Conserved antigen structures and antibody-driven variations on foot-and-mouth disease virus serotype A revealed by bovine neutralizing monoclonal antibodies

Foot-and-mouth disease virus (FMDV) serotype A is antigenically most variable within serotypes. The structures of conserved and variable antigenic sites were not well resolved. Here, a historical A/AF72 strain from A22 lineage and a latest A/GDMM/2013 strain from G2 genotype of Sea97 lineage were respectively used as bait antigen to screen single B cell antibodies from bovine sequentially vaccinated with A/WH/CHA/09 (G1 genotype of Sea97 lineage), A/GDMM/2013 and A/AF72 antigens. Total of 39 strain-specific and 5 broad neutralizing antibodies (bnAbs) were isolated and characterized. Two conserved antigenic sites were revealed by the Cryo-EM structures of FMDV serotype A with two bnAbs W2 and W125. The contact sites with both VH and VL of W125 were closely around icosahedral threefold axis and covered the B-C, E-F, and H-I loops on VP2 and the B-B knob and H-I loop on VP3; while contact sites with only VH of W2 concentrated on B-B knob, B-C and E-F loops on VP3 scattering around the three-fold axis of viral particle. Additional highly conserved epitopes also involved key residues of VP158, VP1147 and both VP272 / VP1147 as determined respectively by bnAb W153, W145 and W151-resistant mutants. Furthermore, the epitopes recognized by 20 strain-specific neutralization antibodies involved the key residues located on VP3 68 for A/AF72 (11/20) and VP3 175 position for A/GDMM/2013 (9/19), respectively, which revealed antigenic variation between different strains of serotype A. Analysis of antibody-driven variations on capsid of two virus strains showed a relatively stable VP2 and more variable VP3 and VP1. This study provided important information on conserve and variable antigen structures to design broad-spectrum molecular vaccine against FMDV serotype A. Bovine is susceptible host to foot-and-mouth disease virus (FMDV) and neutralization antibodies provide vital protection in defending viral infection, concurrently driving viral evolution in host. Herein, using single B cell antibody technology, we isolated and characterized a panel of 44 bovine-derived neutralizing monoclonal antibodies against FMDV serotype A, including 39 strain-specific and 5 broad neutralizing antibodies (bnAbs) against both A22 and Sea97 lineages representative strains. We revealed at least four conserved antigen sites including two sites on VP1 and each one on VP2/VP3, which exist on viral capsid surface and can induce bnAb response to FMDV serotype A in vivo. Additionally, antibody-driven variations showed shrinkage and appearance of strain-specific antigen epitopes were found on VP3 68 and 175 positions of FMDV serotype A. To sum up, this study provided conserved antigen structures and strain-specific epitopes information to guide the design of broad vaccine molecular against FMDV serotype A.