Aggregation and opsonization of type A but not type B Hemophilus influenzae by surfactant protein A

Surfactant protein A promotes atherosclerosis through mediating macrophage foam cell formation

BACKGROUND

Atherosclerosis is a progressive inflammatory disease where macrophage foam cells play a central role in the pathogenesis. Surfactant protein A (SPA) is a lipid-associating protein involved with regulating macrophage function in various inflammatory diseases. However, the role of SPA in atherosclerosis and macrophage foam cell formation has not been investigated.

METHODS

Primary resident peritoneal macrophages were extracted from wildtype (WT) and SPA deficient (SPA -/- ) mice to determine the functional effects of SPA in macrophage foam cell formation. SPA expression was assessed in healthy vessels and atherosclerotic aortic tissue from the human coronary artery and WT or apolipoprotein e-deficient (ApoE -/- ) mice brachiocephalic arteries fed high fat diets (HFD) for 4 weeks. Hypercholesteremic WT and SPA -/- mice fed a HFD for 6 weeks were investigated for atherosclerotic lesions in vivo .

RESULTS

In vitro experiments revealed that global SPA deficiency reduced intracellular cholesterol accumulation and macrophage foam cell formation. Mechanistically, SPA -/- dramatically decreased CD36 cellular and mRNA expression. SPA expression was increased in atherosclerotic lesions in humans and ApoE -/- mice. In vivo SPA deficiency attenuated atherosclerosis and reduced the number of lesion-associated macrophage foam cells.

CONCLUSIONS

Our results elucidate that SPA is a novel factor for atherosclerosis development. SPA enhances macrophage foam cell formation and atherosclerosis through increasing scavenger receptor cluster of differentiation antigen 36 (CD36) expression.

Role of Surfactant Protein D in Experimental Otitis Media

Surfactant protein D (SP-D) is a C-type collectin and plays an important role in innate immunity and homeostasis in the lung. This study studied SP-D role in the nontypeable Haemophilus influenzae (NTHi)-induced otitis media (OM) mouse model. Wild-type C57BL/6 (WT) and SP-D knockout (KO) mice were used in this study. Mice were injected in the middle ear (ME) with 5 μL of NTHi bacterial solution (3.5 × 105 CFU/ear) or with the same volume of sterile saline (control). Mice were sacrificed at 3 time points, days 1, 3, and 7, after treatment. We found SP-D expression in the Eustachian tube (ET) and ME mucosa of WT mice but not in SP-D KO mice. After infection, SP-D KO mice showed more intense inflammatory changes evidenced by the increased mucosal thickness and inflammatory cell infiltration in the ME and ET compared to WT mice (p < 0.05). Increased bacterial colony-forming units and cytokine (IL-6 and IL-1β) levels in the ear washing fluid of infected SP-D KO mice were compared to infected WT mice. Molecular analysis revealed higher levels of NF-κB and NLRP3 activation in infected SP-D KO compared to WT mice (p < 0.05). In vitro studies demonstrated that SP-D significantly induced NTHi bacterial aggregation and enhanced bacterial phagocytosis by macrophages (p < 0.05). Furthermore, human ME epithelial cells showed a dose-dependent increased expression of NLRP3 and SP-D proteins after LPS treatment. We conclude that SP-D plays a critical role in innate immunity and disease resolution through enhancing host defense and regulating inflammatory NF-κB and NLRP3 activation in experimental OM mice.

Collectins: Innate Immune Pattern Recognition Molecules

Collectins are collagen-containing C-type (calcium-dependent) lectins which are important pathogen pattern recognising innate immune molecules. Their primary structure is characterised by an N-terminal, triple-helical collagenous region made up of Gly-X-Y repeats, an a-helical coiled-coil trimerising neck region, and a C-terminal C-type lectin or carbohydrate recognition domain (CRD). Further oligomerisation of this primary structure can give rise to more complex and multimeric structures that can be seen under electron microscope. Collectins can be found in serum as well as in a range of tissues at the mucosal surfaces. Mannanbinding lectin can activate the complement system while other members of the collectin family are extremely versatile in recognising a diverse range of pathogens via their CRDs and bring about effector functions designed at the clearance of invading pathogens. These mechanisms include opsonisation, enhancement of phagocytosis, triggering superoxidative burst and nitric oxide production. Collectins can also potentiate the adaptive immune response via antigen presenting cells such as macrophages and dendritic cells through modulation of cytokines and chemokines, thus they can act as a link between innate and adaptive immunity. This chapter describes the structure-function relationships of collectins, their diverse functions, and their interaction with viruses, bacteria, fungi and parasites.

Innate immunity of surfactant protein A in experimental otitis media

Surfactant protein A (SP-A) plays an important role in innate immune response and host defense against various microorganisms through opsonization and complement activation. To investigate the role of SP-A in non-typeable Haemophilus influenzae (NTHi)-induced acute otitis media, this study used wild type C57BL/6 (WT) and SP-A knockout (KO) mice. We divided mice into an infection group in which the middle ear (ME) was injected with NTHi and a control group that received the same treatment using normal saline. Mice were sacrificed on d 1, 3, and 7 after treatment. Temporal bone samples were fixed for histological, cellular, and molecular analyses. Ear washing fluid (EWF) was collected for culture and analyses of pro-inflammatory cytokines and inflammatory cells. SP-A-mediated bacterial aggregation and killing and phagocytosis by macrophages were studied in vitro. SP-A expression was detected in the ME and Eustachian tube mucosa of WT mice but not KO mice. After infection, KO mice showed more severe inflammation evidenced by increased ME mucosal thickness and inflammatory cell infiltration and higher NF-κB activation compared to WT mice. The levels of IL-6 and IL-1β in the EWF of infected KO mice were higher compared to infected WT mice on d 1. Our studies demonstrated that SP-A mediated NTHi aggregation and killing and enhanced bacterial phagocytosis by macrophages in vitro and modulated inflammation of the ME in otitis media in vivo.

Structure, genetics and function of the pulmonary associated surfactant proteins A and D: The extra-pulmonary role of these C type lectins

The collectins family encompasses several collagenous Ca²⁺-dependent defense lectins that are described as pathogen recognition molecules. They play an important role in both adaptive and innate immunity. Surfactant proteins A and D are two of these proteins which were initially discovered in association with surfactant in the pulmonary system. The structure, immune and inflammatory functions, and genetic variations have been well described in relation to their roles, function and pathophysiology in the pulmonary system. Subsequently, these proteins have been discovered in a wide range of other organs and organ systems. The role of these proteins outside the pulmonary system is currently an active area of research. This review intends to provide a current overview of the genetics, structure and extra-pulmonary functions of the surfactant collectin proteins.

Identification of CD245 as myosin 18A, a receptor for surfactant A: A novel pathway for activating human NK lymphocytes

CD245 is a human surface antigen expressed on peripheral blood lymphocytes, initially delineated by two monoclonal antibodies DY12 and DY35. Until now, CD245 molecular and functional characteristics remained largely unknown. We combined immunological and proteomic approaches and identified CD245 as the unconventional myosin 18A, a highly conserved motor enzyme reported as a receptor for the surfactant protein A (SP-A), that plays a critical role in cytoskeleton organization and Golgi budding. We report that the recruitment of CD245 strongly enhanced NK cell cytotoxicity. Further, we show that the enhancement of the NK lymphocytes killing ability toward CD137-ligand expressing target cells could result from the induction of CD137 expression following CD245 engagement. The SP-A receptor could therefore represent a novel and promising target in cancer immunotherapy.

Type IV pilus of Pseudomonas aeruginosa confers resistance to antimicrobial activities of the pulmonary surfactant protein-A

Pseudomonas aeruginosa(PA) is a Gram-negative bacterial pathogen commonly associated with chronic lung infections. Previously, we have identified several PA virulence factors that are important for resistance to the surfactant protein-A (SP-A), a pulmonary innate immunity protein that mediates bacterial opsonization and membrane permeabilization. In this study, we demonstrate that the type IV pilus (Tfp) is important in the resistance of PA to the antibacterial effects of SP-A. The Tfp-deficient mutant ΔpilA is severely attenuated in an acute pneumonia model of infection in the lungs of wild-type mice, but is virulent in the lungs of SP-A(-/-) mice. The ΔpilA bacteria are more susceptible to SP-A-mediated aggregation and opsonization. In addition, the integrity of the outer membranes of ΔpilA bacteria is compromised, rendering them more susceptible to SP-A-mediated membrane permeabilization. By comparing Tfp extension and retraction mutants, we demonstrate that the increased susceptibility of ΔpilA to SP-A-mediated opsonization requires the total absence of Tfp from PA cells. Finally, we provide evidence of increased expression of nonpilus adhesin OprH that may serve as an SP-A ligand, resulting in increased phagocytosis and preferential pulmonary clearance of ΔpilA.

Genetic complexity of the human surfactant-associated proteins SP-A1 and SP-A2

Pulmonary surfactant protein A (SP-A) plays a key role in innate lung host defense, in surfactant-related functions, and in parturition. In the course of evolution, the genetic complexity of SP-A has increased, particularly in the regulatory regions (i.e. promoter, untranslated regions). Although most species have a single SP-A gene, two genes encode SP-A in humans and primates (SFTPA1 and SFTPA2). This may account for the multiple functions attributed to human SP-A, as well as the regulatory complexity of its expression by a relatively diverse set of protein and non-protein cellular factors. The interplay between enhancer cis-acting DNA sequences and trans-acting proteins that recognize these DNA elements is essential for gene regulation, primarily at the transcription initiation level. Furthermore, regulation at the mRNA level is essential to ensure proper physiological levels of SP-A under different conditions. To date, numerous studies have shown significant complexity of the regulation of SP-A expression at different levels, including transcription, splicing, mRNA decay, and translation. A number of trans-acting factors have also been described to play a role in the control of SP-A expression. The aim of this report is to describe the genetic complexity of the SFTPA1 and SFTPA2 genes, as well as to review regulatory mechanisms that control SP-A expression in humans and other animal species.

An Insight into the Diverse Roles of Surfactant Proteins, SP-A and SP-D in Innate and Adaptive Immunity

Surfactant proteins SP-A and SP-D are hydrophilic, collagen-containing calcium-dependent lectins, which appear to have a range of innate immune functions at pulmonary as well as extrapulmonary sites. These proteins bind to target ligands on pathogens, allergens, and apoptotic cells, via C-terminal homotrimeric carbohydrate recognition domains, while the collagen region brings about the effector functions via its interaction with cell surface receptors. SP-A and SP-D deal with various pathogens, using a range of innate immune mechanisms such as agglutination/aggregation, enhancement of phagocytosis, and killing mechanisms by phagocytic cells and direct growth inhibition. SP-A and SP-D have also been shown to be involved in the control of pulmonary inflammation including allergy and asthma. Emerging evidence suggest that SP-A and SP-D are capable of linking innate immunity with adaptive immunity that includes modulation of dendritic cell function and helper T cell polarization. This review enumerates immunological properties of SP-A and SP-D inside and outside lungs and discusses their importance in human health and disease.

Toll-like receptors and their adapter molecules

Toll-like receptors (TLR) are among key receptors of the innate mammalian immune system. Receptors of this family are able to recognize specific highly conserved molecular regions (patterns) in pathogen structures, thus initiating reactions of both innate and acquired immune response finally resulting in the elimination of the pathogen. In this case every individual TLR type is able to bind a broad spectrum of molecules of microbial origin characterized by different chemical properties and structures. Recent data demonstrate the existence of a multistep mechanism of the TLR recognition of the pathogen in which, in addition to receptors proper, the involvement of different adapter molecules is necessary. However, functions of separate adapter molecules as well as the principles of formation of a multicomponent system of ligand-specific recognition are still not quite understandable. We describe all identified as well as possible (candidate) adapter TLR molecules by giving their brief characteristics, and we also propose generalized possible variants of the TLR ligand-specific recognition with involvement of adapter molecules.

Empiric antibiotics are justified for infants with respiratory syncytial virus lower respiratory tract infection presenting with respiratory failure: a prospective study and evidence review

OBJECTIVE

Although some studies indicate a low risk of serious bacterial infection in infants with respiratory syncytial virus (RSV), these studies focused on patients who did not progress to respiratory failure. We hypothesized the composite diagnosis of concomitant bacterial pneumonia (CBP) is common in lower risk infants with RSV who present in respiratory failure. The aim of the study was to investigate the incidence of CBP in low-risk infants mechanically ventilated for RSV respiratory failure and to compare the results with other studies searched for in MEDLINE.

DESIGN

Prospective, descriptive study, and literature review. Two MEDLINE searches were done using the terms 1) respiratory syncytial virus (RSV) and pneumonia, and 2) RSV, pneumonia, and antibiotics.

SETTING

Tertiary pediatric intensive care unit (PICU) in the Northeast United States.

PATIENTS

We prospectively enrolled 23 infants admitted to our PICU with RSV infection and respiratory failure over a 27-month period.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

All infants were intubated on arrival or soon thereafter; 22 had diagnostic tracheal aspiration performed, and 20 had blood cultures obtained shortly after admission. All had white blood cell count, temperature measured, and chest radiograph. Only one had antibiotics before culture. The length of mechanical ventilation, PICU course, and hospital stay were recorded.The primary outcome variable was the composite diagnosis of CBP as determined by the following criteria: 1) isolation of pathogenic bacteria from a tracheal aspirate, 2) blood culture, 3) chest radiograph, 4) temperature abnormality, and 5) peripheral white blood cell count. In our study, 7 infants met four criteria (probable pneumonia); 6 met three criteria (possible pneumonia); and 10 infants met less than three criteria. By tracheal aspirate criteria alone, 9 of 23 (39%) had probable pneumonia and 9 of 23 had possible pneumonia by previously published criteria. The mean length of mechanical ventilation for 7 infants who met four criteria was 10 +/- 2.7 (sem) days; for 6 infants who met three criteria, 10.5 +/- 2.1 days; and for infants who met less than three criteria 7.4 +/- 0.9 days. The mean PICU stay was 14.3 +/- 3.6 days for infants who met four criteria; 14.3 +/- 3.0 days for infants who met three criteria; and 9.9 +/- 1.4 days for infants who met less than three criteria. The mean hospital stay was 16.3 +/- 3.4 for infants who met four criteria; 18.7 +/- 2.8 days for infants who met three criteria; and 24.8 +/- 9.6 days for infants who met less than three criteria. These differences were not statistically significant. A MEDLINE search was performed using the terms 1) RSV and pneumonia, and 2) RSV, pneumonia, and antibiotics.

CONCLUSIONS

While the small size of this study does not permit definitive conclusions, these data, in combination with other data from the literature, suggest that composite evidence of bacterial pneumonia in otherwise low-risk infants with RSV presenting with respiratory failure is 20% or higher and the use of empirical antibiotics for 24 to 48 hrs pending culture results may be justified and could be used until CBP is excluded.

CYTOKINE STIMULATION BYPSEUDOMONAS AERUGINOSA—STRAIN VARIATION AND MODULATION BY PULMONARY SURFACTANT

Pulmonary surfactant and its components are part of the first-line immune defense within the lung. Here the authors show that the surfactant protein (SP) SP-D, but not SP-A, agglutinates some clinical isolates of Pseudomonas aeruginosa and Stenotrophomonas maltophilia. No agglutination of Staphylococcus aureus or Burkholderia cepacia was observed. The SP-D-induced agglutination of P. aeruginosa was not dependent on a specific lipopolysaccharide (LPS) serotype. The authors also show that SP-D, but not SP-A, increased the tumor necrosis factor (TNF alpha) release from human monocytic cells in response to a subset of P. aeruginosa and P. aeruginosa LPS. A clinical preparation of surfactant (Alveofact) blocked the TNF alpha release from monocytic cells induced by P. aeruginosa or its LPS. SP-A reversed the inhibitory effect of Alveofact in 6/8 strains of P. aeruginosa and 2/9 preparations of P. aeruginosa LPS. SP-D did not significantly alter the TNF alpha production induced by vital P. aeruginosa in the presence of Alveofact but markedly increased the TNF alpha release induced by a preparation of rough and smooth P. aeruginosa LPS. In summary, this study shows that the immunomodulatory properties of SP-A and SP-D specifically depend on the colonizing strain of P. aeruginosa. In addition, the authors show that the function of SP-A and SP-D is modulated in the presence of surfactant lipids.

On the significance of Surfactant Protein-A within the human lungs

Surfactant Protein-A (SP-A) is the most prominent among four proteins in the pulmonary surfactant-system. SP-A is expressed by alveolar epithelial cells type II as well as by a portion of non small cell lung carcinomas (NSCLC).

The expression of SP-A is complexly regulated on the transcriptional and the chromosomal level. SP-A is a major player in the pulmonary cytokine-network and moreover has been described to act in the pulmonary host defense.

By the use of cell culture or animal models the functional properties have been repeatedly shown in many aspects, often bearing surprising properties which strongly indicate the physiological importance of SP-A. To date SP-A is recognized as a molecule essential for pulmonary development, structure and function. An upcoming number of reports deals with the role of SP-A for pulmonary pathology. This article gives an overview about the state of knowledge on SP-A focused in applications for human pulmonary disorders and points out the importance for pathology-orientated research approaches using immunohistochemistry or in situ hybridization as promising methods to further elucidate the role of this molecule in adult lung diseases.

Role of lung surfactant in respiratory disease: current knowledge in large animal medicine

Lung surfactant is produced by type II alveolar cells as a mixture of phospholipids, surfactant proteins, and neutral lipids. Surfactant lowers alveolar surface tension and is crucial for the prevention of alveolar collapse. In addition, surfactant contributes to smaller airway patency and improves mucociliary clearance. Surfactant-specific proteins are part of the innate immune defense mechanisms of the lung. Lung surfactant alterations have been described in a number of respiratory diseases. Surfactant deficiency (quantitative deficit of surfactant) in premature animals causes neonatal respiratory distress syndrome. Surfactant dysfunction (qualitative changes in surfactant) has been implicated in the pathophysiology of acute respiratory distress syndrome and asthma. Analysis of surfactant from amniotic fluid allows assessment of fetal lung maturity (FLM) in the human fetus and exogenous surfactant replacement therapy is part of the standard care in premature human infants. In contrast to human medicine, use and success of FLM testing or surfactant replacement therapy remain limited in veterinary medicine. Lung surfactant has been studied in large animal models of human disease. However, only a few reports exist on lung surfactant alterations in naturally occurring respiratory disease in large animals. This article gives a general review on the role of lung surfactant in respiratory disease followed by an overview of our current knowledge on surfactant in large animal veterinary medicine.

Lung surfactant proteins A and D as pattern recognition proteins

Lung surfactant proteins A and D belong to a group of soluble humoral pattern recognition receptors, called collectins, which modulate the immune response to microorganisms. They bind essential carbohydrate and lipid antigens found on the surface of microorganisms via low affinity C-type lectin domains and regulate the host's response by binding to immune cell surface receptors. They form multimeric structures that bind, agglutinate, opsonise and neutralize many different pathogenic microorganisms including bacteria, yeast, fungi and viruses. They modulate the uptake of these microorganisms by phagocytic cells as well as both the inflammatory and the adaptive immune responses. Recent data have also highlighted their involvement in clearance of apoptotic cells, hypersensitivity and a number of lung diseases.

Pulmonary collectins in innate immunity of the lung

Pulmonary collectins, hydrophilic surfactant proteins A and D (SP-A and SP-D), have been implicated in the regulation of pulmonary host defence and inflammation. SP-A and SP-D directly interact with a variety of microorganisms including bacteria and viruses, and attenuate the growth of Gram-negative bacteria, Histoplasma capsulatum and Mycoplasma pneumoniae. The collectins are thought to contribute to bacterial clearance. These lectins augment the phagocytosis of the bacteria by macrophages. SP-A serves as an opsonin and stimulates the uptake of bacteria and bacillus Calmette-Guérin through a C1q receptor- and an SP-R210-mediated processes. The collectin also stimulates FcR- and CR1-mediated phagocytosis by activating the macrophages. In addition, SP-A and SP-D directly interact with macrophages and enhance the phagocytosis of Streptococcus pneumoniae and Mycobacterium by increasing cell surface localization of the phagocytic receptors, scavenger receptor A and mannose receptor. The collectins also modulate pulmonary inflammation. SP-A and SP-D bind to cell surface receptors including Toll-like receptors, SIRPalpha and calreticulin/CD91, and attenuate or enhance inflammation in a microbial ligand-specific manner. In this article we review the immunomodulatory functions of SP-A and SP-D and their possible mechanisms in direct actions on microbes, macrophage phagocytosis and modulation of inflammation.

Characterization of the receptor-ligand pathways important for entry and survival of Francisella tularensis in human macrophages

Inhalational pneumonic tularemia, caused by Francisella tularensis, is lethal in humans. F. tularensis is phagocytosed by macrophages followed by escape from phagosomes into the cytoplasm. Little is known of the phagocytic mechanisms for Francisella, particularly as they relate to the lung and alveolar macrophages. Here we examined receptors on primary human monocytes and macrophages which mediate the phagocytosis and intracellular survival of F. novicida. F. novicida association with monocyte-derived macrophages (MDM) was greater than with monocytes. Bacteria were readily ingested, as shown by electron microscopy. Bacterial association was significantly increased in fresh serum and only partially decreased in heat-inactivated serum. A role for both complement receptor 3 (CR3) and Fcgamma receptors in uptake was supported by studies using a CR3-expressing cell line and by down-modulation of Fcgamma receptors on MDM, respectively. Consistent with Fcgamma receptor involvement, antibody in nonimmune human serum was detected on the surface of Francisella. In the absence of serum opsonins, competitive inhibition of mannose receptor (MR) activity on MDM with mannan decreased the association of F. novicida and opsonization of F. novicida with lung collectin surfactant protein A (SP-A) increased bacterial association and intracellular survival. This study demonstrates that human macrophages phagocytose more Francisella than monocytes with contributions from CR3, Fcgamma receptors, the MR, and SP-A present in lung alveoli.

Alloiococcus otitidis is a ligand for collectins and Toll-like receptor 2, and its phagocytosis is enhanced by collectins

Alloiococcus otitidis has been found to be associated with otitis media with effusion. In this study we investigated whether TLR2 and collectins, surfactant protein A (SP-A) and mannose-binding lectin (MBL), interacted with A. otitidis. Both SP-A and MBL bound to A. otitidis in a Ca(2+)-dependent manner. A. otitidis induced IL-8 secretion from U937 cells and NF-kappaB activation in TLR2-transfected HEK293 cells. However, the cells transfected with the mutant TLR2(P681H) did not respond to A. otitidis. In addition, A. otitidis co-sedimented a recombinant soluble form of the extracellular TLR2 domain, indicating direct binding of the bacterium to TLR2. SP-A and MBL augmented the phagocytosis of A. otitidis by J774A.1 cells. The collectin-stimulated phagocytosis of A. otitidis was significantly attenuated when fucoidan and polyinosinic acid were co-incubated. Immunoblotting analysis revealed that MBL was present in the middle ear effusion from patients with otitis media. These results demonstrate that A. otitidis is a ligand for the collectins and TLR2, and that the collectins enhance the phagocytosis of A. otitidis by macrophages, suggesting important roles of the collectins and TLR2 in the innate immunity of the middle ear against A. otitidis infection.

Pulmonary collectins selectively permeabilize model bacterial membranes containing rough lipopolysaccharide

We have reported that Gram-negative organisms decorated with rough lipopolysaccharide (LPS) are particularly susceptible to the direct antimicrobial actions of the pulmonary collectins, surfactant proteins A (SP-A) and D (SP-D). In this study, we examined the lipid and LPS components required for the permeabilizing effects of the collectins on model bacterial membranes. Liposomes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), with or without rough Escherichia coli LPS (J5), smooth E. coli LPS (B5), or cholesterol, were loaded with self-quenching probes and exposed to native or oxidatively modified SP-A. Fluorescence that resulted from permeabilization of liposomes and diffusion of dyes was assessed by microscopy or fluorimetry. Human SP-A and melittin increased the permeability of J5 LPS/POPE liposomes, but not B5 LPS/POPE liposomes or control (POPE only) liposomes. At a human SP-A concentration of 100 microg/mL, the permeability of the J5 LPS/POPE membranes increased 4.4-fold (p < 0.02) compared to the control with no added SP-A. Rat SP-A and SP-D also permeabilized the J5-containing liposomes. Incorporation of cholesterol into J5 LPS/POPE liposomes at a POPE:cholesterol molar ratio of 1:0.15 blocked human SP-A or melittin-induced permeability (p < 0.05) compared to cholesterol-free liposomes. Exposure of human SP-A to surfactant lipid peroxidation blocked the permeabilizing activity of the protein. We conclude that SP-A permeabilizes phospholipid membranes in an LPS-dependent and rough LPS-specific manner, that the effect is neither SP-A- nor species-specific, and that oxidative damage to SP-A abolishes its membrane destabilizing properties. Incorporation of cholesterol into the membrane enhances resistance to permeabilization by SP-A, most likely by increasing the packing density and membrane rigidity.

Agglutination of Pseudomonas aeruginosa by surfactant protein D

Surfactant protein D (SP-D) is part of the innate host defense system, and may bind and agglutinate invading microorganisms to enhance their removal. The ability of bronchoalveolar lavage (BAL) fluid to agglutinate bacteria and the relationship to its SP-D content are of interest and not yet known. A micromethod on slides was used to assess the agglutination of Pseudomonas aeruginosa by recombinant SP-D and native human BAL fluid. The SP-D-induced agglutination was blocked by calcium depletion, alkaline pH, or the presence of maltose. Twenty-three of 30 BAL fluids from outpatients carrying a chronic tracheostoma clearly agglutinated P. aeruginosa, which was completely inhibited by maltose. The extent of the agglutination correlated weakly to the concentration of SP-D in the BAL fluid, but not to that of SP-A. The functional property, i.e., the agglutination of P. aeruginosa by BAL fluid, was characterized and appeared related in part to the concentration of SP-D. Additional factors, such as the multimeric organization of SP-D, are likely to contribute to the agglutination of microorganisms by BAL or other body fluids. The assay presented will allow the systematic evaluation of small-volume samples for SP-D agglutinating ability from subjects with various lung diseases.

Surfactant proteins SP-A and SP-D: structure, function and receptors

Surfactant proteins, SP-A and SP-D, are collagen-containing C-type (calcium dependent) lectins called collectins, which contribute significantly to surfactant homeostasis and pulmonary immunity. These highly versatile innate immune molecules are involved in a range of immune functions including viral neutralization, clearance of bacteria, fungi and apoptotic and necrotic cells, down regulation of allergic reaction and resolution of inflammation. Their basic structures include a triple-helical collagen region and a C-terminal homotrimeric lectin or carbohydrate recognition domain (CRD). The trimeric CRDs can recognize carbohydrate or charge patterns on microbes, allergens and dying cells, while the collagen region can interact with receptor molecules present on a variety of immune cells in order to initiate clearance mechanisms. Studies involving gene knock-out mice, murine models of lung hypersensitivity and infection, and functional characterization of cell surface receptors have revealed the diverse roles of SP-A and SP-D in the control of lung inflammation. A recently proposed model based on studies with the calreticulin-CD91 complex as a receptor for SP-A and SP-D has suggested an anti-inflammatory role for SP-A and SP-D in naïve lungs which would help minimise the potential damage that continual low level exposure to pathogens, allergens and apoptosis can cause. However, when the lungs are overwhelmed with exogenous insults, SP-A and SP-D can assume pro-inflammatory roles in order to complement pulmonary innate and adaptive immunity. This review is an update on the structural and functional aspects of SP-A and SP-D, with emphasis on their roles in controlling pulmonary infection, allergy and inflammation. We also try to put in perspective the controversial subject of the candidate receptor molecules for SP-A and SP-D.

Immunoregulatory functions of surfactant proteins

Because the lungs function as the body's gas-exchange organ, they are inevitably exposed to air that is contaminated with pathogens, allergens and pollutants. Host-defence mechanisms within the lungs must facilitate clearance of inhaled pathogens and particles while minimizing an inflammatory response that could damage the thin, delicate gas-exchanging epithelium. Pulmonary surfactant is a complex of lipids and proteins that enhances pathogen clearance and regulates adaptive and innate immune-cell functions. In this article, I review the structure and functions of the surfactant proteins SP-A and SP-D in regulating host immune defence and in modulating inflammatory responses.

The lung collectins, SP-A and SP-D, modulate pulmonary innate immunity

Pulmonary surfactant, which covers the peripheral airway, is a mixture of lipids and proteins. The hydrophilic surfactant proteins A (SP-A) and D (SP-D) play important roles in host defense mechanisms of the lung. These proteins belong to a collectin subgroup in which lectin domains are associated with collagenous structures. Collectins involve mannose-binding lectin, and are considered to function in innate immune systems. SP-A and SP-D interact with various microorganisms and pathogen-derived components. They act as opsonins by binding and agglutinating pathogens. The lung collectins also possess direct inhibitory effects on bacterial growth. SP-A and SP-D associate with immune cells, and activate various cellular functions. The direct interactions of SP-A and SP-D with macrophages result in modulation of phagocytosis or the production of reactive oxygen species. Moreover, by associating with cell surface pattern-recognition receptors, SP-A and SP-D regulate inflammatory cellular responses such as the release of lipopolysaccharides-induced proinflammatory cytokines. Animal models of SP-A- or SP-D-deficiency reveal significant defect in host defense. Significant susceptibility to bacterial and viral infections, delayed microbial clearance, and overexpression of proinflammatory cytokines are observed in SP-A or SP-D knockout mice. A more complete understanding of the mechanisms is required, but the biological relevance of SP-A and SP-D against various respiratory infections has been increasingly recognized.

Interactions of pulmonary collectins with Bordetella bronchiseptica and Bordetella pertussis lipopolysaccharide elucidate the structural basis of their antimicrobial activities

Surfactant proteins A (SP-A) and D (SP-D) play an important role in the innate immune defenses of the respiratory tract. SP-A binds to the lipid A region of lipopolysaccharide (LPS), and SP-D binds to the core oligosaccharide region. Both proteins induce aggregation, act as opsonins for neutrophils and macrophages, and have direct antimicrobial activity. Bordetella pertussis LPS has a branched core structure and a nonrepeating terminal trisaccharide. Bordetella bronchiseptica LPS has the same structure, but lipid A is palmitoylated and there is a repeating O-antigen polysaccharide. The ability of SP-A and SP-D to agglutinate and permeabilize wild-type and LPS mutants of B. pertussis and B. bronchiseptica was examined. Previously, wild-type B. pertussis was shown to resist the effects of SP-A; however, LPS mutants lacking the terminal trisaccharide were susceptible to SP-A. In this study, SP-A was found to aggregate and permeabilize a B. bronchiseptica mutant lacking the terminal trisaccharide, while wild-type B. bronchiseptica and mutants lacking only the palmitoyl transferase or O antigen were resistant to SP-A. Wild-type B. pertussis and B. bronchiseptica were both resistant to SP-D; however, LPS mutants of either strain lacking the terminal trisaccharide were aggregated and permeabilized by SP-D. We conclude that the terminal trisaccharide protects Bordetella species from the bactericidal functions of SP-A and SP-D. The O antigen and palmitoylated lipid A of B. bronchiseptica play no role in this resistance.

Bordetella pertussis lipopolysaccharide resists the bactericidal effects of pulmonary surfactant protein A

Surfactant protein A (SP-A) plays an important role in the innate immune defense of the respiratory tract. SP-A binds to lipid A of bacterial LPS, induces aggregation, destabilizes bacterial membranes, and promotes phagocytosis by neutrophils and macrophages. In this study, SP-A interaction with wild-type and mutant LPS of Bordetella pertussis, the causative agent of whooping cough, was examined. B. pertussis LPS has a branched core structure with a nonrepeating trisaccharide, rather than a long-chain repeating O-Ag. SP-A did not bind, aggregate, nor permeabilize wild-type B. pertussis. LPS mutants lacking even one of the sugars in the terminal trisaccharide were bound and aggregated by SP-A. SP-A enhanced phagocytosis by human monocytes of LPS mutants that were able to bind SP-A, but not wild-type bacteria. SP-A enhanced phagocytosis by human neutrophils of LPS-mutant strains, but only in the absence of functional adenylate cyclase toxin, a B. pertussis toxin that has been shown to depress neutrophil activity. We conclude that the LPS of wild-type B. pertussis shields the bacteria from SP-A-mediated clearance, possibly by sterically limiting access to the lipid A region.

Pulmonary collectins enhance phagocytosis of Mycobacterium avium through increased activity of mannose receptor

Collectins, including surfactant proteins A (SP-A) and D (SP-D) and mannose binding lectin (MBL), are the important constituents of the innate immune system. Mycobacterium avium, a facultative intracellular pathogen, has developed numerous mechanisms for entering mononuclear phagocytes. In this study, we investigated the interactions of collectins with M. avium and the effects of these lectins on phagocytosis of M. avium by macrophages. SP-A, SP-D, and MBL exhibited a concentration-dependent binding to M. avium. The binding of SP-A to M. avium was Ca(2+)-dependent but that of SP-D and MBL was Ca(2+)-independent. SP-A and SP-D but not MBL enhanced the phagocytosis of FITC-labeled M. avium by rat alveolar macrophages and human monocyte-derived macrophages. Excess mannan, zymosan, and lipoarabinomannan derived from the M. avium-intracellular complex, significantly decreased the collectin-stimulated phagocytosis of M. avium. Enhanced phagocytosis was not affected by the presence of cycloheximide or chelation of Ca(2+). The mutated collectin, SP-A(E195Q, R197D) exhibited decreased binding to M. avium but stimulated phagocytosis to a level comparable to wild-type SP-A. Enhanced phagocytosis by cells persisted even after preincubation and removal of SP-A or SP-D. Rat alveolar macrophages that had been incubated with SP-A or SP-D also exhibited enhanced uptake of (125)I-mannosylated BSA. Analysis by confocal microscopy and flow cytometry revealed that the lung collectins up-regulated the cell surface expression of mannose receptor on monocyte-derived macrophages. These results provide compelling evidence that SP-A and SP-D enhance mannose receptor-mediated phagocytosis of M. avium by macrophages.

Mannose-binding lectin binds to two major outer membrane proteins, opacity protein and porin, of Neisseria meningitidis

Human mannose-binding lectin (MBL) provides a first line of defense against microorganisms by complement activation and/or opsonization in the absence of specific Ab. This serum collectin has been shown to activate complement when bound to repeating sugar moieties on several microorganisms, including encapsulated serogroup B and C meningococci, which leads to increased bacterial killing. In the present study, we sought to identify the meningococcal cell surface components to which MBL bound and to characterize such binding. Outer membrane complex containing both lipooligosaccharide (LOS) and proteins and LOS from Neisseria meningitidis were examined for MBL binding by dot blot and ELISA. MBL bound outer membrane complex but not LOS. The binding to bacteria by whole-cell ELISA did not require calcium and was not inhibited by N-acetyl-glucosamine or mannose. With the use of SDS-PAGE, immunoblot analysis, and mAbs specific for meningococcal opacity (Opa) proteins and porin proteins, we determined that MBL bound to Opa and porin protein B (porB). The N-terminal amino acid sequences of the two MBL binding proteins confirmed Opa and PorB. Purified PorB inhibited the binding of MBL to meningococci. Escherichia coli with surface-expressed gonococcal Opa bound significantly more MBL than did the control strain. The binding of human factor H to purified PorB was markedly inhibited by MBL in a dose-dependent manner. Meningococci incubated with human serum bound MBL as detected by ELISA. We conclude that MBL binds to meningococci by a novel target recognition of two nonglycosylated outer membrane proteins, Opa and PorB.

Collectins

Collectins are a family of collagenous calcium-dependent defense lectins in animals. Their polypeptide chains consist of four regions: a cysteine-rich N-terminal domain, a collagen-like region, an alpha-helical coiled-coil neck domain and a C-terminal lectin or carbohydrate-recognition domain. These polypeptide chains form trimers that may assemble into larger oligomers. The best studied family members are the mannan-binding lectin, which is secreted into the blood by the liver, and the surfactant proteins A and D, which are secreted into the pulmonary alveolar and airway lining fluid. The collectins represent an important group of pattern recognition molecules, which bind to oligosaccharide structures and/or lipid moities on the surface of microorganisms. They bind preferentially to monosaccharide units of the mannose type, which present two vicinal hydroxyl groups in an equatorial position. High-affinity interactions between collectins and microorganisms depend, on the one hand, on the high density of the carbohydrate ligands on the microbial surface, and on the other, on the degree of oligomerization of the collectin. Apart from binding to microorganisms, the collectins can interact with receptors on host cells. Binding of collectins to microorganisms may facilitate microbial clearance through aggregation, complement activation, opsonization and activation of phagocytosis, and inhibition of microbial growth. In addition, the collectins can modulate inflammatory and allergic responses, affect apoptotic cell clearance and modulate the adaptive immune system.

Resistant Haemophilus influenzae in community-acquired respiratory tract infections: a role for cefixime

An increase in Haemophilus influenzae resistance has been documented around the world during the last 30 years. Resistance is due to the production of beta-lactamases, and/or changes to penicillin-binding protein (PBP) targets. The resistance problem has led to the need for new therapeutic strategies aimed at maintaining effective management of both upper respiratory tract infections (URTIs) and lower respiratory tract infections (LRTIs). Among antimicrobial agents tested, third-generation cephalosporins have been shown to possess excellent in vitro activity against beta-lactamase-positive and -negative isolates, corresponding with proven clinical efficacy in a wide range of RTIs. The role of H. influenzae in RTIs is outlined, changing trends in epidemiological surveillance studies monitored and implications for therapy, based upon results of clinical trials discussed.

Surfactant proteins A and D inhibit the growth of Gram-negative bacteria by increasing membrane permeability

The pulmonary collectins, surfactant proteins A (SP-A) and D (SP-D), have been reported to bind lipopolysaccharide (LPS), opsonize microorganisms, and enhance the clearance of lung pathogens. In this study, we examined the effect of SP-A and SP-D on the growth and viability of Gram-negative bacteria. The pulmonary clearance of Escherichia coli K12 was reduced in SP-A-null mice and was increased in SP-D-overexpressing mice, compared with strain-matched wild-type controls. Purified SP-A and SP-D inhibited bacterial synthetic functions of several, but not all, strains of E. coli, Klebsiella pneumoniae, and Enterobacter aerogenes. In general, rough E. coli strains were more susceptible than smooth strains, and collectin-mediated growth inhibition was partially blocked by coincubation with rough LPS vesicles. Although both SP-A and SP-D agglutinated E. coli K12 in a calcium-dependent manner, microbial growth inhibition was independent of bacterial aggregation. At least part of the antimicrobial activity of SP-A and SP-D was localized to their C-terminal domains using truncated recombinant proteins. Incubation of E. coli K12 with SP-A or SP-D increased bacterial permeability. Deletion of the E. coli OmpA gene from a collectin-resistant smooth E. coli strain enhanced SP-A and SP-D-mediated growth inhibition. These data indicate that SP-A and SP-D are antimicrobial proteins that directly inhibit the proliferation of Gram-negative bacteria in a macrophage- and aggregation-independent manner by increasing the permeability of the microbial cell membrane.

Surfactant protein A and D differently regulate the immune response to nonmucoid Pseudomonas aeruginosa and its lipopolysaccharide

We investigated the role of the surfactant proteins (SPs) A and D in the pulmonary immune defense of nonmucoid strains of Pseudomonas aeruginosa, the most etiologic agents of nosocomial Pseudomonas pneumonia. We first examined the interactions of recombinant human SP-D dodecamers and purified natural or recombinant human SP-A with two smooth, and two rough, clinical isolates of nonmucoid P. aeruginosa. SP-D bound to all four isolates, but agglutinated only one rough and one smooth strain. SP-D functioned as an opsonin to enhance the uptake of all four strains by the human monocytic cell line Mono Mac 6 (MM6). SP-D also enhanced tumor necrosis factor-alpha secretion by MM6 cells in response to purified lipopolysaccharide (LPS) isolated from the rough, but not the smooth, strains. Although SP-A bound to all four strains, it did not cause bacterial aggregation or enhance uptake. It showed small but statistically significant inhibitory effects on the cytokine response of MM6 cells to one strain of smooth organisms, but did not significantly alter the response to purified LPS. This study in combination with previously published data strongly suggests that SP-D may play important roles in the local innate pulmonary defense against nonmucoid P. aeruginosa of diverse LPS phenotypes, and preferentially augments the cellular response to rough P. aeruginosa endotoxin.

Surfactant protein A enhances the phagocytosis of C1q-coated particles by alveolar macrophages

Surfactant protein-A (SP-A) plays multiple roles in pulmonary host defense, including stimulating bacterial phagocytosis by innate immune cells. Previously, SP-A was shown to interact with complement protein C1q. Our goal was to further characterize this interaction and elucidate its functional consequences. Radiolabeled SP-A bound solid-phase C1q but not other complement proteins tested. The lectin activity of SP-A was not required for binding to C1q. Because C1q is involved in bacterial clearance but alone does not efficiently enhance the phagocytosis of most bacteria, we hypothesize that SP-A enhances phagocytosis of C1q-coated antigens. SP-A enhanced by sixfold the percentage of rat alveolar macrophages in suspension that phagocytosed C1q-coated fluorescent beads. Furthermore, uptake of C1q-coated beads was enhanced when either beads or alveolar macrophages were preincubated with SP-A. In contrast, SP-A had no significant effect on the uptake of C1q-coated beads by alveolar macrophages adhered to plastic slides. We conclude that SP-A may serve a protective role in the lung by interacting with C1q to enhance the clearance of foreign particles.

Lipid-restricted recognition of mycobacterial lipoglycans by human pulmonary surfactant protein A: a surface-plasmon-resonance study

The human pulmonary surfactant protein A (hSP-A), a member of the mammalian collectin family, is thought to play a key defensive role against airborne invading pulmonary pathogens, among which is Mycobacterium tuberculosis, the aetiologic agent of tuberculosis. hSP-A has been shown to promote the uptake and the phagocytosis of pathogenic bacilli through the recognition and the binding of carbohydrate motifs on the invading pathogen surface. Recently we identified lipomannan and mannosylated lipoarabinomannan (ManLAM), two major mycobacterial cell-wall lipoglycans, as potential ligands for binding of hSP-A. We demonstrated that both the terminal mannose residues and the fatty acids are critical for binding, whereas the inner arabinosyl and mannosyl domains do not participate. In the present study we developed a surface-plasmon-resonance assay to analyse the molecular basis for the recognition of ManLAM by hSP-A and to try to define further the role of the lipidic aglycone moiety. Binding of ManLAM to immobilized hSP-A was consistent with the simplest one-to-one interaction model involving a single class of carbohydrate-binding site. This observation strongly suggests that the lipid moiety of ManLAM does not directly interact with hSP-A, but is rather responsible for the macromolecular organization of the lipoglycan, which may be necessary for efficient recognition of the terminal mannosyl epitopes. The indirect, structural role of the lipoglycan lipidic component is further supported by the complete lack of interaction with hSP-A in the presence of a low concentration of mild detergent.

Surfactant protein A regulates surfactant phospholipid clearance after LPS-induced injury in vivo

Previous in vitro studies have suggested that surfactant protein A (SP-A) may play a role in pulmonary surfactant homeostasis by mediating surfactant secretion and clearance. However, mice made deficient in SP-A [SP-A (-/-) animals] have relatively normal levels of surfactant compared with wild-type SP-A (+/+) animals. We hypothesize that SP-A may play a role in surfactant homeostasis after acute lung injury. Bacterial lipopolysaccharide was instilled into the lungs of SP-A (-/-) mice and SP-A (+/+) mice to induce injury. Surfactant phospholipid levels were increased 1.6-fold in injured SP-A (-/-) animals, although injury did not alter [3H]choline or [14C]palmitate incorporation into dipalmitoylphosphatidylcholine (DPPC), suggesting no change in surfactant synthesis/secretion 12 h after injury. Clearance of [3H]DPPC from the lungs of injured SP-A (-/-) animals was decreased by approximately 40%. Instillation of 50 microg of exogenous SP-A rescued both the clearance defect and the increased phospholipid defect in injured SP-A (-/-) animals, suggesting that SP-A may play a role in regulating clearance of surfactant phospholipids after acute lung injury.

Alterations in lung collectins in an adaptive allergic immune response

Although surfactant apoproteins are known to be mediators of innate responses, their relationship to adaptive responses has not been examined extensively. We investigated possible links between surfactant apoproteins and responses to allergens by studying alterations in surfactant apoproteins A, B, and D in a murine model of allergic pulmonary inflammation. Three murine strains (BALB/c, C57BL/6, and 129J) demonstrated increased immunostaining of surfactant apoproteins A and D in nonciliated epithelial cells of noncartilaginous airways after aerosolized challenge. In contrast, surfactant apoprotein B immunostaining was unchanged. Immunoblotting demonstrated increased surfactant A in bronchoalveolar lavage fluid after allergen sensitization and challenge. Surfactant apoprotein A and D induction required T and/or B lymphocyte responses to allergen, since the induction was absent in recombinase-activating gene-deficient mice, which lack functional lymphocytes. We conclude that increased immunoreactivity of two collectins, surfactant apoproteins A and D, occurs within the response to allergen. Our findings support a model in which surfactant apoproteins A and D are important to both innate immunity and adaptive immune responses to allergens.

Surfactant protein A inhibits peptidoglycan-induced tumor necrosis factor-alpha secretion in U937 cells and alveolar macrophages by direct interaction with toll-like receptor 2

Pulmonary surfactant protein A (SP-A) plays an important role in modulation of the innate immune system of the lung. Peptidoglycan (PGN), a cell wall component of Gram-positive bacteria, is known to elicit excessive proinflammatory cytokine production from immune cells. In this study we investigated whether SP-A interacts with PGN and alters PGN-elicited cellular responses. Binding studies demonstrate that PGN is not a ligand for SP-A. However, SP-A significantly reduced PGN-elicited tumor necrosis factor alpha (TNF-alpha) secretion by U937 cells and rat alveolar macrophages. The inhibitory effect on TNF-alpha secretion was dependent upon SP-A concentrations in physiological range. Coincubation of SP-A and PGN with human embryonic kidney 293 cells that had been transiently transfected with the cDNA of Toll-like receptor 2 (TLR2), a cell signaling receptor for PGN, significantly attenuated PGN-induced nuclear factor-kappaB activity. SP-A directly bound to a soluble form of the recombinant extracellular TLR2 domain (sTLR2). Coincubation of sTLR2 with SP-A significantly reduced the binding of sTLR2 to PGN. These results indicate that the direct interaction of SP-A with TLR2 alters PGN-induced cell signaling. We propose that SP-A modulates inflammatory responses against the bacterial components by interactions with pattern-recognition receptors.

Clearance of surfactant lipids by neutrophils and macrophages isolated from the acutely inflamed lung

Pulmonary surfactant reduces surface tension at the lung air-liquid interface and defends the host against infection. Several lines of evidence show that surfactant levels are altered in animal models and patients with inflammatory or infectious lung diseases. We tested the hypothesis that cells responding to lung injury alter surfactant levels through increased phospholipid clearance. Acute lung injury was induced by intratracheal administration of lipopolysaccharide (LPS; Escherichia coli 026:B6) into rats. LPS exposure resulted in a 12-fold increase in the number of cells isolated by lavage, the majority of which were neutrophils. Isolated macrophages and neutrophils from LPS-treated lungs internalized and degraded lipids in vitro, and LPS injury stimulated uptake by macrophages twofold. We estimate that lipid clearance by lavage cells in LPS-treated lungs could be enhanced 6- to 13-fold with both activated macrophages and increased numbers of neutrophils contributing to the process. These data show that the increased number of cells in the alveolar space after acute lung injury may lead to alterations in surfactant pools via enhanced clearance and degradation of lipids.

Collections and ficolins: humoral lectins of the innate immune defense

Collectins and ficolins, present in plasma and on mucosal surfaces, are humoral molecules of the innate immune systems, which recognize pathogen-associated molecular patterns. The human collectins, mannan-binding lectin (MBL) and surfactant protein A and D (SP-A and SP-D), are oligomeric proteins composed of carbohydrate-recognition domains (CRDs) attached to collagenous regions and are thus structurally similar to the ficolins, L-ficolin, M-ficolin, and H-ficolin. However, they make use of different CRD structures: C-type lectin domains for the collectins and fibrinogen-like domains for the ficolins. Upon recognition of the infectious agent, MBL and the ficolins initiate the lectin pathway of complement activation through attached serine proteases (MASPs), whereas SP-A and SP-D rely on other effector mechanisms: direct opsonization, neutralization, and agglutination. This limits the infection and concurrently orchestrates the subsequent adaptive immune response. Deficiencies of the proteins may predispose to infections or other complications, e.g., reperfusion injuries or autoimmune diseases. Structure, function, clinical implications, and phylogeny are reviewed.

Surfactant protein A regulates complement activation

Complement proteins aid in the recognition and clearance of pathogens from the body. C1, the first protein of the classical pathway of complement activation, is a calcium-dependent complex of one molecule of C1q and two molecules each of C1r and C1s, the serine proteases that cleave complement proteins. Upon binding of C1q to Ag-bound IgG or IgM, C1r and C1s are sequentially activated and initiate the classical pathway of complement. Because of structural and functional similarities between C1q and members of the collectin family of proteins, including pulmonary surfactant protein A (SP-A), we hypothesized that SP-A may interact with and regulate proteins of the complement system. Previously, SP-A was shown to bind to C1q, but the functional significance of this interaction has not been investigated. Binding studies confirmed that SP-A binds directly to C1q, but only weakly to intact C1. Further investigation revealed that the binding of SP-A to C1q prevents the association of C1q with C1r and C1s, and therefore the formation of the active C1 complex required for classical pathway activation. This finding suggests that SP-A may share a common binding site for C1r and C1s or Clq. SP-A also prevented C1q and C1 from binding to immune complexes. Furthermore, SP-A blocked the ability of C1q to restore classical pathway activity to C1q-depleted serum. SP-A may down-regulate complement activity through its association with C1q. We hypothesize that SP-A may serve a protective role in the lung by preventing C1q-mediated complement activation and inflammation along the delicate alveolar epithelium.

Surfactant proteins A and D in Eustachian tube epithelium

Surfactant protein (SP) A and SP-D are collectins that have roles in host defense. The Eustachian tube (ET) maintains the patency between the upper airways and the middle ear. Dysfunction of local mucosal immunity in ET may predispose infants to recurrent otitis media. We recently described preliminary evidence of the expression of SP-A and SP-D in the ET. Our present aim was to establish the sites of SP-A and SP-D expression within the epithelium of the ET in vivo. With in situ hybridization, electron microscopy, and immunoelectron microscopy, the cells responsible for SP-A and SP-D expression and storage were identified. SP-A expression was localized within the ET epithelium, and the protein was found in the electron-dense granules of microvillar epithelial cells. Being concentrated in the epithelial lining, only a few cells revealed intracellular SP-D, and it was not associated with granules. The SP-A and SP-D immunoreactivities in ET lavage fluid, as shown by Western blot analyses, were similar to those in bronchoalveolar lavage fluid. We propose that there are specialized cells in the ET epithelium expressing and secreting SP-A and SP-D. SP-A and SP-D may be important for antibody-independent protection of the middle ear against infections.

Surfactant proteins a and d and pulmonary host defense

The lung collectins, SP-A and SP-D, are important components of the innate immune response to microbial challenge and participate in other aspects of immune and inflammatory regulation within the lung. Both proteins bind to surface structures expressed by a wide variety of microorganisms and have the capacity to modulate multiple leukocyte functions, including the enhanced internalization and killing of certain microorganisms in vitro. In addition, transgenic mice with deficiencies in SP-A and SP-D show defective or altered responses to challenge with bacterial, fungal, and viral microorganisms and to bacterial lipopolysaccharides in vivo. Thus collectins could play particularly important roles in settings of inadequate or impaired specific immunity, and acquired alterations in the levels of active collectins within the airspaces and distal airways may increase susceptibility to infection.

Pulmonary collectins and innate host defense of the lung

Surfactant proteins A and D (SP-A and SP-D) are members of the collectin family of polypeptides expressed in the respiratory tract that bind bacterial, fungal and viral pathogens, enhancing their opsonization and killing by phagocytic cells. Clearance of bacterial pathogens including group B streptococci, Haemophilus influenza, Pseudomonas aeruginosa and viral pathogens, respiratory syncytial virus, adenovirus and influenza A virus, was deficient in SP-A(-/-) mice. SP-A deficiency was associated with enhanced inflammation and synthesis of proinflammatory cytokines. SP-D(-/-) mice cleared these bacteria as efficiently as wild-type mice; however, clearance of viral pathogens was deficient in SP-D(-/-) mice and associated with increased inflammation. SP-A and SP-D play critical and distinct roles in the regulation of alveolar macrophage function and inflammation, contributing to innate defense of the lung.

Complement-mediated host defense in the lung

Complement is a system of plasma proteins that aids in the elimination of pathogens from the body. We hypothesized that there is a functional complement system present in the lung that aids in the removal of pathogens. Western blot analysis revealed complement proteins of the alternative and classical pathways of complement in bronchoalveolar lavage fluids (BALF) from healthy volunteers. Functional classical pathway activity was detected in human BALF, but there was no significant alternative pathway activity in lavage fluid, a finding that correlates with the low level of the alternative pathway protein, factor B, in these samples. Although the classical pathway of complement was functional in lavage fluid, the level of the classical pathway activator C1q was very low. We tested the ability of the lung- specific surfactant proteins, surfactant protein A (SP-A) and surfactant protein D (SP-D), to substitute for C1q in classical pathway activation, since they have structural homology to C1q. However, neither SP-A nor SP-D restored classical pathway activity to C1q-depleted serum. These data suggest that the classical pathway of complement is functionally active in the lung where it may play a role in the recognition and clearance of bacteria.

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.