Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Bronchopneumonia with interstitial pneumonia in beef feedlot cattle: Characterization and laboratory investigation

Bronchopneumonia with interstitial pneumonia (BIP) has been considered a variant of acute interstitial pneumonia (AIP) rather than a distinct disease. This study compared 18 BIP, 24 bronchopneumonia (BP), and 13 AIP cases in feedlot beef cattle. Grossly, BIP cases typically had cranioventral lung lesions of similar morphology and extent as BP cases, but the caudodorsal lung appeared overinflated, bulged on section, and had interlobular edema and emphysema. Gross diagnosis of BIP had 83% sensitivity and 73% specificity relative to histopathology. Histologic lesions of BIP in cranioventral areas were of chronic BP, while caudodorsal lesions included alveolar and bronchiolar damage and inflammation, interstitial hypercellularity, and multifocal hemorrhages. In BIP cases, cranioventral lung lesions were more chronic than caudodorsal lesions. Histologic scores and microbiology data were comparable in cranioventral lung of BIP versus BP cases and caudodorsal lung of BIP versus AIP cases, with differences reflecting a more chronic disease involving less virulent bacteria in BIP versus BP. Mycoplasma bovis infection was similarly frequent among groups, and a viral cause of BIP was not identified. Lesion morphology and similar blood cytokine concentrations among groups argued against sepsis as a cause of lung injury. Surfactant dysfunction was identified in BIP and BP, and was only partially the result of protein exudation. These and other findings establish BIP as a distinct condition in which chronic cranioventral BP precedes acute caudodorsal interstitial lung disease, supporting a role of chronic inflammation in heightened sensitivity to 3-methylindole or another lung toxicant.

PepBiotics, novel cathelicidin-inspired antimicrobials to fight pulmonary bacterial infections

BACKGROUND

Antimicrobial peptides are considered potential alternatives to antibiotics. Here we describe the antibacterial properties of a family of novel cathelicidin-related (CR-) peptides, which we named PepBiotics, against bacteria typically present in cystic fibrosis (CF) patients.

METHODS

Broth dilution assays were used to determine antibacterial activity of PepBiotics under physiological conditions, as well as development of bacterial resistance against these peptides. Toxicity was tested in mice and cell cultures while molecular interactions of PepBiotics with bacterial membrane components was determined using CD, ITC and LPS/LTA induced macrophage studies.

RESULTS

A relatively small number of PepBiotics remained highly antibacterial against CF-related respiratory pathogens Pseudomonas aeruginosa and Staphylococcus aureus, at high ionic strength and low pH. Interestingly, these PepBiotics also prevented LPS/LTA induced activation of macrophages and was shown to be non-toxic to primary human nasal epithelial cells. Furthermore, both P. aeruginosa and S. aureus were unable to induce resistance against CR-163 and CR-172, two PepBiotics selected for their excellent antimicrobial and immunomodulatory properties. Toxicity studies in mice indicated that intratracheal administration of CR-163 was well tolerated in vivo. Finally, interaction of CR-163 with bacterial-type anionic membranes but not with mammalian-type (zwitterionic lipid) membranes was confirmed using ITC and ³¹P solid state NMR.

CONCLUSIONS

PepBiotics are a promising novel class of highly active antimicrobial peptides, of which CR-163 showed the most potential for treatment of clinically relevant (CF-) pathogens in physiological conditions.

GENERAL SIGNIFICANCE

These observations emphasize the therapeutic potential of PepBiotics against CF-related bacterial respiratory infections.

National Preclinical Sepsis Platform: developing a framework for accelerating innovation in Canadian sepsis research

Despite decades of preclinical research, no experimentally derived therapies for sepsis have been successfully adopted into routine clinical practice. Factors that contribute to this crisis of translation include poor representation by preclinical models of the complex human condition of sepsis, bias in preclinical studies, as well as limitations of single-laboratory methodology. To overcome some of these shortcomings, multicentre preclinical studies—defined as a research experiment conducted in two or more research laboratories with a common protocol and analysis—are expected to maximize transparency, improve reproducibility, and enhance generalizability. The ultimate objective is to increase the efficiency and efficacy of bench-to-bedside translation for preclinical sepsis research and improve outcomes for patients with life-threatening infection. To this end, we organized the first meeting of the National Preclinical Sepsis Platform (NPSP). This multicentre preclinical  research collaboration of Canadian sepsis researchers and stakeholders was established to study the pathophysiology of sepsis and accelerate movement of promising therapeutics into early phase clinical trials. Integrated knowledge translation and shared decision-making were emphasized to ensure the goals of the platform align with clinical researchers and patient partners. 29 participants from 10 independent labs attended and discussed four main topics: (1) objectives of the platform; (2) animal models of sepsis; (3) multicentre methodology and (4) outcomes for evaluation. A PIRO model (predisposition, insult, response, organ dysfunction) for experimental design was proposed to strengthen linkages with interdisciplinary researchers and key stakeholders. This platform represents an important resource for maximizing translational impact of preclinical sepsis research.

The COVID-19 pandemic: a target for surfactant therapy?

INTRODUCTION

The dramatic impact of COVID-19 on humans worldwide has initiated an extraordinary search for effective treatment approaches. One of these is the administration of exogenous surfactant, which is being tested in ongoing clinical trials.

AREAS COVERED

Exogenous surfactant is a life-saving treatment for premature infants with neonatal respiratory distress syndrome. This treatment has also been tested for acute respiratory distress syndrome (ARDS) with limited success possibly due to the complexity of that syndrome. The 60-year history of successes and failures associated with surfactant therapy distinguishes it from many other treatments currently being tested for COVID-19 and provides the opportunity to discuss the factors that may influence the success of this therapy.

EXPERT OPINION

Clinical data provide a strong rationale for using exogenous surfactant in COVID-19 patients. Success of this therapy may be influenced by the mechanical ventilation strategy, the timing of treatment, the doses delivered, the method of delivery and the preparations utilized. In addition, future development of enhanced preparations may improve this treatment approach. Overall, results from ongoing trials may not only provide data to indicate if this therapy is effective for COVID-19 patients, but also lead to further scientific understanding and improved treatment strategies.

The effect of maternal protein restriction during perinatal life on the inflammatory response in pediatric rats

Fetal growth restriction can affect health outcomes in postnatal life. This study tested the hypothesis that the response to an inflammatory pulmonary insult is altered in pediatric fetal growth restricted rats. Using a low-protein diet during gestation and postnatal life, growth-restricted male and female rats and healthy control rats were exposed to an inflammatory insult via the intratracheal instillation of heat-killed bacteria. After 6 h, animal lungs were examined for lung inflammation and status of the surfactant system. The results showed that in response to an inflammatory insult, neutrophil infiltration was decreased in both male and female rats in the growth-restricted animals compared with the control rats. The amount of surfactant was increased in the growth-restricted animals compared with the control rats, regardless of the inflammatory insult. It is concluded that fetal growth restriction results in increased surfactant and altered neutrophil responses following pulmonary insult.

The effects of aging and exercise on lung mechanics, surfactant and alveolar macrophages

Purpose: Advancing age leads to changes to the respiratory system associated with increased susceptibility to lung diseases, and exercise may counteract this effect. To explore the underlying processes, we investigated the effects of aging and exercise on lung mechanics, alveolar macrophage function, and surfactant pools and activity, in mice. It was hypothesized that aging would impact lung mechanics, macrophage polarization, and the status of the surfactant system, and that these changes would be mitigated by exercise. Methods: Male C57BL/6 mice were housed from 2-3 to 22 months, for the aged group, or until 4 months of age for young mice. Mice in both groups were randomized to voluntarily running exercise or to non-exercise, for a 2-month period. Mice were euthanized and lung mechanics were analyzed using a flexiVent ventilator. Subsequently, the lungs were lavaged to obtain pulmonary surfactant and alveolar macrophages. Pulmonary surfactant was analyzed for pool sizes and activity whereas alveolar macrophages were examined for response to pro and anti-inflammatory stimuli. Results: Changes in lung mechanics, such as increased compliance and decreased airway resistance, were associated with aging but were not affected by exercise. The quantity as well as the biophysical activity of the pulmonary surfactant system was unaffected by either aging or exercise. More alveolar macrophages were recovered from exercising aged mice compared to both the young and non-exercising groups. Macrophages in this aged exercise group were more responsive to an anti-inflammatory stimulus. Conclusions: Our data supports previous literature that suggest the development of emphysema-like alterations to lung mechanics with aging. This effect was independent of exercise. Our data also indicates that surfactant is unaffected by aging and exercise. Alveolar macrophage properties and numbers were affected by exercise in the aging lung and may represent the main, potentially beneficial, effect of exercise on the pulmonary system.

Maternal protein restriction during perinatal life affects lung mechanics and the surfactant system during early postnatal life in female rats

Limited information is available on how fetal growth retardation (FGR) affects the lung in the neonatal period in males and females. This led us to test the hypothesis that FGR alters lung mechanics and the surfactant system during the neonatal period. To test this hypothesis a model of FGR was utilized in which pregnant rat dams were fed a low protein diet during both the gestation and lactation period. We subsequently analyzed lung mechanics using a FlexiVent ventilator in male and female pups at postnatal day 7 and 21. Lung lavage material was obtained at postnatal day 1, 7 and 21, and was used for analysis of the surfactant system which included measurement of the pool size of surfactant and its subfraction as well as the surface tension reducing ability of the surfactant. The main result of the study was a significantly lower lung compliance and higher tissue elastance which was observed in FGR female offspring at day 21 compared to control offspring. In addition, female LP offspring exhibited lower surfactant pool sizes at postnatal day 1compared to controls. These changes were not observed in the male offspring. It is concluded that FGR has a different impact on pulmonary function and on surfactant in female, as compared to male, offspring.

A respiratory-gated micro-CT comparison of respiratory patterns in free-breathing and mechanically ventilated rats

In this study, we aim to quantify the differences in lung metrics measured in free-breathing and mechanically ventilated rodents using respiratory-gated micro-computed tomography. Healthy male Sprague-Dawley rats were anesthetized with ketamine/xylazine and scanned with a retrospective respiratory gating protocol on a GE Locus Ultra micro-CT scanner. Each animal was scanned while free-breathing, then intubated and mechanically ventilated (MV) and rescanned with a standard ventilation protocol (56 bpm, 8 mL/kg and PEEP of 5 cm H₂O) and again with a ventilation protocol that approximates the free-breathing parameters (88 bpm, 2.14 mL/kg and PEEP of 2.5 cm H₂O). Images were reconstructed representing inspiration and end expiration with 0.15 mm voxel spacing. Image-based measurements of the lung lengths, airway diameters, lung volume, and air content were compared and used to calculate the functional residual capacity (FRC) and tidal volume. Images acquired during MV appeared darker in the airspaces and the airways appeared larger. Image-based measurements showed an increase in lung volume and air content during standard MV, for both respiratory phases, compared with matched MV and free-breathing. Comparisons of the functional metrics showed an increase in FRC for mechanically ventilated rats, but only the standard MV exhibited a significantly higher tidal volume than free-breathing or matched MV Although standard mechanical ventilation protocols may be useful in promoting consistent respiratory patterns, the amount of air in the lungs is higher than in free-breathing animals. Matching the respiratory patterns with the free-breathing case allowed similar lung morphology and physiology measurements while reducing the variability in the measurements.

The Antibacterial and Anti-inflammatory Activity of Chicken Cathelicidin-2 combined with Exogenous Surfactant for the Treatment of Cystic Fibrosis-Associated Pathogens

Cystic fibrosis (CF) is characterized by recurrent airway infections with antibiotic-resistant bacteria and chronic inflammation. Chicken cathelicin-2 (CATH-2) has been shown to exhibit antimicrobial activity against antibiotic-resistant bacteria and to reduce inflammation. In addition, exogenous pulmonary surfactant has been suggested to enhance pulmonary drug delivery. It was hypothesized that CATH-2 when combined with an exogenous surfactant delivery vehicle, bovine lipid extract surfactant (BLES), would exhibit antimicrobial activity against CF-derived bacteria and downregulate inflammation. Twelve strains of CF-pathogens were exposed to BLES+CATH-2 in vitro and killing curves were obtained to determine bactericidal activity. Secondly, heat-killed bacteria were administered in vivo to elicit a pro-inflammatory response with either a co-administration or delayed administration of BLES+CATH-2 to assess the antimicrobial-independent, anti-inflammatory properties of BLES+CATH-2. CATH-2 alone exhibited potent antimicrobial activity against all clinical strains of antibiotic-resistant bacteria, while BLES+CATH-2 demonstrated a reduction, but significant antimicrobial activity against bacterial isolates. Furthermore, BLES+CATH-2 reduced inflammation in vivo when either co-administered with killed bacteria or after delayed administration. The use of a host-defense peptide combined with an exogenous surfactant compound, BLES+CATH-2, is shown to exhibit antimicrobial activity against antibiotic-resistant CF bacterial isolates and reduce inflammation.

Voluntary running exercise protects against sepsis-induced early inflammatory and pro-coagulant responses in aged mice

Background

Despite many animal studies and clinical trials, mortality in sepsis remains high. This may be due to the fact that most experimental studies of sepsis employ young animals, whereas the majority of septic patients are elderly (60 − 70 years). The objective of the present study was to examine the sepsis-induced inflammatory and pro-coagulant responses in aged mice. Since running exercise protects against a variety of diseases, we also examined the effect of voluntary running on septic responses in aged mice.

Methods

Male C57BL/6 mice were housed in our institute from 2–3 to 22 months (an age mimicking that of the elderly). Mice were prevented from becoming obese by food restriction (given 70–90% of ad libitum consumption amount). Between 20 and 22 months, a subgroup of mice ran voluntarily on wheels, alternating 1–3 days of running with 1–2 days of rest. At 22 months, mice were intraperitoneally injected with sterile saline (control) or 3.75 g/kg fecal slurry (septic). At 7 h post injection, we examined (1) neutrophil influx in the lung and liver by measuring myeloperoxidase and/or neutrophil elastase in the tissue homogenates by spectrophotometry, (2) interleukin 6 (IL6) and KC in the lung lavage by ELISA, (3) pulmonary surfactant function by measuring percentage of large aggregates, (4) capillary plugging (pro-coagulant response) in skeletal muscle by intravital microscopy, (5) endothelial nitric oxide synthase (eNOS) protein in skeletal muscle (eNOS-derived NO is putative inhibitor of capillary plugging) by immunoblotting, and (6) systemic blood platelet counts by hemocytometry.

Results

Sepsis caused high levels of pulmonary myeloperoxidase, elastase, IL6, KC, liver myeloperoxidase, and capillary plugging. Sepsis also caused low levels of surfactant function and platelet counts. Running exercise increased eNOS protein and attenuated the septic responses.

Conclusions

Voluntary running protects against exacerbated sepsis-induced inflammatory and pro-coagulant responses in aged mice. Protection against pro-coagulant responses may involve eNOS upregulation. The present discovery in aged mice calls for clinical investigation into potential beneficial effects of exercise on septic outcomes in the elderly.

Electronic supplementary material

The online version of this article (doi:10.1186/s13054-017-1783-1) contains supplementary material, which is available to authorized users.

Lack of matrix metalloproteinase 3 in mouse models of lung injury ameliorates the pulmonary inflammatory response in female but not in male mice

BACKGROUND

The acute respiratory distress syndrome (ARDS) is a complex pulmonary disorder in which the local release of cytokines and chemokines appears central to the pathophysiology.

OBJECTIVE

Based on the known role of matrix metalloproteinase-3 (MMP3) in inflammatory processes, the objective was to examine the role of MMP3 in the pathogenesis of ARDS through the modulation of pulmonary inflammation.

MATERIALS AND METHODS

Female and male, wild type (MMP3^+/+) and knock out (MMP3^-/-) mice were exposed to two, clinically relevant models of ARDS including (i) lipopolysaccharide (LPS)-induced lung injury, and (ii) hydrochloric acid-induced lung injury. Parameters of lung injury and inflammation were assessed through measurements in lung lavage including total protein content, inflammatory cell influx, and concentrations of mediators such as TNF-α, IL-6, G-CSF, CXCL1, CXCL2, and CCL2. Lung histology and compliance were also evaluated in the LPS model of injury.

RESULTS

Following intra-tracheal LPS instillation, all mice developed lung injury, as measured by an increase in lavage neutrophils, and decrease in lung compliance, with no overall effect of genotype observed. Increased concentrations of lavage inflammatory cytokines and chemokines were also observed following LPS injury, however, LPS-instilled female MMP3^-/- mice had lower levels of inflammatory mediators compared to LPS-instilled female MMP3^+/+ mice. This effect of the genotype was not observed in male mice. Similar findings, including the MMP3-related sex differences, were also observed after acid-induced lung injury.

CONCLUSION

MMP3 contributes to the pathogenesis of ARDS, by affecting the pulmonary inflammatory response in female mice in relevant models of lung injury.

The effect of matrix metalloproteinase-3 deficiency on pulmonary surfactant in a mouse model of acute lung injury

The acute respiratory distress syndrome (ARDS) is characterized by arterial hypoxemia accompanied by severe inflammation and alterations to the pulmonary surfactant system. Published data has demonstrated a protective effect of matrix metalloproteinase-3 (Mmp3) deficiency against the inflammatory response associated with ARDS; however, the effect of Mmp3 on physiologic parameters and alterations to surfactant have not been previously studied. It was hypothesized that Mmp3 deficient (Mmp3(-/-)) mice would be protected against lung dysfunction associated with ARDS and maintain a functional pulmonary surfactant system. Wild type (WT) and Mmp3(-/-) mice were subjected to acid-aspiration followed by mechanical ventilation. Mmp3(-/-) mice maintained higher arterial oxygenation compared with WT mice at the completion of ventilation. Significant increase in functional large aggregate surfactant forms were observed in Mmp3(-/-) mice compared with WT mice. These findings further support a role of Mmp3 as an attractive therapeutic target for drug development in the setting of ARDS.

Cholesterol-mediated surfactant dysfunction is mitigated by surfactant protein A

The ability of pulmonary surfactant to reduce surface tension at the alveolar surface is impaired in various lung diseases. Recent animal studies indicate that elevated levels of cholesterol within surfactant may contribute to its inhibition. It was hypothesized that elevated cholesterol levels within surfactant inhibit human surfactant biophysical function and that these effects can be reversed by surfactant protein A (SP-A). The initial experiment examined the function of surfactant from mechanically ventilated trauma patients in the presence and absence of a cholesterol sequestering agent, methyl-β-cyclodextrin. The results demonstrated improved surface activity when cholesterol was sequestered in vitro using a captive bubble surfactometer (CBS). These results were explored further by reconstitution of surfactant with various concentrations of cholesterol with and without SP-A, and testing of the functionality of these samples in vitro with the CBS and in vivo using surfactant depleted rats. Overall, the results consistently demonstrated that surfactant function was inhibited by levels of cholesterol of 10% (w/w phospholipid) but this inhibition was mitigated by the presence of SP-A. It is concluded that cholesterol-induced surfactant inhibition can actively contribute to physiological impairment of the lungs in mechanically ventilated patients and that SP-A levels may be important to maintain surfactant function in the presence of high cholesterol within surfactant.

Apolipoprotein E-deficient mice are susceptible to the development of acute lung injury

BACKGROUND

Apolipoprotein E (apoE) has been shown to play a pivotal role in the development of cardiovascular disease, attributable to its function in lipid trafficking and immune modulating properties; however, its role in modulating inflammation in the setting of acute lung injury (ALI) is unknown.

OBJECTIVE

To determine whether apoE-deficient mice (apoE-/-) are more susceptible to ALI compared to wild-type (WT) animals.

METHODS

Two independent models of ALI were employed. Firstly, WT and apoE-/- mice were randomized to acid aspiration (50 μl of 0.1 N hydrochloric acid) followed by 4 h of mechanical ventilation. Secondly, WT and apoE-/- mice were randomized to 72 h of hyperoxia exposure or room air. Thereafter, the intrinsic responses of WT and apoE-/- mice were assessed using the isolated perfused mouse lung (IPML) setup. Finally, based on elevated levels of oxidized low-density lipoprotein (oxLDL) in apoE-/-, the effect of oxLDL on lung endothelial permeability and inflammation was assessed.

RESULTS

In both in vivo models, apoE-/- mice demonstrated greater increases in lung lavage protein levels, neutrophil counts, and cytokine expression (p < 0.05) compared to WT mice. Experiments utilizing the IPML setup demonstrated no differences in intrinsic lung responses to injury between apoE-/- and WT mice, suggesting the presence of a circulating factor as being responsible for the in vivo observations. Finally, the exposure of lung endothelial cells to oxLDL resulted in increased monolayer permeability and IL-6 release compared to native (nonoxidized) LDL.

CONCLUSIONS

Our findings demonstrate a susceptibility of apoE-/- animals to ALI that may occur, in part, due to elevated levels of oxLDL.

The effects of exogenous surfactant administration on ventilation-induced inflammation in mouse models of lung injury

Background

Mechanical ventilation (MV) is an essential supportive therapy for acute lung injury (ALI); however it can also contribute to systemic inflammation. Since pulmonary surfactant has anti-inflammatory properties, the aim of the study was to investigate the effect of exogenous surfactant administration on ventilation-induced systemic inflammation.

Methods

Mice were randomized to receive an intra-tracheal instillation of a natural exogenous surfactant preparation (bLES, 50 mg/kg) or no treatment as a control. MV was then performed using the isolated and perfused mouse lung (IPML) set up. This model allowed for lung perfusion during MV. In experiment 1, mice were exposed to mechanical ventilation only (tidal volume =20 mL/kg, 2 hours). In experiment 2, hydrochloric acid or air was instilled intra-tracheally four hours before applying exogenous surfactant and ventilation (tidal volume =5 mL/kg, 2 hours).

Results

For both experiments, exogenous surfactant administration led to increased total and functional surfactant in the treated groups compared to the controls. Exogenous surfactant administration in mice exposed to MV only did not affect peak inspiratory pressure (PIP), lung IL-6 levels and the development of perfusate inflammation compared to non-treated controls. Acid injured mice exposed to conventional MV showed elevated PIP, lung IL-6 and protein levels and greater perfusate inflammation compared to air instilled controls. Instillation of exogenous surfactant did not influence the development of lung injury. Moreover, exogenous surfactant was not effective in reducing the concentration of inflammatory cytokines in the perfusate.

Conclusions

The data indicates that exogenous surfactant did not mitigate ventilation-induced systemic inflammation in our models. Future studies will focus on altering surfactant composition to improve its immuno-modulating activity.

Adaptation to low body temperature influences pulmonary surfactant composition thereby increasing fluidity while maintaining appropriately ordered membrane structure and surface activity

The interfacial surface tension of the lung is regulated by phospholipid-rich pulmonary surfactant films. Small changes in temperature affect surfactant structure and function in vitro. We compared the compositional, thermodynamic and functional properties of surfactant from hibernating and summer-active 13-lined ground squirrels (Ictidomys tridecemlineatus) with porcine surfactant to understand structure-function relationships in surfactant membranes and films. Hibernating squirrels had more surfactant large aggregates with more fluid monounsaturated molecular species than summer-active animals. The latter had more unsaturated species than porcine surfactant. Cold-adapted surfactant membranes displayed gel-to-fluid transitions at lower phase transition temperatures with reduced enthalpy. Both hibernating and summer-active squirrel surfactants exhibited lower enthalpy than porcine surfactant. LAURDAN fluorescence and DPH anisotropy revealed that surfactant bilayers from both groups of squirrels possessed similar ordered phase characteristics at low temperatures. While ground squirrel surfactants functioned well during dynamic cycling at 3, 25, and 37 degrees C, porcine surfactant demonstrated poorer activity at 3 degrees C but was superior at 37 degrees C. Consequently the surfactant composition of ground squirrels confers a greater thermal flexibility relative to homeothermic mammals, while retaining tight lipid packing at low body temperatures. This may represent the most critical feature contributing to sustained stability of the respiratory interface at low lung volumes. Thus, while less effective than porcine surfactant at 37 degrees C, summer-active surfactant functions adequately at both 37 degrees C and 3 degrees C allowing these animals to enter hibernation. Here further compositional alterations occur which improve function at low temperatures by maintaining adequate stability at low lung volumes and when temperature increases during arousal from hibernation.

Human alveolar epithelial cells attenuate pulmonary microvascular endothelial cell permeability under septic conditions

Acute lung injury (ALI) and its most severe form, acute respiratory distress syndrome (ARDS), are characterised by high-protein pulmonary edema and severe hypoxaemic respiratory failure due to increased permeability of pulmonary microvascular endothelial cells (PMVEC). Alveolar epithelial cells (AEC) contribute importantly to normal alveolar function, and AEC dysfunction in ALI/ARDS is associated with worse outcomes. We hypothesized that AEC can modulate human PMVEC barrier function, and investigated the effects of AEC presence on human PMVEC barrier under septic conditions in vitro. PMVEC isolated from human lung were treated in vitro with septic stimulation (lipopolysaccharide [LPS], a mixture of clinically-relevant cytokines [cytomix], or plasma from patients with severe sepsis), and the trans-PMVEC leak of Evans Blue dye-labeled albumin assessed. PMVEC septic responses were compared in the presence/absence of co-cultured A549 epithelial cell line or primary human AEC. Septic stimulation with LPS, cytomix, or septic plasma induced marked PMVEC hyper-permeability (10.2±1.8, 8.9±2.2, and 3.7±0.2 fold-increase vs. control, respectively, p<0.01 for all). The presence of A549 cells or primary human AEC in a non-contact co-culture model attenuated septic PMVEC hyper-permeability by 39±4% to 100±3%, depending on the septic stimulation (p<0.05). Septic PMVEC hyper-permeability was also attenuated following treatment with culture medium conditioned by previous incubation with either naïve or cytomix-treated A549 cells (p<0.05), and this protective effect of A549 cell-conditioned medium was both heat-stable and transferable following lipid extraction. Cytomix-stimulated PMN-dependent PMVEC hyper-permeability and trans-PMVEC PMN migration were also inhibited in the presence of A549 cells or A549 cell-conditioned medium (p<0.05). Human AEC appear to protect human PMVEC barrier function under septic conditions in vitro, through release of a soluble mediator(s), which are at least partly lipid in nature. This study suggests a scientific and potential clinical therapeutic importance of epithelial-endothelial cross talk in maintaining alveolar integrity in ALI/ARDS.

A modified squeeze-out mechanism for generating high surface pressures with pulmonary surfactant

The exact mechanism by which pulmonary surfactant films reach the very low surface tensions required to stabilize the alveoli at end expiration remains uncertain. We utilized the nanoscale sensitivity of atomic force microscopy (AFM) to examine phospholipid (PL) phase transition and multilayer formation for two Langmuir-Blodgett (LB) systems: a simple 3 PL surfactant-like mixture and the more complex bovine lipid extract surfactant (BLES). AFM height images demonstrated that both systems develop two types of liquid condensed (LC) domains (micro- and nano-sized) within a liquid expanded phase (LE). The 3 PL mixture failed to form significant multilayers at high surface pressure (π while BLES forms an extensive network of multilayer structures containing up to three bilayers. A close examination of the progression of multilayer formation reveals that multilayers start to form at the edge of the solid-like LC domains and also in the fluid-like LE phase. We used the elemental analysis capability of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to show that multilayer structures are enriched in unsaturated PLs while the saturated PLs are concentrated in the remaining interfacial monolayer. This supports a modified squeeze-out model where film compression results in the hydrophobic surfactant protein-dependent formation of unsaturated PL-rich multilayers which remain functionally associated with a monolayer enriched in disaturated PL species. This allows the surface film to attain low surface tensions during compression and maintain values near equilibrium during expansion.

The biological effects of lung-derived mediators on the liver

Despite the use of lung-protective mechanical ventilation (MV), the mortality of patients with acute lung injury remains at 30 to 40%, predominantly due to multiorgan failure. The objective of this study was to determine the biological significance of lung-derived mediators on peripheral organ inflammation. The authors utilized an isolated perfused mouse lung model of lipopolysaccharide (LPS)-induced lung inflammation and protective MV to collect lung-derived mediators. Aliquots of perfusate from these animals (or appropriate controls) were then injected intravenously into a cohort of normal animals whose livers were subsequently assessed in vivo using intravital video microscopy. Perfusate from LPS-inflamed lungs contained significantly higher concentrations of inflammatory mediators than perfusate from saline-instilled lungs. Assessment of livers in the second cohort of animals 120 minutes after perfusate injection revealed decreased sinusoidal blood flow, leukocytosis, and increased cell death in those receiving perfusate from LPS-inflamed lungs compared to perfusate from saline controls. There were no differences between control animals that received pure perfusate or pure LPS mixed with perfusate. These results showed that lung-derived mediators had a significant biological effect on nonpulmonary organs within a short period of time after administration. Therapies targeting these mediators may prevent multiorgan failure and death in patients with acute lung injury.

The effect of tidal volume on systemic inflammation in Acid-induced lung injury

BACKGROUND

Overwhelming systemic inflammation has been implicated in the progression of acute lung injury (ALI) leading to multiple organ failure (MOF) and death. Previous studies suggest that mechanical ventilation (MV) may be a key mediator of MOF through an upregulation of the systemic inflammatory response.

OBJECTIVES

It was the aim of this study to investigate mechanisms whereby mechanical stress induced by different tidal volumes may contribute to the development of systemic inflammation and maladaptive peripheral organ responses in the setting of ALI.

METHODS

An acid aspiration model of ALI was employed in 129X1/SVJ mice through an intratracheal administration of hydrochloric acid followed by MV employing either a low (5 ml/kg) or high (12.5 ml/kg) tidal volume ventilation for 120 min. The isolated perfused mouse lung setup was used to assess the specific contribution of the lung to systemic inflammation during MV. Furthermore, lung perfusate collected over the course of MV was used to assess the effects of lung-derived mediators on activation (expression of a proadhesive phenotype) of liver endothelial cells.

RESULTS

High tidal volume MV of acid-injured lungs resulted in greater physiologic and histological indices of lung injury compared to control groups. Additionally, there was an immediate and significant release of multiple inflammatory mediators from the lung into the systemic circulation which resulted in greater levels of mRNA adhesion molecule expression in liver endothelial cells in vitro.

CONCLUSIONS

This study suggests that MV, specifically tidal volume strategy, influences the development of MOF through an upregulation of lung-derived systemic inflammation resulting in maladaptive cellular changes in peripheral organs.

Surfactant protein-A reduces translocation of mediators from the lung into the circulation

The objective of this study was to characterize a mouse model of lung inflammation and determine the effect of surfactant protein A (SP-A, or sftpa) on the transfer of inflammatory mediators from these injured lungs into the systemic circulation. Lung inflammation was induced in either sftpa-deficient (-/-) or wild-type (+/+) spontaneously breathing, adult mice via intranasal lipopolysaccharide (LPS). Four hours later, lungs were isolated, perfused, and mechanically ventilated for 2 hours. Perfusate was collected for analysis over the duration of ventilation and lung lavage was obtained in groups of animals immediately before and after mechanical ventilation (MV). Lavage analysis showed an increase in interleukin-6 (IL6) and tumor necrosis factor-alpha (TNFalpha) 4 hours after LPS, with a further increase in IL6 following MV. LPS and MV also caused an increase in total cell and neutrophil numbers as well as total protein in the lavage compared to controls. Perfusate analysis revealed a significant increase in IL6 and TNFalpha after LPS and MV, with significantly greater levels of these mediators in sftpa (-/-) versus (+/+) mice. The authors conclude that LPS followed by MV resulted in lung inflammation and injury, and that SP-A significantly influenced inflammatory mediator release from these inflamed lungs into the perfusate.

A ToF-SIMS study of the lateral organization of lipids and proteins in pulmonary surfactant systems

Pulmonary surfactant is a complex lipid-protein mixture whose main function is to reduce the surface tension at the air-liquid interface of alveoli to minimize the work of breathing. The exact mechanism by which surfactant monolayers and multilayers are formed and how they lower surface tension to very low values during lateral compression remains uncertain. We used time-of-flight secondary ion mass spectrometry to study the lateral organization of lipids and peptide in surfactant preparations ranging in complexity. We show that we can successfully determine the location of phospholipids, cholesterol and a peptide in surfactant Langmuir-Blodgett films and we can determine the effect of cholesterol and peptide addition. A thorough understanding of the lateral organization of PS interfacial films will aid in our understanding of the role of each component as well as different lipid-lipid and lipid-protein interactions. This may further our understanding of pulmonary surfactant function.

Recent advances in alveolar biology: some new looks at the alveolar interface

This article examines the manner in which some new methodologies and novel concepts have contributed to our understanding of how pulmonary surfactant reduces alveolar surface tension. Investigations utilizing small angle X-ray diffraction, inverted interface fluorescence microscopy, time of flight-secondary ion mass spectroscopy, atomic force microscopy, two-photon fluorescence microscopy and electrospray mass spectroscopy are highlighted and a new model of ventilation-induced acute lung injury described. This contribution attempts to emphasize how these new approaches have resulted in a fuller appreciation of events presumably occurring at the alveolar interface.

IL-6 and TNFalpha across the umbilical circulation in term pregnancies: relationship with labour events

OBJECTIVE

We have determined venous and arterial cord blood levels for IL-6 and TNFalpha at the time of delivery to assess gestational tissue versus fetal sources in labouring and non-labouring patients at term, and the relationship to labour events.

METHODS

Fifty-five patients were studied (elective cesarean section n=24, and labouring n=31) with blood sampling from a clamped segment of cord after delivery of the fetus and from the cord at its insertion into the placenta after delivery of the placenta, with subsequent measurement of blood gases, pH, IL-6 and TNFalpha.

RESULTS

Umbilical cord levels for IL-6 were increased by 4 fold in low risk labouring patients, and a further 6 fold when showing histologic chorioamnionitis, but with no evident effect of nuchal cord with 'variable' fetal heart rate decelerations, fetal acidemia, nor of labour duration. IL-6 levels from the cord at its insertion into the placenta were generally higher than those from the respective umbilical levels indicating that placental release of IL-6 into cord blood must be occurring. However, a consistent venoarterial difference for IL-6 and thereby a net flux from the placenta could not be demonstrated. TNFalpha levels for both patient groups were uniformly low for all of the cord measurements with no significant differences noted.

CONCLUSION

Umbilical cord levels for IL-6 are increased in low risk labouring patients at term in the absence of evident infection which likely involves both gestational tissue and fetal contributions. Cord levels for IL-6 are further increased in low risk labouring patients showing histologic chorioamnionitis which might then contribute to newborn morbidity in these pregnancies.

Role of cholesterol in the biophysical dysfunction of surfactant in ventilator-induced lung injury

Mechanical ventilation may lead to an impairment of the endogenous surfactant system, which is one of the mechanisms by which this intervention contributes to the progression of acute lung injury. The most extensively studied mechanism of surfactant dysfunction is serum protein inhibition. However, recent studies indicate that hydrophobic components of surfactant may also contribute. It was hypothesized that elevated levels of cholesterol significantly contribute to surfactant dysfunction in ventilation-induced lung injury. Sprague-Dawley rats ( n = 30) were randomized to either high-tidal volume or low-tidal volume ventilation and monitored for 2 h. Subsequently, the lungs were lavaged, surfactant was isolated, and the biophysical properties of this isolated surfactant were analyzed on a captive bubble surfactometer with and without the removal of cholesterol using methyl-β-cyclodextrin. The results showed lower oxygenation values in the high-tidal volume group during the last 30 min of ventilation compared with the low-tidal volume group. Surfactant obtained from the high-tidal volume animals had a significant impairment in function compared with material from the low-tidal volume group. Removal of cholesterol from the high-tidal volume group improved the ability of the surfactant to reduce the surface tension to low values. Subsequent reconstitution of high-cholesterol values led to an impairment in surface activity. It is concluded that increased levels of cholesterol associated with endogenous surfactant represent a major contributor to the inhibition of surfactant function in ventilation-induced lung injury.

Mediators released from LPS-challenged lungs induce inflammatory responses in liver vascular endothelial cells and neutrophilic leukocytes

The systemic inflammatory response plays an important role in the progression of acute lung injury (ALI) to multiple organ dysfunction syndrome (MODS). However, the role of lung-derived inflammatory mediators in induction of the inflammatory response in remote organs is poorly understood. To address the above, we investigated the effects of lung inflammation on induction of inflammatory response(s) in the liver in vitro. Inflammation in mouse lungs was induced by intranasal administration of lipopolysaccharide (LPS; 1 mg/ml) followed by mechanical ventilation using the isolated perfused mouse lung method to obtain and characterize lung perfusate from the pulmonary circulation. LPS administration to mouse lungs resulted in an increased release of inflammation-relevant cytokines and chemokines into the perfusate (Luminex assay) compared with the saline-controls. Subsequently, primary mouse liver vascular endothelial cells (LVEC) or mouse polymorphonuclear leukocytes (PMN) in vitro were stimulated with the perfusate obtained from saline- or LPS-challenged lungs and assessed for various inflammation-relevant end points. The obtained results indicate that stimulation of LVEC with perfusate obtained from LPS-challenged lungs results in 1) reactive oxygen species (ROS) production; 2) activation of NF-kappaB; and 3) expression of E-selectin, ICAM-1, and VCAM-1 and a subsequent increase in PMN rolling and adhesion to LVEC. In addition, perfusate from LPS-challenged lung induced activation of PMN with respect to increased ROS production and upregulation of cell surface levels of adhesion molecules MAC-1 and VLA-4. Heat-inactivation of the perfusate obtained from LPS-challenged lungs was very effective in suppressing increased proadhesive phenotype (i.e., E-selectin and ICAM-1 expression) in LVEC, whereas targeted inhibition (immunoneutralization) of TNF-alpha and/or IL-6 in LPS-lung perfusate had no effect. Taken together, these findings indicate that multiple proinflammatory mediators (proteinaceous in nature) released from inflamed lungs act synergistically to induce systemic activation of circulating PMN and promote inflammatory responses in liver vascular endothelial cells.

PROTEIN INHIBITION OF SURFACTANT DURING MECHANICAL VENTILATION OF ISOLATED RAT LUNGS

This study tested the hypothesis that material leaking into the airspace from the vasculature during ventilation interferes with surfactant function and contributes to decreases in lung compliance. Rats were euthanized and the lungs were isolated either with or without flushing of the vasculature, followed by mechanical ventilation and analysis of lung compliance and lung lavage analysis. Flushed lungs had higher lung compliance compared to the non-flushed lungs. This was associated with lower protein concentrations and improved surfactant activity. It is concluded that during mechanical ventilation, leakage of proteins results in surfactant inhibition and thereby contribute to decreased lung compliance.

INTERLEUKIN-6 HAS NO EFFECT ON SURFACTANT OR LUNG FUNCTION IN DIFFERENT LUNG INSULTS

The current study determined if interleukin-6 (IL-6) had a causative role in the lung dysfunction and/or surfactant alterations associated with three different lung insults. IL-6 (or saline) was instilled into rats followed by mechanical ventilation in vivo for 4 hours. Also, IL-6 (-/-) and wild-type mice were subjected to 3 insults: ex vivo injurious mechanical ventilation; cecal ligation and perforation; and hyperoxia exposure. In all experiments, the presence or absence of IL-6 did not significantly influence gas exchange, lung compliance, or various surfactant measurements. These results suggest that IL-6 may have a limited role in the surfactant alterations observed in acute lung injury.

LUNG MECHANICS IN THE TIMP3 NULL MOUSE AND ITS RESPONSE TO MECHANICAL VENTILATION

Tissue inhibitor of metalloproteinase-3 (TIMP3) null mice develop emphysema-like airspace enlargement due to an enzymatic imbalance. This study investigates how these abnormalities alter lung mechanics and the response to 2 different mechanical ventilation strategies. Phenotypically, TIMP3 null mice had increased compliance, and decreased resistance, tissue damping, and tissue elastance over wild-type controls. Decreased compliance and increased resistance were observed following the injurious ventilation strategy; however, the TIMP3 null response to both ventilation strategies was similar to wild-type mice. In conclusion, TIMP3 null mice have significant alterations in lung mechanics; however, this does not affect their response to ventilation.

Alterations to surfactant precede physiological deterioration during high tidal volume ventilation

Lung injury due to mechanical ventilation is associated with an impairment of endogenous surfactant. It is unknown whether this impairment is a consequence of or an active contributor to the development and progression of lung injury. To investigate this issue, the present study addressed three questions: Do alterations to surfactant precede physiological lung dysfunction during mechanical ventilation? Which components are responsible for surfactant's biophysical dysfunction? Does exogenous surfactant supplementation offer a physiological benefit in ventilation-induced lung injury? Adult rats were exposed to either a low-stretch [tidal volume (Vt) = 8 ml/kg, positive end-expiratory pressure (PEEP) = 5 cmH2O, respiratory rate (RR) = 54–56 breaths/min (bpm), fractional inspired oxygen (FiO2) = 1.0] or high-stretch (Vt = 30 ml/kg, PEEP = 0 cmH2O, RR = 14–16 bpm, FiO2 = 1.0) ventilation strategy and monitored for either 1 or 2 h. Subsequently, animals were lavaged and the composition and function of surfactant was analyzed. Separate groups of animals received exogenous surfactant after 1 h of high-stretch ventilation and were monitored for an additional 2 h. High stretch induced a significant decrease in blood oxygenation after 2 h of ventilation. Alterations in surfactant pool sizes and activity were observed at 1 h of high-stretch ventilation and progressed over time. The functional impairment of surfactant appeared to be caused by alterations to the hydrophobic components of surfactant. Exogenous surfactant treatment after a period of high-stretch ventilation mitigated subsequent physiological lung dysfunction. Together, these results suggest that alterations of surfactant are a consequence of the ventilation strategy that impair the biophysical activity of this material and thereby contribute directly to lung dysfunction over time.

Current perspectives in pulmonary surfactant--inhibition, enhancement and evaluation

Pulmonary surfactant (PS) is a complicated mixture of approximately 90% lipids and 10% proteins. It plays an important role in maintaining normal respiratory mechanics by reducing alveolar surface tension to near-zero values. Supplementing exogenous surfactant to newborns suffering from respiratory distress syndrome (RDS), a leading cause of perinatal mortality, has completely altered neonatal care in industrialized countries. Surfactant therapy has also been applied to the acute respiratory distress syndrome (ARDS) but with only limited success. Biophysical studies suggest that surfactant inhibition is partially responsible for this unsatisfactory performance. This paper reviews the biophysical properties of functional and dysfunctional PS. The biophysical properties of PS are further limited to surface activity, i.e., properties related to highly dynamic and very low surface tensions. Three main perspectives are reviewed. (1) How does PS permit both rapid adsorption and the ability to reach very low surface tensions? (2) How is PS inactivated by different inhibitory substances and how can this inhibition be counteracted? A recent research focus of using water-soluble polymers as additives to enhance the surface activity of clinical PS and to overcome inhibition is extensively discussed. (3) Which in vivo, in situ, and in vitro methods are available for evaluating the surface activity of PS and what are their relative merits? A better understanding of the biophysical properties of functional and dysfunctional PS is important for the further development of surfactant therapy, especially for its potential application in ARDS.

Protective effects of elevated endogenous surfactant pools to injurious mechanical ventilation

Depletion of alveolar macrophages (AM) leads to an increase in endogenous surfactant that lasts several days beyond the repletion of AM. Furthermore, impairment to the endogenous pulmonary surfactant system contributes to ventilation-induced lung injury. The objective of the current study was to determine whether increased endogenous surfactant pools induced via AM depletion was protective against ventilation-induced lung injury. Adult rats were intratracheally instilled with either control or dichloromethylene diphosphonic acid (DMDP) containing liposomes to deplete AMs and thereby increase endogenous surfactant pools. Either 3 or 7 days following instillation, rats were exposed to 2 h of injurious ventilation using either an ex vivo or in vivo ventilation protocol and were compared with nonventilated controls. The measured outcomes were oxygenation, lung compliance, lavage protein, and inflammatory cytokine concentrations. Compared with controls, the DMDP-treated animals had significantly reduced AM numbers and increased surfactant pools 3 days after instillation. Seven days after instillation, AM numbers had returned to normal, but surfactant pools were still elevated. DMDP-treated animals at both time points exhibited protection against ventilation-induced lung injury, which included superior physiological parameters, lower protein leakage, and lower inflammatory mediator release into the air space, compared with animals not receiving DMDP. It is concluded that DMDP-liposome administration protects against ventilation-induced lung injury. This effect appears to be due to the presence of elevated endogenous surfactant pools.

Contribution of alveolar macrophages to the response of the TIMP-3 null lung during a septic insult

Mice deficient in tissue inhibitor of metalloproteinase-3 (TIMP-3) develop an emphysema-like phenotype involving increased pulmonary compliance, tissue degradation, and matrix metalloproteinase (MMP) activity. After a septic insult, they develop a further increase in compliance that is thought to be a result of heightened metalloproteinase activity produced by the alveolar macrophage, potentially modeling an emphysemic exacerbation. Therefore, we hypothesized that TIMP-3 null mice lacking alveolar macrophages would not be susceptible to the altered lung function associated with a septic insult. TIMP-3 null and wild-type (WT) mice were depleted of alveolar macrophages before the induction of a septic insult and assessed for alteration in lung mechanics, alveolar structure, metalloproteinase levels, and inflammation. The results showed that TIMP-3 null mice lacking alveolar macrophages were protected from sepsis-induced alterations in lung mechanics, particularly pulmonary compliance, a finding that was supported by changes in alveolar structure. Additionally, changes in lung mechanics involved primarily peripheral tissue vs. central airways as determined using the flexiVent system. From investigation into possible molecules that could cause these alterations, it was found that although several proteases and inflammatory mediators were increased during the septic response, only MMP-7 was attenuated after macrophage depletion. In conclusion, the alveolar macrophage is essential for the TIMP-3 null sepsis-induced compliance alterations. This response may be mediated in part by MMP-7 activity but occurs independently of inflammatory cytokine and/or chemokine concentrations.

In vivo characterization of lung morphology and function in anesthetized free-breathing mice using micro-computed tomography

Lung morphology and function in human subjects can be monitored with computed tomography (CT). Because many human respiratory diseases are routinely modeled in rodents, a means of monitoring the changes in the structure and function of the rodent lung is desired. High-resolution images of the rodent lung can be attained with specialized micro-CT equipment, which provides a means of monitoring rodent models of lung disease noninvasively with a clinically relevant method. Previous studies have shown respiratory-gated images of intubated and respirated mice. Although the image quality and resolution are sufficient in these studies to make quantitative measurements, these measurements of lung structure will depend on the settings of the ventilator and not on the respiratory mechanics of the individual animals. In addition, intubation and ventilation can have unnatural effects on the respiratory dynamics of the animal, because the airway pressure, tidal volume, and respiratory rate are selected by the operator. In these experiments, important information about the symptoms of the respiratory disease being studied may be missed because the respiration is forced to conform to the ventilator settings. In this study, we implement a method of respiratory-gated micro-CT for use with anesthetized free-breathing rodents. From the micro-CT images, quantitative analysis of the structure of the lungs of healthy unconscious mice was performed to obtain airway diameters, lung and airway volumes, and CT densities at end expiration and during inspiration. Because the animals were free breathing, we were able to calculate tidal volume (0.09 +/- 0.03 ml) and functional residual capacity (0.16 +/- 0.03 ml).

The future of surfactant therapy during ALI/ARDS

The importance of pulmonary surfactant in maintaining normal lung function, and the observations that alterations in endogenous surfactant contribute to the lung dysfunction associated with acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS), provide a rationale for administering exogenous surfactant in this setting. The results of clinical trials have been variable, however, in part due to the various surfactant preparations used, the different delivery and dosing methods employed, and the types of patients targeted for this therapy. Based on the insight gained from these studies, ongoing trials have modified these factors to optimize outcome, including one trial that is focusing on patients with direct lung insults such as pneumonia and aspiration. The future of surfactant therapy may well take advantage of the recently described host defense functions of this material. Based on extensive in vitro data as well as in vivo animal studies demonstrating the anti-inflammatory and antibacterial functions of various surfactant components, administration of surfactant earlier in the course of the disease, when lung inflammation is present but before severe lung dysfunction occurs, may prove to be optimal. This review discusses both the biophysical and host defense functions of surfactant in the context of novel therapeutic approaches for patients with ALI/ARDS.

Physiological effects of oxidized exogenous surfactant in vivo: effects of high tidal volume and surfactant protein A

Oxidative damage to surfactant can decrease lung function in vivo. In the current study, our two objectives were: 1) to examine whether the adverse effects of oxidized surfactant would be accentuated in animals exposed to high tidal volume ventilation, and 2) to test whether supplementation with surfactant protein A (SP-A) could improve the function of oxidized surfactant in vivo. The first objective was addressed by evaluating the response of surfactant-deficient rats administered normal or oxidized surfactant and then subjected to low tidal volume (6 ml/kg) or high tidal volume (12 ml/kg) mechanical ventilation. Under low tidal volume conditions, rats administered oxidized surfactant had impaired lung function, as determined by lung compliance and arterial blood gas analysis, compared with nonoxidized controls. Animals subjected to high tidal volume ventilation had impaired lung function compared with low tidal volume groups, regardless of the oxidative status of the surfactant. The second experiment demonstrated a significantly superior physiological response in surfactant-deficient rats receiving SP-A containing oxidized surfactant compared with oxidized surfactant. Lavage analysis at the end of the in vivo experimentation showed no differences in the recovery of oxidized surfactant compared with nonoxidized surfactant. We conclude that minimizing excessive lung stretch during mechanical ventilation is important in the context of exogenous surfactant supplementation and that SP-A has an important biophysical role in surfactant function in conditions of oxidative stress. Furthermore, the oxidative status of the surfactant does not appear to affect the alveolar metabolism of this material.

Mechanisms responsible for surfactant changes in sepsis-induced lung injury

Pulmonary surfactant is altered in sepsis, and these changes contribute to the predisposition of septic lungs to subsequent insults, ultimately leading to acute lung injury. Specifically, the total amount of surfactant is lower in sepsis, mainly due to decreased small aggregate (SA) surfactant pools. The amount of large aggregate (LA) surfactant is not altered. To evaluate the mechanisms responsible for these alterations, trace doses of tritium-labelled dipalmitoylphosphatidylcholine (3H-DPPC)-labelled LA were instilled intratracheally into adult rats 20 hrs after caecal ligation and perforation (CLP) or sham surgery. Animals were sacrificed at 0, 1 and 4 h after instillation and recovery of 3H-DPPC in alveolar macrophages (AM), LA and SA was measured. In separate in vitro experiments, AM isolated from CLP/sham rats were incubated with LA or SA isolated from normal animals to evaluate the uptake of these aggregates into the AM. Results showed increased surfactant radioactivity associated with AM of CLP animals compared with sham animals both in vivo and in vitro. In addition, more 3H-DPPC label remained in LA forms in the CLP animals in vivo compared with sham. These findings indicate that differences in surfactant aggregate uptake and large aggregate conversion occur in septic lungs, resulting in changes in surfactant pools.

Pre-emptive and continuous inhaled NO counteracts the cardiopulmonary consequences of extracorporeal circulation in a pig model

Cardiopulmonary bypass (CPB) activates a systemic inflammatory response characterized clinically by alterations in cardiovascular and pulmonary function. The aim of this study was to measure the cardiopulmonary consequences in sham-operated pigs, and in animals subjected to CPB in the presence or absence of lipopolysaccharide (LPS). We also investigated, if the perioperative administration of inhaled NO exerts significant cardiopulmonary effects in an anaesthetized and mechanically ventilated pig model of extracorporeal circulation. Thirty pigs were randomized into six equal groups (sham; sham+INO; CPB; CPB+INO; CPB+LPS; CPB+LPS+INO) and subjected to anaesthesia with mechanical ventilation for up to 24h. We found that CPB+LPS group has the highest degree of lung injury. We also demonstrated that there was a significant difference on the cardiovascular parameters (heart rate, central venous pressure, stroke volume index, and mean systemic arterial blood pressure) between the CPB groups and the sham groups. The deteriorated lung mechanics was associated with a decrease in active subfraction of surfactant (LA) with time during the procedure (P=0.0003), on which inhaled NO had only an initial beneficial effect. In our model, inhaled NO had no long-term beneficial effect on lung mechanics and surfactant homeostasis despite improving lung haemodynamics, inflammation, and oxygenation. We conclude from this study that the use of pre-emptive and continuous inhaled NO therapy has protective and safe effects against lung ischemia/reperfusion associated with CPB.

Differential response of TIMP-3 null mice to the lung insults of sepsis, mechanical ventilation, and hyperoxia

An imbalance in matrix metalloproteinases (MMPs) and the tissue inhibitors of metalloproteinases (TIMPs) leads to excessive or insufficient tissue breakdown, which is associated with many disease processes. The TIMP-3 null mouse is a model of MMP/TIMP imbalance, which develops air space enlargement and decreased lung function. These mice responded differently to cecal ligation and perforation (CLP)-induced septic lung injury than wild-type controls. The current study addresses whether the TIMP-3 knockout lung is susceptible to different types of insults or only those involving sepsis, by examining its response to lipopolysaccharide (LPS)-induced sepsis, mechanical ventilation (MV), and hyperoxia. TIMP-3 null noninjured controls of each insult consistently demonstrated significantly higher compliance vs. wild-type mice. Null mice treated with LPS had a further significantly increased compliance compared with untreated controls. Conversely, MV and hyperoxia did not alter compliance in the null lung. MMP abundance and activity increased in response to LPS but were generally unaltered following MV or hyperoxia, correlating with compliance alterations. All three insults produced inflammatory cytokines; however, the response of the null vs. wild-type lung was dependent on the type of insult. Overall, this study demonstrated that 1) LPS-induced sepsis produced a similar response in null mice to CLP-induced sepsis, 2) the null lung responded differently to various insults, and 3) the null susceptibility to compliance changes correlated with increased MMPs. In conclusion, this study provides insight into the role of TIMP-3 in response to various lung insults, specifically its importance in regulating MMPs to maintain compliance during a sepsis.

Host response to intratracheally instilled bacteria in ventilated and nonventilated rats

OBJECTIVE

Pneumonia occurs in approximately 7% of hospitalized patients. Susceptibility to certain bacteria such as Pseudomonas aeruginosa increases in critically ill patients, particularly those requiring mechanical ventilation. Previous studies investigating this susceptibility have used injurious modes of ventilation. The objective of this study was to evaluate the host's response to intratracheal instillation of P. aeruginosa in the setting of noninjurious mechanical ventilation and compare this with normal, spontaneously breathing animals receiving bacteria.

DESIGN

Randomized, controlled in vivo animal study.

SETTING

Research laboratory at a university-affiliated institution.

SUBJECTS

Adult male Sprague-Dawley rats.

INTERVENTIONS

Rats were randomized into four groups: spontaneously breathing given saline, spontaneously breathing given bacteria, mechanically ventilated given saline, and mechanically ventilated given bacteria. The ventilation strategy used involved low stretch (tidal volume of 8 mL/kg) with a positive end-expiratory pressure of 5 cm H2O.

MEASUREMENTS AND MAIN RESULTS

Lung compliance, bacterial recovery, surfactant, total cells, and cytokine concentrations in the lung lavage were analyzed after 4 hrs. Results showed that neither ventilation nor bacteria alone altered lung function, although the combination of ventilation and Pseudomonas significantly decreased arterial oxygenation and lung compliance. Increases in lavage cell counts, cytokines, and surfactant were observed in both groups administered bacteria compared with animals given saline. However, there were no significant differences in bacterial recovery, cell counts, cytokines, and surfactant measurements in the groups given bacteria.

CONCLUSIONS

These data suggest that bacterial instillation with low-stretch ventilation had a significant effect on lung function but did not alter the inflammatory response to a bacterial challenge over this time course compared with spontaneously breathing animals.

Pseudomonas aeruginosa protease IV degrades surfactant proteins and inhibits surfactant host defense and biophysical functions

Pulmonary surfactant has two distinct functions within the lung: reduction of surface tension at the air-liquid interface and participation in innate host defense. Both functions are dependent on surfactant-associated proteins. Pseudomonas aeruginosa is primarily responsible for respiratory dysfunction and death in cystic fibrosis patients and is also a leading pathogen in nosocomial pneumonia. P. aeruginosa secretes a number of proteases that contribute to its virulence. We hypothesized that P. aeruginosa protease IV degrades surfactant proteins and results in a reduction in pulmonary surfactant host defense and biophysical functions. Protease IV was isolated from cultured supernatant of P. aeruginosa by gel chromatography. Incubation of cell-free bronchoalveolar lavage fluid with protease IV resulted in degradation of surfactant proteins (SP)-A, -D, and -B. SPs were degraded in a time- and dose-dependent fashion by protease IV, and degradation was inhibited by the trypsin-like serine protease inhibitor Nalpha-p-tosyl-L-lysine-chloromethyl ketone (TLCK). Degradation by protease IV inhibited SP-A- and SP-D-mediated bacterial aggregation and uptake by macrophages. Surfactant treated with protease IV was unable to reduce surface tension as effectively as untreated surfactant, and this effect was inhibited by TLCK. We speculate that protease IV may be an important contributing factor to the development and propagation of acute lung injury associated with P. aeruginosa via loss of surfactant function within the lung.

Effect of Acute Lung Injury on Structure and Function of Pulmonary Surfactant Films

The structural and functional alterations in pulmonary surfactant that occur during acute lung injury were studied using rat lung surfactant large aggregates (LA) isolated from normal nonventilated lungs (N), and from standard ventilated (V) and injuriously ventilated (IV) excised lungs. N lungs inflated significantly better than IV lungs, with V lungs intermediate. Although IV LA phosphatidylcholine levels were unchanged, cholesterol and protein were elevated. V LA exhibited PC/cholesterol and PC/protein ratios intermediate between N and IV. In contrast to total cholesterol and protein levels, these ratios were not significantly different from IV LA. N and V LA, but not IV LA, adsorbed rapidly and were able to generate surface pressures (pi) near 70 mN/m during surface area reduction at 37 degrees C on a captive bubble tensiometer. Langmuir-Wilhelmy surface balance studies at 23 degrees C showed N LA films consistently attained pi approaching 70 mN/m during ten compression-expansion cycles. IV films were less effective and failed to achieve high pi consistently after the sixth cycle. V films were intermediate. Epifluorescence studies revealed compression of adsorbed N LA films formed well-defined liquid-condensed (LC) domains, but fewer, smaller domains were observed with IV films and, to a lesser extent, V films. Atomic force microscopy on Langmuir-Blodgett N films transferred at pi = 30 mN/m showed high, well-defined LC domains. IV films showed thinner, intermediate height, possibly fluid domains, which contain large numbers of small higher domains with heights corresponding to LC domains. V films were intermediate. We conclude that acute lung injury induced by hyperventilation, and to a lesser extent standard ventilation, of excised lungs alters surfactant surface activity and the ability of natural surfactant to form surface structures at the air-water interface.

Physiological and inflammatory response to instillation of an oxidized surfactant in a rat model of surfactant deficiency

Pulmonary surfactant is a mixture of phospholipids ( approximately 90%) and surfactant-associated proteins (SPs) ( approximately 10%) that stabilize the lung by reducing the surface tension. One proposed mechanism by which surfactant is altered during acute lung injury is via direct oxidative damage to surfactant. In vitro studies have revealed that the surface activity of oxidized surfactant was impaired and that this effect could be overcome by adding SP-A. On the basis of this information, we hypothesized that animals receiving oxidized surfactant preparations would exhibit an inferior physiological and inflammatory response and that the addition of SP-A to the oxidized preparations would ameliorate this response. To test this hypothesis, mechanically ventilated, surfactant-deficient rats were administered either bovine lipid extract surfactant (BLES) or in vitro oxidized BLES of three doses: 10 mg/kg, 50 mg/kg, or 10 mg/kg + SP-A. When instilled with 10 mg/kg normal surfactant, the rats had a significantly superior arterial Po2 responses compared with the rats receiving oxidized surfactant. Interestingly, increasing the dose five times mitigated this physiological effect, and the addition of SP-A to the surfactant preparation had little impact on improving oxygenation. There were no differences in alveolar surfactant pools and the indexes of pulmonary inflammation between the 10 mg/kg dose groups, nor was there any differences observed between either of the groups supplemented with SP-A. However, there was significantly more surfactant and more inflammatory cytokines in the 50 mg/kg oxidized BLES group compared with the 50 mg/kg BLES group. We conclude that instillation of an in vitro oxidized surfactant causes an inferior physiological response in a surfactant-deficient rat.

Negative impact of tissue inhibitor of metalloproteinase-3 null mutation on lung structure and function in response to sepsis

Matrix metalloproteinases (MMPs) are degradative enzymes, which act to remodel tissue. Their activity is regulated by the tissue inhibitors of metalloproteinases (TIMPs). An imbalance in the degradation/inhibition activities has been associated with many diseases, including sepsis. We have previously shown that TIMP-3 knockout animals develop spontaneous, progressive air space enlargement. The objectives of this study were to determine the effects of a septic lung stress induced by cecal ligation and perforation (CLP) on lung function, structure, pulmonary surfactant, and inflammation in TIMP-3 null mice. Knockout and wild-type animals were randomized to either sham or CLP surgery, allowed to recover for 6 h, and then euthanized. TIMP-3 null animals exposed to sham surgery had a significant increase in lung compliance when compared with sham wild-type mice. Additionally, the TIMP-3 knockout mice showed a significant increase in compliance following CLP. Rapid compliance changes were accompanied by significantly decreased collagen and fibronectin levels and increased gelatinase (MMP-2 and -9) abundance and activation. Additionally, in situ zymography showed increased airway-associated gelatinase activity in the knockout animals enhanced following CLP. In conclusion, exposing TIMP-3 null animals to sepsis rapidly enhances the phenotypic abnormalities of these mice, due to increased MMP activity induced by CLP.

High-frequency oscillation and exogenous surfactant administration in lung-injured adult sheep

OBJECTIVE

To evaluate the effects of high-frequency oscillation on the response to exogenous surfactant in lung-injured adult sheep.

DESIGN

A prospective, controlled, in vivo, animal laboratory study.

SETTING

Animal research facility of a health sciences university.

SUBJECTS

Twenty-eight adult sheep.

INTERVENTIONS

Animals were anesthetized and instrumented with a tracheostomy and vascular catheters. Following whole lung saline lavage, animals were randomized to one of four groups: Group S-CMV received surfactant and was ventilated for 4 hrs using a conventional mechanical ventilation strategy, group S-HFOV/CMV received surfactant and was ventilated with a high-frequency oscillation technique for 2 hrs and a conventional mechanical strategy for 2 hrs, group HFOV/CMV underwent the latter ventilatory strategies without receiving surfactant, and group HFOV was ventilated with high-frequency oscillation only for 4 hrs. At the end of the ventilatory period, the distributions of ventilation and surfactant were evaluated in animals that received surfactant.

MEASUREMENTS AND MAIN RESULTS

Animals in the S-CMV group had a significantly greater mean PaO2 value at the end of the experimental period than animals in the S-HFOV/CMV or HFOV/CMV groups. Evaluation of the distribution of ventilation relative to surfactant demonstrated that animals ventilated with high-frequency oscillation followed by conventional mechanical ventilation had a significantly greater disproportionate distribution of ventilation relative to surfactant compared with the CMV-only group.

CONCLUSIONS

A period of high-frequency oscillation, as used in this study, immediately following exogenous surfactant administration mitigates the host's response to surfactant when subsequently switched to conventional mechanical ventilation.