Innate response to focal necrotic injury inside the blood-brain barrier.

We have studied the initial innate immune response to focal necrotic injury on different sides of the mouse blood-brain barrier by two-photon intravital microscopy. Transgenic mice in which the promoter of the myeloid isoform of lysozyme drives GFP were used to track granulocytes and monocytes. Necrotic injury in the meninges, but not the brain parenchyma, recruited GFP+ cells within minutes that fully surrounded the necrotic site within a day. Recently, it has been suggested that microglial cells and astrocytes cooperate to mount a distinct response to laser injury behind the blood-brain barrier. We followed the microglial response in heterozygous knockin mice in which GFP replaces CX3CR1 coding sequence. Prior to injury, microglial cell bodies were immobile over days, but moved to the laser injury site within 1 day. We followed astrocytes, which have been proposed to cooperate with microglial cells in response to focal injury, using transgenic mice in which glial fibrillary acidic protein promoter drives GFP expression. Before injury fine astrocyte processes permeate the parenchyma. Astrocytes polarized toward the injury in an ATP, connexin hemichannels, and intracellular Ca2+ -dependent process. The astrocytes network established a cytoplasmic Ca2+ gradient that preceded the microglial response. This is consistent with astrocyte-microglial collaboration to mount this innate response that excludes blood leukocytes.