Local cellular immune response plays a key role in protecting chickens against hepatitis-hydropericardium syndrome (HHS) by vaccination with a recombinant fowl adenovirus (FAdV) chimeric fiber protein

Abstract

Fowl adenovirus (FAdV)-induced diseases hepatitis-hydropericardium syndrome (HHS) and inclusion body hepatitis (IBH) have been affecting the poultry industry with increasing severity in the last two decades. Recently, a subunit vaccine based on a chimeric fiber protein with epitopes from different fowl adenovirus serotypes (named crecFib-4/11) has been shown to confer simultaneous protection against both HHS and IBH. However, the underlying immune mechanisms in chickens are still enigmatic, especially because of frequently absent neutralizing response despite high levels of protection. In this study, we investigated the kinetics of the humoral and cellular immune responses in specific pathogen-free chickens after vaccination with crecFib-4/11 and/or challenge with a HHS-causing strain, on a systemic level, as well as locally in target and lymphoid organs. The humoral response was assessed via enzyme-linked immunosorbent assay (ELISA) and virus neutralization test in serum, while the cellular immune response was determined by phenotyping using flow cytometry. Although vaccination induced serum antibodies, as confirmed by ELISA, such antibodies exhibited no pre-challenge neutralizing activity against FAdV-4. Nevertheless, immunized birds experienced a significant B cell increase in the liver upon challenge, remaining high throughout the experiment. Furthermore, vaccination stimulated the proliferation of cytotoxic T lymphocytes, with earlier circulation in the blood compared to the challenge control and subsequent increase in liver and spleen. Overall, these findings imply that protection of chickens from HHS after crecFib-4/11 vaccination relies on a prominent local immune response in the target organs, instead of circulating neutralizing antibodies.

Research Note: Overview of fowl adenovirus serotype 4: structure, pathogenicity, and progress in vaccine development

“… The development of neutralizing antibodies was limited against FAdV-11 and absent against FAdV-4, indicating that protection granted by such an antigen may be linked to different immunization pathways. De Luca et al, 2022 Construction of expression plasmid and expression and purification of recombinant fiber-2 protein. The lowest dose of recombinant fiber-2 protein that could provide 100% protection against challenge with virulent FAdV-4 strain HB1501 as well as elicit protective immunity was 2.5 μg/bird.…”

“…Furthermore, vaccination stimulated the proliferation of cytotoxic T lymphocytes, with earlier circulation in the blood compared to the challenge control and subsequent increase in liver and spleen. De Luca et al, 2022 A subunit vaccine candidate derived from the bacterially expressed recombinant Fiber2 protein (termed as rFiber2 subunit vaccine) of FAdV-4 GZ-QL strain and a recombinant plasmid pVAX1-Fiber2 as DNA vaccine candidate (termed as Fiber2 DNA vaccine). Three injections of the rFiber2 subunit vaccine or the Fiber2 DNA vaccine induced robust humoral and cellular immune responses in chickens.…”

The Development of a Novel Fiber-2 Subunit Vaccine against Fowl Adenovirus Serotype 4 Formulated with Oil Adjuvants

“…Recent studies have shown that although the fiber-2 protein cannot induce neutralizing antibodies, it can still provide protective effects against FAdV-4 and suppress virus replication. This may rely on the local cellular immune response in target organs [ 45 ]. Our research results suggest that, although the viral loads in the tissues from chickens vaccinated with fiber-2 protein subunit vaccines were significantly lower than those in the challenged control group, the virus was still detected in the cloacal swabs from vaccinated chickens.…”

The Immunological Basis for Vaccination

“…The development of the B-cell memory response may depend not only on the vaccine type and antigen, but also on the host genotype (115). Recently it was shown in an adenovirus vaccination challenge model that B-cell frequencies are increased locally in the target organ in vaccinated birds already at 3 days postchallenge (dpc), compared to the challenge-only group, which showed a 2-day delayed increase starting at 5 dpc, supporting the concept of the fast memory response in chickens (85).…”

“…Neither are local antibody responses, as they are determined in tears, tracheal washes, or gut lavage, which are more labor intensive to collect and may require sacrificing the birds. But often mucosal immunity provides the dominant mode of protection, as recently demonstrated, for example, in a fowl adenovirus-vaccination challenge experiment using baculovirus-expressed recombinant virus proteins (85). Another recent study provided evidence that enhancement of mucosal immunity may improve vaccine protection against H9N2 avian influenza outbreaks (86).…”