Characterization of a cell-adapted completely attenuated genotype GIIa porcine epidemic diarrhea virus strain

Porcine epidemic diarrhea virus (PEDV) has caused significant harm to the global pig industry since its discovery. In this study, a highly pathogenic strain of GIIa PEDV CH/HBXT/2018, isolated previously, was continuously passaged in Vero cells up to passage (P)240, resulting in a completely attenuated virus. The proliferation characteristics of different passages of the strain in Vero cells, pathogenicity in newborn piglets, and mutations in S gene sequence indicated that as the passage number increased, the replication efficiency of PEDV in Vero cells gradually improved, with a more pronounced cytopathic effect. However, its pathogenicity in piglets decreased progressively, evident as reduced viral loads in the feces and intestinal tissues, less-severe clinical symptoms, less-severe histopathological damage, and lower antigen expression in intestinal tissues. At P240, the strain was completely attenuated. A sequence analysis revealed 17 amino acid mutations in the structural spike protein, which may have contributed to the biological changes observed at P240. Furthermore, compared with P10, the strain's dependence on trypsin had decreased significantly at P200. A differential transcriptomic analysis revealed 1712 differentially expressed genes (DEGs) between the P10 and P200 infection groups, of which 458 were upregulated and 1254 downregulated. These DEGs were primarily involved in signaling pathways such as cytokine-cytokine receptor interaction, inflammatory response, and MHC protein complex. Our findings provide valuable insights into the mechanisms of PEDV attenuation and should facilitate the development of live vaccines.