Ischemia-reperfusion injury in stroke: impact of the brain barriers and brain immune privilege on neutrophil function

Abstract

Reperfusion injury following ischemic stroke is a complex pathophysiological process involving numerous mechanisms ranging from the release of excitatory amino acids and ion disequilibrium to the induction of apoptosis and necrosis, to oxidative stress and inflammation. The migration of neutrophils into the brain parenchyma and release of their abundant proteases are generally considered the main cause of neuronal cell death and acute reperfusion injury following ischemic stroke. Recent findings in experimental and human stroke have challenged this view, as the majority of neutrophils were rather found to accumulate within the neurovascular unit (NVU) and the subarachnoid space (SAS) where they remain separated from the brain parenchyma by the glia limitans. The brain parenchyma is an immune-privileged site that is not readily accessible to immune cells and does not elicit stereotypic adaptive or innate immune responses. Understanding brain immune privilege requires intimate knowledge of its unique anatomy in which the brain barriers, that include the glia limitans, establish compartments that differ remarkably with regard to their accessibility to the immune system. We here propose that the brain immune privilege also extends to an ischemic insult, where the brain parenchyma does not evoke a rapid infiltration of neutrophils as observed in ischemic events in peripheral organs. Rather, neutrophil accumulation in the NVU and SAS could have a potential impact on cerebrospinal fluid (CSF) drainage from the central nervous system (CNS) and thus on edema formation and reperfusion injury after ischemic stroke. Integrating the anatomical and functional implications of the brain immune privilege with the unquestionable role of neutrophils in reperfusion injury is a prerequisite to exploit appropriate strategies for therapeutic interventions aiming to reduce neuronal cell death after ischemic stroke.

Keywords: brain barriers; innate immunity; ischemia; reperfusion; vascular biology.

Figure 1.

Accumulation of neutrophils outside of the brain parenchyma following transient cerebral ischemia and reperfusion in the mouse. Neutrophils localize primarily to the subarachnoid space and penetrating arterioles during the acute phase of I/R (a). The font colors of the legend to the cartoon highlight the various cerebral structures involved in I/R. Male C57BL/6 mice were subjected to 45 min ischemia and 24 h of reperfusion. Cryosections were stained with anti-Ly6G (green) to label neutrophils and anti-pan-laminin antibodies to outline the extracellular matrix of blood vessels. There, neutrophils are shown to associate to venous blood vessels [(b), arrow] and distribute in the subarachnoid space during the acute phase of reperfusion [(b), star]. The scale bar is 50 µm. I/R, injury/reperfusion.

Figure 2.

Schematic representation of the vascular and perivascular accumulation of neutrophils in the brain after tMCAO and reperfusion. Neutrophils can cross the endothelial layer of cerebral microvessels but are confined to the perivascular space of postcapillary venules following I/R. BBB, blood–brain barrier; I/R, injury/reperfusion.