The influence of glucan polymer structure and solution conformation on binding to (1-->3)-beta-D-glucan receptors in a human monocyte-like cell line

Glucans are (1-3)-beta-D-linked polymers of glucose that are produced as fungal cell wall constituents and are also released into the extracellular milieu. Glucans modulate immune function via macrophage participation. The first step in macrophage activation by (1-3)-beta-D-glucans is thought to be the binding of the polymer to specific macrophage receptors. We examined the binding/uptake of a variety of water soluble (1-3)-beta-D-glucans and control polymers with different physicochemical properties to investigate the relationship between polymer structure and receptor binding in the CR3- human promonocytic cell line, U937. We observed that the U937 receptors were specific for (1-->3)-beta-D-glucan binding, since mannan, dextran, or barley glucan did not bind. Scleroglucan exhibited the highest binding affinity with an IC(50)of 23 nM, three orders of magnitude greater than the other (1-->3)-beta-D-glucan polymers examined. The rank order competitive binding affinities for the glucan polymers were scleroglucan>schizophyllan > laminarin > glucan phosphate > glucan sulfate. Scleroglucan also exhibited a triple helical solution structure (nu = 1.82, beta = 0.8). There were two different binding/uptake sites on U937 cells. Glucan phosphate and schizophyllan interacted nonselectively with the two sites. Scleroglucan and glucan sulfate interacted preferentially with one site, while laminarin interacted preferentially with the other site. These data indicate that U937 cells have at least two non-CR3 receptor(s) which specifically interact with (1-->3)-beta-D-glucans and that the triple helical solution conformation, molecular weight and charge of the glucan polymer may be important determinants in receptor ligand interaction.