|dc.description.abstracteng||Neuromyelitis optica (NMO) is a demyelinating autoimmune disease of the central nervous system (CNS), with serum anti-aquaporin4 (AQP4) antibodies (Ab) detected in the majority of patients. Binding of NMO-Ab to AQP4 results in complement- and cell-mediated astrocyte depletion. In addition, a severe impairment of the blood-brain barrier (BBB) is observed as evidenced by gadolinium-enhanced lesions on magnetic resonance imaging (MRI). To investigate the structural, molecular and cellular correlates of the BBB breakdown, we employed a focal model of NMO-like lesions in rats which is based on the intracerebral injection of a human recombinant NMO-Ab directed against AQP4, and human complement.
Astrocytes and pericytes are considered crucial for maintenance and repair of the BBB. In NMO-like lesions, we demonstrated that a transient breakdown of the BBB coincided with the onset of astrocyte loss. However, the BBB integrity to vascular tracers was rapidly restored, even in the absence of astrocytes. No loss of pericytes from NMO-like lesions was observed indicating that pericytes might contribute to the rapid restoration of the BBB.
Tight junctions (TJ) restrict the paracellular diffusion of solutes across the BBB. Therefore, we analyzed the TJ in NMO-like lesions and detected a transient loss of the TJ protein occludin, while the expression of claudin-3 and claudin-5 was not altered. However, no morphological alterations of the TJ were observed on the ultrastructural level, and the integrity of the BBB to vascular tracers was re-established in the absence of occludin.
Inflammatory cells infiltrating the CNS were shown to contribute to BBB disruption. In early NMO-like lesions we demonstrated that polymorphonuclear cells (PMN) were the most abundant infiltrating leukocytes and that the number of PMN in the lesions correlated with the extent of vascular tracer extravasation. Depletion of PMN prevented not only the breakdown of the BBB, but interestingly also the loss of astrocytes. Furthermore, we could show that inhibition of the complement component C5a receptor (C5aR), which is important for PMN attraction and activation, reduced the area of astrocyte loss in NMO-like lesions. In addition, proteases released from PMN granules upon activation were implied in BBB disruption. We demonstrated that matrix metalloproteinase 9 (MMP9) decreased the electrical resistance of endothelial monolayers and that inhibition of the neutrophil elastase (NE), another PMN protease, reduced the loss of astrocytes and PMN infiltration in vivo.
This study affirms the importance of PMN in the development of NMO-like lesions and is the first to show that infiltrating PMN mediate the breakdown of the BBB. These findings, together with the detection of PMN in early human NMO lesions, identify the PMN mediated breakdown of the BBB as a promising target for future therapeutic approaches.||de