium alone (control) or medium supplemented with anti-IgM mAbs at the concentrations indicated above each of the contour maps. This stain allows us to gate on the vital cell population (DiOC₆^bright, PI^exclusion), apoptotic cells (DiOC₆^dim, PI^exclusion), and the dead cells (DiOC₆^dim, PI^positive). The percentage of each cell population is indicated next to each of these gates. In this case, anti-IgM stimulation protected approximately 20% of the cells from undergoing apoptosis, resulting in a viable population of 56.5% at 14 μg/mL anti-IgM and 55.1% at 42 μg/mL anti-IgM. The bar diagram on the lower right displays an increased viability of CLL cells after culture with anti-IgM in 4 different CLL samples. CLL cells were cultured in medium alone (control) or medium supplemented with anti-IgM mAbs at the indicated concentrations, and the viability was assessed after 24 and 48 hours by staining with DiOC₆ and PI. Displayed are the means plus or minus SD of 4 different patient samples.