β 2 -Glycoprotein I (Apolipoprotein H) Modulates Uptake and Endocytosis Associated Chemiluminescence in Rat Kupffer Cells

Abstract

g 2-Glycoprotein I (g 2GPI) is known to influence macrophage uptake of particles with phosphatidylserine containing surfaces, as apoptotic thymocytes and unilamellar vesicles in vitro. Nevertheless, effects upon macrophage activation induced by this interaction are still unknown. g 2GPI influence upon the reactive species production by Kupffer cells was evaluted in order to investigate whether g 2GPI modulates the macrophage response to negatively charged surfaces. Chemiluminescence of isolated non-parenchymal rat liver cells was measured after phagocytosis of opsonized zymosan or phorbolmyristate acetate (PMA) stimulation, in the presence and absence of large unilamellar vesicles (LUVs) containing 25mol% phosphatidylserine (PS) or 50mol% cardiolipin (CL) and complementary molar ratio of phosphatidylcholine (PC). g 2GPI decreased by 50% the chemiluminescence response induced by opsonized zymosan, with a 66% reduction of the initial light emission rate. PMA stimulated Kupffer cell chemiluminescence was insensitive to human or rat g 2GPI. Albumin (500 w g/ml) showed no effect upon chemiluminescence. g 2GPI increased PS/PC LUV uptake and degradation by Kupffer cells in a concentration-dependent manner, without leakage of the internal contents of the LUVs, as shown by fluorescence intensity enhancement. LUVs opsonized with antiphospholipid antibodies (aPL) from syphilitic patients increased light emission by Kupffer cells. Addition of g 2GPI to the assay reduced chemiluminescence due to opsonization with purified IgG antibodies from systemic lupus erythematosus (SLE or syphilis (Sy) patient sera. A marked net increase in chemiluminescence is observed in the presence of Sy aPL antibodies, whereas a decrease was found when SLE aPL were added to the assay, in the presence or absence of g 2GPI. At a concentration of 125 w g/ml, g 2 GPI significantly reduced Kupffer cell Candida albicans phagocytosis index and killing score by 50 and 10%, respectively. The present data strongly suggest that particle uptake in the presence of g 2GPI is coupled to an inhibition of reactive species production by liver macrophages during the respiratory burst, supporting the role of g 2GPI as a mediator of senescent cell removal.