Cardiotoxin-III selectively enhances activation-induced apoptosis of human CD8+ T lymphocytes

Cardiotoxin-III (CTX-III), a major cardiotoxin isolated from the venom of the Taiwan cobra (Naja naja atra), is a highly basic, hydrophobic, toxic protein, which can induce lysis of mononuclear cells by an unknown mechanism. This study was undertaken to investigate the effects of CTX-III on untreated and PHA-activated peripheral blood mononuclear cells (PBMCs) in vitro. The results show that treatment of PHA-activated lymphocytes with CTX-III (10 μg/ml) induced apoptosis and depletion of the CD8+ population. In both untreated and PHA-treated lymphocytes, interferon-γ production was dramatically reduced and interleukin-2 (IL-2) production was moderately reduced by CTX-III treatment. In PHA-activated lymphocytes, CD4 expression was increased, whereas CD8 and IL-2R β chain (CD25) expression were decreased. In contrast, CTX-III had no effect on the viability of PHA-activated monocytes but significantly enhanced their tumor necrosis factor-α production. These results show that CTX-III selectively enhanced activation-induced apoptosis in CD8+ T cells. CTX-III was found to bind to the cell membrane of PHA-stimulated PBMCs, and three CTX-III-binding proteins, with molecular weights of 92, 77, and 68 kDa, were identified. We therefore propose that CTX-III interacts with one or more cell surface proteins and initiates a signal pathway causing functional changes. These findings provide an insight into the immunomodulatory properties of CTX-III and suggest a novel method for the selective induction of apoptosis in CD8+ T lymphocytes