Cytokines: powerful regulators of glial cell activation.

It is now clear that cytokines function as powerful regulators of glial cell function in the central nervous system (CNS), either inhibiting or promoting their contribution to CNS pathology. Although these interactions are complex, the availability of animals with targeted deletions of these genes and/or their receptors, as well as transgenic mice in which cytokine expression has been targeted to specific cell types, and the availability of purified populations of glia that can be studied in vitro, has provided a wealth of interesting and frequently surprising data relevant to this activity. A particular feature of many of these studies is that it is the nature of the receptor that is expressed, rather than the cytokine itself, that regulates the functional properties of these cytokines. Because cytokine receptors are themselves modulated by cytokines, it becomes evident that the effects of these cytokines may change dramatically depending upon the cytokine milieu present in the immediate environment. An additional exciting aspect of these studies is the previously underappreciated role of these factors in repair to the CNS. In this review, we focus on current information that has helped to define the role of cytokines in regulating glial cell function as it relates to the properties of microglia and astrocytes.