Using Pseudotypes To Study Heterosubtypic Antibody Responses Elicted By Seasonal Influenza Vaccination

Influenza viruses represent an important public health burden since they cause annual epidemics associated with severe illness and mortality in high-risk populations. Additionally, zoonotic influenza virus infections have potential to produce intermittent pandemics, which have led to millions of deaths globally. However, strategies to prevent influenza severity and spread can be implemented. It is known that antibodies against the haemagglutinin play a key role in protection from influenza virus infection, thus both seasonal and pandemic influenza vaccines aim to elicit such antibodies. Generally, they are directed against haemagglutinin globular head epitopes and are able to neutralize closely related influenza strains, but recently antibodies able to neutralize multiple influenza strains and subtypes have also been described. The discovery of these antibodies, primarily directed against the haemagglutinin stalk, has generated interest in understanding how they are generated and how widespread they are in the human population. Furthermore, eliciting such antibodies has become the aim of many ‘universal’ vaccine approaches. However, the study of these cross-reactive antibodies using classical serological assays is problematic since the current assays have been shown to be relatively insensitive in detecting them.

The main objective of this thesis was to study the presence and breadth of cross-reactive neutralizing responses in human populations. To overcome the limitations of current serological tests in detecting these responses, lentiviral pseudotype particles bearing the haemagglutinins of different influenza A subtypes and influenza B strains were used as surrogate antigens in neutralization assays. After the generation of these novel tools and the establishment of appropriate controls, pseudotype particle neutralization assays were employed to investigate cross-reactive antibody responses in pre- and post-vaccination sera. Next, the use of chimeric haemagglutinins, in which the globular head was substituted with the head of a different subtype, was incorporated into the pseudotype system. This allowed the differentiation between haemagglutinin head-directed and stalk-directed antibody responses. The ability to efficiently detect broadly neutralizing antibody responses, including those directed against the haemagglutinin stalk, shows that pseudotype particles are tools that should be further characterised and implemented to be used in sero-epidemiological studies and for ‘universal’ vaccine immunogenicity studies.