**2. Overview of viral encephalitides**

There are numerous viruses that can cause encephalitis. For some of these viruses the encephalitis is such a prominent part of the subsequent disease that "encephalitis" is part of the name. Examples include Eastern, Western and Venezuelan encephalitis viruses and St. Louis encephalitis virus all caused by arthropod borne (arbo) viruses that belong to several different virus families (Togaviridae, Flaiviridae) (Adams 2008 and Ciota 2009). In most instances these "encephalitis viruses" cause disease by targeting neurons. In contrast lentivirurses of the family Retroviridae, such as HIV and SIV, which are the major focus of this review, cause disease by a much less direct means.

### **2.1 Lentiviral encephalitis**

The precise mechanism of lentiviral entry to brain is still a subject of some debate. In 1982, Bill Narayan postulated the "Trojan Horse" hypothesis whereby visna virus (one of the first lentiviruses described in detail) entered brains of sheep and goats by hiding within circulating monocytes and then once in the brain, emerged to cause disease (Narayan et al., 1982). It has been proposed that circulating monocytes enter the brain during normal immune surveillance (Williams and Hickey, 1995). However, this remains an issue of debate (Fischer-Smith and Rappaport, 2005). Regardless, the predominate cell infected early in infection of the brain is the CD14+ monocyte-derived macrophage (Williams et al., 2001).

#### **2.2 Simian immunodeficiency virus infection**

The simian and human immunodeficiency viruses are closely related with HIV having originated from at least two cross species transmission events from monkeys to humans in West Africa in the mid 20th century (P Marx, personal communication and (Worobey et al., 2010). Infection of macaques (primarily rhesus macaques—*Macaca mulatta* and Pigtail macaques – *M. nemestrina*) with SIV follows a near identical course to HIV infection of humans with a peak viral load approximately two weeks following infection, subsiding to a viral set point which rises again with development of AIDS. The disease in brain also follows a similar course. SIV and HIV infection have, in addition to acute and terminal phases, a chronic, relatively asymptomatic, phase, during which very little is known about the physiology and pathology (or lack thereof) in brain. For this reason, we will focus on the acute and terminal stages of infection when encephalitis is an issue.

#### **2.2.1 Acute infection**

The precise mechanisms involved in the recruitment of the first viral-infected cell into the brain is the topic of much speculation and debate, although increased expression of VCAM-1 (CD106) by brain endothelial cells is a possibility (Sasseville et al., 1995, Sasseville et al., 1992). CD106 is one of several vascular adhesion molecules involved in directing leukocyte recruitment to tissues (Luster et al., 2005). Expression of CD106 was not limited to areas immediately adjacent to viral-infected cells, but was diffuse throughout brain, remaining elevated through at least 23 weeks post infection (well beyond peak viral load and establishment of viral set point). We have shown that CD106 expression is upregulated on endothelial cells and astrocytes following incubation with either viral-infected cells or their supernatants (MacLean et al., 2004a, MacLean et al., 2004b), and by others on astrocytes using Theiler's Murine Encephalomyelitis Virus (Rubio et al., 2010). That cell-free virus was able to stimulate endothelial cells to express CD106 may explain the diffuse staining earlier observed by Sasseville *et al*.

Both HIV and SIV use two cellular receptors in combination for infection: the CD4 molecule and a chemokine receptor, the two most common being CCR5 and CXCR4 (Moore et al., 2004). Monocyte/macrophages express these receptors and thus SIV and HIV are macrophage tropic (Salazar-Gonzalez et al., 2009). During early SIV infection, the predominate cell type productively-infected in brain is the monocyte-derived macrophage (Williams et al., 2001). Due to the many similarities between HIV infection of humans and SIV infection of macaques, SIV infection of macaques, particularly of Indian-origin rhesus macaques, has become the most widely used model for HIV pathogenesis studies.

### **2.2.2 Terminal disease**

88 Non-Flavivirus Encephalitis

involved in propagating and limiting inflammation (Kielian, 2004). Astrocytes and microglia are the primary cell types found in glia scar formation. They serve a vital role during injury to the brain: both astrocytes and microglia are capable of promoting an inflammatory response, but are also known to have cytoprotective and anti-inflammatory effects (Hauwel et al., 2005, Park et al., 2003). The complex nature of astrocytes' chemokine response has

There are numerous viruses that can cause encephalitis. For some of these viruses the encephalitis is such a prominent part of the subsequent disease that "encephalitis" is part of the name. Examples include Eastern, Western and Venezuelan encephalitis viruses and St. Louis encephalitis virus all caused by arthropod borne (arbo) viruses that belong to several different virus families (Togaviridae, Flaiviridae) (Adams 2008 and Ciota 2009). In most instances these "encephalitis viruses" cause disease by targeting neurons. In contrast lentivirurses of the family Retroviridae, such as HIV and SIV, which are the major focus of

The precise mechanism of lentiviral entry to brain is still a subject of some debate. In 1982, Bill Narayan postulated the "Trojan Horse" hypothesis whereby visna virus (one of the first lentiviruses described in detail) entered brains of sheep and goats by hiding within circulating monocytes and then once in the brain, emerged to cause disease (Narayan et al., 1982). It has been proposed that circulating monocytes enter the brain during normal immune surveillance (Williams and Hickey, 1995). However, this remains an issue of debate (Fischer-Smith and Rappaport, 2005). Regardless, the predominate cell infected early in infection of the brain is the CD14+ monocyte-derived macrophage (Williams et al., 2001).

The simian and human immunodeficiency viruses are closely related with HIV having originated from at least two cross species transmission events from monkeys to humans in West Africa in the mid 20th century (P Marx, personal communication and (Worobey et al., 2010). Infection of macaques (primarily rhesus macaques—*Macaca mulatta* and Pigtail macaques – *M. nemestrina*) with SIV follows a near identical course to HIV infection of humans with a peak viral load approximately two weeks following infection, subsiding to a viral set point which rises again with development of AIDS. The disease in brain also follows a similar course. SIV and HIV infection have, in addition to acute and terminal phases, a chronic, relatively asymptomatic, phase, during which very little is known about the physiology and pathology (or lack thereof) in brain. For this reason, we will focus on the

The precise mechanisms involved in the recruitment of the first viral-infected cell into the brain is the topic of much speculation and debate, although increased expression of VCAM-1 (CD106) by brain endothelial cells is a possibility (Sasseville et al., 1995, Sasseville et al., 1992). CD106 is one of several vascular adhesion molecules involved in directing leukocyte

recently been shown to vary by pathogen (McKimmie and Graham, 2010).

**2. Overview of viral encephalitides** 

**2.1 Lentiviral encephalitis** 

**2.2.1 Acute infection** 

this review, cause disease by a much less direct means.

**2.2 Simian immunodeficiency virus infection** 

acute and terminal stages of infection when encephalitis is an issue.

In humans with symptoms of AIDS dementia complex (the clinical spectrum of illness that includes individuals with HIV encephalitis), there are altered subpopulations of circulating monocytes; CD14 expression is lower, and CD16 and CD69 are both increased (Pulliam et al., 1997, Zhou et al., 2007, Munsaka et al., 2009). Similar changes in monocyte / macrophage populations are also observed throughout disease progression in macaques infected with SIV (Bissel et al., 2006b, Bissel et al., 2006a, Kuroda, 2010, Kim et al., 2005, Williams and Hickey, 2002).

While circulating monocytes are not thought to be productively-infected, the increased numbers of primed monocytes would likely lead to an increased potential for trafficking of cells capable of being infected to brain. The presence of infected monocytes is known to activate endothelial cells of the BBB to express CD106 (MacLean et al., 2004a, MacLean et al., 2004b) and leads to disruption of tight junction proteins including ZO-1 and claudin 5 (Andras et al., 2003, Ivey et al., 2009b, Kanmogne et al., 2007, Luabeya et al., 2000, Persidsky et al., 2006, Huang et al., 2009).

In contradistinction to CD106 expression, the loss of tight junction proteins is largely limited to areas close to viral infected cells (Luabeya et al., 2000, Andras et al., 2003, Kanmogne et al., 2005, Kanmogne et al., 2007, Persidsky et al., 2006), and Renner et al, in press. In those areas where encephalitis is observed, the loss of tight junction protein expression can extend over 150µm (MacLean et al., 2005). As with primary infection, productively-infected cells in brain are largely monocyte-derived macrophages, including microglia in close proximity to blood vessels (Roberts et al., 2004b, Gonzalez-Scarano and Martin-Garcia, 2005). The conceptual framework for interactions of the various cell types involved is summarized in Figure 1.
