Toll-like receptors in CNS viral infections

Protection against viral infections is critically dependent upon the early production of significant levels of type 1 interferons and the expression of interferon-stimulated genes that function as the effectors of innate antiviral immunity. Activation of Toll-like receptors on cells of the immune system is known to play an important role in this process. In this chapter we review evidence for a role of TLRs in innate immune responses against viral infections of the central nervous system. By far the most extensive literature pertains to TLR3. Data from various laboratories have shown that TLR3 is expressed in cells endogenous to the CNS, particularly in astrocytes and microglia. Triggering TLR3 by synthetic dsRNA, poly I:C effectively induces innate antiviral responses as well as boosts adaptive immune responses. Additional experiments show cooperative responses between TLRs (3, 7/8 and 9) in mounting an effective antiviral immune response in the periphery. Perhaps the most exciting data are from patient populations that document the critical role that specific TLRs play in specific CNS infections. Studies also suggest that inappropriate activation of the TLRs can result in a pathogenic outcome rather than a protective one. Since TLR ligands are being actively considered for their antiviral and potential adjuvant effects, this will be an important issue to address in the context of the CNS environment.