Inflammation and bronchopulmonary dysplasia: A continuing story

Seminars in Fetal & Neonatal Medicine (2006) 11, 354e362

a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s i n y

Inflammation and bronchopulmonary dysplasia: A continuing story Christian P. Speer* University Children’s Hospital, Josef-Schneider-Str. 2, 97080 Wuerzburg, Germany

KEYWORDS Preterm infants; Bronchopulmonary dysplasia; Cytokines; Proteases; Oxidative damage; Growth factors

Summary Increasing evidence indicates that bronchopulmonary dysplasia (BPD) results, at least in part, from an imbalance between pro-inflammatory and anti-inflammatory mechanisms, with a persistent imbalance that favours pro-inflammatory mechanisms. The inflammatory response is characterised by an accumulation of neutrophils and macrophages in the airways and pulmonary tissue of preterm infants and, moreover, by an arsenal of pro-inflammatory mediators which affect the alveolar capillary unit and tissue integrity. As well as pro-inflammatory cytokines and toxic oxygen radicals, various lipid mediators as well as potent proteases may be responsible for acute lung injury. During the last decade it has become evident that multiple pre- and postnatal events contribute to the development of BPD in preterm infants. Chorioamnionitis and cytokine exposure in utero, plus sequential lung injury caused by postnatal resuscitation, oxygen toxicity, volu-, barotrauma and infection all lead to a pulmonary inflammatory response which is most probably associated with aberrant wound healing and an inhibition of alveolarisation as well as vascular development in the immature lungs of very preterm infants, causing the ‘new BPD’. ª 2006 Elsevier Ltd. All rights reserved.

Introduction Increased use of antenatal glucocorticosteroids, more gentle ventilation techniques and early surfactant treatment have definitely minimised the severity of lung injury in more mature infants with respiratory distress syndrome (RDS) and significantly reduced the incidence of severe bronchopulmonary dysplasia (BPD) in this group of patients. The most severe form of BPD is characterised by chronic fibroproliferative changes with areas of emphysema and atelectasis and

* Corresponding author. Tel.: C49 931 201 27830; fax: C49 931 201 27833. E-mail address: [email protected]

is clearly associated with long term pulmonary morbidity and impaired neurodevelopmental outcome.1e4 However, there is a new category of very immature infants with a ‘new’ BPD who initially have minimal or absent signs of RDS but who subsequently develop oxygen dependency and ventilatory needs within the first 2 weeks of life.3 Affected infants may be oxygen dependent for weeks and even months. A considerable number of these infants may have been exposed in utero to chorioamnionitis and may be born with inflamed lungs. Various postnatal factors such as pulmonary or systemic infections, inappropriate resuscitation, high airway concentrations of inspired oxygen and mechanical ventilation induce an injurious inflammatory response in the immature airways and the interstitium of preterm infants. These risk factors may act synergistically or additively and

1744-165X/$ - see front matter ª 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.siny.2006.03.004

Inflammation and BPD may amplify and perpetuate the inflammatory reaction and subsequently affect normal alveolarisation and pulmonary vascular development in preterm infants with ‘new’ BDP. This article presents, in a condensed form, the current pathogenetic concepts on the possible role of inflammation in the pathogenesis of BPD and elaborates on recently published reviews.3,5,6

Inflammatory cells Neutrophils and macrophages play a pivotal and crucial role in pulmonary inflammation. Preterm infants at various stages of developing BPD have much higher and persisting numbers of neutrophils and macrophages in their bronchoalveolar lavage fluid compared with infants who have recovered from RDS.7e11 A neutrophil influx into the airways occurs within minutes after the initiation of mechanical ventilation and is associated with a decrease in the number of circulating neutrophils. This phenomenon correlates with the extent of pulmonary oedema formation and an increased risk of developing BPD.3,12 Most probably, neutrophils become activated during the inflammatory process and adhere to the endothelium of the pulmonary vascular system, thus initiating a sequence of injurious events. Recently, a prolonged survival of neonatal neutrophils due to an inappropriate suppression of neutrophil apoptosis has been reported.13e15 Since apoptosis of inflammatory neutrophils and their timely removal by resident macrophages are critical to the resolution of inflammation, neonatal neutrophils with prolonged survival may have the functional capacity to contribute to pulmonary inflammation.15 Neutrophil apoptosis is induced by endogenous ligands such as Fas, which engage various death receptors including Fas receptor (FasR). A decreased expression of FasR was recently observed in neonatal neutrophils when compared with adult phagocytes,14 which could explain the prolonged survival of neonatal neutrophils. Alveolar and pulmonary macrophages play a central role in all stages of BPD-associated inflammation.3,11 CD68-positive macrophages appear to be the predominant cell population in lung tissues of preterm infants who die during the early stages of RDS.16

Cellular-endothelial interaction After activation by systemic or local factors, primed endothelial cells interact with neutrophils through adhesion molecules that include selectins and integrins.17 These adhesion mechanisms initiate a process that allows the extravasation of neutrophils and macrophages with a subsequent migration towards the area of inflammation. Increased concentrations of various soluble cellular and endothelial adhesion molecules such as intercellular adhesion molecule (ICAM-1) and selectins are present in airway secretions and the systemic circulation of preterm infants with BPD18e21 reflecting greater shedding of these molecules in response to inflammation. The reduced expression of b2-integrins (CD18) on neonatal neutrophils is in keeping with these findings.19,22 These data provide indirect evidence for the recruitment of circulating neutrophils into the airways and the pulmonary tissue. Most recently, a strong up-regulation of ICAM-1 on endothelial cord cells

355 and increased serum concentrations of soluble ICAM-1 in preterm infants exposed to chorioamnionitis have been reported.23

Chemotaxis Besides markers of endothelial activation, airway secretions of infants with BPD contain a number of well defined chemotactic and chemokinetic factors that reflect the high chemotactic activity of airway secretions and are responsible for the recruitment of neutrophils and macrophages: C5a, tumour necrosis factor-a (TNF-a), interleukin (IL)-1, IL-16, the chemokine CXCL8 (previously IL-8), lipoxygenase products, leukotriene B4, elastin fragments, fibronectin, monocyte chemotactic protein, macrophage inflammatory protein and others.9,11,24e28 Not surprisingly, the chemotactic activity and the concentrations of numerous chemotactic and chemokinetic factors are considerably higher in infants with BPD compared with babies who recovered from RDS. CXCL8 is involved in the initiation of cellular endothelial interactions and is probably the most important chemotactic factor in the lung. Increased CXCL8 levels in bronchoalveolar secretions from infants with developing BPD clearly preceded the masked neutrophil influx observed in these infants.3 It was convincingly demonstrated in animals exposed to 60% oxygen that the application of a selective chemokine receptor antagonist completely inhibited neutrophil influx into the lungs. This strategy suppressed pulmonary inflammation and enhanced lung growth.29

Pro- and anti-inflammatory cytokines Besides CXCL8 (IL-8), other pro-inflammatory cytokines such as TNF-a, IL-1 as well as interleukin-6 (IL-6), are important mediators in the early inflammatory response and in the evolution of the inflammatory events. These cytokines are synthesised by alveolar macrophages, airway epithelial cells, fibroblasts, type II pneumocytes and the endothelial cells of preterm infants upon stimulation by hypoxia, hyperoxia, microorganisms, endotoxin, other bacterial cell wall constituents and biophysical factors30 such as baro- and volutrauma of the bronchoalveolar system. It has convincingly been demonstrated that pro-inflammatory cytokines do not cross the placenta.31 Increased protein levels and high mRNA expression of these pro-inflammatory cytokines and chemokines (TNF-a, IL-1, IL-6, CXCL8) have been detected in airway secretions and bronchoalveolar cells of infants with developing BPD.3,9,32 It was recently shown that IL-1 present in the airway fluid of mechanically ventilated preterm infants induces airway epithelial CXCL8 expression via a nuclear factor (NF)-kB-dependent pathway.33 NF-kB activation has also been observed in airway neutrophils and macrophages as well as in tracheobronchial secretion from infants with RDS.34e36 Since there are heterogeneous triggers that initiate the inflammatory response in the neonatal lung, one may speculate that distinct mechanisms of NF-kB activation may exist. In preterm infants who had died of severe RDS, the influx of TNF-a positive macrophages in pulmonary tissue was found to be associated with a striking loss of

356 endothelial basement membrane and a destruction of interstitial glycosaminoglycanes.16 A pronounced CXCL8 mRNA-expression could also be detected in the bronchoalveolar epithelium and in a scattered pattern in the interstitial tissue of postmortem lung tissue of infants with RDS.37 These findings underline the importance of CXCL8 expression and other pro-inflammatory cytokines in the initiation and perpetuation of injurious events in pulmonary tissue of preterm infants. The increased levels and enhanced mRNA expression of pro-inflammatory cytokines present in the airways and pulmonary tissue of preterm infants may reflect an inability to regulate inflammation through an adequate expression of the anti-inflammatory cytokines IL-4, IL-10, IL-12, IL-13 and IL-18 and the receptor antagonists for or IL-1.38e42 Cellular IL-10 mRNA is undetectable in most airway samples of preterm infants with RDS, but is expressed in all cell samples from term infants with meconium aspiration syndrome.38 Interestingly, lung inflammatory cells of preterm infants exposed to IL-10 in vitro responded with a reduced expression of pro-inflammatory cytokines.43 An imbalance between pro-inflammatory and anti-inflammatory cytokinesdfavouring pro-inflammatory cytokinesdcan be considered as an important feature of lung injury. In this context, substances which exert antiinflammatory effects or which interfere with neutrophil influx into the airways or lung tissue may be helpful in down-regulating the inflammatory process. Clara cell protein 10 (CC10) is a small molecule generated by Clara cells in the lung and it has various inhibitory in vitro effects on inflammatory responses including the inhibition of phospholipase A2. In animal models, CC10 deficiency has been associated with high expression of cytokines in the lung and an infiltration of pulmonary tissue by inflammatory cells.44,45 In a recent pilot study, a single dose of CC10 administered to premature infants shortly after birth resulted in lower numbers of neutrophils and a decreased protein concentration when compared with controls.46

Proteolytic and oxidative damage

C.P. Speer addition, an increased elastin deposition in the pulmonary tissue of an animal model with BPD has been reported.49 Moreover, disruption of sulphated glycosaminoglycans, changes in hyaluronan deposition and increased laminin concentrations in airway secretions of infants with BPD have been attributed to elastolytic destruction.50e52 An imbalance between the cystein proteases cathepsins B, H, L and S and their inhibitors, cistatins B and C, has also been recently described in a baboon model of BPD.53 All cathepsins were immunolocalised to macrophages. Similarly, high concentrations of different matrix metalloproteinases, which play a significant role in remodelling throughout all stages of lung development, have been identified in airway secretions of infants with BPD and, while overexpressed, they cause disruption of the extracellular matrix.54e57 Protective levels of various tissue inhibitors of metalloproteinases were rather low in these infants, a finding that is suggestive of an imbalance within the metalloproteinase system. In respiratory epithelium, activation of proteinase-activated receptor-2 (PAR2) by trypsin, stimulates the release of inflammatory mediators such as IL-6, CXCL8 and metalloproteinase-9 and induces vascular permeability and infiltration of neutrophils. A high expression of PAR2 has recently been reported.58 Oxygen metabolites and radicals are released by neutrophils and macrophages at sites of inflammation or are generated under hyperoxic conditions by free iron or the cell bound xanthine-oxidase system. They have clearly been shown to cause tissue damage, to contribute to the oxidative inactivation of protective antioxidant systems in the airways and lung tissue,59 and to up-regulate the activity of matrix metalloproteinases. Free iron was detected in airway

O2 Xanthine Oxidase

O2, OH

Airway

Oxidative Damage O2 Elastin

Data from in vitro studies, animal experiments and observations in preterm infants with BPD clearly indicate that an imbalance between proteases and protease inhibitors may contribute to the pathogenesis of BPD. Neutrophils and macrophages present at sites of inflammation release various potent proteases that are thought to play an essential role in the destruction of the alveolar-capillary unit or the extracellular matrix. An imbalance between elastaseda powerful neutral protease stored in neutrophilsdand a1-proteinase inhibitor (a1-PI) within the airways has clearly been demonstrated in preterm infants with RDS and BPD (Fig. 1).4,47,48 a1-PI is presumably functionally inactivated by oxygen intermediates with the consequence that oxidised a1-PI is degraded by proteolytic cleavage.47 As a result of elastolytic damage, an increased urinary excretion of desmosine, a degradation product of mature cross-linked elastic fibres, was identified in infants with free elastase activity in their airway secretions. This is of particular concern in the light of animal and human studies showing that alveolar septation is markedly reduced in the lungs of infants with severe BPD.1 In

O2, OH

Elastase

Elastase-α1- Proteinase Inhibitor (α1-PI)

Figure 1 Schematic presentation of elastolytic and oxidative damage to the alveolar capillary unit. Elastase, a powerful neutral protease stored in the azurophilic granules of neutrophils that have migrated into the airways, is released upon stimulation. Under normal circumstances, elastase is rapidly inactivated by a1-proteinase inhibitor (a1-PI). Under conditions in which oxidative inactivation of a1-PI takes place, free elastase may attack pulmonary elastin, the primary substrate of neutrophil elastase. Toxic oxygen radicals O2 and ÿOH are generated by neutrophils and macrophages as well as by xanthine oxidase and may directly cause oxidative damage by lipid peroxidation.

Inflammation and BPD secretions in the vast majority of ventilated preterm infants with RDS.60 Interestingly, free elastase, cathepsin G and trypsin stored in neutrophils can prime macrophages for an increased release of toxic oxygen metabolites.61 Moreover, resting and stimulated alveolar macrophages of infants with BPD generate increased amounts of hydrogen peroxide compared with cells from control infants.4 Animal experiments indicate that oxidative stress, as reflected by the generation of toxic oxygen radicals, is a very early and crucial event in the initiation of pulmonary inflammation.62 The activity of reactive oxygen species (ROS) is normally balanced by the antioxidant system. However, preterm infants are particularly susceptible to hyperoxia and the damage caused by ROS since the antioxidant system has yet to mature. Following term birth, enzymes such as superoxide dismutase (SOD), catalase and glutathione peroxidase have protective activities against ROS. However, there is insufficient activity of these enzymes at lower gestational ages.63 This means that preterm infants will be deficient in antioxidant enzymes at a time when they are receiving oxygen. However, the exact pathogenetic sequence and the interaction of various systems involved in pulmonary inflammation is largely descriptive and the relative importance of individual inflammatory mediators is rather speculative and has not yet been well defined. Several factors may have detrimental effects on microvascular and alveolar permeability: inflammatory cells and various mediators including pro-inflammatory cytokines, lipid mediators, toxic oxygen radicals, microbial colonisation and infection of the airways and inactivation of the surfactant system by serum proteins.

Increased alveolar capillary permeability and the systemic inflammatory response The increased alveolar capillary permeability is pathognomic for the early stages of pulmonary inflammation and is clearly associated with a deterioration in lung function.3,4,7 Protein leakage into the alveoli and airways of preterm infants occurs within 1 h of the initiation of mechanical ventilation.12 At a postnatal age of 10e14 days, preterm infants who later develop BPD have a drastic increase in albumin concentrations in airway secretions compared with infants who recovered from RDS.9 One recent magnetic resonance imaging study suggested that infants with BPD have increased lung water content and are susceptible to gravity-induced collapse and alveolar flooding of the lung.64 Furthermore, compounds of the plasma protein system are activated after the initiation of RDS and affect the alveolar-capillary membrane directly and indirectly by the sequestration of activated neutrophils and platelets in the pulmonary vascular bed. In mechanically ventilated infants with RDS simultaneous activations of clotting, fibrinolysis, kinin kallikrein and the complement system were observed.65e67 An early and nearly identical activation of inflammation and the clotting system has also been described in animals conventionally ventilated or treated with high frequency oscillatory ventilation.68 The activation of plasma protein systems in preterm infants is associated with a stimulation of neutrophils and platelets as indicated by an increased release of cellular constituents. In addition, an increased, but transient, CD11b expression on

357 neutrophils reflects cellular activation in mechanically ventilated preterm infants in the early stages of RDS.69 One may speculate that various factors that induce injury to the pulmonary capillary endothelium may subsequently promote neutrophil and platelet activation and induce pulmonary as well as systemic inflammation and activation of the clotting system.70

Repair, lung and vessel growth Inflammation-induced tissue injury is normally followed by a phase of repair,71 a complex process that has only partially been studied and understood in BPD. Lung injury and the associated inflammatory process leads to an induction of transforming growth factor-b (TGF-b), which limits some of the inflammatory reactions and plays a key role in mediating tissue remodelling and repair.72 However, if the reparative processes are exaggerated, normal lung development may be inhibited. Furthermore, over-expression of TGF-b and of platelet-derived growth factor-BB has been shown to result in severe pulmonary fibrosis.73,74 In preterm infants with BPD, increased concentrations of TGF-b have been detected in the airways.75e77 Moreover, in preterm animals with inflamed lungs, an increased expression of TGF-b has recently been observed. Interestingly, expression of connective tissue growth factor (CTG-F), a second important key mediator in the induction of pulmonary fibrosis, was decreased in this model.78 These preliminary findings could be an important step forward in the understanding of the pathogenesis of the ‘new BPD,’ which is characterised by growth arrest of lung tissue and pulmonary vessels rather than by fibrosis. One may speculate that overexpression of TGF-b and the subsequent down-regulation of CTGF, together with low or suboptimal levels of various pulmonary and vascular growth factors, may add to the pathogenetic sequence of ‘new BPD.’ Low airway concentrations of keratinocyte and hepatocyte growth factors (which are thought to participate in normal lung development and in tissue regeneration after lung injury) as well as low levels of hypoxia-inducible factors (which promote angiogenic responses) have been found to be associated with BPD.79e81 Similarly, impaired vascular endothelial growth factor (VEGF) and VEGF receptor mRNA expression in lungs from extremely premature baboons developing BPD were shown to contribute to dysmorphic microvasculature and disrupted alveolarisation.82 In addition, in a hyperoxia-induced BPD rat model, air space enlargement and loss of lung capillaries were associated with decreased lung VEGF and VEGF receptor expression. Postnatal administration of intratracheal adenovirus-mediated VEGF gene therapy improved survival and promoted lung capillary formation. Moreover, alveolar development was preserved in this model of irreversible lung injury.83 These findings underscore the importance of the vasculature in what is traditionally thought of as an airway disease and open new therapeutic avenues for lung disease characterised by an irreversible loss of alveoli through the modulation of angiogenic factors.83 VEGF may also be of importance in the early phase of neonatal RDS by contributing to pulmonary maturation and surfactant secretion. Recently, premature infants with higher cord blood levels of VEGF were shown to have a lower risk of developing RDS.84

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Factors inducing pulmonary inflammation Hyperoxia In preterm and term animals, hyperoxia is a strong and independent inducer of various mediators involved in pulmonary inflammation.85,86 Recently, differential gene expression with DNA microarray analysis in premature rat lungs exposed to prolonged hyperoxia during the saccular stage has been studied; this developmental stage closely resembles the pulmonary development of preterm infants receiving intensive care treatment. Oxidative stress affected a complex orchestra of genes involved in inflammation, extracellular matrix turnover, coagulation and other events.85 The majority of pro-inflammatory genes was considerably upregulated, while VEGF receptor-2 was down-regulated. These findings were associated with an increased influx of inflammatory cells especially macrophages in pulmonary tissue. It has previously been demonstrated that exposure of premature baboons,87 neonatal mice88,89 and rats90e92 to hyperoxia results in progressive lung disease that strongly resembles BPD. When macrophages obtained from preterm and term rabbits were incubated in 95% oxygen overnight, only ‘preterm’ macrophages showed a significant increase in IL-1 and CXCL8 mRNA expression and an intracellular oxygen radical content.86 In addition, hyperoxic ventilated premature baboons had increased oxidative DNA damage and decreased VEGF expression. As a potential mechanism of hyperoxia on VEGF expression, an increased induction of p53, a transcription factor that represses VEGF gene transcription could be identified.93

Mechanical ventilation Mechanical ventilation is particularly harmful to preterm infants. Initiation of mechanical ventilation in preterm animals has shown that overdistension of the lungs causes disruption of structural elements and the release of proinflammatory mediators, with subsequent leukocyte influx, suggesting that any baro-/volutrauma of the immature lung may be injurious.94e99 However, certain ventilation strategies may cause more damage than others. For example, in an isolated rat lung model, high volume ventilation with zero positive end expiratory pressure (PEEP) caused significantly greater production and release of pro-inflammatory cytokines than a moderate volume with high PEEP, which allowed stabilisation of alveoli.94 Selection of the least harmful ventilation strategy is, therefore, of the utmost importance if lung damage is to be minimised. Interestingly, high frequency ventilation compared to conventional ventilation induced an identical pro-inflammatory response in preterm infants.98 Recently, the effect of mechanical ventilation on various pro- and anti-inflammatory cytokines in the presence or absence of endotoxin (lipopolysaccharide (LPS))-induced sepsis was studied.97 If animals were pretreated with LPS, bronchoalveolar lavage fluid (BALF) concentrations of pro-inflammatory cytokines in an isolated, non-perfused lung model were impressively increased even with a ‘less’ injurious ventilation strategy.100 ‘Priming’ of the fetal lung by intrauterine endotoxin or exposure

C.P. Speer to pro-inflammatory cytokines generated during chorioamnionitis is most probably a considerable pathogenetic factor in the initiation of the pulmonary inflammatory sequence. As a consequence, basically every form of mechanical ventilation or even a relatively ‘ traumatic’ bag and mask resuscitation may act as a ‘second strike’ and may amplify as well as aggravate the inflammatory reaction in the immature lung. Surprisingly, even the application of continuous positive airway pressure (CPAP) was shown to induce inflammatory changes in rat lungs following the administration of LPS.101

Chorioamnionitis In lung tissue of human fetuses exposed to chorioamnionitis, a pronounced inflammatory response reflected by a marked infiltration of inflammatory cells and an increased expression of the pro-inflammatory chemokine CXCL8 mRNA has been demonstrated.37 In animal models it was clearly confirmed that the preterm fetus developed a brisk and adequate inflammatory response. However, it was also demonstrated that the fetus could quickly modulate and down-regulate inflammation, probably by an endotoxin tolerance-type response.102 In addition, intrauterine exposure to pro-inflammatory cytokines and other mediators during chorioamnionitis resulted in an impressively increased number of apoptotic airway cells in human fetuses.103 Epidemiological data now suggest a strong association between chorioamnionitis and the development of BPD. Furthermore, increased concentrations of pro-inflammatory cytokines in human amniotic fluid and fetal cord blooddindicating a systemic fetal inflammatory response during chorioamnionitisdhave been shown to be independent risk factors for BPD.104,105 Although the mechanisms by which a fetal systemic inflammatory response increases the risk of BPD are incompletely understood, the early intrauterine initiation of pulmonary inflammation seems to be a crucial step in the pathogenesis sequence of BPD. Chorioamnionitis, mechanical ventilation and postnatal sepsis have clearly been identified as modulators of BPD. The two postnatal factors interact either separately or in combination with antenatal inflammation/infection to further increase the risk of BPD.106

Infection An association between early onset systemic bacterial infections and the development of BPD in very low birth weight infants has well been described.32,107 In addition, systemic nosocomial infections have been identified as a risk factor for BPD.107e109 Most probably, vasoactive prostaglandin mediators released during sepsis prevent ductal closure or induce re-opening.110 As well as the direct effects of systemic infections and inflammatory mediators on endothelial pulmonary and bronchoalveolar cells, haemodynamic changes in the pulmonary vascular bed associated with a persistent ductus arteriosus seem to play an essential role in the development of BPD.111 However, the possible impact of airway colonisation or even infection by coagulase-negative staphylococci and Gram-negative bacteria on the development of BPD is less clear and has generated contradictory results.32 The potential role of

Inflammation and BPD Ureaplasma urealyticum (Uu) in the pathogenesis of BPD is also controversial.112 Uu is the microorganism most frequently isolated from the amniotic fluid in preterm births and is a predominant pathogen detected in the airway secretions immediately after birth.107,113 The presence of Uu in the respiratory tract of preterm infantsdeven without clinical or laboratory signs of infectiondhas been correlated with elevated cellular and molecular markers of inflammation and is associated with an increased risk for BPD.32,114,115 In baboons antenatally and intraamniotically colonised with Uu two different patterns of disease were observed: one group with persistently Uu positive tracheal cultures manifested continuously elevated proinflammatory cytokine levels and significantly worse lung function than control infants. The other group, which cleared Uu from the trachea by 48 h of postnatal age showed a reduction in airway cytokine levels and white blood cell numbers, and this was associated with significantly improved lung function.113 Inherent maternal-fetal immune system responses to antenatal Uu, which are not yet understood, most probably determine the pulmonary outcome of Uu colonisation.113

Conclusion Various pre- and postnatal risk factors that act additively or synergistically induce an injurious inflammatory response in the airways and the pulmonary interstitium of preterm infants with BPD. Increasing evidence indicates that BPD resultsdat least in partdfrom an imbalance between proinflammatory and anti-inflammatory mechanisms, with a persistent imbalance that favours pro-inflammatory mechanisms. Recently, impaired generation of the vascular and pulmonary growth factors crucial for normal lung development has been implicated as a possible risk factor for BPD.

Practice points  The most severe form of bronchopulmonary dysplasia (BPD) is characterised by chronic fibroproliferative changes with areas of emphysema and atelectasis and is clearly associated with longterm pulmonary morbidity and impaired neurodevelopmental outcome.  A new category of very immature infants may present with ‘new’ BPD. These infants have minimal or even absent signs of respiratory distress syndrome (RDS) but may subsequently develop oxygen dependency or ventilatory needs within the first 2 weeks of life and may be oxygen dependent for weeks.  Chorioamnionitis and various postnatal risk factors induce an injurious inflammatory response in the immature airways and pulmonary interstitium of preterm infants and may subsequently affect normal alveolarisation and pulmonary vascular development in very immature infants with ‘new’ BPD.

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Research directions  To identify molecular interactions between inflammatory mechanisms and growth arrest of lung tissue as well as pulmonary vessel formation.  To define preventative and therapeutic strategies that could effectively reduce the pulmonary inflammatory response and promote lung growth.

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