autoimmunity ͉ neuroprotection ͉ neurodegenerative conditions ͉ hippocampal slice cultures ͉ immune regulation T he systemic adaptive T cell-dependent immune response plays a key role in the ability of neurons in the central nervous system (CNS) to withstand injurious conditions (1-3). Studies from our laboratory have shown that, after a CNS injury, T cells nonselectively migrate to the site of injury (4), and suggest that homing T cells, which encounter their relevant antigens at the lesion site, are the ones that contribute to the repair. Such T cells become locally activated to produce neurotrophic factors (5) and cytokines, which are capable of affecting the activity of resident microglia and hence the fate of threatened neurons. We further showed that the T cell-dependent protection evoked after an axonal injury, being an anti-self-response (6), is constitutively limited by naturally occurring CD4 ϩ CD25 ϩ regulatory T cells (Treg) (6). It is, however, amenable to therapeutic boosting, either by postinjury immunization with antigens derived from dominant proteins residing in the site of damage or by depletion of Treg (2).Treg comprise Ϸ10% of the CD4 ϩ T cell population, and have been widely viewed as safeguards against the pathogenic effects of autoimmune CD4 ϩ CD25