Abstract
Viruses have developed strategies to counteract the apoptotic response of the infected host cells. Modulation of apoptosis is also thought to be a major component of viral persistence and progression to leukemia induced by retroviruses like human T-lymphotropic virus type 1 (HTLV-1) and bovine leukemia virus (BLV). Here, we analyzed the mechanism of ex vivo apoptosis occurring after isolation of peripheral blood mononuclear cells from BLV-infected sheep. We show that spontaneous apoptosis of ovine B lymphocytes requires at least in part a caspase 8-dependent pathway regardless of viral infection. Cell death is independent of cytotoxic response and does not involve the tumor necrosis factor alpha/NF-kappaB/nitric oxide synthase/cyclooxygenase pathway. In contrast, pharmaceutical depletion of reduced glutathione (namely, gamma-glutamyl-l-cysteinyl-glycine [GSH]) by using ethacrynic acid or 1-pyrrolidinecarbodithioic acid specifically reverts inhibition of spontaneous apoptosis conferred indirectly by protective BLV-conditioned media; inversely, exogenously provided membrane-permeable GSH-monoethyl ester restores cell viability in B lymphocytes of BLV-infected sheep. Most importantly, intracellular GSH levels correlate with virus-associated protection against apoptosis but not with general inhibition of cell death induced by polyclonal activators, such as phorbol esters and ionomycin. Finally, inhibition of apoptosis does not correlate with the activities of GSH peroxidase and GSH reductase. In summary, our data fit into a model in which modulation of the glutathione system is a key event involved in indirect inhibition of apoptosis associated with BLV. These observations could have decisive effects during therapeutic treatment of delta-retroviral pathogenesis.