Tryptophan has been shown to play several roles in the modulation of fish stress and immune responses, and might serve as a complementary strategy alongside vaccination in aquaculture. The present study evaluated the synergistic effects of dietary tryptophan supplementation and vaccination in European seabass (Dicentrarchus labrax) juveniles’ immunity and resistance to infection with Tenacibaculum maritimum.
European seabass juveniles (26.23 ± 7.22 g) were randomly distributed into three recirculating seawater systems. In a complete randomized design, fish were fed a control diet (CTRL) and two CTRL-based diets supplemented with tryptophan (TRP1 0.05% DM and TRP2 0.5% DM) in triplicate tanks of each system for 3 days. Fish from system 1 were then sampled for blood, liver, and head-kidney (0h). The remaining fish from system 1 were bath vaccinated and similarly sampled after 1 and 6 hours and 21 days. Fish were fed the same diets for 3 days more and after that, all fish were fed the CTRL diet until the end of the trial (21 days). Fish from system 2 were left undisturbed until the end of the trial, but were subjected to the same feeding strategy. Fish from system 3 were subjected to a bath challenge with T. maritimum (5 x 106 CFU mL-1) and cumulative mortality was monitored daily for 10 days.
Tryptophan surplus for 3 days did not significantly affect disease resistance. However, results suggest that tryptophan surplus may induce variable physiological effects. Non-vaccinated fish fed TRP diets presented higher hepatic superoxide dismutase (SOD), elevated plasma cortisol and down-regulation of immune-related genes, in a dose-related manner. These results suggest that TRP diets per se, might have triggered regulatory mechanisms by increasing cortisol production.
At 6 h post vaccination, TRP1 dietary treatment inhibited SOD activity and plasma cortisol levels. In addition, the expression of head-kidney tryptophan hydroxylase was downregulated, while plasma total IgM levels and humoral immune parameters were enhanced. These data could indicate a beneficial role of the low tryptophan supplementation level (TRP1) upon vaccination, enhancing humoral responses while counteracting stress-associated cortisol increase.
Regarding the long-term synergistic effects of a 3 days-tryptophan supplementation with vaccination (sampling at 21 days), fish fed TRP2 showed lower expression of indoleamine 2,3 dioxygenase and plasma alternative complement pathway activity compared to TRP1. However, when vaccinated fish were provided with a lower level of tryptophan supplementation (TRP1) their molecular profile was more similar to that of CTRL-fed, non-vaccinated fish. Despite no signals of increased vaccination efficiency were verified (plasma IgM levels), results suggest that tryptophan might at least have a homeostatic effect on the immune system of fish subjected to an acute stress such as a bath vaccination procedure.
Overall, the findings suggest a complex interplay between dietary tryptophan supplementation, immune responses, and immunization mechanisms in fish.
This work was supported by IMMUNAA project (PTDC/CVT-CVT/7741/2020), financed through Fundação para a Ciência e a Tecnologia (FCT). DP, IC, MM, BC, and RA were supported by FCT, Portugal (UI/BD/150900/2021, 2021.04867.BD, 2022.03304.CEECIND, 2020.00290.CEECIND and 2022.03248.CEECIND, respectively).