Cytotoxic activity generated from channel catfish peripheral blood leukocytes in mixed leukocyte cultures

In previous work, lysis of allotargets was routinely observed with PBL from nonimmune channel catfish. In the work reported here, greatly increased (approximately 100-fold) cytotoxic responses were generated by stimulation of channel catfish PBL with irradiated cells of allogeneic cloned B cell lines in mixed leukocyte cultures (MLC). This increased cytotoxicity did not appear to be simply a consequence of cell proliferation since stimulation of catfish PBL proliferative responses with polyclonal mitogens did not result in increased lysis. Somewhat surprisingly, the MLC-generated cytotoxicity did not exhibit allospecificity; i.e., allogeneic targets from other fish were as susceptible to lysis as were the cells used as stimulators. This apparent lack of allospecificity in MLC-generated cytotoxicity was confirmed by "cold" target inhibition assays. However, autologous targets were not killed, clearly demonstrating that MLC-generated effectors could distinguish "self" from "nonself" at the level of lysis/recognition. Although their origin is unresolved, the MLC-generated effectors may be a source of highly enriched fish cytotoxic cells and thus facilitate directly addressing questions pertaining to the evolution of such cells.