Unique features of a recombinant haemagglutinin influenza vaccine that influence vaccine performance

Abstract

The influenza vaccine field has been constantly evolving to improve the speed, scalability, and flexibility of manufacturing, and to improve the breadth and longevity of the protective immune response across age groups, giving rise to an array of next generation vaccines in development. Among these, the recombinant influenza vaccine tetravalent (RIV4), using a baculovirus expression vector system to express recombinant haemagglutinin (rHA) in insect cells, is the only one to have reached the market and has been studied extensively. We describe how the unique structural features of rHA in RIV4 improve protective immune responses compared to conventional influenza vaccines made from propagated influenza virus. In addition to the sequence integrity, characteristic of recombinant proteins, unique post-translational processing of the rHA in insect cells instills favourable tertiary and quaternary structural features. The absence of protease-driven cleavage and addition of simple N-linked glycans help to preserve and expose certain conserved epitopes on HA molecules, which are likely responsible for the high levels of broadly cross-reactive and protective antibodies with rare specificities observed with RIV4. Furthermore, the presence of uniform compact HA oligomers and absence of egg proteins, viral RNA or process impurities, typically found in conventional vaccines, are expected to eliminate potential adverse reactions to these components in susceptible individuals with the use of RIV4. These distinct structural features and purity of the recombinant HA vaccine thus provide a number of benefits in vaccine performance which can be extended to other viral targets, such as for COVID-19.

Fig. 1. Generation of rHA using the baculovirus-insect cell expression system for the manufacture of RIV4.

BV baculovirus. Figure adapted from Cox, M. M. & Hashimoto, Y (2011). A fast track influenza virus vaccine produced in insect cells. J Invertebr Pathol 107 Suppl, S31-41 ^© 2011 Elsevier Inc, with permission from Elsevier.

Fig. 2. Structural features of native HA expressed on influenza virus and rHA produced in insect cells using the baculovirus expression vector system.

a HA protein (shown as monomer) extracted from influenza virus is a heterodimer comprising HA1 (turquoise) and HA2 (orange) linked though an inter-disulfide (S–S) bond and contains complex-type sialylated N-linked glycans. Cleaved fusion peptides (green) and complex glycans eliminate and mask unique epitopes on HA respectively. b Recombinant HA protein (shown as monomer) expressed in insect cells as a single precursor polypeptide (HA0) with amino acid sequence identical to wildtype HA sequence and contains unsialylated paucimannosidic N-linked glycans. HA0 is coloured in turquoise and orange to show HA1 and HA2 segments combined as a single polypeptide. Uncleaved fusion-peptide loop (green) and simpler glycans preserve and expose unique epitopes on rHA.