The 1957 and 1968 pandemic
influenza viruses emerged via reassortment and contained HA
and PB1 gene segments of avian origin. It is well known that
the acquisition of an antigenically novel avian HA allowed
the emerging virus to overcome pre-existing immunity in the
human population. At the same time, the biological
significance of the presence of the avian PB1 in pandemic
influenza viruses remains elusive. In this work we wished to
address this longstanding question, and to determine whether
avian PB1 provided pandemic influenza viruses with some
replicative advantage in humans. In the first part of the
thesis, I modelled the reassortment event that led to the
emergence of the 1968 pandemic influenza virus from its avian
and human precursors. Using human H2N2 virus
A/California/1/66 (Cal66), two recombinant Cal66 virus pairs
were generated, in which the human-origin PB1 and/or HA gene
segments were substituted by corresponding avian-like gene
segments of the pandemic strain A/Hong Kong/1/68 (HK68). To
test, whether avian PB1 facilitated viral replication and
transmission of the pandemic virus, competitive replication
experiments in human bronchial epithelial cells (Calu-3), as
well as competitive contact transmission experiments in
guinea pigs were performed. Viral competition in Calu-3 cells
was studied by making consecutive passages starting from a
1:1 mixture of viruses and determining composition of the
passage 3 material by sequencing. In addition competitive
rescue experiments were performed by transfecting cells with
a whole set of reverse genetics plasmids which included a 1:1
mixture of PB1Cal and PB1HK plasmids. The composition of
rescued virus mixtures was determined by sequencing. To study
contact transmission efficiency of the viruses, a group of
four guinea pigs was inoculated with an equal mixture of
viruses. One day post-infection (p.i.) four naïve animals
were co-housed with infected animals and at different days
p.i. nasal washes were taken and characterized by sequencing.
These studies demonstrated, that avian PB1 enhances viral
replication independently of the origin of the HA gene
segment, possibly due to an enhanced viral polymerase
activity. The human-adapted avian PB1 of the 1968 pandemic
virus diff...