Structural similarity between the macrophage lectin specific for galactose/N-acetylgalactosamine and the hepatic asialoglycoprotein binding protein

Alveolar macrophages bind and phagocytose allergen-containing pollen starch granules via C-type lectin and integrin receptors: implications for airway inflammatory disease

Recent studies suggest that IgE-independent mechanisms of airway inflammation contribute significantly to the pathophysiology of allergic airway inflammatory diseases such as asthma. Such mechanisms may involve direct interactions between inhaled allergens and cells of the respiratory tract such as macrophages, dendritic cells, and epithelial cells. In this study, we investigated receptor-mediated interactions occurring between alveolar macrophages and allergen-containing pollen starch granules (PSG). We report here that PSG are released from a range of grass species and are rapidly bound and phagocytosed by alveolar macrophages. Human monocyte-derived dendritic cells also bound PSG but no internalization was observed. Phagocytosis of PSG was dependent on Mg2+ and Ca2+ and was inhibited by neo-glycoproteins such as galactose-BSA and N-acetylgalactose-BSA. Partial inhibition of phagocytosis was also seen with the Arg-Gly-Asp-Ser (RGDS) motif and with an anti-CD18 mAb (OX42). The combination of both neo-glycoprotein and anti-CD18 achieved the greatest degree of inhibition (>90%). Together, these data suggest a role for both C-type lectins and beta2-integrins in the binding and internalization of PSG. The consequences of this interaction included a rapid up-regulation of inducible NO synthase mRNA and subsequent release of NO by alveolar macrophages. Thus, receptor-mediated recognition of inhaled allergenic particles by alveolar macrophages may represent a potential mechanism for modulating the inflammatory response associated with allergic airway diseases such as asthma.

The differences in structural specificity for recognition and binding between asialoglycoprotein receptors of liver and macrophages

The Gal/GalNAc-specific lectin on the surface of rat peritoneal macrophages (macrophage asialoglycoprotein binding protein, M-ASGP-BP), which consists of a single polypeptide chain of 42 kDa, can form a homo-oligomeric receptor exhibiting high affinity for asialoorosomucoid (ASOR) [Ozaki K., Ii M., Itoh N., Kawasaki T. (1992) J Biol Chem 267: 9229-35]. In this study, the binding affinity of M-ASGP-BP was studied by using a series of synthetic or natural glycosides as inhibitors of 125I-ASOR binding to recombinant M-ASGP-BP expressed on COS-1 cells (rM-ASGP-BP), and the results were compared with those of human hepatic lectin (HHL) on Hep G2 cells. Clustering of multiple Gal (or GalNAc) residues increased the binding affinity to M-ASGP-BP as well as to HHL. In contrast to HHL and other mammalian hepatic lectins, rM-ASGP-BP bound Gal residues tighter than GalNAc residues. A galactose-terminated triantennary N-glycoside, having one N-acetyl-lactosamine unit on the 6 branch and two N-acetyl-lactosamine units on the 3 branch of the trimannosyl core structure, showed affinity enhancement of approximately 10(5) over a monovalent ligand for HHL, while the same glycopeptide showed enhancement of about 2000-fold for rM-ASGP-BP. These results suggest that spatial arrangements of sugar combining sites and subunit organization of macrophage and hepatic lectins are different.

The major form of the murine asialoglycoprotein receptor: cDNA sequence and expression in liver, testis and epididymis

Northern blot analysis of poly(A)+RNAs isolated from mouse liver or mouse testis (Te)/epididymis (Ep) reveals that both tissues express 1.5- and 7.5-kb transcripts which have extensive homology to the major form of the rat asialoglyco-protein receptor (ASGP-R). In situ hybridization studies have localized the expression of this ASGP-R-like transcript to late-stage sperm from Te and Ep of several different strains of mice. Swiss Webster mice express this ASGP-R-like transcript in late-stage spermatids at the time of release into the seminiferous tubule and in Ep sperm, while Balb/C, NIH Swiss and C57Bl/6 mice express this ASGP-R-like transcript predominantly in Ep sperm. cDNAs containing the entire coding region for this ASGP-R-like transcript have been cloned from mouse liver and mouse Te/Ep. These cDNAs are 100% identical in the coding region and 3'-untranslated region (UTR), but differ in the 5'-UTR. The gene encoding these cDNAs is called MHL-1, designating the major form of the mouse ASGP-R. The deduced amino acid (aa) sequence of MHL-1 shares 88% homology to the rat hepatic (He) lectin form 1 (RHL-1) and 78% homology to the human asialoglycoprotein receptor form 1 (H1). The three sites for N-linked glycosylation in the RHL-1 sequence are all conserved in the deduced MHL-1 sequence. Taken collectively, these data describe the cloning and sequencing of the MHL-1 cDNA and illustrate its deduced aa homology to RHL-1 and H1.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and upregulation of galactose/N-acetylgalactosamine macrophage lectin in rat cardiac allografts with arteriosclerosis

Using differential mRNA display to uncover potential mediators associated with chronic rejection, we identified a cDNA fragment induced in Lewis to F344 rat cardiac allografts with arteriosclerosis but not Lewis syngrafts. The full-length cDNA (1.4 kb) isolated from a rat cardiac allograft cDNA library was 99% identical to galactose/N-acetylgalactosamine (Gal/GalNAc) macrophage lectin, a cell-surface receptor. This cDNA hybridized in Northern analysis with total RNA from eight cardiac allografts but not with host hearts, syngrafts, or other organs. There was a significant allograft-specific increase in transcript levels measured by reverse transcriptase PCR at days 7, 14, 28, and 75 in comparison with paired F344 host hearts (subject to same circulation but histologically normal), day-0 hearts, and syngrafts (P < 0.008, n = 4 at each time). Transcript levels in cardiac allografts were higher than those in paired host spleens (a major source of inflammatory cells) (P < 0.0001), indicating the localized nature of Gal/GalNAc lectin induction. By in situ hybridization and immunostaining, Gal/GalNAc lectin expression localized to a subset of inflammatory cells in cardiac allografts. These findings link Gal/GalNAc macrophage lectin to the chronic rejection process, as a possible mediator of macrophage infiltration.

Role of saccharides in immunoglobulin--Fc receptor interactions

The rosettes formed by mouse peritoneal macrophages or DCH-5 cells and TNP-erythrocytes coated with anti-TNP antibodies of different isotypes were inhibited to various extent by monosaccharides. The most effective inhibitors were N-acetylglucosamine, glucosamine, mannose and N-acetylneuraminic acid in 1-5 mmol/L concentrations. Even more efficient were glycopeptides isolated from IgG molecules. The Fc receptors (FcRs) released from DCH-5 cells during cultivation and gradually separated by affinity chromatography on immobilized IgG reacted with aggregated IgG and inhibited the rosette formation. The FcRs eluted by monosaccharides influenced mainly the number of rosettes mediated by IgA and IgE while those eluted with a glycine-HCl buffer inhibited preferentially IgG rosettes. As shown by SDS-PAGE the heterogeneity of the fraction eluted with a mixture of monosaccharides revealed one main component with an effective molar mass of 50 kg/mol. The glycine-HCl eluate contained two major components of 55 and 38 kg/mol. The IgG-Sepharose 4B bound all the fractions but only the binding of the 50 kg/mol molecule could be inhibited by monosaccharides.