Interleukin-6 (IL-6), a cytokine consisting of 184 amino acids, belongs to a grou of polypeptide hormones and cytokines which according to a prediction by Bazan share a structure characterized by a bundle of four antiparallel a-helices. This family which also includes growth hormone, prolactin, and many cytokines-e. J., granulocyte colony stimulating factor (G-CSF), erythropoietin (EPO), leukemia inhibitory factor (LIF), oncostatin (OM), and ciliary neurotrophic factor (CNTF)-has therefore been called the a-helical cytokine family. The predicted tertiary structure was confirmed by X-ray analysis of growth hormone, IL-4, and G-CSF?-* and IL-6 is expected to have a similar structure. So far, however, attempts to crystallize IL-6 have failed in several laboratories.

IL-6 is produced by many different cell types, particularly after stimulation with IL-1. In monocytes, bacterial lipopolysaccharides (LPS) are important inducers of IL-6 synthesis and secretion. In turn, IL-6 acts on a wide spectrum of target cells, showing a pleiotropic spectrum of IL-6 triggers B-cells to differentiate to plasma cells and to synthesize and secrete antibodies, and is a maturation factor for T-cells. Furthermore, IL-6 has been reported to be an autocrine growth factor for