Astrocytes protect the CNS: antigen-specific T helper cell responses are inhibited by astrocyte-induced upregulation of CTLA-4 (CD152).

Astrocytes are the first cells that are encountered by T cells invading the central nervous system (CNS) by crossing the blood-brain barrier. We show that primary astrocytes contribute to the immune privilege of the CNS by suppressing Th1 and Th2 cell activation, proliferation and effector function. Moreover, this astrocyte-mediated inhibition of Th effector cells was effective on already activated, proliferating cells. Transforming growth factor (TGF)-β secreted by astrocytes or T cells was not the major factor in the inhibition. The inhibition of T-cell proliferation induced by astrocytes was mainly mediated by upregulation of CTLA-4 on already activated T cells, which occurred both with and without cell-cell contact. Upregulation of the inhibitory molecule CTLA-4 on autoreactive Th cells, as mediated by astrocytes, thus represents a novel mechanism for securing the immune privilege of the CNS.