Differential responses of human brain cells to West Nile virus infection

Abstract

In recent years, West Nile virus (WNV) has emerged as a major cause of encephalitis in the United States. However, the neuropathogenesis of this flavivirus is poorly understood. In the present study, the authors used primary human brain cell cultures to investigate two neuropathogenic features: viral replication and induction of cytokines. Although neurons and astrocytes were found to support productive WNV infection, viral growth was poorly permissive in microglial cells. Compared to neuronal cultures that sustained viral growth for at least 2 weeks, replication peaked in astrocytes by 72 h post infection. In response to viral infection, astrocytes produced chemokines (CXCL10 and CCL5), but none of the cytokines (tumor necrosis factor [TNF]-alpha, interleukin [IL]-1beta, IL-6, interferon alpha or gamma) tested could be detected. Although microglial cells failed to support viral replication, WNV induced production of the proinflammatory cytokines IL-6 and TNF-alpha. Microglial cells also released robust amounts of the chemokines CXCL10 and CCL2, as well as lower levels of CCL5, in response to WNV infection. WNV-induced chemokine and cytokine production by microglia was coupled with activation of mitogen-activated protein kinase (MAPK) intracellular signaling pathways. Inhibition of p38 MAPK decreased chemokine production in response to WNV. Taken together, these findings suggest that microglial cell responses may influence the neuropathogenesis of WNV infection.

The Immunopathogenesis of Neurotropic Flavivirus Infection

“…All cells of the CNS are readily susceptible to WNV infection in vitro within 16-24 h, including neurons, Schwann cells and astrocytes, although microglia are more resistant (Y. Liu et al, 1989;Argall et al, 1991;Shrestha et al, 2003;Cheeran et al, 2005). In contrast, in animal models, detectable infection is seen in the brain between d3 and 6 post infection (p.i.…”

“…They also secrete various proinflammatory mediators and cytokines as part of the response to local and systemic infection. The increased microglial production of leukocyte-recruiting chemokines, CXCL10 and CCL2, is triggered by the activation of the p53-mitogen-activated protein kinase and extracellular signal-regulated kinase intracellular signaling pathways by WNV infection (Cheeran et al, 2005). Neutralisation of CCL2 by antibody on d6 p.i., results in prolonged survival, strongly implicating microglia as contributors to fatal neuropathology (Getts et al, 2008).…”

“…Endothelial tight junction proteins (TJP), such as ZO-1 and claudin-1, are crucial components of BBB integrity and this seal is maintained, inter alia, by astrocytes, nonneuronal support cells of the CNS that are susceptible to WNV infection in vitro (Y. Liu et al, 1989;Cheeran et al, 2005;Verma et al, 2010). WNV-infected astrocytes increase matrix metalloproteinase (MMP) production to degrade TJP (Verma et al, 2010).…”

“…Cells of the CNS each respond differently towards infiltrating virus. In vitro one of the responses to viral infection of astrocytes and microglia is the production of chemoattractants like CCL5 and CXCL10 (Cheeran et al, 2005), which recruit T cells and possibly monocytes; both astrocytes and microglia can produce CXCL9, which similarly attracts T cells (Getts, Unpublished). Infected neurons produce copious amounts of CCL2, which recruit macrophages and microglia from the bone marrow (Getts et al, 2008).…”

“…Virus undoubtedly spreads via the bloodstream at some point to further infect peripheral organs, including the CNS, where neurons are the main target (Xiao et al, 2001;Cheeran et al, 2005;Samuel & Diamond, 2009). However, the factors controlling these phases of virus spread are virtually unknown.…”