The meninges host a unique compartment of regulatory T cells that bulwarks adult hippocampal neurogenesis [RNA-Seq]
Our knowledge about the meningeal immune system has recently burgeoned, particularly how innate and adaptive effector cells are mobilized in response to brain challenges. However, information on how meningeal immunocytes guard brain homeostasis in healthy individuals remains sparse. This study highlights the heterogeneous and polyfunctional regulatory T (Treg) cell compartment in mouse meninges. A Treg subtype specialized in controlling Th1-cell responses and another subtype devoted to controlling responses in B-cell follicles were substantial components of this compartment, foretelling the finding that punctual Treg-cell ablation rapidly unleashed interferon-gamma production by meningeal lymphocytes, unlocked their access to the brain parenchyma, and altered meningeal B-cell profiles. Distally, the hippocampus took on a reactive state, with activation of multiple glial-cell types; within the dentate gyrus, neural stem cells showed exacerbated death and desisted from further differentiation, associated with inhibition of spatial-reference memory. Thus, meningeal Treg cells are a multifaceted bulwark to brain homeostasis at steady-state. Overall design: For characterizing meningeal Treg cell identity and changes upon physiological variations (sex, age), and anatomical location we double sorted Treg cells from the meninges, spleen and skull bone marrow at different ages. To study how Tregs regulate the meningeal niche, Tregs were punctually depleted in two different ways: via diphtheria toxin (DT) injections into Foxp3-DTR+ and littermate Foxp3-DTR- controls, or via intra-cisterna magna injection of Ultra-LEAF™ purified mouse anti-CD25 mAb (BioLegend, clone PC61) or the recommended rat IgG1? isotype control mAb (BioLegend); then  the meninges were digested in QIAzol for RNA extraction, and followed whole-tissue RNA-seq. performed. To study how microglia responded upon punctual Treg depletion (DTR- and DTR+ littermates), we double sorted microglia from the hippocampus. To understand how different brain regions responded upon punctual Treg depletion, we isolated the hippocampus, cortex, thalamus and hypothalamus, and cerebellum, digested using QIAzol for RNA extraction, and followed whole-tissue RNA-seq.