The meninges form a three-layered structure that covers the brain and spinal cord and contains large numbers of immune cells, especially in its outer layer, the dura mater. These meningeal immune cells are known to affect brain responses to ageing, injury and infection. Mathis and colleagues now show that regulatory T (Treg) cells in the meninges are important regulators of brain homeostasis.

The authors conclude that meningeal Treg cells limit the activation, proliferation, IFNγ production and parenchymal infiltration of T cells and NK cells. This seems to be mediated by competition for the growth and survival factor IL-2, as neutralization of IL-2 abrogated the effects of Treg cell depletion. They note that a pattern of hippocampal changes very similar to that induced by acute Treg cell depletion was recently observed in several human neurodegenerative diseases.