Isolation of an in Vitro Affinity-Matured, Thermostable “Headless” HA Stem Fragment That Binds Broadly Neutralizing Antibodies with High Affinity

The surface glycoprotein hemagglutinin (HA) of influenza virus is the primary target for design of an effective universal influenza vaccine as it is capable of eliciting broadly cross-reactive antibodies against different HA subtypes. Several monoclonal antibodies targeting the stem region of HA that are able to neutralize various subtypes of influenza virus have been isolated in the recent past. Designing a stable, HA stem immunogen that attains a native-like conformation and can elicit such antibodies has been a challenge. We describe the affinity maturation of a previously designed stem immunogen (H1HA6) by random mutagenesis, followed by selection using yeast surface display. The affinity-matured mutant protein (H1HA6P2), upon bacterial expression, attained a stable, native-like, trimeric conformation without any heterologous trimerization motif and showed a significant improvement in thermal stability and binding to several stem-specific, conformation-sensitive, broadly neutralizing antibodies (bnAbs) relative to H1HA6. These results point to an effective strategy for design of stabilized HA stem immunogens that can be tested for their protective ability.