Host Factor Determinants of Influenza A Virus Infection and Severity in Pigs

Influenza A virus (IAV) causes respiratory infection and many cases of disease associated with IAV occur annually during seasonal epidemics. During the past 100 years, zoonotic events followed by IAV pandemics have led to deaths of hundreds of millions of people. IAV has a broad host range and can infect a variety of avian and mammalian species in which novel strains of IAVs can emerge. Transmission of novel IAV to a susceptible human population with no or limited preexisting immunity may lead to a new pandemic with fatal outcomes. The most recent IAV pandemic in 2009 was caused by a virus originating from swine, but to date there is still a lack of knowledge in molecular determinants involved in the ability of IAV to break the species barrier. When IAV infect respiratory epithelial cells of a host, it is recognized by innate immune sensors, which initiate induction of several antiviral immune factors, including proand antiinflammatory cytokines and interferons. The interferon response induces the production of several antiviral molecules both in the infected cell and in neighbouring cells. However, IAV can alter and evade the host immune response through several mechanisms, making hostpathogen interactions highly complex and relevant to study.

Paper 1 is a review, which emphasizes the threat of zoonotic and reverse zoonotic transmission of viruses between humans and animals. Especially spillover events between pigs and humans are likely to occur as pigs and humans share a great number of viruses and are in close contact during pig farming worldwide. Paper 2 is a thorough exposition of the experimental infection study of pigs with different IAV (H1N1) strains that underlies the data generated in Paper 3 and Paper 4. Pigs were inoculated with IAV strains with different adaptation levels to the host, which revealed significant differences in viral dynamics and pathological manifestations. Infection with IAV well-adapted to the host (swine-adapted) resulted in high viral load, but reduced pathogenicity and clinical impact compared to a human-adapted IAV and a less host-adapted ”pre-pandemic” IAV. The observed differences in viral dynamics and pathological changes could be connected to the immune dynamics, which is described in Paper 3 and Paper 4. In Paper 3, the kinetics and dynamics of the antiviral innate immune response were shown to differ depending on the infecting type of IAV strain in nasopharyngeal swabs after IAV challenge. IAV well-adapted to the host induced a fast and strong expression of innate factors compared to a more dampened response when infected with an IAV adapted to another host (in this case humans). Furthermore, infection with a less host-adapted ”pre-pandemic” strain resulted in a prolonged immune response. Importantly, the well-adapted IAV was able to bypass an important first line of defence by downregulation of both secreted and transmembrane mucins. In Paper 4 it was further revealed that the well-adapted IAV regulated the host metabolism to improve viral replication in tracheal tissue. In Paper 5, the use of respiratory explant cultures to study antiviral host immune responses was described. The immune responses observed after IAV infection of explants were comparable to responses after in vivo infection of pigs. In addition, the use of air-liquid interface (ALI) cultures has likewise been investigated as a tool to study immune responses after IAV infection. Both models are promising 3R compliant tools to study host-pathogen interactions under very controlled conditions.

The work described in this thesis has contributed to our knowledge of kinetics and dynamics of the antiviral innate immune response after infection with IAV with different host adaptation levels. Host adaptation and the ability to evade the host immune response impacted the outcome of the antiviral response. Furthermore, it was shown that the innate immune response could be linked to viral load and severity of infection.