Interleukin-6: molecular pathophysiology.

The cytokine interleukin-6 (IL-6) has emerged as a major systemic alarm signal which appears to be produced by essentially every injured tissue. Recent evidence points to the skin, particularly the injured skin, as one of the major sites of IL-6 production. The hallmark of IL-6 gene regulation is its induction by inflammation-associated cytokines, bacterial products, virus infection, and activation of any of the three major signal transduction pathways (diacylglycerol-, cAMP-, and Ca(++)-activated). Many of these inducers act largely through a 23-bp "multiple-response element" in the IL-6 promoter. Different cell types, including keratinocytes, secrete multiple post-translationally modified forms of IL-6. This cytokine, in turn, plays a key role in activating a variety of local and systemic host defense mechanisms that are aimed at limiting tissue injury. Thus, IL-6 elicits major changes in the biochemical, physiologic, and immunologic status of the host (e.g., the "acute phase" plasma protein response; activation of B, T, and NK-cell function). IL-6 enhances the proliferation of human keratinocytes and of many B-cell lines but inhibits that of certain carcinoma cell lines; nevertheless, IL-6 can enhance the motility of these carcinoma cells. Elevated levels of IL-6 are observed in human body fluids during acute and chronic infections, neoplasia, autoimmune diseases, and psoriasis and following third-degree burns. It is likely that IL-6 produced by cellular elements in the skin represents an important means of communication between the external environment and the millieu interieur.