Astrocyte-derived Interleukin-33 promotes microglial synapse engulfment and neural circuit development II

Neuronal synapse formation and remodeling is essential to central nervous system (CNS) development and is dysfunctional in neurodevelopmental diseases. Innate immune signals regulate tissue remodeling in the periphery, but how this impacts CNS synapses is largely unknown. Here we show that the IL-1 family cytokine Interleukin-33 (IL-33) is produced by developing astrocytes and is developmentally required for normal synapse numbers and neural circuit function in the spinal cord and thalamus. We find that IL-33 signals primarily to microglia under physiologic conditions, that it promotes microglial synapse engulfment, and that it can drive microglial-dependent synapse depletion in vivo. These data reveal a cytokine-mediated mechanism required to maintain synapse homeostasis during CNS development. Overall design: 13 samples consisting of 3-4 biological replicates each of flow sorted astrocytes (Aldh1l1-GFP+) and microglia (CD11b+) from spinal cord P9-P12 Il33 KO and littermates control mice (Pichery et al., 2012; J Immunology)