Influenza hemagglutinin stem-fragment immunogen elicits broadly neutralizing antibodies and confers heterologous protection

TLDR

Hemagglutinin is the primary target of neutralizing antibodies, yet its wide diversity necessitates annual vaccination and limits cross‑reactive broadly neutralizing antibodies, and the metastable stem conformation poses challenges for designing a stable, independently folding immunogen. The authors aimed to rationally design a stem‑fragment immunogen that mimics the native HA stem and binds conformation‑specific broadly neutralizing antibodies with high affinity. They engineered a thermotolerant, disulfide‑free stem fragment that folds independently, binds bnAbs, and can be expressed solubly in bacteria for rapid scale‑up during outbreaks. The immunogen elicited broadly neutralizing antibodies and provided robust protection against lethal heterologous virus challenge, while its bacterial expression platform enables rapid production.

Abstract

Significance Hemagglutinin (HA), the major influenza virus envelope glycoprotein, is the principal target of neutralizing antibodies. Wide diversity and variation of HA entails annual vaccination, as current vaccines typically fail to elicit/boost cross-reactive, broadly neutralizing antibodies (bnAbs). Although several bnAbs bind at the conserved stem of HA making it an attractive universal vaccine candidate, the metastable conformation of this domain imposes challenges in designing a stable, independently folding HA stem immunogen. We rationally designed a stem-fragment immunogen, mimicking the native HA stem that binds conformation-specific bnAbs with high affinity. The immunogen elicited bnAbs and conferred robust protection against lethal, heterologous virus challenge in vivo. Additionally, soluble bacterial expression of such a thermotolerant, disulfide-free immunogen allows for rapid scale-up during pandemic outbreak.