Dependence of granzyme B-mediated cell death on a pathway regulated by Bcl-2 or its viral homolog, BHRF1

The molecular pathways responsible for apoptosis in response to granzyme B have remained unresolved. Here we present data supporting the notion that granzyme B-mediated cell death is largely dependent on a pathway that is inhibitable by Bcl-2 or its viral analog BHRF1. We used a panel of stably transfected FDC-P1 mouse myeloid cell lines to show that overexpression of functional, wild-type Bcl-2 or BHRF1 rescued cells from granzyme B-mediated apoptosis, whereas mutated (Gly145-->Glu) Bcl-2, or wild-type Bcl-2 directed to the plasma membrane conferred no protection. Overexpression of Bcl-2 resulted in inhibition of multiple parameters of apoptosis in response to purified perforin and granzyme B, including DNA fragmentation, changes in light scatter profile indicating cell shrinkage and increased refractivity, loss of mitochondrial membrane potential and inhibited colony formation in clonogenic assays. Nevertheless, when exposed to cytotoxic lymphocytes, FDC-P1 and YAC-1 cells overexpressing Bcl-2 remained susceptible to death imparted by cytolytic granules, irrespective of whether the granules contained granzyme B. Thus, alternative granzyme B-independent pathways can be activated by intact lymphocytes to overcome Bcl-2-like inhibitors of apoptosis, enabling CTLs to overcome potential viral blocks to granzyme B-mediated cell death.