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This dissertation, Virological and Host Factors Contributing to Severe Influenza by Kai-wang, Kelvin, To, 杜啟泓, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: The pandemic influenza virus A(H1N1) of 2009 (A[H1N1]pdm09) and the avian influenza virus A(H7N9) of 2013 provided unique opportunities for studying severe influenza. This thesis investigated the pathogenesis of severe influenza and the virological or host factors contributing to disease severity in the hope of finding more effective treatment and prevention options.During the 2009 pandemic, both A(H1N1)pdm09 and pre-existing seasonal influenza viruses co-circulated and were associated with similar morbidity and mortality. When compared with seasonal influenza patients, A(H1N1)pdm09 patients were younger, more likely to be obese or pregnant, but less likely to have chronic illness. By assessing the change in type-specific and age-specific hemagglutinating antibody titers between the pre-pandemic and post-pandemic period, and combining data from laboratory-confirmed cases, patients aged >=51 years were shown to be at a higher risk of severe A(H1N1)pdm09 infection when compared to younger patients. Patients with severe A(H1N1)pdm09 infection had rapidly progressive pneumonia. Postmortem histopathological studies showed diffuse alveolar damage in patients who died early and fibroproliferation in those who died late. Extrapulmonary complications were commonly observed. The clinical and histopathological features of A(H7N9) infection are similar to those of severe A(H1N1)pdm09 infections. Severe A(H1N1)pdm09 cases had a slower decline of viral load but persistently higher levels of IL-6, IL-10 and IL-15 than those of mild cases. There was no significant difference in the nasopharyngeal viral load between pandemic and seasonal A(H1N1)pdm09 patients. However, viremia was detected more frequently in patients with severe A(H1N1)pdm09 infection than those with mild infection.Viral polymorphisms which contribute to disease severity of A(H1N1)pdm09 or human adaptation of A(H5N1) and A(H7N9) were assessed. Hemagglutinin D225G substitution was found to be associated with severe A(H1N1)pdm09 infection, which were enriched in the lower respiratory tract and blood. A systematic bioinformatic analysis showed that hemagglutinin substitutions which enhance binding to human-type sialic acid receptors were more prevalent among A(H5N1) strains isolated from Egypt, where the prevalence of A(H5N1) human infection is highest in the world. L336M substitution of the PA protein, which is associated with enhanced viral replication, emerged as a novel A(H7N9) variant in Hong Kong. Humoral immunity is associated with antiviral effect and host inflammatory response. High titers of high avidity non-neutralizing antibodies that appeared within 2-4 days of symptom onset were associated with severe A(H1N1)pdm09 infection. Furthermore, a low plasma IgG2 titer was associated with dysregulated cytokine/chemokine response in severe cases. Genetic susceptibility study showed that variations in the CD55 and surfactant protein B gene were associated with severe A(H1N1)pdm09 infection. Obesity is a risk factor for severe A(H1N1)pdm09 infection. A high level of leptin was associated with mortality in obese mice with influenza virus infection, and anti-leptin antibody could protect obese mice from lethal challenge by A(H1N1)pdm09. Novel treatment and vaccination strategies were explored. Hyperimmune intravenous immunoglobulin treatment improved survival of severe A(H1N1)pdm09 infection, which demonstrated the impor

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