Introduction: Microglia become activated and monocytes become macrophages following intracerebral hemorrhage (ICH). They are often called “microglia/macrophage” since it is difficult to differentiate between them. The neutrophils also infiltrate into the injured brain after ICH. Those cells are capable of releasing numerous detrimental mediators.
Hypothesis: The activation of microglia/macrophage and the infiltrated neutrophils promote brain injury after ICH and that inhibition both of them attenuates brain injury following ICH.
Methods: 10-μl of autologous blood obtained from tail was introduced into the right striatum of adult male mice to produce ICH injury. We used transgenic CD11b thymidine kinase (CD11b-TK) mice where proliferating microglia/macrophage were removed by ganciclovir treatment. Functional grade purified anti-mouse Ly6G/Gr-1 antibodies were used to reduce the neutrophil infiltration. Groups of CD11b-TK mice were treated with ganciclovir and/or anti-mouse Ly6G/Gr-1 antibodies. These mice were killed at 3 days after ICH injury and brain sections were stained for various parameters.
Results: ICH injury resulted in activation of microglia/macrophages through 1-7 days of ICH insult. The area of brain damage peaked at 2-3 days; the number of dying neurons peaked at 1-3 days, the number of infiltrated neutrophils peaked at 1-3 days, while the activation of microglia/macrophages peaked at days 3-4. These findings led us to choose day 3 for further studies. The activation of microglia/macrophage was significantly reduced in CD11b-TK transgenic mice with ganciclovir treatment after ICH. The numbers of infiltrated neutrophils were significantly reduced with anti-mouse Ly6G/Gr-1 antibodies treatment after ICH. Correspondingly, the area of brain damage and the extent of neuronal death were minimal in CD11b-TK mice with ganciclovir and anti-LyG6/Gr-1 antibodies treatment after ICH.
Conclusions: Both activated microglia/macrophages and the infiltrated neutrophils after ICH promote brain injury. Thus inhibition of their activities reduces ICH brain injury. These results shed light on the advent of new medications for ICH patients, including microglia deactivators and the antibodies to alleviate neutrophil infiltration.