Chimeric Porcine Deltacoronaviruses with Sparrow Coronavirus Spike Protein or the Receptor-Binding Domain Infect Pigs but Lose Virulence and Intestinal Tropism.

Porcine deltacoronavirus (PDCoV) strain OH-FD22 infects poultry and shares high nucleotide identity with sparrow-origin deltacoronaviruses (SpDCoV) ISU73347 and HKU17 strains. We hypothesized that the spike (S) protein or receptor-binding domain (RBD) from these SpDCoVs would alter the host and tissue tropism of PDCoV. First, an infectious cDNA clone of PDCoV OH-FD22 strain (icPDCoV) was generated and used to construct chimeric icPDCoVs harboring the S protein of HKU17 (icPDCoV-S) or the RBD of ISU73347 (icPDCoV-RBD). To evaluate their pathogenesis, neonatal gnotobiotic pigs were inoculated orally/oronasally with the recombinant viruses or PDCoV OH-FD22. All pigs inoculated with icPDCoV or OH-FD22 developed severe diarrhea and shed viral RNA at moderate-high levels (7.62-10.56 log copies/mL) in feces, and low-moderate levels in nasal swabs (4.92-8.48 log copies/mL). No pigs in the icPDCoV-S and icPDCoV-RBD groups showed clinical signs. Interestingly, low-moderate levels (5.07-7.06 log copies/mL) of nasal but not fecal viral RNA shedding were detected transiently at 1-4 days post-inoculation in 40% (2/5) of icPDCoV-S- and 50% (1/2) of icPDCoV-RBD-inoculated pigs. These results confirm that PDCoV infected both the upper respiratory and intestinal tracts of pigs. The chimeric viruses displayed an attenuated phenotype with the loss of tropism for the pig intestine. The SpDCoV S protein and RBD reduced viral replication in pigs, suggesting limited potential for cross-species spillover upon initial passage.