Novel Capsid-Specific Single-Domain Antibodies with Broad Foot-and-Mouth Disease Strain Recognition Reveal Differences in Antigenicity of Virions, Empty Capsids, and Virus-Like Particles.

Foot-and-mouth disease (FMD) vaccine efficacy is mainly determined by the content of intact virions (146S) and empty capsids (75S). Both particles may dissociate into 12S subunits upon vaccine manufacturing, formulation, and storage, reducing vaccine potency. We report the isolation of capsid-specific llama single-domain antibodies (VHHs) with broad strain recognition that can be used to quantify intact capsids in FMD vaccines by double antibody sandwich (DAS) ELISA. One capsid-specific VHH displayed remarkably broad strain reactivity, recognizing 14 strains representing the 13 most important lineages of serotype A, and two VHHs cross-reacted with other serotypes. We additionally show that the newly isolated VHHs, as well as previously characterized VHHs, can be used to identify antigenic differences between authentic 146S and 75S capsids, as well as corresponding genetically engineered virus-like particles (VLPs). Our work underscores that VHHs are excellent tools for monitoring the quantity and stability of intact capsids during vaccine manufacturing, formulation, and storage, and additionally shows that VHHs can be used to predict the native-like structure of VLPs.