Immunomodulation by dietary vitamin C and glucan in rainbow trout (Oncorhynchus mykiss)

The aim of this study was to determine the influence of dietary glucan and vitamin C, alone and in combination, on the cellular and humoral immune responses of rainbow trout. After 4 weeks of adaptation on a control diet containing glucan at 0 ppm and vitamin C at 150 ppm, four groups of rainbow trout were fed the experimental diets containing vitamin C at 150 and 1000 ppm plus a fixed amount of glucan, or no glucan for 2 weeks. The fish were then switched back to the control diet. Non-specific immunity was investigated at the end of the two-week experimental feeding period, and then again 4 weeks later. Macrophage activities such as oxidative burst and pinocytosis were determined along with complement activation and lysozyme levels. The specific immune response was evaluated as: (1) proliferation of lymphocytes induced by mitogen and (2) the antibody response after vaccination against enteric redmouth disease. Fish were vaccinated at the end of the 2-week experimental feeding period and antibody titres were determined every second week for the 16 following weeks. The results of this trial showed a significant effect of dietary vitamin C but not glucan on the non-specific immune response of rainbow trout such as increased oxidative burst, pino-cytosis and lysozyme activity. Dietary treatments had no effect on alternative pathway of complement activation. A significant enhancing effect of dietary beta-1,3/1,6 yeast glucan was observed on concanavalin A-induced proliferation of lymphocytes and the antibody response after vaccination against enteric redmouth disease. When considering the antibody titres at the peak time, glucan or vitamin C significantly enhanced the response. This was also observed on serum complement levels measured at the same time. Results from this trial helped to establish the beneficial effect of a dietary combination of glucan and vitamin C on the immune response of rainbow trout, as well as to evaluate the specific effect of each substance.