Effect of canine surfactant protein (SP-A) on the respiratory burst of phagocytic cells

Surfactant protein A enhances production of secretory leukoprotease inhibitor and protects it from cleavage by matrix metalloproteinases

Mice lacking surfactant protein A (SP-A) are susceptible to bacterial infection associated with an excessive inflammatory response in the lung. To determine mechanisms by which SP-A is antiinflammatory in the lung during bacterial infection, SP-A regulation of secretory leukoprotease inhibitor (SLPI), an inhibitor of serine proteases, was assessed. SLPI protein expression and antineutrophil elastase activity were reduced in bronchoalveolar fluid of SP-A(-/-) compared with SP-A(+/+) mice. Intratracheal administration of SP-A to SP-A(-/-) mice enhanced SLPI protein expression and antineutrophil elastase activity in the lung. SLPI mRNA was similar in whole lung and alveolar type II cells; however, it was significantly reduced in alveolar macrophages from SP-A(-/-) compared with SP-A(+/+) mice. In vitro, SP-A enhanced SLPI production by macrophage THP-1 cells but not respiratory epithelial A549 cells. SP-A inhibited LPS induced IkappaB-alpha degradation in THP-1 cells, which was partially reversed with knockdown of SLPI. Matrix metalloproteinase (MMP)-12 cleaved SLPI and incubation with SP-A reduced MMP-12-mediated SLPI cleavage. The collagen-like region of SP-A conferred protection of SLPI against MMP mediated cleavage. SP-A plays an important role in the lung during bacterial infection regulating protease and antiprotease activity.

Bordetella pertussis lipopolysaccharide resists the bactericidal effects of pulmonary surfactant protein A

Surfactant protein A (SP-A) plays an important role in the innate immune defense of the respiratory tract. SP-A binds to lipid A of bacterial LPS, induces aggregation, destabilizes bacterial membranes, and promotes phagocytosis by neutrophils and macrophages. In this study, SP-A interaction with wild-type and mutant LPS of Bordetella pertussis, the causative agent of whooping cough, was examined. B. pertussis LPS has a branched core structure with a nonrepeating trisaccharide, rather than a long-chain repeating O-Ag. SP-A did not bind, aggregate, nor permeabilize wild-type B. pertussis. LPS mutants lacking even one of the sugars in the terminal trisaccharide were bound and aggregated by SP-A. SP-A enhanced phagocytosis by human monocytes of LPS mutants that were able to bind SP-A, but not wild-type bacteria. SP-A enhanced phagocytosis by human neutrophils of LPS-mutant strains, but only in the absence of functional adenylate cyclase toxin, a B. pertussis toxin that has been shown to depress neutrophil activity. We conclude that the LPS of wild-type B. pertussis shields the bacteria from SP-A-mediated clearance, possibly by sterically limiting access to the lipid A region.

Pulmonary surfactant protein a inhibits macrophage reactive oxygen intermediate production in response to stimuli by reducing NADPH oxidase activity

Alveolar macrophages are important host defense cells in the human lung that continuously phagocytose environmental and infectious particles that invade the alveolar space. Alveolar macrophages are prototypical alternatively activated macrophages, with up-regulated innate immune receptor expression, down-regulated costimulatory molecule expression, and limited production of reactive oxygen intermediates (ROI) in response to stimuli. Surfactant protein A (SP-A) is an abundant protein in pulmonary surfactant that has been shown to alter several macrophage (Mphi) immune functions. Data regarding SP-A effects on ROI production are contradictory, and lacking with regard to human Mphi. In this study, we examined the effects of SP-A on the oxidative response of human Mphi to particulate and soluble stimuli using fluorescent and biochemical assays, as well as electron paramagnetic resonance spectroscopy. SP-A significantly reduced Mphi superoxide production in response to the phorbol ester PMA and to serum-opsonized zymosan (OpZy), independent of any effect by SP-A on zymosan phagocytosis. SP-A was not found to scavenge superoxide. We measured Mphi oxygen consumption in response to stimuli using a new oxygen-sensitive electron paramagnetic resonance probe to determine the effects of SP-A on NADPH oxidase activity. SP-A significantly decreased Mphi oxygen consumption in response to PMA and OpZy. Additionally, SP-A reduced the association of NADPH oxidase component p47(phox) with OpZy phagosomes as determined by confocal microscopy, suggesting that SP-A inhibits NADPH oxidase activity by altering oxidase assembly on phagosomal membranes. These data support an anti-inflammatory role for SP-A in pulmonary homeostasis by inhibiting Mphi production of ROI through a reduction in NADPH oxidase activity.

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.

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.

Surfactant protein-A binds group B streptococcus enhancing phagocytosis and clearance from lungs of surfactant protein-A-deficient mice

Surfactant protein-A (SP-A) gene-targeted mice clear group B streptococcus (GBS) from the lungs at a slower rate than wild-type mice. To determine mechanisms by which SP-A enhances pulmonary clearance of GBS, the role of SP-A in binding and phagocytosis of GBS was assessed in SP-A (-/-) mice infected with GBS in the presence and absence of exogenous SP-A. Coadministration of GBS with exogenous SP-A decreased GBS colony counts in lung homogenates of SP-A (-/-) mice. SP-A bound to GBS in a calcium-dependent manner. Although pulmonary infiltration with macrophages was not altered in SP-A (-/-) versus wild-type mice after GBS infection, the number of alveolar macrophages with phagocytosed bacteria was lower in the SP-A (-/-) mice than in the wild-type mice. When SP-A was coadministered with GBS, phagocytosis was significantly increased. Oxygen radical production by alveolar macrophages from SP-A (-/-) mice infected with GBS was decreased compared with wild-type controls and was increased when SP-A (-/-) mice were infected in the presence of exogenous SP-A. Superoxide (SO) radical generation was deficient in macrophages from SP-A (-/-) mice. SP-A plays an important role in GBS clearance in vivo, mediated in part by binding to and enhancing GBS phagocytosis and by increasing SO production by alveolar macrophages.

Artificial surfactant (Surfactant TA) modulates adherence and superoxide production of neutrophils

Neutrophils cause lung injuries by releasing proteases and active oxygen radicals in patients with acute respiratory distress syndrome (ARDS). Artificial surfactant is used to replace native surfactant whose functions are deteriorated by serum-derived inhibitors in these patients. We investigated potential interactions between exogenous surfactant (Surfactant TA) and neutrophils in in vivo and in vitro experimental models. Neutrophil alveolitis was induced in hamster lungs by the intratracheal administration of bleomycin (5 mg/kg) on Day 0. Some of the animals were followed by replacement with Surfactant TA (5 and 10 mg/100 g body weight) on Day 1. Alveolar cells were harvested by lung lavage on Day 2. The numbers of the neutrophils obtained from the lungs treated with bleomycin and Surfactant TA were unchanged, but the superoxide production from these cells was significantly decreased when compared with control animals (no Surfactant TA). From the in vitro experiments, Surfactant TA was shown to inhibit adherence and superoxide production of human neutrophils. These effects were derived from the heat-resistant components of Surfactant TA and were mimicked by treatment with liposomes of dipalmitoyl phosphatidylcholine. Surfactant-TA-treated neutrophils were demonstrated to have picnotic nuclei and to express Fas antigens, which were characteristic of apoptotic cells. These results suggest that exogenous Surfactant TA may play an important role not only in improving surfactant functions but in preventing neutrophils from further activation, probably through enhancing apoptosis.

Interactions of surfactant protein A with epithelial cells and phagocytes

Surfactant protein A (SP-A) has been shown to bind to and regulate the functions of both alveolar type II cells and immune cells including alveolar macrophages. The interaction of SP-A with type II cells has been shown in vitro to inhibit lipid secretion and to promote the uptake of lipid by these cells and these observations led to the hypothesis that SP-A plays an important role in regulating surfactant turnover and metabolism. The finding that mice made deficient in SP-A by homologous recombination (SP-A -/- mice) have relatively normal surfactant pool sizes has raised the possibility that either redundant mechanisms function in vivo to keep pool sizes normal in the absence of SP-A or that the in vitro findings are not significant in the context of the whole, unstressed animal. The interaction of SP-A with immune cells has been shown to affect a variety of responses which, in general, function to promote host defense against infection. Although SP-A receptors have been identified, additional studies will be required to elucidate the mechanism of interaction of SP-A with these cells and the relative importance of the different receptors in SP-A mediated regulation of cell function.

Surfactant prevents quartz induced down-regulation of complement receptor 1 in human granulocytes

Quartz is known to induce an inflammatory response in the alveolar space by recruitment of different effector cells. We investigated the interaction between granulocytes and quartz with respect to expression of complement receptor type 1 (CR1) and CR3, with and without the presence of surfactant. Granulocytes from hemolyzed blood were stimulated by N-formyl-methionyl-leucyl-phenylalanine (fMLP), which mobilize the intracellular pool of CR1 to the surface, and the mean fluorescence intensity (MFI) measured by cytofluorometry was 47.4 (46-63.6) (median; interquartile range). Quartz exposure reduced the CR1 expression to 23.2 (22.8-30.6) MFI units (P < 0.01), a porcine surfactant preparation added during quartz exposure abolished the down-regulation completely, 47.7 (43.2-62.3) MFI units (P < 0.001). Similar results were obtained after preincubation of the cells with surfactant followed by quartz exposure. No significant influence on CR1 expression was found by a synthetic lipid mixture, nor was the CR3 expression affected. In conclusion, this study demonstrates that the presence of surfactant inhibits quartz induced down-regulation of CR1 on activated granulocytes.

Altered regulation of surfactant phospholipid and protein A during acute pulmonary inflammation

Biochemical changes in the pulmonary surfactant system caused by exposure to toxicants are often accompanied by an influx of inflammatory cells into the lungs. We have investigated the possibility that the inflammatory and surfactant biochemical effects might be connected. Co-treatment with dexamethasone, a synthetic anti-inflammatory glucocorticoid, mitigated the increases in free cells and total intracellular surfactant phospholipid normally seen in animals given silica alone, suggesting a relationship between the free cell population of the alveoli and the surfactant system during alveolitis. Furthermore, we have investigated whether induction of the surfactant system is a universal response to alveolar inflammation. Inflammation was induced in the lungs by intratracheal injections of titanium dioxide, silica, bleomycin or lipopolysaccharide (LPS) suspended in isotonic saline. Inflammatory cell and surfactant responses were measured at 3 days and 14 days following injection. There was a distinct alveolar inflammatory cell profile following administration of each agent, at each time point, indicating a dynamic inflammatory cell population during the course of the study. Furthermore, surfactant phospholipid and protein A (SP-A) pools exhibited unique responses to the inflammatory agents. Only silica-treated lungs maintained elevated levels of surfactant phospholipids and SP-A throughout the course of the experiment. We conclude that both the surfactant components and the inflammatory cell population of the alveoli undergo dynamic changes following treatment with these inflammatory agents and that activation of the surfactant system is not a universal response to alveolar inflammation, since surfactant components were not always elevated during times of increased alveolar cellularity. The unique inflammatory cell infiltrate elicited by silica is of particular interest in that surfactant components were elevated throughout the course of the experiment in this group. Indeed, we have shown that the size of the intracellular pool of surfactant is directly proportional to the number of polymorphonuclear leukocytes but not alveolar macrophages or lymphocytes in the alveoli following silica treatment. Finally, our data suggest that the phospholipid and SP-A components of surfactant respond differentially to the pulmonary toxicants in this study.

Inhibitory effect of porcine surfactant on the respiratory burst oxidase in human neutrophils. Attenuation of p47phox and p67phox membrane translocation as the mechanism

Surfactant has been shown to inhibit the production of reactive oxygen intermediates by various cells including alveolar macrophages and peripheral blood neutrophils. Superoxide O2-. production by the respiratory burst oxidase in isolated plasma membranes prepared from PMA-treated human neutrophils was significantly attenuated by prior treatment with native porcine surfactant. The effect was concentration dependent with half-maximal inhibition seen at approximately 0.050 mg surfactant phospholipid/ml. Kinetic analyses of the membrane-bound enzyme prepared from neutrophils stimulated by PMA in the presence or absence of surfactant demonstrated that surfactant treatment led to a decrease in the maximal velocity of O2-. production when NADPH was used as substrate, but there was no effect on enzyme substrate affinity. Immunoblotting studies demonstrated that surfactant treatment induced a decrease in the association of two oxidase components, p47phox and p67phox, with the isolated plasma membrane. In contrast, surfactant treatment of the cells did not alter the phosphorylation of p47phox. A mixture of phospholipids (phosphatidylcholine and phosphatidylglycerol in a 7:3 ratio) showed similar inhibition of the PMA-induced O2-. generation. Taken together, these data suggest the mechanism of surfactant-induced inhibition of O2-. production by human neutrophils involves attenuation of translocation of cytosolic components of the respiratory burst oxidase to the plasma membrane. The phospholipid components of surfactant appear to play a significant role in this mechanism.

A novel procedure for the rapid isolation of surfactant protein A with retention of its alveolar-macrophage-stimulating properties

Previous studies have shown that surfactant protein A (SP-A) derived from alveolar-proteinosis patients activates rat alveolar macrophages. However, it is not known if normal rat, dog and human SP-A can also stimulate alveolar macrophages. As alveolar-proteinosis SP-A has a slightly different structure from ordinary SP-A, it would be possible that the ascribed alveolar-macrophage-stimulating properties of SP-A are restricted to alveolar-proteinosis SP-A. To clarify this issue, we isolated SP-A from normal rat and dog pulmonary surfactants, using the same isolation technique commonly used for the isolation of alveolar-proteinosis SP-A, i.e. by butanol precipitation. In contrast with human alveolar-proteinosis SP-A, rat and dog SP-A obtained thus could not activate rat alveolar macrophages to produce oxygen radicals or enhance the phagocytosis of fluorescein isothiocyanate-labelled herpes simplex virus. However, rat, dog and normal human SP-A isolated by a novel method, involving extraction from pulmonary surfactant by using n-octyl beta-D-glucopyranoside and subsequent purification by cation-exchange chromatography, were able to elicit an oxidative burst in rat as well as normal human alveolar macrophages. In addition, dog and rat SP-A obtained thus stimulated the phagocytosis of herpes simplex virus by rat alveolar macrophages. These findings indicate that normal human, rat and dog SP-A have the same alveolar-macrophage-stimulating properties as human alveolar proteinosis SP-A. Dog and rat SP-A isolated by this novel method had the same Ca(2+)-dependent self-aggregation and lipid-aggregation properties as SP-A isolated by butanol precipitation. The new and milder isolation procedure yielded SP-A of high purity, as judged by SDS/PAGE and ELISA.

In vitro and in vivo transfer and expression of human surfactant SP-A- and SP-B-associated protein cDNAs mediated by replication-deficient, recombinant adenoviral vectors

Congenital pulmonary alveolar proteinosis (CPAP) is a fatal disease of full-term infants that is unresponsive to current medical therapy. It is now recognized that at least some forms of this disorder are associated with a deficiency of SP-B, one of the surfactant-associated proteins, as well as probable aberrations in the surfactant-associated proteins SP-A and SP-C. Given these developments, it is logical to hypothesize that CPAP may be amenable to gene therapy, in which the human SP-B cDNA, and possibly the cDNAs of the other surfactant associated proteins, are transferred to the epithelium of the lower respiratory tract. We constructed replication-deficient, recombinant adenovirus vectors in which a constitutive viral promoter drives the expression of the DNAs for the surfactant-associated proteins, SP-B (AdCMV.SP-B) and SP-A (AdCMV.SP-A). Following infection of the human lung A549 epithelial cell line with these vectors in vitro, the appropriately sized mRNAs for these cDNAs were detected, whereas cells infected with a control virus or uninfected cells produced none. Western blots demonstrated expression of these proteins, including appropriate processing of the hydrophobic protein, SP-B. Following in vivo intratracheal infection of rats with these vectors, Northern analysis of the lungs revealed appropriately sized mRNAs for these cDNAs whereas rats infected with control virus or uninfected rats show no hybridization with the human surfactant-associated protein probes. In the AdCM-V.SP-A-infected rats, Western blots confirmed the overproduction of the human SP-A protein in both the bronchoalveolar lavage and lung homogenates compared to controls. Thus, it is feasible to utilize adenovirus vectors to transfer and express the human surfactant associated protein cDNAs in vitro and in vivo, presenting a possible mode of therapy for CPAP, as well as other surfactant deficiency states such as the neonatal respiratory distress syndrome and possibly the adult respiratory distress syndrome.

Host defence capacities of pulmonary surfactant: evidence for 'non-surfactant' functions of the surfactant system

The most well characterized function of pulmonary surfactant is its ability to reduce surface tension at the alveolar air-liquid interface, thereby preventing lung collapse. However, several lines of evidence suggest that surfactant may also have 'non-surfactant' functions: specific components of surfactant (proteins and phospholipids) may interact with different alveolar cells, inhaled particles and micro-organisms modulating pulmonary host defence systems. SP-A, the most abundant surfactant protein, binds to alveolar macrophages via a specific surface receptor with high affinity [128]. Such binding effects the release of reactive oxygen species from resident alveolar macrophages if SP-A is properly presented to the target cell. SP-A also stimulates chemotaxis of alveolar macrophages [142], and serves as an opsonin in the phagocytosis of herpes simplex virus [161] Candida tropicalis [138] and various bacteria [137]. In addition, SP-A enhances the uptake of particles by monocytes and culture-derived macrophages [140] and improves bacterial killing. SP-D, another hydrophobic surfactant-associated protein, might interact with alveolar macrophages as well, stimulating the release of oxygen radicals [148], while for the hydrophilic surfactant proteins SP-B and SP-C no macrophage interactions have been described so far. SP-A and SP-D are members of the so-called 'collectins', pattern recognition molecules involved in first line defence. While some surfactant proteins appear to stimulate certain macrophage defence functions, surfactant phospholipids seem to inhibit those of lymphocytes. Suppressed lymphocyte functions include lymphoproliferation in response to mitogens and alloantigens, B cell immunoglobulin production and natural killer cell cytotoxicity. Concerning surfactant's phospholipid composition phosphatidylglycerol is more suppressive than phosphatidylcholine on a molar basis [38]. Bovine surfactant has an immunosuppressive effect on the development of hypersensitivity pneumonitis in a guinea pig model [150]. Despite these interesting observations, several important questions concerning the interactions of surfactant components with pulmonary host defence systems remain unanswered. Sufficient host defence in the lungs works through various humoral-cellular systems in conjunction with the specific anatomy of the airways and the gas exchange surface--how does the surfactant system fit into this network? Surfactant and alveolar cells are both altered during lung injury--is there a relationship between alveolar cells from RDS patients and the endogenous surfactant isolated from such patients? How does exogenous surfactant as used for substitution therapy modulate the defence system of the host? Some of those artificial surfactants have been shown to inhibit the endotoxin-alveolar macrophages, PMNs and monocytes including IL-1, IL-6 and TNF [139,152].(ABSTRACT TRUNCATED AT 400 WORDS)

Rat surfactant apoprotein A (SP-A) exhibits antioxidant effects on alveolar macrophages

The effects of surfactant apoprotein A (SP-A) on the superoxide production of rat alveolar macrophages (AM) were studied. Superoxide production was measured by the ferricytochrome c reduction method. When AM were incubated with SP-A only during the measurement of superoxide production, superoxide production was not influenced by SP-A. However, when AM were preincubated with SP-A at a concentration of 1, 2, and 10 micrograms/ml, superoxide production by AM was significantly inhibited (P < 0.05, P < 0.01, P < 0.01, respectively). The superoxide production of AM stimulated by PMA was significantly inhibited by SP-A at a concentration of 1 microgram/ml (P < 0.01), and superoxide production stimulated by zymosan was also inhibited by SP-A at a concentration of 10 micrograms/ml (P < 0.05). Suppression of superoxide production of unstimulated and PMA-stimulated AM was significantly inhibited by anti-SP-A antibody. Superoxide generation by the xanthine and xanthine oxidase system was not affected by the presence of SP-A. Our results suggest that superoxide production of AM can be inhibited by SP-A and that this inhibitory effect on AM is due to a specific effect of SP-A. From these results, it is speculated that SP-A may have a protective role for oxidant injury by AM in the lung.

Localization of the receptor-binding site in the collectin family of proteins

Collectin receptor (Clq receptor) has been shown to bind human Clq, mannose-binding protein (MBP), lung surfactant protein A (SP-A) and bovine conglutinin. These ligands have a similar ultrastructure, each consisting of collagenous and globular domains, but do not show a high degree of sequence similarity. For Clq and SP-A, it has been shown that both bind to cell-surface-expressed receptor(s) via their collagenous regions and this is likely to be the case with the other ligands. Within the collagenous region, near the 'bend' region of the collagen triple helix in Clq, MBP and SP-A, a cluster of similar charged residues is observed. This region has been suggested to be associated with receptor binding. A similar region of charge density occurs close to the N-terminus of conglutinin. In this paper we describe a truncated form of conglutinin, which has 55 amino acids missing from the N-terminus and does not bind to the collectin receptor. The results presented here strongly indicate that receptor-ligand interaction is mediated via the N-terminal region of conglutinin, consistent with the earlier proposal for the binding site.

Surfactant and the adult respiratory distress syndrome

ARDS includes a complex series of events leading to alveolar damage, high permeability pulmonary edema, and respiratory failure. The endogenous pulmonary surfactant system is crucial to maintaining normal lung function, and only recently has it been appreciated that alterations in the surfactant system significantly contributed to the pathophysiology of the lung injury of patients with ARDS. Through a combination of analyzing BAL samples from patients with ARDS and extensive animal studies, there have been significant insights into the variety of surfactant abnormalities that can occur in injured lungs. These include altered surfactant composition and pool sizes, abnormal surfactant metabolism, and inactivation of alveolar surfactant by serum proteins present within the airspace. Positive effects of exogenous surfactant administration on acute lung injury have been reported. There is now a prospective, randomized clinical trial evaluating the efficacy of aerosolized exogenous surfactant in patients with ARDS. This trial has demonstrated improvements in gas exchange and a trend toward decreased mortality in response to the surfactant. Despite these encouraging results, there are multiple factors requiring further investigation in the development of optimal surfactant treatment strategies for patients with ARDS. Such factors include the development of optimal surfactant delivery techniques, determining the ideal time for surfactant administration during the course of injury, and the development of optimal exogenous surfactant preparations that will be used to treat these patients. With further clinical trials and continued research efforts, exogenous surfactant administration should play a useful role in the future therapeutic approach to patients with ARDS.

Interaction of C1q receptor with lung surfactant protein A

Earlier we reported the purification of C1q receptor (C1qR) from U937 cells and human tonsil lymphocytes (Malhotra, R. and Sim, R. B., Biochem. J. 1989. 218: 625) and showed that C1qR interacts with the ligands C1q, mannose-binding protein, conglutinin and lung surfactant protein A (SP-A) (Malhotra, R., Thiel, S., Reid, K. B. M. and Sim, R. B., J. Exp. Med. 1990. 172: 955). C1qR was characterized as an acidic glycoprotein, which, when solubilized, exists as a dimer of Mr 115,000 under non-denaturing conditions. In this article we provide evidence for binding of radioiodinated SP-A to U937 cells and show that binding of radioiodinated SP-A to U937 cells is specific, saturable, salt dependent and is inhibited by purified C1qR and by C1q. The interaction of SP-A with U937 cells was found to up-regulate the surface expression of C1qR. Incubation of SP-A with U937 cells at 37 degrees C for 80 min was found to increase the receptor number per cell. Increase in receptor number was inhibited in the presence of sodium azide and monensin. Incubation of cells with calcium ionophore A23187 induced increased surface expression in the absence of SP-A. The results indicate that interaction of SP-A with U937 cells triggers the expression of an intracellular pool of C1qR.

Ventilation and secretion of pulmonary surfactant

Various factors are involved in the regulation of surfactant secretion: chemical agonist; local environmental factors such as mediators, locally produced hormones, and possibly pH; and finally, mechanical stress occurring during lung inflation. Here we suggest a model of regulation which is grouped into three levels: a basal autoregulatory mechanism with local factors being superimposed and a systemic level acting through hormones reaching the lung via the bloodstream. Depending on the situation, the different levels may vary in their importance. For the normal situation, in the absence of stress factors, we suggest the autoregulation of stretch-induced secretion and SP-A inhibition as indicated by in vitro experiments to be the prominent regulatory mechanism for surfactant secretion. From this model, mechanisms can be derived which indicate involvement of the surfactant system in, for example, obstructive lung disease. Support from the literature for this hypothesis is reviewed. Because quantitative measurement of the amount of surfactant-associated phospholipids cannot be done adequately at this time, we suggest testing the relatively risk-free application of exogenous surfactant in a pilot phase based on indications for its involvement and usefulness derived from animal and in vitro experiments.