Microglia polarization : an innovative approach to alter immunological response in hemorrhagic stroke

Abstract

Introduction: Hemorrhagic stroke (HS) results in both acute and late phase neurological injury through various neuroinflammatory pathways. Microglia in HS are double edged swords that may exacerbate or attenuate such insults. This candidate hypothesized that a previous hemorrhagic stroke could modulate the inflammatory response in a subsequent stroke, with a specific focus on microglia polarization via the M1/M2 pathways and cellular mechanisms. Methods: Samples were harvested respectively from mouse brains that had undergone a previous stroke and those that encountered a stroke for the first time. Collagenase was injected into the basal ganglia of mice to induce HS. Behavioral parameters, MRI imaging, and histopathological sections were used to quantify the degree of neurological damage. Flow cytometry and qPCR were used to analyze alterations in microglia M1/M2 polarization pattern. To examine whether similar alterations would occur within microglia in vitro, and to further explore my hypothesis on a cellular level, primary cultured microglia from Day 0-3 pups were treated with a combination of red blood cell stressor, T cell coculture, and underwent immunofluorescence-labeled phagocytosis assay. Results: Animals with a past ‘mini’ stroke had better neurological function recovery, faster attenuation of hematoma and lesion volume, and better survival. In mice with a previous stroke, the expression of M2 markers was significantly higher, and M2 predominance appeared markedly in advance compared to mice without a previous HS. Results showed an altered polarization pattern of microglia within the subacute phase after a previous stroke, which may contribute to the modulation of the inflammatory response in subsequent strokes. Mechanistically, the RBC-mediated M2 polarisation of microglia was synergistically enhanced by T cells: microglia cocultured with RBCs alone resulted in mild alterations to M2 markers, whereas in the presence of T cells, they expressed an early and sustained M2 response. Conclusion: This study demonstrated microglia as possessing innate immune memory in a T cell-dependent manner, suggesting that harnessing the microglia-mediated M2 polarisation response could help ameliorate devastating HS sequelae before a prospective HS even happens.