Induction of Target Cell Apoptosis by Channel Catfish Cytotoxic Cells

This study examines cytotoxic mechanisms used by channel catfish peripheral blood-derived effector cells. Transmission electron microscopy (TEM), coupled with [3H]thymidine DNA fragmentation (JAM) and terminal deoxynucleotidyl nick-end labeling (TUNEL) assays, provided the first evidence that catfish peripheral blood cytotoxic effectors killed allogeneic targets via an apoptotic pathway. TEM demonstrated that the effector cell population present within peripheral blood leukocytes (PBLs) was composed of agranular lymphocytes that formed conjugates with, and induced apoptosis in, allogeneic target cells. Both JAM and TUNEL assays showed that PBLs induced target cell DNA fragmentation within 1 h of coculture. In addition, fixed effectors did not induce target cell necrosis or apoptosis, and target cell lysis was completely inhibited by chelation of free Ca2+ by EGTA. These results suggest that catfish peripheral blood-derived effector cells utilize a secretory mechanism rather than a ligand-based mechanism to trigger apoptosis.