Diversity for endoribonuclease nsp15-mediated regulation of alpha-coronavirus propagation and virulence.

Endoribonuclease non-structural protein 15 (nsp15) (EndoU) is conserved among coronaviruses (CoVs) and is crucial for viral replication, evasion of the innate immune system, and virulence. EndoU-deficient CoVs can activate the interferon (IFN) response and attenuate their virulence, and nsp15 is considered the target of attenuated vaccine development. Among alpha-CoVs, transmissible gastroenteritis virus (TGEV) and feline infectious peritonitis virus (FIPV) cause lethal diseases in piglets and cats, but the role of EndoU in viral propagation and virulence remains unclear. Here, we verified the TGEV and FIPV EndoU active sites His226 and His241 and found that the antagonization of SeV-induced IFN-β production by nsp15 depends on its EndoU activity. Furthermore, we constructed infectious clones of wild-type (WT) and EndoU-deficient (EnUmt) TGEV and FIPV. Unexpectedly, we found that both the WT and EnUmt viruses propagated efficiently in multiple types of immunocompetent (PK-15, IPI-2I, ST, CRFK, F81, and Fcwf-4) cells. Moreover, the results of infection experiments showed that compared with piglets and cats infected with the WT, the EnUmt virus-infected piglets and cats did not exhibit significantly reduced mortality, tissue injury, or viral shedding. Specially, the death of cats infected with EnUmt-FIPV occurred earlier than that of cats infected with WT. Hence, our results suggest that the function of EndoU is conserved, but nsp15-mediated regulation of the propagation and pathogenesis of CoVs are diverse. Our findings provide a reference for an in-depth understanding of EndoU-mediated immune escape and pathogenicity in CoVs.IMPORTANCEUnderstanding the role of the endoribonuclease non-structural protein 15 (nsp15) (EndoU) in coronavirus (CoV) infection and pathogenesis is essential for vaccine target discovery. Whether the EndoU activity of CoV nsp15, as a virulence-related protein, has a diverse effect on viral virulence needs to be further explored. Here, we found that the transmissible gastroenteritis virus (TGEV) and feline infectious peritonitis virus (FIPV) nsp15 proteins antagonize SeV-induced interferon-β (IFN-β) production in human embryonic kidney 293 cells. Interestingly, compared with wild-type infection, infection with EnUmt-TGEV or EnUmt-FIPV did not change the IFN-β response or reduce viral propagation in immunocompetent cells. The results of animal experiments showed that EnUmt viruses did not reduce the clinical presentation and mortality caused by TGEV and FIPV. Our findings enrich the understanding of nsp15-mediated regulation of alpha-CoV propagation and virulence and reveal that the conserved functions of nonstructural proteins have diverse effects on the pathogenicity of CoVs.