Structural basis of antigenic drift in FMDV O/Cathay topotype variants revealed by porcine-derived single B-cell neutralizing antibodies.

The porcinophilic O/Cathay topotype variants of foot-and-mouth disease virus (FMDV) continuously accumulated mutations over time and produced antigenic drift, leading to diminished serum neutralization and compromised vaccine efficacy. However, the structural basis underlying this divergence remains poorly defined. Herein, we observed that the newly emerging O/Cathay variants in China exhibited marked neutralization escape from porcine immune sera. Furthermore, using a panel of seven porcine-derived neutralizing monoclonal antibodies (pnAbs), we mapped two antigenic regions with crucial determinants responsible for neutralization escape in recent O/Cathay variants. The charge change at +2 residue (Q → H/R) downstream of the conserved RGD receptor-binding motif on VP1, which modulates sensitivity to multiple pnAbs (pO18-34, pO18-40 and pO18-43) and vaccine-induced sera. Cryo-electron microscopy (cryo-EM) structures of pO18-29 in complex with the ancient O/HN/CHA/93 strain and the recent variant O/18074 revealed a novel VP3-centered antigenic structure and showed that mutations at positions 73 (V → K) and 134 (K → T) on VP3 mediated neutralization escape in O/Cathay variants via disruption of hydrophobic interaction as well as the hydrogen bonds and salt bridge networks. Moreover, residues 73 and 134 synergistically regulate pnAbs (pO18-2, pO18-18, pO18-28 and pO18-29) recognition. Together, our findings elucidate structural basis of neutralizing evasion in O/Cathay FMDV and provide a molecular framework for the rational design of next-generation broad-spectrum FMDV vaccine against emerging O/Cathay variants.