Our appreciation of the contributions of immune signalling to brain development and function is growing, but we still know little about the specific immune cell types and mediators involved. Barron et al. now reveal a crucial role for a subset of innate lymphoid cells (ILCs) in inhibitory synapse formation in the postnatal mouse brain.

In the CNS, many immune cells reside in the brain’s border regions: the meninges, perivascular spaces and choroid plexus. Using a combination of single-cell RNA sequencing and flow cytometry, Barron et al. demonstrated an expansion of the population of group 2 ILCs (ILC2s) present in the meninges of mice during the first weeks of postnatal life and showed that these cells produce a wave of type-2 cytokines (including interleukin-13 (IL-13)) between postnatal day 5 (P5) and P22.