In previous work, lysis of allotargets was routinely observed with PBL
from nonimmune channel catfish. In the work reported here, greatly in
creased (similar to 100-fold) cytotoxic responses were generated by st
imulation of channel catfish PBL with irradiated cells of allogeneic c
loned B cell lines in mixed leukocyte cultures (MLC). This increased c
ytotoxicity did not appear to be simply a consequence of cell prolifer
ation since stimulation of catfish PBL proliferative responses with po
lyclonal mitogens did not result in increased lysis. Somewhat surprisi
ngly, 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 allospeci
ficity in MLC-generated cytotoxicity was confirmed by ''cold'' target
inhibition assays. However, autologous targets were not killed, clearl
y demonstrating that MLC-generated effecters could distinguish ''self'
from ''nonself' at the level of lysis/recognition. Although their ori
gin is unresolved, the MLC-generated effecters may be a source of high
ly enriched fish cytotoxic cells and thus facilitate directly addressi
ng questions pertaining to the evolution of such cells. (C) 1997 Acade
mic Press.