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

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

Blood-Brain Barrier Disruption and Encephalitis in Animal Models of AIDS 91

subjacent to the endothelial cells, and anchoring them to the underlying tissues, is the basement membrane which is composed largely of Type IV collagen and laminin. Perivascular macrophages and the foot processes of astrocytes and microglia surround the endothelial cells (Hickey and Kimura, 1988, Graeber et al., 1992, Streit and Graeber, 1993,

Tight junctions are a fibrillary network of transmembrane proteins that can be phosphorylated to regulate physiologic processes such as replacement of perivascular macrophages by circulating monocytes. This phosphorylation can function differently depending if the stimulus is from the luminal or parenchymal side of the BBB. The presence of adhesion molecules on the luminal surface of BMEC is very important for leukocyte extravasation into the CNS (see our recent review for further details (Ivey et al., 2009a)). Tight junctions consist of at least 40 transmembrane proteins, anchorage proteins and tight junction-associated proteins in the membrane and cytosol of endothelial cells. Tight junctions are characterized as having high transendothelial electrical resistance values

A recent study by Strayer et. al. has shown that either cell-free or cell-associated gp120 (the outer envelope glycoprotein of HIV) leads to increased matrix metallopeptidase 9 (MMP9) expression which causes decreased expression of laminin and the tight junction protein claudin 5 (Louboutin et al., 2010). A possible mechanism for this could be mediated through focal adhesion kinase, which has been shown to be upregulated in areas of increased

We have recently shown that viral infected macrophages are important in disruption of the BBB *in vitro* (MacLean et al., 2004a, MacLean et al., 2004b), *ex vivo* (Ivey et al., 2009b, Renner et al., 2011) and *in vivo* (Renner *et al.*, in press). The precise mechanisms of BBB disruption are a subject of active research by numerous groups (Luabeya et al., 2000, Andras et al., 2003, Kanmogne et al., 2005, Kanmogne et al., 2007, Persidsky et al., 2006). All of these distinct signal transduction mechanisms have a common factor, however: the tight junctions are linked to the actin cytoskeleton, and the dynamics of the cytoskeleton are therefore important regulators. The importance of cytoskeleton activation will be revisited later when

As illustrated in Figure 2, astrocyte foot processes are closely apposed to BMEC and ensheath more than 60% of the vessel exterior (Mathiisen et al., 2010). Through these contacts astrocytes are able to affect changes in BBB integrity during health and disease, and

The BBB is formed during early infancy in primates (Bayer et al., 1993). The exact mechanisms underlying BBB formation are not clear, but it is known that astrocytes are critical in both maturation and maintenance of the barrier integrity (Willis et al., 2004, Al Ahmad et al., 2010). Astrocytes also act to repel circulating immune cells through secretion

Lassmann et al., 1991).

between 1000 and 1500 Ω/cm2 (Butt *et al*, 1990).

neurovascular permeability (Lee et al., 2010).

**4.1 Summary of astrocytes in encephalitis** 

**4.2 Role of astrocytes in BBB physiology** 

recruit or repel inflammatory cells through cytokines (Figure 2).

discussing glial cell activation.

**4. Astrocytes** 

**3.2 Signalling pathways in BBB disruption in HIVE/SIVE** 

determine the reasons underlying increased monocyte migration into brain following lentiviral infection. HIV-infected leukocytes are primed for adhesion (Hallett, 1995), having already shed L-selectin, and increased expression of CD11b/CD18 compared with monocytes from healthy controls (Elbim et al., 1999). Therefore, it is possible that even barely increased levels of chemokines expressed within the parenchyma would lead to increased migration of monocytes. Recent studies have shown that glial cells are stimulated to produce chemokines in response to inflammatory cytokines (Renner et al., 2011, Thompson and Van Eldik, 2009) known to be secreted by SIV-infected macrophages (Orandle et al., 2002). Therefore, the role played by glial cells and tight junctions requires further discussion.

Fig. 1. Schematic of key players in the development of SIV encephalitis. At left, a cutaway section of a cerebral microvessel showing circulating blood cells. At center, normal blood vessel with tight junctions (solid lines) evident between endothelial cells. Astrocyte foot processes are highly evident with occasional microglia. Low levels of cytokines and chemokines are expressed. At right a microvessel in an encephalitic lesion, showing disrupted junctions (dashed lines), leakage of serum proteins into parenchyma, displaced astrocyte foot processes and increased cytokines/ chemokines concomitant with increased numbers of perivascular macrophages.

### **3.1 Tight junction proteins**

The primary defining feature of the blood brain barrier (BBB) is the presence of tight junction proteins between brain microvascular endothelial cells (BMEC). Immediately subjacent to the endothelial cells, and anchoring them to the underlying tissues, is the basement membrane which is composed largely of Type IV collagen and laminin. Perivascular macrophages and the foot processes of astrocytes and microglia surround the endothelial cells (Hickey and Kimura, 1988, Graeber et al., 1992, Streit and Graeber, 1993, Lassmann et al., 1991).

Tight junctions are a fibrillary network of transmembrane proteins that can be phosphorylated to regulate physiologic processes such as replacement of perivascular macrophages by circulating monocytes. This phosphorylation can function differently depending if the stimulus is from the luminal or parenchymal side of the BBB. The presence of adhesion molecules on the luminal surface of BMEC is very important for leukocyte extravasation into the CNS (see our recent review for further details (Ivey et al., 2009a)). Tight junctions consist of at least 40 transmembrane proteins, anchorage proteins and tight junction-associated proteins in the membrane and cytosol of endothelial cells. Tight junctions are characterized as having high transendothelial electrical resistance values between 1000 and 1500 Ω/cm2 (Butt *et al*, 1990).

A recent study by Strayer et. al. has shown that either cell-free or cell-associated gp120 (the outer envelope glycoprotein of HIV) leads to increased matrix metallopeptidase 9 (MMP9) expression which causes decreased expression of laminin and the tight junction protein claudin 5 (Louboutin et al., 2010). A possible mechanism for this could be mediated through focal adhesion kinase, which has been shown to be upregulated in areas of increased neurovascular permeability (Lee et al., 2010).
