Reovirus μ2 Protein Inhibits Interferon Signaling through a Novel Mechanism Involving Nuclear Accumulation of Interferon Regulatory Factor 9

Abstract

The secreted cytokine alpha/beta interferon (IFN-␣/␤) binds its receptor to activate the Jak-STAT signal transduction pathway, leading to formation of the heterotrimeric IFN-stimulated gene factor 3 (ISGF3) transcription complex for induction of IFN-stimulated genes (ISGs) and establishment of an antiviral state. Many viruses have evolved countermeasures to inhibit the IFN pathway, thereby subverting the innate antiviral response. Here, we demonstrate that the mildly myocarditic reovirus type 1 Lang (T1L), but not the nonmyocarditic reovirus type 3 Dearing, represses IFN induction of a subset of ISGs and that this repressor function segregates with the T1L M1 gene. Concordantly, the T1L M1 gene product, 2, dramatically inhibits IFN-␤-induced reporter gene expression. Surprisingly, T1L infection does not degrade components of the ISGF3 complex or interfere with STAT1 or STAT2 nuclear translocation as has been observed for other viruses. Instead, infection with T1L or reassortant or recombinant viruses containing the T1L M1 gene results in accumulation of interferon regulatory factor 9 (IRF9) in the nucleus. This effect has not been previously described for any virus and suggests that 2 modulates IRF9 interactions with STATs for both ISGF3 function and nuclear export. The M1 gene is a determinant of virus strain-specific differences in the IFN response, which are linked to virus strain-specific differences in induction of murine myocarditis. We find that virusinduced myocarditis is associated with repression of IFN function, providing new insights into the pathophysiology of this disease. Together, these data provide the first report of an increase in IRF9 nuclear accumulation associated with viral subversion of the IFN response and couple virus strain-specific differences in IFN antagonism to the pathogenesis of viral myocarditis.

Apoptosis Induction Influences Reovirus Replication and Virulence in Newborn Mice

“…The genomes of AP and AD viruses were constructed to contain two gene segments (M1 and L2) from strain T1L and the remaining eight gene segments from strain T3D. The T1L M1 gene is associated with interferon antagonism, and the T1L L2 gene has been linked to reduced interferon sensitivity and enhanced infectivity of reovirus in the mouse intestine (49)(50)(51). The S1 genes of both AP and AD viruses were engineered to encode a 1 attachment protein with a threonine-to-isoleucine (T249I) substitution, which renders the attachment protein resistant to cleavage by intestinal proteases (52).…”

The Type I Interferon-α Mediates a More Severe Neurological Disease in the Absence of the Canonical Signaling Molecule Interferon Regulatory Factor 9

“…Moreover, the results here give deeper insight into our understanding of the pathogenesis of proposed neurological "interferonopathies" such as AGS and suggest that some key neuropathological changes in these disorders may arise from noncanonical signaling mediated by IFN-␣. Finally, modifications to type I IFN signaling pathways [as can occur following infection with certain viruses (Leonard and Sen, 1996;Miller et al, 1999;Palosaari et al, 2003;Zurney et al, 2009] may cause dramatic changes in the biological actions of this cytokine, including enhanced development of disease.…”

Interferon Regulatory Factor 3 Attenuates Reovirus Myocarditis and Contributes to Viral Clearance

“…This effect is likely IFN independent, because in some cell types, including cardiac myocytes, T1L has a greater replication capacity and is less sensitive to the antiviral effects of IFN than are type 3 strains. Additionally, T1L antagonizes the IFN signaling response (44), suggesting that IRF-3 impedes T1L replication independently of IFN. Alternatively, IRF-3 deficiency may alter the capacity of T1L to induce apoptosis.…”