Interleukin 1/3 Propeptide Is Detected Intracellularly and Extracellularly When Human Monocytes Are Stimulated with LPS In Vitro

Human interleukin 1/$ (IL-1B) is synthesized as an inactive precursor that is cleaved by IL-1 converting enzyme (ICE) between Asp 116 and Ala 11y to form COOH-terminal mature IL-1/3 and NH2-terminal IL-1B propeptide. Little is known about the fate of the NH2-terminal cleavage product. In this study, human recombinant (hr)IL-1/3 propeptide (amino acids 2-116) was produced and used to prepare specific antibodies which do not recognize mature human IL-1B. These anti-propeptide antibodies were used for immunoprecipitation of biosynthetically labeled proteins from lipopolysaccharide-stimulated human monocytes. Analysis of immunopredpitates by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography revealed that these antibodies recognize precursor IL-I~ and two unique proteins: one migrating at 17.5 kD and one at 14 kD. The larger of these two proteins has a migration nearly identical to that of the recombinant IL-1/3 propeptide, and most likely represents naturally derived propeptide. The protein migrating at 14 kD may result from a second cleavage by ICE, between Asp 27 and Gly 2s. These proteins accumulate intracellularly and extracellularly during pulse-chase experiments, and therefore represent stable products of precursor IL-1B cleavage.