Four critical epitopes of the nucleocapsid protein contribute to serological cross-reactivity between porcine epidemic diarrhea virus and porcine deltacoronavirus

Porcine epidemic diarrhea virus (PEDV) and porcine deltacoronavirus (PDCoV) coronaviruses cause highly similar histopathological damage and clinical symptoms, and often appear as mixed infections in clinic, leading to great economic losses in the pig industry. Previous studies have demonstrated a certain degree of serological cross-reactivity between PDCoV and PEDV. However, the antigens and crucial epitopes contributing to serological cross-reactivity remain unclear. In this study, the specific viral antigen and crucial epitopes leading to serological cross-reactivity between PDCoV and PEDV were investigated. The nucleocapsid (N) protein was the contributing viral antigen. Truncation and mutation of the PDCoV N protein revealed that the region for serological cross-reactivity was located in the N-terminal domain. The crucial epitopes were amino acid aa19−62, aa79−91, aa102−196, and aa285−342. In addition, these crucial epitopes were highly homologous between the N proteins of PDCoV and other coronaviruses, suggesting that these sites may be the crucial epitopes inducing serological cross-reactivity between different coronaviruses. Moreover, an indirect ELISA capable of specifically differentiating PDCoV and PEDV sera was established by virtue of the C-terminal-truncated portion of N protein, which had low serological cross-reactivity. In conclusion, the main antigen and crucial epitopes leading to serological cross-reactivity between PDCoV and PEDV were identified. The specific ELISA created in this study provided important clues and effective methods for subsequent establishment of the diagnostic methods for PDCoV and PEDV, as well as effective differentiation between them in clinical practice.