Development of an S protein-based indirect ELISA for detecting IgA antibodies against porcine deltacoronavirus

Porcine deltacoronavirus (PDCoV) is an emerging coronavirus that primarily infects piglets causing acute enteric disease characterized by watery diarrhea, vomiting, dehydration, and growth retardation. As PDCoV spreads via the fecal-oral route and mainly targets the mucosal immune system, immunoglobulin A (IgA) has been identified as a key biomarker in infection detection and vaccine evaluation. During PDCoV, the viral spike (S) protein mediates receptor binding and membrane fusion and is also a major inducer of antibody responses, making it an ideal antigen for PDCoV antibody detection. In this study, the PDCoV S protein was expressed in a monoclonal Chinese Hamster Ovary (CHO) cell line and used as a coating antigen to establish an indirect enzyme-linked immunosorbent assay (iELISA) for PDCoV-specific IgA detection. The assay showed no cross-reaction with positive porcine epidemic diarrhea virus (PEDV), African swine fever virus (ASFV), porcine rotavirus (PoRV), foot and mouth disease virus (FMDV), transmissible gastroenteritis virus (TGEV), and swine acute diarrhea syndrome coronavirus (SADS-CoV) samples, indicating high specificity. The assay also successfully detected positive serum (1:4500 dilution) and milk samples (1:4000 dilution), with intra- and inter-assay coefficients of variation below 10%, demonstrating high sensitivity and repeatability. When compared with an indirect immunofluorescence assay (IFA), the iELISA demonstrated 95.68% and 93.02% concordance for serum and milk samples, respectively. Thus, our iELISA is a specific, sensitive, and reliable tool for PDCoV IgA antibody detection in serum and milk, and applies to infection surveillance and vaccine assessment.