Threonine requirement and its effect on intestinal defense mechanisms in poultry
Threonine (Thr) is competitively priced into least cost formulations, however, estimates for Thr requirement of ducks are not included in NRC (1994). A series of experiments were conducted to study the Thr requirement of Pekin ducks from hatch to 14 d and 15 to 35 d post hatching, based on growth performance, carcass traits, serology and intestinal characteristics. For 0-14 d Pekin duck, the estimated Thr requirement ranged from 0.81 to 1.00%, or 0.90 to 0.98% by quadratic broken-line (QBL) regression or quadratic regression, respectively. For 15-35 d Pekin ducks, the estimated Thr requirement ranged from 0.62 to 0.73%, or 0.70 to 0.80% by QBL or quadratic regression, respectively. Because Thr is a major structural component of mucins and secretory IgA (sIgA), which constitute the first lines of intestinal defense, the development of mucin2 (MUC2), IgA and its transcytosis receptor, polymeric Ig receptor (pIgR) was determined. In intestinal tissues, MUC2 was expressed with a rapid increase at hatching, followed by steady expression through 21 d post-hatch. Expression of IgA did not increase until the second wk post-hatch and reached at a steady expression at 21 d post-hatch. The expression of pIgR was highly correlated with both MUC2 and IgA expressions with its expression gradually increasing from hatching until 21 d post-hatch. Subsequently, the ileal tissues from 21 d of broilers were used to develop a 2 h short-term ex vivo explant culture model, which was applied for the measurement of Thr effect on MUC2, IgA and pIgR expression with LPS challenge. Deprivation of Thr for 2 h from the culture medium induced a compensatory increase of interleukin-8 (IL-8), MUC2 and IgA gene expression in the presence of LPS challenge. However, at 2 h of incubation, explants from birds fed Thr deficient diets showed higher IL-8 gene expression regardless of LPS challenge, and did not response to Thr deprivation from culture medium, which indicated in vivo Thr deficiency had impaired inflammatory and secretory immune responses. Further, the effect of Thr deficiency on intestinal integrity and immunity were determined with a feed withdrawal combined with coccidial challenge in broiler chicks. Under combined challenge, Thr-deficient birds had lower BW gain, lower jejunal crypt depth, higher number of oocysts, higher gut permeability with decreased IgA production in the ileum as well as a limited increase of CD3 lymphocytes with lower IFN-γ but higher IL-10 mRNA expression. Overall, inadequate Thr supplementation could impair intestinal morphology, barrier function, lymphocyte profiles as well as inflammatory and immune responses, which may worsen the detrimental impact on growth performance when birds are exposed to challenges.
Ajuwon, Purdue University.
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