Research Article: Cell-Specific IRF-3 Responses Protect against West Nile Virus Infection by Interferon-Dependent and -Independent Mechanisms

Date Published: July 27, 2007

Publisher: Public Library of Science

Author(s): Stephane Daffis, Melanie A Samuel, Brian C Keller, Michael Gale, Michael S Diamond, Ganes Seb.


Interferon regulatory factor (IRF)-3 is a master transcription factor that activates host antiviral defense programs. Although cell culture studies suggest that IRF-3 promotes antiviral control by inducing interferon (IFN)-β, near normal levels of IFN-α and IFN-β were observed in IRF-3−/− mice after infection by several RNA and DNA viruses. Thus, the specific mechanisms by which IRF-3 modulates viral infection remain controversial. Some of this disparity could reflect direct IRF-3-dependent antiviral responses in specific cell types to control infection. To address this and determine how IRF-3 coordinates an antiviral response, we infected IRF-3−/− mice and two primary cells relevant for West Nile virus (WNV) pathogenesis, macrophages and cortical neurons. IRF-3−/− mice were uniformly vulnerable to infection and developed elevated WNV burdens in peripheral and central nervous system tissues, though peripheral IFN responses were largely normal. Whereas wild-type macrophages basally expressed key host defense molecules, including RIG-I, MDA5, ISG54, and ISG56, and restricted WNV infection, IRF-3−/− macrophages lacked basal expression of these host defense genes and supported increased WNV infection and IFN-α and IFN-β production. In contrast, wild-type cortical neurons were highly permissive to WNV and did not basally express RIG-I, MDA5, ISG54, and ISG56. IRF-3−/− neurons lacked induction of host defense genes and had blunted IFN-α and IFN-β production, yet exhibited only modestly increased viral titers. Collectively, our data suggest that cell-specific IRF-3 responses protect against WNV infection through both IFN-dependent and -independent programs.

Partial Text

West Nile virus (WNV) is a mosquito-borne, positive polarity, single-stranded RNA virus in the Flaviviridae family. In humans, WNV causes a spectrum of illness that ranges from a self-limiting WNV fever to flaccid paralysis and fatal encephalitis [1]. WNV is endemic in Africa, Europe, Asia, and Australia, and has emerged in the Western hemisphere with annual outbreaks in North America; additionally, WNV has recently spread to Mexico and Central and South America.

An intact type I IFN response controls WNV infection in mice and limits replication in specific cell populations [9]. In vitro studies in MEFs suggested that the host response restricts WNV spread, in part, through activation of the IRF-3 pathway [14]. In this study, we established IRF-3 as an essential regulator of the early host response in vivo after WNV infection. Mice lacking IRF-3 were uniformly vulnerable with increased viremia, higher viral burden in peripheral tissues, altered tissue tropism, and earlier entry into the CNS. Ex vivo studies showed that specific primary cell types utilize IRF-3 with distinct functional consequences: in macrophages, IRF-3 was required for basal ISG expression and control of WNV infection, whereas in neurons, IRF-3 was necessary for efficient IFN induction. Remarkably, in vitro studies in myeloid cells and in vivo experiments suggested that a deficiency of IRF-3 did not dramatically diminish the peripheral IFN response to WNV infection, yet was still required for early control of WNV replication and spread. Thus, IRF-3 plays distinct roles in diverse cell types to limit WNV through IFN-dependent and -independent mechanisms.