Signaling events involved in anti-CD20-induced apoptosis of malignant human B cells.

Publication Type:

Journal Article


Cancer immunology, immunotherapy : CII, Volume 48, Issue 12, p.673-83 (2000)


Amino Acid Chloromethyl Ketones, Animals, Antibodies, Monoclonal, Antibodies, Monoclonal, Murine-Derived, Antigens, CD20, Antigens, CD95, APOPTOSIS, B-Lymphocytes, bcl-Associated Death Protein, Burkitt Lymphoma, Calcium, Calcium Signaling, Carrier Proteins, Caspases, Chelating Agents, Cysteine Proteinase Inhibitors, Enzyme Activation, Humans, Immunoglobulin Fc Fragments, Ionomycin, Lymphoma, B-Cell, Lymphoma, T-Cell, MICE, Phosphoprotein Phosphatases, PHOSPHORYLATION, Poly(ADP-ribose) Polymerases, Protein Processing, Post-Translational, Protein-Tyrosine Kinases, Proto-Oncogene Proteins c-bcl-2, Receptor Aggregation, Recombinant Fusion Proteins, Signal Transduction, Tumor Cells, Cultured


Anti-CD20 monoclonal antibodies have been successfully employed in the clinical treatment of non-Hodgkin's lymphomas in both unmodified and radio-labeled forms. Previous publications have demonstrated that the antitumor effects of unmodified anti-CD20 mAb are mediated by several mechanisms including antibody-dependent cellular cytotoxicity, complement-mediated cell lysis, and induction of apoptosis by CD20 cross-linking. In this report, we demonstrate induction of apoptosis by three anti-CD20 monoclonal antibodies [1F5, anti-B1, and C2B8 (Rituximab)]. The magnitude of apoptosis induction was greater with the chimeric Rituximab antibody than with the murine 1F5 and anti-B1 antibodies. Apoptosis could be enhanced with any of the antibodies by cross-linking with secondary antibodies (or Fc-receptor-bearing accessory cells). The signaling events involved in anti-CD20-induced apoptosis were investigated, including activation of protein tyrosine kinases, increases in intracellular Ca2+ concentrations, caspase activation, and cleavage of caspase substrates. Our results indicate that anti-CD20-induced apoptosis can be attenuated by PP1, an inhibitor of protein tyrosine kinases Lck and Fyn, chelators of extracellular or intracellular Ca2+, and inhibitors of caspases, suggesting that anti-CD20-induced apoptosis may involve modulation of these signaling molecules. We also demonstrated that varying the expression of Bc1-2 did not affect the magnitude of anti-B1-induced apoptosis, possibly because of the sequestering effects of other Bc1-2 family members, such as Bad. These studies identify several of the signal-transduction events involved in the apoptosis of malignant B cells that transpire following ligation of CD20 by anti-CD20 antibodies in the presence of Fc-receptor-expressing cells or secondary goat anti-(mouse Ig) antibodies and which may contribute to the tumor regressions observed in mouse models and clinical trials.