Simultaneous activation of border-associated immune cells and glial cells at the CNS-meningeal interface after subarachnoid haemorrhage in rats

Border-associated macrophages (BAM) and mast cells are resident immune cells at the peripheral CNS borders, strategically located close to the brain surface, potentially influencing the homeostasis of the underlying parenchyma. Subarachnoid haemorrhage (SAH), when blood enters between the meningeal layers that cover the brain, is associated with neuroinflammation, which has been shown to play a critical role in subsequent brain damage; however, the impact of the activation of border-associated immune cells on the pathomechanism of the disease has not been investigated. Our aim was to examine inflammatory reactions that occur simultaneously at the cellular level in various compartments of the CNS: meningeal, subdural space, and parenchyma after experimental SAH in rats. Using immunohistochemistry, we performed the morphological characterisation of the BAM subpopulations in meningeal preparations. Additionally, confocal microscopy and image analysis were used to evaluate the reactive state of microglia cells and the integrity of the glial boundary in the upper fronto-parietal cortex of the rat 72 h after SAH. We demonstrated morphological alterations of BAM populations in the meninges and parallel disintegration of the astrocyte barrier due to haemorrhage. Furthermore, we confirmed the crucial role of mast cells in subsequent glial reactions. Our results suggest that activation of border-associated immune cells, contemporaneously with the early neuroinflammatory reactions that take place in the brain parenchyma, proposes a feasible signalling between these compartments following haemorrhage. Further studies are to be performed to reveal the importance of CNS meningeal border as a communication interface in the pathomechanism of SAH.