Effect of different routes of inoculation on plant-derived VP2 immunogenicity and ability to confer protection against infectious bursal disease

Abstract

Infectious Bursal Disease Virus (IBDV) is the etiological agent of an immunosuppressive and highly contagious disease that affects young birds causing important economic losses in the poultry industry. The structural protein VP2 has been used for the development of subunit vaccines in a variety of heterologous platforms. We have previously demonstrated that plant-derived VP2 (pVP2) is able to elicit a neutralizing antibody response in chickens when administered intramuscularly (i.m.) in a prime/boost scheme. However, administration via injection is impractical and carries the risk of needle stick injury or pain. Mucosal vaccination is noninvasive and has several advantages over traditional systemic vaccines. Taking this into account and the fact that natural infections with IBDV occur by the oral route, we decided to investigate whether pVP2 was also immunogenic when given intranasally (i.n.) or orally to chickens. In addition, we evaluated if intramuscular vaccination with VP2 plant extract in a more welfarefriendly scheme with less injections and without adjuvant was able to elicit a protective immune response against IBDV as previously seen. We determined that animals inoculated i.m., but not i.n., with the experimental vaccine developed high titres of specific antibodies, with virus neutralizing activity. Also, bursae of animals vaccinated i.m. with pVP2 presented few infiltrating T cells, low viral charge and normal morphology. However, chickens that received the immunogen via nasal or oral route were not protected after challenge. Considering the disadvantages of conventional live-attenuated and inactivated vaccines, a plant-based subunit vaccine represents a viable alternative in the veterinary field. Once again pVP2 has proven to be immunogenic when parentally inoculated. However, further investigations need to be done in order to find an alternative route of administration which is more practical than the intramuscular injection and capable of eliciting a mucosal immune response.