# Porcine B cell receptor repertoire uncovers balanced recognition of antigenic structures on serotype Asia1 foot-and-mouth disease virus

## TL;DR

This study explores how pigs recognize and respond to a specific strain of foot-and-mouth disease virus, revealing a balanced immune response that may explain its strong vaccine effectiveness.

## Contribution

The study identifies a balanced antibody response to FMDV serotype Asia1, revealing novel antigenic structures and epitopes that may guide vaccine design.

## Key findings

- Seventeen neutralizing monoclonal antibodies were identified from high-frequency B-cell clonotypes.
- Cryo-EM structures revealed antigenic sites on VP2 and a novel epitope spanning VP2 and VP3.
- Vaccinated pigs showed a balanced antibody response across VP1, VP2, and VP3 antigenic structures.

## Abstract

Of the seven serotypes of foot-and-mouth disease virus (FMDV) strains circulating globally, serotype Asia1 has been effectively eradicated in China through systematic vaccination in livestock. The structural characteristics of serotype Asia1 may enhance its immunogenicity compared to other serotypes. Herein, we present a preliminary exploration of Asia1-binding B-cell receptor repertoire, containing 3571 clones, and identified 17 porcine-derived neutralizing monoclonal antibodies (pnAbs) from the top 33 high-frequency clonotypes. The majority of pnAbs (14/17) recognized the epitopes on VP2, with a common determinant at residue 72 (D) on the B-C loop; two pnAbs (2/17) recognized a novel epitope spanning VP2 and VP3; and the remaining one (1/17) bound to the C-terminus of VP1. Furthermore, the antigenic structures on VP2 and spanning VP2 and VP3 were respectively elucidated by determining the cryo-EM structures of FMDV serotype Asia1 in complexes with two pnAbs, PAS5 and PAS12. The light chain of PAS5, forming the majority of contact sites with the viral particle, focuses on the βB, B-C loop, βC and H-I loop of VP2, with key determinants at residues 68, 72 and 77 around the three-fold axis, corresponding to antigenic site 2. The contact sites of both VH and VL of PAS12 uncover a novel antigenic structure comprising the B-C, and H-I loops on VP2, and the B-B knob and βB on VP3, with key determinants at residue 73 on VP2 and 59 on VP3. Subsequently, site-directed competitive ELISA analysis of sera from primary and booster vaccinated pigs revealed a balanced antibody response profile, suggesting a potentially even immunodominance among antigenic site 2, VP1 G-H loop, and the novel antigenic structure spanning VP2 and VP3 on FMDV serotype Asia1. Compared to the focused immunodominance observed in other serotypes, this balanced antigenic recognition across VP1, VP2, and VP3 of FMDV serotype Asia1 reflects a diversified antibody response that may contribute to effective neutralization and protection.

Foot-and-mouth disease virus serotype Asia1, having good immunogenicity, was effectively eradicated in China through livestock vaccination. However, the antigenic properties of Asia1 remain unknown. We constructed a porcine B-cell receptor repertoire against FMDV serotype Asia1 and identified a panel of neutralizing antibodies from high-frequency clonotypes. Furthermore, the antigenic structures on VP2 and spanning VP2 and VP3 of serotype Asia1 were elucidated from cryo-EM complexes of virus-antibodies. Next, by evaluating competitive antibodies in sera from vaccinated pigs, we observed that FMDV serotype Asia1 exhibits a more balanced antibody response profile across its structural proteins. The evenly distributed immunodominance of antigenic structures on viral particle reflects a diversified antibody response, which may contribute to the effective immunogenicity of FMDV serotype Asia1. This configuration of antigenic structures is beneficial for guiding rational vaccine design.