Resistance to Alpha/Beta Interferon Is a Determinant of West Nile Virus Replication Fitness and Virulence

Abstract

The emergence of West Nile virus (WNV) in the WesternWest Nile virus (WNV) is a positive-sense single-stranded RNA virus in the family Flaviviridae. Isolates of WNV are subdivided into two lineages: lineage I viruses are represented by emergent strains distributed throughout the world and have been associated with outbreaks of encephalitis and meningitis in Europe, the Middle East, and, most recently, in North America, whereas lineage II isolates are largely nonemergent/ endemic strains that are confined to the African subcontinent and the island countries of Madagascar and Cyprus (5,7,25,26). In most cases, WNV infection of humans can be characterized as asymptomatic or as a mild, febrile illness termed West Nile fever. However, a significant increase in the global incidence of severe neurological disease associated with WNV lineage I infections arose in the mid-1990s, culminating in the U.S. outbreak in 2003, which included 9,862 reported cases and 264 deaths (CDC website, http://www.cdc.gov/ncidod/dvbid /westnile/index.htm). After its introduction in New York City in 1999, WNV rapidly spread across the continent and now appears to have firmly established itself in the ecology of North America. The rapid emergence of WNV and its virulence within a naïve population suggest that epidemic forms of the virus may encode mechanisms to evade host immunity.Infection with WNV triggers a delayed host response that includes the activation of interferon regulatory factor-3 (IRF-3) and the subsequent production of alpha/beta interferon (IFN-␣/␤) (14,15,38). IFNs are a family of immunomodulatory cytokines that are produced in response to virus infection and serve as integral signal initiators of host intracellular defenses (40,46). Binding of IFN to the cognate IFN-␣/␤ receptor (IFNAR) on target cells results in the activation of the JAK-STAT pathway, which includes the receptor-associated kinases JAK1 and Tyk2 that in turn phosphorylate and activate their downstream effectors, STAT1 and STAT2. Activated phospho-STAT1/STAT2 heterodimers translocate to the nucleus to form a heterotrimeric complex with IRF-9 and induce the transcription of hundreds of interferon-stimulated genes (ISGs), whose products can direct antiviral and antiproliferative actions that limit virus replication and spread. Many viruses encode proteins that direct mechanisms to disrupt innate antiviral defenses and IFN-induced JAK-STAT signaling, and these processes have been linked to viral emergence in new host populations and species (16,23,44) and to pathogenic outcomes of infection (reviewed in references 11, 40, and 46). Importantly, virulent isolates of WNV have been shown to be capable of attenuating IFN actions by preventing STAT1 and STAT2 activation, although the mechanisms of this regulation and its influence in vivo were not defined (17, 29). Here we describe in vitro and in vivo studies comparing the genetic and phenotypic properties of a lineage I/emergent strain and a lineage II/nonemergent strain of WNV. Our data show that viral control of I...

Genetic and Phenotypic Properties of Vero Cell-Adapted Japanese Encephalitis Virus SA14-14-2 Vaccine Strain Variants and a Recombinant Clone, Which Demonstrates Attenuation and Immunogenicity in Mice

“…The host type I IFN response is critical for restricting replication and dissemination of flaviviruses 10,[54][55][56][57] and JEV SA14-14-2 is capable of inducing morbidity in IFN-deficient mice, 58 indicating that innate-immune antiviral mechanisms contribute to its attenuated phenotype. Studies in primary human monocyte-derived macrophages showed that JEV SA14-14-2 induced similar levels of type I IFN as WT JEV but is more sensitive to its affects.…”

Differential Replication of Pathogenic and Nonpathogenic Strains of West Nile Virus within Astrocytes

“…However, the observation that exogenous disruption of the BBB enables a nonneuropathogenic strain of WNV to enter the CNS (13) suggests that the capacity to traverse the BBB is a determining factor for neuropathogenicity. Here, we investigated the nature of the restriction at the BBB by comparing the ability of an avirulent lineage 2 African isolate, WNV-MAD78 (12), to a highly virulent lineage 1 North American strain isolated in 2000, West Nile virus New York (WNV-NY), (12,14,15) to replicate in various cell types comprising the NVU. While both strains replicated efficiently in brain microvascular endothelial cells and neurons, WNV-MAD78 replication was restricted within astrocytes compared to that of WNV-NY.…”

Innate immune evasion by hepatitis C virus and West Nile virus

“…The importance of this fine tuning of JAK-Stat signaling was demonstrated by comparing a highly pathogenic WNV strain (WNV-TX02) and a traditionally nonpathogenic strain (WNV-MAD78) during infection of wild type cells or cells recovered from mice lacking a functional α/β IFN receptor. In cells from wild type animals the nonpathogenic WNV-MAD78 strain was attenuated in its ability to antagonize IFN signaling compared to pathogenic WNV-TX02 [51]. This phenotype correlated with a completely avirulent phenotype of the nonpathogenic virus in vivo during infection of wild type mice.…”

“…Several groups have recently reported that WNV is capable of inhibiting activation of JAK-STAT signaling components [48][49][50][51]. However, the exact mechanism of this inhibition is not clear, as it has been proposed that the NS2A, NS2B3, NS4A and NS4B viral proteins each have inhibitory activity against IFN signaling.…”

“…Further work using viral genetic approaches is needed to define the precise mechanisms by which WNV antagonizes α/β IFN signaling. It is clear, however, that ISG induction still occurs during WNV infection, suggesting that viral control of α/β IFN signaling is not complete, and that continuous induction of α/β IFN expression may occur through PRR signaling processes triggered during asynchronous cell to cell virus spread [12,45,51,52]. Thus, WNV may attenuate or "fine-tune" α/β IFN signaling sufficiently to support virus replication.…”