| Summary: | 博士 === 國立中興大學 === 獸醫學系 === 93 === The study was aimed to evaluate the effect of dexamethasone (DEX), a stress mimic, on the immune system in weanling pigs, to test the efficacy of sugar can extract (SCE) as an immunostimulating agent on the modulation of nature immunity of pigs in experimental and field conditions, and to evaluate SCE’s antiviral effect in experimental challenged model. Piglets were administered with DEX (1 mg/kg, IM) every 12 h for two consecutive days in short-term treatment group, with DEX (1 mg/kg, IM) daily for 2 weeks in long-term treatment group, or with saline in control group. In short-term DEX experiment, there was transient significant increase (P＜0.05) in the numbers of neutrophils at 12 h after DEX treatment (hours post injection, HPI) in both DEX and saline treated groups when compared to sample at 0 HPI of both groups. In long-term DEX experiment, the DEX treated group has lower (P＜0.05) on the numbers of lymphocytes when compared to in its saline treated group on days 7 and 14 after DEX treatment (days post injection, DPI ) and 0 DPI of DEX treated group. But the numbers of nuetrophils in both saline and DEX treated groups were greater (P < 0.05) on 7 and 14 DPI when compared to sample on 0 DPI of both groups. Comparing to the control group, the relative percentages of CD8+ T cells in PBMC were significantly decreased (P < 0.05) in both short- and long-term DEX treated groups, while the highest percentage of IgM+ cells was observed in long-term DEX treated group (P < 0.05). In addition, neutrophil phagocytosis was significantly reduced (P < 0.05) at 12 HPI in the short-term DEX treated group as compared with saline treated group, but was increased in both saline treated and DEX treated groups (P < 0.05) at 24 and 36 HPI as compared to at 0 HPI. The cytotoxicity of NK cells was significantly enhanced (P < 0.05) at 24 HPI in short-term DEX treated piglets as compared with saline treated group. In long-term DEX treated group, phagocytosis of neutrophils was significantly greater (P < 0.05) than that in its saline treated group, but there was no significant difference in cytotoxic activity of NK cells between DEX treated and saline treated groups. However, at day 7 and day 14 post injection of DEX and saline, cytotoxicity of NK cells and phagocytosis of neutrophils in long-term DEX treated and saline treated groups were all significantly decreased (P < 0.05) as compared to at day 0.
In experiments assessing SCE for its modulatory effect on innate immunity, the cytotoxicity of NK cells and phagocytic function of phagocytes were significantly enhanced in pigs fed with SCE at a dose of 500 to 1000 mg/kg for short- or long-term administration (P < 0.05). Therefore, the effect of SCE was further studied in field conditions. In these studies, weanling pigs were fed with SCE (500 mg/kg) for 3 consecutive days per week for 4 weeks, and the effects of SCE on the leukocyte functions and growth performance, and its role as an adjuvant were evaluated. Significant enhancement of cytotoxicity of NK cells and phagocytosis of neutrophils and monocytes were detected in pigs fed with SCE (500 mg/kg/day) for 2 and 4 weeks, but not in pigs fed with SCE for 1 week. SCE-treated pigs showed a 7.87 % enhancement in growth performance as compared to untreated controls. Furthermore, an adjuvant effect of SCE on M. hyopneumoniae vaccination was demonstrated as the application of SCE increased antibody titer against M. hyopneumoniae. Administration of SCE also decreased seroconversion rates of porcine reproductive respiratory syndrome (PRRS) in pigs. To validate the antiviral effect of SCE, twelve-week-old experimental pigs were fed with SCE (500 mg/kg/day) for 3 days, challenged with PrV (2 × 105 TCID50) at the second day, and the effects of SCE were assessed at the 7th and the 14th day post-PrV inoculation. Pigs that only challenged with PrV and without SCE treatment were served as the control group. Our results indicated that the cytotoxicity of nature killer cells, proliferation of lymphocytes, phagocytic function of monocytes, and IFN-γ production by CD4+ and γδT cells were all greatly enhanced (P < 0.05) in the SCE-treated pigs as compared to control group. SCE administration also reduced the severity of clinical signs and course of disease in PrV challenged pigs. In addition, there was a 12 % increase in body-weight-gain in SCE-treated pigs as compared with the control pigs. In summary, function of leucocytes was temporally increased after short-term DEX treatment, but was decreased after long-term DEX treatment. SCE administration had an immunomodulatory effect on the porcine innate immunity which in turn may provide protective activities against pathogenic infections.
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