Porcine epidemic diarrhea virus (PEDV) causes devastating enteric disease in piglets, yet the mechanistic basis of antibody-mediated neutralization remains poorly understood. Here, we determined the structure of PEDV HNXX-strain spike domain B (S1B) simultaneously bound by C62, a neutralizing porcine monoclonal antibody against PEDV G2 strains, and N34, a non-neutralizing porcine PEDV antibody. The structure reveals that C62 targets a conserved, cryptic epitope that is accessible only when S1B adopts an “up” conformation. Functionally, we showed that C62 has substantially stronger activity than N34 in triggering S-trimer disassembly and inducing the formation of proteinase K-resistant, post-fusion-like S2 structures. Despite the weaker triggering activity of N34, both C62 and N34 can function as artificial receptors. Notably, although the C62 epitope is conserved across both G1 and G2 strains, C62 exhibits G2-strain-biased neutralizing activity. We further showed that differences in cell-surface membrane fusion activity among PEDV spikes correlate with distinct viral entry pathways and are jointly determined by the S1A and S1B sequences. Together, our findings identify a strain-specific vulnerable site on the PEDV S-trimer and provide insight into how cell-surface membrane fusion activity may influence viral entry pathway selection and antibody neutralization efficacy
