SP-A binding sites on bovine alveolar macrophages

Endocytic pathway for surfactant protein A in human macrophages: binding, clathrin-mediated uptake, and trafficking through the endolysosomal pathway

In the noninflamed lung, surfactant protein A (SP-A) acts as an anti-inflammatory molecule through its effects on macrophage (MΦ) function, modulating cytokine and reactive oxygen and nitrogen intermediate production. The receptors responsible for these effects of SP-A on human MΦ are not clear, although SP-A binding to several proteins has been described. In this study, we demonstrate high-affinity specific binding of SP-A to primary human MΦ. SP-A binding was inhibited by EGTA, indicating calcium dependence. However, mannan did not inhibit SP-A binding, suggesting that binding is mediated by a direct protein-protein interaction that does not involve carbohydrate recognition. Our laboratory has previously shown that SP-A is rapidly endocytosed by human MΦ into discrete vesicles. Although previous work indicates that SP-A is ultimately degraded by murine MΦ over time, the trafficking pathway of SP-A through MΦ after uptake has not been reported and is of potential biological importance. We examined trafficking of SP-A in human MΦ by electron and confocal microscopy and show for the first time that SP-A is endocytosed by primary human MΦ through clathrin-coated pits and colocalizes sequentially over time with the early endosome marker EEA1, late endosome marker lamp-1, and lysosome marker cathepsin D. We conclude that SP-A binds to receptor(s) on human MΦ, is endocytosed by a receptor-mediated, clathrin-dependent process, and trafficks through the endolysosomal pathway. These studies provide further insight into the interactions of SP-A with the MΦ cell surface and intracellular compartments that play important roles in SP-A modulation of lung MΦ biology.

Collectins

Collectins are a family of collagenous calcium-dependent defense lectins in animals. Their polypeptide chains consist of four regions: a cysteine-rich N-terminal domain, a collagen-like region, an alpha-helical coiled-coil neck domain and a C-terminal lectin or carbohydrate-recognition domain. These polypeptide chains form trimers that may assemble into larger oligomers. The best studied family members are the mannan-binding lectin, which is secreted into the blood by the liver, and the surfactant proteins A and D, which are secreted into the pulmonary alveolar and airway lining fluid. The collectins represent an important group of pattern recognition molecules, which bind to oligosaccharide structures and/or lipid moities on the surface of microorganisms. They bind preferentially to monosaccharide units of the mannose type, which present two vicinal hydroxyl groups in an equatorial position. High-affinity interactions between collectins and microorganisms depend, on the one hand, on the high density of the carbohydrate ligands on the microbial surface, and on the other, on the degree of oligomerization of the collectin. Apart from binding to microorganisms, the collectins can interact with receptors on host cells. Binding of collectins to microorganisms may facilitate microbial clearance through aggregation, complement activation, opsonization and activation of phagocytosis, and inhibition of microbial growth. In addition, the collectins can modulate inflammatory and allergic responses, affect apoptotic cell clearance and modulate the adaptive immune system.

Surfactant protein A enhances mycobacterial killing by rat macrophages through a nitric oxide-dependent pathway

Surfactant-associated protein A (SP-A) is involved in surfactant homeostasis and host defense in the lung. We have previously demonstrated that SP-A specifically binds to and enhances the ingestion of bacillus Calmette-Guerin (BCG) organisms by macrophages. In the current study, we investigated the effect of SP-A on the generation of inflammatory mediators induced by BCG and the subsequent fate of ingested BCG organisms. Rat macrophages were incubated with BCG in the presence and absence of SP-A. Noningested BCG organisms were removed, and the release of tumor necrosis factor-alpha (TNF-alpha) and nitric oxide were measured at varying times. TNF-alpha and nitric oxide production induced by BCG were enhanced by SP-A. In addition, SP-A enhanced the BCG-induced increase in the level of inducible nitric oxide synthase protein. Addition of antibodies directed against SPR210, a specific macrophage SP-A receptor, inhibited the SP-A-enhanced mediator production. BCG in the absence of SP-A showed increased growth over a 5-day period, whereas inclusion of SP-A dramatically inhibited BCG growth. Inhibition of nitric oxide production blocked BCG killing in the presence and absence of SP-A. These results demonstrate that ingestion of SP-A-BCG complexes by rat macrophages leads to production of inflammatory mediators and increased mycobacterial killing.