TH1/TH2 cytokines in the central nervous system.

For the past 20 years it has become increasingly evident that cytokines play an important role in both the normal development of the brain, acting as neurotrophic factors, and in brain injuries. Although cytokines and their receptors are synthesized and expressed in the brain (normally at low levels), increased cytokine production levels are now associated with various neurological disorders. T lymphocytes are the cells responsible for coordinating the immune response and a major source of cytokines. Different cytokines induce different subsets of T cells or have different effects on proliferation within a particular subset. Recent studies suggest that the immune response is in fact regulated by the balance between Th1 and Th2 cytokines. These two pathways are often mutually exclusive, the one resulting in protection and the other in progression of disease. Various studies describe the function and production of proinflammatory cytokines in the central nervous system (CNS) and their role in health and disease. Inflammation is upregulated following activation of Th1 cells, whereas Th2 cells may play a significant role in downregulating Th1 proinflammatory responses in those instances in which there is overproduction of Th2 cytokines. Although both Th1 and Th2 cytokines may influence CNS functioning, most studies have so far dealt with proinflammatory cytokines, probably because they directly affect CNS cells and are thought to be implicated in CNS pathology. It is of interest that endogenous glucocorticoids also control Th1-Th2 balance, favoring Th2 cell development. This review presents the evidence that cytokines have important functions in the CNS, both during development and as a part of brain pathology. In particular, the author highlighted recent work that supports a major role for the so-called inflammatory cytokines, Th1, and the anti-inflammatory Th2 cytokines.