Secretory leukocyte protease inhibitor interferes with uptake of lipopolysaccharide by macrophages

Macrophages are among the most sensitive targets of bacterial endotoxin (LPS), responding to minute amounts of LPS by releasing a battery of inflammatory mediators. Transfection of macrophages with secretory leukocyte protease inhibitor (SLPI) renders these cells refractory to LPS stimulation. Here we show that uptake of LPS from soluble CD14 (sCD14)-LPS complexes by SLPI-overexpressing cells was only 50% of that seen in control cells. SLPI transfectants and mock transfectants did not differ in the surface expression of CD14 or CD18, We show, in addition, that recombinant human SLPI can bind to purified endotoxin in vitro, SLPI caused a decrease in the binding of LPS to sCD14 as assessed both by fluorescence quenching of labeled LPS and by nondenaturing polyacrylamide gel electrophoresis, These results suggest that the inhibitory effect of SLPI on macrophage responses to LPS may, in part, be due to its blockade of LPS transfer to soluble CD14 and its interference with uptake of LPS from LPS-sCD14 complexes by macrophages.