Alveolar Air-liquid Interface Research Articles

Cytokines and production of surfactant components

The production of pulmonary surfactant, a complex of lipids and proteins that reduces surface tension at the alveolar air-liquid interface, is developmentally regulated. Several hormones, most notably glucocorticoids, are known to accelerate maturation of the surfactant system. Cytokines are polypeptides that act mostly in a paracrine fashion and possess a wide spectrum of activities on multiple types of cells. Many cytokines are produced by different lung cells a various stages of fetal development or under pathological conditions affecting the fetus. In addition, cytokines present in amniotic fluid or in the blood stream may reach the fetal lungs. Some cytokines, including epidermal growth factor, transforming growth factor-alpha, and interferon-gamma have been shown to stimulate the production of surfactant components. On the other hand, tumor necrosis factor and transforming growth factor-beta downregulate the production of surfactant lipids and proteins. We have recently shown that the proinflammatory cytokine interleukin-1 (IL-I) enhances the expression of surfactant protein A (SP-A) in fetal rabbit lung explants. In addition, injection of IL-I into the amniotic fluid of fetal rabbits enhances the expression of surfactant proteins and improves the lung compliance of preterm animals. Preterm delivery is often associated with subclinical intraamniotic infection. In these cases, amniotic fluid concentrations of IL-I are often elevated. We propose that this cytokine accelerates maturation of the surfactant system in fetal lungs and thus prepares the fetus for extrauterine life.

Dynamic elastance and tissue resistance of isolated liquid-filled rat lungs

The effect of the surface forces of the alveolar air-liquid interface on the dynamic behavior of lung tissue was investigated in five isolated liquid-filled rat lungs. The lungs were subjected to 0.04-Hz sinusoidal oscillation (1.5-ml tidal volume) at lung volume (VL) levels ranging from volume at zero pressure (V0) + 4 ml to V0 + 10 ml. Oscillations were performed at each VL after inflation of the lungs from V0. Alveolar pressure (PA) was measured with an alveolar capsule attached to the visceral pleura. Dynamic elastance (Edyn), tissue resistance (Rti), and hysteresivity [eta = Rti omega/Edyn, where omega is angular frequency (2 pi x frequency)] were computed from PA and VL changes. Edyn was 59.6 +/- 4.3 Pa/ml at V0 + 4 ml and varied little up to V0 + 7 ml. Thereafter, Edyn increased markedly with VL, reaching 102 +/- 16 Pa/ml at V0 + 10 ml. No significant difference was found between elastance computed from PA and that computed from pressure recorded at the airway opening. Rti was 35.2 +/- 3.6 Pa.s.ml-1 and exhibited a VL dependence similar to that of Edyn. As a result, eta was 0.16 and did not vary significantly in the explored VL range. We conclude that PA can be reliably measured in the liquid-filled lung by means of alveolar capsules. In the liquid-filled lung, Edyn was smaller than and eta was similar to values reported for air-filled lungs. Hence, surface tension accounts for a considerable part of elastance and Rti of the air-filled lung within the volume range of normal breathing.

Alveolar lining layer is thin and continuous: low-temperature scanning electron microscopy of rat lung.

The low-temperature electron microscope, which preserves aqueous structures as solid water at liquid nitrogen temperature, was used to image the alveolar lining layer, including surfactant and its aqueous subphase, of air-filled lungs frozen in anesthetized rats at 15-cmH2O transpulmonary pressure. Lining layer thickness was measured on cross fractures of walls of the outermost subpleural alveoli that could be solidified with metal mirror cryofixation at rates sufficient to limit ice crystal growth to 10 nm and prevent appreciable water movement. The thickness of the liquid layer averaged 0.14 micron over relatively flat portions of the alveolar walls, 0.89 micron at the alveolar wall junctions, and 0.09 micron over the protruding features (9 rats, 20 walls, 16 junctions, and 146 areas), for an area-weighted average thickness of 0.2 micron. The alveolar lining layer appears continuous, submerging epithelial cell microvilli and intercellular junctional ridges; varies from a few nanometers to several micrometers in thickness, and serves to smooth the alveolar air-liquid interface in lungs inflated to zone 1 or 2 conditions.

Adenoviral-mediated gene transfer of human surfactant protein B to respiratory epithelial cells.

Human surfactant protein B (SP-B) is a 79-amino acid, phospholipid-associated polypeptide expressed by respiratory epithelial cells of the lung. SP-B is essential for lung function, enhancing the spreading and stability of surfactant phospholipids that serve to reduce surface tension at the alveolar air-liquid interface. Congenital absence of SP-B results in neonatal respiratory failure and death. In the present work, we constructed a replication-deficient adenoviral vector, Av1SP-B1, in which the human SP-B cDNA is expressed under control of the Rous sarcoma virus (RSV) promoter in an E1-E3-deleted adenovirus type 5 (Ad5)-based vector system. Av1SP-B1 was produced in 293 kidney cells, directing the synthesis of the SP-B protein and SP-B peptides. Av1SP-B1 directed the synthesis of SP-B mRNA, precursor and active 8-9 kD polypeptide in immortalized mouse lung epithelial cells (MLE-12 cells), demonstrating complete processing to the human SP-B protein by these cells. Synthesis of human SP-B mRNA was detected as early as 12 h after infection and was maximal 48 h after infection in vitro. Northern blot analysis demonstrated that human SP-B mRNA was expressed in the lungs of cotton rats infected with Av1SP-B1 but not in those of uninfected animals or in animals infected with a reporter adenoviral vector, Av1LacZ4. In situ hybridization demonstrated the abundance and localization of the transferred human SP-B mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Pathophysiology of congenital diaphragmatic hernia II: The fetal lamb CDH model is surfactant deficient

The high mortality for congenital diaphragmatic hernia (CDH) has been attributed to a combination of pulmonary hypoplasia and pulmonary hypertension. We hypothesize that a surfactant deficiency may in part be contributing to the pathophysiology of CDH. This study documents the functional, quantitative, and qualitative aspects of the surfactant status of the alveolar air-liquid interface and the type II pneumocyte in the fetal lamb CDH model. Ten lamb fetuses (gestational age, 80 days) had a CDH created via a left thoracotomy and then were allowed to continue in utero development until term. Three litter mates and three nonoperated time-dated fetuses served as controls. At term, pressure-volume curves were performed to measure pulmonary compliance and total lung capacity. Alveolar lavage was then performed to measure the quantitative and the qualitative aspects of pulmonary surfactant. Finally, isolation of type II pneumocytes allowed quantification of phospholipid synthesis. When compared with controls (N = 6), the CDH lambs (N = 5) had significantly smaller lungs ( P = .009), decreased total lung capacity ( P < .001) and compliance ( P < .001), reduced total lavaged phospholipids ( P = .006), and decreased percent phosphatidylcholine ( P = .02). CDH lambs also had increased total lavaged proteins ( P = .05) and higher minimum dynamic surface tension ( P < .001). A surfactant deficiency may be contributing to the pathophysiology of CDH. Surfactant replacement therapy in premature infants has been shown to improve lung compliance, decrease morbidity, and improve survival. Exogenous surfactant may also benefit infants with CDH.

Pulmonary surfactant: Surface properties and function of alveolar and airway surfactant

Abstract

Experimental Neonatal Respiratory Failure Induced by a Monoclonal Antibody to the Hydrophobic Surfactant-Associated Protein SP-B

The present experiments were designed to test whether selective blocking of the surfactant-associated hydrophobic polypeptide SP-B (8.7 kD) would interfere with lung function during neonatal adaptation. A MAb to porcine SP-B was produced by hybridoma cell line (8B5E); this antibody cross-reacts with rabbit SP-B. Six mg of MAb to SP-B, dissolved in 0.2 mL of saline, was instilled into the airways of near-term newborn rabbits (gestational age 29 d), before the onset of ventilation. Control animals received the same amount of nonspecific rabbit IgG in saline, or were untreated. The animals were ventilated for 120 min with a standardized tidal volume (10 mL/kg). The specific antibody caused a prominent, immediate decrease in lung-thorax compliance, associated with acute inflammatory and exudative lung lesions including hyaline membranes. IgG alone had no such effects. Our data suggest that the MAb to SP-B inhibits surfactant function in the neonatal period by blocking one of the mechanisms responsible for fast adsorption of the surfactant phospholipids to the alveolar air-liquid interface. In addition, an acute inflammatory reaction is probably triggered in the lung parenchyma by the immune reaction.

Effect of Fibrinogen Degradation Products and Lung Ground Substance on Surfactant Function

Acute lung injury syndromes have many characteristics including protein-rich alveolar edema, hyaline membranes, and abnormal surface tension at the alveolar air-liquid interface. Increased surface tension can occur because of a relative surfactant deficiency and/or dysfunction. It has been previously demonstrated that surfactant dysfunction occurs when plasma protein inhibitors leak into the alveolar space during the induction of the lung injury and edema formation. The present study investigated whether inhibitors that would be generated during the stage of repair from lung injury could impair surfactant function. We determined whether fibrinogen degradation products (FDP) which would be released during lysis of the fibrin(ogen)-containing alveolar exudate and hyaline membranes, and components of the lungs' ground substance could inhibit the in vitro function of a lipid extract surfactant preparation. FDP were prepared by incubating human fibrinogen with plasmin or neutrophil elastase for 4 min to 60 h and were characterized by SDS-PAGE. Early (fragment X and Y) and late (fragment D and E) plasmin-derived FDP (MW greater than 40,000) inhibited surfactant function as assessed by a bubble surfactometer. The early elastase-derived FDP also inhibited surfactant, but the later and much smaller fragments (MW less than 15,000) did not affect surfactant function. Laminin also inhibited surfactant in a dose-dependent manner. Neither hyaluronic acid nor heparan sulfate affected surfactant performance in vitro. We conclude that plasmin-induced lysis of intraalveolar fibrinogen and hyaline membranes will result in prolonged generation (i.e. days) of surfactant inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

Composition and metabolism of the apolipoproteins of pulmonary surfactant.

It seems difficult to believe, in light of to day's sophisticated morphology, that as recently as 40 years ago pulmonary biologists still thought it possible that gas exchange might occur across capillaries that were directly exposed to the alveolar air (30). With the development of a more realistic viewpoint of alveolar morphology, encompassing an alveolar epithelium with an overlying fluid layer, much older questions (47) concerning the stability of air spaces received renewed interest. Macklin (33), Pattie (37), Clements (9), and Mead (36) and their collaborators asked how it was possible that alveoli were able to resist the collapsing forces that would result from the surface tension of the alveolar air-liquid interface. Pattle and Clements provided partial answers to these questions through their isolation and characterization of a material se­ creted by alveolar epithelial type II cells, which they then showed to be capable of lowering surface tension to values less than had ever been observed for a biological substance, This material is now generally referred to as pulmonary surfactant. When isolated from alveolar lavage fluid it has been found to be comprised of several lipids, specific apolipoproteins, and carbohydrates (20). The structure is complex and poorly understood (48). This paper will review information related to the composition and metabo­ lism of this material, focusing on the newer results that are pertinent. Studies on the physicochemical properties and physiological functions of surfactant

Pulmonary surfactant.

Pulmonary surfactant reduces the surface tension of the alveolar air-liquid interface, thereby providing mechanical stability and preventing alveolar atelectasis. More than 50% of surfactant is dipalmitoyl phosphatidylcholine, a material that is capable of reducing the surface tension of the alveolar interface to uniquely low values. The functions of the remaining 25% unsaturated phosphatidylcholines, 5-10% phosphatidylglycerol, 5% cholesterol, and 8-10% protein are unknown. Surfactant is synthesized by alveolar epithelial type II cells and is probably secreted as a lipoprotein complex. Lamellar bodies, which distinguish type II cells, are likely to be intracellular sites of transport of processing. The catabolism of surfactant after it is secreted into the alveolar lumen is complicated and involves different turnover times for the phosphatidylcholines, phosphatidylglycerol, and the proteins. The metabolic events are under hormonal control and may involve an interplay between beta-adrenergic agonists cAMP, and prostaglandins. In disease, such as the neonatal and adult respiratory distress syndromes, derangements in the metabolic processes may produce surfactant that is abnormal with respect to its chemical and physical properties.

Relation between alveolar surface tension and pulmonary vascular resistance.

In order to examine the hypothesis that surface forces at the alveolar air-liquid interface may play a role in determining the course of pulmonary vascular resistance on negative-pressure inflation, resistance was studied as a function of lung volume for both air and dextran-filled dog lungs in excised preparations perfused with fresh dog blood under carefully controlled conditions. Results show that abolishing the interface by liquid filling does not alter the course of resistance significantly. It is thus concluded that the presence of the interface can have only a minor effect on the course of resistance and that purely volume-dependent factors are primarily responsible. Submitted on September 19, 1960