Oral vaccination with plant-derived VP2 against IBDV

LUCERO, MARÍA SOLEDAD; GOMEZ, EVANGELINA; CHIMENO ZOTH, SILVINA; CARBALLEDA, JUAN MANUEL; GRAVISACO, MARÍA JOSÉ; BERINSTEIN, ANALÍA

Verona

Conferencia; Fifth International Conference on Plant-based Vaccines, Antibodies and Biologics; 2013

Introduction-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. VP2, which contains the major neutralizing epitopes, has been used for the development of subunit vaccines in a variety of heterologous platforms in order to meet the needs of new generation vaccines as effective as the traditional virus based ones. We have previously demonstrated that VP2 transiently expressed in Nicotiana benthamiana is able to elicit a neutralizing antibody response in chickens when administered intramuscularly in a prime/boost scheme. We have also observed a decrease in T cells infiltration into the bursa after the challenge in those animals vaccinated with plant derived VP2, suggesting that humoral response prompted by the recombinant immunogen neutralizes totally or partially the entrance of IBDV. Given these previous findings and the fact that natural infections with IBDV occur by the oral route we decided to investigate whether VP2 produced in plants was also immunogenic when given orally to chickens. Objectives- The objectives of the present work were to transiently express VP2 of IBDV and to investigate whether this recombinant immunogen can be used as an oral vaccine in chickens. Materials and Methods- Transient expression was performed by the agroinfiltration of N. benthamiana leaves with a suspension of recombinant bacteria harboring the VP2 gene. VP2 expression was analyzed by Western blot utilizing an anti-VP2 polyclonal antibody raised in rabbit. Specific pathogen free chickens of 18 days old were randomly assigned to experimental groups. During 23 consecutive days chickens were fed with 1ml of plant extract containing 30 µg of VP2 (group 1), or a non-related antigen as control group (group 2). Animals in group 3 were vaccinated at day 0 with the commercial vaccine. All animals were weekly bled by the wing vein. On the last day of vaccination (23rd day) animals were challenged by oral inoculation with 500 µl of classical IBDV strain LZD (6934 TCID50/ml). Five days later animals were euthanized and bursae were removed for lymphocyte isolation and flow cytometry analysis. Serum samples were evaluated for the presence of specific antibodies against IBDV with a commercial ELISA and for virus neutralizing activity using chicken embryo fibroblasts (CEFs). Results- Three out of four chickens orally vaccinated showed a specific humoral response with low titers in a serum working dilution of 1/20, at 23 dpi. Animals vaccinated with VP2 barely developed a neutralizing response (titers of 1/16). Knowing that after infection IBDV replication in the bursa involves an infiltration of T cells into this organ and tissue damage, we also investigated the frequency of T cells in the bursa of vaccinated animals after challenge to determine if plant-derived VP2 elicited a protective immune response. Bursae of animals vaccinated with VP2 presented fewer infiltrating T cells compared to animals in the control group. Conclusion and Discussion- The immunogenicity and protective efficacy of an oral plant-based vaccine against IBDV were investigated. Although the humoral response was not as strong as we expected, animals orally vaccinated with VP2 seemed to be protected against challenge. Probably different mechanisms are involved in the immune state of protection acquired by vaccination with VP2 plant extract delivered by the i.m. and oral routes. Further investigations need to be done to elucidate this issue. Our results are encouraging and we believe it is worth to continue working on this approach to find an oral vaccine against IBDV based on a plant expression platform.