Perforin -

Cytotoxic T Lymphocyte-Mediated Apoptosis Cytotoxic T lymphocytes (CTLs) recognize foreign antigens presented on the surface of infected host cells, Upon activation, CTLs secrete perforin, a transmembrane pore-forming molecule, which promotes entry of the CTL granule serine proteases called granzymes. Granzymes cleave proteins at aspartate residues and thus activate a variety of cellular caspases. In this way, the CTL kills target cells by directly inducing the effector phase of apoptosis.