Prevention of B cell antigen receptor-induced apoptosis by ligation of CD40 occurs downstream of cell cycle regulation.
Cross-linking of the B cell antigen receptor (BCR) on germinal center B cells can induce growth arrest and apoptosis, thereby eliminating potentially autoreactive B cells. Using the Burkitt lymphoma cell line Ramos as a model, we studied the commitment to apoptosis following growth arrest, as well as how triggering of CD40 or addition of tumor necrosis factor (TNF)-alpha can interfere to block cell death. Both BCR triggering and direct induction of growth arrest by sodium butyrate (n-But) caused hypophosphorylation of the retinoblastoma protein (pRb), followed by apoptosis. Interestingly, although CD40 ligation or TNF-alpha efficiently prevented BCR-induced and n-But-induced apoptosis, these co-stimuli did not inhibit, but rather augmented, growth arrest. Analysis of cell cycle regulators showed that each apoptotic and T(h) stimulus distinctly affected cyclins or cyclin-dependent kinase inhibitors, indicating that growth arrest can be uncoupled from apoptosis. BCR ligation and growth arrest activated the intrinsic or mitochondrial route of apoptosis. CD40 ligation and TNF-alpha prevented release of cytochrome c and activation of caspase-3, which could not be explained by effects on the expression of Bcl-2, Bcl-x(L) or Bax. Finally, the onset of BCR-induced apoptosis occurred after 10-12 h and addition of CD40 mAb or TNF-alpha at that point still prevented further execution of apoptosis. We conclude that in mature B cells apoptosis is not an obligatory event following growth arrest. Instead, commitment to apoptosis can be rapidly controlled by T cells via CD40 ligand and TNF-alpha, downstream of the pRb-regulated restriction point of the cell cycle, but prior to mitochondrial cytochrome c release.
Mackus
Wendelina J M
WJ
Department of Hematology, Academic Medical Center, PO Box 22660, 1100 DD Amsterdam, The Netherlands.
Lens
Susanne M A
SM
Medema
René H
RH
Kwakkenbos
Mark J
MJ
Evers
Ludo M
LM
Oers
Marinus H J van
MH
Lier
René A W van
RA
Eldering
Eric
E
D017209:Apoptosis / Q000187:drug effects / Q000276:immunology / Q000502:physiology*; D019013:CD40 Antigens / Q000276:immunology / Q000378:metabolism*; D002453:Cell Cycle / Q000187:drug effects / Q000502:physiology*; D003574:Cytochrome c Group / Q000502:physiology; D006801:Humans; D011947:Receptors, Antigen, B-Cell / Q000502:physiology*; D014407:Tumor Cells, Cultured; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology
D019013:CD40 Antigens; D003574:Cytochrome c Group; D011947:Receptors, Antigen, B-Cell; D014409:Tumor Necrosis Factor-alpha