We have shown that Leishmania lipophosphoglycan (LPG) inhibits IL-1 beta gene expression in human monocytes. Here, we show that LPG can bind in a time-dependent manner and suppress endothelial cell activation, possibly via specific LPG domains. Endotoxin (10 ng/ml, 4 h) consistently caused endothelium to increase monocyte adhesion (approximately 20-fold). LPG pretreatment (2 microM, 2 h) completely blocked endotoxin-mediated monocyte adhesion. LPG did not grossly suppress endothelial functions because TNF-alpha- and IL-1 beta-mediated adhesion toward monocytes were not affected. Using four highly purified LPG fragments (namely, repeating phosphodisaccharide (PGM), phosphoglycan, phosphosaccharide core-lyso-alkyl-phosphatidylinositol (core-PI), and lyso-alkyl-phosphatidylinositol (lyso-PI)), we examined whether these fragments can independently inhibit endothelial adhesion. In contrast to that of intact LPG, neither the four LPG fragments (2 microM, 2 h) independently nor the co-addition of phosphoglycan and core-P1 fragments blocked the endotoxin-mediated adhesion to monocytes. To determine whether the fragments can reverse the effect of intact LPG, endothelial cells were first pretreated with the LPG fragments (10 microM, 15 min), followed by the addition of LPG (2 microM). All four LPG fragments fully reversed the effect of LPG. Simultaneous addition of LPG fragments and intact LPG caused only partial suppression (approximately 45%), while the addition of LPG fragments 14 min later had no reversal effect. Flow cytometry revealed that only core-P1 and lyso-P1 competitively inhibited (approximately 30%) LPG binding. Conversely, LPG competed with the binding of [3H]lyso-P1 (approximately 30%). Furthermore, mAb against the PGM reversed (approximately 70%) the effect of LPG. Thus, the lyso-P1 domain on LPG mediates binding to endothelial cells, whereas the PGM domain mediates the cell inhibitory effect.