The effects of the adaptive immune system on the cognitive performance and abnormal behaviors seen in mental disorders such as schizophrenia have never been documented. Here, we show that mice deprived of mature T cells manifested cognitive deficits and behavioral abnormalities, which were remediable by T cell restoration. T cell-based vaccination, using glatiramer acetate (copolymer-1, a weak agonist of numerous self-reactive T cells), can overcome the behavioral and cognitive abnormalities that accompany neurotransmitter imbalance induced by (+)dizocilpine maleate (MK-801) or amphetamine. The results, by suggesting that peripheral T cell deficit can lead to cognitive and behavioral impairment, highlight the importance of properly functioning adaptive immunity in the maintenance of mental activity and in coping with conditions leading to cognitive deficits. These findings point to critical factors likely to contribute to age- and AIDS-related dementias and might herald the development of a therapeutic vaccination for fighting off cognitive dysfunction and psychiatric conditions.

The adaptive arm of the immune system has been suggested as an important factor in brain function. However, given the fact that interactions of neurons or glial cells with T lymphocytes rarely occur within the healthy CNS parenchyma, the underlying mechanism is still a mystery. Here we found that at the interface between the brain and blood circulation, the epithelial layers of the choroid plexus (CP) are constitutively populated with CD4 + effector memory cells with a T-cell receptor repertoire specific to CNS antigens. With age, whereas CNS specificity in this compartment was largely maintained, the cytokine balance shifted in favor of the T helper type 2 (Th2) response; the Th2-derived cytokine IL-4 was elevated in the CP of old mice, relative to IFN-γ, which decreased. We found this local cytokine shift to critically affect the CP epithelium, triggering it to produce the chemokine CCL11 shown to be associated with cognitive dysfunction. Partial restoration of cognitive ability in aged mice, by lymphopenia-induced homeostasis-driven proliferation of memory T cells, was correlated with restoration of the IL-4:IFN-γ ratio at the CP and modulated the expression of plasticity-related genes at the hippocampus. Our data indicate that the cytokine milieu at the CP epithelium is affected by peripheral immunosenescence, with detrimental consequences to the aged brain. Amenable to immunomodulation, this interface is a unique target for arresting age-related cognitive decline.blood-cerebrospinal fluid barrier | brain senescence | neuroinflammation C irculating immune cells have been repeatedly shown to be essential for central nervous system (CNS) maintenance (1-3). Specifically, T cells that recognize CNS antigens contribute to the functional integrity of the CNS under both normal and pathological conditions (2, 4-6), supporting hippocampus-dependent learning and memory, adult neurogenesis, and neurotrophic factor production (2).Under physiological conditions, T cells are rarely found in the brain parenchyma and are mainly observed at the borders of the CNS: the choroid plexus (CP) of the brain's ventricles, forming the blood-cerebrospinal fluid barrier (BCSFB), the meningeal spaces, and the cerebrospinal fluid (CSF) (7). T cells were shown to accumulate in these compartments in response to signals from the CNS, specifically in the meninges after performance of cognitive tasks (8) and in the CP after exposure to mental stress (9). In the meningeal spaces, these cells were further characterized as producing the cytokine interleukin 4 (IL-4), known for its beneficial role in CNS maintenance and neuroprotection (8, 10-13). However, the questions of why, where, and how T-cell specificity is needed for brain plasticity remained mysterious.The CP is strategically positioned at the lining between the CNS and the immune system and, in addition to its classically known role in generating the CSF, can enable bidirectional communication between the CNS parenchyma and blood circulation (14). Accordingly, we envisioned that T cells...

Fighting off neuronal degeneration requires a well controlled T-cell response against self-antigens residing in sites of the CNS damage. The ability to evoke this response is normally suppressed by naturally occurring CD4 ϩ CD25ϩ regulatory T-cells (Treg). No physiological compound that controls Treg activity has yet been identified. Here, we show that dopamine, acting via type 1 dopamine receptors (found here to be preferentially expressed by Treg), reduces the suppressive activity and the adhesive and migratory abilities of Treg. Treg activity was correlated with activation of the ERK1/2 (extracellular signal-regulated kinase 1/2) signaling pathway. Systemic injection of dopamine or an agonist of its type 1 receptors significantly enhanced, via a T-cell-dependent mechanism, protection against neuronal death after CNS mechanical and biochemical injury. These findings shed light on the physiological mechanisms controlling Treg and might open the way to novel therapeutic strategies for downregulating Treg activity (e.g., in neuronal degeneration) or for strengthening it (in autoimmune diseases).