Dietary Histidine, Threonine, or Taurine Supplementation Affects Gilthead Seabream (Sparus aurata) Immune Status

SIMPLE SUMMARY: The concept of supporting animal health through the best possible nutrition is well-accepted in modern aquaculture, and functional amino acids (AAs) appear to be good candidates to improve health and growth performance. For instance, histidine (His), taurine (Tau), and threonine (Thr) appear to play important roles in homeostatic maintenance, detoxification of reactive species, and immune function. The present study aimed to evaluate the effects of His, Tau, and Thr supplementation on the gilthead seabream (Sparus aurata) immune status. In general, the results suggest that dietary supplementation with His, Tau, or Thr above their nominal requirements for this species has relatively mild effects. Still, some effects of threonine and taurine supplementation on the fish immune system were observed, particularly after a short-term feeding period (four weeks), which reinforces the importance of feeding period when aiming to improve immune alertness. Hence, further studies with other supplementation levels and eventually duration of supplementation could help to clarify the potential immunomodulatory role of these AAs for gilthead seabream. ABSTRACT: AAs have become interesting feed ingredients to be used in functional fish feeds as not only are they protein building blocks, but they also participate in several other key metabolic processes. In the present study, a comprehensive analysis of transcriptomics, hematology, and humoral immune parameters (plasma and skin mucus) were measured twice over the course of the feeding trial (four weeks). Plasma antiprotease activity increased in fish fed Thr compared to those fed the CTRL and Tau treatments, regardless of sampling time. The bactericidal activity in skin mucus decreased in fish fed Tau and His treatments compared to those fed the CTRL diet after two weeks. The membrane IgT (mIgT) was upregulated in fish fed Tau after four weeks, while C-type lectin domain family domain 10 member (clec10a) was downregulated in fish fed Thr after two weeks of feeding. By comparing the molecular signatures of head-kidney by means of a PLS-DA, it is possible to visualize that the main difference is between the two sampling points, regardless of diet. Altogether, these results suggest that dietary supplementation with these AAs at the tested levels causes mild immune-modulation effects in gilthead seabream, which should be further studied under disease challenge conditions.