Endotoxin-stimulated monocytes release multiple forms of IL-1 beta, including a proIL-1 beta form whose detection is affected by export

The processing and release of 31-kDa proIL-1 beta to the mature 17-kDa form of IL-1 beta are still poorly understood. To help elucidate the mechanisms involved in IL-1 beta processing and release, we measured IL-1 beta forms released from endotoxin stimulated monocytes by inmunoprecipitation of [S-3 5] methionine-labeled protein, by Western blots, and by our recently develo ped ELISA specific for proIL-1 beta. Our studies demonstrate that in additi on to the 17-kDa mature IL-1 beta, IL-1 beta is also released as 31-, 28-, and 3-kDa molecules. The 31-kWDa-released form of proIL-1 beta represented 20-40% of the total released IL-1 beta, as measured by SDS-PAGE with densit ometry. This released proIL-1 beta was susceptible to ICE processing: howev er, this proIL-1 beta was not detectable by either a mature or proIL-1 beta -specific ELISA, suggesting that release induces a conformational change. T he ELISA, inability to detect proIL-1 beta was not due to inadequate sensit ivity or subsequent degradation in the ELISA. Furthermore, while immunoaffi nity-purified cytosolic proIL-1 beta could complex the type II IL-1R, relea sed proIL-1 beta did not. Finally, the absence of a hand shift in nondenatu ring gel electrophoresis excluded proIL-1 beta binding to another protein. These findings imply that IL-1 beta is exported from monocytes as 3-, 17-, 28-, and 31-kDa forms and that the released 31-kDa form differs from cytoso lic proIL-1 beta.