Receptor-mediated endocytosis of IL-8: A fluorescent microscopic evidence and implication of the process in ligand-induced biological response in human neutrophils

Interleukin 8 (IL-8), a neutrophil-activating and chemotactic cytokine, is known to play a key role in the pathogenesis of a large number of neutrophil-driven inflammatory diseases. Although the cytokine is rapidly internalized at 37 degrees C with its receptors, there was no direct evidence for the ligand-induced endocytosis of the receptor or that of the interaction of receptor ligand complex at 37 degrees C. As a result, our understanding about the regulation of IL-8 induced biological response is very limited. In the present study, using FITC-IL-8 conjugate as a probe, we have demonstrated the time- and temperature-dependent endocytosis of IL-8 under fluorescent microscope. We have also shown that the bright fluorescent light on the surface of neutrophils gradually disappears and it becomes almost dark after 120 min of incubation. Monodansyl cadaverine (MDC, 900 mu M), however, was found to retain the fluorescent light of FITC coupled with IL-8 on the cells. MDC and ouabain (2.5 mM) can inhibit the ligand induced endocytosis by 76% and 96%, respectively, compared to control. With respect to control, IL-8 induced biological responses e.g. IL-8 directed migration, intracellular Ca2+ release and superoxide release are significantly reduced by 77%, 94% and 76%, respectively, in presence of MDC. The study presents a direct visual evidence of the time and temperature-dependent receptor-mediated endocytosis of IL-8 which is inhibited by MDC and ouabain. This information is useful for understanding the ligand receptor interaction at 37 degrees C and mag be useful for developing anti-inflammatory agents against IL-8. (C) 1997 Academic Press Limited.