The expression of cytokines in the organs of normal individuals: role in homeostasis. A review.

Cytokines such as interferons (IFN), tumor necrosis factor (TNF), interleukins (IL), and growth factors are suggested to occupy a central position in a putative network of cytokine interactions in vivo which act to maintain homeostasis in normal tissues. Recent results suggest that specific interferon genes are transcribed at low levels in the organs of normal individuals in the apparent absence of virus infection while other interferon genes remain unexpressed in the same tissues. In contrast, the genes of cytokines such as IL-1, TNF, and IFN beta-2/IL-6 appear to be regulated in vivo in a manner quite distinct from that of interferon genes as these cytokines are expressed at high levels in the spleen, liver, and peripheral blood leukocytes of normal individuals. The localized production and action of a cytokine could provide a mean of attaining specificity of action for multifunctional cytokines. Thus under physiological conditions the activity expressed by a particular cytokine may be determined by tissue specific interactions which may in turn be influenced by other cytokines. Abnormal production of a particular cytokine would lead to a perturbation of homeostasis and may contribute to the pathogenesis of certain autoimmune or inflammatory diseases.