Surfactant protein A suppresses reactive nitrogen intermediates by alveolar macrophages in response to Mycobacterium tuberculosis.
Mycobacterium tuberculosis attaches to, enters, and replicates within alveolar macrophages (AMs). Our previous studies suggest that surfactant protein A (SP-A) can act as a ligand in the attachment of M. tuberculosis to AMs. Reactive nitrogen intermediates (RNIs) play a significant role in the killing of mycobacteria. We have demonstrated that RNI levels generated by AMs were significantly increased when interferon-gamma-primed AMs were incubated with M. tuberculosis. However, the RNI levels were significantly suppressed in the presence of SP-A (10 microg/ml). The specificity of SP-A's effect was demonstrated by the use of F(ab')2 fragments of anti-SP-A monoclonal antibodies and by the use of mannosyl-BSA, which blocked the suppression of RNI levels by SP-A. Furthermore, incubation of deglycosylated SP-A with M. tuberculosis failed to suppress RNI by AMs, suggesting that the oligosaccharide component of SP-A, which binds to M. tuberculosis, is necessary for this effect. These results show that SP-A-mediated binding of M. tuberculosis to AMs significantly decreased RNI levels, suggesting that this may be one mechanism by which M. tuberculosis diminishes the cytotoxic response of activated AMs. (+info)
Changes in surfactant-associated protein mRNA profile in growth-restricted fetal sheep.
To test the hypothesis that chronic placental insufficiency resulting in fetal growth restriction causes an increase in fetal lung surfactant-associated protein (SP) gene expression, we embolized chronically catheterized fetal sheep (n = 6) daily using nonradioactive microspheres in the abdominal aorta for 21 days (between 0.74 and 0.88 of gestation) until fetal arterial oxygen content was reduced by approximately 40-50%. Control animals (n = 7) received saline only. Basal fetal plasma cortisol concentration was monitored. At the end of the experiment, fetal lung tissues were collected, and ratios of tissue levels of SP-A, SP-B, and SP-C mRNA to 18S rRNA were determined by standard Northern blot analysis. Total DNA content of fetal lungs was reduced by 30% in the embolized group compared with control group (P = 0.01). There was a 2.7-fold increase in fetal lung SP-A mRNA (P < 0.05) and a 3.2-fold increase in SP-B mRNA (P < 0.01) in the chronically embolized group compared with those in the control group. SP-A and SP-B mRNA tissue levels were highly correlated with the mean fetal plasma cortisol levels on days 20-21 (r = 0.90, P < 0.01 for SP-A mRNA and r = 0.94, P < 0.01 for SP-B mRNA). SP-C mRNA tissue levels were not significantly affected by placental insufficiency. We conclude that fetal growth restriction due to placental insufficiency is associated with alterations in fetal lung SP, suggesting enhanced lung maturation that was highly dependent on the degree of increase in fetal plasma cortisol levels. (+info)
Surfactant protein A enhances the binding and deacylation of E. coli LPS by alveolar macrophages.
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)
Surfactant protein-A enhances respiratory syncytial virus clearance in vivo.
To determine the role of surfactant protein-A(SP-A) in antiviral host defense, mice lacking SP-A (SP-A-/-) were produced by targeted gene inactivation. SP-A-/- and control mice (SP-A+/+) were infected with respiratory syncytial virus (RSV) by intratracheal instillation. Pulmonary infiltration after infection was more severe in SP-A-/- than in SP-A+/+ mice and was associated with increased RSV plaque-forming units in lung homogenates. Pulmonary infiltration with polymorphonuclear leukocytes was greater in the SP-A-/- mice. Levels of proinflammatory cytokines tumor necrosis factor-alpha and interleukin-6 were enhanced in lungs of SP-A-/- mice. After RSV infection, superoxide and hydrogen peroxide generation was deficient in macrophages from SP-A-/- mice, demonstrating a critical role of SP-A in oxidant production associated with RSV infection. Coadministration of RSV with exogenous SP-A reduced viral titers and inflammatory cells in the lung of SP-A-/- mice. These findings demonstrate that SP-A plays an important host defense role against RSV in vivo. (+info)
Filaments of surfactant protein A specifically interact with corrugated surfaces of phospholipid membranes.
Pulmonary surfactant, a mixture of lipids and surfactant proteins (SPs), plays an important role in respiration and gas exchange. SP-A, the major SP, exists as an octadecamer that can self-associate to form elongated protein filaments in vitro. We have studied here the association of purified bovine SP-A with lipid vesicle bilayers in vitro with negative staining with uranyl acetate and transmission electron microscopy. Native bovine surfactant was also examined by transmission electron microscopy of thinly sectioned embedded material. Lipid vesicles made from dipalmitoylphosphatidylcholine and egg phosphatidylcholine (1:1 wt/wt) generally showed a smooth surface morphology, but some large vesicles showed a corrugated one. On the smooth-surfaced vesicles, SP-As primarily interacted in the form of separate octadecamers or as multidirectional protein networks. On the surfaces of the striated vesicles, SP-As primarily formed regularly spaced unidirectional filaments. The mean spacing between adjacent striations and between adjacent filaments was 49 nm. The striated surfaces were not essential for the formation of filaments but appeared to stabilize them. In native surfactant preparations, SP-A was detected in the dense layers. This latter arrangement of the lipid bilayer-associated SP-As supported the potential relevance of the in vitro structures to the in vivo situation. (+info)
Formation of membrane lattice structures and their specific interactions with surfactant protein A.
Biological membranes exist in many forms, one of which is known as tubular myelin (TM). This pulmonary surfactant membranous structure contains elongated tubes that form square lattices. To understand the interaction of surfactant protein (SP) A and various lipids commonly found in TM, we undertook a series of transmission-electron-microscopic studies using purified SP-A and lipid vesicles made in vitro and also native surfactant from bovine lung. Specimens from in vitro experiments were negatively stained with 2% uranyl acetate, whereas fixed native surfactant was delipidated, embedded, and sectioned. We found that dipalmitoylphosphatidylcholine-egg phosphatidylcholine (1:1 wt/wt) bilayers formed corrugations, folds, and predominantly 47-nm-square latticelike structures. SP-A specifically interacted with these lipid bilayers and folds. We visualized other proteolipid structures that could act as intermediates for reorganizing lipids and SP-As. Such a reorganization could lead to the localization of SP-A in the lattice corners and could explain, in part, the formation of TM-like structures in vivo. (+info)
Effects of endotoxin on surfactant protein A and D stimulation of NO production by alveolar macrophages.
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 A mediates mycoplasmacidal activity of alveolar macrophages by production of peroxynitrite.
We have previously shown that surfactant protein A (SP-A) mediates in vitro killing of mycoplasmas by alveolar macrophages (AMs) from resistant C57BL/6 mice through a nitric oxide (.NO)-dependent mechanism. Herein, SP-A-deficient [SP-A(-/-)] and inducible.NO synthase-deficient [iNOS(-/-)] mice were infected intranasally with 10(5) or 10(7) colony-forming units of Mycoplasma pulmonis. SP-A(-/-) mice were as susceptible to mycoplasmal infection as highly susceptible C3H/He mice, and far more susceptible than resistant C57BL/6 mice. iNOS(-/-) mice had significantly greater numbers of mycoplasmas and severity of lung lesions than iNOS(+/+) controls. In vitro, AMs isolated from C57BL/6 mice, activated with IFN-gamma, incubated with SP-A (25 micrograms/ml), and infected with 10(10) colony-forming units of M. pulmonis, killed mycoplasmas within 6 h. Mycoplasmal killing was abrogated by 1,000 units/ml of copper-zinc superoxide dismutase. In the absence of AMs, incubation of M. pulmonis with the peroxynitrite generator 3-morpholinosynodiomine.HCl (SIN-1) effected complete killing of mycoplasmas by 90 min in a dose-dependent manner. Addition of copper-zinc superoxide dismutase (3,000 units/ml), which converts SIN-1 to a.NO donor, prevented this killing. Neither of the reactive oxygen species generated by xanthine oxidase (10 milliunits/ml, plus 500 microM xanthine and 100 microM FeCl3), nor.NO generated by 1-propanamine-3-(2-hydroxy-2-nitroso-1-propylhydrazine (PAPA NONOate) (100 microM) killed mycoplasmas. These data establish that peroxynitrite generation by AMs is necessary for the killing of a pathogen in vitro and in vivo. (+info)