Microglia require CD4 T cells to complete the fetal to adult transition

The blood brain barrier has long been thought to isolate the brain from the adaptive immune system. While CD4 T cells have been reported in the central nervous system, their presence in the healthy brain remains controversial, and their function at this site unknown. Here we have used a combination of imaging, single cell and surgical approaches to identify a unique CD69+ CD4 T cell population in both the mouse and human brain, which is distinct from circulating CD4 T cells. The brain-resident population was derived through in situ differentiation from activated circulatory cells, and was shaped by interaction with the gut microbiome and self-antigen in the brain. Single cell sequencing revealed that in the absence of murine CD4 T cells, resident microglia remained suspended between a fetal and adult state. This maturation defect resulted in excess immature neuronal synapses and behavioral abnormalities in the adult mouse. These results illuminate a role for CD4 T cells in the basic developmental processes of the brain, and a potential interconnected dynamic between the evolution of the immunological and neurological systems. Overall design: Quantification and characterisation of the brain CD4 T cell population in two wild type mice and one MHC class II-deficient mouse with a near-complete absence of CD4 T cells