Genetic and Antigenic Evolution of Influenza A (H3N2) Virus Neuraminidase

There is still much uncertainty about the underlying mechanisms that govern antigenic drift of influenza viruses, and several theories have been proposed. These theories consider hemagglutinin (HA) to be the primary driving force, while the second key surface glycoprotein (neuraminidase, NA) and the other viral proteins have largely been ignored. This thesis focuses on all influenza A (H3N2) virus proteins, with special emphasis on NA, to better understand—and ultimately predict—the complex evolution of A(H3N2) viruses.

The genetic evolution, with respect to antigenic change, was analyzed by comparing HA and NA (chapter 2) and by studying the whole-genome (chapter 3). We next optimized the enzyme-linked lectin assay (ELLA) as an NA inhibition (NI) assay for human serology (chapter 5) and rapid antigenic characterization (chapter 6). With the optimized NI ELLA, we analyzed NA from 1968 till the 2010-2011 season and subsequently compared it to HA (chapter 7).

Our work clearly shows that there is antigenic evolution for NA, thus warranting the inclusion of NAs representing emerging influenza A strains in vaccines. Increasing knowledge on which mutations cause changes in the phenotype of NA can help to perform more targeted influenza surveillance. It would then be advisable to integrate genetic and antigenic NA data with sequence and antigenic data of HA, epidemiological data, and geographical data during influenza surveillance. This will facilitate consideration of NA content, and improve next generation influenza vaccines.