Japanese encephalitis virus and its mechanisms of neuroinvasion

Abstract

Japanese encephalitis virus (JEV) is a positive-sense single-stranded RNA virus of the Flavivirus genus that is spread by Culex mosquitos. It is maintained in an enzootic cycle in pigs and wild birds in which humans are dead-end hosts [1]. Despite having effective vaccines, JEV is the leading cause of viral encephalitis in Asia [1]. As a neuroinvasive virus, it can effectively cross the blood-brain barrier (BBB) to cause acute encephalitis. Twenty-five percent to 30% of Japanese encephalitis (JE) cases are fatal, and 50% result in permanent neuropsychiatric complications [2]. There are currently no treatments for JE, partly due to an incomplete understanding of the mechanisms promoting encephalitis. The central nervous system (CNS) relies on the BBB, a tightly regulated barrier between the peripheral circulation and the CNS, to prevent entry of pathogens, including viruses. Yet, JEV and other neuroinvasive viruses can overcome the BBB, which usually excludes foreign substances. It is formed primarily by tight junctions (TJ) between endothelial cells, comprised of proteins such as claudin-5, zonula occludens (ZO)-1, and occludin. The BBB is sustained by supporting cell types, including astrocytes, pericytes, microglia, and mast cells (MCs) [1, 3]. Together, these cells form neurovascular units that maintain a barrier along the cerebrovascular microvessels to promote immune privilege and CNS homeostasis [3, 4]. Neuroinvasive viruses use several mechanisms to access the CNS: (1) direct infection of endothelial cells and subsequent transcellular release of virus into the brain parenchyma, (2) infection of peripheral immune cells that enter the CNS in a "Trojan Horse" mechanism, (3) paracellular entry following breakdown of the BBB, (4) retrograde transport of virus from the peripheral nervous system (PNS) into the CNS, and (5) translocation from the blood to the cerebral spinal fluid (CSF) [5, 6] (Fig 1). JEV infection through the natural subcutaneous route leads to widespread infection in various parts of the brain [7], suggesting a hematological route of infection, such as would occur for mechanisms 1-3. Here we provide an overview of a few described mechanisms of JEV penetration of the BBB and processes that amplify CNS infection. Transcellular infection of endothelial cells and activation of the neurovascular unit Some neurotropic viruses directly infect endothelial cells to reach the brain from the circulation and travel transcellularly to release viruses into the brain parenchyma [6]. JEV has been

Efficient control of Japanese encephalitis virus in the central nervous system of infected pigs occurs in the absence of a pronounced inflammatory immune response

“…In our study, JEV showed a particular tropism for the CNS without causing any neurologic symptoms, although signs of non-suppurative encephalitis were found during histopathology. It is nevertheless still not completely clear how flaviviruses enter the brain, but multiple strategies are probably exploited [23][24][25]. A first route to be considered is a hematogenous route of entry to the brain parenchyma, which can occur via direct infection of endothelial cells, a passage between disrupted endothelial tight junctions, or via hijacking of peripheral leukocytes migrating into the CNS.…”

“…A first route to be considered is a hematogenous route of entry to the brain parenchyma, which can occur via direct infection of endothelial cells, a passage between disrupted endothelial tight junctions, or via hijacking of peripheral leukocytes migrating into the CNS. The hematogenous route has since a long time been described as a major entry route of JEV in the CNS of multiple hosts [ 24 , 25 ]. Our observation that the moment of peak JEV detection in the serum and blood leukocytes during the short viremia coincides with the moment of first JEV detection in the cerebrum and cerebellum of most animals might support a JEV entry in the porcine brain via this route.…”

“…Our observation that the moment of peak JEV detection in the serum and blood leukocytes during the short viremia coincides with the moment of first JEV detection in the cerebrum and cerebellum of most animals might support a JEV entry in the porcine brain via this route. As described for WNV and JEV in other hosts like humans and mice [ 25 – 28 ], infectious virus present in the serum could directly infect endothelial cells followed by transcellular release of the virus into the brain parenchyma or infected leukocytes could enter JEV in the CNS via a “Trojan Horse” mechanism. The third route via disruption of the BBB seems to be secondary to the entry into the CNS and is probably the result of aberrant cytokine production induced by the infection of microglia cells in the CNS [ 29 , 30 ].…”

Pathogenicity and virulence of Japanese encephalitis virus: Neuroinflammation and neuronal cell damage

“…The interaction of neurovasculature cells with each other and their cooperation with endothelial cells is also vital for tight junctions and regulation of the BBB integrity [ 9 , 10 ]. The mechanisms by which JEV crosses the BBB to enter into the CNS are supported by four possible modes: (1) proliferation of virus within endothelial cells without affecting the cell viability, followed by passive transport of viral particles across the endothelial cells, (2) diapedesis of virus-infected leukocytes between endothelial cell junctions, (3) transport via the peripheral nervous system, and (4) disruption of the BBB through the release of virus-induced inflammatory mediators from the cells of apical (blood) and basolateral (brain) sides of the BBB, which is considered to be the most common mode [ 11 , 12 ].…”

Severe acute respiratory syndrome coronavirus 2 may be an underappreciated pathogen of the central nervous system

“…2a). A recent study on Japanese encephalitis virus (JEV) (an RNA virus) suggests that the paracellular mode of trafficking could be one of the potential routes of entry into the CNS [76]. JEV infected mast cells release chymase, a vasoactive protease, which cleaves TJ proteins, including zona occludens-1 and -2, claudin-5 and occludin, breaking down the BBB and facilitating entry of JEV into the CNS.…”