Defining the association between virulence and fitness of seasonal influenza viruses

Summary We expect to detect a correlation between severity of illness and virus titre, fitness and/or specific infectivity achieved in human respiratory tract cells.

Project Details

The severity of influenza illness is influenced by host and viral factors. Host factors include age, prior immunity, underlying health conditions and genetic susceptibility. Some influenza viruses, particularly A(H3N2) viruses, are clearly associated with more severe illness and epidemics than others. Viral determinants include antigenic drift that permits escape from immunity, tropism and replicative fitness in the human respiratory tract. Additionally, the ratio of infectious and non-infectious (defective) viral particles produced from different substrates can differ; this can be measured as specific infectivity. Madin Darby canine kidney (MDCK) cells are the standard cell line used for propagation of influenza viruses in laboratories but they are not the natural substrate for human influenza viruses. We hypothesise that influenza viruses associated with severe disease will replicate more efficiently in cells derived from the human respiratory tract than viruses associated with mild outbreaks. As part of global virologic surveillance activities, the World Health Organization Collaborating Centre for Reference and Research on Influenza (WHO CC) monitors genetic and antigenic drift by characterising approximately 4000 influenza viruses each year. Seasonal influenza viruses associated with severe or mild illness will be identified among viruses submitted to the WHO CC.

Their replicative fitness and specific infectivity will be assessed by measuring virus titre and number of infectious virions by plaque assay/number of genome copies by digital drop PCR, respectively in (1) monolayers of A549 cells, a cell line derived from a human lung adenocarcinoma; 2) normal human tracheobronchial epithelial (NHBE) cells grown at an air liquid interface from a commercial source, and (3) recently described human airway organoids established through a collaboration with Professor Elizabeth Vincan from the Doherty Institute, Professor Michael Chan from the University of Hong Kong, and Professor Hans Clever from the University of Utrecht, in addition to (4) standard MDCK cells.

Data on replication and specific infectivity will be correlated with nucleotide sequences generated by next generation sequencing, which is routinely performed in the WHO CC. We expect to detect a correlation between severity of illness and virus titre, fitness and/or specific infectivity achieved in human respiratory tract cells.