Soluble murine IL-1 receptor type I induces release of constitutive IL-1 alpha

IL-1 alpha and IL-1 beta are proinflammatory cytokines involved in the pathogenesis of many infectious and noninfectious inflammatory diseases. To reduce IL-1 toxicity, extracellular domains of the soluble (s) IL-1R are shed from cell membranes and prevent triggering of cell-bound receptors. We investigated to what extent murine sIL-1RI can neutralize the IL-1 produced by LPS-stimulated macrophages. When mouse peritoneal macrophages were incubated with LPS, addition of sIL-1RI significantly inhibited the bioactivity of IL-1. Stimulation of cells with sIL-1RI alone induced no bioactive IL-1. When immunoreactive cytokine concentrations were measured with specific radioimmunoassays, sIL-1RI alone appeared to induce a significant release of IL-1 alpha in a concentration-dependent manner. This effect was independent of new protein synthesis. The production of IL-1 beta or TNF-alpha was not influenced by sIL-1RI. There was no interference of sIL-1RI with the IL-1 alpha radioimmunoassay. In mice, an i.v. injection of sIL-RI alone induced a rapid release of IL-1 alpha, but not of TNF-alpha or IL-1 beta. Treatment of mice with sIL-1RI improved the survival during a lethal infection with Candida albicans. In conclusion, sIL-1RI induces a rapid release of IL-1 alpha from cells, as well as into the systemic circulation. Although this IL-1 alpha may be inactivated in circulation by the same sIL-1RI, this phenomenon probably has immunostimulatory effects at local levels where the sIL-1RI-induced IL-1 alpha acts in a paracrine or autocrine manner.