Early immune responses to systemic inflammation in the postnatal mouse brain initiated by migrating macrophages and leptomeningeal fibroblasts

Bacterial infection is a key trigger of inflammatory pathways that damage the preterm brain, even without direct bacterial brain invasion. However, the mechanisms underlying the involvement of intracranial tissues and cells in early immune responses to acute systemic inflammation in preterm infants remain unknown. Lipopolysaccharide (LPS) was administered with a single intraperitoneal injection into postnatal day (P) 7 mice. Four hours later, the leptomeningeal macrophages initiated an intracranial reaction by producing IL-1β, which set the stage for the arachnoid, pial, and perivascular fibroblasts to act. These brain fibroblasts produced CCL2, which increased the number of brain parenchymal macrophages/microglia and led to their hypertrophy. The macrophages/microglia were then isolated from the brain 24 hours after LPS administration. Microarray analysis showed marked transcript expression of genes including Saa3, Irg1, Lcn2, and Cxcl9. RT-qPCR assays and histological staining showed that the expression level of Saa3 was upregulated in the leptomeningeal macrophages, cerebral perivascular macrophages, and cerebellar medulla macrophages, as well as in the fibroblasts of the leptomeninges, choroid plexus stroma, and perivascular space. The leptomeningeal macrophages expressing Saa3 continued their developmental migration from the leptomeninges to the parenchyma, such as the bundles that traverse the roof of the third ventricle and the corpus callosum. The expression level of Saa3 reached their peak 4 and 12 hours after LPS administration. The blood-brain barrier (BBB) of P7 pups remained intact up to 72 hours after LPS administration. The expression level of Irg1 was remarkably increased in the leptomeningeal macrophages and reached their peak 4 and 12 hours after LPS administration. The expression level of Lcn2 was the most increased in the perivascular fibroblasts and choroid plexus epithelial cells, peaking 4 and 12 hours after LPS administration. The expression level of Cxcl9 was also increased in the leptomeningeal fibroblasts, peaking 4 and 12 hours after LPS administration. The early enhancement of Saa3 expression played a key role in priming inflammation, initiated by macrophages and fibroblasts. Early enhancement of Irg1 expression was crucial for determining the direction of the inflammatory response. The increase of Lcn2 was vital, sequestering the iron siderophore to limit bacterial growth. It was interesting to observe how the early immune responses to systemic inflammation in the postnatal mouse brain were initiated by migrating macrophages and leptomeningeal fibroblasts, leading to immediate gene upregulation to protect immature brain tissue.