MMP9 and brain ischemia: This damaged BBB is the site of neutrophil and monocyte infiltration and matrix metalloproteinase 9 (MMP9) release.203 Microglia, the innate immune effector cells of the central nervous system, are activated during ischemic stroke.202,204,205 In the early stage of cerebral ischemia, microglia undergo a phenotype switch from the anti-inflammatory M2 phenotype to the pro-inflammatory M1 phenotype.206,207 The Wnt signaling pathway is involved in the toll-like receptor (TLR)-mediated central nervous system immune response.208