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

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

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; Weiss et al., 2009).

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

Virus-Induced Encephalitis and

**Virus family** Alpha

(+)DNA double stranded linear

Trigeminal Nerve Olfactory nerve

Nectin-1, Nectin-2, HSPG

Encephalitis Herpetic neuralgia Meningitis Myelitis

**Genome** 

**Route of brain entry** 

**Receptors** 

**Targets** 

**Clinics** 

**Geographic** 

**distribution** Worldwide

(See text for abbreviation)

Innate Immune Responses – A Focus on Emerging or Re-Emerging Viruses 65

nectin-1 or poliovirus receptor related 1 (PRR1 also called HveC, CD111, HIgR), nectin-2 (also termed PRR2, HveB, CD112), nectin-3 and nectin-4. Each of them contains an extracellular region with three Ig-like domains, a single transmembrane region, and a cytoplasmic tail region. Nectin-1, -2 and -3 are expressed ubiquitously in multiple cell types such as epithelia, fibroblasts and neurons, whereas nectin-4 is mainly expressed in the human placenta. Nectin-1 and -2 have been identified as HSV entry mediators (Hve) **(see Table 1)**. Nectin-1 can serve as entry receptors for both HSV-1 and HSV-2 (Geraghty et al., 1998). In contrast, nectin-2 has more limited entry activity. Indeed, human nectin-2 is only a weak entry receptor for HSV-2 and certain strains of HSV-1 carrying out mutations in the glycoprotein D (Lopez et al., 2000). A more recent study also assessed the roles of the two known entry receptors, HVEM and nectin-1, in neuronal infection in the CNS and the development of encephalitis in a mouse model (Kopp et al., 2009). Intracranial injection of HSV was performed directly into the hippocampus of Wild-type (WT), HVEM KO, nectin-1

**Name** HSV Rabies HIV Polio WNV CHIKV

(+)RNA single stranded linear

**Envelope** yes yes yes No yes yes

Cell mediated BBB crossing

CD4, CCR5, CXCR4

Dementia Encephalitis Myelitis Neuropathy

Worldwide India

(+)RNA single stranded linear

Retrograde Axonal + BBB crossing

> Nectin-1(PRR1), Nectin-2(PRR2)

microglia Neurons Myeloid cells

Paralysis Respiratory arrest

Africa

virus Flavivirus Alphavirus

(+)RNA Single stranded linear

> Blood- CSF crossing

Apoptotic blebmediated cell entry

Epithelial cells Ependymal cells Neurons Glia

Encephalopathy Encephalitis Guillain Barre

> Africa Asia India (Europe) (Reunion)

(+)RNA single stranded linear

Direct BBB crossing

DC-SIGN (myeloid cells)

Microglia

Encephalitis Meningitis Myopathy

> Europe Americas Africa

herpesvirus Rhabdovirus Retrovirus Picorna

(-)RNA single stranded linear

Retrograde axonal

NCAM (CD56) AChR P75 NTR

Guillain Barre like syndrome Progressive encephalitis paralysis

Europe Asia Africa Americas

Table 1. Encephalitis due to emerging and re-emerging viruses

**(CNS)** Neurons Neurons Myeloid cells

encephalitis, capillary and endothelial inflammation of cortical vessels is a striking pathological finding, occurring primarily in the grey matter or grey white junction and this observation may facilitate virus entry. For example, HIV may gain access to CNS *in vivo* by this paracellular route as a result of endothelium activation by TNFα and other proinflammatory cytokines (Fiala et al., 1997).

Second, viruses may transmigrate across the BBB within virally infected leukocytes. For HIV, several studies suggest that virus shedding from infected CD4+ T cells, macrophages, and monocytes during migration through the BBB can instigate CNS replication into the parenchyma (Nottet et al., 1996; Persidsky et al., 1997; Schmidtmayerova et al., 1996). CMV can also be transferred to CNS by virus infected mononuclear phagocytes and bi-directional cell to cell transmission between infected monocytes and endothelial cells (Drevets and Leenen, 2000).

Third, viruses can also penetrate the CNS by taking advantage of incomplete closure of the BBB. Despite the intercellular tight junctions between the capillary endothelial cells in most regions of the BBB, certain areas of the CNS such as the choroid plexus, posterior pituitary, and circumventricular organs are not completely protected by the BBB due to a fenestrated endothelial cell layer and sparse basement membrane (Zhang and Tuomanen, 1999). A number of blood-borne viruses including mumps (Herndon et al., 1974), HIV (Bagasra et al., 1996) and CHIKV (Couderc et al., 2008) have been suggested to penetrate across the choroid plexus micro- vessels and infect the epithelium. In the CSF space, viruses can subsequently infect the ependymal cells and the surrounding brain tissue.

Finally, viruses can spread to the CNS through peripheral neuronal routes, like the motor neurons of the spinal cord ,olfactory neurons, retinal neurons, oculomotor neurons and trigeminal nerves , which are directly connected to the CNS, thus providing a convenient route for neurotropic viruses (Mori et al., 2005; Tirabassi et al., 1998). Viruses including HSV (Barnett et al., 1993), rabies virus (Jackson, 2003) and VEEV (Charles et al., 1995) are able to replicate within peripheral nerves and are transported into the CNS from the PNS along axons as the result of axonal transport of neurons. Certain enteroviruses can also spread to the CNS by infecting enteric neurons (Morrison and Fields, 1991). Virus spread within the CNS by retrograde, anterograde or cell to cell diffusion mechanisms.
