Immune and inflammatory responses in the CNS: modulation by astrocytes.

Because the skull bones, the cerebrospinal fluid, the blood-brain barrier (BBB), and the meninges effectively shield the central nervous system from other tissues, it was proposed that the brain is an 'immunologically privileged' organ. However, with recent evidence that in response to invasion by microorganisms, resident cells, such as astrocytes and microglia can fully mount an immune response, this long-standing view has been rethought and revised. Over the last two decades, both astrocytes and microglia have been shown to secrete numerous cytokines, and, therefore, it is presently widely accepted that these cells actively participate in an integrative communicative pathway between resident immune cells of the CNS and those of the periphery. While clearly implicated in the initiation, maintenance, and suppression of immune responses, cytokines produced by these cells (e.g. astrocytes and microglia), as well as the responses of these cells to cytokines produced elsewhere, has also been shown to propagate CNS damage. Therefore the potential involvement of these cells in neurodegenerative disorders has been raised and subjected to intense experimentation. The objective of this synopsis is to review the role played by astrocytes in the initiation and modulation of immune responses.