Immune protection duration and efficacy stability of DNA vaccine encoding Eimeria tenella TA4 and chicken IL-2 against coccidiosis
Introduction
Avian coccidiosis caused by Eimeria infection is one of the most common and economically important diseases of poultry worldwide (Kaboudi et al., 2016, Shirley et al., 2005). Eimeria tenella is one of the seven Eimeria species that infects domestic chickens and results in severe lesion of caeca, body weight loss, hemorrhagic diarrhea, haemorrhaging and death (Witcombe and Smith, 2014). The annual cost of losses to the global poultry industry caused by coccidiosis has been predicted to exceed US $3 billion (Blake and Tomley, 2014).
Current control strategy relies primarily on routine chemoprophylaxis using anticoccidial drugs. However, alternative methods are needed due to the rapid emergence of drug-resistant parasites and drug residues in chicken products (Chapman and Jeffers, 2014, Clarke et al., 2014). Immunization is a practical alternative to chemotherapy for the control of coccidiosis. However, most current commercial vaccines consisting of live oocysts of attenuated or non-attenuated coccidia are expensive and difficult for large-scale manufacturing. Furthermore, live vaccines have the risk of vaccinal pathogenicity, the potential reversion to virulence and coccidiosis breakout (Del Cacho et al., 2016, Williams, 2002). Therefore, it is urgent to develop safe and effective vaccine against avian coccidiosis.
DNA vaccines have been suggested as a promising alternative strategy against coccidiosis without the disadvantages associated with chemoprophylaxis and live vaccines (Blake and Tomley, 2014, Pereira et al., 2014, Song et al., 2015). In our previous work, a DNA vaccine encoding Eimeria tenella TA4 and chicken IL-2 was constructed and induced effective protection against challenge of Eimeria (Xu et al., 2008). The immunization procedure of the DNA vaccine including immunization doses, routes, times and age was optimized and the storage period was evaluated (Song et al., 2009). A practical vaccine against coccidiosis should confer stable protection between different bathes as well as long-lasting protection covering the production cycle of chickens. In order to address these issues, the present study was performed to evaluate the efficacy stability and immune protection duration of DNA vaccine pVAX1.0-TA4-IL-2 against E. tenella to provide reference for clinical application of the DNA vaccine.
Section snippets
Plasmid, parasite and experimental animals
Plasmid pET28a-TA4 and pVAX1.0-TA4-IL-2 encoding E. tenella TA4 and chicken IL-2 were prepared in our lab (Xu et al., 2008, Song et al., 2009). Three batches of plasmid were prepared using HighPure Maxi Plasmid Kit (TIANGEN, China) according to manufacturer's protocol. Oocysts of E. tenella were collected and sporulated 7 days before the challenge infection using a previously described protocol (Tomley, 1997). The purity of E. tenella was determined with ITS1-PCR (Jenkins et al., 2006, Haug et
Optimization of immunization doses
Protective efficacies of immunization doses of 2 μg, 10 μg, 25 μg, 50 μg, 100 μg and 200 μg were shown in Table 1. The immunized chickens had significantly increased body weight gain, alleviated cecal lesions and reduced oocyst output compared with those of the chickens treated with PBS (P < 0.05). Immunization dose of 25 μg resulted in relative body weight gain of 108.66% and oocyst decrease ratio of 74.06% which were higher than the other doses. It also resulted in the lowest mean lesion scores of
Discussion
In this study, the immunization dose of the DNA vaccine was optimized and the optimal dose was 25 μg which was in agreement with that in our previous study (Song et al., 2009). In our previous study, 25 μg was the lowest dose among the designed doses (25, 50,100 and 200 μg), while whether a lower dose could provide better efficacy than 25 μg remained unknown. In this study, two lower doses (2 μg and 15 μg) were designed and did not provide better efficacy than 25 μg. Our data demonstrated 25 μg was
Acknowledgments
This work was supported by the Natural Science Foundation of Jiangsu Province of China (No. BK20161442), the Fundamental Research Funds for the Central Universities (KYZ201631), the National Natural Science Foundation of China (31372428, 31672545) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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