Th17 effector cytokines induce shared and distinct microglial and endothelial cell responses in post-streptococcal encephalitis.

Group A Streptococcus (GAS) infections can lead to neuropsychiatric sequelae in children, yet the mechanisms driving post-infectious brain pathology remain poorly defined. In a mouse disease model, Th17 lymphocytes induce microglial activation, blood-brain barrier (BBB) dysfunction, and neural circuit impairment; however, the transcriptional programs underlying these effects, and the specific Th17-derived cytokines involved are unclear. Using mouse genetics, single-cell RNA sequencing, and spatial transcriptomics, we show that GAS infections induce inflammatory gene programs in microglia and brain endothelial cells (BECs), accompanied by downregulation of BBB-associated transcripts in BECs. Spatial transcriptomic analyses reveal that GAS-responsive microglia are enriched near infiltrating T cells. Several chemokines upregulated in microglia following GAS infection in mice are elevated in sera from affected patients. Conditional ablation of GM-CSF in CD4+ T cells partially attenuates microglial chemokine gene expression, but does not restore BBB integrity. Neutralization of IL-17A partially rescues BBB transcriptional changes in BECs and reduces microglial chemokine expression; however, compensatory peripheral immune responses associated with persistent infection exacerbate BBB disruption. In contrast, microglia/macrophage-specific deletion of IL-17 receptor A partially rescues BBB deficits following GAS infection. Together, these findings identify IL-17A-IL-17RA signaling in microglia as a critical driver of BBB dysfunction after GAS infections.