LIPOPOLYSACCHARIDE-BINDING PROTEIN AND CD14 MODULATE THE SYNTHESIS OFPLATELET-ACTIVATING-FACTOR BY HUMAN MONOCYTES AND MESANGIAL AND ENDOTHELIAL-CELLS STIMULATED WITH LIPOPOLYSACCHARIDE

The biosynthesis of platelet-activating factor (PAF) during Gram-negat ive sepsis involves the interaction of LPS with the cells of the host. We have investigated the molecular mechanism that controls. cell reco gnition and PAF biosynthetic response to LPS in human monocytes (MO), glomerular mesangial cells (MC), and HUVEC in culture. The synthesis o f PAF by MO and MC involves two proteins, plasma LPS binding protein ( LBP) and cell membrane CD14 (mCD14). As MO, MC were shown to express t he mCD14 molecule by several mAbs, MO and mCD14-positive MC were stimu lated to synthesize PAF either by the 63D3 and IOM-2 mAbs or by the na tural ligand LBP-LPS complex. Moreover, LeuM3, 28C5, and 18E12 mAbs th at were themselves unable to stimulate the synthesis of PAF blocked PA F synthesis initiated by LBP-LPS complex. LBP was required for synthes is of PAF by MO. In MC, which synthesize PAF also after stimulation by LPS alone, the LBP was shown to speed and significantly enhance the s ynthesis of PAF. The soluble form of CD14 (sCD14), when added to MO st imulated with LBP-LPS complexes, inhibited the synthesis of PAF possib ly by competing with mCD14. In contrast, sCD14 was shown to be require d for LPS-induced synthesis of PAF by HUVEC, which did not express mCD 14. Therefore, membrane receptors (mCD14) and plasma soluble proteins (LBP and sCD14) may enable different human cell types to synthesize PA F after LPS stimulation.