| Oreochromis niloticus is an important economically farmed fish in the coastal areas of south China.In recent years,the large scale outbreak of streptococcicosis causing by Streptococcus agalactiae has caused great economic losses to the tilapia industry.Vaccination is a relatively safe and effective way to prevent streptococcicosis in fish,but immunization is a complex physiological phenomenon and can be affected by organism,antigen and environment.In order to assess whether the resistance of vaccinated Nile tilapia to S.agalactiae was affected by environmental stress,in this paper,temperature,intermittent hypoxia,elevated salinities,and fasting was used as stress factors to establish biological models and study the effect of stress on the immune responses in vaccinated Nile tilapia by inactivated S.agalactiae vaccine.This study is not only necessary to evaluate how environmental stress affect the immune responses and vaccine efficacy in Nile tilapia,but also to provide a scientific reference for the optimization of vaccine efficacy to prevent tilapia from S.agalactiae infection after vaccination.The main research results are as follows:1.In order to assess the effect of temperature on the immune response and vaccine efficacy,Nile tilapia were reared at 21,25,29,or 33°C,and immunized intraperitoneally with inactivated S.agalactiae vaccine.The results show that four of immune genes(IgM,IL-1β,TNF-α,and IFN-γ)expression levels in spleen and head kidney were significantly inhibited in fish vaccinated at 21 and 33°C.Among the vaccinated fish at 28 d post vaccination,total WBC counts,absolute counts of neutrophil,monocyte,lymphocyte,bactericidal activity and phagocytic activity were lowest at 21°C.Respiratory burst activity level and specific antibody level first increased and then decreased significantly with the increase of temperature.All the vaccinated fish were challeaged with live S.agalactiae at the same temperature,and the results show that the survival rate was lowest in vaccinated at 21°C,and there were no significant differences in survival rate among vaccinated fish reared at 25,29,and 33°C.These results suggest that the immune responses and vaccine efficacy following vaccination were markedly suppressed in vaccinated fish reared at 21°C.2.We studied the effect of intermittent hypoxia under different temperature on the immunomodulation in vaccinated Nile tilapia.Study conditions were intermittently hypoxic(4.0 ± 1.0 mg/L DO)and normoxic(8.0 ± 0.5 mg/L DO)at 30 or 35°C.The results show that immune genes(IL-1β,TNF-α,and IFN-γ)expression levels,autioxidant enzyme(SOD,CAT,and GSH-Px)activities,phagocytic activity of leukocyts,and respiratory burst activity in head kidney cells were all significantly lower in vaccinated hypoxic fish compared to the vaccinated normoxic fish at either 30 or 35°C.Villus of distal intestine showed some degree of shedding,and head kidney showed intercellular hyperplasia and healthy red cells decreasing.At 1 d post challenge,the bacterial burdens in spleen and head kidney were significantly higher in vaccinated hypoxic fish compared to vaccinated normoxic fish at either 30°C or 35°C.The survival rate after challenge with S.agalactiae was lower in vaccinated hypoxic fish,and the lowest survival rate was in vaccinated hypoxic fish at 35°C.Taken together,intermittent hypoxia at either 30°C or 35°C could suppress immunomodulation in vaccinated Nile tilapia.3.In order to assess the effect of salinity on the immune response and vaccine efficacy,Nile tilapia were acclimatized to 0,10,20,or 30‰ salinity and then immunized with inactivated S.agalactiae vaccine.As the salinity rose to 30‰,some of the fish began to diet.The body surfaces of some moribund fish suffered different degrees of skin lesions and subcutaneous hemorrhages,and the gill lamellae were severe shedding in fish at 30‰ salinigy.Elevated cortisol levels were detected in all of the fish exposure to salinity.Expression levels of immune genes(IgM,IL-1β,and IFN-γ),white blood cell counts,absolute blood lymphocyte counts,serum bactericidal activity levels,and specific antibody levels were all significantly lower in vaccinated fish at 20 and 30‰ salinity.A suppressed immune response and decreased vaccine efficacy were also indicated by the lower survival rate of vaccinated fish at 20‰ salinity when challenged with S.agalactiae.Therefore,salinities ≥ 20‰ negatively affected antibody production in Nile tilapia,ultimately affecting vaccine efficacy.4.In order to assess the effect of fasting on the resistance of Nile tilapia to S.agalactiae,vaccinated and unvaccinated fish were fasted for 0,1,3,and 7 days.The results show that the cortisol levels of vaccinated and unvaccinated fish first decreased and then increased significantly with the prolongation of fasting time.While antioxidant enzymes(SOD,CAT,and GXH-Px)activities,expression levels of cytokines(IL-1β and IL-6),and specific antibody levels of vaccinated fish were opposite to that of cortisol,which were all lowest at 7 d post fasting.There were no significantly differences in glucose contents at 3 and 7 d post fasting,which may be related to the self-regulation of glucose metabolism,inculding significant decrease in liver glycogen,triglycerides,total cholesterol and HK and PK activities,and the significant increase in PEPCK activity.After the challenge of S.agalactiae infection,the survival rate of unvaccinated fish in 3 d fasting group was significantly higher than that in 7 d fasting group.However,there were no significant differences in the survival rate of vaccinated fish among different fasting treatment groups.These results suggest that appropriate short-term fasting may improve the resistance to S.agalactiae in unvaccinated Nile tilapia. |
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