Inhibition of Beta Interferon Induction by Severe Acute Respiratory Syndrome Coronavirus Suggests a Two-Step Model for Activation of Interferon Regulatory Factor 3

Abstract

Severe acute respiratory syndrome (SARS) is caused by a novel coronavirus termed SARS-CoV. We and others have previously shown that the replication of SARS-CoV can be suppressed by exogenously added interferon (IFN), a cytokine which is normally synthesized by cells as a reaction to virus infection. Here, we demonstrate that SARS-CoV escapes IFN-mediated growth inhibition by preventing the induction of IFN-␤. In SARS-CoV-infected cells, no endogenous IFN-␤ transcripts and no IFN-␤ promoter activity were detected. Nevertheless, the transcription factor interferon regulatory factor 3 (IRF-3), which is essential for IFN-␤ promoter activity, was transported from the cytoplasm to the nucleus early after infection with SARS-CoV. However, at a later time point in infection, IRF-3 was again localized in the cytoplasm. By contrast, IRF-3 remained in the nucleus of cells infected with the IFN-inducing control virus Bunyamwera delNSs. Other signs of IRF-3 activation such as hyperphosphorylation, homodimer formation, and recruitment of the coactivator CREB-binding protein (CBP) were found late after infection with the control virus but not with SARS-CoV. Our data suggest that nuclear transport of IRF-3 is an immediate-early reaction to virus infection and may precede its hyperphosphorylation, homodimer formation, and binding to CBP. In order to escape activation of the IFN system, SARS-CoV appears to block a step after the early nuclear transport of IRF-3.

Severe acute respiratory syndrome coronavirus nsp1 protein suppresses host gene expression by promoting host mRNA degradation

“…We suspect that nsp1 promoted degradation of IFN-␤ mRNA in SeV-infected cells. Others (18) demonstrated that IRF-3 dimerization is blocked in SCoV-infected 293 cells, so we think that at least two different mechanisms inhibit IFN-␤ mRNA accumulation in SCoV-infected cells: One suppresses a signaling pathway that activates IRF-3 and the other is the nsp1-mediated promotion of IFN-␤ mRNA degradation. Dual means of inhibiting IFN-␤ mRNA accumulation would seem to be crucial for SCoV to establish infection in the host, because SCoV is known to be susceptible to IFN treatment (19,20).…”

“…IRF-3 phosphorylation, dimerization, nuclear translocation, and association with CBP͞p300, all of which are essential for IFN-␤ mRNA transcription, do not occur in SCoV-infected 293 cells (18). We next examined the effect of nsp1 expression on the SeV-induced IRF-3 homodimerization and found that neither nsp1 nor NSs expression blocked dimerization (Fig.…”

“…To know whether nsp1 was responsible for inhibition of IFN-␤ mRNA accumulation in SCoV-infected cells (18), we examined the effect of nsp1 expression on Sendai virus (SeV)-induced IFN-␤ mRNA accumulation. Cultures of 293 cells were cotransfected with an IFN-␤ promoter-driven luciferase (Luc) reporter plasmid and pCAGGS-nsp1.…”

“…Many viruses developed defensive mechanisms to suppress and͞or evade host innate immune functions (17). SCoV replication in human 293 cells suppresses IFN-␤ mRNA accumulation by inhibiting IFN regulatory factor (IRF)-3 phosphorylation, dimerization, nuclear translocation, and association with CBP͞p300 (18). Because SCoV is susceptible to the antiviral action of IFNs (19,20), inhibition of IFN-␤ transcription appears to be one of SCoV's defenses against host innate immune response.…”

Characterization of the antiviral effects of interferon-α against a SARS-like coronoavirus infection in vitro

“…Indeed, many different viruses evade an immune response by interfering with transcriptional activation of IFN-α/β genes [59]. Notably, in order to escape activation of the IFN system, the SARS-CoV appears to block a step after the early nuclear transport of IRF-3, the transcription factor essential for IFN-β and IFN-α4 promoter activity [60].…”