Cerebral inflammation and mobilization of the peripheral immune system following global hypoxia-ischemia in preterm sheep.
BACKGROUND
Hypoxic-ischemic encephalopathy (HIE) is one of the most important causes of brain injury in preterm infants. Preterm HIE is predominantly caused by global hypoxia-ischemia (HI). In contrast, focal ischemia is most common in the adult brain and known to result in cerebral inflammation and activation of the peripheral immune system. These inflammatory responses are considered to play an important role in the adverse outcomes following brain ischemia. In this study, we hypothesize that cerebral and peripheral immune activation is also involved in preterm brain injury after global HI.


METHODS
Preterm instrumented fetal sheep were exposed to 25 minutes of umbilical cord occlusion (UCO) (n = 8) at 0.7 gestation. Sham-treated animals (n = 8) were used as a control group. Brain sections were stained for ionized calcium binding adaptor molecule 1 (IBA-1) to investigate microglial proliferation and activation. The peripheral immune system was studied by assessment of circulating white blood cell counts, cellular changes of the spleen and influx of peripheral immune cells (MPO-positive neutrophils) into the brain. Pre-oligodendrocytes (preOLs) and myelin basic protein (MBP) were detected to determine white matter injury. Electro-encephalography (EEG) was recorded to assess functional impairment by interburst interval (IBI) length analysis.


RESULTS
Global HI resulted in profound activation and proliferation of microglia in the hippocampus, periventricular and subcortical white matter. In addition, non-preferential mobilization of white blood cells into the circulation was observed within 1 day after global HI and a significant influx of neutrophils into the brain was detected 7 days after the global HI insult. Furthermore, global HI resulted in marked involution of the spleen, which could not be explained by increased splenic apoptosis. In concordance with cerebral inflammation, global HI induced severe brain atrophy, region-specific preOL vulnerability, hypomyelination and persistent suppressed brain function.


CONCLUSIONS
Our data provided evidence that global HI in preterm ovine fetuses resulted in profound cerebral inflammation and mobilization of the peripheral innate immune system. These inflammatory responses were paralleled by marked injury and functional loss of the preterm brain. Further understanding of the interplay between preterm brain inflammation and activation of the peripheral immune system following global HI will contribute to the development of future therapeutic interventions in preterm HIE.
School of Mental Health and Neuroscience, Maastricht University, Universiteitssingel 40, Maastricht, 6229 ER, The Netherlands.
Reint K
RK
Jellema
Valéria
V
Lima Passos
Alex
A
Zwanenburg
Daan R M G
DR
Ophelders
Stephanie
S
De Munter
Joris
J
Vanderlocht
Wilfred T V
WT
Germeraad
Elke
E
Kuypers
Jennifer J P
JJ
Collins
Jack P M
JP
Cleutjens
Ward
W
Jennekens
Antonio W D
AW
Gavilanes
Matthias
M
Seehase
Hans J
HJ
Vles
Harry
H
Steinbusch
Peter
P
Andriessen
Tim G A M
TG
Wolfs
Boris W
BW
Kramer
D000818:Animals; D000831:Animals, Newborn; D001921:Brain / Q000276:immunology* / Q000473:pathology*; D002465:Cell Movement / Q000276:immunology*; D005260:Female; D005333:Fetus / Q000276:immunology / Q000473:pathology; D020925:Hypoxia-Ischemia, Brain / Q000276:immunology* / Q000473:pathology*; D007113:Immunity, Innate; D017628:Microglia / Q000276:immunology / Q000473:pathology; D011247:Pregnancy; D012756:Sheep