Inhibition of the human neutrophil respiratory burst by native and synthetic surfactant

Radiation Mitigating Properties of Intranasally Administered KL4 Surfactant in a Murine Model of Radiation-Induced Lung Damage

The threat of exposure to ionizing radiation from a nuclear reactor accident or deliberate terrorist actions is a significant public health concern. The lung is particularly susceptible to radiation-induced injury from external sources or inhalation of radioactive particles from radioactive fallout. Radiation-induced lung disease can manifest with an acute radiation pneumonitis and/or delayed effects leading to pulmonary fibrosis. As prior warning of radiation exposure is unlikely, medical countermeasures (MCMs) to mitigate radiation-induced lung disease that can be given in mass-casualty situations many hours or days postirradiation are needed to prevent both early and late lung damage. In this study, KL₄ surfactant (lucinactant) was evaluated as a radiation mitigator in a well-characterized mouse model of targeted thoracic radiation exposure, for its effect on both early (several weeks) and late (18 weeks) lung damage. Here, 120 mg/kg total phospholipid of KL₄ surfactant was administered twice daily intranasally, (enabling intrapulmonary inhalation of drug) to C57BL/6 mice 24 h after a single 13.5 Gy dose of thoracic irradiation (LD₅₀ dose). Both early and chronic phase (2 and 4 weeks and 18 weeks postirradiation, respectively) assessments were performed. Mice were evaluated for evidence of reduced arterial blood oxygenation and early and chronic lung and systemic inflammation, lung fibrosis and oxidative stress. Analysis was done by performing lung function/respiration dynamics and measuring cellular protein content of bronchoalveolar lavage fluid (BALF), and levels of cytokines, 8-iso-prostaglandin F2α, hydroxyproline in lung and plasma, along with evaluating lung histology. The results of this study showed that intranasal delivery of KL₄ surfactant was able to preserve lung function as evidenced by adequate arterial oxygen saturation and reduced lung inflammation and oxidative stress; total white count and absolute neutrophil count was decreased in BALF, as were plasma pro-inflammatory cytokine levels and biomarker of oxidative stress. KL₄ surfactant is a promising MCM for mitigation of lung tissue damage after targeted, thoracic irradiation and has the potential to be developed as a broad-spectrum, multi-use MCM against chemical, biological, radiological or nuclear threat agents with potential to cause lung injury.

Surfactant Releases Internal Calcium Stores in Neutrophils by G Protein–Activated Pathway

Pulmonary surfactant with surfactant-associated proteins (PS+SAP) decreases pulmonary inflammation by suppressing neutrophil activation. We have observed that PS+SAP inserts channels into artificial membranes, depolarizes neutrophils, and depresses calcium influx and function in stimulated neutrophils. We hypothesize that PS+SAP suppresses neutrophil activation by depletion of internal Ca++ stores and that PS+SAP induces depletion through release of Ca++ stores and through inhibition of Ca++ influx. Our model predicts that PS+SAP releases Ca++ stores through insertion of channels, depolarization of neutrophils, and activation of a G protein–dependent pathway. If the model of channel insertion and membrane depolarization is accurate, then gramicidin—a channel protein with properties similar to those of PS+SAP—is expected to mimic these effects. Human neutrophils were monitored for [Ca++] responses after exposure to one of two different PS+SAP preparations, a PS-SAP preparation, gramicidin alone, and gramicidin reconstituted with phospholipid (PLG). [Ca++] responses were reexamined following preexposure to inhibitors of internal Ca++ release or the G protein pathway. We observed that (i) 1% PS+SAP—but not PS-SAP—causes transient increase of neutrophil [Ca++] within seconds of exposure; (ii) 1% PLG—but not gramicidin alone—closely mimics the effect of PS+SAP on Ca++ response; (iii) PS+SAP and PLG equally depolarize neutrophils; (iv) direct inhibition of internal Ca++ stores releases or of G protein activation suppresses Ca++ responses to PS+SAP and PLG; and (v) preexposure to either PS+SAP or PLG inhibits Ca++ influx following fMLP stimulation. We conclude that PS+SAP independently depolarizes neutrophils, releases Ca++ from internal stores by a G protein-mediated pathway, and alters subsequent neutrophil response to physiologic stimulants by depleting internal Ca++ stores and by inhibiting Ca++ influx during subsequent fMLP activation. The mimicking of these results by PLG supports the hypothesis that PS+SAP initiates depolarization via channel insertion into neutrophil plasma membrane.

Immunomodulatory properties of surfactant preparations

Surfactant replacement significantly decreased acute pulmonary morbidity and mortality among preterm neonates with respiratory distress syndrome. Besides improving lung function and oxygenation, surfactant is also a key modulator of pulmonary innate and acquired immunity regulating lung inflammatory processes. In this review, we describe the immunomodulatory features of surfactant preparations. Various surfactant preparations decrease the proinflammatory cytokine and chemokine release, the oxidative burst activity, and the nitric oxide production in lung inflammatory cells such as alveolar neutrophils, monocytes and macrophages; they also affect lymphocyte proliferative response and immunoglobulin production, as well as natural killer and lymphokine-activated killer cell activity. In addition, surfactant preparations are involved in airway remodeling, as they decrease lung fibroblast proliferation capacity and the release of mediators involved in remodeling. Moreover, they increase cell transepithelial resistance and VEGF synthesis in lung epithelial cells. A number of different signaling pathways and molecules are involved in these processes. Because the inhibition of local immune response may decrease lung injury, surfactant therapeutic efficacy may be related not only to its biophysical characteristics but, at least in part, to its anti-inflammatory features and its effects on remodeling processes. However, further studies are required to identify which surfactant preparation ensures the highest anti-inflammatory activity, thereby potentially decreasing the inflammatory process underlying respiratory distress syndrome. In perspective, detailed characterization of these anti-inflammatory effects could help to improve the next generation of surfactant preparations.

Poractant alfa (Curosurf®) increases phagocytosis of apoptotic neutrophils by alveolar macrophages in vivo

BACKGROUND

Clearance of apoptotic neutrophils in the lung is an essential process to limit inflammation, since they could become a pro-inflammatory stimulus themselves. The clearance is partially mediated by alveolar macrophages, which phagocytose these apoptotic cells. The phagocytosis of apoptotic immune cells by monocytes in vitro has been shown to be augmented by several constituents of pulmonary surfactant, e.g. phospholipids and hydrophobic surfactant proteins. In this study, we assessed the influence of exogenous poractant alfa (Curosurf®) instillation on the in vivo phagocytosis of apoptotic neutrophils by alveolar macrophages.

METHODS

Poractant alfa (200 mg/kg) was instilled intratracheally in the lungs of three months old adult male C57/Black 6 mice, followed by apoptotic neutrophil instillation. Bronchoalveloar lavage was performed and alveolar macrophages and neutrophils were counted. Phagocytosis of apoptotic neutrophils was quantified by determining the number of apoptotic neutrophils per alveolar macrophages.

RESULTS

Exogenous surfactant increased the number of alveolar macrophages engulfing apoptotic neutrophils 2.6 fold. The phagocytosis of apoptotic neutrophils was increased in the presence of exogenous surfactant by a 4.7 fold increase in phagocytosed apoptotic neutrophils per alveolar macrophage.

CONCLUSIONS

We conclude that the anti-inflammatory properties of surfactant therapy may be mediated in part by increased numbers of alveolar macrophages and increased phagocytosis of apoptotic neutrophils by alveolar macrophages.

Anti-inflammatory, antinociceptive and anti-angiogenic activities of a phospholipid mixture purified from porcine lung tissues

This work aimed to assess anti-inflammatory and related properties of a phospholipid mixture purified from porcine lung tissues, named KT&G101, which is being developed as a novel topical remedy for atopic dermatitis. KT&G101 consists of pure phospholipids, mainly phosphatidylcholine (PC) and other phospholipids such as phosphatidylinositol (PI) and phosphatidylserine (PS). Its predominant PC species is 1,2-dipalmitoylphosphatidylcholine (DPPC). KT&G101 exhibited an anti-angiogenic activity in the chick chorioallantoic membrane (CAM) assay. Oral administration of KT&G101 at the dosages of 100, 200 and 400 mg/kg body weight gave rise to an inhibition of 15.4%, 25.3% and 30.1% in the vascular permeability assay, respectively. In the carrageenan-induced inflammation in the air pouches, KT&G101 significantly diminished the volume of exudates in the pouches, the number of polymorphonuclear leukocytes and nitrite content in exudates. In the acetic acid-induced writhing response, oral administration of KT&G101 at the dosages of 50, 100 and 200 mg/kg body weight showed the reduction of 21.6%, 51.6% and 60.8% in the pain response of mice, respectively. It was also able to diminish the nitric oxide (NO) and reactive oxygen species (ROS) levels in the lipopolysaccharide (LPS)-stimulated RAW264.7 macrophage cells. KT&G101 displayed a significant suppression on the induction of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in the stimulated RAW264.7 cells. However, the free radical scavenging activity of KT&G101 was detected to be very weak in the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. Taken together, KT&G101 possesses anti-inflammatory and related antinociceptive and anti-angiogenic activities, which indirectly supports its use as an anti-atopic therapy.

Surfactant lipids regulate LPS-induced interleukin-8 production in A549 lung epithelial cells by inhibiting translocation of TLR4 into lipid raft domains

In addition to providing mechanical stability, growing evidence suggests that surfactant lipid components can modulate inflammatory responses in the lung. However, little is known of the molecular mechanisms involved in the immunomodulatory action of surfactant lipids. This study investigates the effect of the lipid-rich surfactant preparations Survanta, Curosurf, and the major surfactant phospholipid dipalmitoylphosphatidylcholine (DPPC) on interleukin-8 (IL-8) gene and protein expression in human A549 lung epithelial cells using immunoassay and PCR techniques. To examine potential mechanisms of the surfactant lipid effects, Toll-like receptor 4 (TLR4) expression was analyzed by flow cytometry, and membrane lipid raft domains were separated by density gradient ultracentrifugation and analyzed by immunoblotting with anti-TLR4 antibody. The lipid-rich surfactant preparations Survanta, Curosurf, and DPPC, at physiological concentrations, significantly downregulated lipopolysaccharide (LPS)-induced IL-8 expression in A549 cells both at the mRNA and protein levels. The surfactant preparations did not affect the cell surface expression of TLR4 or the binding of LPS to the cells. However, LPS treatment induced translocation of TLR4 into membrane lipid raft microdomains, and this translocation was inhibited by incubation of the cells with the surfactant lipid. This study provides important mechanistic details of the immune-modulating action of pulmonary surfactant lipids.

DPPC regulates COX-2 expression in monocytes via phosphorylation of CREB

The major phospholipid in pulmonary surfactant dipalmitoyl phosphatidylcholine (DPPC) has been shown to modulate inflammatory responses. Using human monocytes, this study demonstrates that DPPC significantly increased PGE(2) (P<0.05) production by 2.5-fold when compared to untreated monocyte controls. Mechanistically, this effect was concomitant with an increase in COX-2 expression which was abrogated in the presence of a COX-2 inhibitor. The regulation of COX-2 expression was independent of NF-kappaB activity. Further, DPPC increased the phosphorylation of the cyclic AMP response element binding protein (CREB; an important nuclear transcription factor important in regulating COX-2 expression). In addition, we also show that changing the fatty acid groups of PC (e.g. using l-alpha-phosphatidylcholine beta-arachidonoyl-gamma-palmitoyl (PAPC)) has a profound effect on the regulation of COX-2 expression and CREB activation. This study provides new evidence for the anti-inflammatory activity of DPPC and that this activity is at least in part mediated via CREB activation of COX-2.

Adjunctive therapy to mechanical ventilation: surfactant therapy, liquid ventilation, and prone position

Acute lung injury and acute respiratory distress syndrome are associated with significant morbidity and mortality in critically ill patients. Although lung protective mechanical ventilation is the only therapy shown to reduce mortality and development of organ failure, several biologic pathways have been identified and provided an opportunity for therapeutic interventions. No pharmacologic or adjunctive treatments are available. Clinical studies demonstrated that prone position results in significant and clinically relevant improvement in oxygenation and ventilation, which persist when patients are returned to supine position; the beneficial response is not limited to patients turned early in disease course. Few complications are associated with prone ventilation. Clinical experience suggests that prone ventilation may protect the lung from potential detrimental effects of mechanical ventilation. Further studies are needed.

The role of surfactant protein D in the colonisation of the respiratory tract and onset of bacteraemia during pneumococcal pneumonia

We have shown previously that surfactant protein D (SP-D) binds and agglutinates Streptococcus pneumoniae in vitro. In this study, the role of SP-D in innate immunity against S. pneumoniae was investigated in vivo, by comparing the outcome of intranasal infection in surfactant protein D deficient (SP-D-/-) to wildtype mice (SP-D+/+). Deficiency of SP-D was associated with enhanced colonisation and infection of the upper and lower respiratory tract and earlier onset and longer persistence of bacteraemia. Recruitment of neutrophils to inflammatory sites in the lung was similar in both strains mice in the first 24 hrs post-infection, but different by 48 hrs. T cell influx was greatly enhanced in SP-D-/- mice as compared to SP-D+/+ mice. Our data provides evidence that SP-D has a significant role to play in the clearance of pneumococci during the early stages of infection in both pulmonary sites and blood.

Bilateral lavage with diluted surfactant improves lung function after unilateral lung contusion in pigs*

OBJECTIVE

This study evaluates the effects of bronchoalveolar lavage with diluted surfactant on unilateral lung contusion-induced lung dysfunction.

DESIGN

Randomized prospective animal study.

SETTING

An animal laboratory.

SUBJECTS

Twenty adult pigs, weighing 25-35 kg.

INTERVENTIONS

Animals were randomly assigned to controls and surfactant treatment. Bilateral lavage with surfactant treatment began 30 mins after unilateral lung contusion. Then 25 mg/kg of body weight diluted Curosurf (5 mg/mL) was applied in a volume of 5 mL/kg of body weight. Observation time was 8 hrs postinjury.

MEASUREMENTS AND MAIN RESULTS

The Pao2/Fio2 ratio fell from 500 to 250 and then recovered gradually in controls and surfactant-treated pigs. After another 4 hrs, the Pao2/Fio2 ratio deteriorated again in controls, but not in surfactant-treated animals. Total compliance fell by 50% after injury but was completely restored by surfactant treatment. Lung contusion increased the median number of neutrophils in bronchoalveolar lavage fluid from 2% to 30% of total cells and peaked >60% at 480 mins in the contused lungs of control pigs. Surfactant-treated pigs had 40% neutrophils at 480 mins without reaching significant difference to controls. The leukocyte neutral proteinase inhibitor increased to 500 ng/mL at 30 mins postinjury in the contused lungs and increased to 2000 ng/mL after surfactant treatment.

CONCLUSIONS

Bilateral bronchoalveolar lavage with diluted surfactant can effectively improve lung function after experimental unilateral lung contusion in pigs.

Respiratory innate immune proteins differentially modulate the neutrophil respiratory burst response to influenza A virus

Oxidants and neutrophils contribute to lung injury during influenza A virus (IAV) infection. Surfactant protein (SP)-D plays a pivotal role in restricting IAV replication and inflammation in the first several days after infection. Despite its potent anti-inflammatory effects in vivo, preincubation of IAV with SP-D in vitro strongly increases neutrophil respiratory burst responses to the virus. Several factors are shown to modify this apparent proinflammatory effect of SP-D. Although multimeric forms of SP-D show dose-dependent augmentation of respiratory burst responses, trimeric, single-arm forms either show no effect or inhibit these responses. Furthermore, if neutrophils are preincubated with multimeric SP-D before IAV is added, oxidant responses to the virus are significantly reduced. The ability of SP-D to increase neutrophil uptake of IAV can be dissociated from enhancement of oxidant responses. Finally, several other innate immune proteins that bind to SP-D and/or IAV (i.e., SP-A, lung glycoprotein-340 or mucin) significantly reduce the ability of SP-D to promote neutrophil oxidant response. As a result, the net effect of bronchoalveolar lavage fluids is to increase neutrophil uptake of IAV while reducing the respiratory burst response to virus.

Surfactant phospholipid DPPC downregulates monocyte respiratory burst via modulation of PKC

Pulmonary surfactant phospholipids have been shown previously to regulate inflammatory functions of human monocytes. This study was undertaken to delineate the mechanisms by which pulmonary surfactant modulates the respiratory burst in a human monocytic cell line, MonoMac-6 (MM6). Preincubation of MM6 cells with the surfactant preparations Survanta, Curosurf, or Exosurf Neonatal inhibited the oxidative response to either lipopolysaccharide (LPS) and zymosan or phorbol 12-myristate 13-acetate (PMA) by up to 50% ( P < 0.01). Preincubation of MM6 cells and human peripheral blood monocytes with dipalmitoyl phosphatidylcholine (DPPC), the major phospholipid component of surfactant, inhibited the oxidative response to zymosan. DPPC did not directly affect the activity of the NADPH oxidase in a MM6 reconstituted cell system, suggesting that DPPC does not affect the assembly of the individual components of this enzyme into a functional unit. The effects of DPPC were evaluated on both LPS/zymosan and PMA activation of protein kinase C (PKC), a ubiquitous intracellular kinase, in MM6 cells. We found that DPPC significantly inhibited the activity of PKC in stimulated cells by 70% ( P < 0.01). Western blotting experiments demonstrated that DPPC was able to attenuate the activation of the PKCδ isoform but not PKCα. These results suggest that DPPC, the major component of pulmonary surfactant, plays a role in modulating leukocyte inflammatory responses in the lung via downregulation of PKC, a mechanism that may involve the PKCδ isoform.

From birds to humans: new concepts on airways relative to alveolar surfactant

Pulmonary surfactant is a surface-active mixture of phospholipids and specific proteins that lines the epithelial surfaces of mammalian lungs. In the alveoli, its main function is to reduce surface tension to ensure that these structures can remain open during respiratory cycles of contraction and expansion. However, surfactant is also present in the conducting airways, even though they are relatively rigid and do not need a system capable of rapidly lowering surface tension in response to compression. This has raised the question whether there is a difference in composition and function between airway and alveolar surfactant. Interest in this question has been stimulated further by the recognition that surfactant also has important functions in the immune defenses of the respiratory tract. In this review, we describe differences that have been reported between human airway and alveolar surfactant. In addition, we draw parallels between human airway surfactant and surfactant from the lungs of birds. The latter are tubular and rigid and do not undergo cycles of contraction and expansion, thus more resembling the human conducting airways than alveoli. Using this as a model, we propose a new hypothesis to explain structural and functional differences between human airway and alveolar surfactant. We suggest that the molecular composition of surfactant is adapted to differences in the architecture of pulmonary surfaces and to the dynamics of surface area changes during respiration.

Treatment of acute respiratory distress syndrome with recombinant surfactant protein C surfactant

We performed a phase I/II trial in North America of a recombinant surfactant protein C-based surfactant (Venticute) as treatment for the acute respiratory distress syndrome. Patients were prospectively randomized to receive either standard therapy or standard therapy plus one of two doses of exogenous surfactant given four times over 24 hours. Surfactant administration was well tolerated. No significant treatment benefit was associated with surfactant treatment. Bronchoalveolar lavage of treated patients at 48 hours reflected the presence of exogenous surfactant components, did not show evidence of improved surface tension lowering function, and had interleukin-6 concentrations that were significantly lower than control group values, consistent with an antiinflammatory treatment effect. The presence of exogenous surfactant was not detected in lavage fluid obtained at 120 hours. Future studies might rationally employ larger surfactant doses and a more prolonged dosing schedule.

Pulmonary surfactant and inflammation in septic adult mice: role of surfactant protein A

Surfactant alterations, alveolar cytokine changes, and the role of surfactant protein (SP)-A in septic mice were investigated. Sepsis was induced via cecal ligation and perforation (CLP). Septic and sham mice were euthanized at 0, 3, 6, 9, 12, 15, and 18 h after surgery. Mice deficient in SP-A and mice that overexpressed SP-A were euthanized 18 h after surgery. In wild-type, sham-operated mice, surfactant pool sizes were similar at all time points, whereas in the CLP groups there was a significant decrease in small-aggregate surfactant pool sizes beginning 6 h after CLP. Interleukin-6 concentrations in bronchoalveolar lavage fluid from septic animals increased from 6 to 18 h after surgery. Identical surfactant alterations and concentrations of cytokines were observed in septic mice that were SP-A deficient or that overexpressed SP-A. In conclusion, alterations of pulmonary surfactant and alveolar cytokines occur simultaneously, 6 h after a systemic insult. In addition, we did not detect a role for SP-A in regulating surfactant phospholipid pool sizes or pulmonary inflammation in septic mice.

Phosphatidylcholine molecular species in lung surfactant: composition in relation to respiratory rate and lung development

Surfactant reduces surface tension at the air-liquid interface of lung alveoli. While dipalmitoylphosphatidylcholine (PC16:0/ 16:0) is its main component, proteins and other phospholipids contribute to the dynamic properties and homeostasis of alveolar surfactant. Among these components are significant amounts of palmitoylmyristoylphosphatidylcholine (PC16:0/ 14:0) and palmitoylpalmitoleoylphosphatidylcholine (PC16:0/ 16:1), whereas in surfactant from the rigid tubular bird lung, PC16:0/14:0 is absent and PC16:0/16:1 strongly diminished. We therefore hypothesized that the concentrations of PC16:0/14:0 and PC16:0/16:1 in surfactants correlate with differences in the respiratory physiology of mammalian species. In surfactants from newborn and adult mice, rats, and pigs, molar fractions of PC16:0/14:0 and PC16:0/16:1 correlated with respiratory rate. Labeling experiments with [methyl-(3)H]choline in mice and perfused rat lungs demonstrated identical alveolar proportions of total and newly synthesized PC16:0/14:0, PC16:0/16:1, and PC16:0/16:0, which were much higher than those of other phosphatidylcholine species. In surfactant from human term and preterm neonates, fractional concentrations not only of PC16:0/16:0 but also of PC16:0/14:0 and PC16:0/ 16:1 increased with maturation. Our data emphasize that PC16:0/14:0 and PC16:0/16:1 may be important surfactant components in alveolar lungs, and that their concentrations are adapted to respiratory physiology.

Dipalmitoylphosphatidylcholine modulates inflammatory functions of monocytic cells independently of mitogen activated protein kinases

Phosphatidylcholine (PC) is the major phospholipid of pulmonary surfactant and it is hypothesized that PC and its subspecies modulate the functions of alveolar macrophages. The most abundant of these subspecies is dipalmitoylphosphatidylcholine (DPPC). This study was undertaken to determine the effect of PC on monocyte function using a human monocytic cell line, MonoMac-6 (MM6). This study showed that preincubation of MM6 cells with DPPC at 125 microg/ml for 2 h inhibited the oxidative response to either zymosan or phorbol-12-myristate-13-acetate (PMA) by 30% (P < 0.001). This inhibition with DPPC was independent of LPS priming. When DPPC was replaced with 1-palmitoyl-2-arachidonoyl phosphatidylcholine (PAPC) there was no inhibition and in contrast a significant increase in oxidant production was observed. We also demonstrated that total PC (tPC; a heterogeneous species of PC from egg) and DPPC but not PAPC significantly inhibited the release of TNF-alpha from MM6 cells (P < 0.05). DPPC did not inhibit phosphorylation of the mitogen activated protein kinases (MAPKs) p44/p42 or p38 in stimulated cells. Measurements of membrane fluidity with spin label EPR spectroscopy indicate that DPPC incorporation significantly alters the membrane fluidity of MM6 cells. These results suggest that DPPC, the major component of pulmonary surfactant, may play a role in modulating leucocyte inflammatory responses in the lung. This may in part be related to membrane effects but does not include alterations in p44/p42 or p38 MAPK signalling.

Surfactant proteins: role in lung physiology and disease in early life

Pulmonary surfactant is an amalgam of proteins and phospholipids which serves to maintain a low surface tension within the alveolar regions of the lungs during changes in lung volume. Recently, two of the surfactant proteins--A and D--have been characterised within the collectin family and found to play important roles in the non-specific host defence of the lung. The field of surfactant biology has attracted the attention of physiologists, biochemists, molecular biologists and clinical scientists in an effort to describe the nature and role of pulmonary surfactant in health and disease. This paper will review the history and content of discoveries in the field of surfactant biology together with pulmonary diseases related to surfactant deficiency or dysfunction.

Distinct effects of surfactant protein A or D deficiency during bacterial infection on the lung

Mice lacking surfactant protein (SP)-A (SP-A-/-) or SP-D (SP-D-/-) and wild-type mice were infected with group B streptococcus or Haemophilus influenzae by intratracheal instillation. Although decreased killing of group B streptococcus and H. influenzae was observed in SP-A-/- mice but not in SP-D-/- mice, deficiency of either SP-A or SP-D was associated with increased inflammation and inflammatory cell recruitment in the lung after infection. Deficient uptake of bacteria by alveolar macrophages was observed in both SP-A- and SP-D-deficient mice. Isolated alveolar macrophages from SP-A-/- mice generated significantly less, whereas those from SP-D-/- mice generated significantly greater superoxide and hydrogen peroxide compared with wild-type alveolar macrophages. In SP-D-/- mice, bacterial killing was associated with increased lung inflammation, increased oxidant production, and decreased macrophage phagocytosis. In contrast, in the absence of SP-A, bacterial killing was decreased and associated with increased lung inflammation, decreased oxidant production, and decreased macrophage phagocytosis. Increased oxidant production likely contributes to effective bacterial killing in the lungs of SP-D-/- mice. The collectins, SP-A and SP-D, play distinct roles during bacterial infection of the lung.

Thirty years of clinical trials in acute respiratory distress syndrome

OBJECTIVE

To systematically review clinical trials in acute respiratory distress syndrome (ARDS).

DATA SOURCES

Computerized bibliographic search of published research and citation review of relevant articles.

STUDY SELECTION

All clinical trials of therapies for ARDS were reviewed. Therapies that have been compared in prospective, randomized trials were the focus of this analysis.

DATA EXTRACTION

Data on population, interventions, and outcomes were obtained by review. Studies were graded for quality of scientific evidence.

MAIN RESULTS

Lung protective ventilator strategy is supported by improved outcome in a single large, prospective trial and a second smaller trial. Other therapies for ARDS, including noninvasive positive pressure ventilation, inverse ratio ventilation, fluid restriction, inhaled nitric oxide, almitrine, prostacyclin, liquid ventilation, surfactant, and immune-modulating therapies, cannot be recommended at this time. Results of small trials using corticosteroids in late ARDS support the need for confirmatory large clinical trials.

CONCLUSIONS

Lung protective ventilator strategy is the first therapy found to improve outcome in ARDS. Trials of prone ventilation and fluid restriction in ARDS and corticosteroids in late ARDS support the need for large, prospective, randomized trials.

Surfactant replacement therapy

Dysfunction of the surfactant system of the lung in the setting of acute lung injury (ALI) is likely to contribute to the pathophysiology of that syndrome. Multiple mechanisms, including injury to alveolar type II cells and inhibition by plasma proteins contribute to this loss of function. Similar injury occurs in animal models of acute lung injury and, in that setting, treatment with exogenous surfactant causes marked improvement in gas exchange. Clinical studies of surfactant treatment of ALI suggest benefit, and definitive phase III trials are now in progress.

Surfactant modulates calcium response of neutrophils to physiologic stimulation via cell membrane depolarization

Pulmonary surfactant (PS) reduces inflammation in the lung by poorly understood mechanisms. We have observed that surfactant-associated proteins (SAP) insert monovalent cation channels in artificial membranes. Neutrophils are primary mediators of acute pulmonary inflammation, and their functions are activated by increases in cytosolic ionized calcium concentration ([Ca2+]) and by changes in membrane potential. We hypothesize that PS inserts SAP-dependent cation channels in neutrophils, causing membrane depolarization, altered [Ca2+] response, and depressed activation. Human neutrophils were isolated, exposed to PS+SAP (1% Survanta), PS-SAP (1% Exosurf), or buffer, and washed before activating with selected stimulants. PS+SAP reduced phorbol ester- and formyl peptide-stimulated adherence and aggregation by 38% (p < 0.05) and 54% (p < 0.02), respectively. PS+SAP also inhibited the formyl peptide-induced [Ca2+] response of neutrophils (p < 0.01), but only in the presence of external Ca2+. Further characterization of this inhibition demonstrated that PS+SAP blocked formyl peptide-induced influx of both Ca2+ and Mn2+, and that this inhibition was present during activation by other neutrophil stimulants (IL-8, immune complexes). Prior depolarization of neutrophils with gramicidin-D similarly inhibited the [Ca2+] response of neutrophils to formyl peptide, and analysis of neutrophil membrane potential by 3,3'-dipentyloxaearbocyanine iodide (diOC5(3)) fluorescence revealed that PS+SAP induced rapid neutrophil depolarization. In contrast, PS-SAP exhibited little effect on neutrophil function, [Ca2+], or membrane potential. We conclude that PS+SAP decreases neutrophil adherence and aggregation responses, blocks Ca2+ influx after physiologic stimulation, and decreases membrane potential. We speculate that these effects are caused by membrane depolarization via SAP-dependent cation channel insertion, and that all of these effects contribute to the antiinflammatory properties of PS+SAP.

Curosurf modulates cAMP accumulation in human monocytes through a membrane-controlled mechanism

The cellular mechanisms by which pulmonary surfactant exerts its effects, including anti-inflammatory or proinflammatory effects, have remained elusive. To address the issue of whether plasma membrane modifications represent a target for these mechanisms, we designed an experimental protocol involving the determination of changes in cAMP levels under membrane-dependent or -independent stimulatory pathways. The effects of a modified natural porcine surfactant, Curosurf, and the major surfactant protein A were evaluated on resting and stimulated cAMP levels of human monocytes. We found that agents that elevate intracellular cAMP exhibit different susceptibilities toward a preexposure to Curosurf. The rise in cAMP induced by membrane-active agents such as cholera toxin or the diterpene forskolin was significantly inhibited by monocyte preexposure to Curosurf. In contrast, the rise in cAMP induced by the membrane-permeant phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine or by the Bordetella pertussis toxin adenylate cyclase-hemolysin was unaffected by Curosurf. Surfactant protein A did not affect either cAMP levels or the inhibitory capacity of Curosurf. We suggest that a plasma membrane-associated event affecting the mechanism underlying the effects of cholera toxin or forskolin is involved in the inhibition of cAMP accumulation caused by Curosurf.

Phagocyte-induced lipid peroxidation of lung surfactant

Surfactant therapy is given routinely to premature newborns with respiratory failure. However, alterations in surfactants have been shown to be a significant factor in some forms of respiratory failure in newborns in animal models of lung injury. To investigate whether antioxidant supplementation might help to protect exogenous surfactant from damage by oxygen free radicals, we examined the influence of vitamin E in combination with surfactant on superoxide production as estimated by the nitroblue tetrazolium reduction test, and measured surfactant peroxidation with a new colorimetric method with or without addition of superoxide dismutase (SOD) or vitamin E. Our results showed that surfactant interacts with free radicals; surfactant reduced superoxide production by neutrophils and was peroxidized when incubated with resting and with stimulated cells. Vitamin E supplementation decreased superoxide radical production and in a dose-dependent manner decreased surfactant peroxidation. The decrease in lipid peroxidation by SOD was not significant. These findings suggest that phagocytes induce lipid peroxidation of lung surfactant, a reaction that might be prevented by antioxidants.

GM-CSF-deficient mice are susceptible to pulmonary group B streptococcal infection

Granulocyte-macrophage colony-stimulating factor (GM-CSF) gene-targeted mice (GM-/-) cleared group B streptococcus (GBS) from the lungs more slowly than wild-type mice. Expression of GM-CSF in the respiratory epithelium of GM-/- mice improved bacterial clearance to levels greater than that in wild-type GM+/+ mice. Acute aerosolization of GM-CSF to GM+/+ mice significantly enhanced clearance of GBS at 24 hours. GBS infection was associated with increased neutrophilic infiltration in lungs of GM-/- mice, while macrophage infiltrates predominated in wild-type mice, suggesting an abnormality in macrophage clearance of bacteria in the absence of GM-CSF. While phagocytosis of GBS was unaltered, production of superoxide radicals and hydrogen peroxide was markedly deficient in macrophages from GM-/- mice. Lipid peroxidation, assessed by measuring the isoprostane 8-iso-PGF2alpha, was decreased in the lungs of GM-/- mice. GM-CSF plays an important role in GBS clearance in vivo, mediated in part by its role in enhancing superoxide and hydrogen peroxide production and bacterial killing by alveolar macrophages.

Modified natural porcine surfactant modulates tobacco smoke-induced stress response in human monocytes

Tobacco smoke (TS) is a potent source of oxidants and oxidative stress is an important mechanism by which TS exerts its toxicity in the lung. We have shown that TS induces heat shock (HS)/stress protein (HSP) synthesis in human monocytes. Pulmonary surfactant (PS) whose major physiological function is to confer mechanical stability to alveoli, also modulates oxidative metabolism and other pro-inflammatory functions of monocytes-macrophages. In order to determine whether PS alters the stress response induced by TS, we incubated human peripheral blood monocytes overnight with modified natural porcine surfactant (Curosurf) (1 mg/ml) before exposure to TS. Curosurf decreased TS-induced, but not HS-induced, expression of the major cytosolic, inducible 72 kD HSP (Hsp70). Furthermore, TS-generated superoxide anions production was significantly decreased by Curosurf in an acellular system, suggesting a direct scavenging effect of PS. We also examined the effects of TS and PS on monocytes ultrastructure. Monocytes incubated with Curosurf presented smoother cell membranes than control monocytes, while TS-induced monocyte vacuolization was, at least in part, prevented by Curosurf. Taken together, our data suggest that PS plays a protective role against oxygen radical-mediated, TS-induced cellular stress responses.

Artificial surfactant (Surfactant TA) modulates adherence and superoxide production of neutrophils

Neutrophils cause lung injuries by releasing proteases and active oxygen radicals in patients with acute respiratory distress syndrome (ARDS). Artificial surfactant is used to replace native surfactant whose functions are deteriorated by serum-derived inhibitors in these patients. We investigated potential interactions between exogenous surfactant (Surfactant TA) and neutrophils in in vivo and in vitro experimental models. Neutrophil alveolitis was induced in hamster lungs by the intratracheal administration of bleomycin (5 mg/kg) on Day 0. Some of the animals were followed by replacement with Surfactant TA (5 and 10 mg/100 g body weight) on Day 1. Alveolar cells were harvested by lung lavage on Day 2. The numbers of the neutrophils obtained from the lungs treated with bleomycin and Surfactant TA were unchanged, but the superoxide production from these cells was significantly decreased when compared with control animals (no Surfactant TA). From the in vitro experiments, Surfactant TA was shown to inhibit adherence and superoxide production of human neutrophils. These effects were derived from the heat-resistant components of Surfactant TA and were mimicked by treatment with liposomes of dipalmitoyl phosphatidylcholine. Surfactant-TA-treated neutrophils were demonstrated to have picnotic nuclei and to express Fas antigens, which were characteristic of apoptotic cells. These results suggest that exogenous Surfactant TA may play an important role not only in improving surfactant functions but in preventing neutrophils from further activation, probably through enhancing apoptosis.

Bronchoalveolar lavage with KL4-surfactant in models of meconium aspiration syndrome

As a model of the meconium aspiration syndrome (MAS) of human infants, adult rabbits and newborn rhesus monkeys received intratracheal instillation of human meconium to induce pulmonary injury. Injured rabbits were ventilated with 100% O2 and divided into four treatment groups, receiving: 1) bronchoalveolar lavages (BAL) with dilute KL4-Surfactant; 2) lavages with equal volumes of sterile saline; 3) a single intratracheal bolus of KL4-Surfactant, 100 mg/kg; and 4) no treatment. The untreated rabbits developed atelectasis, a fall in pressure-volume levels and in partial pressure of O2 in arterial blood (PaO2) from approximately 500 to < 100 mm Hg, and severe pulmonary inflammation between 3 and 5 h after instillation of meconium. Rabbits treated by BAL with dilute KL4-Surfactant showed rapid and sustained recovery of PaO2 to approximately 300 mm Hg within minutes, a return toward normal pressure-volume levels, and diminished inflammation. Rabbits receiving BAL with saline failed to show recovery, and rabbits treated with a bolus of surfactant intratracheally exhibited a transient response by 1-2 h after treatment, but then returned to the initial atelectatic state. Newborn rhesus monkeys, after receiving human meconium intratracheally before the first breath, developed severe loss of pulmonary function. Treatment of these monkeys 1-5 h after birth with BAL with dilute KL4-Surfactant produced clearing of chest radiographs and a rapid improvement in pulmonary function with ratios of partial pressure of O2 in arterial blood to the fraction of O2 in the inspired air rising into the normal range where they remained through the 20-h period of study. The studies indicate that pulmonary function in two models of severe meconium injury respond rapidly to BAL with dilute KL4-Surfactant.

Surfactant protein-A-deficient mice are susceptible to Pseudomonas aeruginosa infection

To determine the role of surfactant protein-A (SP-A) in host defense, the murine SP-A locus was targeted by homologous recombination to produce mice lacking SP-A. SP-A-/- and wild-type mice were infected with mucoid Pseudomonas aeruginosa by intratracheal instillation. Pulmonary bacterial loads were greater in SP-A-/- than in wild-type mice, with increased numbers of mucoid P. aeruginosa in lung homogenates at 6 and 24 h after infection. Pulmonary infiltration with polymorphonuclear leukocytes (PMN) was similar in both groups; however, an earlier influx of PMN into the lung occurred in the SP-A-/- mice. The number of bacteria phagocytosed by alveolar macrophages was decreased in the SP-A-/- mice at 1 h after infection. Superoxide-radical generation by PMN was similar for the SP-A-/- and wild-type mice, but nitrite levels were increased in SP-A-/- mice. Concentrations of tumor necrosis factor-alpha, interleukin-6, and macrophage inflammatory protein-2 (proinflammatory cytokines) were greater in bronchoalveolar lavage fluid at 2 h after infection in SP-A-/- mice. SP-A plays an important role in the pathogenesis of mucoid P. aeruginosa infection in the lung in vivo by enhancing macrophage phagocytosis and clearance of bacteria, and by modifying the inflammatory response.

Endotoxin-stimulated alveolar macrophage recruitment of neutrophils and modulation with exogenous surfactant

OBJECTIVE

To determine whether endotoxin-stimulated alveolar macrophages would attract neutrophils and whether exogenous surfactant treatment would modulate this chemoattraction.

DESIGN

Alveolar macrophages were harvested from bronchoalveolar lavage fluid and neutrophils from the blood of anesthetized guinea pigs.

SUBJECTS

Hartley guinea pigs.

INTERVENTIONS

Alveolar macrophages were suspended in RPMI 1640 and stimulated with 1 microg/mL of lipopolysaccharide (LPS), the supernatant removed and the alveolar macrophages were incubated in either RPMI or RPMI with surfactant at two different doses (292 microg/mL or 875 microg/mL) for 16 hrs.

MEASUREMENTS AND MAIN RESULTS

The supernatant was extracted from the alveolar macrophages and placed in a chemotaxis plate and the migration of neutrophils was measured. Chemotaxis of all cell types to be tested was measured by a change of absorbance on a microplate reader set at 492 nm. Results were compared with alveolar macrophages not stimulated with LPS, RPMI alone, and N formyl-methionyl-leucyl-phenylalanine (FMLP). The supernatant of the stimulated alveolar macrophages increased neutrophil chemotaxis as compared with unstimulated alveolar macrophages, and RPMI (p < .05). Surfactant treatment with 292 microg/mL significantly decreased LPS-stimulated alveolar macrophages induced neutrophil chemotaxis. Treatment with 875 microg/mL of surfactant did not alter neutrophil chemotaxis.

CONCLUSIONS

Alveolar macrophages stimulation with LPS increased the chemotaxis of neutrophils. Treatment with surfactant at a concentration of 875 microg/mL did not alter neutrophil migration; however, treatment with 292 microg/mL significantly decreased neutrophil chemotaxis suggesting that at low concentrations, surfactant inhibits chemokine release and may reduce pulmonary neutrophil sequestration in vivo.

Surfactant replacement therapy. An update on applications

Surfactant replacement therapy has been shown to be an effective and often life-saving treatment for newborn infants with respiratory distress syndrome (RDS). This article provides the clinician with an update regarding the various other applications of surfactant replacement therapy, as well as issues related to surfactant administration for the preparations approved for use in pediatric patients.

Characterization of N-acetylcysteine and ambroxol in anti-oxidant therapy

Reactive free oxygen radicals are known to play an important role in the pathogenesis of various lung diseases such as idiopathic pulmonary fibrosis (IPF), adult respiratory distress syndrome (ARDS) or cystic fibrosis (CF). They can originate from endogenous processes or can be part of exogenous exposures (e.g. ozone, cigarette smoke, asbestos fibres). Consequently, therapeutic enhancement of anti-oxidant defence mechanisms in these lung disorders seems a rational approach. In this regard, N-acetyl-L-cysteine (NAC) and ambroxol have both been frequently investigated. Because of its SH group, NAC scavenges H2O2 (hydrogen peroxide), .OH (hydroxol radical), and HOCl (hypochlorous acid). Furthermore, NAC can easily be deacetylated to cysteine, an important precursor of cellular glutathione synthesis, and thus stimulate the cellular glutathione system. This is most evident in pulmonary diseases characterized by low glutathione levels and high oxidant production by inflammatory cells (e.g. in IPF and ARDS). NAC is an effective drug in the treatment of paracetamol intoxication and may even be protective against side-effects of mutagenic agents. In addition NAC reduces cellular production of pro-inflammatory mediators (e.g. TNF-alpha, IL-1). Also, ambroxol [trans-4-(2-amino-3,5-dibromobenzylamino)-cyclohexane hydrochloride] scavenges oxidants (e.g. .OH, HOCl). Moreover, ambroxol reduces bronchial hyperreactivity, and it is known to stimulate cellular surfactant production. In addition, ambroxol has anti-inflammatory properties owing to its inhibitory effect on the production of cellular cytokines and arachidonic acid metabolites. For both substances effective anti-oxidant and anti-inflammatory function has been validated when used in micromolar concentrations. These levels are attainable in vivo in humans. This paper gives an up-to-date overview about the current knowledge of the hypothesis that oxidant-induced cellular damage underlies the pathogenesis of many human pulmonary diseases, and it discusses the feasibility of anti-oxidant augmentation therapy to the lung by using NAC or ambroxol.