Microglia/Macrophage Polarization After Experimental Intracerebral Hemorrhage

Excerpt

Microglia are the resident macrophages of the brain and are the first responders to immune system activation in that tissue. Under resting conditions, microglia are ramified and monitor the brain microenvironment maintaining neuron, astrocyte, and oligodendrocyte coordination. In conditions such as intracerebral hemorrhage (ICH), traumatic brain injury (TBI), subarachnoid hemorrhage, and cerebral ischemia, microglia become activated as part of the injury response [1‐7]. Upon activation, microglia transform into an amoeboid phenotype and consequently play roles in inflammatory reactions and phagocytosis of necrotic tissue. However, based on protein and mRNA expression, activated microglia have different phenotypes. Classically, these have been termed M1- and M2-polarized microglia/macrophages and they have different roles in brain injury [4]. Until now, most studies on microglial polarization have focused on TBI and cerebral ischemia, and relatively little is known about the polarization of microglia/macrophages following ICH. In this commentary, we will discuss some known molecular mechanisms that mediate microglial phenotype transformation, discuss new therapeutic approaches to altering the balance of microglial activation phenotypes, and identify future directions that could aid in long-term recovery after ICH. …