Analysis of the sugar specificity and molecular location of the β-glucan-binding lectin site of complement receptor type 3 (CD11b/CD18)

Abstract

Zymosan, the cell wall from Saccharomyces cerevisiae, was reported to be a macrophage activator through its β-glucan over 30 yr ago. Nevertheless, the identity of the β-glucan receptor has been controversial. This study showed that the α M β 2-integrin, CR3 (Mac-1, CD11b/CD18) served as the β-glucan receptor through one or more lectin sites located outside of the CD11b I-domain that contains the binding sites for iC3b, ICAM-1, and fibrinogen. Sugar specificity, analyzed with FITC-labeled soluble polysaccharides and flow cytometry, showed CR3-specific staining with several pure β-glucans but not with α-mannan. However, a 10-kDa soluble zymosan polysaccharide (SZP) with high affinity (6.7 x 10(-8) M) for CR3 consisted largely of mannose and approximately 5% glucose. Binding of either SZP-FITC or β-glucan-FITC to CR3 was blocked not only by pure β-glucans from yeast, mushroom, seaweed, or barley, but also by N-acetyl-D-glucosamine (NADG), α- or β-methylmannoside, and α- or β-methyl-glucoside. SZP-FITC and β-glucan-FITC stained all leukocyte types similarly to anti-CR3-FITC, and polysaccharide-FITC staining was inhibited > or = 95% by unlabeled anti-CR3. SZP-FITC staining of cells expressing recombinant chimeras between CR3 and CR4 (p150,95, CD11c/CD18) suggested that both the divalent cation-binding region of CD11b and the region C-terminal to it may regulate binding of polysaccharides to CR3. Unlabeled SZP or β-glucan also blocked CR3 staining by 11 mAb to C-terminal domain epitopes of CD11b but had no effect on staining by mAb directed to the I-domain. In conclusion, CR3 serves as the leukocyte β-glucan receptor through a cation-independent lectin site located C-terminal to the I-domain of CD11b. Its sugar specificity is broader than originally appreciated, allowing it to react with certain polysaccharides containing mannose or NADG, as well as glucose.