Release of IL-1 from mononuclear phagocytes.
IL-1 alpha and -beta are 31- and 34-kDa cytokines produced by stimulated monocytes, macrophages, and a variety of other cells. These proteins are thought to function primarily as intercellular mediators and can be detected in plasma and the supernatants of cultured cells; however, IL-1 alpha and -beta contain no identifiable signal peptides and are not secreted via the classical secretory pathway. To understand the mechanism of IL-1 release, we have analyzed IL-1 production by LPS-stimulated mononuclear cells. IL-1 was quantified by bioassay, immunoprecipitation, and ELISA. Of these techniques, only immunoprecipitation permitted the quantitative detection of intracellular pro-IL-1. Both the full-length pro-forms and proteolytically processed mature forms of IL-1 were detected in culture supernatants; however, for macrophages the released material represented less than 5% of the total IL-1 alpha and -beta synthesized. Freshly isolated human monocytes released a higher fraction of their total IL-1 (up to 22%): however, monocytes cultured in vitro for 24 h showed very little fractional release, similar to macrophages. Nonspecific release of intracellular contents was determined by measurement of release of lactate dehydrogenase activity and was found to parallel IL-1 release. In fact the higher release of IL-1 from freshly cultured human monocytes correlated also with an increase in the release of lactate dehydrogenase. We conclude that, in cultured LPS-stimulated monocytic cells, IL-1 is not released via a novel secretory pathway, but exits the cell via a nonspecific pathway, most likely as a consequence of cellular injury.
Hogquist
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Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110.
Unanue
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Chaplin
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D000818:Animals; D002478:Cells, Cultured; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D007375:Interleukin-1 / Q000032:analysis / Q000276:immunology / Q000378:metabolism*; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D008264:Macrophages / Q000378:metabolism*; D051379:Mice; D008815:Mice, Inbred Strains; D009000:Monocytes / Q000378:metabolism*; D011233:Precipitin Tests
D007375:Interleukin-1; D007770:L-Lactate Dehydrogenase