SP-A permeabilizes lipopolysaccharide membranes by forming protein aggregates that extract lipids from the membrane

Effect of lipid-polymer hybrid nanoparticles on the biophysical function and lateral structure of pulmonary surfactant: Mechanistic in vitro studies

The interaction between inhaled drug-loaded nanoparticles and pulmonary surfactant (PS) is critical for the efficacy and safety of inhaled nanomedicines. Here, we investigated the effect of small interfering RNA (siRNA)-loaded lipid-polymer hybrid nanoparticles (LPNs), which are designed for treatment of lung inflammation, on the physiological function of PS. By using biophysical in vitro methods we show that siRNA-loaded LPNs affect the biophysical function and lateral structure of PS. We used the Langmuir monolayer technique to demonstrate that LPNs display intrinsic surface activity by forming interfacial films that collapse at 49 mN/m, and they competitively inhibit the adsorption and spreading of PS components at the air-liquid interface. However, LPNs are excluded from the interface into the aqueous subphase at surface pressures above 49 mN/m, and hence they overcome the PS monolayer film barrier. Epifluorescence microscopy data revealed that LPNs influence the lateral structure of PS by: (i) affecting the nucleation, shape, and growth of compression-driven segregated condensed PS domains, and (ii) facilitating intermixing of liquid-expanded and tilted-condensed domains. However, the total surface area occupied by a highly condensed phase, presumably enriched in the highly surface tension-reducing dipalmitoylphosphatidylcholine, remained constant upon exposure to LPNs. These results suggest that surface-active LPNs influence the lateral structure of PS during translocation from the interface into the subphase, but LPNs do apparently not affect the biophysical function of PS under physiologically relevant conditions.

Cooperative action of SP-A and its trimeric recombinant fragment with polymyxins against Gram-negative respiratory bacteria

The exploration of therapies combining antimicrobial lung proteins and conventional antibiotics is important due to the growing problem of multidrug-resistant bacteria. The aim of this study was to investigate whether human SP-A and a recombinant trimeric fragment (rfhSP-A) have cooperative antimicrobial activity with antibiotics against pathogenic Gram-negative bacteria. We found that SP-A bound the cationic peptide polymyxin B (PMB) with an apparent dissociation constant (K D) of 0.32 ± 0.04 µM. SP-A showed synergistic microbicidal activity with polymyxin B and E, but not with other antibiotics, against three SP-A-resistant pathogenic bacteria: Klebsiella pneumoniae, non-typable Haemophilus influenzae (NTHi), and Pseudomonas aeruginosa. SP-A was not able to bind to K. pneumoniae, NTHi, or to mutant strains thereof expressing long-chain lipopolysaccharides (or lipooligosaccharides) and/or polysaccharide capsules. In the presence of PMB, SP-A induced the formation of SP-A/PMB aggregates that enhance PMB-induced bacterial membrane permeabilization. Furthermore, SP-A bound to a molecular derivative of PMB lacking the acyl chain (PMBN) with a K D of 0.26 ± 0.02 μM, forming SP-A/PMBN aggregates. PMBN has no bactericidal activity but can bind to the outer membrane of Gram-negative bacteria. Surprisingly, SP-A and PMBN showed synergistic bactericidal activity against Gram-negative bacteria. Unlike native supratrimeric SP-A, the trimeric rfhSP-A fragment had small but significant direct bactericidal activity against K. pneumoniae, NTHi, and P. aeruginosa. rfhSP-A did not bind to PMB under physiological conditions but acted additively with PMB and other antibiotics against these pathogenic bacteria. In summary, our results significantly improve our understanding of the antimicrobial actions of SP-A and its synergistic action with PMB. A peptide based on SP-A may aid the therapeutic use of PMB, a relatively cytotoxic antibiotic that is currently being reintroduced into clinics due to the global problem of antibiotic resistance.

Relationship of Notch Signal, Surfactant Protein A, and Indomethacin in Cervix During Preterm Birth: Mast Cell and Jagged-2 May Be Key in Understanding Infection-mediated Preterm Birth

Although it is thought that there is a close relationship between Notch signal and preterm birth, the functioning of this mechanism in the cervix is unknown. The efficacy of surfactants and prostaglandin inhibitors in preterm labor is also still unclear. In this study, 48 female CD-1 mice were distributed to pregnant control (PC), Sham, PBS, indomethacin (2 mg/kg; intraperitoneally), lipopolysaccharides (LPS) (25 μg/100 μl; intrauterine), LPS + IND, and Surfactant Protein A Block (SP-A Block: SP-A B; the anti-SP-A antibody was applied 20 µg/100μl; intrauterine) groups. Tissues were examined by immunohistochemistry, immunofluorescence, and Western blot analysis. LPS administration increased the expression of N1 Dll-1 and Jagged-2 (Jag-2). Although Toll-like receptor (Tlr)-2 significantly increased in the LPS-treated and SP-A-blocked groups, Tlr-4 significantly increased only in the LPS-exposed groups. It was observed that Jag-2 is specifically expressed by mast cells. Overall, this experimental model shows that some protein responses increase throughout the uterus, starting at a specific point on the cervix epithelium. Surfactant Protein A, which we observed to be significantly reduced by LPS, may be associated with the regulation of the epithelial response, especially during preterm delivery due to infection. On the contrary, prostaglandin inhibitors can be considered an option to delay infection-related preterm labor with their dose-dependent effects. Finally, the link between mast cells and Jag-2 could potentially be a control switch for preterm birth.

Mammals' humoral immune proteins and peptides targeting the bacterial envelope: from natural protection to therapeutic applications against multidrug‐resistant Gram ‐negatives

Mammalian innate immunity employs several humoral ‘weapons’ that target the bacterial envelope. The threats posed by the multidrug‐resistant ‘ESKAPE’ Gram‐negative pathogens (Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) are forcing researchers to explore new therapeutic options, including the use of these immune elements. Here we review bacterial envelope‐targeting (peptidoglycan and/or membrane‐targeting) proteins/peptides of the mammalian immune system that are most likely to have therapeutic applications. Firstly we discuss their general features and protective activity against ESKAPE Gram‐negatives in the host. We then gather, integrate, and discuss recent research on experimental therapeutics harnessing their bactericidal power, based on their exogenous administration and also on the discovery of bacterial and/or host targets that improve the performance of this endogenous immunity, as a novel therapeutic concept. We identify weak points and knowledge gaps in current research in this field and suggest areas for future work to obtain successful envelope‐targeting therapeutic options to tackle the challenge of antimicrobial resistance.

Pulmonary surfactant inactivation by β-D-glucan and protective role of surfactant protein A

Pulmonary fungal infections lead to damage of the endogenous lung surfactant system. However, the molecular mechanism underlying surfactant inhibition is unknown. β-D-glucan is the major component of pathogenic fungal cell walls and is also present in organic dust, which increases the risk of respiratory diseases. The objective of this study was to characterize the interaction of this D-glucopyranose polymer with pulmonary surfactant. Our results show that β-D-glucan induced a concentration-dependent inhibition of the surface adsorption, respreading, and surface tension-lowering activity of surfactant preparations containing surfactant proteins SP-B and SP-C. Our data support a new mechanism of surfactant inhibition that consists in the extraction of phospholipid molecules from surfactant membranes by β-D-glucan. As a result, surfactant membranes became more fluid, as demonstrated by fluorescence anisotropy, and showed decreased T_m and transition enthalpy. Surfactant preparations containing surfactant protein A (SP-A) were more resistant to β-D-glucan inhibition. SP-A bound to different β-D-glucans with high affinity (K_d = 1.5 ± 0.1 nM), preventing and reverting β-D-glucan inhibitory effects on surfactant interfacial adsorption and partially abrogating β-D-glucan inhibitory effects on surfactant's reduction of surface tension. We conclude that β-D-glucan inhibits the biophysical function of surfactant preparations lacking SP-A by subtraction of phospholipids from surfactant bilayers and monolayers. The increased resistance of SP-A-containing surfactant preparations to β-D-glucan reinforces its use in surfactant replacement therapy.

Synergistic Action of Antimicrobial Lung Proteins against Klebsiella pneumoniae

As key components of innate immunity, lung antimicrobial proteins play a critical role in warding off invading respiratory pathogens. Lung surfactant protein A (SP-A) exerts synergistic antimicrobial activity with the N-terminal segment of the SP-B proprotein (SP-B^N) against Klebsiella pneumoniae K2 in vivo. However, the factors that govern SP-A/SP-B^N antimicrobial activity are still unclear. The aim of this study was to identify the mechanisms by which SP-A and SP-B^N act synergistically against K. pneumoniae, which is resistant to either protein alone. The effect of these proteins on K. pneumoniae was studied by membrane permeabilization and depolarization assays and transmission electron microscopy. Their effects on model membranes of the outer and inner bacterial membranes were analyzed by differential scanning calorimetry and membrane leakage assays. Our results indicate that the SP-A/SP-B^N complex alters the ultrastructure of K. pneumoniae by binding to lipopolysaccharide molecules present in the outer membrane, forming packing defects in the membrane that may favor the translocation of both proteins to the periplasmic space. The SP-A/SP-B^N complex depolarized and permeabilized the inner membrane, perhaps through the induction of toroidal pores. We conclude that the synergistic antimicrobial activity of SP-A/SP-B^N is based on the capability of this complex, but not either protein alone, to alter the integrity of bacterial membranes.

Restoration of surfactant activity by polymyxin B in lipopolysaccharide-potentiated injury of immature rabbit lungs

During postnatal adaptation pulmonary surfactant may be inactivated by lipopolysaccharide (LPS). We evaluated the effect of surfactant therapy in combination with antibiotic polymyxin B (PxB) in double-hit model of neonatal lung injury. Surfactant (poractant alfa, Curosurf) was exposed to smooth (S) LPS without/with PxB and tested in captive bubble surfactometer. Preterm rabbits received intratracheally saline (control) or S-LPS and were ventilated with 100% oxygen. After 30 min, LPS-treated animals received no treatment, or surfactant (200 mg/kg) without/with 3% PxB; controls received the same dose of surfactant. Animals were ventilated for further 2 h. In vitro, addition of 5% S-LPS to surfactant increased minimum surface tension (γmin) and addition of 1-3% PxB to surfactant/S-LPS mixture restored γmin to low values. Animals only given S-LPS had lower lung compliance and lung gas volume (LGV) compared to surfactant groups. Treatment with surfactant/PxB, but not with surfactant only, restored LGV. Addition of PxB to the surfactant increased the alveolar expansion. S-LPS interferes with surface activity of the pulmonary surfactant and PxB improves the resistance of surfactant to LPS-induced inactivation. In our neonatal model of respiratory distress syndrome surfactant gives positive response even in simultaneous exposure to S-LPS, when enriched with PxB.

Structure of Fungal α Mating Pheromone in Membrane Mimetics Suggests a Possible Role for Regulation at the Water-Membrane Interface

Fusarium oxysporum is a highly destructive plant pathogen and an emerging pathogen of humans. Like other ascomycete fungi, F. oxysporum secretes α-pheromone, a small peptide that functions both as a chemoattractant and as a quorum-sensing signal. Three of the ten amino acid residues of α-pheromone are tryptophan, an amino acid whose sidechain has high affinity for lipid bilayers, suggesting a possible interaction with biological membranes. Here we tested the effect of different lipid environments on α-pheromone structure and function. Using spectroscopic and calorimetric approaches, we show that this peptide interacts with negatively charged model phospholipid vesicles. Fluorescence emission spectroscopy and nuclear magnetic resonance (NMR) measurements revealed a key role of the positively charged groups and Trp residues. Furthermore, NMR-based calculation of the 3D structure in the presence of micelles, formed by lipid surfactants, suggests that α-pheromone can establish an intramolecular disulfide bond between the two cysteine residues during interaction with membranes, but not in the absence of lipid mimetics. Remarkably, this oxidized version of α-pheromone lacks biological activity as a chemoattractant and quorum-sensing molecule. These results suggest the presence of a previously unidentified redox regulated control of α-pheromone activity at the surface of the plasma membrane that could influence the interaction with its cognate G-protein coupled receptor.

Lipid-Protein and Protein-Protein Interactions in the Pulmonary Surfactant System and Their Role in Lung Homeostasis

Pulmonary surfactant is a lipid/protein complex synthesized by the alveolar epithelium and secreted into the airspaces, where it coats and protects the large respiratory air–liquid interface. Surfactant, assembled as a complex network of membranous structures, integrates elements in charge of reducing surface tension to a minimum along the breathing cycle, thus maintaining a large surface open to gas exchange and also protecting the lung and the body from the entrance of a myriad of potentially pathogenic entities. Different molecules in the surfactant establish a multivalent crosstalk with the epithelium, the immune system and the lung microbiota, constituting a crucial platform to sustain homeostasis, under health and disease. This review summarizes some of the most important molecules and interactions within lung surfactant and how multiple lipid–protein and protein–protein interactions contribute to the proper maintenance of an operative respiratory surface.

Alveolar-Capillary Membrane-Related Pulmonary Cells as a Target in Endotoxin-Induced Acute Lung Injury

The main function of the lungs is oxygen transport from the atmosphere into the blood circulation, while it is necessary to keep the pulmonary tissue relatively free of pathogens. This is a difficult task because the respiratory system is constantly exposed to harmful substances entering the lungs by inhalation or via the blood stream. Individual types of lung cells are equipped with the mechanisms that maintain pulmonary homeostasis. Because of the clinical significance of acute respiratory distress syndrome (ARDS) the article refers to the physiological role of alveolar epithelial cells type I and II, endothelial cells, alveolar macrophages, and fibroblasts. However, all these cells can be damaged by lipopolysaccharide (LPS) which can reach the airspaces as the major component of the outer membrane of Gram-negative bacteria, and lead to local and systemic inflammation and toxicity. We also highlight a negative effect of LPS on lung cells related to alveolar-capillary barrier and their response to LPS exposure. Additionally, we describe the molecular mechanism of LPS signal transduction pathway in lung cells.

The Perturbation of Pulmonary Surfactant by Bacterial Lipopolysaccharide and Its Reversal by Polymyxin B: Function and Structure

After inhalation, lipopolysaccharide (LPS) molecules interfere with a pulmonary surfactant, a unique mixture of phospholipids (PLs) and specific proteins that decreases surface tension at the air⁻liquid interphase. We evaluated the behaviour of a clinically used modified porcine pulmonary surfactant (PSUR) in the presence of LPS in a dynamic system mimicking the respiratory cycle. Polymyxin B (PxB), a cyclic amphipathic antibiotic, is able to bind to LPS and to PSUR membranes. We investigated the effect of PxB on the surface properties of the PSUR/LPS system. Particular attention was paid to mechanisms underlying the structural changes in surface-reducing features. The function and structure of the porcine surfactant mixed with LPS and PxB were tested with a pulsating bubble surfactometer, optical microscopy, and small- and wide-angle X-ray scattering (SAXS/WAXS). Only 1% LPS (w/w to surfactant PLs) prevented the PSUR from reaching the necessary low surface tension during area compression. LPS bound to the lipid bilayer of PSUR and disturbed its lamellar structure by swelling. The structural changes were attributed to the surface charge unbalance of the lipid bilayers due to LPS insertion. PxB acts as an inhibitor of structural disarrangement induced by LPS and restores original lamellar packing, as detected by polarised light microscopy and SAXS.

Pulmonary Surfactant and Bacterial Lipopolysaccharide: The Interaction and its Functional Consequences

The respiratory system is constantly exposed to pathogens which enter the lungs by inhalation or via blood stream. Lipopolysaccharide (LPS), also named endotoxin, can reach the airspaces as the major component of the outer membrane of Gram-negative bacteria, and lead to local inflammation and systemic toxicity. LPS affects alveolar type II (ATII) cells and pulmonary surfactant and although surfactant molecule has the effective protective mechanisms, excessive amount of LPS interacts with surfactant film and leads to its inactivation. From immunological point of view, surfactant specific proteins (SPs) SP-A and SP-D are best characterized, however, there is increasing evidence on the involvement of SP-B and SP-C and certain phospholipids in immune reactions. In animal models, the instillation of LPS to the respiratory system induces acute lung injury (ALI). It is of clinical importance that endotoxin-induced lung injury can be favorably influenced by intratracheal instillation of exogenous surfactant. The beneficial effect of this treatment was confirmed for both natural porcine and synthetic surfactants. It is believed that the surfactant preparations have anti-inflammatory properties through regulating cytokine production by inflammatory cells. The mechanism by which LPS interferes with ATII cells and surfactant layer, and its consequences are discussed below.

Elucidation of Lipid Binding Sites on Lung Surfactant Protein A Using X-ray Crystallography, Mutagenesis, and Molecular Dynamics Simulations

Surfactant protein A (SP-A) is a collagenous C-type lectin (collectin) that is critical for pulmonary defense against inhaled microorganisms. Bifunctional avidity of SP-A for pathogen-associated molecular patterns (PAMPs) such as lipid A and for dipalmitoylphosphatidylcholine (DPPC), the major component of surfactant membranes lining the air-liquid interface of the lung, ensures that the protein is poised for first-line interactions with inhaled pathogens. To improve our understanding of the motifs that are required for interactions with microbes and surfactant structures, we explored the role of the tyrosine-rich binding surface on the carbohydrate recognition domain of SP-A in the interaction with DPPC and lipid A using crystallography, site-directed mutagenesis, and molecular dynamics simulations. Critical binding features for DPPC binding include a three-walled tyrosine cage that binds the choline headgroup through cation-π interactions and a positively charged cluster that binds the phosphoryl group. This basic cluster is also critical for binding of lipid A, a bacterial PAMP and target for SP-A. Molecular dynamics simulations further predict that SP-A binds lipid A more tightly than DPPC. These results suggest that the differential binding properties of SP-A favor transfer of the protein from surfactant DPPC to pathogen membranes containing appropriate lipid PAMPs to effect key host defense functions.

Natural Anti-Infective Pulmonary Proteins: In Vivo Cooperative Action of Surfactant Protein SP-A and the Lung Antimicrobial Peptide SP-BN

The anionic antimicrobial peptide SP-B(N), derived from the N-terminal saposin-like domain of the surfactant protein (SP)-B proprotein, and SP-A are lung anti-infective proteins. SP-A-deficient mice are more susceptible than wild-type mice to lung infections, and bacterial killing is enhanced in transgenic mice overexpressing SP-B(N). Despite their potential anti-infective action, in vitro studies indicate that several microorganisms are resistant to SP-A and SP-B(N). In this study, we test the hypothesis that these proteins act synergistically or cooperatively to strengthen each other's microbicidal activity. The results indicate that the proteins acted synergistically in vitro against SP-A- and SP-B(N)-resistant capsulated Klebsiella pneumoniae (serotype K2) at neutral pH. SP-A and SP-B(N) were able to interact in solution (Kd = 0.4 μM), which enabled their binding to bacteria with which SP-A or SP-B(N) alone could not interact. In vivo, we found that treatment of K. pneumoniae-infected mice with SP-A and SP-B(N) conferred more protection against K. pneumoniae infection than each protein individually. SP-A/SP-B(N)-treated infected mice showed significant reduction of bacterial burden, enhanced neutrophil recruitment, and ameliorated lung histopathology with respect to untreated infected mice. In addition, the concentrations of inflammatory mediators in lung homogenates increased early in infection in contrast with the weak inflammatory response of untreated K. pneumoniae-infected mice. Finally, we found that therapeutic treatment with SP-A and SP-B(N) 6 or 24 h after bacterial challenge conferred significant protection against K. pneumoniae infection. These studies show novel anti-infective pathways that could drive development of new strategies against pulmonary infections.

Folding and Intramembraneous BRICHOS Binding of the Prosurfactant Protein C Transmembrane Segment

Surfactant protein C (SP-C) is a novel amyloid protein found in the lung tissue of patients suffering from interstitial lung disease (ILD) due to mutations in the gene of the precursor protein pro-SP-C. SP-C is a small α-helical hydrophobic protein with an unusually high content of valine residues. SP-C is prone to convert into β-sheet aggregates, forming amyloid fibrils. Nature's way of solving this folding problem is to include a BRICHOS domain in pro-SP-C, which functions as a chaperone for SP-C during biosynthesis. Mutations in the pro-SP-C BRICHOS domain or linker region lead to amyloid formation of the SP-C protein and ILD. In this study, we used an in vitro transcription/translation system to study translocon-mediated folding of the WT pro-SP-C poly-Val and a designed poly-Leu transmembrane (TM) segment in the endoplasmic reticulum (ER) membrane. Furthermore, to understand how the pro-SP-C BRICHOS domain present in the ER lumen can interact with the TM segment of pro-SP-C, we studied the membrane insertion properties of the recombinant form of the pro-SP-C BRICHOS domain and two ILD-associated mutants. The results show that the co-translational folding of the WT pro-SP-C TM segment is inefficient, that the BRICHOS domain inserts into superficial parts of fluid membranes, and that BRICHOS membrane insertion is promoted by poly-Val peptides present in the membrane. In contrast, one BRICHOS and one non-BRICHOS ILD-associated mutant could not insert into membranes. These findings support a chaperone function of the BRICHOS domain, possibly together with the linker region, during pro-SP-C biosynthesis in the ER.

In vitro interaction between SURFACEN® and surfactant protein A against Leishmania amazonensis

Leishmaniasis is caused by a parasite of the Leishmania genus, affecting more than 12 million people in 98 countries. The control of leishmaniasis remains a serious problem. There are currently no vaccines for leishmaniasis. The drugs available are toxic, expensive and frequently ineffective. The in vitro activity of SURFACEN® and SP-A against Leishmania amazonensis was evaluated. The combination of both products resulted in a synergic pharmacology effect, demonstrated by a fractional inhibitory concentration index <0.5. A more effective combination was a SURFACEN/SP-A ratio of 4:1, using a method of fixed ratio. The therapeutic effect of SURFACEN and SP-A as antileishmanial compounds was demonstrated, with a potentiation of activity when they were incubated in conjunction. Our results propose an exploration of these products in order to design new formulations against the Leishmania parasite.

Pulmonary surfactant protein A-induced changes in the molecular conformation of bacterial deep-rough LPS lead to reduced activity on human macrophages

The lung is constantly exposed to immune stimulation by LPS from inhaled microorganisms. A primary mechanism to maintain immune homeostasis is based on anti-inflammatory regulation by surfactant protein A (SP-A), a secreted component of lung innate immunity. The architecture of LPS aggregates is strongly associated with biological activity. We therefore investigated whether SP-A affects the physico-chemical properties of LPS. Determination of the three-dimensional aggregate structure of LPS by small-angle X-ray scattering demonstrated that SP-A induced the formation of multi-lamellar aggregate structures. Determination of the acyl-chain-fluidity of LPS aggregates by Fourier transform infrared (FTIR) spectroscopy showed that the phase transition temperature of LPS was reduced in the presence of SP-A. The phosphate groups at the diglucosamine backbone of LPS represent important functional groups for the bioactivity of LPS. FTIR analysis revealed changes in the vibrational bands νas PO-(2), indicating an interaction of SP-A with the 1-phosphate, but not with the 4'-phosphate. The physico-chemical changes induced by SP-A were associated with up to 90% reduction in LPS-induced TNF-α-production by human macrophages. In conclusion, our data demonstrate that the SP-A/LPS interaction induces conformational changes in LPS aggregates leading to biologically less active structures, thereby providing a new molecular mechanism of immune modulation by SP-A.

Differential scanning calorimetry of protein-lipid interactions

Differential scanning calorimetry (DSC) is a highly sensitive non-perturbing technique for measuring the thermodynamic properties of thermally induced transitions. This technique is particularly useful for the characterization of lipid/protein interactions. This chapter presents an introduction to DSC instrumentation, basic theory, and methods and describes DSC applications for characterizing protein effects on model lipid membranes. Examples of the use of DSC for the evaluation of protein effects on modulation of membrane domains and membrane stability are given.

Staphylococcus aureus proteases degrade lung surfactant protein A potentially impairing innate immunity of the lung

The pulmonary surfactant is a complex mixture of lipids and proteins that is important for respiratory lung functions, which also provides the first line of innate immune defense. Pulmonary surfactant protein-A (SP-A) is a major surfactant component with immune functions with importance during Staphylococcus aureus infections that has been demonstrated in numerous studies. The current study showed that S. aureus can efficiently cleave the SP-A protein using its arsenal of proteolytic enzymes. This degradation appears to be mediated by cysteine proteases, in particular staphopain A (ScpA). The staphopain-mediated proteolysis of SP-A resulted in a decrease or complete abolishment of SP-A biological activity, including the promotion of S. aureus phagocytosis by neutrophils, aggregation of Gram-negative bacteria and bacterial cell adherence to epithelium. Significantly, ScpA has also efficiently degraded SP-A in complete bronchi-alveolar lavage fluid from human lungs. This indicates that staphopain activity in the lungs is resistant to protease inhibitors, thus suggesting that SP-A can be cleaved in vivo. Collectively, this study showed that the S. aureus protease ScpA is an important virulence factor that may impair innate immunity of the lungs.

Pseudomonas aeruginosa elastase provides an escape from phagocytosis by degrading the pulmonary surfactant protein-A

Pseudomonas aeruginosa is an opportunistic pathogen that causes both acute pneumonitis in immunocompromised patients and chronic lung infections in individuals with cystic fibrosis and other bronchiectasis. Over 75% of clinical isolates of P. aeruginosa secrete elastase B (LasB), an elastolytic metalloproteinase that is encoded by the lasB gene. Previously, in vitro studies have demonstrated that LasB degrades a number of components in both the innate and adaptive immune systems. These include surfactant proteins, antibacterial peptides, cytokines, chemokines and immunoglobulins. However, the contribution of LasB to lung infection by P. aeruginosa and to inactivation of pulmonary innate immunity in vivo needs more clarification. In this study, we examined the mechanisms underlying enhanced clearance of the ΔlasB mutant in mouse lungs. The ΔlasB mutant was attenuated in virulence when compared to the wild-type strain PAO1 during lung infection in SP-A+/+ mice. However, the ΔlasB mutant was as virulent as PAO1 in the lungs of SP-A⁻/⁻ mice. Detailed analysis showed that the ΔlasB mutant was more susceptible to SP-A-mediated opsonization but not membrane permeabilization. In vitro and in vivo phagocytosis experiments revealed that SP-A augmented the phagocytosis of ΔlasB mutant bacteria more efficiently than the isogenic wild-type PAO1. The ΔlasB mutant was found to have a severely reduced ability to degrade SP-A, consequently making it unable to evade opsonization by the collectin during phagocytosis. These results suggest that P. aeruginosa LasB protects against SP-A-mediated opsonization by degrading the collectin.

Bacterial lipopolysaccharide promotes destabilization of lung surfactant-like films

The airspaces are lined with a dipalmitoylphosphatidylcholine (DPPC)-rich film called pulmonary surfactant, which is named for its ability to maintain normal respiratory mechanics by reducing surface tension at the air-liquid interface. Inhaled airborne particles containing bacterial lipopolysaccharide (LPS) may incorporate into the surfactant monolayer. In this study, we evaluated the effect of smooth LPS (S-LPS), containing the entire core oligosaccharide region and the O-antigen, on the biophysical properties of lung surfactant-like films composed of either DPPC or DPPC/palmitoyloleoylphosphatidylglycerol (POPG)/palmitic acid (PA) (28:9:5.6, w/w/w). Our results show that low amounts of S-LPS fluidized DPPC monolayers, as demonstrated by fluorescence microscopy and changes in the compressibility modulus. This promoted early collapse and prevented the attainment of high surface pressures. These destabilizing effects could not be relieved by repeated compression-expansion cycles. Similar effects were observed with surfactant-like films composed of DPPC/POPG/PA. On the other hand, the interaction of SP-A, a surfactant membrane-associated alveolar protein that also binds to LPS, with surfactant-like films containing S-LPS increased monolayer destabilization due to the extraction of lipid molecules from the monolayer, leading to the dissolution of monolayer material in the aqueous subphase. This suggests that SP-A may act as an LPS scavenger.

Surfactant protein A (SP-A)-tacrolimus complexes have a greater anti-inflammatory effect than either SP-A or tacrolimus alone on human macrophage-like U937 cells

Intratracheal administration of immunosuppressive agents to the lung is a novel treatment after lung transplantation. Nanoparticles of tacrolimus (FK506) might interact with human SP-A, which is the most abundant lipoprotein in the alveolar fluid. This study was undertaken to determine whether the formation of FK506/SP-A complexes interferes with FK506 immunosuppressive actions on stimulated human macrophage-like U937 cells. We found that SP-A was avidly bound to FK506 (K(d) = 35 ± 4nM), as determined by solid phase-binding assays and dynamic light scattering. Free FK506, at concentrations ≤ 1 μM, had no effect on the inflammatory response of LPS-stimulated U937 macrophages. However, coincubation of FK506 and SP-A, at concentrations where each component alone did not affect LPS-stimulated macrophage response, significantly inhibited LPS-induced NF-κB activation and TNF-alpha secretion. Free FK506, but not FK506/SP-A, functioned as substrate for the efflux transporter P-glycoprotein. FK506 bound to SP-A was delivered to macrophages by endocytosis, since several endocytosis inhibitors blocked FK506/SP-A anti-inflammatory effects. This process depended partly on SP-A binding to its receptor, SP-R210. These results indicate that FK506/SP-A complexes have a greater anti-inflammatory effect than either FK506 or SP-A alone and suggest that SP-A strengthened FK506 anti-inflammatory activity by facilitating FK506 entrance into the cell, overcoming P-glycoprotein.

Surfactant protein A (SP-A)-mediated clearance of Staphylococcus aureus involves binding of SP-A to the staphylococcal adhesin eap and the macrophage receptors SP-A receptor 210 and scavenger receptor class A

Staphylococcus aureus causes life-threatening pneumonia in hospitals and deadly superinfection during viral influenza. The current study investigated the role of surfactant protein A (SP-A) in opsonization and clearance of S. aureus. Previous studies showed that SP-A mediates phagocytosis via the SP-A receptor 210 (SP-R210). Here, we show that SP-R210 mediates binding and control of SP-A-opsonized S. aureus by macrophages. We determined that SP-A binds S. aureus through the extracellular adhesin Eap. Consequently, SP-A enhanced macrophage uptake of Eap-expressing (Eap(+)) but not Eap-deficient (Eap(-)) S. aureus. In a reciprocal fashion, SP-A failed to enhance uptake of Eap(+) S. aureus in peritoneal Raw264.7 macrophages with a dominant negative mutation (SP-R210(DN)) blocking surface expression of SP-R210. Accordingly, WT mice cleared infection with Eap(+) but succumbed to sublethal infection with Eap- S. aureus. However, SP-R210(DN) cells compensated by increasing non-opsonic phagocytosis of Eap(+) S. aureus via the scavenger receptor scavenger receptor class A (SR-A), while non-opsonic uptake of Eap(-) S. aureus was impaired. Macrophages express two isoforms: SP-R210(L) and SP-R210(S). The results show that WT alveolar macrophages are distinguished by expression of SP-R210(L), whereas SR-A(-/-) alveolar macrophages are deficient in SP-R210(L) expressing only SP-R210(S). Accordingly, SR-A(-/-) mice were highly susceptible to both Eap(+) and Eap(-) S. aureus. The lungs of susceptible mice generated abnormal inflammatory responses that were associated with impaired killing and persistence of S. aureus infection in the lung. In conclusion, alveolar macrophage SP-R210(L) mediates recognition and killing of SP-A-opsonized S. aureus in vivo, coordinating inflammatory responses and resolution of S. aureus pneumonia through interaction with SR-A.

Pulmonary surfactant: an immunological perspective

Pulmonary surfactant has two crucial roles in respiratory function; first, as a biophysical entity it reduces surface tension at the air water interface, facilitating gas exchange and alveolar stability during breathing, and, second, as an innate component of the lung's immune system it helps maintain sterility and balance immune reactions in the distal airways. Pulmonary surfactant consists of 90% lipids and 10% protein. There are four surfactant proteins named SP-A, SP-B, SP-C, and SP-D; their distinct interactions with surfactant phospholipids are necessary for the ultra-structural organization, stability, metabolism, and lowering of surface tension. In addition, SP-A and SP-D bind pathogens, inflict damage to microbial membranes, and regulate microbial phagocytosis and activation or deactivation of inflammatory responses by alveolar macrophages. SP-A and SP-D, also known as pulmonary collectins, mediate microbial phagocytosis via SP-A and SP-D receptors and the coordinated induction of other innate receptors. Several receptors (SP-R210, CD91/calreticulin, SIRPalpha, and toll-like receptors) mediate the immunological functions of SP-A and SP-D. However, accumulating evidence indicate that SP-B and SP-C and one or more lipid constituents of surfactant share similar immuno-regulatory properties as SP-A and SP-D. The present review discusses current knowledge on the interaction of surfactant with lung innate host defense.