Dietary Tryptophan Plays a Role as an Anti-Inflammatory Agent in European Seabass (Dicentrarchus labrax) Juveniles during Chronic Inflammation

Abstract

Where teleost fish are concerned, studies in tryptophan immunomodulation generally point to immunosuppressive properties, thus presenting a potential anti-inflammatory dietary strategy. The goal of the present work was to evaluate the effects of tryptophan dietary supplementation on immune and neuroendocrine responses of the European seabass, Dicentrarchus labrax, undergoing chronic inflammation. Juvenile European seabass were intraperitoneally injected with either Freund's Incomplete Adjuvant (FIA, inflamed group) or a saline solution (control group). Within each group, fish were fed a control (CTRL) and a CTRL-based diet supplemented with tryptophan (0.3% DM basis; TRP) for 4 weeks. Different tissues were sampled every week for the assessment of immune-related parameters. When TRP was provided to FIA-injected fish, mcsfr gene expression increased from 1 to 2 weeks and remained high until the end of the experiment. The same fish showed a concurrent increase in peripheral monocyte counts. Moreover, il34 expression at 1 week post-FIA injection was higher in TRP-fed than in CTRL-fed fish. After one week, molecular patterns of anti-inflammatory processes seemed to be favoured by TRP (mcsfr, gr1, il34 and tgfβ). Altogether, the results show that the feeding period seems to be critical where tryptophan supplementation is concerned since at later inflammatory stages-and longer feeding periods-fish fed TRP displayed a molecular profile similar to that of the CTRL group. In contrast, shorter administration periods might accelerate immune regulatory pathways.

Keywords: functional ingredient; inflammation; innate immunity; serotonergic activity.

Figure 1

Peripheral neutrophils (A), monocytes (B), lymphocytes (C) and thrombocytes concentrations (D) in European seabass at 1, 2, 3 and 4 weeks following intraperitoneal injection with HBSS or FIA (means ± SD, n = 9). Capital letters stand for significant differences attributed to dietary treatment (A < B); lowercase letters indicate significant differences attributed to sampling time (a < b); different symbols stand for significant differences between stimuli (* < #); (multifactorial ANOVA followed by Tukey post hoc test to identify significantly different groups; p ≤ 0.05).

Figure 2

Total WBC (A), macrophages (B), neutrophils (C) and lymphocyte concentrations (D) in the peritoneal cavity of European seabass at 1, 2, 3 and 4 weeks following intraperitoneal injection with HBSS or FIA (means ± SD, n = 9). Lowercase letters indicate significant differences attributed to sampling time (a < b); different symbols stand for significant differences between stimuli (* < #); (multifactorial ANOVA followed by Tukey post hoc test to identify significantly different groups; p ≤ 0.05).

Figure 3

Cortisol (A) and total bactericidal activity (B) in plasma of European seabass before (0 h) or at 1, 2, 3 and 4 weeks following intraperitoneal injection with HBSS or FIA (means ± SD, n = 9). multifactorial ANOVA followed by Tukey post hoc test to identify significantly different groups (p ≤ 0.05).

Figure 4

Superoxide dismutase (A), catalase (B) and total bactericidal activity (C) in the gut of European seabass before (0 h) or at 1, 2, 3 and 4 weeks following intraperitoneal injection with HBSS or FIA (means ± SD, n = 9). Capital letters stand for significant differences attributed to dietary treatment (A < B); lowercase letters indicate significant differences attributed to sampling time (a < b); different symbols stand for significant differences between stimuli (* < #); (multifactorial ANOVA followed by Tukey post hoc test to identify significantly different groups; p ≤ 0.05).

Figure 5

Gene expression of transforming growth factor β (A, tgfβ), interleukin 10 (B, il10), macrophage colony-stimulating factor receptor (C, mcsfr) and interleukin 34 (D, il34) in the head kidney of European seabass before (0 h) or at 1, 2, 3 and 4 weeks following intraperitoneal injection with HBSS or FIA (means ± SD, n = 9). Capital letters stand for significant differences attributed to dietary treatment (A < B); lowercase letters indicate significant differences attributed to sampling time (a < b); different symbols stand for significant differences between stimuli (* < #); (multifactorial ANOVA followed by Tukey post hoc test to identify significantly different groups; p ≤ 0.05).

Figure 6

Canonical discriminant analysis of gene expression in the head kidney of European seabass sampled at one or four weeks following intra-peritoneal injection with FIA: (A) correlation variables/factors (factor loads) for two main discriminant functions (F1 and F2); (B) canonical discriminant scores of each group. Group centroids are marked by a small diamond, whereas circles indicate data distribution per group.