Adaptive immune response to viral infections in the central nervous system.

Historically, the central nervous system (CNS) has been considered to be an immunologically privileged site within the body (Bailey et al., 2006; Galea et al. 2007; Engelhardt, 2008; Prendergast and Anderton, 2009). By definition, immunologically privileged sites, to include the brain, cornea, testis, and pregnant uterus, have a reduced/delayed ability to reject foreign tissue grafts compared to conventional sites within the body, such as skin (Streilein, 2003; Bailey et al., 2006; Carson et al., 2006; Mrass and Weninger, 2006; Kaplan and Niederkorn, 2007). In addition and conversely, tissue grafts prepared from immunologically privileged sites have increased survival, compared to tissue grafts prepared from conventional sites, when implanted at conventional sites (Streilein, 2003). The imune privilege of the CNS has been shown to be confined to the parenchyma, whereas the immune reactivity of the meninges and the ventricles, containing the choroid plexus, cerebrospinal fluid (CSF), and the circumventricular organs, is similar to conventionalsites (Carson et al., 2006; Engelhardt, 2006; Galea et al., 2007). This confinement of the imm une privilege to the parenchyma has also been demonstrated for experimental influenza virus infection in which confinement of the infection to the brain parenchyma did not result in efficient immune system priming whereas infection of the CSF elicited a virus-specific immune response comparable to that of intranasal infection (Stevenson et al. 1997). An important functional aspect of immune privilege is that damage due to the immune response and inflammation is limited within sensitive organs containing cell types that regenerate poorly, such as neurons within the brain (Mrass and Weninger, 2006; Galea et al.. 2007; Kaplan and Niederkorn, 2007).