The ontogeny of humoral immunity in rainbow trout, Salmo gairdneri

The ontogeny of the humoral immune response to a 'thymus dependent' and a 'thymus independent' antigen, human gamma globulin (HGG) and Aeromonas salmonicida (AS) respectively, was investigated in the fry of rainbow trout, Salmo gairdneri, by direct immersion vaccination in the antigens (dose 5 mg/L HGG; 10(8) cells/ml AS; 30 minutes) at known ages/weights from 7 days post hatch, and at 1, 2, 3, and 4 months post hatch. Half the fry in each group were tested for antibodies 4 weeks after vaccination, the remainder were reimmunised and tested again after a further 4 weeks. Appropriate controls to test for tolerance induction and memory responses were included. The results indicate that fry are capable of mounting a humoral immune response very early in ontogeny. There is a period of 'unresponsiveness' which persists for longer against HGG, than against AS, though it was not thought to be tolerance as such. Memory could be detected to HGG in fry given a first immunisation at 2 months. The results are compared with preliminary experiments in which fry were first thymectomised 4 weeks before the first immunisation. In fry thymectomised at 1 month post hatch, and tested for primary and secondary responses at 2 and 3 months, the primary response to HGG is unaltered, but the secondary response is reduced. Both the primary and secondary response to AS is unaltered. When thymectomy is performed later, the effect on the secondary response to HGG is no longer apparent, but the primary response to AS is slightly reduced.