Antiviral activity of navy bean ( <i>Phaseolus vulgaris</i> ) extract against influenza A virus <i>via</i> haemagglutinin interaction and interferon pathway modulation

<i>Phaseolus vulgaris</i> agglutinins (PHAs), lectins derived from navy beans, are recognised for their limited antiviral effects and toxicity in Madin-Darby canine kidney (MDCK) cells. In this study, fermented navy bean extract (FBE) produced with <i>Bacillus subtilis</i> was analysed using ultra-performance liquid chromatography coupled with tandem mass spectrometry, identifying various lectins, including monomeric forms of PHA-L and PHA-E (∼32 kDa). Pre-treatment with FBE exhibited 2.6- and 12.6-fold greater inhibition of influenza A virus (IAV) H1N1 replication than commercially available PHA-E and PHA-L (12.5 μg mL^-1), respectively. Additionally, a synthesised 10-amino acid peptide derived from PHA-L (antifungal lectin, AFL) exhibited antiviral properties, reducing the viral load by 1.95 log under pre-treatment conditions. This effect was attributed to AFL-induced type I interferon responses, which led to the upregulation of key antiviral genes (<i>IFN-α</i>, <i>IFN-β</i>, <i>STAT1</i>, and <i>STAT2</i>). Co-treatment with FBE and IAV H1N1 effectively inhibited viral-host interaction and entry. To further investigate the underlying mechanism, FBE was neutralised with haemagglutinin (HA)-His proteins, and immunoprecipitation-mass spectrometry analysis identified a 36 kDa protein (UniProt accession number: V7BGE3), previously annotated but newly implicated as an HA-binding protein. Transmission electron microscopy revealed that FBE binding to HA glycoproteins induced structural alterations on the outer surface of IAV H1N1 particles, potentially blocking viral entry. These findings highlight the multifunctional antiviral mechanisms of FBE and its potential as a novel therapeutic agent against IAV, warranting further exploration.