Ligation of CD22 with blocking MoAbs induces apoptosis in several Burkitt lymphoma cell lines. Twenty micrograms per milliliter of the blocking (HB22.7 and HB22.33) or nonblocking (HB22.27) MoAbs were incubated with the Burkitt lymphoma cell lines Ramos, ST486, and BL41 for 48 hours. Apoptosis was assessed by propidium iodide staining and BRDU incorporation as described in Fig 5.