Mechanisms of Antigen Receptor-Dependent Apoptosis of Human B Lymphoma Cells Probed with a Panel of 27 Monoclonal Antibodies

The present study has used a panel of 23 monoclonal antibodies to IgM and 4 to IgD in order to probe parameters influencing sIg-dependent apoptosis in an IgM/IgD-expressing Burkitt lymphoma line. No direct correlation was observed between the capacity of the different anti-μ to drive cells into apoptosis and either their domain specificity or their affinity for sIgM. There was, however, a direct correlation between the functional outcome and the ability of the monoclonal antibodies to elicit a rise in intracellular Ca2+. For apoptosis to occur, the Ca2+response had to attain a threshold value of approximately 100 nM. A direct role for Ca2+in the delivery of the apoptotic signal was demonstrated using thapsigargin to raise intracellular Ca2+levels. Antigen receptor ligation was linked to Ca2+increases by tyrosine kinases as revealed by direct analysis of protein tyrosine phosphorylation and the effects of selective protein tyrosine kinase-inhibiting tyrphostins. These findings reveal a central role for the antigen receptor-generated Ca2+signal in driving apoptosis in human B lymphoma cells and stresses the need to use a panel of reagents when probing function with presumed ligand–mimetic monoclonal antibodies.