Abstract
This study aims to investigate the effects of threonine (Thr) on immunoregulation in vivo and in vitro of teleost grass carp (Ctenopharyngodon idella). Juveniles (9.53 ± 0.02 g) were reared for 8 weeks with respective Thr diet (3.99, 7.70, 10.72, 14.10, 17.96, and 21.66 g/kg) and then challenged with Aeromonas hydrophila for in vivo study. Macrophages isolated from head kidney were treated in vitro for 48 h with L-Thr (0, 0.5, 1.0, 2.0, 4.0, and 8.0 mM) after 6 h of lipopolysaccharide induction. The results showed that, compared with Thr deficiency (3.99 g/kg), the optimal dietary Thr (14.10g/kg) affected the immunocyte activation in the head kidney (HK) and spleen (SP) by downregulating the mRNA expressions of MHC-II and upregulating CD4 (not CD8), and it mediated the innate immune by enhancing the activities of lysozyme (LZ), acid phosphatase content of complement 3 (C3) and C4, increasing the mRNA abundances of hepcidin, liver expressed antimicrobial peptide-2A (LEAP-2A), LEAP-2B, β-defensin1, downregulating tumor necrosis factor α (TNF-α), IL-6, IL-1β, IL-12p35, IL-12p40, IL-17AF1, and IL-17D partly by attenuating RORγ1 transcriptional factor and nuclear factor kappa B p65 (NF-κBp65) signaling cascades [IKKβ/IκBα/NF-κBp65] and upregulating transforming growth factor β1 (TGF-β1), IL-4/13A, -4/13B, IL-10, and IL-22 partly by GATA-3. Besides these, the optimal dietary Thr regulated the adaptive immune by upregulating the mRNAs of immunoglobulin M (IgM) and IgZ (not IgD). Moreover, 2 mM Thr downregulated in vitro the mRNA abundances of colony stimulating factor-1, inducible nitric oxide synthase, mannose receptor 1, matrix metalloproteinase2 (MMP-2), and MMP-9 significantly (P < 0.05), indicating that Thr could attenuate the M1-type macrophages’ activation. Moreover, L-Thr downregulated the mRNA transcripts of TNF-α, IL-6, and IL-1β associated with impairing the SOCS1/STAT1 signaling and upregulated IL-10 and TGF-β1 partly by accentuating the SOCS3/STAT3 pathway. The above-mentioned observations suggested that Thr improved the immune status in the immune organs of fish by enhancing the immune defense and mediating the inflammation process. Finally, based on the immune indices of LZ activity in HK and C3 content in SP, the optimal Thr for immune enhancement in juvenile grass carp (9.53–53.43 g) was determined to be 15.70 g/kg diet (4.85 g/100 g protein) and 14.49 g/kg diet (4.47 g/100 g protein), respectively.
Highlights
The medium supplemented with 2.0 mM L-Thr decreased the mRNA levels of IL-1b, CSF-1, MMP2, MMP-9, MRC1, and SOCS3 and increased the TGF-b1, IL-10, and SOCS1 expression abundances significantly (P < 0.01)
We have found that the dietary Thr promoted the TGFb1 and GATA-3 gene transcripts in the head kidney (HK) and SP, supporting our hypothesis
We found that the release of MMP-2 and MMP9, potentially by canonical macrophage activation, was downregulated by treatment with 2 mM Thr
Summary
Nutrition-immune have been a focal point with the aim of improving animal health [1, 2]. Teleosts have a reserved hematopoietic function in the spleen but lack mature lymphatic organs with a substitution of the head kidney, being considered as the key core that coordinated immunoregulation as well as emphasized with highly expressed clusters of immune enzymes and metabolism- and transcriptional factor-related genes [17–20]. A mammalian study demonstrated that an immune reaction is initiated by, but not limited to, the antigenstimulated activation of antigen-presenting cells (APCs), antimicrobial substances involved defenses, cytokines that mediated the migration of macrophages, and inflammatory regulated T lymphocytes that processed immunological balances [21–23]. Evidence focused on the systematical administration of amino acids on teleost’s immune regulation from immune cell activation, innate immune component production to inflammation network-mediated signaling cascade crosstalk as well as the underlying mechanism in the head kidney (HK) and spleen (SP) of fish, the data of which are still limited
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