Date Published: September 1, 2011
Publisher: Public Library of Science
Author(s): James D. Brien, Stephane Daffis, Helen M. Lazear, Hyelim Cho, Mehul S. Suthar, Michael Gale, Michael S. Diamond, Jean K. Lim.
Interferon regulatory factor (IRF)-1 is an immunomodulatory transcription factor that functions downstream of pathogen recognition receptor signaling and has been implicated as a regulator of type I interferon (IFN)-αβ expression and the immune response to virus infections. However, this role for IRF-1 remains controversial because altered type I IFN responses have not been systemically observed in IRF-1-/- mice. To evaluate the relationship of IRF-1 and immune regulation, we assessed West Nile virus (WNV) infectivity and the host response in IRF-1-/- cells and mice. IRF-1-/- mice were highly vulnerable to WNV infection with enhanced viral replication in peripheral tissues and rapid dissemination into the central nervous system. Ex vivo analysis revealed a cell-type specific antiviral role as IRF-1-/- macrophages supported enhanced WNV replication but infection was unaltered in IRF-1-/- fibroblasts. IRF-1 also had an independent and paradoxical effect on CD8+ T cell expansion. Although markedly fewer CD8+ T cells were observed in naïve animals as described previously, remarkably, IRF-1-/- mice rapidly expanded their pool of WNV-specific cytolytic CD8+ T cells. Adoptive transfer and in vitro proliferation experiments established both cell-intrinsic and cell-extrinsic effects of IRF-1 on the expansion of CD8+ T cells. Thus, IRF-1 restricts WNV infection by modulating the expression of innate antiviral effector molecules while shaping the antigen-specific CD8+ T cell response.
The rapid triggering of an IFN-αβ response results in the early control of virus infection in mammalian cells. Detection of RNA viruses occurs through the recognition of specific sequence motifs, secondary structure, or modification of viral nucleic acids by pattern recognition receptors (PRR) in the cytosol (RIG-I and MDA5) and endosome (TLR3, TLR7, and TLR8) . PRR binding of viral RNA signals constituent adaptor molecules (IPS-1, TRIF, and MyD88) to activate transcription factors and induce expression of IFN-αβ genes. A current paradigm for IFN production after RNA virus infection describes a positive feedback model that is modulated by the transcription factors interferon regulatory factors (IRF)-3 and IRF-7 , . In the initial phase, viral nucleic acid sensing induces nuclear localization of IRF-3, which stimulates gene transcription and production of IFN-β and IFN-α4 by infected cells. In the second phase, these IFNs bind to the common IFN-αβ receptor in a paracrine and autocrine manner and signal through the JAK-STAT pathway resulting in the induced expression of hundreds of interferon stimulated genes (ISG) (e.g., PKR, RNAse L, viperin, ISG15, ISG54, ISG56, IFITM3, and ISG20), which limit viral replication through multiple mechanisms , , , , , . IRF-7 is both an ISG and a transcriptional activator and participates in an IFN amplification loop by inducing IFN-β and many subtypes of IFN-α , .
In this study, we identified IRF-1 as an essential regulator of the host immune response against WNV infection, and show that it governs processes of both the innate and adaptive immune responses that control outcome. IRF-1-/- mice were vulnerable to lethal infection with enhanced viremia, increased viral replication in peripheral tissues, altered tropism, and rapid dissemination into the CNS. Ex vivo analysis showed a cell-specific utilization of IRF-1 in controlling WNV replication. Although an absence of IRF-1 did not alter WNV infection in MEF, IRF-1-/- Mφ supported enhanced viral replication. Additionally, we identified a surprising phenotype with respect to the effects of IRF-1 on the induction of WNV-specific CD8+ T cells. Despite markedly fewer CD8+ T cells in naïve IRF-1-/- mice at baseline, we observed a rapid expansion in the periphery of antigen-specific CD8+ T cells that were IFN-γ+ and GrB+, which was associated with increased accumulation in the brain. Although IRF-1-/- CD8+ T cells expanded rapidly and were capable of killing targets and efficiently clearing virus from infected neurons, they did not attain sufficient numbers quickly enough to mitigate the increased infection in the CNS caused by the compromised early innate control of WNV in peripheral tissues.