Structural basis of porcine reproductive and respiratory syndrome virus 2 neutralization by a GP4-targeting monoclonal antibody

Porcine reproductive and respiratory syndrome virus (PRRSV) is a genetically diverse RNA virus that causes reproductive failure and respiratory disease in pigs, leading to major economic losses worldwide. Although inactivated and live-attenuated vaccines are available, they provide only partial protection, allowing endemic circulation, persistent transmission and recurrent outbreaks. The limited efficacy of current vaccines reflects the incomplete understanding of immune correlates of protection. Here, we describe a murine monoclonal antibody (IgG #18) with potent, strain-specific neutralizing activity against PRRSV-2. Epitope mapping localized its binding site to the N-terminal region of the minor glycoprotein GP4. Functional and biochemical analyses demonstrated high-affinity binding and effective neutralization, while crystallographic studies revealed its atomic structure in complex with an epitope peptide. To our knowledge, this structure provides the first structural snapshot of a neutralization epitope within the PRRSV glycoproteins, providing insights into the conformation of this key component of the PRRSV envelope complex. This structure also explains the restricted specificity of IgG #18, highlighting the challenge of epitope variability across PRRSV species. Our findings advance understanding of antibody-mediated neutralization and provide an initial framework for the use of structural information in the design of next-generation immunogens aimed at eliciting protective responses against genetically diverse PRRSV strains.