How Flaviviruses Activate and Suppress the Interferon Response

The flavivirus genus includes viruses with a remarkable ability to produce disease on a large scale. The expansion and increased endemicity of dengue and West Nile viruses in the Americas exemplifies their medical and epidemiological importance. The rapid detection of viral infection and induction of the innate antiviral response are crucial to determining the outcome of infection. The intracellular pathogen receptors RIG-I and MDA5 play a central role in detecting flavivirus infections and initiating a robust antiviral response. Yet, these viruses are still capable of producing acute illness in humans. It is now clear that flaviviruses utilize a variety of mechanisms to modulate the interferon response. The non-structural proteins of the various flaviviruses reduce expression of interferon dependent genes by blocking phosphorylation, enhancing degradation or down-regulating expression of major components of the JAK/STAT pathway. Recent studies indicate that interferon modulation is an important factor in the development of severe flaviviral illness. This suggests that an increased understanding of viral-host interactions will facilitate the development of novel therapeutics to treat these viral infections and improved biological models to study flavivirus pathogenesis

Differential replication of pathogenic and nonpathogenic strains of West Nile virus within astrocytes

The severity of West Nile virus (WNV) infection in immunocompetent animals is highly strain dependent, ranging from avirulent to highly neuropathogenic. Here, we investigate the nature of this strain-specific restriction by analyzing the replication of avirulent (WNV-MAD78) and highly virulent (WNV-NY) strains in neurons, astrocytes, and microvascular endothelial cells, which comprise the neurovascular unit within the central nervous system (CNS). We demonstrate that WNV-MAD78 replicated in and traversed brain microvascular endothelial cells as efficiently as WNV-NY. Likewise, similar levels of replication were detected in neurons. Thus, WNV-MAD78's nonneuropathogenic phenotype is not due to an intrinsic inability to replicate in key target cells within the CNS. In contrast, replication of WNV-MAD78 was delayed and reduced compared to that of WNV-NY in astrocytes. The reduced susceptibility of astrocytes to WNV-MAD78 was due to a delay in viral genome replication and an interferon-independent reduction in cell-to-cell spread. Together, our data suggest that astrocytes regulate WNV spread within the CNS and therefore are an attractive target for ameliorating WNV-induced neuropathology

West Nile virus (WNV) replication is independent of autophagy in mammalian cells.

Autophagy is a homeostatic process responsible for recycling cytosolic proteins and organelles. Moreover, this pathway contributes to the cell's intrinsic innate defenses. While many viruses have evolved mechanisms to antagonize the antiviral effects of the autophagy pathway, others subvert autophagy to facilitate replication. Here, we have investigated the role of autophagy in West Nile virus (WNV) replication. Experiments in cell lines derived from a variety of sources, including the kidney, liver, skin, and brain, indicated that WNV replication does not upregulate the autophagy pathway. Furthermore, WNV infection did not inhibit rapamycin-induced autophagy, suggesting that WNV does not disrupt the authophagy signaling cascade. Perturbation of the autophagy pathway by depletion of the major autophagy factors Atg5 or Atg7 had no effect on WNV infectious particle production, indicating that WNV does not require a functional autophagy pathway for replication. Taken together, the results of our study provide evidence that WNV, unlike several other viruses of the family Flaviviridae, does not significantly interact with the conventional autophagy pathway in mammalian cells

IFN-Dependent and -Independent Reduction in West Nile Virus Infectivity in Human Dermal Fibroblasts

Although dermal fibroblasts are one of the first cell types exposed to West Nile virus (WNV) during a blood meal by an infected mosquito, little is known about WNV replication within this cell type. Here, we demonstrate that neuroinvasive, WNV-New York (WNV-NY), and nonneuroinvasive, WNV-Australia (WNV-AUS60) strains are able to infect and replicate in primary human dermal fibroblasts (HDFs). However, WNV-AUS60 replication and spread within HDFs was reduced compared to that of WNV-NY due to an interferon (IFN)-independent reduction in viral infectivity early in infection. Additionally, replication of both strains was constrained late in infection by an IFN-β-dependent reduction in particle infectivity. Overall, our data indicates that human dermal fibroblasts are capable of supporting WNV replication; however, the low infectivity of particles produced from HDFs late in infection suggests that this cell type likely plays a limited role as a viral reservoir in vivo

Primary human cell lines do not upregulate autophagy in response to WNV infection.

<p>(A-B) Effect of WNV infection on steady-state LC3B-II levels. Human foreskin fibroblasts (A) or human brain cortical astrocytes (B) were mock-infected or infected with WNV-NY (MOI = 3). Following infection, cells were grown in medium containing 10 µg/mL Pepstatin A and E64d. Whole cell lysates prepared at the indicated times (h) post-infection were subjected to immunoblot analysis for expression of LC3B (top), GAPDH (middle) and WNV (bottom).</p

WNV-MAD78 infection does not induce autophagy.

<p>293T monolayers were mock-infected or infected with WNV-MAD78 (MOI = 3). Following infection, cells were grown in medium containing 10 µg/mL Pepstatin A and E64d. Whole cell lysates prepared at the indicated times (h) post-infection were subjected to immunoblot analysis for LC3B (top), GAPDH (middle) and WNV (bottom).</p

LC3B-II accumulation in 293T cells.

<p>(A) Effect of WNV infection on LC3-II levels. Whole cell lysates were prepared from mock-infected or WNV-infected (MOI = 3) 293T cells at 24 hours post-infection. Immunoblot anyalysis was used to determine steady-state levels of LC3B (top), GAPDH (middle), and WNV (bottom). (B) Effect of rapamycin on LC3B. Whole cell lysates prepared at the indicated times (h) from 293T cells treated with 100 nM rapamycin (rap) were subjected to immunoblot analysis to determine the steady-state levels of LC3B (top) and GAPDH (bottom). (C) Effect of protease inhibitors on LC3B-II levels. Whole cell lysates were prepared from 293T monolayers treated with rapamycin (Rap) in the presence or absence of pepstatin A (Pep) and/or E64d at 24 hours post-treatment. Lysates were subjected to immunoblot analysis as described in Panel B.</p