Interleukin-6 infusion during human endotoxaemia inhibits in vitro release of the urokinase receptor from peripheral blood mononuclear cells

Leucocyte expression of the urokinase receptor [urokinase-type plasminogen activator receptor (uPAR)] is regulated by inflammatory mediators. This study investigated the in vivo effect of endotoxin, interleukin (IL)-6 and tumour necrosis factor (TNF)-alpha on uPAR-release in vivo and in vitro in humans. Healthy subjects received intravenous endotoxin injection [high-dose, 2 ng/kg (n = 8) and low-dose, 0.06 ng/kg (n = 7)], coadministration of 0.06 ng/kg endotoxin and 3 h recombinant human (rh)IL-6 infusion (n = 7) or 3 h infusion of rhIL-6 (n = 6), rhTNF-alpha (n = 6) or NaCl (n = 5). Soluble uPAR (suPAR) was measured by enzyme-linked immunosorbent assay in plasma and supernatants from unstimulated and phytohaemagglutinin and lipopolysaccharide-stimulated peripheral blood mononuclear cell (PBMC) cultures incubated for 24 h. The spontaneous and stimulated uPAR-release from PBMC cultures was enhanced 5 h after low-dose endotoxin (both P < 0.05), but coadministration of rhIL-6 during low-dose endotoxaemia abolished this enhanced uPAR release. High-dose endotoxin increased plasma suPAR levels (P < 0.001) whereas low-dose endotoxin, rhIL-6 or TNF-alpha did not influence uPAR release in vivo to such degree that a systemic effect on the plasma suPAR level was detectable. Even subclinical doses of endotoxin in vivo enhance the capacity of PBMC to release uPAR after incubation in vitro. The inhibitory effect of IL-6 on endotoxin-mediated uPAR-release in vitro suggests that IL-6 has anti-inflammatory effects on endotoxin-mediated inflammation.