Development and Comparison of Indirect ELISAs for Detecting IgG and IgA Antibodies Against Major Structural Proteins of Porcine Deltacoronavirus With Virus Neutralization as a Benchmark.

Porcine deltacoronavirus (PDCoV) is an emerging enteric pathogen that causes substantial economic losses in the global swine industry. Although neutralizing antibodies (NAbs) are a key indicator of vaccine efficacy, their diagnostic concordance with IgG/IgA levels measured by indirect enzyme-linked immunosorbent assays (iELISAs) targeting major structural proteins has not been systematically evaluated. In this study, the full-length spike (S), S1 domain, and receptor-binding domains (RBD) from a highly virulent PDCoV strain were expressed in CHO cells. At the same time, the membrane (M) and nucleocapsid (N) proteins were produced in Escherichia coli (E. coli). Following initial reactivity screening via protein microarray, S, S1, RBD, and N were chosen to establish iELISAs for detecting IgG and IgA antibodies in serum and milk samples. Evaluating iELISAs' specificity revealed cross-reactivity of anti-S IgA and anti-N IgA with porcine epidemic diarrhea virus (PEDV) antibody-positive sera. Analysis of 75 clinical pig serum samples, and 75 colostrum samples demonstrated that IgA-based iELISAs had superior diagnostic concordance with virus neutralization test (VNT) results than IgG-based iELISAs, with the S1-IgA iELISA showing the highest concordance (93.3%). Furthermore, IgA antibody levels correlated more strongly with neutralizing titers (NTs) than IgG. These findings validate the S1-IgA iELISA as a robust, high-throughput serological tool for assessing PDCoV immunity in pigs.