Astrocytes as mediators of immune and inflammatory responses in the CNS

The long-standing view that the brain is isolated from the effects of the immune system has recently been challenged, with experimental evidence suggesting that in response to invasion by microorganisms, the CNS can mount its own defense by resident cells, such as the microglia and astrocytes. Both cell types produce and secrete a number of cytokines and therefore can potentially modulate and integrate the communication between hematogenous cells and resident cells of the CNS. This manuscript will commence with a brief overview of astrocytic functions in the CNS, and proceed to discuss astrocytic responses that may regulate CNS inflammation. Specifically, it will address (1) the function of astrocytes as the antigen presenting cells (APCs) of the CNS, and (2) the role afforded by astrocyte-derived cytokines, and astrocytic responses to cytokines secreted elsewhere, in mediating and sustaining immune responses. Finally, some recent experimental evidence on the possibility that astroglial impairment by pathogens may contribute to the etiology of neurologic diseases will be highlighted.