In this study we have investigated the specificity of bioassays in which interleukin (IL) 1 and/or IL 6 are active. The thymocyte assay cannot be used to discriminate between IL 1 and IL 6; both monokines are active in this assay. Moreover the detection limit for both IL 1 and IL 6 is around 100 pg/ml. IL 6 activity can be measured with a murine hybridoma cell line (B9). The detection limit for human as well as murine IL 6 is about 0.5 pg/ml. The assay is specific for IL 6 and is not influenced by a variety of other cytokines except for murine IL 4 which shows some activity in this variety of other cytokines except for murine IL 4 which shows some activity in this assay. IL 1 can be measured specifically with D10 cells. The detection limit for IL 1 alpha and IL 1 beta is around 1 pg/ml whereas IL 6 is not active in this assay at all. Upon stimulation by IL 1 and/or IL 2 D10 cells produce IL 6. However, this IL 6 does not seem to be involved in the proliferation of these cells.
Human monocytes produce a number of soluble mediators involved in regulation of inflammation and lymphocyte growth and differentiation such as interleukin 1 (IL 1) and tumor necrosis factor. Recently, the cDNA of another monocyte-derived factor, interleukin 6 (IL 6), was cloned. Herein we show that purified E. coli-derived recombinant IL 6 (rIL 6) is as active as IL 1 in the thymocyte assay. In addition, IL 1 and IL 6 synergize strongly in stimulating thymocyte proliferation. Another property shared by IL 1 and IL 6 is their pyrogenicity. Human rIL 6 induces a monophasic fever after i.v. injection into rabbits. Together with the observation that IL 1 induces IL 6 in a variety of cells including thymocytes, our data suggest that IL 6 is involved in many of the pleiotropic effects of IL 1.
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