Tryptophan Modulatory Role in European Seabass (Dicentrarchus labrax) Immune Response to Acute Inflammation under Stressful Conditions

Abstract

The present work aimed to study the role of dietary tryptophan supplementation in modulating the European seabass (Dicentrarchus labrax) immune condition during stressful rearing conditions (i.e., 15 days exposure to high density), as well as the immune response to acute inflammation after intraperitoneal injection of a bacterial pathogen. Stress alone did not compromise seabass health indicators. In contrast, a clear peripheral and local inflammatory response was observed in response to the inoculated bacteria. Moreover, exposure to a high stocking density seemed to exacerbate the inflammatory response at early sampling points, compared to fish stocked at a lower density. In contrast, stressed fish presented some immune-suppressing effects on the T-cell surface glycoprotein receptor expressions at a late sampling point following inflammation. Regarding the effects of dietary tryptophan, no changes were observed on seabass immune indicators prior to inflammation, while a small number of immunosuppressive effects were observed in response to inflammation, supporting tryptophan's role in the promotion of immune-tolerance signals during inflammation. Nonetheless, tryptophan dietary supplementation improved the inflammatory response against a bacterial pathogen during stressful conditions, supported by a reduction of plasma cortisol levels, an up-regulation of several immune-related genes at 48 h, and an inversion of the previously observed, stress-induced T-cell suppression. Finally, the involvement of tryptophan catabolism in macrophages was confirmed by the up-regulation of genes involved in the kynurenine pathway. The present study brings new insights regarding the immune modulatory role of tryptophan during stressful conditions in fish, thus allowing for the development of novel prophylactic protocols during vaccination by intraperitoneal injection in the European seabass.

Keywords: aquaculture; functional diets; innate immunity; stress; tryptophan.

Figure 1

Quantitative expression of (A) transforming growth factor-beta, (B) tumor necrosis factor alpha, (C) chemokine CXC receptor 4, (D) interleukin 1 beta, (E) cluster of differentiation 8 alpha chain, (F) cluster of differentiation 8 beta chain in head kidney of European seabass reared at high density, fed dietary treatments for 15 days (0 h), and sampled 4, 24, 48, and 72 h post-bacterial challenge. Quantitative expression of (G) T-cell surface glycoprotein cd4, (H) T-cell surface glycoprotein cd3 zeta chain, (I) T-cell receptor alpha chain, (J) melanocortin 2 receptor, (K) arylformamidase-like and (L) indoleamine dioxygenase 2 in head kidney of European seabass reared at high density, fed dietary treatments for 15 days (0 h), and sampled 4, 24, 48, and 72 h post-bacterial challenge. Values represent means ± SD (n = 10). Different symbols stand for statistically significant differences attributed to stress. Capital letters stand for statistically significant differences attributed to sampling time. Lower case letters stand for significant differences attributed to dietary treatment (multifactorial ANOVA; Tukey post-hoc test; ns: non-significant; p ≤ 0.05).

Figure 1

Quantitative expression of (A) transforming growth factor-beta, (B) tumor necrosis factor alpha, (C) chemokine CXC receptor 4, (D) interleukin 1 beta, (E) cluster of differentiation 8 alpha chain, (F) cluster of differentiation 8 beta chain in head kidney of European seabass reared at high density, fed dietary treatments for 15 days (0 h), and sampled 4, 24, 48, and 72 h post-bacterial challenge. Quantitative expression of (G) T-cell surface glycoprotein cd4, (H) T-cell surface glycoprotein cd3 zeta chain, (I) T-cell receptor alpha chain, (J) melanocortin 2 receptor, (K) arylformamidase-like and (L) indoleamine dioxygenase 2 in head kidney of European seabass reared at high density, fed dietary treatments for 15 days (0 h), and sampled 4, 24, 48, and 72 h post-bacterial challenge. Values represent means ± SD (n = 10). Different symbols stand for statistically significant differences attributed to stress. Capital letters stand for statistically significant differences attributed to sampling time. Lower case letters stand for significant differences attributed to dietary treatment (multifactorial ANOVA; Tukey post-hoc test; ns: non-significant; p ≤ 0.05).

Figure 2

Trial design.