Proinflammatory mediators released by activated microglia induces neuronal death in Japanese encephalitis

Abstract

While a number of studies have documented the importance of microglia in central nervous system (CNS) response to injury, infection and disease, little is known regarding its role in viral encephalitis. We therefore, exploited an experimental model of Japanese Encephalitis, to better understand the role played by microglia in Japanese Encephalitis Virus (JEV) infection. Lectin staining performed to assess microglial activation indicated a robust increase in reactive microglia following infection. A difference in the topographic distribution of activated, resting, and phagocytic microglia was also observed. The levels of various proinflammatory mediators, such as inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (Cox-2), IL-6, IL-1beta, TNF-alpha, and MCP-1 that have been implicated in microglial response to an activational state was significantly elevated following infection. These cytokines exhibited region selective expression in the brains of infected animals, with the highest expression observed in the hippocampus. Moreover, the expression of neuronal specific nuclear protein NeuN was markedly downregulated during progressive infection indicating neuronal loss. In vitro studies further confirmed that microglial activation and subsequent release of various proinflammatory mediators induces neuronal death following JEV infection. Although initiation of immune responses by microglial cells is an important protective mechanism in the CNS, unrestrained inflammatory responses may result in irreparable brain damage. Our findings suggest that the increased microglial activation following JEV infection influences the outcome of viral pathogenesis. It is likely that the increased microglial activation triggers bystander damage, as the animals eventually succumb to infection.

Glial activation involvement in neuronal death by Japanese encephalitis virus infection

“…1b). Evidence suggests that this type of morphological switch represents a transformation from resting microglia to reactive/amoeboid microglia (Ghoshal et al, 2007). These results implied that neuronal death after JEV infection was accompanied by the activation of glia, especially microglia.…”

“…The CNS response to ischaemia, degenerative diseases and viral infection is largely manifested by neuronal loss and inflammatory responses (Gao et al, 2002;Liao et al, 2002;Chen et al, 2004;Raung et al, 2005;German et al, 2006;Ovanesov et al, 2006;Ghoshal et al, 2007;van Marle et al, 2007;Saxena et al, 2008;Swarup et al, 2008). Generally, the onset and/or interactive crosstalk of neuronal injury and neuroinflammation play a critical role in the pathogenesis of neurological disorders.…”

“…Although JEV causes productive infection in most cell lines derived from different tissue origins, clinical and animal studies suggest that the principal target cells for JEV in the CNS are neurons (German et al, 2006;Ghoshal et al, 2007). As a result of infection, massive dysfunction and/or destruction of neurons is a hallmark of JEV-associated encephalitis (Kumar et al, 1990).…”

“…It has been demonstrated that the mortality rate increases with increasing concentrations of cytokines in the serum and CSF in Japanese encephalitis patients (Ravi et al, 1997). Regarding JEV-associated neuroinflammation, a number of studies have shown that JEV infects microglia and astrocytes, and their consequent activation contributes to neuronal death (Chen et al, 2000(Chen et al, , 2004Raung et al, 2005Raung et al, , 2007Abraham & Manjunath, 2006;Bhowmick et al, 2007;Ghoshal et al, 2007;Mishra et al, 2007;Das et al, 2008;Swarup et al, 2008). These studies suggest that, in addition to direct viral infection of neurons, bystander damage caused by activated microglia/ astrocytes is involved in JEV-induced neuronal death.…”

“…Although activation of microglia and astrocytes is found after JEV infection, activated microglia rather than activated astrocytes are proposed to mediate severe neurotoxicity (Raung et al, 2005;Ghoshal et al, 2007;Mishra et al, 2007). Currently, the discrepancy between JEV-activated microglia and astrocytes is not fully understood.…”