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As little as 1 microliter of serum-free supernatant from Mo(t), an established lymphocyte line, when added to a 500-microliters incubation of macrophages derived from human monocytes, significantly decreased the receptor-mediated uptake and degradation of three cholesterol-rich molecules: low density lipoprotein (LDL); LDL complexed to dextran sulfate; and LDL modified by malondialdehyde (MDA-LDL). In contrast, the receptor-mediated uptake and degradation of mannosyl bovine serum albumin was increased three-fold. The Mo(t) supernatant did not contain competitive inhibitors of the cholesterol-rich ligands, and it did not alter macrophage receptor-independent endocytosis, protein synthesis, or phagocytosis of heat-killed yeast. The effect of the Mo(t) supernatant was specific for macrophages and was abolished by preincubation of the supernatant with trypsin, which indicates that the active substances are protein in nature. The decrease in the uptake and degradation of MDA-LDL induced by preincubating the macrophages with Mo(t) supernatant appeared to result from a decrease in the number of receptors for this ligand at the cell surface. The isolation of these lymphokines should offer new insights into macrophage receptor-mediated endocytosis, and may yield substances useful in preventing foam cell formation in atherosclerosis.

It has been previously demonstrated that maley-lated-BSA (maleyl-albumin) induces functional activation in murine peritoneal macrophages. Furthermore, maleyl-albumin has been shown to interact with two distinct sites on human monocytes; one site is the scavenger receptor, a 260-kDa oligomeric protein which recognizes modified forms of low density lipoprotein (LDL), and the second is a lower affinity site which has yet to be structurally characterized. In the present study, we wished to quantitatively assess the number and character of maleyl-albumin-binding sites on murine peritoneal macrophages and to determine which site or sites are involved in signaling the macrophage to undergo extensive functional development. Binding studies. demonstrate at least two distinct receptors for maleyl-albumin on murine peritoneal macrophages. Scatchard analyses of the binding isotherms reveal two sites characterized by dissociation constants (Kd) of 17.6 nM and 4.9 microM and maximal binding of 1.2 x 10(5) and 1 x 10(6) sites/cell, respectively. The contribution of the scavenger receptor, determined by binding analyses of malondialdehyde-LDL, is described by two sites with Kd of 39.4 pM and 9.6 nM, and maximal binding of 2.7 x 10(3) and 1.9 x 10(4) sites/cell, respectively. Maleyl-albumin blocks binding of malondialdehyde-LDL, whereas modified LDL fails to inhibit binding of maleyl-albumin. Maleyl-albumin, at concentrations producing lower affinity binding, stimulates tumor cytolysis, expression of mRNA encoding TNF, and suppression of INF-gamma-induced expression of Ia Ag. Malondialdehyde-LDL fails to elicit these responses. We conclude that macrophage activation produced by maleyl-albumin is mediated by interaction with the low affinity, high capacity binding site for maleyl-albumin rather than the scavenger receptor.

Concentrations of bacterial lipopolysaccharide (LPS) as low as 1 ng/ml suppressed the activity of the scavenger receptor on cultured human monocyte-macrophages. In contrast, concentrations of LPS as high as 100 ng/ml had no effect on the activity of the low density lipoprotein (LDL) receptor. LPS and purified forms of the lipid A moiety of LPS were effective in suppressing scavenger receptor activity. However, acid hydrolysis of the labile phosphate group of the native diphosphorylated lipid A to form monophosphoryl lipid A rendered the molecule ineffective in suppressing scavenger receptor activity. LPS at a concentration of 100 ng/ml had no effect on the secretion of apolipoprotein E, phagocytic activity, tumoricidal activity, or the protein content of monocyte-macrophages. We conclude that the active component of LPS that mediates suppression of scavenger receptor activity is diphosphoryl lipid A.