Pathophysiologic, morphometric, and biochemical studies of the premature baboon with bronchopulmonary dysplasia

Oxygen in the neonatal ICU: a complicated history and where are we now?

Despite major advances in neonatal care, oxygen remains the most commonly used medication in the neonatal intensive care unit (NICU). Supplemental oxygen can be life-saving for term and preterm neonates in the resuscitation period and beyond, however use of oxygen in the neonatal period must be judicious as there can be toxic effects. Newborns experience substantial hemodynamic changes at birth, rapid energy consumption, and decreased antioxidant capacity, which requires a delicate balance of sufficient oxygen while mitigating reactive oxygen species causing oxidative stress. In this review, we will discuss the physiology of neonates in relation to hypoxia and hyperoxic injury, the history of supplemental oxygen in the delivery room and beyond, supporting clinical research guiding trends for oxygen therapy in neonatal care, current practices, and future directions.

Biomarkers of lung alveolarization and microvascular maturation in response to intermittent hypoxia and/or early antioxidant/fish oil supplementation in neonatal rats

OBJECTIVE

Extremely preterm infants experience frequent intermittent hypoxia (IH) episodes during oxygen therapy which causes significant damage to the lungs and curtails important signaling pathways that regulate normal lung alveolarization and microvascular maturation. We tested the hypothesis that early supplementation with fish oil and/or antioxidants in rats exposed to neonatal IH improves expression of lung biomarkers of alveolarization and microvascular maturation, and reduces IH-induced lung injury.

STUDY DESIGN/METHODS

From birth (P0) to P14, rat pups were exposed to room air (RA) or neonatal IH during which they received daily oral supplementation with either: (1) olive oil (OO) (control); (2) Coenzyme Q10 (CoQ10) in OO; (3) fish oil; (4) glutathione nanoparticles (nGSH); or (5) fish oil +CoQ10. At P14 pups were placed in RA until P21 with no further treatment. RA controls were similarly treated. Lung growth and alveolarization, histopathology, apoptosis, oxidative stress and biomarkers of alveolarization and microvascular maturation were determined.

RESULTS

Neonatal IH was associated with reduced lung weights and severe histopathological outcomes. These effects were curtailed with fish oil and nGSH. nGSH was also protective against apoptosis, while CoQ10 prevented IH-induced ROS production. Of all treatments, nGSH and CoQ10 + fish oil-induced vascular endothelial growth factor165 and CD31 (Platelet endothelial cell adhesion molecule-1), which are associated with angiogenesis. CoQ10 + fish oil improved alveolarization in RA and IH despite evidence of hemorrhage.

CONCLUSIONS

The benefits of nGSH and CoQ10 + fish oil suggest an antioxidant effect which may be required to curtail IH-induced lung injury. Further clinical assessment of the effectiveness of nGSH is warranted.

The Role of the Interleukin-1 Family in Complications of Prematurity

Preterm birth is a major contributor to neonatal morbidity and mortality. Complications of prematurity such as bronchopulmonary dysplasia (BPD, affecting the lung), pulmonary hypertension associated with BPD (BPD-PH, heart), white matter injury (WMI, brain), retinopathy of prematurity (ROP, eyes), necrotizing enterocolitis (NEC, gut) and sepsis are among the major causes of long-term morbidity in infants born prematurely. Though the origins are multifactorial, inflammation and in particular the imbalance of pro- and anti-inflammatory mediators is now recognized as a key driver of the pathophysiology underlying these illnesses. Here, we review the involvement of the interleukin (IL)-1 family in perinatal inflammation and its clinical implications, with a focus on the potential of these cytokines as therapeutic targets for the development of safe and effective treatments for early life inflammatory diseases.

Reactive Oxygen Species, Biomarkers of Microvascular Maturation and Alveolarization, and Antioxidants in Oxidative Lung Injury

The lungs of extremely low gestational age neonates (ELGANs) are deficient in pulmonary surfactant and are incapable of efficient gas exchange necessary for successful transition from a hypoxic intrauterine environment to ambient air. To improve gas exchange and survival, ELGANs often receive supplemental oxygen with mechanical ventilation which disrupts normal lung developmental processes, including microvascular maturation and alveolarization. Factors that regulate these developmental processes include vascular endothelial growth factor and matrix metalloproteinases, both of which are influenced by generation of oxygen byproducts, or reactive oxygen species (ROS). ELGANs are also deficient in antioxidants necessary to scavenge excessive ROS. Thus, the accumulation of ROS in the preterm lungs exposed to prolonged hyperoxia, results in inflammation and development of bronchopulmonary dysplasia (BPD), a form of chronic lung disease (CLD). Despite advances in neonatal care, BPD/CLD remains a major cause of neonatal morbidity and mortality. The underlying mechanisms are not completely understood, and the benefits of current therapeutic interventions are limited. The association between ROS and biomarkers of microvascular maturation and alveolarization, as well as antioxidant therapies in the setting of hyperoxia-induced neonatal lung injury are reviewed in this article.

The NLRP3 inflammasome is critically involved in the development of bronchopulmonary dysplasia

The pathogenesis of bronchopulmonary dysplasia (BPD), a devastating lung disease in preterm infants, includes inflammation, the mechanisms of which are not fully characterized. Here we report that the activation of the NLRP3 inflammasome is associated with the development of BPD. Hyperoxia-exposed neonatal mice have increased caspase-1 activation, IL1β and inflammation, and decreased alveolarization. Nlrp3(-/-) mice have no caspase-1 activity, no IL1β, no inflammatory response and undergo normal alveolarization. Treatment of hyperoxia-exposed mice with either IL1 receptor antagonist to block IL1β or glyburide to block the Nlrp3 inflammasome results in decreased inflammation and increased alveolarization. Ventilated preterm baboons show activation of the NLRP3 inflammasome with increased IL1β:IL1ra ratio. The IL1β:IL1ra ratio in tracheal aspirates from preterm infants with respiratory failure is predictive of the development of BPD. We conclude that early activation of the NLRP3 inflammasome is a key mechanism in the development of BPD, and represents a novel therapeutic target for BPD.

Utility of large-animal models of BPD: chronically ventilated preterm lambs

This paper is focused on unique insights provided by the preterm lamb physiological model of bronchopulmonary dysplasia (BPD). Connections are also made to insights provided by the former preterm baboon model of BPD, as well as to rodent models of lung injury to the immature, postnatal lung. The preterm lamb and baboon models recapitulate the clinical setting of preterm birth and respiratory failure that require prolonged ventilation support for days or weeks with oxygen-rich gas. An advantage of the preterm lamb model is the large size of preterm lambs, which facilitates physiological studies for days or weeks during the evolution of neonatal chronic lung disease (CLD). To this advantage is linked an integrated array of morphological, biochemical, and molecular analyses that are identifying the role of individual genes in the pathogenesis of neonatal CLD. Results indicate that the mode of ventilation, invasive mechanical ventilation vs. less invasive high-frequency nasal ventilation, is related to outcomes. Our approach also includes pharmacological interventions that test causality of specific molecular players, such as vitamin A supplementation in the pathogenesis of neonatal CLD. The new insights that are being gained from our preterm lamb model may have important translational implications about the pathogenesis and treatment of BPD in preterm human infants.

Animal models of bronchopulmonary dysplasia. The preterm baboon models

Much of the progress in improved neonatal care, particularly management of underdeveloped preterm lungs, has been aided by investigations of multiple animal models, including the neonatal baboon (Papio species). In this article we highlight how the preterm baboon model at both 140 and 125 days gestation (term equivalent 185 days) has advanced our understanding and management of the immature human infant with neonatal lung disease. Not only is the 125-day baboon model extremely relevant to the condition of bronchopulmonary dysplasia but there are also critical neurodevelopmental and other end-organ pathological features associated with this model not fully discussed in this limited forum. We also describe efforts to incorporate perinatal infection into these preterm models, both fetal and neonatal, and particularly associated with Ureaplasma/Mycoplasma organisms. Efforts to rekindle the preterm primate model for future evaluations of therapies such as stem cell replacement, early lung recruitment interventions coupled with noninvasive surfactant and high-frequency nasal ventilation, and surfactant therapy coupled with antioxidant or anti-inflammatory medications, to name a few, should be undertaken.

Biphasic response of checkpoint control proteins in hyperoxia: exposure to lower levels of oxygen induces genome maintenance genes in experimental baboon BPD

Breathing high concentrations of oxygen (hyperoxia) causes lung injury and is associated with lung diseases such as bronchopulmonary dysplasia (BPD), respiratory distress syndrome and persistent pulmonary hypertension of the newborns. Hyperoxia (95-100 %O2) causes DNA damage and growth arrest of lung cells and consequently cells die by apoptosis or necrosis. Although supplemental oxygen therapy is clinically important, the level and duration of hyperoxic exposure that would allow lung cells to reenter the cell cycle remains unclear. We hypothesized that cells exposed to lower concentrations of hyperoxia will retain the capacity to enter cell cycle when recovered in room air. We employed varying concentrations of oxygen (21-95 %) to determine the response of lung cells to hyperoxia. Our results indicate that cells were growth arrested and failed to reenter the cell cycle when exposed to greater than 60 % oxygen. Cell cycle checkpoint proteins were increased in a biphasic manner, increasing until 70 % oxygen, but declined in greater than 90 % oxygen. Microarray analysis shows that there is significant decrease in the abundance of Cdks 6-8 and retinoblastoma protein (Rb), p107 and p130 in exposure to 90 % oxygen for 48 h. We further tested the effect of clinically relevant as needed oxygen [(pro-re-nata (prn)] in premature infant (125-days and 140-days) baboon model of BPD. The microarray results show that 6 or 14d PRN oxygen-exposed animals had induced expression of chromosomal maintenance genes (MCMs), genes related to anti-inflammation, proliferation, and differentiation.

A hyperoxic lung injury model in premature rabbits: the influence of different gestational ages and oxygen concentrations

BACKGROUND

Many animal models have been developed to study bronchopulmonary dysplasia (BPD). The preterm rabbit is a low-cost, easy-to-handle model, but it has a high mortality rate in response to the high oxygen concentrations used to induce lung injury. The aim of this study was to compare the mortality rates of two models of hyperoxia-induced lung injury in preterm rabbits.

METHODS

Pregnant New Zealand white rabbits were subjected to caesarean section on gestational day 28 or 29 (full term  = 31 days). The premature rabbits in the 28-day gestation group were exposed to room air or FiO₂ ≥95%, and the rabbits in the 29-day gestation group were exposed to room air or FiO₂  = 80% for 11 days. The mean linear intercept (Lm), internal surface area (ISA), number of alveoli, septal thickness and proportion of elastic and collagen fibers were quantified.

RESULTS

The survival rates in the 29-day groups were improved compared with the 28-day groups. Hyperoxia impaired the normal development of the lung, as demonstrated by an increase in the Lm, the septal thickness and the proportion of elastic fibers. Hyperoxia also decreased the ISA, the number of alveoli and the proportion of collagen fibers in the 28-day oxygen-exposed group compared with the control 28-day group. A reduced number of alveoli was found in the 29-day oxygen exposed animals compared with the control 29-day group.

CONCLUSIONS

The 29-day preterm rabbits had a reduced mortality rate compared with the 28-day preterm rabbits and maintained a reduction in the alveoli number, which is comparable to BPD in humans.

Neonatal hyperoxia stimulates the expansion of alveolar epithelial type II cells

Supplemental oxygen used to treat infants born prematurely disrupts angiogenesis and is a risk factor for persistent pulmonary disease later in life. Although it is unclear how neonatal oxygen affects development of the respiratory epithelium, alveolar simplification and depletion of type II cells has been observed in adult mice exposed to hyperoxia between postnatal Days 0 and 4. Because hyperoxia inhibits cell proliferation, we hypothesized that it depleted the adult lung of type II cells by inhibiting their proliferation at birth. Newborn mice were exposed to room air (RA) or hyperoxia, and the oxygen-exposed mice were recovered in RA. Hyperoxia stimulated mRNA expressed by type II (Sftpc, Abca3) and type I (T1α, Aquaporin 5) cells and inhibited Pecam expressed by endothelial cells. 5-Bromo-2'-deoxyuridine labeling and fate mapping with enhanced green fluorescence protein controlled statically by the Sftpc promoter or conditionally by the Scgb1a1 promoter revealed increased Sftpc and Abca3 mRNA seen on Day 4 reflected an increase in expansion of type II cells shortly after birth. When mice were returned to RA, this expanded population of type II cells was slowly depleted until few were detected by 8 weeks. These findings reveal that hyperoxia stimulates alveolar epithelial cell expansion when it disrupts angiogenesis. The loss of type II cells during recovery in RA may contribute to persistent pulmonary diseases such as those reported in children born preterm who were exposed to supplemental oxygen.

Mechanical ventilation injury and repair in extremely and very preterm lungs

Background

Extremely preterm infants often receive mechanical ventilation (MV), which can contribute to bronchopulmonary dysplasia (BPD). However, the effects of MV alone on the extremely preterm lung and the lung’s capacity for repair are poorly understood.

Aim

To characterise lung injury induced by MV alone, and mechanisms of injury and repair, in extremely preterm lungs and to compare them with very preterm lungs.

Methods

Extremely preterm lambs (0.75 of term) were transiently exposed by hysterotomy and underwent 2 h of injurious MV. Lungs were collected 24 h and at 15 d after MV. Immunohistochemistry and morphometry were used to characterise injury and repair processes. qRT-PCR was performed on extremely and very preterm (0.85 of term) lungs 24 h after MV to assess molecular injury and repair responses.

Results

24 h after MV at 0.75 of term, lung parenchyma and bronchioles were severely injured; tissue space and myofibroblast density were increased, collagen and elastin fibres were deformed and secondary crest density was reduced. Bronchioles contained debris and their epithelium was injured and thickened. 24 h after MV at 0.75 and 0.85 of term, mRNA expression of potential mediators of lung repair were significantly increased. By 15 days after MV, most lung injury had resolved without treatment.

Conclusions

Extremely immature lungs, particularly bronchioles, are severely injured by 2 h of MV. In the absence of continued ventilation these injured lungs are capable of repair. At 24 h after MV, genes associated with injurious MV are unaltered, while potential repair genes are activated in both extremely and very preterm lungs.

Progress in understanding the pathogenesis of BPD using the baboon and sheep models

Bronchopulmonary dysplasia (BPD) is among the most common chronic lung diseases in infants in the US. Improved survival of preterm infants who developed BPD is becoming increasingly important because of the high risk for persistent pulmonary morbidities such as poor respiratory gas exchange, pulmonary hypertension, and excess airway expiratory resistance later in life. This review focuses on unique insights provided by the two large-animal, physiological models of neonatal chronic lung disease: preterm baboons and preterm lambs. The models' are valuable because they contribute to better understanding of the underlying molecular pathogenic mechanisms. An epigenetic hypothesis is proposed as a pathogenic mechanism for BPD and its persistent pulmonary morbidities.

Late administration of surfactant replacement therapy increases surfactant protein-B content: a randomized pilot study

BACKGROUND

Surfactant dysfunction may contribute to the development of bronchopulmonary dysplasia (BPD) in persistently ventilated preterm infants. We conducted a multicenter randomized, blinded, pilot study to assess the safety and efficacy of late administration of doses of a surfactant protein-B (SP-B)-containing surfactant (calfactant) in combination with prolonged inhaled nitric oxide (iNO) in infants ≤1,000 g birth weight (BW).

METHODS

We randomized 85 preterm infants ventilated at 7-14 d after birth to receive either late administration of surfactant (up to 5 doses) plus prolonged iNO or iNO alone. Large aggregate surfactant was isolated from daily tracheal aspirates (TAs) for measurement of SP-B content, total protein, and phospholipid (PL).

RESULTS

Late administration of surfactant had minimal acute adverse effects. Clinical status as well as surfactant recovery and SP-B content in tracheal aspirate were transiently improved as compared to the controls; these effects waned after 1 d. The change in SP-B content with surfactant dosing was positively correlated with SP-B levels during treatment (r = 0.50, P = 0.02).

CONCLUSION

Low SP-B values increased with calfactant administration, but the relationship of this response to SP-B levels suggests that degradation is a contributing mechanism for SP-B deficiency and surfactant dysfunction. We conclude that late therapy with surfactant in combination with iNO is safe and transiently increases surfactant SP-B content, possibly leading to improved short- and long-term respiratory outcomes.

High bias gas flows increase lung injury in the ventilated preterm lamb

BACKGROUND

Mechanical ventilation of preterm babies increases survival but can also cause ventilator-induced lung injury (VILI), leading to the development of bronchopulmonary dysplasia (BPD). It is not known whether shear stress injury from gases flowing into the preterm lung during ventilation contributes to VILI.

METHODS

Preterm lambs of 131 days' gestation (term = 147 d) were ventilated for 2 hours with a bias gas flow of 8 L/min (n = 13), 18 L/min (n = 12) or 28 L/min (n = 14). Physiological parameters were measured continuously and lung injury was assessed by measuring mRNA expression of early injury response genes and by histological analysis. Control lung tissue was collected from unventilated age-matched fetuses. Data were analysed by ANOVA with a Tukey post-hoc test when appropriate.

RESULTS

High bias gas flows resulted in higher ventilator pressures, shorter inflation times and decreased ventilator efficiency. The rate of rise of inspiratory gas flow was greatest, and pulmonary mRNA levels of the injury markers, EGR1 and CTGF, were highest in lambs ventilated with bias gas flows of 18 L/min. High bias gas flows resulted in increased cellular proliferation and abnormal deposition of elastin, collagen and myofibroblasts in the lung.

CONCLUSIONS

High ventilator bias gas flows resulted in increased lung injury, with up-regulation of acute early response genes and increased histological lung injury. Bias gas flows may, therefore, contribute to VILI and BPD.

Elastase inhibitory activity of airway α1-antitrypsin is protected by treatment with a catalytic antioxidant in a baboon model of severe bronchopulmonary dysplasia

Recent studies in animal models of bronchopulmonary dysplasia (BPD) suggest that antioxidant treatments may be beneficial for the disease. However, the mechanisms by which these drugs improve the course of BPD are not completely known. Alpha1-antitrypsin (α1-AT) is one of the major serine protease inhibitors in human plasma that has antielastase and antiapoptotic activities. Both activities of α1-AT are dependent on its reactive site loop (RSL), which is highly susceptible to oxidative inactivation. In this study, we investigated the elastase inhibitory activity of α1-AT in two different baboon models of BPD, the "new BPD" and the "severe BPD" models, and determined the effect of treatment with a catalytic antioxidant, Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP), on the elastase inhibitory activity of α1-AT in the severe BPD model. Our results demonstrate the presence of sufficient elastase inhibitory activity of the airway α1-AT in the new but not in the severe BPD model. Treatment of severe BPD group baboons with the catalytic antioxidant MnTE-2-PyP resulted in augmentation of the elastase inhibitory activity of α1-AT. These findings suggest that prevention of the oxidative inactivation of α1-AT may be one of the mechanisms by which antioxidant therapy improves the pulmonary outcomes in animal models of severe BPD.

Injury and repair in the very immature lung following brief mechanical ventilation

Mechanical ventilation (MV) of very premature infants contributes to lung injury and bronchopulmonary dysplasia (BPD), the effects of which can be long-lasting. Little is currently known about the ability of the very immature lung to recover from ventilator-induced lung injury. Our objective was to determine the ability of the injured very immature lung to repair in the absence of continued ventilation and to identify potential mechanisms. At 125 days gestational age (days GA, 0.85 of term), fetal sheep were partially exposed by hysterotomy under anesthesia and aseptic conditions; they were intubated and ventilated for 2 h with an injurious MV protocol and then returned to the uterus to continue development. Necropsy was performed at either 1 day (short-term group, 126 days GA, n = 6) or 15 days (long-term group, 140 days GA, n = 5) after MV; controls were unventilated (n = 7-8). At 1 day after MV, lungs displayed signs of injury, including hemorrhage, disorganized elastin and collagen deposition in the distal airspaces, altered morphology, significantly reduced secondary septal crest density, and decreased airspace. Bronchioles had thickened epithelium with evidence of injury and sloughing. Relative mRNA levels of early response genes (connective tissue growth factor, cysteine-rich 61, and early growth response-1) and proinflammatory cytokines [interleukins (IL)-1β, IL-6, IL-8, tumor necrosis factor-α, and transforming growth factor-β] were not different between groups 1 day after MV. At 15 days after MV, lung structure was normal with no evidence of injury. We conclude that 2 h of MV induces severe injury in the very immature lung and that these lungs have the capacity to repair spontaneously in the absence of further ventilation.

Inhibition of apoptosis by 60% oxygen: a novel pathway contributing to lung injury in neonatal rats

During early postnatal alveolar formation, the lung tissue of rat pups undergoes a physiological remodeling involving apoptosis of distal lung cells. Exposure of neonatal rats to severe hyperoxia (≥95% O2) both arrests lung growth and results in increased lung cell apoptosis. In contrast, exposure to moderate hyperoxia (60% O2) for 14 days does not completely arrest lung cell proliferation and is associated with parenchymal thickening. On the basis of similarities in lung architecture observed following either exposure to 60% O2, or pharmacological inhibition of physiological apoptosis, we hypothesized that exposure to 60% O2 would result in an inhibition of physiological lung cell apoptosis. Consistent with this hypothesis, we observed that the parenchymal thickening induced by exposure to 60% O2 was associated with decreased numbers of apoptotic cells, increased expressions of the antiapoptotic regulator Bcl-xL, and the putative antiapoptotic protein survivin, and decreased expressions of the proapoptotic cleaved caspases-3 and -7. In summary, exposure of the neonatal rat lung to moderate hyperoxia results in an inhibition of physiological apoptosis, which contributes to the parenchymal thickening observed in the resultant lung injury.

A role for matrix metalloproteinase 9 in IFNγ-mediated injury in developing lungs: relevance to bronchopulmonary dysplasia

We noted a marked increase in IFNγ mRNA in newborn (NB) murine lungs after exposure to hyperoxia. We sought to evaluate the role of IFNγ in lung injury in newborns. Using a unique triple-transgenic (TTG), IFNγ-overexpressing, lung-targeted, externally regulatable NB murine model, we describe a lung phenotype of impaired alveolarization, resembling human bronchopulmonary dysplasia (BPD). IFNγ-mediated abnormal lung architecture was associated with increased cell death and the upregulation of cell death pathway mediators caspases 3, 6, 8, and 9, and angiopoietin 2. Moreover, an increase was evident in cathepsins B, H, K, L, and S, and in matrix metalloproteinases (MMPs) 2, 9, 12, and 14. The IFNγ-mediated abnormal lung architecture was found to be MMP9-dependent, as indicated by the rescue of the IFNγ-induced pulmonary phenotype and survival during hyperoxia with a concomitant partial deficiency of MMP9. This result was concomitant with a decrease in caspases 3, 6, 8, and 9 and angiopoietin 2, but an increase in the expression of angiopoietin 1. In addition, NB IFNγ TTG mice exhibited significantly decreased survival during hyperoxia, compared with littermate controls. Furthermore, as evidence of clinical relevance, we show increased concentrations of the downstream targets of IFNγ chemokine (C-X-C motif) ligands (CXCL10 and CXCL11) in baboon and human lungs with BPD. IFNγ and its downstream targets may contribute significantly to the final common pathway of hyperoxia-induced injury in the developing lung and in human BPD.

Highly conserved transcriptional responses to mechanical ventilation of the lung

Cross-species analysis of microarray data has shown improved discriminating power between healthy and diseased states. Computational approaches have proven effective in deciphering the complexity of human disease by identifying upstream regulatory elements and the transcription factors that interact with them. Here we used both methods to identify highly conserved transcriptional responses during mechanical ventilation, an important therapeutic treatment that has injurious side effects. We generated control and ventilated whole lung samples from the premature baboon model of bronchopulmonary dysplasia (BPD), processed them for microarray, and combined them with existing whole lung oligonucleotide microarray data from 85 additional control samples from mouse, rat, and human and 19 additional ventilated samples from mouse and rat. Of the 2,531 orthologs shared by all 114 samples, 60 were modulated by mechanical ventilation [false discovery rate (FDR)-adjusted q value (q(FDR)) = 0.005, ANOVA]. These included transcripts encoding the transcription factors ATF3 and FOS. Because of compelling known roles for these transcription factors, we used computational methods to predict their targets in the premature baboon model of BPD, which included elastin (ELN), gastrin-releasing polypeptide (GRP), and connective tissue growth factor (CTGF). This approach identified highly conserved transcriptional responses to mechanical ventilation and may facilitate identification of therapeutic targets to reduce the side effects of this valuable treatment.

Persistent bronchiolar remodeling following brief ventilation of the very immature ovine lung

Children and adults who were mechanically ventilated following preterm birth are at increased risk of reduced lung function, suggesting small airway dysfunction. We hypothesized that short periods of mechanical ventilation of very immature lungs can induce persistent bronchiolar remodeling that may adversely affect later lung function. Our objectives were to characterize the effects of brief, positive-pressure ventilation per se on the small airways in very immature, surfactant-deficient lungs and to determine whether the effects persist after the cessation of ventilation. Fetal sheep (0.75 of term) were mechanically ventilated in utero with room air (peak inspiratory pressure 40 cmH2O, positive end-expiratory pressure 4 cmH2O, 65 breaths/min) for 6 or 12 h, after which tissues were collected; another group was studied 7 days after 12-h ventilation. Age-matched unventilated fetuses were controls. The mean basement membrane perimeter of airways analyzed was 548.6+/-8.5 microm and was not different between groups. Immediately after ventilation, 21% of airways had epithelial injury; in airways with intact epithelium, there was more airway smooth muscle (ASM) and less collagen, and the epithelium contained more mucin-containing and apoptotic cells and fewer proliferating cells. Seven days after ventilation, epithelial injury was absent but the epithelium was thicker, with greater cell turnover; there were increased amounts of bronchiolar collagen and ASM and fewer alveolar attachments. The increase in ASM was likely due to cellular hypertrophy rather than hyperplasia. We conclude that brief mechanical ventilation of the very immature lung induces remodeling of the bronchiolar epithelium and walls that lasts for at least 7 days; such changes could contribute to later airway dysfunction.

Interleukin-1β in the bronchoalveolar lavage fluid of premature neonates: a marker for maternal chorioamnionitis and predictor of adverse neonatal outcome

OBJECTIVE

To determine whether the presence of the proinflammatory cytokine interleukin (IL)-1beta in the lungs of preterm infants immediately after birth was associated with maternal inflammation and could predict adverse neonatal outcome.

STUDY DESIGN

Prospective evaluation of serially obtained tracheal aspirates for the presence of IL-1beta in 25 preterm infants (birth weight 595-1700 g; gestational age 24-32 weeks) with respiratory distress syndrome. The initial tracheal aspirate was obtained within 1 h after delivery.

RESULTS

An initial tracheal aspirate positive for IL-1beta had a highly significant correlation with documented maternal chorioamnionitis for the given patient. In addition, the presence of IL-1beta correlated significantly with elevated total cell count (2.62 vs. 0.96 x 10(6)/ml, p = 0.0097), granulocyte count (2.12 vs. 0.22 x 10(6)/ml, p = 0.001), macrophage count (0.28 vs. 0.01 x 10(6)/ml, p = 0.02) and the presence of proinflammatory cytokines IL-6, IL-8 and tumor necrosis factor (TNF)-alpha. Preterm neonates positive for IL-1beta in their initial sample were on prolonged assisted ventilation (38 vs. 16 days, p = 0.013) and oxygen supplementation (62 vs. 40.5 days, p = 0.0462) and required prolonged hospitalization (69 vs. 46 days, p = 0.0165).

CONCLUSIONS

The concentration of IL-1beta in the initial tracheal aspirate obtained from the lungs of preterm infants within the first hour of life may serve as a marker of antenatal/perinatal inflammation, probably due to maternal chorioamnionitis, and could predict an adverse clinical course and short-term outcome.

Chest radiograph thoracic areas and lung volumes in infants developing bronchopulmonary dysplasia

OBJECTIVES

To determine whether chest radiograph (CXR) thoracic areas and lung volumes differed between infants who did and did not develop BPD and according to the severity of BPD developed.

WORKING HYPOTHESIS

Infants developing BPD, particularly if moderate or severe, would have low CXR thoracic areas and lung volumes in the perinatal period.

STUDY DESIGN

Prospective study. PATIENT-SUBJECT SELECTION: 53 infants with a median gestational age of 28 (range 24-32) weeks.

METHODOLOGY

CXR thoracic areas were calculated using a Picture Archiving and Communicating System (PACS) and lung volume assessed by measurement of functional residual capacity (FRC) in the first 72 hr after birth. BPD was diagnosed if the infants were oxygen dependent beyond 28 days, mild BPD in infants no longer oxygen dependent at 36 weeks post-menstrual age (PMA) and moderate/severe BPD in infants who required supplementary oxygen with or without respiratory support at 36 weeks PMA.

RESULTS

Thirty two infants developed BPD, 21 had moderate/severe BPD. The median CXR thoracic areas were higher (P < 0.0001) and FRCs were lower (P < 0.0001) in the BPD compared to no BPD infants. The median CXR thoracic areas of the moderate/severe group (P < 0.001) and the mild group (P < 0.05) were greater than that of the no BPD group and the median FRC of the moderate/severe BPD group was lower than the no BPD group (<0.001) and the mild BPD group (P < 0.05).

CONCLUSION

These results highlight that in the perinatal period infants developing BPD, particularly if moderate/severe, have low functional lung volumes and may have gas trapping, which likely reflects ventilation inhomogeneity.

Airway dimensions in bronchopulmonary dysplasia: implications for airflow obstruction

The cause of lung function abnormalities in bronchopulmonary dysplasia (BPD) is incompletely understood, even in the "new era" of this disease. Altered airway wall dimensions are important in the pathogenesis of airflow obstruction in diseases such as asthma and chronic obstructive pulmonary disease. Whether airway wall dimensions contribute to lung function abnormalities in BPD is unknown. The purpose of this study was to investigate airway wall dimensions in relation to airway size in BPD. Lung tissue of patients with BPD was obtained at autopsy, and lung tissue from children who died from sudden infant death syndrome (SIDS) served as control. Airway wall dimensions and epithelial loss were measured in 75 airways from 5 BPD patients and 176 airways from 11 SIDS patients. Repeated measures analysis of variance was used to assess the relationships between airway wall dimensions and airway size for BPD and SIDS patients. Little epithelial loss was present in the BPD patients while extensive loss was observed in some of the SIDS patients. The inner wall area, outer wall area, epithelium area and smooth muscle area were all substantially larger (all P < 0.001) in BPD than in SIDS patients. It is likely that the increased thickness of the airway wall components contributes to airflow obstruction in BPD patients.

Ventilation of the very immature lung in utero induces injury and BPD-like changes in lung structure in fetal sheep

Preterm infants are at high risk of developing ventilator-induced lung injury (VILI), which contributes to bronchopulmonary dysplasia. To investigate causes of VILI, we have developed an animal model of in utero ventilation (IUV). Our aim was to characterize the effects of IUV on the very immature lung, in the absence of nonventilatory factors that could contribute to lung pathology. Fetal sheep were ventilated in utero at 110 d gestation for 1, 6, or 12 h (two groups; n = 5 each). Lung tissue was collected at 12 h after initiating IUV in the 1, 6, and one 12 h IUV groups. Lung liquid was replaced in the second 12 h IUV group and tissues collected at 117 d. Operated, nonventilated 110 and 117 d fetuses were controls. IUV reduced secondary septal crest densities, simplified distal airsacs, caused abnormal collagen and elastin deposition, and stimulated myofibroblast differentiation and cellular proliferation. IUV causes VILI in very immature lungs in the absence of other complicating factors and reproduces bronchopulmonary dysplasia -like changes in lung morphology. IUV offers a novel method for dissociating VILI from other iatrogenic factors that could contribute to altered lung development caused by VILI.

Inflammatory mediators in the immunobiology of bronchopulmonary dysplasia

Inflammation is important in the development of bronchopulmonary dysplasia (BPD). Polymorphonuclear cells and macrophages and proinflammatory cytokines/chemokines denote early inflammation in clinical scenarios such as in utero inflammation with chorioamnionitis or initial lung injury associated with respiratory distress syndrome or ventilator-induced lung injury. The persistence and non-resolution of lung inflammation contributes greatly to BPD, including altering the lung's ability to repair, contributing to fibrosis, and inhibiting secondary septation, alveolarization, and normal vascular development. Further understanding of the role of inflammation in the pathogenesis of BPD, in particular, during the chronic inflammatory period, offers us the opportunity to develop inflammation-related prevention and treatment strategies of this disease that has long-standing consequences for very premature infants.

Nasal ventilation alters mesenchymal cell turnover and improves alveolarization in preterm lambs

RATIONALE

Bronchopulmonary dysplasia (BPD) is a frequent cause of morbidity in preterm infants that is characterized by prolonged need for ventilatory support in an intensive care environment. BPD is characterized histopathologically by persistently thick, cellular distal airspace walls. In normally developing lungs, by comparison, remodeling of the immature parenchymal architecture is characterized by thinning of the future alveolar walls, a process predicated on cell loss through apoptosis.

OBJECTIVES

We hypothesized that minimizing lung injury, using high-frequency nasal ventilation to provide positive distending pressure with minimal assisted tidal volume displacement, would increase apoptosis and decrease proliferation among mesenchymal cells in the distal airspace walls compared with a conventional mode of support (intermittent mandatory ventilation).

METHODS

Accordingly, we compared two groups of preterm lambs: one group managed by high-frequency nasal ventilation and a second group managed by intermittent mandatory ventilation. Each group was maintained for 3 days.

MEASUREMENTS AND MAIN RESULTS

Oxygenation and ventilation targets were sustained with lower airway pressures and less supplemental oxygen in the high-frequency nasal ventilation group, in which alveolarization progressed. Thinning of the distal airspace walls was accompanied by more apoptosis, and less proliferation, among mesenchymal cells of the high-frequency nasal ventilation group, based on morphometric, protein abundance, and mRNA expression indices of apoptosis and proliferation.

CONCLUSIONS

Our study shows that high-frequency nasal ventilation preserves the balance between mesenchymal cell apoptosis and proliferation in the distal airspace walls, such that alveolarization progresses.

Bombesin-like peptides modulate alveolarization and angiogenesis in bronchopulmonary dysplasia

RATIONALE

The incidence of bronchopulmonary dysplasia (BPD), a chronic lung disease of newborns, is paradoxically rising despite medical advances. We demonstrated elevated bombesin-like peptide levels in infants that later developed BPD. In the 140-day hyperoxic baboon model of BPD, anti-bombesin antibody 2A11 abrogated lung injury.

OBJECTIVES

To test the hypothesis that bombesin-like peptides mediate BPD in extremely premature baboons (born at Gestational Day 125 and given oxygen pro re nata [PRN], called the 125-day PRN model), similar to "modern-day BPD."

METHODS

The 125-day animals were treated with 2A11 on Postnatal Day 1 (P1), P3, and P6. On P14 and P21, lungs were inflation-fixed for histopathologic analyses of alveolarization. Regulation of angiogenesis by bombesin was evaluated using cultured pulmonary microvascular endothelial cells.

MEASUREMENTS AND MAIN RESULTS

In 125-day PRN animals, urine bombesin-like peptide levels at P2-3 are directly correlated with impaired lung function at P14. Gastrin-releasing peptide (the major pulmonary bombesin-like peptide) mRNA was elevated eightfold at P1 and remained high thereafter. At P14, 2A11 reduced alveolar wall thickness and increased the percentage of secondary septa containing endothelial cells. At P21, 2A11-treated 125-day PRN animals had improved alveolarization according to mean linear intercepts and number of branch points per millimeter squared. Bombesin promoted tubulogenesis of cultured pulmonary microvascular endothelial cells, but cocultured fetal lung mesenchymal cells abrogated this effect.

CONCLUSIONS

Early bombesin-like peptide overproduction in 125-day PRN animals predicted alveolarization defects weeks later. Bombesin-like peptide blockade improved septation, with the greatest effects at P21. This could have implications for preventing BPD in premature infants.

Hyperoxia modulates TGF-beta/BMP signaling in a mouse model of bronchopulmonary dysplasia

Prematurely born infants who require oxygen therapy often develop bronchopulmonary dysplasia (BPD), a debilitating disorder characterized by pronounced alveolar hypoplasia. Hyperoxic injury is believed to disrupt critical signaling pathways that direct lung development, causing BPD. We investigated the effects of normobaric hyperoxia on transforming growth factor (TGF)-beta and bone morphogenetic protein (BMP) signaling in neonatal C57BL/6J mice exposed to 21% or 85% O(2) between postnatal days P1 and P28. Growth and respiratory compliance were significantly impaired in pups exposed to 85% O(2), and these pups also exhibited a pronounced arrest of alveolarization, accompanied by dysregulated expression and localization of both receptor (ALK-1, ALK-3, ALK-6, and the TGF-beta type II receptor) and Smad (Smads 1, 3, and 4) proteins. TGF-beta signaling was potentiated, whereas BMP signaling was impaired both in the lungs of pups exposed to 85% O(2) as well as in MLE-12 mouse lung epithelial cells and NIH/3T3 and primary lung fibroblasts cultured in 85% O(2). After exposure to 85% O(2), primary alveolar type II cells were more susceptible to TGF-beta-induced apoptosis, whereas primary pulmonary artery smooth muscle cells were unaffected. Exposure of primary lung fibroblasts to 85% O(2) significantly enhanced the TGF-beta-stimulated production of the alpha(1) subunit of type I collagen (Ialpha(1)), tissue inhibitor of metalloproteinase-1, tropoelastin, and tenascin-C. These data demonstrated that hyperoxia significantly affects TGF-beta/BMP signaling in the lung, including processes central to septation and, hence, alveolarization. The amenability of these pathways to genetic and pharmacological manipulation may provide alternative avenues for the management of BPD.

Pathology of bronchopulmonary dysplasia

Over the past three decades, advances in prenatal and neonatal intensive care have contributed to marked improvements in survival rates for extremely immature infants born during the canalicular phase of lung development at 24 to 26 weeks, a time when alveolar and distal vascular development is rapidly occurring. The histopathological lesions of severe airway injury and alternating sites of overinflation and fibrosis in "old" BPD have been replaced in "new" BPD with the pathologic changes of large, simplified alveolar structures, a dysmorphic capillary configuration, and variable interstitial cellularity and/or fibroproliferation. Airway and vascular lesions, when present, tend to be present in infants, who over time develop more severe disease. The concept that "new" BPD results in an arrest in alveolization should be modified to that of an impairment in alveolization as evidence shows that short ventilatory times and/or the use of nCPAP allow continued alveolar formation.

SERPINB1 upregulation is associated with in vivo complex formation with neutrophil elastase and cathepsin G in a baboon model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) continues to be a major cause of morbidity in premature infants. An imbalance between neutrophil elastase and its inhibitors has been implicated in BPD. Serine protease inhibitor (SERPIN)B1 is an inhibitor of neutrophil proteases, including neutrophil elastase (NE) and cathepsin G (cat G). Recent studies suggest that SERPINB1 could provide protection in the airways by regulating excess protease activity associated with inflammatory lung disorders. In this study, we determined the distribution and ontogeny of SERPINB1 in the baboon lung and characterized the expression of SERPINB1 in baboon models of BPD. SERPINB1 expression was detected in the conducting airway and glandular epithelial cells in addition to neutrophils, macrophages, and mast cells. SERPINB1 mRNA and protein expression increased with advancing gestational age and in the new BPD model. In contrast, SERPINB1 expression levels were decreased in the old BPD model. Furthermore, SERPINB1 was detected as a high-molecular-mass (HMM) complex in lung tissue and bronchoalveolar lavage fluid samples from the BPD group. Analysis of the HMM complex by coimmunoprecipitation showed that these complexes were formed between SERPINB1 and NE or cat G. High-performance liquid chromatography (HPLC) ion trap mass spectrometry verified the presence of SERPINB1 in HMM complexes. Finally, NE activity level was compared between new and old baboon models of BPD and was found to be significantly lower in new BPD. Thus SERPINB1 upregulation in new BPD may be protective by contributing to the regulation of neutrophil proteases NE and cat G.

Neonatal Chronic Lung Disease in the Post-Surfactant Era

This is a brief review of neonatal chronic lung disease, sometimes called the 'new bronchopulmonary dysplasia (BPD)'. The clinical, radiographic and pathological features of this condition have changed considerably in recent years because of major advances in perinatal care, including widespread use of antenatal glucocorticoid therapy, postnatal surfactant replacement and improved respiratory and nutritional support. Authentic animal models, featuring lengthy mechanical ventilation of surfactant-treated, premature neonatal baboons and lambs, have provided important insights on the pathophysiology and treatment of this disease. Lung histopathology after 2-4 weeks of positive-pressure ventilation with oxygen-rich gas results in failed formation of alveoli and lung capillaries, excess disordered elastin accumulation, smooth muscle overgrowth in small pulmonary arteries and airways, chronic inflammation and interstitial edema. Treatment interventions that have been tested in these animal models include nasal application of continuous positive airway pressure, high-frequency mechanical ventilation, inhaled nitric oxide and retinol. The challenge now is to improve understanding of the molecular mechanisms that regulate normal lung growth and development, and to clarify the dysregulation of lung structure and function that occurs with injury and subsequent repair so that effective treatment or prevention strategies can be devised and implemented.

Impaired lung vascular endothelial growth factor in extremely premature baboons developing bronchopulmonary dysplasia/chronic lung disease

BACKGROUND

Preterm infants exposed to O2 with mechanical ventilation often develop bronchopulmonary dysplasia (BPD), a form of chronic lung disease (CLD). The pathogenesis of BPD/CLD involves dysmorphic microvasculature and disrupted alveolarization. This may be due to impaired vascular endothelial growth factor (VEGF) and VEGF receptor expression.

METHODS

To examine the ontogeny of VEGF and VEGF receptors in baboon lungs from 125 to 185 (term) days gestation and to determine whether exposure to O2 and mechanical ventilation alter these ontogenic profiles, we examined lung specimens from three O2-exposed groups: (1) animals delivered at 125 days gestation and exposed to O2 for 14 days as needed; (2) animals delivered at 140 days gestation and exposed to O2 for 10 days as needed; and (3) animals delivered at 140 days gestation and exposed to 100% O2 for 10 days. Lungs from gestational age-matched controls were also examined at 125, 140, 160, 175, and 185 (term) days.

RESULTS

VEGF189 was the most abundant splice variant in the lungs at all stages of development. Extremely premature baboons developing BPD/CLD had higher lung VEGF121 messenger ribonucleic acid (mRNA) expression. However, transcripts for VEGF189, VEGF165, and VEGF receptors (Fms-like tyrosine kinase-1 [Flt-1], kinase-insert domain receptor [KDR]/fetal liver kinase-1 [Flk-1], and neuropilin 1) were suppressed in the BPD models.

CONCLUSIONS

We conclude that impaired VEGF and VEGF receptor mRNA expression in lungs from extremely premature baboons developing BPD/CLD may contribute to dysmorphic microvasculature and disrupted alveolarization.

Does sustained lung inflation at resuscitation reduce lung injury in the preterm infant?

BACKGROUND

Bronchopulmonary dysplasia (BPD) is a common outcome of preterm birth. Experimental animal work has shown that initial ventilation strategies injure the immature lung and may lead to BPD. Studies with asphyxiated babies have shown that, if tidal ventilation at birth is preceded by sustained lung inflation, larger inflation volumes can be achieved, which is thought to lead to clearance of lung fluid and formation of the functional residual capacity (FRC).

OBJECTIVE

To see if sustained lung inflation at initial resuscitation of preterm babies would facilitate the removal of lung fluid, establish the FRC, and allow an even distribution of alveolar opening, permitting less aggressive ventilation, leading to a reduction in pulmonary inflammation and subsequent BPD.

METHOD

The outcomes of 52 babies of less than 31 weeks gestation, resuscitated at birth using either a sustained lung inflation of five seconds or a conventional lung inflation of two seconds for the first assisted breath of resuscitation, were examined. Evidence of pulmonary inflammation was determined by quantification of interleukins 6, 10, and 1beta and tumour necrosis factor alpha in bronchoalveolar lavage fluid by enzyme linked immunosorbent assay.

RESULTS

There were no significant differences in any of the cytokines. Death occurred in 3/26 babies in the conventional group and 6/26 babies in the sustained lung inflation group. Survival without BPD occurred in 13/26 and 14/26 respectively.

CONCLUSION

The use of sustained lung inflation at resuscitation did not reduce lung injury, as measured by inflammatory markers.

Lung volumes in infants who had mild to moderate bronchopulmonary dysplasia

"New" bronchopulmonary dysplasia (BPD) has been suggested to be a maldevelopment sequence with reduced alveolarisation of the lungs; affected infants then would be predicted to have low lung volumes. The aim of this study was to test that hypothesis by comparing the lung volumes of infants who had had mild-moderate BPD with those without BPD of similar postmenstrual age. Lung volumes of 17 infants who had mild-moderate BPD (oxygen dependent beyond 28 days, but not past term) (BPD infants) were compared to those of 17 infants without BPD (non-BPD infants). All were born at less than 33 weeks of gestation and studied at postmenstrual ages of 33 to 39 weeks. Lung volume was assessed by measurement of functional residual capacity (FRC). The BPD infants had lower lung volumes (median 19.1 ml/kg) than the non-BPD infants (median 26.5 ml/kg) (p = 0.0001). The BPD compared to the non-BPD infants were of greater postnatal age (p = 0.0003), born at a lower gestational age (p = 0.0001) and of lighter birthweight (p = 0.0001). Regression analysis, however, demonstrated that lung volume was significantly related to BPD status (p = 0.005), independently of postnatal age, birthweight and gestational age. It is concluded that the lower lung volumes of the infants who had had mild-moderate BPD support the hypothesis that new BPD is associated with poor alveolarisation.

Vasculogenesis drives pulmonary vascular growth in the developing chick embryo

Formation of the pulmonary vasculature has been described as occurring by outgrowth of existing vessels (angiogenesis), de novo formation of new vessels (vasculogenesis), or a combination of both processes. Uncertainty about the contribution of angiogenesis and vasculogenesis to pulmonary vascular formation is partly due to methodologic approaches. Evidence in favor of angiogenesis stems from studies that used vascular-filling methods. Such methods identify only directly continuous lumina. Evidence for vasculogenesis has been provided by the use of molecular markers of blood vessel endothelium. Use of both methods has not been combined in the same species, however. We hypothesized, based on published evidence from quail and mouse, that chick pulmonary vascular formation occurs by vasculogenesis. To test that hypothesis, we used vascular filling, serial section, and immunohistochemical methods to analyze the developing lungs of chick embryos from Hamburger and Hamilton stages 20 to 43. Vascular filling suggested that the lumen of the pulmonary arteries sprouted from the sixth pharyngeal arch arteries. However, serial sections and immunohistochemical localization of fetal liver kinase-1 protein, the receptor for vascular endothelial growth factor, showed that the pulmonary arterial tree formed from endothelial cell precursors and coalescence of isolated blood vessels in the mediastinal splanchnic mesenchyme centrally to the developing lung tissue distally. Pulmonary veins grew from the left atrium to the developing lungs. Pulmonary blood vessel formation occurred continuously throughout the embryonic period studied. Our results show that vasculogenesis is the main process by which the pulmonary vasculature forms in the developing chick embryo.

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.

Antioxidant defenses in the preterm lung: role for hypoxia-inducible factors in BPD?

Pulmonary antioxidants and their therapeutic implications have been extensively studied during past decades. The purpose of this review is to briefly summarize the key findings of these studies as well as to elaborate on some novel approaches with respect to potential preventive treatments for neonatal chronic lung disease bronchopulmonary dysplasia (BPD). Such new ideas include, for example, modification of transcription factors governing the hypoxic response pathways, important in angiogenesis, cell survival, and glycolytic responses. The fundamental strategy behind that approach is that fetal lung normally develops under hypoxic conditions and that this hypoxic, growth-favoring environment is interrupted by a premature birth. Importantly, during fetal lung development, alveolar development appears to be dependent on vascular development. Therefore, enhancement of signaling factors that occur during hypoxic fetal life ('continued fetal life ex utero'), including angiogenic responses, could potentially lead to improved lung growth and thereby alleviate the alveolar and vascular hypoplasia characteristic of BPD.

Chronic endotoxin exposure does not cause sustained structural abnormalities in the fetal sheep lungs

Chronic early gestational chorioamnionitis is associated with development of bronchopulmonary dysplasia in preterm infants. A single intra-amniotic exposure to endotoxin decreased alveolarization and reduced expression of endothelial proteins in 125-day gestational age preterm lambs. We hypothesized that prolonged exposure to intra-amniotic endotoxin would cause progressive lung inflammation and inhibit alveolar and pulmonary vascular development. Endotoxin (1 mg/day) or saline was administered via an intra-amniotic osmotic pump from 80 to 108 days of gestational age (continuous pump) or by four weekly 10-mg intra-amniotic endotoxin injections starting at 100 days of gestational age (multiple dose). Lung morphometry, lung inflammation, vascular effects, and lung maturation were measured at delivery. The continuous pump lambs delivered at 100 days (approximately 70% of total endotoxin exposure) had lung inflammation, fewer saccules, and decreased endothelial proteins endothelial nitric oxide synthase and VEGF receptor 2 expression compared with controls. The continuous pump (delivered at 138 days) and multiple dose lambs (delivered at 130 and 145 days) had mild persistent lung inflammation and no significant differences in lung morphometry or expression of endothelial proteins compared with controls. Surfactant saturated phosphatidylcholine pool sizes were increased in all endotoxin-exposed groups, but lung function was not changed relative to controls. Contrary to our hypothesis, a prolonged fetal exposure to intra-amniotic endotoxin caused mild persistent inflammation but did not lead to progressive structural abnormalities in lungs of near-term gestation lambs.

Increased lung matrix metalloproteinase-9 levels in extremely premature baboons with bronchopulmonary dysplasia

Matrix metalloproteinases (MMPs) regulate the formation of normal lung architecture. Extremely premature infants exposed to hyperoxia and mechanical ventilation often develop lung inflammation and injury. We hypothesized that an imbalance between MMPs and their tissue inhibitors plays a key role. Our hypothesis was tested to: 1) examine the ontogeny of lung MMPs and tissue inhibitors of metalloproteinases (TIMPs); and 2) determine the effects of hyperoxia and mechanical ventilation on lung MMPs and TIMPs in premature newborn baboons developing chronic lung disease/bronchopulmonary dysplasia (CLD/BPD). Lung specimens were obtained from five groups of gestational controls (GCs) sacrificed at 125, 140, 160, 175, and 185 (term) days of gestation, one fetal baboon model of CLD/BPD delivered at 125 days, and two at 140 days of gestation. Paraffin-embedded lung tissue sections were examined for pathological changes, and frozen lung specimens were analyzed for MMPs-1, -2, -8, and -9; TIMPs-1 and -2; and messenger RNA expression of type I collagen. In GCs, MMP-1 and -9 were elevated in the last trimester, whereas MMP-2 and -8 levels were decreased. Significant changes in lung architecture were noted in the BPD models. MMP-1 was increased in the 125-day model, but decreased in both 140-day models. MMP-8 and collagen mRNA levels were decreased, while MMP-9 and MMP-9 to TIMP-1 ratios were increased in all BPD models. We conclude that an imbalance between MMP-9 and TIMP-1 leading to excessive MMP-9 activity contributes to lung inflammation and edema in CLD/BPD.

Pulmonary antioxidant defenses in the preterm newborn with respiratory distress and bronchopulmonary dysplasia in evolution: implications for antioxidant therapy

Preterm neonates with respiratory distress are exposed not only to the relative hyperoxia ex utero, but also to life-saving mechanical ventilation with high inspired oxygen (O2) concentrations, which is considered a major risk factor for the development of bronchopulmonary dysplasia, also referred to as chronic lung disease of infancy. O2 toxicity is mediated through reactive oxygen species (ROS). ROS are constantly generated as byproducts of normal cellular metabolism, but their production is increased in various pathological states, and also upon exposure to exogenous oxidants, such as hyperoxia. Antioxidants, either enzymatic or nonenzymatic, protect the lung against the deleterious effects of ROS. Expression of various pulmonary antioxidants is developmentally regulated in many species so that the expression is increased toward term gestation, as if in anticipation of birth into an O2-rich extrauterine environment. Therefore, the lungs of prematurely born infants may be ill-adapted for protection against ROS. While premature birth interrupts normal lung development, the clinical condition necessitating the administration of high inhaled O2 concentrations may lead to permanent impairment of alveolar development. An understanding of the processes involved in lung growth, especially in alveolarization and vascularization, as well as in repair of injured lung tissue, may facilitate development of strategies to enhance these processes.

Thioredoxin system in premature and newborn biology

Thioredoxin is an important redox protein that is ubiquitously distributed. Thioredoxin exists in dynamic equilibrium between the oxidized and reduced forms, making it an ideal redox-regulatory protein. Thioredoxin, together with thioredoxin reductase and peroxiredoxins, forms a complete redox system that is similar to the glutathione system, but with distinct and divergent functions. This review provides a brief general summary of the thioredoxin system with particular emphasis on its role in premature birth and newborn physiology and disease states. Although extensive studies have examined the role of the thioredoxin system in antioxidant defense, cell proliferation, and signal transduction, further studies are needed to understand its role in embryogenesis and development. Such studies will facilitate our understanding of how thioredoxin may modulate newborn diseases via redox regulation.

Increased apoptosis and expression of p21 and p53 in premature infant baboon model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a major complication of premature infants who receive prolonged ventilatory support. The pathophysiology of BPD involves oxidant injury, baro/volutrauma, and disordered lung repair. Exposure of premature lung that is poorly adapted for air breathing (>3% oxygen in fetal lung) to a higher concentration of oxygen can cause significant oxidant injury. Cell growth and differentiation of the developing lung require selective and ordered cell division. As hyperoxia can increase the expression of cell-cycle checkpoints that can cause growth arrest of lung cells, in this report we examined the expression of checkpoint proteins p53 and p21 in a premature infant the baboon model of BPD. Additionally, we also determined whether enhanced apoptosis occurs in baboon BPD model. We have shown that p53 and p21 expression are increased in 125-day as well as 140-day premature baboons with BPD. We also demonstrate increased apoptosis in lung tissue of premature baboons with BPD. These results demonstrate that cell growth inhibition is a likely factor in the evolution of BPD. Additionally, lung cells may undergo increased apoptosis that can impair the repair process in the postventilatory recovery period.

Altered expression of cyclins and cdks in premature infant baboon model of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of premature infants, which results in substantial morbidity. The pathophysiology of BPD includes oxidant injury, baro/volutrauma, and disordered lung repair. As lung development, differentiation, and repair require cell division, we hypothesized dysregulation of the cell cycle in oxygen exposure of premature infants that may contribute to the evolution of BPD. In this investigation, we studied the expression of cyclins and cyclin-dependent kinases (cdks) that regulate transition from G1 and G2 phases of the cell cycle. We report here that expression of cyclin D1, cyclin E, and cyclin A is modulated in premature baboons in respiratory distress. In addition, the expression of cdk1 or cdk4 was also modulated in these premature animals. The phosphorylation of retinoblastoma protein was progressively decreased in 125-day animals and in 140-day animals exposed to 6 or 14 days of PRN oxygen. These results indicate that due to altered cyclin and cdk expression, the repair of injured epithelium may proceed in a disordered manner that is characteristic of BPD. Thus, altered cell cycle regulation may be an important factor in the evolution of BPD.

How safe is intermittent positive pressure ventilation in preterm babies ventilated from delivery to newborn intensive care unit?

OBJECTIVES

To examine whether clinically determined ventilator settings will produce acceptable arterial blood gas values on arrival, in preterm infants ventilated from delivery to the newborn intensive care unit (NICU). Further, to examine the usefulness of tidal volume and minute ventilation measurements at this time.

DESIGN

A prospective observational cohort study in a tertiary level 3 NICU.

PATIENTS

Twenty six preterm infants requiring intubation and mechanical ventilation at the point of delivery to the NICU.

SETTING

Infants who required mechanical ventilation were monitored with a blinded Ventrak 1550 dynamic lung function monitor from the point of delivery to the NICU. A Dräger Babylog 2000 transport ventilator was set up to achieve adequate chest wall movement, and FIO(2) was adjusted to achieve preductal SaO(2) of 90-98%. Dynamic lung function monitoring data were recorded and related to the arterial blood gas taken on arrival.

RESULTS

Mean gestation was 28 weeks (range 23-34) and mean birth weight was 1180 g (range 480-4200). A quarter (26% (95% confidence interval (CI) 12% to 48%)) were hypocarbic, with 20% (95% CI 7% to 39%) below 25 mm Hg, and 38% (95% CI 20% to 60%) had hyperoxia. Some (20% (95% CI 7% to 39%)) were both hypocarbic and hyperoxic. Total minute ventilation per kilogram correlated significantly with the inverse of PaCO(2) (p < 0.001).

CONCLUSIONS

Clinically determining appropriate mechanical ventilation settings from the point of delivery to the NICU is difficult, and inadvertent overventilation may be common. Severe hyperoxia can occur in spite of adjustment of the FIO(2) concentration to achieve an SaO(2) range of 90-98%. Limiting minute ventilation during resuscitation may prevent hypocarbia.

Pulmonary vascular dysfunction in preterm lambs with chronic lung disease

Chronic lung injury from prolonged mechanical ventilation after premature birth inhibits the normal postnatal decrease in pulmonary vascular resistance (PVR) and leads to structural abnormalities of the lung circulation in newborn sheep. Compared with normal lambs born at term, chronically ventilated preterm lambs have increased pulmonary arterial smooth muscle and elastin, fewer lung microvessels, and reduced abundance of endothelial nitric oxide synthase. These abnormalities may contribute to impaired respiratory gas exchange that often exists in infants with chronic lung disease (CLD). Nitric oxide inhalation (iNO) reduces PVR in human infants and lambs with persistent pulmonary hypertension. We wondered whether iNO might have a similar effect in lambs with CLD. We therefore studied the effect of iNO on PVR in lambs that were delivered prematurely at approximately 125 days of gestation (term = 147 days) and mechanically ventilated for 3 wk. All of the lambs had chronically implanted catheters for measurement of pulmonary vascular pressures and blood flow. During week 2 of mechanical ventilation, iNO at 15 parts/million for 1 h decreased PVR by approximately 20% in 12 lambs with evolving CLD. When the same study was repeated in eight lambs at the end of week 3, iNO had no significant effect on PVR. To see whether this loss of iNO effect on PVR might reflect dysfunction of lung vascular smooth muscle, we infused 8-bromo-guanosine 3',5'-cyclic monophosphate (cGMP; 150 microg. kg(-1). min(-1) iv) for 15-30 min in four of these lambs at the end of week 3. PVR consistently decreased by 30-35%. Lung immunohistochemistry and immunoblot analysis of excised pulmonary arteries from lambs with CLD, compared with control term lambs, showed decreased soluble guanylate cyclase (sGC). These results suggest that loss of pulmonary vascular responsiveness to iNO in preterm lambs with CLD results from impaired signaling, possibly related to deficient or defective activation of sGC, the intermediary enzyme through which iNO induces increased vascular smooth muscle cell cGMP and resultant vasodilation.

Surfactant kinetics in preterm infants on mechanical ventilation who did and did not develop bronchopulmonary dysplasia

OBJECTIVE

To characterize surfactant kinetics in vivo in two groups of premature infants on different levels of mechanical ventilation and at different risk of developing bronchopulmonary dysplasia.

DESIGN

Controlled observational study in two independent groups of infants.

SETTING

Neonatal intensive care unit.

PATIENTS

Thirteen preterm infants (26 +/- 0.5 wks, birth weight 801 +/- 64 g) on high ventilatory setting and who finally all developed bronchopulmonary dysplasia (MechVentBPD), and eight (26 +/- 0.5 wks, birth weight 887 +/- 103 g) who had minimal or no lung disease and of whom none developed bronchopulmonary dysplasia (MechVentNoBPD).

MEASUREMENTS AND MAIN RESULTS

Endotracheal 13C-labeled dipalmitoyl-phosphatidylcholine was administered and subsequent measurements of the 13C enrichment of surfactant-disaturated phosphatidylcholine (DSPC) from serial tracheal aspirates were made by gas chromatography-mass spectrometry. We calculated disaturated phosphatidylcholine pharmacokinetic variables in terms of half-life and apparent pool size from the enrichment decay curves over time. DSPC concentration from tracheal aspirates was expressed as milligrams/milliliter epithelial lining fluid (ELF-DSPC). Data are presented as mean +/- se. In MechVentBPD infants vs. MechVentNoBPD, ELF-DSPC was much reduced, 2.9 +/- 0.6 vs. 9.4 +/- 3.0 mg/mL ELF (p =.03), half-life was shorter, 19.4 +/- 2.8 vs. 42.5 +/- 6.3 hrs (p =.002), and apparent pool size larger, 136 +/- 21 vs. 65.8 +/- 16.0 mg/kg (p =.057). In MechVentBPD, apparent DSPC pool size positively correlated with mean airway pressure x Fio(2) and inversely correlated with ELF-DSPC. ELF-DSPC was inversely correlated with mean airway pressure x Fio(2). No significant correlations were found in the MechVentNoBPD group.

CONCLUSIONS

MechVentBPD infants showed profound alteration of surfactant kinetics compared with preterm infants with minimal lung disease, and these alterations were correlated with severity of ventilatory support.

Volume-guaranteed ventilation

Pressure-limited, time-cycled ventilation has been the primary mode of ventilation for neonates for several decades. But the realization that volume rather than pressure causes ventilator-induced lung injury has led to the development of new strategies for ventilation. Volume guarantee is a mode of ventilation that automatically adjusts the inspiratory pressure to achieve a set tidal volume according to changes in lung compliance or resistance or the patient's respiratory drive. Volume-guaranteed ventilation delivers a specific, preset volume of gas, and inspiration ends when it has been delivered. This mode of ventilation requires careful attention to the infant and to ventilator settings.

A catalytic antioxidant attenuates alveolar structural remodeling in bronchopulmonary dysplasia

Superoxide anion and other oxygen-free radicals have been implicated in the pathogenesis of bronchopulmonary dysplasia. We tested the hypothesis that a catalytic antioxidant metalloporphyrin AEOL 10113 can protect against hyperoxia-induced lung injury using a fetal baboon model of bronchopulmonary dysplasia. Fetal baboons were delivered by hysterotomy at 140 days of gestation (term = 185 days) and given 100% oxygen for 10 days. Morphometric analysis of alveolar structure showed that fetal baboons on 100% oxygen alone had increased parenchymal mast cells and eosinophils, increased alveolar tissue volume and septal thickness, and decreased alveolar surface area compared with animals given oxygen as needed. Treatment with AEOL 10113 (continuous intravenous infusion) during 100% oxygen exposure partially reversed these oxygen-induced changes. Hyperoxia increased the number of neuroendocrine cells in the peripheral lung, which was preceded by increased levels of urine bombesin-like peptide at 48 hours of age. AEOL 10113 inhibited the hyperoxia-induced increases in urine bombesin-like peptide and numbers of neuroendocrine cells. An increasing trend in oxygenation index over time was observed in the 100% oxygen group but not the mimetic-treated group. These results suggest that AEOL 10113 might reduce the risk of pulmonary oxygen toxicity in prematurely born infants.