Surfactant protein profile of pulmonary surfactant in premature infants

Preterm birth affects both surfactant synthesis and lung liquid resorption actors in fetal sheep

INTRODUCTION

After birth, the lungs must resorb the fluid they contain. This process involves multiple actors such as surfactant, aquaporins and ENaC channels. Preterm newborns often exhibit respiratory distress syndrome due to surfactant deficiency, and transitory tachypnea caused by a delay in lung liquid resorption. Our hypothesis is that surfactant, ENaC and aquaporins are involved in respiratory transition to extrauterine life and altered by preterm birth. We compared these candidates in preterm and term fetal sheeps.

MATERIALS AND METHODS

We performed cesarean sections in 8 time-dated pregnant ewes (4 at 100 days and 4 at 140 days of gestation, corresponding to 24 and 36 weeks of gestation in humans), and obtained 13 fetal sheeps in each group. We studied surfactant synthesis (SP-A, SP-B, SP-C), lung liquid resorption (ENaC, aquaporins) and corticosteroid regulation (glucocorticoid receptor, mineralocorticoid receptor and 11-betaHSD2) at mRNA and protein levels.

RESULTS

The mRNA expression level of SFTPA, SFTPB and SFTPC was higher in the term group. These results were confirmed at the protein level for SP-B on Western Blot analysis and for SP-A, SP-B and SP-C on immunohistochemical analysis. Regarding aquaporins, ENaC and receptors, mRNA expression levels for AQP1, AQP3, AQP5, ENaCα, ENaCβ, ENaCγ and 11βHSD2 mRNA were also higher in the term group.

DISCUSSION

Expression of surfactant proteins, aquaporins and ENaC increases between 100 and 140 days of gestation in an ovine model. Further exploring these pathways and their hormonal regulation could highlight some new explanations in the pathophysiology of neonatal respiratory diseases.

Neonatal surfactant therapy beyond respiratory distress syndrome

Whilst exogenous surfactant therapy is central to the management of newborn infants with respiratory distress syndrome, its use in other neonatal lung diseases remains inconsistent and controversial. Here we discuss the evidence and experience in relation to surfactant therapy in newborns with other lung conditions in which surfactant may be deficient or dysfunctional, including meconium aspiration syndrome, pneumonia, congenital diaphragmatic hernia and pulmonary haemorrhage. We find that, for all of these diseases, administration of exogenous surfactant as bolus therapy is frequently associated with transient improvement in oxygenation, likely related to temporary mitigation of surfactant inhibition in the airspaces. However, for none of them is there a lasting clinical benefit of surfactant therapy. By virtue of interrupting disease pathogenesis, lavage therapy with dilute surfactant in MAS offers the greatest possibility of a more pronounced therapeutic effect, but this has yet to be definitively proven. Lavage therapy also involves a greater degree of procedural risk.

SuPreme Study: a protocol to study the neuroprotective potential of sulfate among very/extremely preterm infants

INTRODUCTION

Antenatal maternal magnesium sulfate (MgSO4) administration is a proven efficacious neuroprotective treatment reducing the risk of cerebral palsy (CP) among infants born preterm. Identification of the neuroprotective component with target plasma concentrations could lead to neonatal treatment with greater efficacy and accessibility.

METHODS AND ANALYSIS

This is a prospective observational cohort study, in three tertiary Australian centres. Participants are preterm infants, irrespective of antenatal MgSO4 exposure, born in 2013-2020 at 24+0 to 31+6 weeks gestation, and followed up to 2 years corrected age (CA) (to September 2023). 1595 participants are required (allowing for 17% deaths/loss to follow-up) to detect a clinically significant reduction (30% relative risk reduction) in CP when sulfate concentration at 7 days of age is 1 SD above the mean.A blood sample is collected on day 7 of age for plasma sulfate and magnesium measurement. In a subset of participants multiple blood and urine samples are collected for pharmacokinetic studies, between days 1-28, and in a further subset mother/infant blood is screened for genetic variants of sulfate transporter genes.The primary outcome is CP. Surviving infants are assessed for high risk of CP at 12-14 weeks CA according to Prechtl's Method to assess General Movements. Follow-up at 2 years CA includes assessments for CP, cognitive, language and motor development, and social/behavioural difficulties.Multivariate analyses will examine the association between day 7 plasma sulfate/magnesium concentrations with adverse neurodevelopmental outcomes. A population pharmacokinetic model for sulfate in the preterm infant will be created using non-linear mixed-effects modelling.

ETHICS AND DISSEMINATION

The study has been approved by Mater Misericordiae Ltd Human Research Ethics Committee (HREC/14/MHS/188). Results will be disseminated in peer-reviewed journal publications, and provided to the funding bodies. Using consumer input, a summary will be prepared for participants and consumer groups.

Surfactant proteins analysis in perinatal deceased preterm twins among the Romanian population

The molecular basis of the evaluation of children suspected of having disorders of surfactant proteins is still under discussion. In this study, we aimed to describe the morphological characteristics and to evaluate the immunohistochemical expression of surfactant proteins (surfactant protein A [SPA], surfactant protein B, and pro-surfactant protein C) in the preterm twins that deceased due to unexplained respiratory distress syndrome (n = 12). Results showed statistically significant positive correlations between surfactant protein B expressions and pulmonary hemorrhage (ρ = 0.678; P < .05), SPA levels, and Apgar score (ρ = 0.605; P < .05) and also expressions of SPA and bronchopneumonia (ρ = 0.695; P < .05). The fetuses and neonates of the same gestational age showed differences among surfactant proteins regarding the immunostaining expression. Our data evidence a marked interindividual variability in the expression of all 3 surfactant proteins among the cases analyzed (n = 12), suggesting the intervention of some individual and epigenetic factors during gestation that might influence surfactant protein production and consequently survival rate.

The steroid mometasone alters protein containing lung surfactant monolayers in a concentration-dependent manner

Mometasone is an investigational anti-inflammatory steroidal drug to treat inflammation via pulmonary administration. For steroid drugs to be effective they need to be adsorbed by lung surfactants, a thin monolayer at the air-water interface in alveoli that reduces surface tension. Information on the molecular-level interactions of the drug with lung surfactants is useful to understand the mechanism of adsorption. In this study, we use coarse-grained molecular dynamics simulation to understand the concentration-dependent effect of mometasone on a lung surfactant monolayer (LSM) composed of lipids and surfactant proteins, under two different breathing conditions (exhalation, at surface tension 0 mNm^-1 and inhalation, surface tension 20-25 mNm^-1). A series of fixed-APL and fixed-surface tension simulations were used to demonstrate that in the absence of drugs, the model LSM reproduces the surface tensions for the compressed and expanded states, as well as compressibility at different surface tensions. In-depth analysis of simulations of a LSM in the presence of five different drug concentrations shows that mometasone alters the structure and dynamics of the LSM in a concentration-dependent manner. Mometasone induces a collapse in the monolayer that is affected by the surfactant protein and surface tension. Overall, these findings suggest that the surfactant proteins, surface tension and drug concentration are all critical components affecting monolayer stability and drug adsorption. The outcomes of this study may be beneficial for a more in-depth understanding of how mometasone is adsorbed by lung surfactants.

Shape matters-the interaction of gold nanoparticles with model lung surfactant monolayers

The lung surfactant monolayer (LSM) forms the main biological barrier for any inhaled particles to enter our bloodstream, including gold nanoparticles (AuNPs) present as air pollutants and under investigation for use in biomedical applications. Understanding the interaction of AuNPs with lung surfactant can assist in understanding how AuNPs enter our lungs. In this study, we use coarse-grained molecular dynamics simulations to investigate the effect of four different shape D AuNPs (spherical, box, icosahedron and rod) on the structure and dynamics of a model LSM, with a particular focus on differences resulting from the shape of the AuNP. Monolayer-AuNP systems were simulated in two different states: the compressed state and the expanded state, representing inhalation and exhalation conditions, respectively. Our results indicate that the compressed state is more affected by the presence of the AuNPs than the expanded state. Our results show that in the compressed state, the AuNPs prevent the monolayer from reaching the close to zero surface tension required for normal exhalation. In the compressed state, all four nanoparticles (NPs) reduce the lipid order parameters and cause a thinning of the monolayer where the particles drag surfactant molecules into the water phase. Comparing the different properties shows no trend concerning which shape has the biggest effect on the monolayer, as shape-dependent effects vary among the different properties. Insights from this study might assist future work of how AuNP shapes affect the LSM during inhalation or exhalation conditions.

Surfactant status and respiratory outcome in premature infants receiving late surfactant treatment

BACKGROUND

Many premature infants with respiratory failure are deficient in surfactant, but the relationship to occurrence of bronchopulmonary dysplasia (BPD) is uncertain.

METHODS

Tracheal aspirates were collected from 209 treated and control infants enrolled at 7-14 days in the Trial of Late Surfactant. The content of phospholipid, surfactant protein B, and total protein were determined in large aggregate (active) surfactant.

RESULTS

At 24 h, surfactant treatment transiently increased surfactant protein B content (70%, p < 0.01), but did not affect recovered airway surfactant or total protein/phospholipid. The level of recovered surfactant during dosing was directly associated with content of surfactant protein B (r = 0.50, p < 0.00001) and inversely related to total protein (r = 0.39, p < 0.0001). For all infants, occurrence of BPD was associated with lower levels of recovered large aggregate surfactant, higher protein content, and lower SP-B levels. Tracheal aspirates with lower amounts of recovered surfactant had an increased proportion of small vesicle (inactive) surfactant.

CONCLUSIONS

We conclude that many intubated premature infants are deficient in active surfactant, in part due to increased intra-alveolar metabolism, low SP-B content, and protein inhibition, and that the severity of this deficit is predictive of BPD. Late surfactant treatment at the frequency used did not provide a sustained increase in airway surfactant.

Surfactant Components and Tracheal Aspirate Inflammatory Markers in Preterm Infants with Respiratory Distress Syndrome

In 93 preterm infants ≤32 weeks of gestational age and 12 control infants, epithelial lining fluid disaturated-phosphatidylcholine, surfactant protein A and B, albumin, and myeloperoxidase activity were assessed after intubation and before exogenous surfactant administration. We found that disaturated-phosphatidylcholine, surfactant protein B, and myeloperoxidase were significantly higher in preterms with chorioamnionitis.

Effect of betamethasone, surfactant, and positive end-expiratory pressures on lung aeration at birth in preterm rabbits

Antenatal glucocorticoids, exogenous surfactant, and positive end-expiratory pressure (PEEP) ventilation are commonly provided to preterm infants to enhance respiratory function after birth. It is unclear how these treatments interact to improve the transition to air-breathing at birth. We investigated the relative contribution of antenatal betamethasone, prophylactic surfactant, and PEEP (3 cmH₂O) on functional residual capacity (FRC) and dynamic lung compliance (C_DL) in preterm (28 day GA) rabbit kittens at birth. Kittens were delivered by cesarean section and mechanically ventilated. FRC was calculated from X-ray images, and C_DL was measured using plethysmography. Without betamethasone, PEEP increased FRC recruitment and C_DL Surfactant did not further increase FRC, but significantly increased C_DL Betamethasone abolished the benefit of PEEP on FRC, but surfactant counteracted this effect of betamethasone. These findings indicate that low PEEP levels are insufficient to establish FRC at birth following betamethasone treatment. However, surfactant reversed the effect of betamethasone and when combined, these two treatments enhanced FRC recruitment irrespective of PEEP level.

The Lungfish Transcriptome: A Glimpse into Molecular Evolution Events at the Transition from Water to Land

Lungfish and coelacanths are the only living sarcopterygian fish. The phylogenetic relationship of lungfish to the last common ancestor of tetrapods and their close morphological similarity to their fossil ancestors make this species uniquely interesting. However their genome size, the largest among vertebrates, is hampering the generation of a whole genome sequence. To provide a partial solution to the problem, a high-coverage lungfish reference transcriptome was generated and assembled. The present findings indicate that lungfish, not coelacanths, are the closest relatives to land-adapted vertebrates. Whereas protein-coding genes evolve at a very slow rate, possibly reflecting a “living fossil” status, transposable elements appear to be active and show high diversity, suggesting a role for them in the remarkable expansion of the lungfish genome. Analyses of single genes and gene families documented changes connected to the water to land transition and demonstrated the value of the lungfish reference transcriptome for comparative studies of vertebrate evolution.

Mature Surfactant Protein-B Expression by Immunohistochemistry as a Marker for Surfactant System Development in the Fetal Sheep Lung

Evaluation of the number of type II alveolar epithelial cells (AECs) is an important measure of the lung's ability to produce surfactant. Immunohistochemical staining of these cells in lung tissue commonly uses antibodies directed against mature surfactant protein (SP)-C, which is regarded as a reliable SP marker of type II AECs in rodents. There has been no study demonstrating reliable markers for surfactant system maturation by immunohistochemistry in the fetal sheep lung despite being widely used as a model to study lung development. Here we examine staining of a panel of surfactant pro-proteins (pro-SP-B and pro-SP-C) and mature proteins (SP-B and SP-C) in the fetal sheep lung during late gestation in the saccular/alveolar phase of development (120, 130, and 140 days), with term being 150 ± 3 days, to identify the most reliable marker of surfactant producing cells in this species. Results from this study indicate that during late gestation, use of anti-SP-B antibodies in the sheep lung yields significantly higher cell counts in the alveolar epithelium than SP-C antibodies. Furthermore, this study highlights that mature SP-B antibodies are more reliable markers than SP-C antibodies to evaluate surfactant maturation in the fetal sheep lung by immunohistochemistry.

Surface film formation in vitro by infant and therapeutic surfactants: role of surfactant protein B

BACKGROUND

Pulmonary surfactant provides an alveolar surface-active film that is critical for normal lung function. Our objective was to determine in vitro film formation properties of therapeutic and infant surfactants and the influence of surfactant protein (SP)-B content.

METHODS

We used a multiwell fluorescent assay measuring maximum phospholipid surface accumulation (Max), phospholipid concentration required for half-maximal film formation (½Max), and time for maximal accumulation (tMax).

RESULTS

Among five therapeutic surfactants, calfactant (highest SP-B content) had film formation values similar to natural surfactant, and addition of SP-B to beractant (lowest SP-B) normalized its Max value. Addition of budesonide to calfactant did not adversely affect film formation. In tracheal aspirates of preterm infants with evolving chronic lung disease, SP-B content correlated with ½Max and tMax values, and SP-B supplementation of SP-B-deficient infant surfactant restored normal film formation. Reconstitution of normal surfactant indicated a role for both SP-B and SP-C in film formation.

CONCLUSION

Film formation in vitro differs among therapeutic surfactants and is highly dependent on SP-B content in infant surfactant. The results support a critical role of SP-B for promoting surface film formation.

The impact of vitamin D on fetal and neonatal lung maturation. A systematic review

Respiratory distress syndrome (RDS) and bronchopulmonary dysplasia (BPD) are major complications to preterm birth. Hypovitaminosis D is prevalent in pregnancy. We systematically reviewed the evidence of the impact of vitamin D on lung development, surfactant synthesis, RDS, and BPD searching PubMed, Embase, and Cochrane databases with the terms vitamin D AND (surfactant OR lung maturation OR lung development OR respiratory distress syndrome OR fetal lung OR prematurity OR bronchopulmonary dysplasia). Three human studies, ten animal studies, two laboratory studies, and one combined animal and laboratory study were included. Human evidence was sparse, allowing no conclusions. BPD was not associated with vitamin D receptor polymorphism in a fully adjusted analysis. Animal and laboratory studies showed substantial positive effects of vitamin D on the alveolar type II cell, fibroblast proliferation, surfactant synthesis, and alveolarization. These data support the hypothesis of hypovitaminosis D as a frequent, modifiable risk factor of RDS and BPD, which should be tested in randomized controlled trials on pregnant women, those with threatening preterm delivery, or in the preterm neonates. Future experimental and human studies should aim to identify optimal time windows, vitamin D doses, and cut-off levels for 25-hydroxyvitamin D in interventions against RDS, BPD, and later adverse respiratory outcomes.

Surfactant protein C metabolism in human infants and adult patients by stable isotope tracer and mass spectrometry

Surfactant protein C (SP-C) is deemed as the surfactant protein most specifically expressed in type II alveolar epithelial cells and plays an important role in surfactant function. SP-C turnover in humans and its meaning in the clinical context have never been approached. In this study, we used mass spectrometry to investigate SP-C turnover in humans. We studied four infants and eight adults requiring mechanical ventilation. All patients had no lung disease. Patients received a 24-h continuous infusion of (13)C-leucine as precursor of SP-C, and serial tracheal aspirates and plasma samples were obtained every 6 h till 48 h. SP-C was isolated from tracheal aspirates by sorbent-phase chromatography. (13)C-leucine SP-C enrichment could be successfully measured in three infant and in four adult samples by using mass spectrometry coupled with a gas chromatographer. Median SP-C fractional synthesis rate, secretion time, and peak time were 15.7 (14.1-27.5)%/day, 6.0 (4.7-11.5) h, and 24 (20-27) h. In conclusion, this study shows that it is feasible to accurately determine SP-C turnover in humans by stable isotopes.

Pbx1 activates Fgf10 in the mesenchyme of developing lungs

Insufficiency of surfactants is a core factor in respiratory distress syndrome, which causes apnea and neonatal death, particularly in preterm infants. Surfactant proteins are secreted by alveolar type II cells in the lung epithelium, the differentiation of which is regulated by Fgf10 elaborated by the adjacent mesenchyme. However, the molecular regulation of mesenchymal Fgf10 during lung development has not been fully understood. Here, we show that Pbx1, a homeodomain transcription factor, is required in the lung mesenchyme for the expression of Fgf10. Mouse embryos lacking Pbx1 in the lung mesenchyme show compact terminal saccules and perinatal lethality with failure of postnatal alveolar expansion. Mutant embryos had severely reduced expression of Fgf10 and surfactant genes (Spa, Spb, Spc, and Spd) that are essential for alveolar expansion for gas exchange at birth. Molecularly, Pbx1 directly binds to the Fgf10 promoter and cooperates with Meis and Hox proteins to transcriptionally activate Fgf10. Our results thus show how Pbx1 controls Fgf10 in the developing lung.

Remarkably delayed occurrence of normal surfactant composition in an extremely preterm infant

Reported herein is the first case of a remarkably delayed occurrence of normal surfactant composition in an extremely preterm infant who required a total of 15 doses of artificial pulmonary surfactant (Surfacten®). A male infant, born at 26 weeks gestation, developed respiratory distress at birth. Chest radiography was consistent with respiratory distress syndrome. The infant required repeated doses of surfactant, each resulting in transient periods of decreased ventilator requirement and improved blood gas values. Surfactant proteins (SP)-A, SP-B, SP-C, and SP-D from tracheal aspirate samples were analyzed on the 13th day (deterioration period) and 36th day (recovery period) after birth. On the 13th day sufficient SP-A and SP-D but no SP-B no SP-C were detected on western blot analysis. SP-B and SP-C were eventually detected on the 36th day. This infant therefore required almost 3 months to achieve normal surfactant function.

Mortality in preterm infants with respiratory distress syndrome treated with poractant alfa, calfactant or beractant: a retrospective study

OBJECTIVE

The objective of this study is to compare all-cause in-hospital mortality in preterm infants with respiratory distress syndrome (RDS) treated with poractant alfa, calfactant or beractant.

STUDY DESIGN

A retrospective cohort study of 14 173 preterm infants with RDS, treated with one of three surfactants between 2005 and 2009, using the Premier Database was done. Multilevel, multivariable logistic regression modeling, adjusting for patient- and hospital-level factors was performed.

RESULT

Calfactant treatment was associated with a 49.6% greater likelihood of death than poractant alfa (odds ratio (OR): 1.496, 95% confidence interval (CI): 1.014-2.209, P=0.043). Beractant treatment was associated with a non-significant 37% increase in mortality, compared with poractant alfa (OR: 1.370, 95% CI: 0.996-1.885, P=0.053). No differences in mortality were observed between calfactant and beractant treatment (OR: 1.092, 95% CI: 0.765-1.559, P=0.626).

CONCLUSION

Poractant alfa treatment for RDS was associated with a significantly reduced likelihood of death when compared with calfactant and a trend toward reduced mortality when compared with beractant.

Surfactant protein B amount and kinetics in newborn infants: an optimized procedure

Surfactant protein B (SP-B) plays a key role in surfactant homeostasis affecting its biophysical properties and physiological function. Recently, a method to measure SP-B amount and kinetics from tracheal aspirates (TAs) became available. The main objective of this study was to improve the critical steps of the procedure to obtain a better SP-B sensitivity. We administered a 24 h continuous infusion of 1 mg/kg/h of 1(13)C-leucine to ten newborn infants. SP-B was isolated from serial TAs and its fractional synthesis rate, secretion time, peak time and half life were derived from (13)C enrichment curves obtained by gas chromatography mass spectrometry. SP-B amount in TAs was also assessed. During the extraction step, acidification and organic solvent ratio optimization doubled the recovery of SP-B from TAs, so did the elongation of the propylation time (from 20 min to 1 h) with enhanced leucine derivatization yield. Measurement of (13)C leucine enrichments, and therefore all SP-B kinetics parameters, were successfully calculated in all TAs samples due to the increase of SP-B yield. SP-B amount was 0.29 (0.16-0.41) % of total phospholipids with a minimum value of 0.08% belonging to one of the respiratory distress syndrome (RDS) patients. In conclusion, this new procedure enables accurate determination of SP-B kinetics even in the presence of low protein amount like in preterm RDS patients.

The role of surfactant in respiratory distress syndrome

The key feature of respiratory distress syndrome (RDS) is the insufficient production of surfactant in the lungs of preterm infants. As a result, researchers have looked into the possibility of surfactant replacement therapy as a means of preventing and treating RDS. We sought to identify the role of surfactant in the prevention and management of RDS, comparing the various types, doses, and modes of administration, and the recent development. A PubMed search was carried out up to March 2012 using phrases: surfactant, respiratory distress syndrome, protein-containing surfactant, protein-free surfactant, natural surfactant, animal-derived surfactant, synthetic surfactant, lucinactant, surfaxin, surfactant protein-B, surfactant protein-C.Natural, or animal-derived, surfactant is currently the surfactant of choice in comparison to protein-free synthetic surfactant. However, it is hoped that the development of protein-containing synthetic surfactant, such as lucinactant, will rival the efficacy of natural surfactants, but without the risks of their possible side effects. Administration techniques have also been developed with nasal continuous positive airway pressure (nCPAP) and selective surfactant administration now recommended; multiple surfactant doses have also reported better outcomes. An aerosolised form of surfactant is being trialled in the hope that surfactant can be administered in a non-invasive way. Overall, the advancement, concerning the structure of surfactant and its mode of administration, offers an encouraging future in the management of RDS.

Differential effects of cholesterol and budesonide on biophysical properties of clinical surfactant

INTRODUCTION

Corticosteroids have been widely used in clinical medicine as a first-line therapy to modify the inflammatory response in many pulmonary and systemic diseases. Inhaled and intratracheally administered corticosteroids have a particular interest in that their use allows the clinician to circumvent systemic steroid side effects. However, it is vital that corticosteroids delivered via the lungs not interfere with surface activity of the pulmonary surfactant lining layer.

RESULTS

We found differential effects of cholesterol and budesonide on the biophysical properties of a cholesterol-free clinical surfactant preparation, Curosurf. At a low concentration up to 1%, both steroids play a similar role of fluidizing the surfactant film. However, when steroid concentration is increased to 10%, cholesterol induces a unique phase transition that abolishes the surface activity of the Curosurf film. By contrast, 10% budesonide simply fluidizes the film, thus having only limited effects on surface activity.

DISCUSSION

Together with those of a previous study using a cholesterol-containing surfactant, our findings suggest that cholesterol-free surfactant preparations may be more advantageous than cholesterol-containing preparations as a carrier of budesonide because a larger amount of the drug may be delivered to the lungs without significantly compromising the surface activity of pulmonary surfactant.

METHODS

Langmuir balance was used to study the effect of cholesterol and budesonide added at different concentrations on surface activity of Curosurf. Atomic force microscopy (AFM) was used to reveal their effects on the interfacial molecular organization and lateral structure of Curosurf films.

Distribution and surfactant association of carcinoembryonic cell adhesion molecule 6 in human lung

Carcinoembryonic cell adhesion molecule 6 (CEACAM6) is a glycosylated, glycophosphatidylinositol-anchored protein expressed in epithelial cells of various primate tissues. It binds gram-negative bacteria and is overexpressed in human cancers. CEACAM6 is associated with lamellar bodies of cultured type II cells of human fetal lung and protects surfactant function in vitro. In this study, we characterized CEACAM6 expression in vivo in human lung. CEACAM6 was present in lung lavage of premature infants at birth and increased progressively in intubated infants with lung disease. Of surfactant-associated CEACAM6, ∼80% was the fully glycosylated, 90-kDa form that contains the glycophosphatidylinositol anchor, and the concentration (3.9% of phospholipid for adult lung) was comparable to that for surfactant proteins (SP)-A/B/C. We examined the affinity of CEACAM6 by purification of surfactant on density gradient centrifugation; concentrations of CEACAM6 and SP-B per phospholipid were unchanged, whereas levels of total protein and SP-A decreased by 60%. CEACAM6 mRNA content decreased progressively from upper trachea to peripheral fetal lung, whereas protein levels were similar in all regions of adult lung, suggesting proximal-to-distal developmental expression in lung epithelium. In adult lung, most type I cells and ∼50% of type II cells were immunopositive. We conclude that CEACAM6 is expressed by alveolar and airway epithelial cells of human lung and is secreted into lung-lining fluid, where fully glycosylated protein binds to surfactant. Production appears to be upregulated during neonatal lung disease, perhaps related to roles of CEACAM6 in surfactant function, cell proliferation, and innate immune defense.

Pilot trial of late booster doses of surfactant for ventilated premature infants

OBJECTIVE

Many premature infants at risk for bronchopulmonary dysplasia experience episodes of surfactant dysfunction with reduced surfactant protein B (SP-B). In this study, we investigated the safety and responses to booster doses of surfactant.

STUDY DESIGN

A total of 87 infants, 500 to 1250 g birth weight, who were ventilated at 7 to 10 days received 2 or 3 doses of Infasurf (Calfactant, Forest Pharmaceuticals, St Louis, MO, USA) within a 1-week period.

RESULT

For 184 doses, occurrence rates of transient bradycardia (13) and plugged endotracheal tube (5) were low, and no other adverse effects were noted. Treatment transiently improved the respiratory severity score (FiO(2) × mean airway pressure), SP-B content (+75%) and surface properties of isolated surfactant. Levels of eight proinflammatory cytokines in tracheal aspirate were interrelated and unchanged from baseline after surfactant treatment.

CONCLUSION

Booster doses of surfactant for premature infants with lung disease are safe and transiently improve respiratory status as well as composition and function of endogenous surfactant.

Comparative transcriptome analyses indicate molecular homology of zebrafish swimbladder and mammalian lung

The fish swimbladder is a unique organ in vertebrate evolution and it functions for regulating buoyancy in most teleost species. It has long been postulated as a homolog of the tetrapod lung, but the molecular evidence is scarce. In order to understand the molecular function of swimbladder as well as its relationship with lungs in tetrapods, transcriptomic analyses of zebrafish swimbladder were carried out by RNA-seq. Gene ontology classification showed that genes in cytoskeleton and endoplasmic reticulum were enriched in the swimbladder. Further analyses depicted gene sets and pathways closely related to cytoskeleton constitution and regulation, cell adhesion, and extracellular matrix. Several prominent transcription factor genes in the swimbladder including hoxc4a, hoxc6a, hoxc8a and foxf1 were identified and their expressions in developing swimbladder during embryogenesis were confirmed. By comparison of enriched transcripts in the swimbladder with those in human and mouse lungs, we established the resemblance of transcriptome of the zebrafish swimbladder and mammalian lungs. Based on the transcriptomic data of zebrafish swimbladder, the predominant functions of swimbladder are in its epithelial and muscular tissues. Our comparative analyses also provide molecular evidence of the relatedness of the fish swimbladder and mammalian lung.

Dynamic tracheal occlusion improves lung morphometrics and function in the fetal lamb model of congenital diaphragmatic hernia

BACKGROUND

Congenital diaphragmatic hernia (CDH) is associated with significant neonatal morbidity and mortality. Although prenatal complete tracheal occlusion (cTO) causes hypoplastic CDH lungs to enlarge, improved lung function has not been demonstrated. Furthermore, cTO interferes with the dynamic pressure change and fluid flow associated with fetal breathing.

PURPOSE

The purpose of the study was to assess a novel dynamic tracheal occlusion (dTO) device that preserves pressure changes and fluid flow.

METHODS

In this pilot study, CDH was created in fetal lambs at 65 days of gestational age (GA). At 110 days GA, a cTO device (n = 3) or a dTO device (n = 4) was placed in the fetal trachea. At 135 days GA, lambs were delivered and resuscitated. Unoperated lamb co-twins (n = 5), sham thoracotomy lambs (n = 2), and untreated CDH lambs (n = 3) served as controls.

RESULTS

Tracheal opening pressure, lung volume, lung fluid total protein, and phospholipid were significantly higher in the cTO group than in the dTO and unoperated control groups. Maximal oxygenation and lung compliance were significantly lower in the cTO group when compared with the unoperated control and dTO groups.

CONCLUSION

Preliminary results suggest that in the fetal lamb CDH model, dTO restores normal lung morphometrics and function, whereas cTO leads to enlarged but less functional lungs.

Lipid polymorphism induced by surfactant peptide SP-B(1-25)

Pulmonary surfactant protein B (SP-B) is an essential protein for lowering surface tension in the alveoli. SP-B(1-25), a peptide comprised of the N-terminal 25 amino-acid residues of SP-B, is known to retain much of the biological activity of SP-B. Circular dichroism has shown that when SP-B(1-25) interacts with negatively charged lipid vesicles, it contains significant helical structure for the lipid compositions and peptide/lipid ratios studied here. The effect of SP-B(1-25) on lipid organization and polymorphisms was investigated via DSC, dynamic light scattering, transmission electron microscopy, and solid-state NMR spectroscopy. At 1-3 mol% peptide and physiologic temperature, SP-B(1-25) partitions at the interface of negatively charged PC/PG lipid bilayers. In lipid mixtures containing 1-5 mol% peptide, the structure of SP-B(1-25) remains constant, but (2)H and (31)P NMR spectra show the presence of an isotropic lipid phase in exchange with the lamellar phase below the T(m) of the lipids. This behavior is observed for both DPPC/POPG and POPC/POPG lipid mixtures as well as for both the PC and PG components of the mixtures. For 1-3 mol% SP-B(1-25), a return to a single lamellar phase above the lipid mixture T(m) is observed, but for 5 mol% SP-B(1-25) a significant isotropic component is observed at physiologic temperatures for DPPC and exchange broadening is observed in (2)H and (31)P NMR spectra of the other lipid components in the two mixtures. DLS and TEM rule out the formation of micellar structures and suggest that SP-B(1-25) promotes the formation of a fluid isotropic phase. The ability of SP-B(1-25) to fuse lipid lamellae via this mechanism, particularly those enriched in DPPC, suggests a specific role for the highly conserved N-terminus of SP-B in the packing of lipid lamellae into surfactant lamellar bodies or in stabilizing multilayer structures at the air-liquid interface. Importantly, this behavior has not been seen for the other SP-B fragments of SP-B(8-25) and SP-B(59-80), indicating a critical role for the proline rich first seven amino acids in this protein.

Local cytokine response upon respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the leading cause of childhood hospitalization and respiratory distress and has been recognized for several decades as a major health and economic burden worldwide. This virus has developed several virulence mechanisms to impair the establishment of a protective immune response to re-infection. Accordingly, inefficient immunological memory is usually generated after exposure to this pathogen. Furthermore, it has been shown that RSV can actively promote the induction of an inadequate cellular immune response at the site of infection that causes exacerbated inflammation in the respiratory tract. Such an inflammatory response is both inefficient for clearing the virus and can be responsible for detrimental symptoms, such as asthma and wheezing. Recent data suggest that RSV possesses molecular mechanisms to induce the secretion of pro-inflammatory cytokines that modulate the immune response and impair viral clearance by reducing IFN-γ production. Here, we discuss recent research leading to the identification of RSV virulence factors that are responsible of promoting a pro-inflammatory environment at the airways and their implications on pathogenicity.

Inhaled nitric oxide in premature infants: effect on tracheal aspirate and plasma nitric oxide metabolites

OBJECTIVE

Inhaled nitric oxide (iNO) is a potential new therapy for prevention of bronchopulmonary dysplasia and brain injury in premature infants. This study examined dose-related effects of iNO on NO metabolites as evidence of NO delivery.

STUDY DESIGN

A subset of 102 premature infants in the NO CLD trial, receiving 24 days of iNO (20 p.p.m. decreasing to 2 p.p.m.) or placebo, were analyzed. Tracheal aspirate (TA) and plasma samples collected at enrollment and at intervals during study gas were analyzed for NO metabolites.

RESULT

iNO treatment increased NO metabolites in TA at 20 and 10 p.p.m. (1.7- to 2.3-fold vs control) and in plasma at 20, 10, and 5 p.p.m. (1.6- to 2.3-fold). In post hoc analysis, treated infants with lower metabolite levels at entry had an improved clinical outcome.

CONCLUSION

iNO causes dose-related increases in NO metabolites in the circulation as well as lung fluid, as evidenced by TA analysis, showing NO delivery to these compartments.

1alpha,25(OH)2D3 and its 3-epimer promote rat lung alveolar epithelial-mesenchymal interactions and inhibit lipofibroblast apoptosis

Although alveolar wall thinning has been attributed to apoptosis of interstitial lung lipofibroblasts (LFs), the underlying molecular mechanism(s) remains unknown. Although the physiological vitamin D steroid hormone 1alpha,25(OH)(2)D(3) (1,25D) has been suggested as a local paracrine/autocrine effector of fetal lung maturation and is known to affect fibroblast apoptosis, its effects on LF apoptosis are unknown. We determined the role of 1,25D and its metabolite, C-3-epimer (3-epi-1,25D), on LF and alveolar type II (ATII) cell differentiation, proliferation, and apoptosis. Embryonic day 19 Sprague-Dawley fetal rat lung LFs and ATII cells were treated with 1,25D or 3-epi-1,25D (1 x 10(-10) to 1 x 10(-8) M) for 24 h, and cell proliferation, apoptosis, and differentiation were assessed. Both 1,25D and 3-epi-1,25D exhibited dose-dependent increases in expression of the key homeostatic epithelial-mesenchymal differentiation markers, increased LF and ATII cell proliferation, and decreased apoptosis. Furthermore, rat pups administered 1,25D from postnatal days 0 to 14 showed increased expressions of key LF and ATII cell differentiation markers, increased Bcl-2-to-Bax ratio as an index of decreased spontaneous alveolar LF and ATII cell apoptosis, increased alveolar count, and a paradoxical increase in septal thickness. We conclude that spatial- and temporal-specific actions of vitamin D play a critical role in perinatal lung maturation by stimulating key alveolar epithelial-mesenchymal interactions and by modulating LF proliferation/apoptosis. These data not only provide the biological rationale for the presence of an alveolar vitamin D paracrine system, but also provide the first integrated molecular mechanism for increased surfactant synthesis and alveolar septal thinning during perinatal lung maturation.

Penetration depth of surfactant peptide KL4 into membranes is determined by fatty acid saturation

KL(4) is a 21-residue functional peptide mimic of lung surfactant protein B, an essential protein for lowering surface tension in the alveoli. Its ability to modify lipid properties and restore lung compliance was investigated with circular dichroism, differential scanning calorimetry, and solid-state NMR spectroscopy. KL(4) binds fluid lamellar phase PC/PG lipid membranes and forms an amphipathic helix that alters lipid organization and acyl chain dynamics. The binding and helicity of KL(4) is dependent on the level of monounsaturation in the fatty acid chains. At physiologic temperatures, KL(4) is more peripheral and dynamic in fluid phase POPC/POPG MLVs but is deeply inserted into fluid phase DPPC/POPG vesicles, resulting in immobilization of the peptide. Substantial increases in the acyl chain order are observed in DPPC/POPG lipid vesicles with increasing levels of KL(4), and POPC/POPG lipid vesicles show small decreases in the acyl chain order parameters on addition of KL(4). Additionally, a clear effect of KL(4) on the orientation of the fluid phase PG headgroups is observed, with similar changes in both lipid environments. Near the phase transition temperature of the DPPC/POPG lipid mixtures, which is just below the physiologic temperature of lung surfactant, KL(4) causes phase separation with the DPPC remaining in a gel phase and the POPG partitioned between gel and fluid phases. The ability of KL(4) to differentially partition into lipid lamellae containing varying levels of monounsaturation and subsequent changes in curvature strain suggest a mechanism for peptide-mediated lipid organization and trafficking within the dynamic lung environment.

Carcinoembryonic cell adhesion molecule 6 in human lung: regulated expression of a multifunctional type II cell protein

Carcinoembryonic cell adhesion molecule 6 (CEACAM6) is a glycosylated, glycosylphosphatidylinositol (GPI)-anchored protein expressed in epithelial cells of various human tissues. It binds gram-negative bacteria and is overexpressed in cancers, where it is antiapoptotic and promotes metastases. To characterize CEACAM6 expression in developing lung, we cultured human fetal lung epithelial cells and examined responses to differentiation-promoting hormones, adenovirus expressing thyroid transcription factor-1 (TTF-1), and silencing of TTF-1 with small inhibitory RNA. Glucocorticoid and cAMP had additive stimulatory effects on CEACAM6 content, and combined treatment maximally increased transcription rate, mRNA, and protein approximately 10-fold. Knockdown of TTF-1 reduced hormone induction of CEACAM6 by 80%, and expression of recombinant TTF-1 increased CEACAM6 in a dose-dependent fashion. CEACAM6 content of lung tissue increased during the third trimester and postnatally. By immunostaining, CEACAM6 was present in fetal type II cells, but not mesenchymal cells, and localized to both the plasma membrane and within surfactant-containing lamellar bodies. CEACAM6 was secreted from cultured type II cells and was present in both surfactant and supernatant fractions of infant tracheal aspirates. In functional studies, CEACAM6 reduced inhibition of surfactant surface properties by proteins in vitro and blocked apoptosis of electroporated cultured cells. We conclude that CEACAM6 in fetal lung epithelial cells is developmentally and hormonally regulated and a target protein for TTF-1. Because CEACAM6 acts as an antiapoptotic factor and stabilizes surfactant function, in addition to a putative role in innate defense against bacteria, we propose that it is a multifunctional alveolar protein.

Developmental and genetic regulation of human surfactant protein B in vivo

BACKGROUND

Genetic and developmental disruption of surfactant protein B (SP-B) expression causes neonatal respiratory distress syndrome (RDS).

OBJECTIVES

To assess developmental and genetic regulation of SP-B expression in vivo.

METHODS

To evaluate in vivo developmental regulation of SP-B, we used immunoblotting to compare frequency of detection of mature and pro-SP-B peptides in developmentally distinct cohorts: 24 amniotic fluid samples, unfractionated tracheal aspirates from 101 infants >or=34 weeks' gestation with (75) and without (26) neonatal RDS, and 6 nonsmoking adults. To examine genetic regulation, we used univariate and logistic regression analyses to detect associations between common SP-B (SFTPB) genotypes and SP-B peptides in the neonatal RDS cohort.

RESULTS

We found pro-SP-B peptides in 24/24 amniotic fluid samples and in 100/101 tracheal aspirates from newborn infants but none in bronchoalveolar lavage from normal adults (0/6) (p < 0.001). We detected an association (p = 0.0011) between pro-SP-B peptides (M(r) 40 and 42 kDa) and genotype of a nonsynonymous single nucleotide polymorphism at genomic position 1580 that regulates amino-terminus glycosylation.

CONCLUSIONS

Pro-SP-B peptides are more common in developmentally less mature humans. Association of genotype at genomic position 1580 with pro-SP-B peptides (M(r) 40 and 42 kDa) suggests genetic regulation of amino terminus glycosylation in vivo.

A pilot randomized, controlled trial of later treatment with a peptide-containing, synthetic surfactant for the prevention of bronchopulmonary dysplasia

OBJECTIVE

Oxidant injury and lung inflammation in extremely premature infants are associated with the development of bronchopulmonary dysplasia. Surfactant dysfunction resulting from these events may contribute to the pathogenesis of bronchopulmonary dysplasia. Treatment with exogenous surfactant may decrease the incidence or severity of bronchopulmonary dysplasia. We conducted a masked, multicenter, multinational, randomized, controlled, pilot study to estimate the effects of treating infants at high risk for developing bronchopulmonary dysplasia with lucinactant, a synthetic, peptide-containing surfactant, on safety during dosing and the incidence of death or bronchopulmonary dysplasia.

METHODS

Preterm infants between 600 and 900 g requiring mechanical ventilation and a fraction of inspired oxygen of > or =0.30 between 3 and 10 days of age were randomly assigned to receive either sham air (placebo) or 1 of 2 doses of lucinactant (90 or 175 mg/kg total phospholipid) every 48 hours to a maximum of 5 doses, if they remained on mechanical ventilation.

RESULTS

Of 136 infants enrolled at 34 sites, 44 received placebo, 47 received 90 mg/kg total phospholipid, and 45 received 175 mg/kg total phospholipid. The 90 mg/kg group had a significantly higher percentage of boys (64%) compared with the placebo group (39%); no other significant differences in baseline characteristics among groups were present. Compared with placebo, both the 90 mg/kg and 175 mg/kg groups experienced a significantly higher incidence of desaturation and bradycardia during dosing. Twenty-four hours after dosing, the mean fraction of inspired oxygen was lower in both lucinactant groups (33%) compared with the placebo group (39%). The incidence of mortality or bronchopulmonary dysplasia was 66% in the placebo group, 79% in the 90 mg/kg group, and 58% in the 175 mg/kg group. These differences were not statistically significant. There were no statistical differences among groups for pneumothorax, pulmonary interstitial emphysema, intraventricular hemorrhage, periventricular leukomalacia, retinopathy of prematurity, or mortality.

CONCLUSIONS

There were trends toward lower oxygen requirements and toward a lower incidence of mortality or bronchopulmonary dysplasia at 36 weeks' postmenstrual age in infants who received the higher dose of lucinactant, and this warrants further investigation.

Surfactant status in preterm neonates recovering from respiratory distress syndrome

OBJECTIVE

The goal was to establish whether reduced amounts of pulmonary surfactant contribute to postextubation respiratory failure in preterm infants recovering from respiratory distress syndrome.

METHODS

We prospectively recruited preterm infants who needed mechanical ventilation and exogenous surfactant for treatment of moderate/severe respiratory distress syndrome and could not be extubated before day 3 of life. (13)C-labeled dipalmitoyl-phosphatidylcholine was administered endotracheally as tracer before extubation, for estimation of surfactant disaturated phosphatidylcholine pool size and half-life. Patients were retrospectively divided into 3 groups, that is, extubation failure if, after extubation, they needed reintubation or continuous positive airway pressure treatment of >or=6 cmH(2)O and fraction of inspired oxygen of >0.4, extubation success if they did not meet the failure criteria, and not extubated if they needed ongoing ventilation. Clinical and respiratory parameters were recorded hourly.

RESULTS

Reliable kinetic data could be obtained for 63 of the 88 enrolled neonates. Sixteen, 23, and 24 neonates were categorized in the extubation failure, extubation success, and not extubated groups, respectively. Clinical and demographic characteristics did not differ between the extubation failure and extubation success groups. Disaturated phosphatidylcholine pool size was smaller in the extubation failure group than in the extubation success group (25 +/- 12 vs 43 +/- 24 mg/kg) and was 37 +/- 32 mg/kg in the not extubated group. Disaturated phosphatidylcholine half-life was 19 +/- 7, 24 +/- 12, and 28 +/- 18 hours in the extubation failure, extubation success, and not extubated groups, respectively.

CONCLUSIONS

In a selected population of preterm infants with moderate/severe respiratory distress syndrome who could not be extubated in the first 3 days of life, infants who were reintubated or needed high continuous positive airway pressure settings after extubation had a smaller disaturated phosphatidylcholine pool size than did those who were successfully extubated or needed low continuous positive airway pressure settings.

Dexamethasone therapy in preterm infants developing bronchopulmonary dysplasia: effect on pulmonary surfactant disaturated-phosphatidylcholine kinetics

The role of corticosteroid in severe bronchopulmonary dyplasia (BPD) is still debated. Scanty data are available on the corticosteroids effect on surfactant metabolism. Our objective was to compare surfactant kinetics in preterm infants with developing BPD, before and after dexamethasone (DEXA) treatment. Twenty-eight studies were performed in 14 preterm infants (birth weight 786 +/- 192 g, gestational age 26 +/- 1 wk) on high ventilatory setting, before (age 22 +/- 11 d) and after (age 33 +/- 11 d) DEXA. C-labeled dipalmitoyl-phosphatidylcholine (DPPC) was administered endotrachelly to trace pulmonary surfactant. Surfactant disaturated-phosphatidylcholine (DSPC) kinetics and pools were calculated from DSPC C-enrichment curves of serial tracheal aspirates and bi-compartmental analysis. Total protein and myeloperoxidase (MPO) activity in tracheal aspirates were also measured and expressed per ml of Epithelial Lining Fluid (ELF). After DEXA, DSPC alveolar pool increased significantly from 8.2 +/- 7.6 to 10.6 +/- 11.3 mg/kg (p = 0.039), total proteins and MPO were reduced from 8.8 +/- 8.6 to 3.1 +/- 2.1 mg/ml ELF (p = 0.046) and from 1822 +/- 1224 to 1261 +/- 987 mU/mlELF (p = 0.028) respectively. In conclusion, DEXA treatment in mechanically ventilated preterm infants with severe respiratory failure and at high risk of developing BPD, significantly reduced inflammatory markers and increased alveolar surfactant DSPC pool.

Genetic Abnormalities of Surfactant Metabolism

Pulmonary surfactant is the complex mixture of lipids and proteins needed to reduce alveolar surface tension at the air-liquid interface and prevent alveolar collapse at the end of expiration. It has been recognized for almost 50 years that a deficiency in surfactant production due to pulmonary immaturity is the principal cause of the respiratory distress syndrome (RDS) observed in prematurely born infants.1 Secondary surfactant deficiency due to injury to the cells involved in its production and functional inactivation of surfactant is also important in the pathophysiology of acute respiratory distress syndrome (ARDS) observed in older children and adults.2,3 In the past 15 years, it has been recognized that surfactant deficiency may result from genetic mechanisms involving mutations in genes encoding critical components of the surfactant system or proteins involved in surfactant metabolism.4,5 Although rare, these single gene disorders provide important insights into normal surfactant metabolism and into the genes in which frequently occurring allelic variants may be important in more common pulmonary diseases.

Surfactant function and composition in premature infants treated with inhaled nitric oxide

OBJECTIVES

We hypothesized that inhaled nitric oxide treatment of premature infants at risk for bronchopulmonary dysplasia would not adversely affect endogenous surfactant function or composition.

METHODS

As part of the Nitric Oxide Chronic Lung Disease Trial of inhaled nitric oxide, we examined surfactant in a subpopulation of enrolled infants. Tracheal aspirate fluid was collected at specified intervals from 99 infants with birth weights <1250 g who received inhaled nitric oxide (20 ppm, weaned to 2 ppm) or placebo gas for 24 days. Large-aggregate surfactant was analyzed for surface activity with a pulsating bubble surfactometer and for surfactant protein contents with an immunoassay.

RESULTS

At baseline, before administration of study gas, surfactant function and composition were comparable in the 2 groups, and there was a positive correlation between minimum surface tension and severity of lung disease for all infants. Over the first 4 days of treatment, minimum surface tension increased in placebo-treated infants and decreased in inhaled nitric oxide-treated infants. There were no significant differences between groups in recovery of large-aggregate surfactant or contents of surfactant protein A, surfactant protein B, surfactant protein C, or total protein, normalized to phospholipid.

CONCLUSIONS

We conclude that inhaled nitric oxide treatment for premature infants at risk of bronchopulmonary dysplasia does not alter surfactant recovery or protein composition and may improve surfactant function transiently.

Defects in surfactant synthesis: clinical implications

Since the original description of deficiency of the pulmonary surfactant in premature newborn infants by Avery and Mead in 1959, respiratory distress syndrome has most commonly been attributed to developmental immaturity of surfactant production. Studies of different ethnic groups, gender, targeted gene ablation in murine lineages, and recent clinical reports of monogenic causes of neonatal respiratory distress syndrome have demonstrated that genetic defects disrupt pulmonary surfactant metabolism and cause respiratory distress syndrome, especially in term or near-term infants and in older infants, children, and adults. In contrast to developmental causes of respiratory distress, which may improve as infants and children mature, genetic causes result in both acute and chronic (and potentially irreversible) respiratory failure.

Lipidomics of cellular and secreted phospholipids from differentiated human fetal type II alveolar epithelial cells

Maturation of fetal alveolar type II epithelial cells in utero is characterized by specific changes to lung surfactant phospholipids. Here, we quantified the effects of hormonal differentiation in vitro on the molecular specificity of cellular and secreted phospholipids from human fetal type II epithelial cells using electrospray ionization mass spectrometry. Differentiation, assessed by morphology and changes in gene expression, was accompanied by restricted and specific modifications to cell phospholipids, principally enrichments of shorter chain species of phosphatidylcholine (PC) and phosphatidylinositol, that were not observed in fetal lung fibroblasts. Treatment of differentiated epithelial cells with secretagogues stimulated the secretion of functional surfactant-containing surfactant proteins B and C (SP-B and SP-C). Secreted material was further enriched in this same set of phospholipid species but was characterized by increased contents of short-chain monounsaturated and disaturated species other than dipalmitoyl PC (PC16:0/16:0), principally palmitoylmyristoyl PC (PC16:0/14:0) and palmitoylpalmitoleoyl PC (PC16:0/16:1). Mixtures of these PC molecular species, phosphatidylglycerol, and SP-B and SP-C were functionally active and rapidly generated low surface tension on compression in a pulsating bubble surfactometer. These results suggest that hormonally differentiated human fetal type II cells do not select the molecular composition of surfactant phospholipid on the basis of saturation but, more likely, on the basis of acyl chain length.

RESPIRATORY CONSEQUENCES OF PRETERM BIRTH

Approximately 8% of Australia's 250,000 annual births occur preterm (before 37 weeks completed gestation). Preterm infants represent 75% of all neonatal deaths in Australia, with the vast majority of these deaths caused by pulmonary disease. 2. The respiratory consequences for survivors of preterm birth include the immediate challenges of breathing with underdeveloped lungs, usually manifest as respiratory distress syndrome (RDS), and, in the long term, with persisting pulmonary abnormalities. Therapies to prevent neonatal lung disease now permit survival of preterm infants born as early as 22 weeks gestational age, but not without consequences. 3. Preterm infants are at risk of developing chronic lung disease/bronchopulmonary dysplasia (BPD). The lungs of infants dying from BPD are inflamed and have fewer, larger alveoli than normal and exhibit abnormal pulmonary vascular development. There is now a growing appreciation of the contribution of intrauterine inflammation to the aetiology of BPD. 4. Impaired airway function is commonly reported in follow-up studies of children born preterm. Decreased expiratory flow rates have been associated with preterm birth per se, but airway function appears more affected in survivors of RDS and BPD. Observations in survivors of BPD suggest persisting abnormalities in the structure of the lung parenchyma and airways. 5. Follow-up studies of preterm infants into adulthood are lacking, as are experimental examinations of the long-term physiological and anatomical effects of preterm birth. Both are necessary to understand the causes of the long-term respiratory consequences of preterm birth.

Surfactant composition and function in a primate model of infant chronic lung disease: effects of inhaled nitric oxide

Bronchopulmonary dysplasia, or chronic lung disease (CLD), of premature infants involves injury from hyperoxia and mechanical ventilation to an immature lung. We examined surfactant and nitric oxide (NO), which are developmentally deficient in premature infants, in the baboon model of developing CLD. Fetuses were delivered at 125 d gestation and were managed for 14 d with ventilation and oxygen prn without (controls) or with inhaled NO at 5 ppm. Compared with term infants, premature control infants had reduced maximal lung volume, decreased tissue content of surfactant proteins SP-A, -B, and -C, abnormal lavage surfactant as assessed by pulsating bubble surfactometer, and a low concentration of SP-B/phospholipid. NO treatment significantly increased maximal lung volume and tissue SP-A and SP-C, reduced recovery of lavage surfactant by 33%, decreased the total protein:phospholipid ratio of surfactant by 50%, and had no effect on phospholipid composition or SP content except for SP-C (50%). In both treatment groups, levels of SP-B and SP-C in surfactant were negatively correlated with STmin, with a 5-fold greater SP efficiency for NO versus control animals. By contrast, lung volume and compliance were not correlated with surfactant function. We conclude that surfactant is often dysfunctional in developing CLD secondary to SP-B deficiency. NO treatment improves the apparent ability of hydrophobic SP to promote low surface tension, perhaps secondary to less protein inactivation of surfactant, and improves lung volume by a process unrelated to surfactant function.

Alveolar epithelial cell differentiation and surfactant protein expression after mild preterm birth in sheep

As the transition to extrauterine life at birth alters the proportions of type I and II alveolar epithelial cells (AECs), our aim was to determine the effect of mild preterm birth on AECs and surfactant protein (SP) gene expression. Preterm lambs were born at approximately 133 d of gestational age (DGA); controls were born at term (approximately 147 DGA). Lungs were collected from preterm lambs at term-equivalent age (TEA; approximately 2 wk after preterm birth) and 6 wk post-TEA. Control lung tissue was collected from fetuses (at 132 DGA), as well as from lambs at approximately 6 h (normal term) and 2, 6, and 8 wk of postnatal age (PNA). In controls, the proportion of type I AECs decreased from 65.1 +/- 3.9% at term to 50.9 +/- 3.3%, while the proportion of type II AECs increased from 33.7 +/- 3.9% to 48.5 +/- 3.3% at 6 wk PNA. At 2 wk after preterm birth, the proportions of type I and II AECs were similar in preterm lambs compared to 132-d fetal levels and term controls but differed from control values at 2 wk PNA; differences between control and preterm lambs persisted at 8 wk PNA. At approximately 2 wk after preterm birth, SP-A and SP-B, but not SP-C, mRNA levels were significantly reduced in preterm lambs compared with term controls, but these differences did not persist at 2 and 6 wk PNA. We conclude that mild preterm birth alters the normal postnatal changes in type I and II cell proportions but does not severely affect SP gene expression.

Surfactant disaturated phosphatidylcholine kinetics in infants with bronchopulmonary dysplasia measured with stable isotopes and a two-compartment model

We previously found a shorter surfactant disaturated phosphatidylcholine palmitate (DSPC-PA) half-life in infants with bronchopulmonary dysplasia (BPD) by using a single stable isotope tracer and simple formulas based on a one-exponential fit of the final portion of the enrichment decay curve. The aim of this study was to apply noncompartmental and compartmental analysis on the entire enrichment decay curve of DSPC-PA and to compare the kinetic data with our previous results. We analyzed 10 preterm newborns with BPD (gestational age 26 ± 0.6 wk, weight 777 ± 199 g) and 6 controls (gestational age 26 ± 1.4 wk, weight 787 ± 259 g). All took part in our previous study. Endotracheal13C-labeled dipalmitoyl phosphatidylcholine was administered, and the13C-enrichment of surfactant DSPC-PA was measured from serial tracheal aspirates by gas chromatography-mass spectrometry. Noncompartmental and compartmental models were numerically identified from the tracer-to-tracee ratio and kinetic parameters related to the accessible (pool accessible to sampling, likely to be the lung alveolar pool) and to the nonaccessible pools (pools not accessible to samplings, likely to be the intracellular storage pool) were estimated in the two study groups. Comparison was performed by Mann-Whitney test. A two-compartment model provided the most reliable assessment of DSPC-PA kinetics. In BPD vs. controls, mean ± SE residence time of DSPC-PA in the accessible was 17.5 ± 2.6 vs. 32.2 ± 6.4 h ( P < 0.05), whereas it was 49.7 ± 3.5 vs. 54.4 ± 3.9 h (NS, not significant) in the nonaccessible pool; DSPC-PA recycling was 0.26 ± 0.05 vs. 0.43 ± 0.04% (NS), respectively. A two-compartment model of surfactant DSPC-PA kinetics allowed a thorough assessment of DSPC-PA kinetics, including masses, synthesis, and fluxes between pools. The most important findings of this study are that in BPD infants DSPC-PA loss from the alveolar pool was higher and recycling through the intracellular pool lower than in controls.

Surfactant-associated protein B kinetics in vivo in newborn infants by stable isotopes

Surfactant-associated protein B (SP-B) is critical to the biophysical function of pulmonary surfactant. No information is available on SP-B synthesis and kinetics in humans. We administered a 24-h i.v. infusion of 13C-valine as metabolic precursor of SP-B to six newborn infants (weight 3.5+/-0.5 kg; age 12 d, range 1-43 d). Three of the study infants also received i.v. 2H-palmitate to label surfactant disaturated phosphatidylcholine (DSPC). SP-B and DSPC were isolated from tracheal aspirates, and their respective 13C and 2H enrichments were measured by gas chromatography-mass spectrometry. SP-B kinetics was measured successfully in all six infants. SP-B median (range) fractional synthesis rate was 30% per day (20-78% per day), secretion time was 4.5 h (1-9 h), time to peak was 24 h (12-36 h), and half-life was 21 h (8-35 h). The ascending part of the SP-B kinetic curve was similar to the DSPC curve, suggesting similar secretion pathways. SP-B half-life seemed to be shorter than DSPC half-life. These results agree with existing animal data. We conclude that the measurement of SP-B kinetics is feasible in vivo in humans by stable isotope technology.

Surfactant Therapy for Meconium Aspiration Syndrome

Meconium aspiration syndrome (MAS) is an important cause of respiratory distress in the term infant. Therapy for the disease remains problematic, and newer treatments such as high-frequency ventilation and inhaled nitric oxide are being applied with increasing frequency. There is a significant disturbance of the pulmonary surfactant system in MAS, with a wealth of experimental data indicating that inhibition of surfactant function in the alveolar space is an important element of the pathophysiology of the disease. This inhibition may be mediated by meconium, plasma proteins, haemoglobin and oedema fluid, and, at least in vitro, can be overcome by increasing surfactant phospholipid concentration. These observations have served as the rationale for administration of exogenous surfactant preparations in MAS, initially as standard bolus therapy and, more recently, in association with therapeutic lung lavage. Bolus surfactant therapy in ventilated infants with MAS has been found to improve oxygenation in most studies, although there are a significant proportion of nonresponders and in many cases the effect is transient. Pooled data from randomised controlled trials of surfactant therapy suggest a benefit in terms of a reduction in the requirement for extracorporeal membrane oxygenation (relative risk 0.48 in surfactant-treated infants) but no diminution of air leak or ventilator days. Current evidence would support the use of bolus surfactant therapy on a case by case basis in nurseries with a relatively high mortality associated with MAS, or the lack of availability of other forms of respiratory support such as high-frequency ventilation or nitric oxide. If used, bolus surfactant should be administered as early as practicable to infants who exhibit significant parenchymal disease, at a phospholipid dose of at least 100 mg/kg, rapidly instilled into the trachea. Natural surfactant or a third-generation synthetic surfactant should be used and the dosage repeated every 6 hours until oxygenation has improved. Lung lavage with dilute surfactant has recently emerged as an alternative to bolus therapy in MAS, which has the advantage of removing surfactant inhibitors from the alveolar space in addition to augmenting surfactant phospholipid concentration. Combined animal and human data suggest that lung lavage can remove significant amounts of meconium and alveolar debris, and thereby improve oxygenation and pulmonary mechanics. Arterial oxygen saturation inevitably falls during lavage but has been noted to recover relatively rapidly, even in infants with severe disease. Several randomised controlled trials of surfactant lavage in MAS are underway, and until the results are known, lavage must be considered an unproven and experimental therapy.

Dysfunction of pulmonary surfactant in chronically ventilated premature infants

Infants of <30 wk gestation often require respiratory support for several weeks and may develop bronchopulmonary dysplasia (BPD), which is associated with long-term pulmonary disability or death in severe cases. To examine the status of surfactant in infants at high risk for BPD, this prospective study analyzed 247 tracheal aspirate samples from 68 infants of 23-30 wk gestation who remained intubated for 7-84 d. Seventy-five percent of the infants had one or more surfactant samples with abnormal function (minimum surface tension 5.1-21.7 mN/m by pulsating bubble surfactometer), which were temporally associated with episodes of infection (p = 0.01) and respiratory deterioration (p = 0.005). Comparing normal and abnormal surfactant samples, there were no differences in amount of surfactant phospholipid, normalized to total protein that was recovered from tracheal aspirate, or in relative content of phosphatidylcholine and phosphatidylglycerol. Contents of surfactant proteins (SP) A, B, and C, measured in the surfactant pellet by immunoassay, were reduced by 50%, 80%, and 72%, respectively, in samples with abnormal surface tension (p < or = 0.001). On multivariable analysis of all samples, SP-B content (r = -0.58, p < 0.0001) and SP-C content (r = -0.32, p < 0.001) were correlated with surfactant function. We conclude that most premature infants requiring continued respiratory support after 7 d of age experience transient episodes of dysfunctional surfactant that are associated with a deficiency of SP-B and SP-C.

Surfactant protein profile of pulmonary surfactant in premature infants

Although premature infants are known to be deficient in pulmonary surfactant, there is limited information regarding surfactant protein (SP) composition. To assess the postnatal profile of SPs, tracheal aspirate samples were collected from 35 intubated infants of 23-31 weeks of gestation between 8 and 80 days of age. In 71 large aggregate surfactant samples that had normal in vitro function (minimum surface tension of less than 1 mN/m by pulsating bubble surfactometry), mean +/- SEM contents of SP-A, SP-B, and SP-C (3.7 kD) were 7.1 +/- 1.4%, 1.8 +/- 0.2%, and 4.6 +/- 0.6%, respectively, of phospholipid. To assess SPs in the 1st week of life, we analyzed samples from additional infants receiving only synthetic replacement surfactant. On the 2nd day of life, contents of SP-A, SP-B, and SP-C were 13.4%, 8.4%, and 0.1%, respectively, of the mean levels for Day 8-80 samples. The major postnatal increases for SP-A, SP-B, and SP-C occurred during the 1st, 2nd, and 3rd weeks, respectively. We conclude that surfactant of newborn premature infants is markedly deficient in SPs, in particular SP-C. Despite continuing lung disease, some infants who are more than 1 week of age have surfactant with normal in vitro function that contains SPs at levels comparable to adult surfactant.