**Virus-Induced Encephalitis and Innate Immune Responses – A Focus on Emerging or Re-Emerging Viruses**

Melanie Denizot1, Vincent G. Thon-Hon1, Shiril Kumar1, Jim W. Neal2 and Philippe Gasque1 *1GRI/IRG, Immunopathology and Infectious Disease Research Grouping (IRG, GRI), EA4517, University, CHR Felix Guyon and CYROI of La Reunion 2Neuropathology Laboratory, Dept of Histopathology, Cardiff University Medical School., CF14 4XN 1France 2UK* 

### **1. Introduction**

A wide variety of emerging and re-emerging viruses (e.g. arboviruses, 'arthropod-borne viruses') contributes to neurological diseases. Infections can be associated with new viral variants that are more efficiently transmitted and lead to massive outbreak and increased reports of complicated cases involving the CNS (Tsetsarkin et al., 2007; Vazeille et al., 2007). It is also possible that viruses may have acquired increased neurovirulence by a previously non-neurotropic virus. Viruses that appear to have recently become more neurovirulent include for example the West Nile flavivirus (WNV), Chikungunya alphavirus (CHIKV) and the enterovirus 71 (ENV71) (Griffin, 2010). In addition to these newer challenges, Japanese encephalitis flavivirus (JEV), rabies, polio, measles virus (MV), human immunodeficiency virus (HIV) and human herpes virus (HHV) remain important causes of neurologic diseases. Focusing on CHIKV, this is an alphavirus of the *Togaviridae* family transmitted by mosquitoes of the Aedes (*Ae*) genus. The alphavirus group comprises 29 viruses, six of which of the 'Old World, ie Africa' can cause human joint disorders (arthralgia evolving to arthritis), namely CHIKV, O'nyong-nyong virus (ONNV), Semliki forest virus (SFV), Ross River (RRV), Sindbis virus (SINV), Mayaro virus (MAYV) while the so-called 'New World' such as Eastern equine encephalitis virus (EEEV) and Venezuelan equine encephalitis virus (VEEV) can cause severe brain damage (Das et al., 2010; Jaffar-Bandjee et al., 2009). Interestingly, CHIKV-associated neuropathology was first described in the 1960s but it is the unprecedented incidence rate in the Indian Ocean with efficient clinical facilities that allowed a better description of cases with severe encephalitis, meningoencephalitis, peripheral neuropathies and deaths among newborns (mother-to-child infection), infants and elderly patients (Das et al., 2010; Jaffar-Bandjee et al., 2009). The follow-up of the neonates contaminated by CHIKV clearly indicates poor outcomes and neurodevelopment defects (Jaffar-Bandjee et al., 2011). Neurological manifestations described in adults requiring hospitalization involved cases of encephalopathy frequently associated

Virus-Induced Encephalitis and

and altered mental state.

Weiss et al., 2009).

adenoviruses or coxsackie viruses.

against the infectious challenge (Hoarau et al., 2010).

Innate Immune Responses – A Focus on Emerging or Re-Emerging Viruses 63

cells within the CNS can also be the site of viral replication and for instance with mumps virus which can infect epithelial cells of the choroid plexus. The involvement of ependymal cells of the SVZ can result in hydranencephaly following infection for instance by

Viruses vary greatly in their capacity to induce encephalitis. For some viruses (e.g. mumps),

For others (flaviviruses such as JEV or WNV), although the infection is highly asymptomatic, CNS infection when it occurs is the most prominent clinical feature. A third group of viruses are those which commonly cause infection, but only rarely cause encephalitis (herpes simplex virus, HSV or CHIKV). In this group, newborns and elderly patients are at risk because they either have a poor or inappropriate immune response

A paramyxovirus isolated from a Malaysian patient with encephalitis showed *in vitro*  characteristics similar to Hendra virus (HeV), a new morbillivirus previously isolated from horses and human in 1995. Subsequent virological studies have shown that the Malaysian pathogen, now named Nipah virus (NiV), is closely related to, but distinct from HeV and that the two belong to a new genus within the family paramyxoviridae capable of causing major outbreaks of encephalitis. Patients present with fever, headache, dizziness, vomiting,

Finally, there are viruses for which human infection inevitably and exclusively results in CNS disease (e.g. rabies). In addition to acute pathology, other viruses (e.g. measles) can cause syndromes of post-infectious encephalopathy with the capacity of the virus to hide into tissue sanctuaries. The capacity of these viruses to reactivate the viral cycle is poorly

The clinical manifestations of many virus infections are dependent on whether or not virus gains access to susceptible cells within the CNS. Therefore, the mechanisms by which viruses penetrate the CNS are of prime importance in understanding the pathogenesis of the disease. To understand the invasion process it is necessary to describe one of the defensive structures that prevent microbial invasion. The CNS is enclosed completely by three

A blood -brain barrier (BBB) composed of brain micro-vascular endothelial cells (BMEC) with intercellular tight junctions and supported by astrocytes, pericytes and the basement membrane. The second is highly vascularized and fenestrated barrier localized at the choroid plexus between blood and cerebrospinal fluid (CSF), which also allows passage of some blood components and, thirdly, an interface provided by avascular arachnoid epithelium, underlying the dura, and completely enclosing the CNS (Abbott et al., 2010;

There are at least four different mechanisms by which viruses can gain access into the CNS. First, viruses may gain access by infecting the BMEC or may be transported across the BMEC (Jarvis and Nelson, 2002). Infection of the BMEC may provide a portal for viral entry into the CNS and disrupting the BBB function. A number of viruses such as cytomegalovirus (CMV) (Jarvis and Nelson, 2002), HIV (Moses et al., 1993) and arboviruses (Dropulic and Masters, 1990) are able to infect the BMEC at least *in vitro*. In acute viral

neuroinfection is a common but a relatively benign part of the syndrome.

understood and may be the results of immune escape mechanisms.

different but connected blood-brain interfaces (Abbott et al., 2010).

**3. Molecular and cellular mechanisms for virus entry into the CNS** 

with the presence of IgM anti-CHIKV in the CSF, encephalitis, Guillain-Barre, encephalomyeloradiculitis and rare deaths (Economopoulou et al., 2009; Lemant et al., 2008). In recent histopathological studies, CHIKV infection in adults was associated with bilateral frontoparietal white matter lesions with restricted diffusion, which is described as an early sign of viral encephalitis (Ganesan et al., 2008). Focal perivascular lymphocytic infiltrates were also present in area of active demyelination and some degree of microglial activation was also noted in the gray matter which may contribute to bystander neuronal loss (Ganesan et al., 2008). Although data are still scarce, the number of cases with CNS involvement appears to support the neurotropic/neuroinfectious activity of CHIKV. This unique CNS infection illustrated by subventricular white matter lesions and intraparenchymal haemorrhages have been confirmed experimentally. CHIKV diseases can be reproduced in several animal models and the virus was shown to infect mouse/macaque brains and to replicate in cultures of glial and neuronal cells (Gardner et al., 2010; Labadie et al., 2010; Solignat et al., 2009; Wang et al., 2008; Ziegler et al., 2008). The incidence of neuroinfection is dramatically increased following either the injection of high viral titers, the route of injection (intranasal > intraperitoneal), in young animals (less than two week-old) or in mice failing to mount a robust interferon response (i.e. IFNAR knockout animals). Thus, experimental infections where mice were inoculated subcutaneously showed rapid and robust replication (106-107 PFU/ml) of CHIKV in the brain particularly of newborn mice. Interestingly, infected mice showed signs of illness suggestive of human clinical pathology such as loss of balance, difficulty of walking, dragging of the hind limbs, skin lesions but with rare mortality. CHIKV neuroinfection is particularly severe in IFNAR -/ mice and targets the leptomeninges, the choroid plexus and ependymal cells lining the subventricular zone (SVZ) also known as one of the neural stem cell niche in adult brains. The nature of the receptor(s) mediating cell attachment and infection remains to be characterized but the role of apoptotic blebs carrying the virus from cell to cell has recently been established as in a Trojan-horse paradigm (see below). On the one hand, the local innate immune response is the key to the control of viral infection but could also, on the other, contribute to neuronal loss through the uncontrolled release of cytotoxic inflammatory cytokines, complement proteins or proapoptotic molecules (TNF-, FasL, granzymes) (Hauwel et al., 2005). We will review herein the pathological mechanisms as well as the innate immune mechanisms engaged during encephalitis and with special emphasis on chikungunya.
