In a recent study published in Vaccines, researchers reported on the immunogenicity of a heterologous vaccine regimen against IBV (infectious bronchitis virus), comprising prime doses of QAC (QuilA-loaded chitosan)-encapsulated plasmid and a booster dose of MVA (modified vaccinia Ankara).
IBV causes acute pulmonary infections among chickens. Modified vaccinia Ankara vaccines can revert to virulence, recombine with circulating serological subtypes, and result in tissue injury among vaccinated avian species. The authors previously showed the anti-IBV efficacy and safety of QAC nanoparticle-complexed pCAG-N deoxyribonucleic acid (DNA) vaccinations (pQAC-N) among chickens.
About the study
In the present study, researchers evaluated immune protection conferred by heterologous DNA prime vaccination followed by MVA booster vaccinations compared to homologous MVA vaccinations among chickens against IBV.
The team prepared CEF (chicken embryonic fibroblasts) from embryos of SPF (specific pathogen-free) white leghorn embryonated chicken eggs (ECEs) to confirm IBV Arkansas DPI strain expression from the vaccine construct. The constructs were prepared using p-CAGN-loaded QAC nanoparticles, modified vaccinia Ankara expressing nucleocapsid, and tetramethylbenzidine (TMB) substrates. The QAC-encapsulated plasmid DNA/modified vaccinia Ankara-nucleocapsid construct efficacy was assessed among four groups of chickens.
The first and second groups, of eight chickens each, were intranasally inoculated with phosphate-buffered saline (negative control), and commercial Arkansas modified live virus (MLV, positive control). The third and fourth groups, of 10 chickens each, were vaccinated with 108 plaque-forming units per bird MVA-N on the first day, followed by homologous booster dose administration two weeks later intranasally, or 100.0 µg per bird pQAC-N on the first day followed by heterologous 108 plaque-forming units per bird MVA-N booster dose two weeks later intranasally.
21 days old birds were intranasally challenged with 107 EID50 (50% of the embryo infectious dose) PER bird of virulent IBV Arkansas DPI strains. After 10.0 days and 20.0 days of prime vaccination and 3.0 days of challenge, lachrymal fluid specimens and serum specimens were obtained and analyzed by enzyme-linked immunosorbent assay (ELISA) analysis.
The severity scores for IBV infection signs were documented once daily for all chickens over eight days following the challenge. Tracheal swab specimens and lachrymal fluid specimens were assessed for IBV ribonucleic acid (RNA) by infectious bronchitis virus nucleocapsid gene-targeted quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis.
Subsequently, IBV-targeted ELISA analysis and flow cytometry analysis were performed, and viral loads were measured. N-6xHis antigen expression from MVA-N vaccines was verified by subjecting proteins present in MVA-N-infected cells to Western blot analysis. Antigen-targeted T lymphocyte proliferation assays were performed to assess the capability of the experimental vaccines to induce pulmonary (local) IBV nucleocapsid-targeted cell-mediated immunological responses.
The heterologous vaccination regimen induced robust T lymphocyte responses. Chickens vaccinated with the DNA prime-modified vaccinia Ankara boost regimen exhibited lowered clinical severity scores and >2.0-fold lower viral loads in lachrymal fluid specimens and tracheal swab specimens following viral challenge, in comparison to homologous vaccinations.
IBV-targeted immunoglobulin (Ig)-A and -Y were significantly greater in the modified live virus-immunized chickens compared to the non-immunized PBS control chicken group. However, the Ig titers were not significantly greater among chickens vaccinated with either vaccine regimen compared to non-vaccinated chickens.
Pulmonary cells of QAC-encapsulated plasmid DNA/MVA-nucleocapsid-immunized chickens showed significantly greater proliferation after being stimulated by the nucleocapsid antigen compared to the 2.0X modified vaccinia Ankara-nucleocapsid and control group chickens. An elevation in the proliferating T cell receptor (TCR)-γδ+ and the cluster of differentiation 8+ (CD8+) T lymphocytes was observed following nucleocapsid stimulation among QAC-encapsulated plasmid DNA/modified vaccinia Ankara-nucleocapsid-immunized chickens compared to control chickens, while greater (but non-significant) helper (CD4+) T lymphocyte proliferation was observed among modified live virus-immunized chickens.
The significant reductions in viral loads among lachrymal fluid specimens and tracheal swab specimens of pQAC/ modified vaccinia Ankara-vaccinated birds were similar to levels observed among 2.0X-modified vaccinia Ankara-nucleocapsid-vaccinated chickens, in accordance with the clinical severity scores.
The heterologous doses of QAC-encapsulated plasmid DNA/modified vaccinia Ankara-nucleocapsid vaccines provided significantly greater protection against IBV than the homologous 2.0X modified vaccinia Ankara-nucleocapsid doses. The immune protection may be due to the robust cell-mediated memory response induction regulated by QAC-encapsulated plasmid DNA/modified vaccinia Ankara-nucleocapsid in pulmonary tissues.
Among SPF chickens vaccinated with the MPLA (synthetic monophosphoryl lipid A) + QAC triple adjuvant vaccine, loaded with pCAG-nucleocapsid plasmid on the first day, and modified vaccinia Ankara-nucleocapsid vaccination (pCAG-N or pmQAC/modified vaccinia Ankara) two weeks later, the decrease in clinical severity scores and viral loads among tracheal swab specimens was similar to the modified live virus-immunized chickens.
The protection conferred was similar to that by QAC-encapsulated plasmid DNA-modified vaccinia Ankara-nucleocapsid, indicating that MPLA addition did not enhance the performance of the experimental vaccines.
Overall, the study findings showed that the heterologous DNA prime-modified vaccinia Ankara boost vaccinations were more effective compared to homologous 2.0x modified vaccinia Ankara-nucleocapsid vaccinations in protecting chickens against IBV.