Surfactant protein A enhances the binding and deacylation of E. coli LPS by alveolar macrophages.
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Surfactant protein (SP) A and SP-D are involved in multiple immunomodulatory functions of innate host defense partly via their interaction with alveolar macrophages (AMs). In addition, both SP-A and SP-D bind to bacterial lipopolysaccharide (LPS). To investigate the functional significance of this interaction, we first tested the ability of SP-A and SP-D to enhance the binding of tritium-labeled Escherichia coli LPS to AMs. In contrast to SP-D, SP-A enhanced the binding of LPS by AMs in a time-, temperature-, and concentration-dependent manner. Coincubation with surfactant-like lipids did not affect the SP-A-mediated enhancement of LPS binding. At SP-A-to-LPS molar ratios of 1:2-1:3, the LPS binding by AMs reached 270% of control values. Second, we investigated the role of SP-A in regulating the degradation of LPS by AMs. In the presence of SP-A, deacylation of LPS by AMs increased by approximately 2.3-fold. Pretreatment of AMs with phosphatidylinositol-specific phospholipase C had no effect on the SP-A-enhanced LPS binding but did reduce the amount of serum-enhanced LPS binding by 50%, suggesting that a cell surface molecule distinct from CD14 mediates the effect of SP-A. Together the results for the first time provide direct evidence that SP-A enhances LPS binding and degradation by AMs. (+info)
Effects of endotoxin on surfactant protein A and D stimulation of NO production by alveolar macrophages.
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Surfactant protein (SP) A and SP-D affect numerous functions of immune cells including enhancing phagocytosis of bacteria and production of reactive species. Previous studies have shown that SP-A and SP-D bind to a variety of bacteria and to the lipopolysaccharide (LPS) components of their cell walls. In addition, purified preparations of SPs often contain endotoxin. The goals of this study were 1) to evaluate the effects of SP-A and SP-D and complexes of SPs and LPS on the production of nitric oxide metabolites by rat alveolar macrophages and 2) to evaluate methods for the removal of endotoxin with optimal recovery of SP. Incubation of SP-A or SP-D with polymyxin, 100 mM N-octyl-beta-D-glucopyranoside, and 2 mM EDTA followed by dialysis was the most effective method of those tested for reducing endotoxin levels. Commonly used storage buffers for SP-D, but not for SP-A, inhibited the detection of endotoxin. There was a correlation between the endotoxin content of the SP-A and SP-D preparations and their ability to stimulate production of nitrite by alveolar macrophages. SP-A and SP-D treated as described above to remove endotoxin did not stimulate nitrite production. These studies suggest that the functions of SP-A and SP-D are affected by endotoxin and illustrate the importance of monitoring SP preparations for endotoxin contamination. (+info)
Surfactant protein D binds to Mycobacterium tuberculosis bacilli and lipoarabinomannan via carbohydrate-lectin interactions resulting in reduced phagocytosis of the bacteria by macrophages.
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Surfactant protein-D (SP-D) is a collectin produced in the distal lung airspaces that is believed to play an important role in innate pulmonary immunity. Naive immunologic responses to Mycobacterium tuberculosis (M.tb) are especially important in the lung, since entry of this inhaled pathogen into the alveolar macrophage is a pivotal event in disease pathogenesis. Here we investigated SP-D binding to M.tb and the effect of this binding on the adherence of M. tb to human macrophages. These studies demonstrate specific binding of SP-D to M.tb that is saturable, calcium dependent, and carbohydrate inhibitable. In addition to purified SP-D, SP-D in bronchoalveolar lavage fluids from healthy donors and patients with alveolar proteinosis also binds to M.tb. Incubation of M.tb with SP-D results in agglutination of the bacteria. In contrast to its binding to M.tb, SP-D binds minimally to the avirulent Mycobacterium smegmatis. SP-D binds predominantly to lipoarabinomannan from the virulent Erdman strain of M.tb, but not the lipoarabinomannan from M. smegmatis. The binding of SP-D to Erdman lipoarabinomannan is mediated by the terminal mannosyl oligosaccharides of this lipoglycan. Incubation of M.tb with subagglutinating concentrations of SP-D leads to reduced adherence of the bacteria to macrophages (62.7% of control adherence +/- 3.3% SEM, n = 8), whereas incubation of bacteria with surfactant protein A leads to significantly increased adherence to monocyte-derived macrophages. These data provide evidence for specific binding of SP-D to M. tuberculosis and indicate that SP-D and surfactant protein A serve different roles in the innate host response to this pathogen in the lung. (+info)
Surfactant protein A and D expression in the porcine Eustachian tube.
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Surfactant proteins A and D are collectins which are considered to play an important role in the innate immunity of lungs. Our aim was to investigate whether surfactant protein A or D is expressed in the porcine Eustachian tube originating from the upper airways. Both surfactant proteins A and D were present in the epithelial cells of the Eustachian tube, as shown by strong immunostaining. Using RT-PCR and Northern hybridization, these collectins were detected in the Eustachian tube. The present study is the first report demonstrating surfactant protein gene expression in the Eustachian tube. Surfactant proteins A and D may be important in the antibody-independent protection of the middle ear. (+info)
Aerosolized endotoxin is immediately bound by pulmonary surfactant protein D in vivo.
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Collectins are carbohydrate binding proteins that are implicated in innate host defense. The lung collectins, surfactant proteins A and D (SP-A and SP-D), bind a variety of pathogens in vitro and influence phagocytosis by alveolar macrophages. In this report we show that SP-D binds endotoxin (lipopolysaccharide, LPS) in vivo in a rat model of acute respiratory distress syndrome (ARDS). Intratracheal aerosolization of LPS in rats resulted in the typical features of human ARDS. Total amounts of SP-D, as well as the carbohydrate binding properties of SP-D were measured in lung lavage as a function of time. The amount of SP-D did not change during 24 h. Interestingly, SP-D in lung lavage isolated from rats during the first 2 h after LPS treatment, was not able to bind to carbohydrate. Further analysis revealed that the carbohydrate binding sites of SP-D were occupied by LPS, suggesting that SP-D is an LPS scavenging molecule in vivo. Electron microscopic analysis indicated that, 1 h after LPS aerosolization, aggregates of SP-D with LPS were found in lysosomal structures in alveolar macrophages. We conclude that the lung collectin SP-D binds inhaled endotoxin in vivo, which may help to protect the lung from endotoxin-induced disease. (+info)
Binding of rat and human surfactant proteins A and D to Aspergillus fumigatus conidia.
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Surfactant proteins A (SP-A) and D (SP-D) are thought to play important roles in pulmonary host defense. We investigated the interactions of rat and human SP-A and SP-D with Aspergillus fumigatus conidia. Rat SP-D but not rat SP-A bound the conidia, and the binding was inhibited by EDTA, mannose, glucose, maltose, and inositol. Binding studies using a mutant recombinant rat SP-D with altered carbohydrate recognition but normal structural organization clearly established a role for the carbohydrate recognition domain in binding to conidia. However, neither rat SP-A nor SP-D increased the association of fluorescein isothiocyanate-labeled conidia with rat alveolar macrophages as determined by flow cytometry. Both human SP-A (isolated from normal and alveolar proteinosis lungs) and SP-D (recombinant protein and protein isolated from alveolar proteinosis lungs) bound the conidia. These data indicate that important differences exist between rat and human SP-A in binding to certain fungi. Human SP-A and SP-D binding to conidia was also examined in the presence of hydrophobic surfactant components (HSC), containing both the phospholipid and hydrophobic proteins of surfactant. We found that HSC inhibited but did not eliminate human SP-A binding to Aspergillus conidia. In contrast, the SP-D binding to conidia was unaffected by HSC. These findings indicate that SP-D plays a major role in the recognition of Aspergillus conidia in alveolar fluid. (+info)
Cloning of gp-340, a putative opsonin receptor for lung surfactant protein D.
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Surfactant protein D (SP-D) is an oligomeric C type lectin that promotes phagocytosis by binding to microbial surface carbohydrates. A 340-kDa glycoprotein (gp-340) has been shown to bind SP-D in the presence of calcium but does so independently of carbohydrate recognition. This protein exists both in a soluble form and in association with the membranes of alveolar macrophages. The primary structure of gp-340 has been established by molecular cloning, which yielded a 7,686-bp cDNA sequence encoding a polypeptide chain of 2, 413 amino acids. The domain organization features 13 scavenger receptor cysteine-rich (SRCR) domains, each separated by an SRCR-interspersed domain, except for SRCRs 4 and 5, which are contiguous. The 13 SRCR domains are followed by two C1r/C1s Uegf Bmp1 domains separated by a 14th SRCR domain and a zona pellucida domain. gp-340 seems to be an alternative spliced form of DMBT1. Reverse transcription-PCR analysis showed that the main sites of synthesis of gp-340 are lung, trachea, salivary gland, small intestine, and stomach. Immunohistochemistry revealed strong staining for gp-340 in alveolar and other tissue macrophages. Immunostaining of the macrophage membrane was either uniform or focal in a way that suggested capping, whereas other macrophages showed strong intracellular staining within the phagosome/phagolysosome compartments. In some macrophages, SP-D and gp-340 were located in the same cellular compartment. Immunoreactive gp-340 was also found in epithelial cells of the small intestine and in the ducts of salivary glands. The distribution of gp-340 in macrophages is compatible with a role as an opsonin receptor for SP-D. (+info)
Structural characterization of human and bovine lung surfactant protein D.
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Human and bovine surfactant proteins D (SP-D) were purified from late amniotic fluid and bronchioalveolar lavage on the basis of its Ca(2+)-dependent affinity for maltose. The molecular mass of a trimeric subunit was determined by matrix-assisted laser desorption ionization MS to lie in the range 115-125 kDa for human SP-D and 110-123 kDa for bovine SP-D. A single polypeptide chain was determined at 37-41 and 36-40 kDa for the human and bovine species respectively. The major parts of the primary structures of both SP-D molecules were determined by a combination of MS and Edman degradation. The heterogeneity in SP-D was caused mainly by a high number of post-translational modifications in the collagen-like region. Proline and lysine residues were partly hydroxylated and lysine residues were further O-glycosylated with the disaccharide galactose-glucose. A partly occupied N-linked glycosylation site was characterized in human SP-D. The carbohydrate was determined as a complex type bi-antennary structure, with a small content of mono-antennary and tri-antennary structures. No sialic acid residues were present on the glycan, but some had an attached fucose and/or an N-acetylglucosamine residue linked to the core. Bovine SP-D was determined as having a similar structure. (+info)