Fowl adenovirus capsid proteins as broad-protective subunit vaccines against inclusion body hepatitis (IBH) and hepatitis-hydropericardium syndrome (HHS)

PhD thesis - University of Veterinary Medicine Vienna - 2022

Fowl adenoviruses (FAdVs) are associated with three disease complexes affecting the poultry industry: adenoviral gizzard erosion (AGE), caused by FAdV serotype 1 (FAdV-1), hepatitis-hydropericardium syndrome (HHS), caused by FAdV-4, and inclusion body hepatitis (IBH), caused by FAdV-2, -8a, -8b and -11. Despite the increasing burden that these diseases represent for the poultry industry, a comprehensive immunization strategy available in the field is still lacking. The objective of this thesis was to develop a broad-protective immunization strategy to protect chickens against IBH and, subsequently, HHS, using subunit vaccines based on FAdV recombinant capsid proteins, which are favourable for their efficient cost- and time-production. In the first study, specific pathogen-free (SPF) chickens were vaccinated with a recombinant FAdV-8a fiber protein and challenged with FAdV field isolates belonging to both a homotypic (FAdV-8a) or heterotypic (-8b) IBH-causing strain. Results showed that single fiber-based immunity was only able to protect the birds from homologous challenge, possibly due to the strictly type-specific neutralizing activity of fiber immune sera determined by virus neutralization test (VNT). In protected birds, vaccination prevented a post-challenge drop of peripheral B cells in blood, stimulated helper T lymphocyte proliferation while moderating the cytotoxic T cell response, and prevented challenge-induced changes in systemic monocytes/macrophages and γδ+ T cell subpopulations. In order to overcome the lack of cross-protection linked to single fiber vaccines, the novel concept of recombinant chimeric fiber proteins (crecFibs) retaining epitopes from different FAdV serotypes was developed merging two consecutive segments in the fibers of FAdV-8a and -8b, swapped reciprocally to result in novel chimeras, crecFib-8a/8b and crecFib-8b/8a. The constructs showed the same reactivity as monospecific recombinant fibers in western blot against different FAdV antisera, and crecFib-8b/8a was able to induce cross-neutralizing antibodies against both serotypes in chickens. This highlights distinct epitopes in these fibers: the conserved one detected in western blot and at least two type-specific epitopes participating in neutralization. When administered as single-antigen component, crecFib-8b/8a protected chickens against IBH-causing serotypes, promoting the advancement of broadly protective subunit vaccination strategies against FAdVs. This concept was therefore extended to achieve simultaneous protection against IBH and HHS, with the design of a chimeric construct retaining epitopes from FAdV-4 and -11 fibers (crecFib-4/11). The construct was used to vaccinate SPF chickens before challenge with either FAdV-4 or -11, and was able to protect the birds from both HHS and IBH. Clinical protection was associated with high levels of pre-challenge antibodies measured on enzyme-linked immunosorbent assay (ELISA) plates coated with the vaccination antigen, although the development of neutralizing antibodies was limited against FAdV-11 and absent against FAdV-4, indicating that protection granted by such antigen may be linked to different immunization pathways compared to crecFib-8b/8a. Nevertheless, birds immunized with crecFib-4/11 and challenged with FAdV-4, experienced a significant increase of hepatic B cells and the proliferation of circulating cytotoxic T lymphocytes at an earlier time point compared to the challenge control, with subsequent increase in liver and spleen. Overall, these findings imply a potent local response in the target and lymphoid organs as crucial component for the protection from HHS after crecFib-4/11 vaccination, even more so than the development of neutralizing antibodies. In conclusion, it was proven that the concept of chimeric fiber vaccines represents the first single-component FAdV subunit vaccine providing comprehensive protection against different FAdV-associated diseases.

PhD Thesis - Veterinärmedizinische Universität Wien - 2022

165 Seiten