A (1 -> 3)-beta-D-linked heptasaccharide is the unit ligand for glucan pattern recognition receptors on human monocytes

Glucans are fungal cell wall polysaccharides which stimulate innate immune responses. We determined the minimum unit ligand that would bind to glucan receptors on human U937 cells using laminarin-derived pentaose, hexaose, an d heptaose glucan polymers. When U937 membranes were pretreated with the ol igosaccharides and passed over a glucan surface, only the heptasaccharide i nhibited the interaction of glucan with membrane receptors at a K-d of 31 m uM (95% CI 20-48 muM) and 100% inhibition. However, the glucan heptasacchar ide did not stimulate U937 monocyte NF kappaB signaling, nor did it increas e survival in a murine model of polymicrobial sepsis. Laminarin, a larger a nd more complex glucan polymer (M-w = 7 700 g/mol), only partially inhibite d binding (61 +/- 4%) at a K-d of 2.6 muM (9996 CI 1.7-4.2 muM) with charac teristics of a single binding site. These results indicate that a heptasacc haride is the smallest unit ligand recognized by macrophage glucan receptor s. The data also indicate the presence of at least two glucan-binding sites on U937 cells and that the binding sites on human monocyte/macrophages can discriminate between glucan polymers. The heptasaccharide and laminarin we re receptor antagonists, but they were not receptor agonists with respect t o activation of NF kappaB-dependent signaling pathways or protection agains t experimental sepsis. (C) 2001 Editions scientifiques et medicales Elsevie r SAS.