Optimizing an avian influenza vaccine using a novel Bacterial Enzymatic Combinatorial Chemistry (BECC) TLR4 adjuvant.

The development of broadly protective and dose-sparing influenza vaccines remains a critical challenge, particularly for zoonotic H5N1 strains with pandemic potential. This study evaluates BECC470s, a synthetic TLR4 adjuvant, for its ability to enhance the immunogenicity and protective efficacy of recombinant H5 hemagglutinin (rHA) vaccination in murine models. BECC470s-adjuvanted rHA elicited robust IgG1/IgG2a antibody responses and complete survival following homologous 2004 H5N1 challenge in a prime-boost model. Although BECC470s broadened antibody binding to both variable HA head and conserved stalk domains by ELISA, functional neutralizing antibody responses were restricted to the matched 2004 H5N1 isolate, with no detectable neutralization of H5N1 viruses isolated in 2022 or 2024. These data indicate that BECC470s enhances the magnitude and apparent cross-reactivity of binding antibody responses while maintaining strain-specific neutralizing activity, supporting its potential as an adjuvant for next-generation influenza vaccines while underscoring the need for further optimization to achieve true cross-neutralizing protection.IMPORTANCEBECC470s is a synthetic TLR4 agonist adjuvant that enhances the breadth and potency of recombinant H5 hemagglutinin vaccination in mice. By driving balanced antibody responses, complete protection against homologous challenge, and neutralizing activity targeting both HA head and stalk, BECC470s outperforms the comparator TLR4 adjuvant. These preliminary findings indicate that BECC470s enables antigen sparing while increasing antigen immunogenicity, supporting its potential translational application to current H5N1 vaccines.