Sci. Immunol. 4, eaay5199 (2019)

The brain has long been considered to be a site of immune privilege; however, it is now increasingly appreciated that immune cells can enter the brain at steady state to exert important physiological effects. In Science Immunology, Ribot and colleagues find a distinctive population of Vγ6+ γδ T cells in the mouse meninges but not in the brain parenchyma. These cells are derived from the fetus, are present perinatally and do not require the presence of microbiota or inflammatory signals for their maintenance in the meninges. Phenotypically, these γδ T cells are the preeminent producers of the cytokine IL-17 within the meninges but almost wholly lack production of the cytokine IFN-γ. Their steady-state production of IL-17 acts on glia — and potentially other cells of the brain parenchyma — to elicit production of the nerve growth factor BDNF. This signaling axis via γδ T cell–produced IL-17 supports synaptic plasticity and short-term spatial memory acquisition.