Endotoxin-induced lung maturation in preterm lambs is not mediated by cortisol

Positive end-expiratory pressure and tidal volume during initial ventilation of preterm lambs

Positive end-expiratory pressure (PEEP) protects the lung from injury during sustained ventilation, but its role in protecting the lung from injury during the initiation of ventilation in the delivery room is not established. We aimed to evaluate whether PEEP and/or tidal volume (VT) within the first 15-min of ventilation are protective against lung injury. Operatively delivered preterm lambs (133 +/- 1 d gestation) were randomly assigned to unventilated controls or to one of four 15 min ventilation interventions: 1) VT15 mL/kg, PEEP 0 cm H2O; 2) VT15 mL/kg, PEEP 5 cm H2O; 3) VT8 mL/kg, PEEP 0 cm H2O; and 4) VT8 mL/kg, PEEP 5 cm H2O. Each group was subsequently ventilated with VT 10 mL/kg, PEEP 5 cm H2O for 1 h 45 min. Lung function was assessed and measurements of lung injury were evaluated postmortem. After the 15 min ventilation maneuver, the VT15 groups were hypocarbic, had higher oxygenation, and required lower pressures than the VT8 groups; no consistent effect of PEEP was found. Markers of lung injury were significantly elevated in all ventilation groups compared with unventilated controls; no effect of PEEP was found. Ventilation resulted in localization of IL-6 to the small airways. Initial ventilation of preterm lambs with PEEP and/or VT of 8 mL/kg did not prevent an inflammatory injury to the lung.

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.

Antenatal inflammation and lung injury: prenatal origin of neonatal disease

INTRODUCTION

Antenatal inflammation in utero may be associated with lung injury and subsequent aberrant lung development resulting in bronchopulmonary dysplasia (BPD). BPD has become a developmental disease with a uniform arrest in lung development.

STUDY DESIGN

The role of antenatal inflammation in the induction of lung injury was explored in a sheep model suitable for the study of lung development with respect to human development. Chorioamnionitis was induced by a single injection of endotoxin into the amniotic cavity under ultrasound guidance.

RESULT

Endotoxin-induced chorioamnionitis caused a cascade of lung injury, pulmonary inflammation and remodeling in the fetal lung similar to lung injury previously described in adult animal models. The structural changes in the fetal lung after chorioamnionitis showed little to no fibrosis and alveolar/microvascular simplification similar to new BPD. The identified cytokine networks and regulators may explain the absence of fibrosis and lung simplification after strictly intra-uterine inflammation.

CONCLUSION

The mechanisms of antenatal inflammation in the fetal lung were multifactorial and could be antenatally modulated. Fetal pulmonary inflammation was temporarily suppressed by maternal glucocorticoid therapy. However, pulmonary inflammation could be augmented postnatally by resuscitation, oxygen toxicity, mechanical ventilation and pulmonary and systemic infection, which opens a broad window of clinical options.

Toll-like receptors and agonist responses in the developing fetal sheep lung

Toll-like receptors (TLRs) are pattern recognition molecules that initiate innate immune responses. Intra-amniotic exposure of fetal sheep to pro-inflammatory stimuli causes pulmonary inflammation and induced lung maturation. We examined TLR ontogeny and fetal lung responsiveness to three different TLR agonists. We cloned ovine TLRs 2, 3, and 4 and found 83-88% homology between these ovine and human TLRs. Lung TLR2 and 4 mRNAs increased throughout late gestation to 50% of adult level in the term newborn lamb. Doses of 10 mg of PAMCysK4 (TLR2 agonist), poly I:C dsRNA (TLR3 agonist), or E. coli O55:B5 lipopoysaccharide (LPS) (TLR4 agonist) were given by intra-amniotic injection 2 d or 7 d before operative delivery of preterm lambs at 123 d (n = 4-7/group). The TLR4 agonist induced lung inflammation and maturation, whereas the TLR2 agonist gave less consistent responses. Intra-amniotic LPS increased TLR2 mRNA expression primarily in the inflammatory cells and TLR4 mRNA diffusely in multiple cell types. The TLR3 agonist had no effects, and TLR3 mRNA in the fetal lung did not change after LPS exposure. We conclude that TLR2 and TLR4 mRNAs increase through gestation and expression of TLR2 and TLR4 are induced by LPS in the fetal sheep lung.

A role for macrophage migration inhibitory factor in the neonatal respiratory distress syndrome

Using a mouse model of neonatal respiratory distress syndrome (RDS), we demonstrate a central role for macrophage migration inhibitory factor (MIF) in lung maturation at the developmental stage when human neonates are most susceptible to RDS. We prematurely delivered mouse pups at embryonic day 18, during the early saccular stage of pulmonary development. Only 8% of the prematurely delivered pups genetically deficient in MIF survived 8 h vs 75% of wild-type controls (p<0.001). This phenotype was corrected when pups of all genotypes were bred from dams heterozygote for MIF deficiency. Local production of MIF in the lung increased at embryonic day 18, continued until full-term at embryonic day 19.5, and decreased in adulthood, thus coinciding with this developmental window. The lungs of pups genetically deficient in MIF were less mature upon histological evaluation, and demonstrated lower levels of vascular endothelial growth factor and corticosterone--two factors that promote fetal lung maturation. In vitro studies support a role for MIF in surfactant production by pulmonary epithelial cells. In a cohort of human neonates with RDS, higher intrapulmonary MIF levels were associated with a lower likelihood of developing bronchopulmonary dysplasia, a sequelae of RDS (p<0.03). This study demonstrates for the first time a role for MIF in lung maturation, and supports a protective role for MIF in newborn lung disease.

Pulmonary and systemic endotoxin tolerance in preterm fetal sheep exposed to chorioamnionitis

In a model of human chorioamnionitis, fetal sheep exposed to a single injection, but not repeated injections, of intra-amniotic endotoxin develop lung injury responses. We hypothesized that repeated exposure to intra-amniotic endotoxin induces endotoxin tolerance. Fetal sheep were given intra-amniotic injections of saline (control) or Escherichia coli LPS O55:B5 (10 mg) either 2 days (2-day group, single exposure), 7 days (7-day group, single exposure), or 2 plus 7 days (2- and 7-day repeat exposure) before preterm delivery at 124 days gestation (term=150 days). Endotoxin responses were assessed in vivo in the lung and liver, and in vitro in monocytes from the blood and the lung. Compared with the single 2-day LPS exposure group, the (2 plus 7 days) repeat LPS-exposed lambs had: 1) decreased lung neutrophil and monocyte inducible NO synthase (NOSII) expression, and 2) decreased lung cytokine and liver serum amyloid A3 mRNA expression. In the lung, serum amyloid A3 mRNA expression decreased in the airway epithelial cells but not in the lung inflammatory cells. Unlike the single 7-day LPS exposure group, peripheral blood and lung monocytes from the repeat-LPS group did not increase IL-6 secretion or hydrogen peroxide production in response to in vitro LPS. Compared with controls, TLR4 expression did not change but IL-1R-associated kinase M expression increased in the monocytes from repeat LPS-exposed lambs. These results are consistent with the novel finding of endotoxin tolerance in preterm fetal lungs exposed to intra-amniotic LPS. The findings have implications for preterm infants exposed to chorioamnionitis for both responses to lung injury and postnatal nosocomial infections.

Oxidative stress in fetal lambs exposed to intra-amniotic endotoxin in a chorioamnionitis model

Chorioamnionitis is a risk factor for the development of bronchopulmonary dysplasia. Endotoxin-induced oxidative stress to the fetus in the uniquely hypoxic intrauterine environment has not been reported. Using a model of chorioamnionitis, we measured markers of pulmonary and systemic oxidant exposures in fetal lambs at 124 d gestation (term = 150 d) exposed to 10 mg intra-amniotic endotoxin 2 d (n = 6) or 7 d (n = 6) before delivery, or saline as controls (n = 9). The 7 d endotoxin-exposed animals had 3-fold higher protein carbonyls (0.66 +/- 0.46 versus 0.23 +/- 0.14 nmol/mg protein) and 10-fold greater myeloperoxidase activity (2.38 +/- 1.87 versus 0.27 +/- 0.18 nM) in the bronchoalveolar lavage fluid (BALF), suggestive of neutrophil-derived oxidant activity. However, in the lung tissue, protein carbonyls, superoxide dismutase, and peroxiredoxin 1 were not different between groups. The expression of peroxiredoxin 1 was prominent, primarily in the peri-bronchiolar epithelium. Notably, evidence of oxidant exposure was minimal at 2 d when BALF inflammatory cells, lung IL-1beta, and IL-8 were highest. Intra-amniotic endotoxin induced systemic oxidative stress as plasma protein carbonyl was elevated at 7 d (0.14 +/- 0.04 nmol/mg protein; p = 0.005). Surfactant protein A and B mRNAs were highest at 2 d, suggesting that oxidative stress did not contribute to the lung maturation response. A modest lung oxidative stress in chorioamnionitis could contribute to bronchopulmonary dysplasia.

Maternal betamethasone and chorioamnionitis induce different collagenases during lung maturation in fetal sheep

BACKGROUND

Fetal lung maturation occurs after both maternal corticosteroid administration and chorioamnionitis. The effectors of this antenatally-induced lung maturation are not understood. Matrix metalloproteinases (MMPs) 2 and 9 are type-IV collagenases that can degrade alveolar basement membranes.

OBJECTIVES

We hypothesized that the structural changes of lung maturation by both antenatal corticosteroid treatment and chorioamnionitis would be associated with increases in these MMPs.

METHODS

64 pregnant ewes were randomly assigned to one of four treatment groups: intra-amniotic injection of 10 mg endotoxin, maternal intramuscular injection of 0.5 mg/kg betamethasone, both treatments combined or saline-treated controls. We quantified MMP-2 which is derived from connective tissue and MMP-9 which is predominantly derived from neutrophils in fetal lung fluid of lambs after maternal corticosteroid therapy and induction of chorioamnionitis and the combination of both therapies given at 109-111 days' gestational age with delivery 1, 5 or 15 days later.

RESULTS

Betamethasone, endotoxin and the combined treatments increased both surfactant pool size, lung gas volume and reduced alveolar wall thickness at 15 days. MMP-2 concentration was increased after betamethasone. MMP-9 concentration increased after endotoxin-induced chorioamnionitis but decreased by the combined treatments.

CONCLUSION

Lung maturation via different pathways may use different forms of collagenases for remodelling lung structure.

Fetal cortisol response to intrauterine microbial colonisation identified by the polymerase chain reaction and fetal inflammation

OBJECTIVE

To determine the fetal cortisol response to intrauterine infection.

STUDY DESIGN

16s ribosomal RNA genes or the urease genes of Ureaplasma spp were identified by the polymerase chain reaction in intrauterine samples. Cord blood cortisol, interleukin 6 (IL6) and IL8 were measured in samples from 31 infants born at <32 weeks gestation.

RESULTS

13 infants (median gestation 30 weeks, birth weight 1350 g) had at least one positive intrauterine sample for microbial genes and 18 infants (31 weeks, 1320 g) did not. The cord blood cortisol concentration was significantly higher in fetuses exposed to intrauterine infection and significantly increased in fetuses/mothers presenting in preterm labour with intact membranes compared with infants delivered by elective prelabour caesarean section (p<0.05). The cord blood cortisol concentration was increased in the mothers with prelabour premature rupture of membranes but this was not significant compared with the caesarean section group. The cord blood cortisol concentration was significantly increased in the presence of chorioamnionitis or funisitis and was moderately correlated with cord blood IL6 (r = 0.64, p<0.01) and IL8 (r = 0.52, p<0.01).

CONCLUSIONS

In this study, cord blood cortisol was increased in the colonised group compared with non-colonised infants. It is unclear if infants born following prelabour premature rupture of the membranes mount an adequate anti-inflammatory response.

Experimental amniotic fluid infection in sheep: effects of Ureaplasma parvum serovars 3 and 6 on preterm or term fetal sheep

OBJECTIVE

The objective of the study was to determine the effects in late gestation of Ureaplasma parvum serovar 3 colonization and the effects, preterm, of U. parvum serovar 6.

STUDY DESIGN

Ewes received an intraamniotic (i.a.) injection of U. parvum serovar 6 (20 x 10(6) colony-forming units [cfu]; n = 9), U. parvum serovar 3 (20 x 10(3) cfu; n = 6), vehicle (n = 10), or saline (n = 4) on day 80 of pregnancy (d). The lambs were delivered at 125 d (U. parvum serovar 6, n = 9; saline or media controls, n = 9) or 145 d (U. parvum serovar 3, n = 6; media controls, n = 5) for assessment of inflammation and lung maturation.

RESULTS

I.a. ureaplasmas caused histologic chorioamnionitis but not preterm delivery. Fetal lung epithelium was colonized with ureaplasmas at both gestational ages, and pulmonary interleukin-8 levels had doubled in the ureaplasma-colonized animals, compared with the controls at 145 d. Surfactant levels in bronchoalveolar lavage fluid had increased 8-fold and 2.5-fold at 125 and 145 d, respectively, after ureaplasma injection.

CONCLUSION

Fetal lung inflammation and altered development accompanies ureaplasma colonization, regardless of age at delivery.

The fetal inflammatory response syndrome

The fetal inflammatory response syndrome (FIRS) is a condition characterized by systemic inflammation and an elevation of fetal plasma interleukin-6. This syndrome has been observed in fetuses with preterm labor with intact membranes, preterm prelabor rupture of the membranes, and also fetal viral infections such as cytomegalovirus. FIRS is a risk factor for short-term perinatal morbidity and mortality after adjustment for gestational age at delivery and also for the development of long-term sequelae such as bronchopulmonary dysplasia and brain injury. Multiorgan involvement in FIRS has been demonstrated in the hematopoietic system, thymus, adrenal glands, skin, kidneys, heart, lung, and brain. This article reviews the fetal systemic inflammatory response as a mechanism of disease. Potential interventions to control an exaggerated inflammatory response in utero are also described.

Brief, large tidal volume ventilation initiates lung injury and a systemic response in fetal sheep

RATIONALE

Premature infants are exposed to potentially injurious ventilation in the delivery room. Assessments of lung injury are confounded by effects of subsequent ventilatory support.

OBJECTIVES

To evaluate the injury response to a brief period of large tidal volume (Vt) ventilation, simulating neonatal resuscitation in preterm neonates.

METHODS

Preterm lambs (129 d gestation; term is150 d) were ventilated (Vt = 15 ml/kg, no positive end-expiratory pressure) for 15 minutes to simulate delivery room resuscitation, either with the placental circulation intact (fetal resuscitation [ FR]) or after delivery (neonatal resuscitation [NR]). After the initial 15 minutes, lambs received surfactant and were maintained with either ventilatory support (FR-VS and NR-VS) or placental support (FR-PS) for 2 hours, 45 minutes. A control group received no resuscitation and was maintained with placental support. Samples of bronchoalveolar lavage fluid, lung, and liver were analyzed.

MEASUREMENTS AND MAIN RESULTS

Inflammatory cells and protein in bronchoalveolar lavage fluid, heat shock protein-70 immunostaining, IL-1beta, IL-6, IL-8, monocyte chemotactic protein-1, serum amyloid A (SAA)-3, Toll-like receptor (TLR)-2, and TLR4 mRNA in the lungs were increased in the FR-PS group compared with control animals. There were further elevations in neutrophils, IL-6, and IL-8 mRNA in the FR-VS and NR-VS groups compared with FR-PS. SAA3, TLR2, and TLR4 mRNA increased in the liver in all resuscitation groups relative to control animals.

CONCLUSIONS

Ventilation for 15 minutes with a Vt of 15 ml/kg initiates an injurious process in the preterm lung and a hepatic acute-phase response. Subsequent ventilatory support causes further increases in some injury indicators.

Antenatal infection/inflammation and fetal tissue injury

There is a clear association between antenatal infection/inflammation and preterm labour, with intrauterine infection complicating up to one third of preterm deliveries. In addition to this, there is now accumulating evidence that intrauterine infection and inflammation can lead to the development of a systemic inflammatory response in the fetus and subsequent tissue injury. The fetal inflammatory response is characterized by funisitis, high levels of pro-inflammatory cytokines in the amniotic fluid and cord blood, and systemic immune activation. This review discusses the evidence for this process and focuses on the clinical and experimental data supporting the hypothesis that these inflammatory processes contribute to brain and lung injury in the newborn.

Endotoxin-induced maturation of monocytes in preterm fetal sheep lung

The fetal lung normally contains immature monocytes and very few mature macrophages. The chorioamnionitis frequently associated with preterm birth induces monocyte influx into the fetal lung. Previous studies demonstrated that monocytes in the developing lung can mediate lung injury responses that resemble BPD in humans. We hypothesized that chorioamnionitis would induce maturation of immature monocytes in the fetal lung. Groups of three to seven time-mated ewes received saline or 10 mg of endotoxin (Escherichia coli 055:B5) in saline by intra-amniotic injection for intervals from 1 to 14 days before operative delivery at 124 days of gestational age. Monocytic cells from lung tissue were recovered using Percoll gradients. Monocytic cells consistent with macrophages were identified morphologically and by myosin heavy chain class II expression. An increase in macrophages was preceded by induction of granulocyte-macrophage colony-stimulating factor in the lung and subsequent activation of the transcription factor PU.1. The production of IL-6 by monocytes/macrophages in response to endotoxin challenge in vitro increased 7 and 14 days after exposure to intra-amniotic endotoxin. Recombinant TNF-alpha induced IL-6 production by lung monocytic cells exposed to intra-amniotic endotoxin but not in control cells. Monocytic phagocytosis of apoptotic neutrophils also increased 7 and 14 days after exposure to intra-amniotic endotoxin. Intra-amniotic endotoxin induced lung monocytes to develop into functionally mature cells consistent with macrophages. These findings have implications for lung immune responses after exposure to chorioamnionitis.

A paradoxical temporal response of the PTHrP/PPARgamma signaling pathway to lipopolysaccharide in an in vitro model of the developing rat lung

Chorioamnionitis alters lung development, resulting in a paradoxical decrease in the incidence of respiratory distress syndrome but an increase in the incidence of bronchopulmonary dysplasia (BPD). The mechanism(s) underlying this disparity in the pulmonary outcomes is not known. We hypothesized that specific alterations in alveolar epithelial-mesenchymal interactions might explain this apparent disparity in the pulmonary outcome following chorioamnionitis. We determined the effects of lipopolysaccharide (LPS) on parathyroid hormone-related protein (PTHrP)-driven epithelial-mesenchymal interactions that are essential for normal lung development and homeostasis. Lung explants from embryonic day 19.5 Sprague-Dawley rat fetuses were treated with LPS with or without a PTHrP pathway agonist, prostaglandin J(2) (PGJ(2)). LPS treatment affected the production of proinflammatory cytokines and the expression of the key markers of the epithelial-mesenchymal paracrine interactions in a time-dependent manner. At 6 h, there was a significant increase in the expression of PTHrP and the other key markers of alveolar homeostasis without any significant effect on alpha-smooth muscle actin (alphaSMA). In contrast, at 72 h, there was a significant decrease in the expression of PTHrP and the other key markers of alveolar homeostasis accompanied by a significant increase in alphaSMA expression. The cytokine and molecular changes at 72 h were completely prevented by the concomitant treatment with PGJ(2). We speculate that these data provide a likely mechanism for the acute stimulation of lung differentiation, accompanied paradoxically by BPD following chorioamnionitis, and suggest that by specifically targeting PTHrP signaling, the inflammation-induced molecular injury that is known to result in BPD can be prevented.

The role of inflammation and infection in preterm birth

Inflammation has been implicated in the mechanisms responsible for preterm and term parturition, as well as fetal injury. Out of all of the suspected causes of preterm labor and delivery, infection and/or inflammation is the only pathological process for which both a firm causal link with preterm birth has been established and a molecular pathophysiology defined. Inflammation has also been implicated in the mechanism of spontaneous parturition at term. Most cases of histopathological inflammation and histological chorioamnionitis, both in preterm and term labor, are sub-clinical in nature. The isolation of bacteria in the amniotic fluid, known as microbial invasion of the amniotic cavity, is a pathological finding; the frequency of which is dependent upon the clinical presentation and gestational age. This article reviews the role of inflammation in preterm and term parturition.

The preterm parturition syndrome

The implicit paradigm that has governed the study and clinical management of preterm labour is that term and preterm parturition are the same processes, except for the gestational age at which they occur. Indeed, both share a common pathway composed of uterine contractility, cervical dilatation and activation of the membranes/decidua. This review explores the concept that while term labour results from physiological activation of the components of the common pathway, preterm labour arises from pathological signalling and activation of one or more components of the common pathway of parturition. The term "great obstetrical syndromes" has been coined to reframe the concept of obstetrical disease. Such syndromes are characterised by: (1) multiple aetiology; (2) long preclinical stage; (3) frequent fetal involvement; (4) clinical manifestations that are often adaptive in nature; and (5) gene-environment interactions that may predispose to the syndromes. This article reviews the evidence indicating that the pathological processes implicated in the preterm parturition syndrome include: (1) intrauterine infection/inflammation; (2) uterine ischaemia; (3) uterine overdistension; (4) abnormal allograft reaction; (5) allergy; (6) cervical insufficiency; and (7) hormonal disorders (progesterone related and corticotrophin-releasing factor related). The implications of this conceptual framework for the prevention, diagnosis, and treatment of preterm labour are discussed.

IL-1 alpha causes lung inflammation and maturation by direct effects on preterm fetal lamb lungs

Intra-amniotic endotoxin induces IL-1, causes chorioamnionitis, lung inflammation, lung injury and lung maturation in preterm lambs. Intra-amniotic IL-1alpha also causes chorioamnionitis, lung inflammation and lung maturation. We asked if IL-1alpha effects on the preterm lung are mediated by direct signaling to the lung rather than by indirect effects from the chorioamnionitis. To study IL-1 effects independently of chorioamnionitis, the lungs and the amniotic fluid were surgically separated in fetal sheep by diverting fetal lung fluid via a tracheostomy tube to a sialastic bag. A mini-osmotic pump delivered an intratracheal infusion of recombinant sheep IL-1alpha (10 microg) or saline (control) over 24 h. Preterm lambs were delivered 1d or 7d after the start of the infusion at 124d gestational age (Term = 150d). IL-1alpha recruited inflammatory cells and increased pro-inflammatory cytokine mRNA expression in the fetal lungs. Compared with controls, IL-1alpha did not alter lung antioxidant enzyme activity or alveolar numbers. IL-1alpha had minimal effects on the mRNA or protein expression of proteins essential for vascular development. IL-1alpha induced large increases in alveolar surfactant saturated phosphatidylcholine and increased lung gas volumes. Lung inflammation and maturation result from direct exposure of the fetal lung to a single cytokine - IL-1alpha.

Antenatal inflammation induced TGF-beta1 but suppressed CTGF in preterm lungs

Chorioamnionitis is frequently associated with preterm birth and increases the risk of adverse outcomes such as bronchopulmonary dysplasia (BPD). Transforming growth factor (TGF)-beta1 is a key regulator of lung development, airway remodeling, lung fibrosis, and regulation of inflammation, and all these processes contribute to the development of BPD. Connective tissue growth factor (CTGF) is a downstream mediator of some of the profibrotic effects of TGF-beta1, vascular remodeling, and angiogenesis. TGF-beta1-induced CTGF expression can be blocked by TNF-alpha. We asked whether chorioamnionitis-associated antenatal inflammation would regulate TGF-beta1, the TGF-beta1 signaling pathway, and CTGF in preterm lamb lungs. Fetal sheep were exposed to 4 mg of intra-amniotic endotoxin or saline for 5 h, 24 h, 72 h, or 7 days before preterm delivery at 125 days gestation (full term = 150 days). Intra-amniotic endotoxin increased lung TGF-beta1 mRNA and protein expression. Elevated TGF-beta1 levels were associated with TGF-beta1-induced phosphorylation of Smad2. CTGF was selectively expressed in lung endothelial cells in control lungs, and intra-amniotic endotoxin caused CTGF expression to decrease to 30% of control values and TNF-alpha protein to increase. The antenatal inflammation-induced TGF-beta1 expression and Smad signaling in the fetal lamb lung may contribute to impaired lung alveolarization and reduced lung inflammation. Decreased CTGF expression may inhibit vascular development or remodeling and limit lung fibrosis during remodeling. These effects may contribute to the impaired alveolar and pulmonary vascular development that is the hallmark of the new form of BPD.

Physiological consequences of intrauterine insults

Fetal lung development occurs as a complicated series of interactions between the different cell types in the lung in response to different growth factors and hormones. At birth, the human lung is in the stage of alveolar development in which the gas exchange units (alveoli) are being actively formed. The alveolar growth continues into postnatal life. Different intrauterine insults perturb this sequence of lung development in different ways. The ultimate result of aberrant lung development depends on the type of intrauterine insult, the severity, the duration of the insult and the developmental stage at which the insult occurs. This review article focuses on the common intrauterine insults encountered in clinical practice, such as infections, disorders of amniotic fluid volume, nutrition and maternal smoking. The information derived from clinical studies is juxtaposed with data from animal experiments to discuss the physiological consequences of intrauterine insults on fetal lung growth.

Contribution of inflammation to lung injury and development

Inflammation interferes with lung development in model systems and is present chronically in the lungs of preterm infants who develop bronchopulmonary dysplasia (BPD). Antenatal inflammation is very commonly associated with preterm deliveries, but there is generally minimal information about the duration, intensity, or organisms associated with chorioamnionitis. In preterm lamb models, chorioamnionitis causes a lung injury similar to BPD and also causes clinical lung maturation. Continuous exposure of the developing lung before and after delivery to inflammation may be central to the development of BPD.

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.

Stage-specific regulation of respiratory epithelial cell differentiation by Foxa1

Foxa1 is a member of the winged helix family of transcription factors that is expressed in epithelial cells of the conducting airways and in alveolar type II cells of the lung. To determine the role of Foxa1 during lung morphogenesis, histology and gene expression were assessed in lungs from Foxa1-/- gene-targeted mice from embryonic day (E) 16.5 to postnatal day (PN) 13. Deletion of Foxa1 perturbed maturation of the respiratory epithelium at precise times during lung morphogenesis. While dilatation of peripheral lung saccules was delayed in Foxa1-/- mice at E16.5, sacculation was unperturbed later in development (E17.5-E18.5). At PN5, alveolarization was markedly delayed in Foxa1-/- mice; however, by PN13 lung histology was comparable to wild-type controls. Clara cell secretory protein (CCSP), prosurfactant protein (SP)-C, and SP-B protein content and immunostaining were decreased in Foxa1-/- mice between E16.5 and E18.5 but normalized after birth. Timing and sites of expression of thyroid transcription factor-1, Foxj1, and beta-tubulin were unaltered in lungs of Foxa1-/- mice. In vitro, Foxa1 regulated the activity of CCSP and SP-A, SP-B, SP-C, and SP-D promoters as assessed by luciferase reporter assays in HeLa, H441, and MLE15 cells. Although Foxa1 regulates respiratory epithelial differentiation and structural maturation of the lung at precise developmental periods, the delay in maturation is subsequently compensated at times to enable respiratory function and restore normal lung structure after birth.

The Clever Fetus: Responding to Inflammation to Minimize Lung Injury

Clinical and experimental information indicate that fetal exposure to inflammation can induce lung maturation. This inflammation may be chronic and indolent. We present clinical and experimental information that challenge assumptions about chorioamnionitis associated infection/inflammatory exposures to the fetus. We question the assumption that the fetal compartment is sterile, and that delivery is inevitable if chronic infection/inflammation is present. We demonstrate that the preterm fetus can develop a brisk and adequate inflammatory response, and the fetus also can quickly modulate and downregulate inflammation to prevent injury. The fetus at risk of early preterm delivery may be exposed to both chorioamnionitis and glucocorticoids. Both exposures can either increase or decrease fetal inflammatory responses depending on the timing of the exposures. The immunomodulatory ability of the fetus to fetal exposures remains an unexplored research field.

Recruited inflammatory cells mediate endotoxin-induced lung maturation in preterm fetal lambs

RATIONALE

Chorioamnionitis is paradoxically associated with a decreased incidence of respiratory distress syndrome in preterm infants. In preterm lambs, intraamniotic endotoxin and interleukin 1 (IL-1) induce lung inflammation followed by lung maturation.

OBJECTIVE

To test if inflammatory cells are required to mediate induced lung maturation.

METHODS

Lung inflammation was induced by intraamniotic injection of endotoxin or IL-1. Inflammatory cell recruitment to the lung was inhibited by an anti-CD18 blocking antibody given intramuscularly to the fetus. Preterm lambs were delivered at 124-d gestation (term = 150 d) 2 or 7 d after exposure to endotoxin/IL-1 or endotoxin/IL-1 + anti-CD18 antibody.

MEASUREMENTS

Lung inflammation was measured by bronchoalveolar lavage fluid cell count, inflammatory scoring of lung parenchyma, and expression of proinflammatory cytokines and inducible nitric oxide synthase. Lung maturation was quantitated by surfactant protein mRNA expression, saturated phosphatidylcholine pool size, and pressure-volume curves.

MAIN RESULTS

Inhibition of CD18 significantly reduced endotoxin-induced but not IL-1-induced fetal lung inflammatory cell recruitment and activation as well as expression of proinflammatory cytokines. Compared with control lungs, both endotoxin and IL-1 induced lung maturation. Anti-CD18 antibody administration inhibited only endotoxin-induced but not IL-1-induced increases in surfactant protein mRNA and surfactant saturated phosphatidylcholine. Exposure to anti-CD18 antibody moderated endotoxin-induced increases in lung volumes but had no effect on IL-1-induced increases in lung volumes.

CONCLUSIONS

(1) Endotoxin- but not IL-1-induced inflammatory cell recruitment in the preterm fetal lamb lung is CD18 dependent; (2) recruited inflammatory cells mediate some aspects of fetal lung maturation.

Antenatal associations with lung maturation and infection

Chronic clinically unapparent chorioamnionitis is a common antenatal exposure for very preterm infants, and these infants have variable degrees of lung maturation and a high risk of developing bronchopulmonary dysplasia. Exposure of fetal sheep to intra-amniotic endotoxin or IL-1alpha induces chorioamnionitis and lung injury (decreased alveolarization and microvascular injury), which resolves to a phenotype of striking lung maturation (increased surfactant, improved gas exchange and lung mechanics). The immune responses of the fetus also are suppressed or induced (matured) in time and dose-dependent ways by either chorioamnionitis or antenatal corticosteroids. These experimental observations contribute to explanations of why preterm infants have variable degrees of lung maturation at birth and unpredictably develop bronchopulmonary dysplasia 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.

Endotoxin-induced Chorioamnionitis Modulates Innate Immunity of Monocytes in Preterm Sheep

The preterm fetus is immune naive and has immature innate immune function. Although the preterm fetus is frequently exposed to chorioamnionitis, the effects of exposure of the fetal lung to inflammation on innate immune responses are unknown. Using the fetal sheep model of chorioamnionitis, cord blood monocytes were isolated from preterm lambs 1 to 14 days after intra-amniotic endotoxin injection, cultured for approximately 16 hours, and challenged with endotoxin in vitro. Compared with monocytes from adult sheep, the preterm monocytes produced less H(2)O(2) and interleukin-6, and toll-like receptor 4 expression was decreased. Three days after intra-amniotic endotoxin exposure, preterm monocyte responses to in vitro endotoxin challenge demonstrated decreased H(2)O(2) and interleukin-6 production and decreased CD14 and major histocompatibility complex class II expression. Preterm monocyte responses 7 to 14 days after endotoxin tended to exceed those of adults and preterm control animals indicating augmented function. In contrast, a second intra-amniotic endotoxin injection 7 days after the initial endotoxin exposure suppressed monocyte function at 14 days. The fetal monocytes demonstrated patterns of responses consistent with endotoxin tolerance (immune paralysis) as well as maturation of function. Modulation of fetal innate immune responses by exposure to inflammation may alter subsequent immune adaptation after birth.

Modeling and Remodeling of the Lung in Neonatal Chronic Lung Disease

Neonatal chronic lung disease (CLD) is the major long-term pulmonary complication of preterm birth affecting about 20% of infants who need mechanical ventilation. CLD is the result of abnormal repair processes following inflammatory lung injury that lead to remodeling of the lung. Inflammation may be initiated by a variety of stimuli including mechanical ventilation, oxygen toxicity and infection. The resultant neutrophil chemotaxis and degranulation leads to the release of enzymes such as matrix metalloproteinases that can cause proteolysis of the lung extracellular matrix. Abnormal healing with remodeling leads to poorly compliant lungs with reduced capacity for gas exchange. Drugs can influence the normal process of lung modeling or remodeling. Fetal lung development can be influenced by glucocorticosteroids and inflammation. Both can cause abnormal lung modeling with fewer, larger alveoli and accelerated lung maturation, which confers benefits in terms of reduced morbidity and mortality from respiratory distress syndrome but potentially increases the risk of subsequent lung injury. Antioxidants, such as retinol (vitamin A), administered post-natally may reduce the effects of oxidative stress leading to a modest reduction in CLD but they require repeated intramuscular injections. Postnatal glucocorticosteroid therapy can modify the lung inflammatory response and reduce CLD but it can also have detrimental effects on the developing brain and lung, thereby creating a clinical dilemma for neonatologists. Proteinase inhibitors may be a rational therapy but more research is needed before they can be accepted as a treatment for preterm neonates.'Modeling' is defined as planning or forming that follows a set pattern. The term is used to describe the normal process of lung growth and development that culminates in mature branching alveolar air spaces surrounded by a network of capillaries. Normal lung modeling occurs under a variety of genetic and hormonal influences that can be altered, leading to abnormal patterns of growth. 'Remodeling' is defined as altering the structure of or re-making and, in the case of the lung, is used to describe the abnormal patterns of lung growth that occur after lung injury. Modeling and remodeling of the lungs occur to an extent throughout life but never more rapidly than during the fetal and early neonatal periods, and factors that influence this process may lead to development of neonatal CLD. Some of the factors involved in normal and abnormal lung modeling and inflammation and glucocorticosteroid-induced remodeling in the perinatal period, in the context of neonatal CLD, are reviewed with considerations of how various drugs may influence these processes.

Vascular changes after intra-amniotic endotoxin in preterm lamb lungs

Chorioamnionitis is associated with preterm delivery and bronchopulmonary dysplasia (BPD), characterized by impaired alveolar and pulmonary vascular development and vascular dysfunction. To study the vascular effects in a model of chorioamnionitis, preterm lambs were exposed to 20 mg of intra-amniotic endotoxin or saline for 1, 2, 4, or 7 days and delivered at 122 days gestational age (term = 150 days). This intra-amniotic endotoxin dose was previously shown to induce lung maturation. The effect of intra-amniotic endotoxin on expression of endothelial proteins was evaluated. Muscularization of the media and collagen deposition in adventitia of small pulmonary arteries was used to assess vascular remodeling. Compared with controls, bronchoalveolar lavage fluid protein content was increased 2 days after intra-amniotic endotoxin exposure. Vascular endothelial growth factor (VEGF) 165 isoform mRNA decreased 2–4 days after intra-amniotic endotoxin. VEGF, VEGF receptor-2, endothelial nitric oxide synthase (eNOS), platelet endothelial cell adhesion molecule-1, and Tie-2 protein expression in the lung coordinately decreased 1–7 days after intra-amniotic endotoxin. Intra-amniotic endotoxin appeared to selectively decrease eNOS expression in small pulmonary vessels compared with large vessels. Medial smooth muscle hypertrophy and increased adventitial fibrosis were observed 4 and 7 days after intra-amniotic endotoxin. These results demonstrate that, in the preterm lamb lung, antenatal inflammation inhibits endothelial cell protein expression followed by vascular remodeling changes in small pulmonary arteries. Exposure to antenatal inflammation may cause vascular remodeling and contribute to the development of BPD.

Lipopolysaccharide increases alveolar type II cell number in fetal mouse lungs through Toll-like receptor 4 and NF-κB

Chorioamnionitis is a major cause of preterm delivery. Infants exposed to inflammation in utero and then born preterm may have improved lung function in the immediate postnatal period. We developed a mouse model of chorioamnionitis to study the inflammatory signaling mechanisms that might influence fetal lung maturation. With this in vivo model, we found that Escherichia coli lipopolysaccharide (LPS) increased the number of alveolar type II cells in the fetal mouse lung. LPS also increased type II cell number in cultured fetal lung explants, suggesting that LPS could directly signal the fetal lung in the absence of maternal influences. Using immunostaining, we localized cells within the fetal mouse lung expressing the LPS receptor molecule Toll-like receptor 4 (TLR4). Similar to the signaling pathways in inflammatory cells, LPS activated NF-κB in fetal lung explants. Activation of the TLR4/NF-κB pathway appeared to be required, as LPS did not increase the number of type II cells in C.C3H- Tlr4 Lps-d mice, a congenic strain containing a loss of function mutation in tlr4. In addition, the sesquiterpene lactone parthenolide inhibited NF-κB activation following LPS exposure and blocked the LPS-induced increase in type II cells. On the basis of these data from our mouse model of chorioamnionitis, it appears that LPS specifically activated the TLR4/NF-κB pathway, leading to increased type II cell maturation. These data implicate an important signaling mechanism in chorioamnionitis and suggest the TLR4/NF-κB pathway can influence lung development.

Effects of Anoectochilus formosanus Hayata extract and glucocorticoid on lung maturation in preterm rats

We investigated the effects of maternal administration of Anoectochilus formosanus extract and dexamethasone on lung maturation in preterm rats. A. formosanus group mothers were tube-fed A. formosanus extract (300 mg/kg body wt./day) for 7 days from days 12-18 of gestation. Dexamethasone group mothers were injected intraperitoneally with dexamethasone (0.2 mg/kg body wt.) in saline on day 18 of gestation. Control group mothers were similarly injected with saline alone. On day 19 of gestation, fetuses were delivered by cesarean section. A. formosanus treatment significantly increased the fetal lung/body weight ratio, as compared to dexamethasone treatment. Saturated phosphatidylcholine levels in fetal lung tissue and growth hormone levels in maternal serum were significantly increased in the A. formosanus- and dexamethasone-treated groups as compared to controls. The histological appearance of preterm rat lungs revealed extensive branching of intermediate airways, denser mesenchyme, and more epithelial tubules in the dexamethasone and A. formosanus groups as compared with the control group. These results suggest that antenatal A. formosanus treatment may play a role in accelerating fetal rat lung maturation.

Effects of maternal undernutrition during late gestation on the lung surfactant system and morphometry in rats

Intrauterine growth restriction (IUGR) is associated with reduced lung function during infancy and throughout adulthood. We investigated the effects of maternal undernutrition (50% rations of the control food intake) during the last week of gestation on the pulmonary surfactant system and lung morphometry in postnatal rats. IUGR rats exhibited a significantly lower body weight, lower lung weight, lower lung/body weight ratio, lower lung volume, and lower lung volume/body weight ratio on some postnatal days. IUGR rats had a significantly lower lung saturated phosphatidylcholine and lower plasma corticosterone levels on postnatal d 1 only, and values were comparable between control and IUGR rats in the ensuing weeks. Lung surfactant protein (SP)-A, SP-B, SP-C, and SP-D mRNA expressions were similar between control and IUGR rats. Volume fractions of the alveolar airspace were significantly lower in IUGR rats on postnatal d 7, 14, and 42. Alveolar surface areas were significantly lower in IUGR rats during the study period. The alveolar surface area/body weight ratio reached a peak on postnatal d 7, and values were significantly lower in IUGR rats on postnatal d 1, 14, 28, and 42. We conclude that maternal undernutrition during late gestation decreases lung surfactant lipid levels in the immediate postnatal period and alters the development of lung structure during the postnatal period. Alteration of lung surfactant and structure may be important in the pathogenesis of impaired pulmonary function in IUGR infants and children.

Variability in preterm lamb lung mechanics after intra-amniotic endotoxin is associated with changes in surfactant pool size and morphometry

Antenatal exposure to intra-amniotic (i.a.) endotoxin initiates a complex series of events, including an inflammatory cascade, increased surfactant production, and alterations to lung structure. Using the low frequency forced oscillation technique as a sensitive tool for measurement of respiratory impedance, we aimed to determine which factors contributed most to measured changes in lung mechanics. Respiratory impedance data obtained from sedated preterm lambs exposed to either i.a. injection with saline or 20 mg of endotoxin 1, 2, 4, and 15 days before delivery at 125 days gestation were studied, and association with indexes of standard lung morphometry, inflammatory response, and alveolar surfactant-saturated phosphatidylcholine (Sat PC) pool size was demonstrated. Reduction in tissue impedance with increasing interval between exposure and delivery was evident as early as 4 days after i.a. endotoxin injection, coinciding with resolution of inflammatory reaction, increased alveolar surfactant pools, and contribution of alveolar ducts to the parenchymal fraction, and a later decrease in the tissue component of the parenchymal fraction. Decreases in tissue damping (resistance) were more marked than decreases in tissue elastance. Log alveolar Sat PC accounted for most variability in tissue damping (88.9%) and tissue elastance (73.4%), whereas tissue fraction contributed 2 and 6.4%, respectively. The alveolar Sat PC pool size was the sole factor contributing to change in tissue hysteresivity. No changes were observed in airway resistance. Despite the complex cascade of events initiated by antenatal endotoxin exposure, variability in lung tissue mechanics is associated primarily with changes in alveolar Sat PC pool and lung morphology.

Foxa2 regulates alveolarization and goblet cell hyperplasia

The airways are lined by several distinct epithelial cells that play unique roles in pulmonary homeostasis; however, the mechanisms controlling their differentiation in health and disease are poorly understood. The winged helix transcription factor, FOXA2, is expressed in the foregut endoderm and in subsets of respiratory epithelial cells in the fetal and adult lung. Because targeted mutagenesis of the Foxa2 gene in mice is lethal before formation of the lung, its potential role in lung morphogenesis and homeostasis has not been determined. We selectively deleted Foxa2 in respiratory epithelial cells in the developing mouse lung. Airspace enlargement, goblet cell hyperplasia, increased mucin and neutrophilic infiltration were observed in lungs of the Foxa2-deleted mice. Experimental goblet cell hyperplasia caused by ovalbumin sensitization, interleukin 4 (IL4), IL13 and targeted deletion of the gene encoding surfactant protein C (SP-C), was associated with either absent or decreased expression of Foxa2 in airway epithelial cells. Analysis of lung tissue from patients with a variety of pulmonary diseases revealed a strong inverse correlation between FOXA2 and goblet cell hyperplasia. FOXA2 is required for alveolarization and regulates airway epithelial cell differentiation in the postnatal lung.

Initial responses to ventilation of premature lambs exposed to intra-amniotic endotoxin 4 days before delivery

Preterm delivery is frequently preceded by chorioamnionitis, resulting in exposure of the fetal lung to inflammation. We hypothesized that ventilation of the antenatally inflamed lung would result in amplification of the lung injury. Therefore, we induced fetal lung inflammation with intra-amniotic endotoxin (10 mg of Escherichia coli 055:B5) 4 days before premature delivery at 130 days of gestation. Lung function and lung inflammation after surfactant treatment and 4 h of mechanical ventilation were evaluated. Inflammatory cell numbers in amniotic fluid were increased >10-fold by antenatal endotoxin exposure. Antenatal endotoxin exposure had minimal effects on blood pressure, heart rate, lung compliance, and blood gas values. The endotoxin-exposed lungs required higher ventilation pressures. Ventilation did not increase the number of inflammatory cells or the protein in bronchoalveolar lavage fluid of the endotoxin-exposed animals above that measured in endotoxin-exposed fetuses that were not ventilated. IL-1β, IL-6, and IL-8 mRNA in cells from bronchoalveolar lavage fluid were increased by antenatal endotoxin exposure but not changed by ventilation. IL-1β and IL-8 protein was increased in lung tissue by 4 h of ventilation. Very little inflammation was induced by ventilation in this premature lamb model of surfactant treatment and gentle ventilation. After lung inflammation was induced by intra-amniotic endotoxin injection, ventilation did not increase lung injury.

Chorioamnionitis induced by subchorionic endotoxin infusion in sheep

OBJECTIVE

This study was undertaken to determine whether subchorionic endotoxin infusion causes chorioamnionitis and preterm lung maturation, as occurs after intra-amniotic endotoxin.

STUDY DESIGN

From day 118 of pregnancy, sheep received infusions of endotoxin (subchorionic 7.5 mg/d, n=11; intra-amniotic 2.5 mg/d, n=9) until delivery of lambs at 120 or 124 days. Other sheep received a single intra-amniotic injection of endotoxin (10 mg, n=7) at 118 days before delivery at 124 days. Controls (n=9) received equivalent saline solution treatments.

RESULTS

Chorioamnionitis accompanied all endotoxin treatments. Lung inflammation occurred after intra-amniotic endotoxin infusion or injection but not after subchorionic endotoxin. Umbilical arterial pH was lower and Pco(2) was higher than control after subchorionic endotoxin. Lung compliance and surfactant were increased after intra-amniotic endotoxin infusion or injection but not after subchorionic endotoxin.

CONCLUSION

Chorioamnionitis may result from inflammatory stimuli at various intrauterine sites, with different sites causing different fetal effects and not all cases of chorioamnionitis being accompanied by enhanced lung maturation.

Betamethasone effects on chorioamnionitis induced by intra-amniotic endotoxin in sheep

OBJECTIVE

Intra-amniotic administration of endotoxin in sheep is a model of subclinical chorioamnionitis. Intrauterine inflammation alters lung development to improve postnatal lung function and may predispose the infant to lung and brain injury. We describe the effects of intra-amniotic endotoxin on cytokines and white cell responses in the membranes and amniotic fluid and investigate the hypothesis that betamethasone treatment suppresses these responses.

STUDY DESIGN

Pregnant ewes were allocated at random to receive either intra-amniotic saline solution (control animals), maternal intramuscular betamethasone, intra-amniotic endotoxin by ultrasound guidance (10 mg Escherichia coli 055:B5), or a combination of the betamethasone and endotoxin treatments. Lambs were delivered abdominally at 110 to 125 days of gestation at time points that ranged from 2 hours to 15 days after treatment.

RESULTS

When compared with saline solution-injected control animals, the intra-amniotic injection of endotoxin increased white cell counts in amniotic fluid. Levels of interleukin-8, but not interleukin-6, were significantly increased in amniotic fluid from 5 hours to 15 days after intra-amniotic endotoxin injection, and interleukin-8 levels were not decreased by concurrent treatment with betamethasone. After endotoxin treatment, interleukin-1beta and interleukin-8 messenger RNA were expressed in chorion, and interleukin-6 messenger RNA expression was localized to chorionic blood vessel epithelium. The half-life of endotoxin in the amniotic fluid was 1.7 days, and levels remained measurable 15 days after injection.

CONCLUSION

These findings confirm that the fetus can survive within amniotic fluid that contains endotoxin, white cells, and cytokines for periods of weeks or more. Betamethasone treatment can suppress the initial inflammation in the amnion-chorion, but interleukin-8 levels and inflammatory cells in amniotic fluid were not suppressed 5 and 15 days after betamethasone treatment, presumably because of the slow clearance of bioactive endotoxin from the amniotic fluid.

Intra-amniotic corticosteroids for preterm lung maturation in sheep

OBJECTIVE

The purpose of this study was to compare efficacy on fetal lung maturation of intra-amniotic betamethasone or budesonide with the efficacy of maternal intramuscular betamethasone.

STUDY DESIGN

Pregnant ewes received intra-amniotic betamethasone (0.5 mg/kg or 2 mg/kg fetal weight), intra-amniotic budesonide (0.5 mg/kg or 2 mg/kg), maternal intramuscular betamethasone (0.5 mg/kg maternal weight), intra-amniotic saline solution, or maternal saline solution. Lambs were delivered 2 or 7 days later, at 124 days of gestation for measurement of respiratory system compliance, ventilatory efficiency index, and surfactant levels.

RESULTS

Lung function increased 2 days after maternal betamethasone, intra-amniotic betamethasone (2 mg/kg), and intra-amniotic budesonide (2 mg/kg) administration and 7 days after maternal betamethasone or intra-amniotic budesonide (2.0 mg/kg) administration. Lung function was not improved 7 days after intra-amniotic betamethasone (2.0 mg/kg) administration or 2 days after intra-amniotic betamethasone (0.5 mg/kg) or intra-amniotic budesonide (0.5 mg/kg) administration. Intra-amniotic corticosteroid administration increased fetal death and respiratory morbidity.

CONCLUSION

Intra-amniotic corticosteroid administration improved preterm lung function, but the associated morbidity and mortality rates suggest that they are not suitable for clinical use.

Antenatal corticosteroids for fetal maturation in women at risk for preterm delivery

The available data unambiguously support the beneficial, short-term fetal effects of antenatal corticosteroids in women at risk for preterm delivery. There are still several incompletely addressed questions, including the use of corticosteroids in women with preterm premature rupture of membranes, the optimal corticosteroid preparation to be used, and the impact of repeated dosing. These issues are discussed in this review from the perspective of recent scientific evidence on the mechanisms responsible for positive short-term effects on survival and possible harmful long-term effects.

The effect of antibiotic therapy on intrauterine infection-induced preterm parturition in rabbits

OBJECTIVE

The purpose of this study was to determine whether early antibiotic administration to pregnant rabbits with intrauterine infection could prevent preterm delivery and perinatal mortality.

STUDY DESIGN

Under hysteroscopic guidance, pregnant rabbits at 70% gestation (21 days) were allocated to three groups: (1) control group, transcervical inoculation of 0.2 ml phosphate-buffered saline (n = 16); (2) infection group, transcervical inoculation of 0.2 ml of 10(5) colony-forming units (CFU) of Escherichia coli (n = 21); (3) infection and antibiotics group, transcervical inoculations of 0.2 ml of 10(5) CFU of E. coli and ampicillin-sulbactam 150 mg/kg every 8 h intramuscularly (n = 32). To examine the consequences of treatment delay, animals in the latter group were subdivided to receive antibiotics at different time intervals of 0, 6, 11 and 18 h after bacterial inoculation. The intervals from bacterial inoculation to delivery and litter survival were documented. Systemic (rectal) temperatures were recorded at 4 h intervals through the first 36 h and every 12 h until delivery. A p value of < 0.05 was considered significant.

RESULTS

All rabbits inoculated with E. coli without antibiotic treatment delivered prematurely. The median inoculation-to-delivery interval was significantly shorter in the infected group than in the control group (median 32 h, range 14.9-76.5 h vs. median 219 h, range 173-246 h, respectively; p < 0.0001). Antibiotic administration within 12 h of inoculation, but not after 18 h, increased duration of pregnancy (by reducing the rate of preterm delivery) and neonatal survival (0% vs. 71%; p < 0.0001). The mean temperatures at delivery of animals whose treatments began at 6 and 11 h post-inoculation were significantly lower than those untreated with antibiotics or those treated at 18 h post-inoculation (p < 0.0001 for each comparison).

CONCLUSIONS

Antibiotic administration can prolong pregnancy and reduce perinatal mortality if administered early (within 12 h of microbial inoculation) in a rabbit model of ascending intrauterine infection.

Minimal lung and systemic responses to TNF-alpha in preterm sheep

TNF-alpha has been associated with chorioamnionitis and the subsequent development of bronchopulmonary dysplasia in preterm infants. We asked whether bioactive recombinant ovine TNF-alpha could induce chorioamnionitis, lung inflammation, lung maturation, and systemic effects in fetal sheep. We compared the responses to IL-1alpha, a cytokine known to induce these responses in preterm sheep. Intra-amniotic TNF-alpha caused no chorioamnionitis, no lung maturation, and a very small increase in inflammatory cells in the fetal lung after 5 h, 2 days (d), and 7 d. In contrast, IL-1alpha induced inflammation and lung maturation. TNF-alpha given into the airways at birth increased granulocytes in the bronchoalveolar lavage fluid of ventilated preterm lungs and decreased the mRNA for surfactant protein C but did not adversely effect postnatal lung function. An intravascular injection of IL-1alpha caused a systemic inflammatory response in fetal sheep, whereas there was no fetal response to intravascular TNF-alpha. Fetal and newborn preterm sheep are minimally responsive to TNF-alpha. Therefore, the presence of a mediator such as TNF-alpha in a developing animal does not necessarily mean that it is causing the responses anticipated from previous results in adult animals.

Intraamniotic endotoxin increases lung antioxidant enzyme activity in preterm lambs

Proinflammatory stimulation resulting from intraamniotic endotoxin improves lung function, increases surfactant protein mRNA expression and protein content, increases alveolar and lung saturated phosphatidylcholine pools, and accelerates lung morphometric maturation in fetal sheep. The mechanism for induction of lung maturation does not involve an increase in fetal cortisol. The effect of endotoxin on the maturation of a different lung system, the antioxidant enzyme (AOE) system, has not been examined. Therefore, we hypothesized that intraamniotic endotoxin would produce acceleration of AOE activity in fetal sheep at similar doses and schedule of administration to those producing lung functional and surfactant maturation. In a dose-response study, intraamniotic injections of 1, 4, 20, or 100 mg of Escherichia coli 055:beta5 endotoxin were administered 7 d before preterm delivery of sheep at 125 d gestation. In a study examining time interval of administration before delivery, 20 mg of endotoxin was injected at either 1-, 2-, 4-, 7-, or 15-d intervals before preterm delivery at 125 d. Doses of 1-100 mg of endotoxin produced significant increases in glutathione peroxidase activity; doses of 4-100 mg significantly increased catalase activity, whereas doses of 20-100 mg resulted in significant increases in total superoxide dismutase activity. Glutathione peroxidase activity was elevated within 2 d, whereas superoxide dismutase was increased by 4 d and catalase activity increased by 7 d after endotoxin. No AOE increases were sustained for 15 d. Endotoxin increased fetal lung AOE activity at similar dosing amounts and intervals to those producing maturation of lung function and surfactant. Thus, mechanisms involving proinflammatory stimulation, unrelated to glucocorticoid hormones, can induce maturation of the AOE system of the fetal lung.

Effect of exogenous surfactant on the development of surfactant synthesis in premature rabbit lung

Surfactant replacement is an effective therapy for neonatal respiratory distress syndrome. Full recovery from respiratory distress syndrome requires development of endogenous surfactant synthesis and metabolism. The influence of exogenous surfactant on the development of surfactant synthesis in premature lungs is not known. We hypothesized that different exogenous surfactants have different effects on the development of endogenous surfactant production in the premature lung. We treated organ cultures of d 25 fetal rabbit lung for 3 d with 100 mg/kg body weight of natural rabbit surfactant, Survanta, and Exosurf and measured their effects on the development of surfactant synthesis. Additional experiments tested how these surfactants and Curosurf affected surfactant protein (SP) SP-A, SP-B, and SP-C mRNA expression. Surfactant synthesis was measured as the incorporation of 3H-choline and 14C-glycerol into disaturated phosphatidylcholine recovered from lamellar bodies. Randomized-block ANOVA showed significant differences among treatments for incorporation of both labels (p < 0.01), with natural rabbit surfactant less than control, Survanta greater than control, and Exosurf unchanged. Additional experiments with natural rabbit surfactant alone showed no significant effects in doses up to 1000 mg/kg. Survanta stimulated disaturated phosphatidylcholine synthesis (173 +/- 41% of control; p = 0.01), increased total lamellar body disaturated phosphatidylcholine by 22% (p < 0.05), and increased 14C-disat-PC specific activity by 35% (p < 0.05). The response to Survanta was dose-dependent up to 1000 mg/kg. Survanta did not affect surfactant release. No surfactant altered the expression of mRNA for SP-A, SP-B, or SP-C. We conclude that surfactant replacement therapy can enhance the maturation of surfactant synthesis, but this potential benefit differs with different surfactant preparations.

Maternal glucocorticoids increase endotoxin-induced lung inflammation in preterm lambs

Antenatal betamethasone (Beta) is widely used in women with asymptomatic chorioamnionitis at risk for preterm delivery, but its effects on fetal inflammation are unstudied. Groups of ewes at 109 +/- 1 days of gestation received the following treatments: intra-amniotic (IA) saline (control), 0.5 mg/kg intramuscular Beta, 10 mg IA endotoxin (Endo), and Beta + 2 h later Endo (Beta + Endo). Beta suppressed Endo-induced lung inflammation at 1 day. However, compared with Endo 5 days after treatment, Beta + Endo lambs had increased alveolar neutrophils, proinflammatory cytokine mRNA expression, and serum amyloid A3 (SAA3) mRNA expression. IL-1beta mRNA expression was localized to the inflammatory cells, whereas SAA3 mRNA expression was induced in the bronchial epithelium and the inflammatory cells. Compared with Endo, Beta + Endo lambs had increased lung inflammation but equivalent lung volumes 15 days after treatment. The late increase in inflammation in the Beta + Endo animals suggests that glucocorticoids impair the ability of the preterm lung to downregulate Endo-induced inflammation after fetal clearance of the glucocorticoids. These results have implications for lung inflammation and bronchopulmonary dysplasia in preterm infants exposed to chorioamnionitis and maternal glucocorticoids.

Increased IP-10 and MIG expression after intra-amniotic endotoxin in preterm lamb lung

Subtraction hybridization was performed to explore changes in gene expression in the fetal lung after 20 mg of intra-amniotic (IA) endotoxin. Interferon-gamma-inducible 10-kd protein (IP-10) and monokine induced by interferon-gamma (MIG) constituted 20% of 102 endotoxin-induced clones identified in the preterm lamb lung. IP-10 (CXCL10) and MIG (CXCL9) are T-cell chemoattractants that have angiostatic properties. Both IP-10 and MIG mRNA were induced 30- to 40-fold in the fetal lung at 1 to 2 days after IA endotoxin. Intense IP-10 mRNA expression was detected by in situ hybridization in the bronchiolar and peribronchiolar areas and the vascular endothelium after IA endotoxin at all time points tested. MIG mRNA expression was detected initially focally in infiltrating neutrophils (15 hours after IA endotoxin) and later in the bronchiolar and peribronchiolar areas and vascular endothelium (1 day after IA endotoxin). In contrast to endotoxin, IA tumor necrosis factor-alpha or interleukin-1 alpha did not induce IP-10 or MIG mRNA in the lung. IA endotoxin also caused a modest induction of IP-10 and MIG mRNA in the jejunum, liver, and spleen. The IP-10 and MIG receptor CXCR3 was detected in the bronchiolar epithelium of preterm lambs by immunostaining. IP-10 and MIG are potent angiostatic chemokines that may contribute to lung injury and altered pulmonary vascular development in the preterm exposed to chorioamnionitis.