Low-dose lipopolysaccharide selectively sensitizes hypoxic ischemia-induced white matter injury in the immature brain

Little is known about roles of inflammation and hypoxic ischemia (HI) in the generation of neuroinflammation and damage of blood-brain barrier (BBB) in the white matter (WM) that displays regional vulnerability in preterm infants. We investigated whether low-dose lipopolysaccharide (LPS) sensitizes HI-induced WM injury in postpartum (P) day 2 rat pups by selectively increasing neuroinflammation and BBB damage in the WM. Pups received LPS (0.05 mg/kg) (LPS + HI) or normal saline (NS + HI) followed by 90-min HI. LPS and NS group were the pups that had LPS or NS only. Myelin basic protein immunohistochemistry on P11 showed WM injury in LPS + HI group, but not in NS + HI, LPS, and NS groups. In contrast, no gray matter injury was found in the four groups. LPS + HI group also showed decreased number of oligodendrocytes in the WM 72-h postinsult. In the same brain region, increases of activated microglia, TNF-alpha expression, BBB leakage, and cleaved caspase-3 positive cells were much more prominent in LPS + HI group than in the other three groups 24-h postinsult. The oligodendrocytes were the major cells with cleaved caspase-3 expression. We concluded that low-dose LPS sensitized HI-induced WM injury in the immature brain by selectively up-regulating neuroinflammation and BBB damage in the WM.