Stabilization of the trimeric pre-fusion structures of influenza H1 and H9 hemagglutinins by mutations in the stem helices

Stabilizing the pre-fusion structures of antigenic proteins can enhance the effectiveness of antiviral vaccines. The pre-fusion form of hemagglutinin (HA) from the influenza virus typically adopts a stable trimeric structure. However, the recombinant ectodomain of HA from the A/California/04/2009 (H1N1) influenza virus formed a monomer in solution rather than the expected trimer. To promote trimer formation in the pre-fusion conformation, we redesigned five amino acid residues in the stem region of HA that are involved in trimerization. The engineered HA protein formed a stable trimer at both pH 8.0 and pH 5.5. Additionally, the thermal stability of the modified protein improved, as indicated by an approximately ten-degree increase in its denaturation temperature. Cryo-EM analysis at 2.2 Å resolution confirmed that the mutant HA protein adopted the pre-fusion structure. Furthermore, the stabilized mutant exhibited enhanced immunogenicity in mice. We applied the same optimization strategy to the HA proteins from A/Malaysia/1706215/2007 (H1N1) and A/swine/Hong Kong/2106/98 (H9N2). These engineered proteins demonstrated increased thermal stability and retained a trimeric pre-fusion structure, as confirmed by cryo-EM analysis. Extending this optimization strategy to the equivalent five residues in hemagglutinins from six additional group 1 influenza viruses successfully stabilized their trimeric structures.