T cells in the central nervous system: the delicate balance between viral clearance and disease.

The central nervous system (CNS) is considered an "immunoprivileged" site with restricted access and a unique microenvironment that profoundly affects the capacity of T cells to exert their functions. The lymphocytic choriomeningitis virus model offers a unique system in which to evaluate the contrasting roles of specific T cells in causing lethal CNS disease or curing pervasive and life-long CNS infection. Specific T cell kinetics in the periphery is briefly discussed. The T cell-mediated mechanisms leading to fatal choriomeningitis are reviewed as are recent methodologic advances that will facilitate the study of antigen-specific T cells in disease pathogenesis. Understanding the specific constraints imposed by the CNS on local T cell activity has important consequences for the design of therapeutic strategies aimed at preventing or curing CNS infection.