A Novel Mucosal Adjuvant System for Immunization against Avian Coronavirus Causing Infectious Bronchitis

Abstract

Infectious Bronchitis (IB) caused by Infectious Bronchitis Virus (IBV) is currently a major threat to chicken health with multiple outbreaks being reported in the US over the past decade. Modified live virus (MLV) vaccines used in the field can persist and provide the genetic material needed for recombination and emergence of novel IBV serotypes. Inactivated and subunit vaccines overcome some of the limitations of MLV with no risk of virulence reversion and emergence of new virulent serotypes. However, these vaccines are weakly immunogenic and poorly protective. There is an urgent need to develop more effective vaccines that can elicit a robust, long-lasting immune response. In this study, we evaluate a novel adjuvant system developed from Quil-A and chitosan (QAC) for the intranasal delivery of nucleic acid immunogens to improve protective efficacy. The QAC adjuvant system forms nanocarriers (<100 nm) that efficiently encapsulate nucleic acid cargo, exhibit sustained release of payload and can stably transfect cells. Encapsulation of plasmid DNA vaccine expressing IBV nucleocapsid (N) protein by the QAC adjuvant system (pQAC-N) enhanced immunogenicity as evidenced by robust induction of adaptive humoral and cellular immune responses post vaccination and challenge. Birds immunized with pQAC-N showed reduced clinical severity and viral shedding post challenge on par with protection observed with current commercial vaccines without the associated safety concerns. Presented results indicate that the QAC adjuvant system can offer a safer alternative to the use of live vaccines against avian and other emerging coronaviruses. Importance According to the 2017 US agriculture statistics, the combined value of production and sales from broilers, eggs, turkeys, and chicks was $42.8 billion. Of this number, broiler sales comprised 67 percent of the industry value with the production of > 50 billion pounds of chicken meats. The economic success of the poultry industry in the USA hinges on the extensive use of vaccines to control Infectious Bronchitis Virus (IBV) and other poultry pathogens. Majority of vaccines currently licensed for poultry health include both modified live vaccine and inactivated pathogens. Despite their proven efficacy, modified live vaccine constructs take time to produce and could potentially revert to virulence, which limits their safety. The significance of our research stems from the development of a safer and potent alternative mucosal vaccine to replace live vaccines against IBV and other emerging coronaviruses.

Localized and Systemic Immune Responses against SARS-CoV-2 Following Mucosal Immunization

“…Th2 (IL-13+) and Tc2 responses were significantly lower in the heterologous vaccine group in the lungs and spleen of vaccinated mice ( Figures 5 and 6, respectively). Significantly higher S-specific Tc1 responses (predominantly IFN-γ+) were also present in the lungs of pQAC-CoV (I.N)-immunized mice without significant induction of CD4+ T-cells ( Supplementary Figure S3), an observation noted previously with QAC-DNA vaccination in chickens [11]. In addition, intramuscular (I.M) administration of pQAC-CoV alone induced significantly higher Th1 and Th17 responses in the spleen of vaccinated mice ( Figure 6).…”

“…Earlier studies have shown that heterologous DNA/virus immunization is more immunogenic and protective than homologous virus/virus immunization strategies [38]. Based upon our previous experience with the avian coronavirus [11] and other studies with SARS-CoV [39,40], we selected both nucleocapsid (N) and spike (S) proteins as our antigen targets. Although we immunized mice concurrently with both the N and S vaccine constructs, we focused on characterizing the more biologically relevant humoral responses against spike, which neutralized and limited viral entry [41,42].…”

“…A major challenge with DNA vaccines, however, is the in vivo degradation of the construct by DNases, inefficient uptake by antigen-presenting cells (APC), and low immunogenicity [ 9 , 10 ]. Fortunately, articulate delivery systems such as quil-A-loaded chitosan (QAC), represent a significant improvement over other traditional vaccines by prolonged release of active plasmid expressing antigens as shown before [ 11 ]. QAC particulate adjuvant system is composed of chitosan, a biodegradable natural polysaccharide that readily complexes with DNA owing to its positive charge and quil-A, a potent adjuvant with mild surfactant properties [ 12 , 13 ].…”

“…QAC particulate adjuvant system is composed of chitosan, a biodegradable natural polysaccharide that readily complexes with DNA owing to its positive charge and quil-A, a potent adjuvant with mild surfactant properties [ 12 , 13 ]. The QAC adjuvant system can both mediate direct delivery of plasmid DNA to target cells and act as an antigen depot, releasing DNA payload over time [ 11 ]. QAC-adjuvanted vaccines appear to target local mucosal immunity where airway epithelium T-cells and IgA humoral responses have been shown to be critical for restricting respiratory viral pathogens like SARS- and MERS-CoV [ 14 ].…”

“…Furthermore, the recently approved nucleic acid vaccines for COVID-19 are also administered parenterally [ 15 ]. Previously, we had shown that a 2-dose QAC encapsulated plasmid DNA (pQAC) encoding the nucleocapsid gene against avian coronavirus provided equal protection to live-attenuated vaccines when birds were challenged with Infectious Bronchitis Virus [ 11 ]. Although comparable, only T-cell responses were observed without a complementing humoral response in birds vaccinated with pQAC.…”