Japanese encephalitis virus review

Despite the progress made in vaccine research and development, JEV is still a major public health problem in southern, eastern, and southeastern Asia, as well as northern Oceania, with the potential to become an emerging global pathogen. In viral replication, the entry of JEV into the cell is the first step in a cascade of complex interactions between the virus and target cells that is required for the initiation, dissemination, and maintenance of infection. Our own research indicates that JEV infects and reproduces in endothelial cells but is not cytotoxic to endothelial cells of the microcirculation manuscript in preparation.

Thus, JEV virions circulating in the blood may directly infect the brain endothelial cells, or circulating infected immune cells, including monocytes, dendritic and T cells, may transfer JEV to the brain endothelial cells. Secondly, JEV-infected immune cells may enter through known physiological ways into the brain as in the healthy individual, that is, via the choroid plexus into the ventricular space from where they may spread the infection in the brain tissue [ ].

Consequently, the breakdown of the blood—brain barrier may be only secondary to infection of nerve tissue cells and subsequent to the anti-viral and inflammatory response. However, when it comes to the entry of JEV into the brain tissue, both ways discussed here could certainly apply. JEV enters the brain through two different ways and leads to infection of neurons and encephalitis. On the left, it is proposed that JEV or JEV-infected leucocytes monocytes, dendritic cells, T cells circulating in the blood infect endothelial cells of the brain capillaries.

The endothelial cells, without being harmed by JEV, amplify and transmit JEV to pericytes, or even microglia cells or astrocytes that are in contact with the capillaries. Infected pericytes, microglia, and astrocytes amplify JEV and transmit it to other brain cells, including neurons, astrocytes, and microglia.

On the right, JEV-infected T cells or monocytes circulating in the blood migrate through the choroid plexus into the ventricular space with its cerebrospinal fluid, and from there into the periventricular nervous tissue. There, microglia, astrocytes, and neurons are infected either by cell-to-cell contact, or by newly produced extracellular JEV virions. In the middle, JEV-infected neurons undergo eventually apoptosis. Sometimes, JEV infection of the brain progresses in parallel with aseptic meningitis, which may affect the blood supply to the brain [ ].

Usually, encephalitis is the most severe clinical appearance of JEV infection with a variety of first symptoms, including seizures, as well as acute sensory and neuromuscular functional deficiencies [ ].

JEV infection of microglia [ , ], astrocytes [ ] and neurons [ , , , ] with corresponding cellular response and possible apoptosis, as well as subsequent inflammatory response results in neuronal cell death.

In addition, oedema and vascular damage may enhance damage to the tissue and cells. Accordingly, the basal ganglia, thalamus, and nuclei of the brainstem are most affected [ ]. Japanese encephalitis is often focal affecting only one, or several brain regions and centres.

If visual centres are affected, blindness may be the consequence. Deficiency of other sensory functions conducted through the cranial nerves and integrated in the centres of the midbrain and the brain stem may be targeted by JEV.

Centres of vital functions in the brain stem may also be affected, including for respiration, cardiovascular and digestive regulation, resulting in corresponding acute symptoms like neurogenic respiratory deficiency, cardiovascular shock, and nausea. In addition, the motor neurons in the spinal cord may be affected, resulting in corresponding acute flaccid paralysis [ ]. Substantial periventricular tissue damage may also result in an obstructive internal hydrocephalus with increased intracerebral pressure and subsequent additional brain damage [ ].

Inflammation in the course of a JEV brain infection is due to an inflammatory reaction of the local microglia and astrocyte cell population, which may be infected or just respond to the surrounding cell and tissue damage in the course of the immune response against JEV-infected cells. These cells produce various inflammatory factors and cytokines with the aim to fight the infection. In addition, immune cells may be recruited from the periphery to the infected brain site, including inflammatory monocytes [ 76 ] and JEV-specific T cells [ ], which will target infected local cells, possibly killing them and enhance inflammation, but also neuronal damage.

However, in the best-case scenario, the immune response in the brain may clear the virus with minimal collateral damage. Less severe JEV infection of the brain may result in transient milder symptoms, such as learning deficiency [ ]. Unfortunately, Japanese encephalitis can be reactivated in seropositive children, despite them having built efficient humoral anti-JEV immunity and without being newly infected.

This may be due to chronic infection of T cells or microglia [ , , , ], indicating that those cells are long-term carriers and that those individuals have latent JEV infection. As the infectious JEV material hides in the cells, antibody-based immunity cannot help with controlling spread of JEV in the body, especially if the infectious material is transferred directly through cell-to-cell contact, with JEV-neutralising antibodies not being able to enter in contact with newly produced virions.

Only efficient anti-viral drugs could interfere with intracellular JEV reproduction and interrupt re-activation of encephalitis and new lesions in the brain. Unfortunately, no such drug is yet available. As for all flaviviruses, JEV is persistent and non-pathogenic to susceptible cells of mosquitos. In that respect, one has to consider the long coevolution of flaviviruses in mosquitos with persistent infection [ ].

Detailed information about virus-cell interaction in the mosquito has been covered by the review of Salas-Benito and De Nova-Ocampo [ ]. Many different human cell types are susceptible to JEV, including immune cells monocytes, macrophages, dendritic cells, microglia, T cells , astrocytes, neurons, endothelial cells, pericytes, and fibroblasts, but little is known about the interaction between JEV and human cells.

However, each cell type reacts differently to JEV infection. In addition to methodological and technical experimental restrictions, the multifactorial variability of the host cells makes thorough understanding of JEV biology and pathology quite difficult. Furthermore, minor differences in the genome and consequently in the resulting proteins from the different JEV genotypes lead to slightly different cellular effects.

For example, substitution of one amino acid in the M protein results in deficient assembling of infectious virions in mammalian cells, whereas the mutation does not affect production of infectious JEV in the mosquito [ ]. Early cellular events, including viral entry and unfolding of viral control of the infected cell has been recently reviewed in general and with the focus on hepatitis C virus and DENV, by Neufeldt et al.

Therefore, we cover here only mechanisms used by JEV to control infected human or mammalian cells and use them for its own purpose, as well as how JEV undermines some protective cellular responses. Thereby, information derived from other flaviviruses will also be considered. Mammalian cells have evolved various mechanisms for the recognition of viral components, including TLR3, TLR7, and RIG-1 [ , , ], as well as mechanisms production of type I interferon for subsequent blocking of viral survival and reproduction in the infected cell [ ].

TLR7 deficiency has been shown in a mouse model to result in decreased immune response with decreased type I interferon production and increased viral load in the brain [ ]. Therefore, TLR7 is essential for building a protective immune response. However, TLR7 is up-regulated in the course of a JEV infection in human in immune cells, as well as in the brain, which may also result in an excessive and damaging inflammatory response and damaging encephalitis [ , ].

Unfortunately, the differential expression and regulation of TLR7 and other genes of early intracellular immune response during a JEV infection is not well understood, and it is not known whether and how JEV may interfere with this first step of induction of the immune response [ 77 , , ].

JEV uses and manipulates elements of the cytoskeleton, including actin, vimentin, and microtubules for cellular entry, transport of the various components for replication and assembly, as well as for release of infectious virions [ , , ]. In addition, JEV also controls cell membranes and corresponding membrane proteins, that is, the cell surface membrane for viral entry, the endosomal membrane for viral fusion and delivery of the genomic RNA into the cytoplasm, the membrane complex of the endoplasmic reticulum for viral protein translation, and genomic RNA synthesis and finally assembly of the viral progeny [ 56 , 63 , , , ].

However, the molecular mechanisms of how JEV interferes with cell membranes and the cytoskeleton are not well understood. Furthermore, JEV components enter the nucleus, where viral RNA synthesis has been described in insect and mammalian cells [ ]. There, the virus may also influence transcription regulation of host cell genes that help to inactivate cellular anti-viral mechanisms. JEV may also prevent cell death and manipulate the cell cycle to keep the cell alive and in an optimal metabolic state, as long as viral reproduction is required [ 61 , , , , , , , , , ].

In addition to the non-structural proteins, non-coding viral RNA of the positive and complementary genomic strain interfere with cellular processes [ 57 , , ]. However, more studies are required that investigate how and which host gene expression are manipulated by JEV. Subsequently, this RNA is packed into virions that are able to infect other cells, when released into the extracellular space. Our most recent research indicates that transmission of JEV between cells may not need assembly of virions, but single stranded or double stranded genomic JEV RNA may be transferred from one cell to another [ ].

The mechanisms for that are still under investigation. However, exchange of infectious material between cells may spread JEV in certain tissues e. One possible mechanism for spreading could be the transfer of infectious material through gap junctions, which are plentiful in the nervous tissue connecting neurons, but also astrocytes and microglia, as well as connecting all three cell types with each other [ , ].

Exchange of JEV genomic RNA through gap junctions may just work for the diameter of about or smaller than 2 nm [ ]. It has already been shown for exchange of RNA in cardiomyocytes [ ] and it has been postulated for osteocytes in bone [ ].

Such JEV spreading mechanism would explain the focal appearance of JEV-induced brain lesions and re-activation of encephalitis in seropositive children with latent JEV infection [ ]. Similar findings have already been published, showing that infectious material could also be transferred between cells through tunnelling effects or nanotubes [ , , ] or via extracellular vesicles [ ]. However, more research is needed to investigate the various possible ways of exchange of infectious JEV material between cells.

Better understanding of those mechanisms will then allow a targeted development of new drugs preventing JEV spreading in the body, tissue and between cells in a newly infected individual or in patients with latent infection. The first part of this review summarized essential epidemiologic information about the emerging JEV emphasizing on the increasing worldwide threat that JEV poses to humans, especially considering global warming and the corresponding geographic expansion of mosquitos that are JEV vectors.

In addition, the role of increased world trade with corresponding international exchange of workers and animals was briefly mentioned, as well as bird migration routes of corresponding JEV infected birds as carriers were discussed as unmanageable risk of geographical spreading of JEV. The subsequent sections discussed biological mechanisms that contribute to JEV infection and pathology focusing on human. Thereby, spread of virus in the human body were described and new concepts of JEV entry into the brain were proposed.

In addition, the interaction between JEV and various cell types and the immune system were elucidated, also connecting the clinical appearance of JEV infection with basic scientific concepts.

The most recently discovered way of JEV transmission between cells was also discussed as a new concept. This new concept may well explain re-emerging of JEV in infected and immune individuals, in the absence of reinfection. It may also explain the focal spreading of JEV infection in certain brain areas.

However, a better understanding of JEV infection at cellular and system level is required for the targeted development of new efficient anti-viral treatments. We thank Karl Link, Med. Vet, University of Fribourg, Switzerland, for the English editing. National Center for Biotechnology Information , U. Journal List Pathogens v. Published online Jul Author information Article notes Copyright and License information Disclaimer. Received May 4; Accepted Jul This article has been cited by other articles in PMC.

Keywords: Japanese encephalitis virus, flavivirus, JEV, epidemiology, symptoms, immune response, vaccine, blood—brain barrier, transmission, cellular events. Open in a separate window. Figure 1. Figure 2. Immune Response and Vaccine Humans usually respond with a strong physiological immune response to a first JEV infection, including the adaptive humoral and cellular immunity [ 80 ]. JEV Crossing the Blood—Brain Barrier and Brain Infection Crossing the blood-brain-barrier by JEV, infiltration and infection of the brain cells, as well as induction of inflammation resulting in encephalitis is still not well understood.

Figure 3. Summary and Conclusions The first part of this review summarized essential epidemiologic information about the emerging JEV emphasizing on the increasing worldwide threat that JEV poses to humans, especially considering global warming and the corresponding geographic expansion of mosquitos that are JEV vectors.

Acknowledgments We thank Karl Link, Med. Funding This research received no external funding. Conflicts of Interest The authors declare no conflict of interest.

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Saito M. Since the JEV distribution area has been large and continuously extending toward new Asian and Australasian regions, it is considered an emerging and reemerging pathogen. Despite large effective immunization campaigns, Japanese encephalitis remains a disease of global health concern. JEV zoonotic transmission cycles may be either wild or domestic: the first involves wading birds as wild amplifying hosts; the second involves pigs as the main domestic amplifying hosts.

Culex mosquito species, especially Cx. Abstract Japanese encephalitis is a mosquito-borne disease that occurs in Asia and is caused by Japanese encephalitis virus JEV , a member of the genus Flavivirus. Publication types Research Support, Non-U. Gov't Review.