| Summary: | In this study, the effect of dietary supplementation of mulberry leaf polysaccharides (MLPs) on the immune parameters—i.e., the immune organ weight, serum immunoglobulins, cytokines, nitric oxide (NO) production, and insulin-Like growth factor-1 (<i>IGF1</i>) mRNA expression—of weanling pigs as a model animal was investigated. A total of 120 healthy weanling pigs (aged 28 ± 2 d) with the same body weights were randomly divided into four groups: (1) Control treatment (CT), basal diet (BD), (2) MLP low-dose treatment (MLT), 0.6 g/kg MLP + BD, (3) MLP high-dose treatment (MHT), 1.2 g/kg MLP + BD, and (4) antibiotic treatment (AT), 0.15 g/kg chlortetracycline + BD. The results revealed that the thymus and spleen indices were significantly increased (<i>P</i> < 0.05) in both MLT and MHT groups in comparison with the CT group, while the serum levels of immunoglobulin G (IgG), interleukin (IL)-1<i>β</i>, IL-2, IL-8, and interferon (IFN-<i>γ</i>) in the MLT group and IL-2, IL-6, and IFN-<i>γ</i> in the MHT group were also considerably greater (<i>P</i> < 0.05) than the corresponding levels in the CT group. The serum contents of IgG, IL-1<i>β</i>, IL-2, and IL-8 in the MLT group and IL-2 and IL-6 in the MHT group were significantly increased in comparison with the corresponding contents in the AT group (<i>P</i> < 0.05). The transformation rate of lymphocytes in the MLT and MHT groups was higher compared to the CT and AT groups. However, a notable difference was found between the MLT group and the two control groups. The peripheral lymphocyte NO production in the MLT, MHT, and AT groups was significant relative to the CT group. The expression levels of <i>IGF1</i> mRNA in the liver and muscle longissimus tissues of both the MLT and MHT groups showed significant improvement (<i>P</i> < 0.05) over those in the CT group. Moreover, the <i>IGF1</i> mRNA expression in the muscle longissimus from the MLT group was significantly higher than in the AT group. In conclusion, the results suggest that incorporating MLPs into the diets of weanling pigs improves the animals’ metabolisms and immune functions, and the effects of the MLT group were superior to those of both the MHT and AT groups.
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