Designing a reoriented H5 hemagglutinin to generate stem-directed, heterosubtypic immunity to influenza A viruses

Human infections with the highly pathogenic avian influenza viruses highlight the risk of novel pandemics, against which seasonal vaccines provide limited cross-protection. Whereas antibody responses to current vaccine candidates are typically biased toward the immunodominant and variable head region of hemagglutinin (HA), the stem region of HA is highly conserved and targeted by broadly neutralizing antibodies. Here we describe a reoriented H5 HA (reoH5HAPhe, based on A/Vietnam/1203/2004) displayed on alum as a vaccine candidate to elicit stem-directed antibody responses and confer cross-protective immunity. We show that a single immunization with reoH5HAPhe protected naive mice against heterologous H5N1 lethal challenge (A/Texas/37/2024), and boosting with reoH5HAPhe conferred complete heterologous protection in mice pre-immunized with seasonal vaccines. The stem-directed response elicited by reoH5HAPhe also protected against heterosubtypic H1N1 challenge in both naive and pre-immune mice. Electron microscopy polyclonal epitope mapping revealed that reoH5HAPhe elicited antibodies targeting the central stem and anchor epitopes on heterologous H5 and H1 HAs, alongside antibodies against the (H5) vestigial esterase or (H1) lateral patch epitopes. Despite limited cross-neutralizing activity, increased stem-directed responses with balanced IgG1 and IgG2 subclasses reduced viral replication and improved survival. Our results highlight antigen reorientation as an effective approach for designing variant-resilient vaccines and demonstrate the potential of reoH5HAPhe as an H5 pandemic influenza vaccine candidate that confers protective immunity against both emerging and seasonal influenza viruses.