Spatial analysis of microglial and vascular responses following recurrent intranasal Group A Streptococcus infections

Peripheral infections can result in neurocognitive changes in many contexts, including after recurrent Group A Streptococcus (GAS) infections in children. To explore the underlying mechanisms, here we used an intranasal inoculation model to analyze nearly 200,000 cells from the mouse olfactory bulb (OB) by multiplexed error-robust fluorescent in situ hybridization (MERFISH). Upon repeated GAS inoculations, endothelial cells (ECs) responded by downregulating blood-brain barrier and extracellular matrix genes. Microglia upregulated response to interferon and antigen presentation genes and downregulated homeostatic microglia genes. We determined that microglia with higher expression of Streptococcus-responsive genes were localized to the outer glomerular layer of the OB, possibly due to their proximity to infiltrating CD4 T cells. Expression of homeostatic genes was correspondingly high in the inner granular layer of the OB. Brains were isolated from P53 mice following five repeated inoculations with either a suspension of Group A Streptococcus or with vehicle (PBS), and profiled at 24 hours post-inoculation. After perfusion with cold PBS, brains were embedded in OCT for sectioning onto specialized coverslips. Coronal sections of the olfactory bulb (around bregma 4.28) were collected for analysis by MERFISH