Cytoplasmic stress granules (SGs) are generally triggered by stress-induced translation arrest for storing mRNAs. Recently, it has been shown that SGs exert anti-viral functions due to their involvement in
protein synthesis shut off and recruitment of innate immune signaling intermediates. The largest RNA viruses, coronaviruses, impose great threat to public safety and animal health; however, the significance of SGs in
coronavirus infection is largely unknown. Infectious Bronchitis Virus (IBV) is the first identified coronavirus in 1930s and has been prevalent in poultry farm for many years. In this study, we provided evidence that IBV overcomes the host
antiviral response by inhibiting SGs formation via the virus-encoded
endoribonuclease nsp15. By immunofluorescence analysis, we observed that IBV
infection not only did not trigger SGs formation in approximately 80% of the infected cells, but also impaired the formation of SGs triggered by heat shock,
sodium arsenite, or NaCl stimuli. We further demonstrated that the intrinsic
endoribonuclease activity of nsp15 was responsible for the interference of SGs formation. In fact, nsp15-defective recombinant IBV (rIBV-nsp15-H238A) greatly induced the formation of SGs, along with accumulation of dsRNA and activation of PKR, whereas wild type IBV failed to do so. Consequently,
infection with rIBV-nsp15-H238A strongly triggered transcription of IFN-β which in turn greatly affected rIBV-nsp15-H238A replication. Further analysis showed that SGs function as an
antiviral hub, as demonstrated by the attenuated IRF3-IFN response and increased production of IBV in SG-defective cells. Additional evidence includes the aggregation of
pattern recognition receptors (
PRRs) and signaling intermediates to the IBV-induced SGs. Collectively, our data demonstrate that the
endoribonuclease nsp15 of IBV interferes with the formation of
antiviral hub SGs by regulating the accumulation of viral dsRNA and by antagonizing the activation of PKR, eventually ensuring productive virus replication. We further demonstrated that nsp15s from PEDV, TGEV, SARS-CoV, and SARS-CoV-2 harbor the conserved function to interfere with the formation of chemically-induced SGs. Thus, we speculate that coronaviruses employ similar nsp15-mediated mechanisms to antagonize the host anti-viral SGs formation to ensure efficient virus replication.