Lymphoma cells protected from apoptosis by dysregulated bcl-2 continue to bind Annexin V in response to B-cell receptor engagement: A cautionary tale

Translocation of phosphatidylserine (PS) from the inner to outer leaflet of the surface membrane lipid bilayer, a characteristic early event of cells entering the apoptotic program, is routinely assessed by the Ca2+-dependent binding of Annexin V (AV). Here, we show that lymphoma cells protected from apoptosis by expression of a bcl-2 transgene or by virtue of the t(14;18)(q32;q21) translocation continue to register enhanced AV binding in response to BCR crosslinking. Induced AV binding appeared BCR-selective in that it did not proceed in Bcl-2high cells in response to calcium ionophore or the antidepressant fluoxetine, each of which activate the full apoptotic program in Bcl-2low equivalents. AV-positive cells did increase on crosslinking the BCR co-receptor CD19, despite it being a completely non-apoptotic signal. These findings advise caution when interpreting studies where Annexin V binding is used as a sole, or major, indicator of apoptotic death among lymphoma B cells.