Antibodies directed against the
influenza hemagglutinin (HA)
protein largely mediate virus neutralization and confer protection against
infection. Consequently, many studies and assays of
influenza vaccines are focused on HA-specific immune responses. Recombinant HA (rHA)
proteins can be produced in a number of
protein expression and cell culture systems. These range from baculovirus
infection of insect cell cultures, to transient transfection of plants, to stably transfected human cell lines. Furthermore, the rHA
proteins may contain genetic modifications, such as
histidine tags or trimerization domains, intended to ease purification or enhance protein stability. However, no systematic study of these different forms of the HA
protein have been conducted. It is not clear which, if any, of these different
protein expression systems or structural modifications improve or diminish the
biological behavior of the
proteins as immunogens or
antigens in immune assays. Therefore we set out to perform systematic evaluation of rHA produced in different
proteins expression systems and with varied modifications. Five rHA
proteins based on recent strains of seasonal
influenza A and five based on
influenza B HA were kindly provided by the Biodefense and Emerging
Infections Reagent Repository (BEIR). These
proteins were evaluated in a combination of biochemical and structural assays, in vitro humoral and cellular immune assays, and in an animal vaccination model. Marked differences in the behavior of the individual
proteins was evident suggesting that they are not equal when being used to detect an immune response. They were, nevertheless, similar at eliciting
neutralizing antibody responses.