Meningeal type 2 immunity in cortical synapse remodeling during brain development and injury

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PROJECT SUMMARY Immune signals profoundly influence brain physiology, and immune dysregulation is implicated in neurodevelopmental disorders including autism, schizophrenia, as well as in the remodeling responses engaged after brain injury. As such, there is a critical need to define the cells and molecules that mediate brain-immune communication. Microglia, a type of glial cell, are the dominant immune cell in the brain parenchyma and play key roles in synaptogenesis and synapse refinement during periods of brain remodeling. However, recent discoveries have identified a rich array of immune cells in the meninges and perivascular barrier regions of the brain. This raises the question of how these ‘border’ immune cells in and around the blood brain barrier impact brain remodeling, including during development and after injury. In preliminary data, we demonstrate that a subtype of immune cell known as group 2 innate lymphoid cells (ILC2s) expand and are activated in the developing brain meninges and larger perivascular areas. They secrete their canonical effector cytokine Interleukin-13 with a peak at postnatal days 5-12, which coincides with a robust period of synapse refinement. During development, we show that genetic depletion of ILC2s impaired cortical inhibitory synapse function consistent with defects in GABA receptor subunit composition. These mice had impacts in social recall memory in adulthood. Importantly, global deletion of the IL-4/IL-13 receptor phenocopied these synaptic defects, whereas exogenous IL-13 had the opposite effects. Aim 1 will determine the cellular targets and impacts of IL-13 signaling that mediate these synaptic and behavioral effects in development, based on preliminary data showing that microglia and border associated macrophages express high levels of the IL-13 receptor, and have transcriptional and morphologic responses to IL-13. In preliminary data to Aim 2, we identify a stromal fibroblast niche where ILC2s reside within the developing meninges and use single cell sequencing to define stromal ‘adventitial fibroblasts’ that produce ILC2-supporting signals. We demonstrate that these fibroblasts are sufficient to support ILC2 expansion in vitro, and that they produce the ILC2-regulating signals Interleukin-33. Aim 2 will determine how the meningeal stromal niche supports ILC2 expansion and IL-13 production in vivo, including testing the role of meningeal derived IL-33. In preliminary data to Aim 3, we find that meningeal ILC2s become reactivated and expand after photothrombotic brain injury, limiting post-injury hyperexcitability and damage and promoting the expansion of protective microglial subsets. Aim 3 will determine the impact of ILC2s, IL-13, and their downstream targets in inhibitory synapse remodeling during injury recovery. We will test the alterations in the meninges that sustain this ILC2 expansion and test the hypothesis that IL-33 signaling during damage is required for these changes. Together, these studies will help to define how immune cells in the brain borders impact the developing brain and how these may drive both beneficial and pathologic responses during development and neuroinflammation.