Monocyte depletion prior to intracerebral hemorrhage worsens cognitive and motor outcome
Peters, Nicholas Vette Hubbard
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A number of studies have cited immune cell infiltration as a mechanism for secondary injury following intracerebral hemorrhage. However, the role of monocytes remains poorly understood. We hypothesized that monocytes are a beneficial cell type that help remove extravasated blood and other pathogenic substances that enter brain parenchymal tissue during a hemorrhagic stroke. Using macrophage Fas-induced apoptosis (MAFIA) mice, this study documents a time course of monocyte infiltration into brain tissue. We then systemically knocked down monocytes prior to initiating an intracerebral hemorrhage, and studied the motor and cognitive outcomes relative to a vehicle treated group. The genetic alteration present in MAFIA mice causes the co-expression of green fluorescent protein and a suicide protein, exclusively in monocytes and dendritic cells. We established a time course of monocyte infiltration by counting cells expressing green fluorescent protein in brain parenchymal tissue at four time points after initiating a severe collagenase-induced hemorrhage. We found that at the 48-hour time point there was a significant increase of immune cells infiltrating the core of the lesion relative to that observed at 6 hours and 24 hours, and non-significant decline at the 7-day time point. Furthermore, the administration of AP20187 to MAFIA mice causes the dimerization of the suicide proteins, which initiates Fas-induced apoptosis selectively in cells expressing the suicide protein. By administering10uL of AP20187 per animal for four consecutive days, we were able to knockdown monocytes both systemically and in brain parenchymal tissue following hemorrhage. To investigate the role of monocytes following intracerebral hemorrhage, we compared a monocyte-depleted group of MAFIA mice to a vehicle-treated group. Both groups underwent a collagenase-induced hemorrhage, and then were subjected to wire grip testing and Morris Water Maze. The group of monocyte-depleted MAFIA mice performed worse in wire grip testing (p < 0.02), as well as in the Morris Water Maze test (p < 0.01). Through the course of the 3- week experiment there was 50% mortality in the monocyte-depleted group, while no animals died in the vehicle treated group. Both of the functional studies and the mortality data suggest that monocytes play an important role following intracerebral hemorrhage. The infiltration data suggests that monocytes are recruited to parenchymal tissue after 6 and 24 hours following hemorrhage and remain present for at least one week. This time fame offers a large window for therapeutic modulation of monocyte recruitment and/or function. Future studies should focus on identifying monocytes’ mechanisms of action, as well as improving the MAFIA mouse model by controlling for the weight loss and the cause of profound splenomegaly observed in the monocyte-depleted group.
Thesis (M.A.)--Boston University