Studying Influenza Immune Imprinting in the Porcine Model

Abstract

Influenza A viruses (IAVs) are seasonal respiratory pathogens that cause significant morbidity and mortality each year, as well as incur large economic costs. Seasonal vaccination is effective in the prevention of severe disease, but vaccine responses aren’t uniform across the population. It has been shown that an individual’s first exposure plays a pivotal role in shaping future immune responses, a phenomenon termed Original Antigenic Sin (OAS), immune imprinting, or antigenic seniority. The outcome of this can be beneficial, providing more protection against infection or better vaccine responses, or detrimental, causing increased susceptibility to severe disease or reducing vaccine efficacy. The aim of this dissertation is to characterize a swine model of immune imprinting to better facilitate studying the effect of pre-existing immunity in Influenza disease and immune responses. A hallmark of OAS is an apparent immunological bias in antibody responses, with individuals responding most strongly to strains which circulated around their time of birth. In Chapter II we generated groups of animals with different initial exposures, either following experimental infections or vaccination, and then challenged them with A/CA/07/2009/H1N1 (CA/09/H1N1) in an attempt to recreate this response bias in our swine model. Vaccination produced limited humoral responses which were overtaken by CA/09/H1N1-specific antibodies after challenge. A single group, primed by infection with a season swine virus, A/sw/NC/KH1552516/2016/H3N2 (sw/NC/16/H3N2), continued to respond most strongly to the imprinting strain even following secondary challenge, demonstrating that it is possible to recreate an imprinted phenotype in our swine model. Since imprinting is potentially a major impediment to the generation of vaccine-induced immunity, it is imperative that both current seasonal vaccines and novel vaccine platforms are tested in models with pre-existing immunity to better capture how the general populace will respond. Chapter III of this dissertation describes the testing of a novel universal IAV vaccine, Centi-Flu, in our pre-immune swine model. Three groups of animals with pre-existing immunity were used: the first group was vaccinated with a whole-inactivated bivalent vaccine containing CA/09/H1N1 and A/TX/50/2012/H3N2 (TX/12/H3N2), the second group was vaccinated with A/sw/NC/A02248194/2021/H1N1 (sw/NC/21/H1N1) and sw/NC/16/H3N2, and the third group was IAV-naive. All three groups were subsequently challenged with CA/09/H1N1 six months after vaccination. Four months following challenge all three groups received the Centi-Flu vaccine. Two fully naïve groups were also included, one vaccinated with Centi-Flu and one vaccinated with a seasonal inactivated vaccine, both combined with a water-in-oil-in-water emulsion adjuvant. We demonstrated that any pre-existing immunity scenario included in this study significantly increased both the magnitude and breadth of hemagglutination inhibition antibody responses following vaccination with Centi-Flu. Naïve animals vaccinated with Centi-Flu exhibited less breadth, and responses were mostly below the level considered to be predictive of protection. Naïve animals also displayed much more rapid waning of antibody titers within two months. Our work demonstrates the validity of swine as a model for studying the role of pre-existing immunity in modulating immune responses to Influenza vaccination or infection and can serve as a better model for the development of universal IAV vaccines moving forward.