Identification of a subpopulation of leucocytes from the rainbow trout (Oncorhynchus mykiss) responsive to pokeweed mitogen

Several compounds which are known to be potent polyclonal activators of lymphocytes in mammals have also been demonstrated to have mitogenic activity for leucocytes from a number of fish species, including rainbow trout. In the present study, the rainbow trout leucocyte subpopulation responsive to pokeweed mitogen (PWM) was investigated. The known specificity of mammalian T cell mitogens phytohaemagglutinin (PHA) and concanavalan A (ConA) for fish T cells, and of the mammalian B cell mitogen lipopolysaccharide (LPS) for fish B cells was utilised to elucidate the PWM-responsive population. When used to stimulate trout leucocytes in combination with either PHA, ConA or LPS, PWM was shown to produce an enhanced proliferative effect in combination with LPS. In contrast, PWM failed to augment cell proliferation in co-cultures containing PHA or ConA. Furthermore, trout leucocytes depleted of sIg+cells by panning exhibited enhanced responses to PHA and PWM. Taken together, these results indicate that in trout, T cells represent the predominant leucocyte population responding to stimulation with PWM.