IL-1 beta is secreted by activated murine macrophages as biologically inactive precursor

IL-1 alpha and IL-beta are distinct cytokines, produced by activated macrophages. The temporal sequence in the processing and secretion as well as the mechanism(s) by which IL-1 is secreted from the cells remain undefined. Here we have studied the production of IL-1 from murine macrophages after stimulation with LPS or Listeria monocytogenes by two distinct methods: i) immunoprecipitation of radio-labeled IL-1 peptides from culture supernatants, and ii) determination of IL-1 activity by neutralization with monospecific antisera to either form of IL-1. We confirmed that precursor and mature forms of both IL-1 alpha and IL-1 beta can be detected in the culture supernatants after stimulation of the macrophages with 10 to 20 micrograms LPS/ml but, in addition, we report the novel finding that IL-1 beta is exclusively secreted in its unprocessed precursor form after stimulation of the cells with either 0.5 to 1 microgram LPS/ml or with L. monocytogenes. Exposure of the cells to increasing amounts of LPS led to the appearance of a 20-kDa IL-1 beta peptide in the culture supernatants concomitant with the release of a processing activity for the IL-1 beta precursor. These data therefore suggest that, in a first step, IL-1 beta is secreted as an unprocessed precursor protein that in a second, postsecretory step is cleaved by a LPS-inducible protease, thus generating the 20-kDa IL-1 beta peptide. The latter represents the biologically active IL-1 beta inasmuch as the generation of IL-1 beta activity in the culture supernatants strictly correlated with the appearance of the 20-kDa IL-1 beta peptide.