Microglia have been considered to be the first line of defense in the CNS [91] , a hypothesis that has been supported by the finding that microglia actively screen their microenvironment with highly motile processes; thus, the brain is under continual surveillance by microglia. To do this with high efficiency, microglia must be variable, adaptive to their environment and capable of integrating various inputs and responding appropriately [92, 93] . All of these processes require significant interactions with other components within the same environment, including neurons and astrocytes.