Caspase-independent killing of Burkitt lymphoma cell lines by rituximab

Caspase-independent cell death may have a critical role to play in the therapeutic destruction of tumours. Recently it has been suggested that one of the mechanisms by which rituximab, a therapeutic anti-CD20 antibody, kills B cells is caspase-independent. In this study we show that rituximab can induce death in a variety of Burkitt lymphoma derived cell lines. Rituximab-treated cells show leakage of adenylate kinase, surface expression of phosphatidylserine, upregulation of the cellular stress protein HSP70, phosphorylation of the survival protein Akt, and depolarisation of the mitochondrial membrane but no loss of cytochrome c or apoptosis inducing factor. Caspase inhibitors do not block these events. In support of these data there is no cleavage of caspases 3, 8 and 9, poly(ADP-ribose) polymerase, BH3 interacting domain death agonist or genomic DNA. Morphologically, cells show nuclear enlargement and cytoplasmic vacuolisation. Triggering of receptor mediated death in CD95 responsive lines results in classical apoptosis indicating that the internal machinery necessary for apoptosis is intact in these lines. The results suggest that rituximab can kill human B cells via a caspase-independent form of programmed cell death that shares features of apoptosis and necrosis. This pathway may be relevant to the clinical efficacy of rituximab.