A novel plasma factor initiating complement activation on cetylmannoside-modified liposomes in human plasma

Abstract

The possibility that the human complement (C) system plays a critical role in both destabilization and opsonization of the liposomes modified with cetylmannoside (Man-MLVs) was demonstrated in our previous in vitro study. In this study, our attention was focused on the underlying mechanism of activation of the C system by the Man-MLVs. It was found that the Man-MLVs had significant ability to activate the C system and the activation proceeded through the C1q-dependent classical pathway. Pretreatment of human plasma with Man-MLVs at low temperature (4°C) diminished the ability of the C system to destabilize the homologous liposomes without affecting the ability to lyse rabbit erythrocytes. However, the C-dependent destabilization did not disappear by the pretreatment with PC-MLVs. The results suggest that a plasma factor, which specifically adsorbs to Man-MLVs but not PC-MLVs, is essential for initiating the C activation. The plasma factor was obviously distinguished from any known classical C pathway (CCP) activators, since the adsorption of the plasma factor to Man-MLVs was not inhibited by treatment with EGTA/Mg 2+, soluble sugars (GlcNAc, ManNAc or D-mannose) or Con-A Sepharose. In addition, isolation of the plasma factor was attempted by the following procedure: PEG precipitation, DEAE Sepharose chromatography and gel filtration chromatography. In a series of these isolations, the plasma factor was found to be a protein with higher molecular weight at least than 669 000 Da. It was concluded that the activation of the human C system by the Man-MLVs proceeded through the CCP in the presence of a novel plasma factor.