Induction of nonspecific immunosuppression in a primitive teleost, rainbow trout (Salmo gairdneri)

Induction of immunological non-reactivity to goldfish red blood cells (GFRBC), sheep red blood cells (SRBC) and to formalin-killed Aeromonas salmonicida cells was obtained in yearling rainbow trout raised under laboratory aquarium conditions (at 10 degrees C). Haemolysin and agglutinin titres were measured 1 to 4 weeks after immunization. While trout immunized with low and high doses of antigen by intramuscular (im) and intraperitoneal (ip) injections produced high antibody titres, administration of high doses of antigen by intravascular (iv) injection induced a state of tolerance which was antigen nonspecific. This lack of response persisted for at least 3 weeks. The phagocytic activity of peritoneal exudate cells from immunosuppressed fish was reduced. The numbers of plaque-forming cells amongst peripheral lymphocytes collected from trout receiving a high dose of GFRBC iv was markedly low when compared with those from trout receiving a similar dose ip. These findings are of interest since they demonstrate that the induction of antigen-nonspecific tolerance by iv administration of high doses of antigen in rainbow trout may be due to the generation of nonspecific suppressor cells.