Relationship of TNF to interleukins.

It is evident from this review that TNF exhibits complex interactions with other cytokines at the level of production and in its effects. Studies designed to determine the role of TNF in the animal models or cell culture system using pure recombinant molecules have revealed that TNF never operates by itself, but instead operates within a network of cytokines. First, the multitude of exogenous as well as endogenous signals, which induce TNF production, concomitantly also stimulate the production of a battery of other inflammatory cytokines: IL-1, IL-6, IL-8, multiple CSFs, IFN, and TGF-beta. Moreover, TNF itself stimulates the production of most of these cytokines. Thus even when pure recombinant TNF is used, it readily generates the production of other interactive cytokines. This apparent redundancy in the production of cytokines with overlapping effects presumably has protective advantage for the host. Furthermore, interaction of these cytokines is more economical and amplifies the responses to subtoxic doses of potentially harmful cytokines. Cytokine interaction may lead to either synergistic (as for many TNF-IL-1 interactions) or antagonistic effects (TNF and TGF-beta, for example). These may depend on (1) the modulation of receptor expression of one cytokine by another (IFN-gamma-enhancing receptor expression for TNF, and TGF-beta down-regulation of IL-1 receptors), (2) stabilization of the cytokine message by one another (induction of IL-6 by TNF or IL-1), (3) interactions at the level of signal transduction, (4) gene expression, or (5) at the posttranslational level. Thus the receptor repertoire, which is a function of the cell type and stage of development, actually determines the net effects of a particular combination of interactive cytokines. Clearly, the mechanisms of these interactions will need to be elucidated to better understand their biological function and to permit cytokines to be used clinically to the advantage of the host.