The taming of IL-12: suppressing the production of proinflammatory cytokines.

Interleukin (IL)-12 is a cytokine that possesses both proinflammatory and immunoregulatory activity. IL-12, and the interferon-gamma (IFN-gamma) that is induced by IL-12, play central roles in the development of the Th1-type immune responses that are required for immunity to intracellular pathogens. Recently a number of these pathogens, including Leishmania, measles virus, and human immunodeficiency virus (HIV), have been shown to subvert the development of cell-mediated immunity by actively inhibiting the production of IL-12. Similarly, the ligation of phagocytic receptors on macrophages has also been shown to suppress IL-12 production. The suppression of IL-12 production by receptor ligation occurs by at least two distinct mechanisms: one involves a direct inhibition of gene transcription and the other depends on the production of inhibitory cytokines. We review studies in which IL-12 has been experimentally manipulated, and we compare the mechanisms by which this regulation can occur. Because the IL-12 that is produced during acute inflammation and chronic autoimmune disorders can lead to exacerbated disease, the development of pharmacological means to suppress IL-12 production is currently under investigation. This review focuses on the production of IL-12 by antigen-presenting cells and the methods by which the down-regulation of IL-12 production can be exploited either by pathogens or for therapeutic ends.