Background: Red blood cells (RBC) and other blood components deposited in brain parenchyma during intracerebral hemorrhage (ICH) are the source of secondary brain injury, inflammation, and oxidative stress. Therefore, a fast and efficient removal of the blood from the brain is essential for ameliorating secondary injury and for recovery and repair process. Microglia/macrophages (MΦ)-mediated phagocytosis is a key component of hematoma clearance after ICH. However, the high levels of pro-oxidative molecules (including H2O2) generated by MΦ during phagocytosis could be highly cytotoxic not only to brain cells, but also to MΦ themselves. Thus, an efficient coupling between MΦ-mediated phagocytosis and anti-oxidative processes is essential for the safe and efficient hematoma cleanup.
Methods and Results: H2O2 at higher concentrations is known to be neurotoxic. In this study, we established that H2O2 at submicromolar, biologically relevant concentrations, acts as a pro-survival factor for MΦ. By activating transcription factor nuclear factor-erythroid 2 p45-related factor 2 (Nrf2), a master regulator of anti-oxidative regulation, H2O2 stimulates the expression of many antioxidant proteins, which protects MΦ from oxidative injury and damaging components of inflammation. We established that Nrf2-deficient MΦ are more susceptible to H2O2-mediated or toxic blood components-mediated damage (“ICH-like” injury). Nrf2-KO mice subjected to ICH experienced more severe brain edema and delayed hematoma resolution. In addition, the phagocytosis of RBCs (in vitro model of hematoma clearance) was significantly diminished in MΦ treated with Nrf2 decoy or in MΦ that were harvested from Nrf2-KO mice. On the other hand, pharmacologic activation of Nrf2 or administration of low dosages of H2O2improved phagocytic capacity of MΦ toward RBC. Furthermore, low levels of H2O2 protected MΦ from ICH-like injury, suggesting that improved phagocytic function could involve preservation of MΦ’ integrity by activating Nrf2.
Conclusion: H2O2 at low doses could protect MΦ for oxidative injury and facilitate their phagocytic function, which could benefit hematoma clearance, inflammation resolution, and improve neurological recovery after ICH.