**Blood-Brain Barrier Disruption and Encephalitis in Animal Models of AIDS**

Nicole A. Renner, Andrew A. Lackner and Andrew G. MacLean *Tulane National Primate Research Center, Program in Neuroscience, Covington USA* 

#### **1. Introduction**

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The pathogenesis of HIV/SIV encephalitis (HIVE/SIVE) remains incompletely understood, but is associated with alterations in the blood brain barrier. In animals infected with pathogenic strains of simian immunodeficiency virus (SIV), such as SIVmac239 and SIVmac251, the virus can be consistently found in the central nervous system (CNS) within 10 to 14 days of infection: at the time of peak viremia (Lackner et al., 1994). This also appears to be true in human immunodeficiency virus (HIV)-infected humans, but the number of cases examined during peak viremia is very small (Davis et al., 1992). In SIV-infected macaques at this early time point, endothelial cells of the blood-brain barrier (BBB) are activated and integrity of the BBB is compromised (Stephens et al., 2003). As viral loads decline toward set point at roughly two months post infection the endothelial activation subsides and BBB integrity is largely restored (Sasseville et al., 1995, Lackner et al., 1994, Annunziata, 2003, Zink et al., 1998). However, in the terminal phases of disease, viral loads rise and approximately one third of animals develop SIV encephalitis (SIVE), which is associated with breakdown of the BBB.

The exact mechanisms of BBB disruption are unclear, but it is known that numerous resident and transitory cell populations in the CNS can be infected, with CD14-positive perivascular macrophages being the primary productively-infected cell type (Little et al., 1999, Gorry et al., 2003, Bissel and Wiley, 2004, Ryzhova et al., 2002, Liu et al., 2004, Brack-Werner, 1999, Trillo-Pazos et al., 2003, Williams et al., 2001, Fischer-Smith et al., 2001). Nervous system manifestations associated with HIV infection of humans or SIV infection of rhesus macaques include an encephalitis (SIV or HIV encephalitis, SIVE/HIVE) characterized by astrocytic and microglial activation and scattered perivascular aggregates of mononuclear cells and multinucleated giant cells. These perivascular lesions contain large numbers of HIV/SIV-infected cells, the majority of which are monocyte/macrophages. The presence of cells productively-infected with SIV/HIV in the parenchyma has been shown to induce a response in astrocytes (Nath, 1999, Tyor et al., 1992, Persidsky et al., 1999, Persidsky et al., 2000) which in turn may lead to decreased tight junction protein expression and a leaky BBB (Dallasta et al., 1999, Persidsky et al., 1997, Moses and Nelson, 1994, Boven et al., 2000, Luabeya et al., 2000, Andras et al., 2003, Annunziata, 2003, Kanmogne et al., 2005, Kanmogne et al., 2007, MacLean et al., 2004b, Persidsky, 1999).

Astrocytes, along with microglia, are resident cells in the brain involved in inflammation. Their role during inflammation is not well understood; it is believed that both cell types are

Blood-Brain Barrier Disruption and Encephalitis in Animal Models of AIDS 89

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

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

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

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

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

As outlined above, lentiviruses are thought to enter the brain within circulating infected monocytes during immune surveillance. Numerous studies have been undertaken to

macaques, has become the most widely used model for HIV pathogenesis studies.

observed by Sasseville *et al*.

**2.2.2 Terminal disease** 

et al., 2006, Huang et al., 2009).

**3. Blood-brain barrier disruption in HIVE/SIVE** 

Hickey, 2002).

Figure 1.

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