Immune cells can affect brain development and function, but little is known about the specific cell types and mediators involved. Barron et al. now reveal a crucial role for group 2 innate lymphoid cells (ILC2s) in the formation of inhibitory synapses in the postnatal mouse brain.

The surge in ILC2 numbers coincides with a crucial period of cortical circuit formation in mice. Investigations of synaptic markers, together with electrophysiological recordings, showed that mice with a constitutive lack of ILC2s have fewer inhibitory synapses in several cortical regions at P15 compared with ILC2-sufficient control mice, whereas excitatory synapses were unaffected. Transfer of donor ILC2s into the brains of these mice increased the number of inhibitory (but not excitatory) synapses, which suggests that ILC2s specifically promote the formation of inhibitory synapses.