Conserved Linear Epitopes in the VP1 Protein of Foot-and-Mouth Disease Virus Serotype SAT2: Identification and Characterization.

Foot-and-mouth disease virus (FMDV) serotype Southern African Territories 2 (SAT2), historically endemic to sub-Saharan Africa, has recently expanded its geographical range to the Middle East, posing a significant transboundary threat to global livestock industries. Characterized by high genetic diversity encompassing 14 distinct topotypes, SAT2 poses a challenge to reliable serological diagnosis and underscores the need for conserved, serotype-specific antigenic targets. In this study, we aimed to identify such targets by mapping linear epitopes in the VP1 capsid protein using three monoclonal antibodies (mAbs; 3B2, 6F2, and 2F11) raised against SAT2 VP1. Epitopes were mapped through sequential truncation and alanine‑scanning mutagenesis, and their conservation was assessed across VP1 sequences from 53 SAT strains (35 SAT2, 13 SAT1, and 5 SAT3). The epitope recognized by mAb 3B2 was identified as 204RFDAPIGVE212. It displayed 88.9% sequence identity within SAT2 topotypes I, II, III, and VI, and 100% identity in all other topotypes, while showing minimal similarity with SAT1 and SAT3 VP1 proteins-confirming its serotype specificity. Through systematic alanine scanning, Asp206 was defined as a critical binding residue and was 100% conserved across all SAT2 topotypes, including recent outbreak strains. Structural analysis further indicated that this epitope is surface‑exposed and possesses high solvent accessibility. In contrast, the 47TSFVVDL53 epitope (recognized by mAb 6F2) was predominantly conserved only within SAT2 topotype VII, with limited conservation in other topotypes. The 7GAD9 core recognition sequence (targeted by mAb 2F11) showed variable conservation (66.7%-100%) among SAT strains. Collectively, the broad conservation across topotypes, combined with favorable structural features, positions the 204RFDAPIGVE212 epitope as a promising candidate for developing robust, SAT2‑specific serological diagnostics.