Biosynthesis of surfactant protein D. Contributions of conserved NH2-terminal cysteine residues and collagen helix formation to assembly and secretion

Surfactant protein D (SP-D) as a biomarker of SARS-CoV-2 infection

Background

Surfactant protein-D (SP-D) is a lung-resident protein that has emerged as a potential biomarker for COVID-19. Previous investigations on acute respiratory distress syndrome patients demonstrated a significant increment of SP-D serum levels in pathological conditions. Since SP-D is not physiologically permeable to alveoli-capillary membrane and poorly expressed by other tissues, this enhancement is likely due to an impairment of the pulmonary barrier caused by prolonged inflammation.

Methods

A retrospective study on a relatively large cohort of patients of Hospital Pio XI of Desio was conducted to assess differences of the hematic SP-D concentrations among COVID-19 patients and healthy donors and if SP-D levels resulted a risk factor for disease severity and mortality.

Results

The first analysis, using an ANOVA-model, showed a significant difference in the mean of log SP-D levels between COVID-19 patients and healthy donors. Significant variations were also found between dead vs survived patients. Results confirm that SP-D concentrations were significantly higher for both hospitalized COVID-19 and dead patients, with threshold values of 150 and 250 ng/mL, respectively. Further analysis conducted with Logistic Mixed models, highlighted that higher SP-D levels at admission and increasing differences among follow-up and admission values resulted the strongest significant risk factors of mortality (model predictive accuracy, AUC = 0.844).

Conclusions

The results indicate that SP-D can be a predictive marker of COVID-19 disease and its outcome. Considering its prognostic value in terms of mortality, the early detection of SP-D levels and its follow-up in hospitalized patients should be considered to direct the therapeutic intervention.

Elucidating the enhanced binding affinity of a double mutant SP-D with trimannose on the influenza A virus using molecular dynamics

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The Asp325Ala mutation in SP-D promotes a trimannose conformational change to a more stable state.

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The Arg343Val mutation in SP-D reduces its interaction with Glu333 to increase the binding affinity with trimannose.

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The Arg343Val mutation contributes more to the increase of SP-D’s binding affinity with trimannose than Asp325Ala.

Surfactant protein D (SP-D) is an essential component of the human pulmonary surfactant system, which is crucial in the innate immune response against glycan-containing pathogens, including Influenza A viruses (IAV) and SARS-CoV-2. Previous studies have shown that wild-type (WT) SP-D can bind IAV but exhibits poor antiviral activities. However, a double mutant (DM) SP-D consisting of two point mutations (Asp325Ala and Arg343Val) inhibits IAV more potently. Presently, the structural mechanisms behind the point mutations’ effects on SP-D’s binding affinity with viral surface glycans are not fully understood. Here we use microsecond-scale, full-atomistic molecular dynamics (MD) simulations to understand the molecular mechanism of mutation-induced SP-D’s higher antiviral activity. We find that the Asp325Ala mutation promotes a trimannose conformational change to a more stable state. Arg343Val increases the binding with trimannose by increasing the hydrogen bonding interaction with Glu333. Free energy perturbation (FEP) binding free energy calculations indicate that the Arg343Val mutation contributes more to the increase of SP-D’s binding affinity with trimannose than Asp325Ala. This study provides a molecular-level exploration of how the two mutations increase SP-D binding affinity with trimannose, which is vital for further developing preventative strategies for related diseases.

Surfactant protein D and bronchopulmonary dysplasia: a new way to approach an old problem

Surfactant protein D (SP-D) is a collectin protein synthesized by alveolar type II cells in the lungs. SP-D participates in the innate immune defense of the lungs by helping to clear infectious pathogens and modulating the immune response. SP-D has shown an anti-inflammatory role by down-regulating the release of pro-inflammatory mediators in different signaling pathways such as the TLR4, decreasing the recruitment of inflammatory cells to the lung, and modulating the oxidative metabolism in the lungs. Recombinant human SP-D (rhSP-D) has been successfully produced mimicking the structure and functions of native SP-D. Several in vitro and in vivo experiments using different animal models have shown that treatment with rhSP-D reduces the lung inflammation originated by different insults, and that rhSP-D could be a potential treatment for bronchopulmonary dysplasia (BPD), a rare disease for which there is no effective therapy up to date. BPD is a complex disease in preterm infants whose incidence increases with decreasing gestational age at birth. Lung inflammation, which is caused by different prenatal and postnatal factors like infections, lung hyperoxia and mechanical ventilation, among others, is the key player in BPD. Exacerbated inflammation causes lung tissue injury that results in a deficient gas exchange in the lungs of preterm infants and frequently leads to long-term chronic lung dysfunction during childhood and adulthood. In addition, low SP-D levels and activity in the first days of life in preterm infants have been correlated with a worse pulmonary outcome in BPD. Thus, SP-D mediated functions in the innate immune response could be critical aspects of the pathogenesis in BPD and SP-D could inhibit lung tissue injury in this preterm population. Therefore, administration of rhSP-D has been proposed as promising therapy that could prevent BPD.

SP-A and SP-D: Dual Functioning Immune Molecules With Antiviral and Immunomodulatory Properties

Surfactant proteins A (SP-A) and D (SP-D) are soluble innate immune molecules which maintain lung homeostasis through their dual roles as anti-infectious and immunomodulatory agents. SP-A and SP-D bind numerous viruses including influenza A virus, respiratory syncytial virus (RSV) and human immunodeficiency virus (HIV), enhancing their clearance from mucosal points of entry and modulating the inflammatory response. They also have diverse roles in mediating innate and adaptive cell functions and in clearing apoptotic cells, allergens and other noxious particles. Here, we review how the properties of these first line defense molecules modulate inflammatory responses, as well as host-mediated immunopathology in response to viral infections. Since SP-A and SP-D are known to offer protection from viral and other infections, if their levels are decreased in some disease states as they are in severe asthma and chronic obstructive pulmonary disease (COPD), this may confer an increased risk of viral infection and exacerbations of disease. Recombinant molecules of SP-A and SP-D could be useful in both blocking respiratory viral infection while also modulating the immune system to prevent excessive inflammatory responses seen in, for example, RSV or coronavirus disease 2019 (COVID-19). Recombinant SP-A and SP-D could have therapeutic potential in neutralizing both current and future strains of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus as well as modulating the inflammation-mediated pathology associated with COVID-19. A recombinant fragment of human (rfh)SP-D has recently been shown to neutralize SARS-CoV-2. Further work investigating the potential therapeutic role of SP-A and SP-D in COVID-19 and other infectious and inflammatory diseases is indicated.

Generation of novel trimeric fragments of human SP-A and SP-D after recombinant soluble expression in E. coli

Surfactant treatment for neonatal respiratory distress syndrome has dramatically improved survival of preterm infants. However, this has resulted in a markedly increased incidence of sequelae such as neonatal chronic inflammatory lung disease. The current surfactant preparations in clinical use lack the natural lung defence proteins surfactant proteins (SP)-A and D. These are known to have anti-inflammatory and anti-infective properties essential for maintaining healthy non-inflamed lungs.

Supplementation of currently available animal derived surfactant therapeutics with these anti-inflammatory proteins in the first few days of life could prevent the development of inflammatory lung disease in premature babies. However, current systems for production of recombinant versions of SP-A and SP-D require a complex solubilisation and refolding protocol limiting expression at scale for drug development.

Using a novel solubility tag, we describe the expression and purification of recombinant fragments of human (rfh) SP-A and SP-D using Escherichia coli without the need for refolding. We obtained a mean (± SD) of 23.3 (± 5.4) mg and 86 mg (± 3.5) per litre yield of rfhSP-A and rfhSP-D, respectively. rfhSP-D was trimeric and 68% bound to a ManNAc-affinity column, giving a final yield of 57.5 mg/litre of highly pure protein, substantially higher than the 3.3 mg/litre obtained through the standard refolding protocol. Further optimisation of this novel lab based method could potentially make rfhSP-A and rfhSP-D production more commercially feasible to enable development of novel therapeutics for the treatment of lung infection and inflammation.

Structure and activity of human surfactant protein D from different natural sources

Surfactant protein D (SP-D) is a C-type lectin that participates in the innate immune defense of lungs. It binds pathogens through its carbohydrate recognition domain in a calcium-dependent manner. Human surfactant protein D (hSP-D) has been routinely obtained from bronchoalveolar lavage of patients suffering from pulmonary alveolar proteinosis (PAP) and from amniotic fluid (AF). As a consequence of the disease, hSP-D obtained from PAP is found in higher amounts and is mainly composed of higher order oligomeric forms. However, PAP-hSP-D has never been directly compared with nonpathological human protein in terms of structure and biological activity. Moreover, the quantitative distribution of the different hSP-D oligomeric forms in human protein obtained from a natural source has never been evaluated. In this work, we have determined the quantitative distribution of AF-hSP-D oligomers, characterized the sugars attached through the N-glycosylation site of the protein, and compared the activity of hSP-D from AF and PAP with respect to their ability to bind and agglutinate bacteria. We have found that fuzzy balls (40%) are the most abundant oligomeric form in AF-hSP-D, very closely followed by dodecamers (33%), with both together constituting 73% of the protein mass. The glycan attached to the N-glycosylation site was found to be composed of fucose, galactose, sialic acid, and N-acetylglucosamine. Finally, in the functional assays performed, hSP-D obtained from PAP showed higher potency, probably as a consequence of its higher proportion of large oligomers compared with hSP-D from AF.

Functional characterization of the different oligomeric forms of human surfactant protein SP-D

Surfactant Protein D (SP-D) is a collectin protein that participates in the innate immune defense of the lungs. SP-D mediates the clearance of invading microorganisms by opsonization, aggregation or direct killing, which are lately removed by macrophages. SP-D is found as a mixture of trimers, hexamers, dodecamers and higher order oligomers, "fuzzy balls". However, it is unknown whether there are differences between these oligomeric forms in functions, activity or potency. In the present work, we have obtained fractions enriched in trimers, hexamers and fuzzy balls of full-length recombinant human (rh) SP-D by size exclusion chromatography, in a sufficient amount to perform functional assays. We have evaluated the differences in protein lectin-dependent activity relative to aggregation and binding to E. coli, one of the ligands of SP-D in vivo. Fuzzy balls are the most active oligomeric form in terms of binding and aggregation of bacteria, achieving 2-fold binding higher than hexamers and 50% bacteria aggregation at very short times. Hexamers, recently described as a defined oligomeric form of the protein, have never been isolated or tested in terms of protein activity. rhSP-D hexamers efficiently bind and aggregate bacteria, achieving 50-60% aggregation at final time point and high protein concentrations. Nevertheless, trimers are not able to aggregate bacteria, although they bind to them. Therefore, SP-D potency, in functions that relay on the C-lectin activity of the protein, is proportional to the oligomeric state of the protein.

Surfactant Proteins A and D: Trimerized Innate Immunity Proteins with an Affinity for Viral Fusion Proteins

Innate recognition of viruses is an essential part of the immune response to viral pathogens. This is integral to the maintenance of healthy lungs, which are free from infection and efficient at gaseous exchange. An important component of innate immunity for identifying viruses is the family of C-type collagen-containing lectins, also known as collectins. These secreted, soluble proteins are pattern recognition receptors (PRRs) which recognise pathogen-associated molecular patterns (PAMPs), including viral glycoproteins. These innate immune proteins are composed of trimerized units which oligomerise into higher-order structures and facilitate the clearance of viral pathogens through multiple mechanisms. Similarly, many viral surface proteins form trimeric configurations, despite not showing primary protein sequence similarities across the virus classes and families to which they belong. In this review, we discuss the role of the lung collectins, i.e., surfactant proteins A and D (SP-A and SP-D) in viral recognition. We focus particularly on the structural similarity and complementarity of these trimeric collectins with the trimeric viral fusion proteins with which, we hypothesise, they have elegantly co-evolved. Recombinant versions of these innate immune proteins may have therapeutic potential in a range of infectious and inflammatory lung diseases including anti-viral therapeutics.

The roles of direct recognition by animal lectins in antiviral immunity and viral pathogenesis

Lectins are a group of proteins with carbohydrate recognition activity. Lectins are categorized into many families based on their different cellular locations as well as their specificities for a variety of carbohydrate structures due to the features of their carbohydrate recognition domain (CRD) modules. Many studies have indicated that the direct recognition of particular oligosaccharides on viral components by lectins is important for interactions between hosts and viruses. Herein, we aim to globally review the roles of this recognition by animal lectins in antiviral immune responses and viral pathogenesis. The different classes of mammalian lectins can either recognize carbohydrates to activate host immunity for viral elimination or can exploit those carbohydrates as susceptibility factors to facilitate viral entry, replication or assembly. Additionally, some arthropod C-type lectins were recently identified as key susceptibility factors that directly interact with multiple viruses and then facilitate infection. Summarization of the pleiotropic roles of direct viral recognition by animal lectins will benefit our understanding of host-virus interactions and could provide insight into the role of lectins in antiviral drug and vaccine development.

Surfactant Protein D modulates HIV infection of both T-cells and dendritic cells

Surfactant Protein D (SP-D) is an oligomerized C-type lectin molecule with immunomodulatory properties and involvement in lung surfactant homeostasis in the respiratory tract. SP-D binds to the enveloped viruses, influenza A virus and respiratory syncytial virus and inhibits their replication in vitro and in vivo. SP-D has been shown to bind to HIV via the HIV envelope protein gp120 and inhibit infectivity in vitro. Here we show that SP-D binds to different strains of HIV (BaL and IIIB) and the binding occurs at both pH 7.4 and 5.0 resembling physiological relevant pH values found in the body and the female urogenital tract, respectively. The binding of SP-D to HIV particles and gp120 was inhibited by the presence of several hexoses with mannose found to be the strongest inhibitor. Competition studies showed that soluble CD4 and CVN did not interfere with the interaction between SP-D and gp120. However, soluble recombinant DC-SIGN was shown to inhibit the binding between SP-D and gp120. SP-D agglutinated HIV and gp120 in a calcium dependent manner. SP-D inhibited the infectivity of HIV strains at both pH values of 7.4 and 5.0 in a concentration dependent manner. The inhibition of the infectivity was abolished by the presence of mannose. SP-D enhanced the binding of HIV to immature monocyte derived dendritic cells (iMDDCs) and was also found to enhance HIV capture and transfer to the T-cell like line PM1. These results suggest that SP-D can bind to and inhibit direct infection of T-cells by HIV but also enhance the transfer of infectious HIV particles from DCs to T-cells in vivo.

S-nitrosylation of surfactant protein D as a modulator of pulmonary inflammation

BACKGROUND

Surfactant protein D (SP-D) is a member of the family of proteins termed collagen-like lectins or "collectins" that play a role in non-antibody-mediated innate immune responses [1]. The primary function of SP-D is the modulation of host defense and inflammation [2].

SCOPE OF REVIEW

This review will discuss recent findings on the physiological importance of SP-D S-nitrosylation in biological systems and potential mechanisms that govern SP-D mediated signaling.

MAJOR CONCLUSIONS

SP-D appears to have both pro- and anti-inflammatory signaling functions. SP-D multimerization is a critical feature of its function and plays an important role in efficient innate host defense. Under baseline conditions, SP-D forms a multimer in which the N-termini are hidden in the center and the C-termini are on the surface. This multimeric form of SP-D is limited in its ability to activate inflammation. However, NO can modify key cysteine residues in the hydrophobic tail domain of SP-D resulting in a dissociation of SP-D multimers into trimers, exposing the S-nitrosylated N-termini. The exposed S-nitrosylated tail domain binds to the calreticulin/CD91 receptor complex and initiates a pro-inflammatory response through phosphorylation of p38 and NF-κB activation [3,4]. In addition, the disassembled SP-D loses its ability to block TLR4, which also results in activation of NF-κB.

GENERAL SIGNIFICANCE

Recent studies have highlighted the capability of NO to modify SP-D through S-nitrosylation, causing the activation of a pro-inflammatory role for SP-D [3]. This represents a novel mechanism both for the regulation of SP-D function and NO's role in innate immunity, but also demonstrates that the S-nitrosylation can control protein function by regulating quaternary structure. This article is part of a Special Issue entitled Regulation of Cellular Processes by S-nitrosylation.

The ability of pandemic influenza virus hemagglutinins to induce lower respiratory pathology is associated with decreased surfactant protein D binding

Pandemic influenza viral infections have been associated with viral pneumonia. Chimeric influenza viruses with the hemagglutinin segment of the 1918, 1957, 1968, or 2009 pandemic influenza viruses in the context of a seasonal H1N1 influenza genome were constructed to analyze the role of hemagglutinin (HA) in pathogenesis and cell tropism in a mouse model. We also explored whether there was an association between the ability of lung surfactant protein D (SP-D) to bind to the HA and the ability of the corresponding chimeric virus to infect bronchiolar and alveolar epithelial cells of the lower respiratory tract. Viruses expressing the hemagglutinin of pandemic viruses were associated with significant pathology in the lower respiratory tract, including acute inflammation, and showed low binding activity for SP-D. In contrast, the virus expressing the HA of a seasonal influenza strain induced only mild disease with little lung pathology in infected mice and exhibited strong in vitro binding to SP-D.

Comprehensive characterisation of pulmonary and serum surfactant protein D in COPD

Background

Pulmonary surfactant protein D (SP-D) is considered as a candidate biomarker for the functional integrity of the lung and for disease progression, which can be detected in serum. The origin of SP-D in serum and how serum concentrations are related to pulmonary concentrations under inflammatory conditions is still unclear.

Methods

In a cross-sectional study comprising non-smokers (n = 10), young - (n = 10), elderly smokers (n = 20), and smokers with COPD (n = 20) we simultaneously analysed pulmonary and serum SP-D levels with regard to pulmonary function, exercise, repeatability and its quaternary structure by native gel electrophoresis. Statistical comparisons were conducted by ANOVA and post-hoc testing for multiple comparisons; repeatability was assessed by Bland-Altman analysis.

Results

In COPD, median (IQR) pulmonary SP-D levels were lower (129(68) ng/ml) compared to smokers (young: 299(190), elderly: 296(158) ng/ml; p < 0.01) and non-smokers (967(708) ng/ml; p < 0.001). The opposite was observed in serum, with higher concentrations in COPD (140(89) ng/ml) as compared to non-smokers (76(47) ng/ml; p < 0.01). SP-D levels were reproducible and correlated with the degree of airway obstruction in all smokers. In addition, smoking lead to disruption of the quaternary structure.

Conclusions

Pulmonary and serum SP-D levels are stable markers influenced by smoking and related to airflow obstruction and disease state. Smaller subunits of pulmonary SP-D and the rapid increase of serum SP-D levels in COPD due to exercise support the translocation hypothesis and its use as a COPD biomarker.

Trial registration

no interventional trial

Review: Chemical and structural modifications of pulmonary collectins and their functional consequences

The lung is continuously exposed to inhaled pathogens (toxic pollutants, micro-organisms, environmental antigens, allergens) from the external environment. In the broncho-alveolar space, the critical balance between a measured protective response against harmful pathogens and an inappropriate inflammatory response to harmless particles is discerned by the innate pulmonary immune system. Among its many components, the surfactant proteins and specifically the pulmonary collectins (surfactant proteins A [SP-A] and D [SP-D]) appear to provide important contributions to the modulation of host defense and inflammation in the lung. Many studies have shown that multimerization of SP-A and SP-D are important for efficient local host defense including neutralization and opsonization of influenza A virus, binding Pneumocystis murina and inhibition of LPS-induced inflammatory cell responses. These observations strongly imply that oligomerization of collectins is a critical feature of its function. However, during the inflammatory state, despite normal pool sizes, chemical modification of collectins can result in alteration of their structure and function. Both pulmonary collectins can be altered through proteolytic inactivation, nitration, S-nitrosylation, oxidation and/or crosslinking as a consequence of the inflammatory milieu facilitated by cytokines, nitric oxide, proteases, and other chemical mediators released by inflammatory cells. Thus, this review will summarize recent developments in our understanding of the relationship between post-translational assembly of collectins and their modification by inflammation as an important molecular switch for the regulation of local innate host defense.

Immune reconstitution during Pneumocystis lung infection: disruption of surfactant component expression and function by S-nitrosylation

Pneumocystis pneumonia (PCP), the most common opportunistic pulmonary infection associated with HIV infection, is marked by impaired gas exchange and significant hypoxemia. Immune reconstitution disease (IRD) represents a syndrome of paradoxical respiratory failure in patients with active or recently treated PCP subjected to immune reconstitution. To model IRD, C57BL/6 mice were selectively depleted of CD4(+) T cells using mAb GK1.5. Following inoculation with Pneumocystis murina cysts, infection was allowed to progress for 2 wk, GK1.5 was withdrawn, and mice were followed for another 2 or 4 wk. Flow cytometry of spleen cells demonstrated recovery of CD4(+) cells to >65% of nondepleted controls. Lung tissue and bronchoalveolar lavage fluid harvested from IRD mice were analyzed in tandem with samples from CD4-depleted mice that manifested progressive PCP for 6 wks. Despite significantly decreased pathogen burdens, IRD mice had persistent parenchymal lung inflammation, increased bronchoalveolar lavage fluid cellularity, markedly impaired surfactant biophysical function, and decreased amounts of surfactant phospholipid and surfactant protein (SP)-B. Paradoxically, IRD mice also had substantial increases in the lung collectin SP-D, including significant amounts of an S-nitrosylated form. By native PAGE, formation of S-nitrosylated SP-D in vivo resulted in disruption of SP-D multimers. Bronchoalveolar lavage fluid from IRD mice selectively enhanced macrophage chemotaxis in vitro, an effect that was blocked by ascorbate treatment. We conclude that while PCP impairs pulmonary function and produces abnormalities in surfactant components and biophysics, these responses are exacerbated by IRD. This worsening of pulmonary inflammation, in response to persistent Pneumocystis Ags, is mediated by recruitment of effector cells modulated by S-nitrosylated SP-D.

Reduced influenza viral neutralizing activity of natural human trimers of surfactant protein D

Background

Surfactant protein D (SP-D) plays important roles in innate host defense against influenza A virus (IAV) infection. Common human polymorphisms of SP-D have been found in many human populations and associated with increased risk of certain infections. We recently reported that the Thr/Thr 11 form of SP-D is associated with low serum levels and assembles predominantly as trimers as opposed to the more common multimeric forms of SP-D.

Methods

Preliminary experiments were done to establish the effects of different monoclonal antibodies against SP-D on ability of SP-D to bind to or neutralize the virus. We then purified natural human trimeric and multimeric forms of SP-D from amniotic fluid and tested ability of these preparations to bind to IAV, to inhibit infectivity and hemagglutination activity of IAV in vitro.

Results

In initial experiments mAbs directed against different areas on the CRD of SP-D were found to have differing effects on antiviral activity. Using an mAb that did not interfere with antiviral activity of SP-D, we confirm that natural SP-D trimers had reduced ability to bind to IAV. In addition, the trimers had reduced ability to neutralize IAV as compared to natural human SP-D multimers as well as reduced hemagglutination inhibiting activity against several strains of IAV. Natural SP-D trimers also had different interactions with human neutrophil peptide defensins (HNPs) in viral neutralization assays as compared to multimeric SP-D.

Conclusion

These studies indicate that a common human polymorphic form of SP-D may modulate host defense against IAV and give impetus to clinical studies correlating this genotype with risk for IAV infection in susceptible groups. We also show that mAbs directed against different areas on the carbohydrate recognition domain of SP-D can be useful for dissecting out different functional properties of the protein.

Oxidative damage to surfactant protein D in pulmonary diseases

Surfactant protein D is an important innate host defence molecule that has been shown to interact with a variety of pathogens and to play a role in surfactant homeostasis. The aim of this study was to examine the influence of oxidation on surfactant protein D in different lung diseases. Bronchoalveolar lavage fluids (BALFs) from patients with different grade of protein oxidation were examined for changes in the primary chain and the quaternary structure of surfactant protein D. Significant changes of quaternary surfactant protein-D (SP-D) structure were detected under oxidative conditions in vitro and in vivo. The functional capacity of surfactant protein D to agglutinate bacteria was impaired by oxidation. We conclude that surfactant protein D is an important target of free radicals generated in the lungs. Host defence may be impaired due to the oxidation of surfactant protein D and may contribute to the suppurative lung diseases like cystic fibrosis (CF).

Surfactant protein D decreases pollen-induced IgE-dependent mast cell degranulation

Mast cells play a key role in allergy and asthma. They reside at the host-environment interface and are among the first cells to make contact with inhaled microorganisms and particulate antigens. Pulmonary surfactant proteins A and D (SP-A and SP-D) function in lung host defense by enhancing microbe phagocytosis and mediating other immune cell functions, but little is known about their effects on mast cells. We hypothesized that SP-A and/or SP-D modulate IgE-dependent mast cell functions. Pollen starch granules (PSG) extracted from Dactylis glomerata and coated with trinitrophenol (TNP) were used as a model of an inhaled organic particulate allergen. Our data revealed that SP-D inhibited by 50% the release of beta-hexosaminidase by peritoneal mast cells sensitized with IgE anti-TNP and stimulated with TNP-PSG. In contrast, SP-A had no effect. Furthermore, SP-D aggregated PSG in a dose-dependent manner, and this aggregation was mediated by SP-D's carbohydrate recognition domain. A single arm SP-D mutant (RrSP-Dser15,20) neither aggregated PSG nor inhibited degranulation, suggesting that multimerization of SP-D is required for maximal PSG aggregation and inhibition of PSG-induced mast cell degranulation. This study is the first to demonstrate that SP-D modulates IgE-mediated mast cell functions, which are important in asthma and allergic inflammation.

[Classification of pulmonary alveolar proteinosis in newborns, infants, and children]

Pulmonary alveolar proteinoses are rare pulmonary diseases characterised by an intraalveolar accumulation of surfactant protein A. Subtyping of alveolar proteinoses: Type I alveolar proteinoses: severe respiratory insufficiency in newborns, which will take a lethal course without lung transplant; hereditary SP-B deficiency and an intraalveolar accumulation of N-terminal incompletely processed SP-C. Type II alveolar proteinoses: occur in newborns and infants; often take a lethal course; show intraalveolar accumulation of precursors of SP-B and mature SP-B as well as an accompanying interstitial lung disease of variable severity. Type III alveolar proteinoses: in infants and children; do not generally take a lethal course; they are characterised by an intraalveolar accumulation of precursors of SP-B and mature SP-B without accompanying interstitial lung disease. "Cryptogenic" congenital, acquired (idiopathic), and secondary type III alveolar proteinoses can be distinguished. In newborns, infants, and children with pulmonary alveolar proteinosis, a detailed pathological-anatomical examination including immunohistochemical and molecular genetic analyses, should be performed in order to optimise the therapeutical management.

Neutrophil Serine Proteinases Inactivate Surfactant Protein D by Cleaving within a Conserved Subregion of the Carbohydrate Recognition Domain

Surfactant protein D (SP-D) plays important roles in innate immunity including the defense against bacteria, fungi, and respiratory viruses. Because SP-D specifically interacts with neutrophils that infiltrate the lung in response to acute inflammation and infection, we examined the hypothesis that the neutrophil-derived serine proteinases (NSPs): neutrophil elastase, proteinase-3, and cathepsin G degrade SP-D. All three human NSPs specifically cleaved recombinant rat and natural human SP-D dodecamers in a time- and dose-dependent manner, which was reciprocally dependent on calcium concentration. The NSPs generated similar, relatively stable, disulfide cross-linked immunoreactive fragments of approximately 35 kDa (reduced), and sequencing of a major catheptic fragment definitively localized the major sites of cleavage to a highly conserved subregion of the carbohydrate recognition domain. Cleavage markedly reduced the ability of SP-D to promote bacterial aggregation and to bind to yeast mannan in vitro. Incubation of SP-D with isolated murine neutrophils led to the generation of similar fragments, and cleavage was inhibited with synthetic and natural serine proteinase inhibitors. In addition, neutrophils genetically deficient in neutrophil elastase and/or cathepsin G were impaired in their ability to degrade SP-D. Using a mouse model of acute bacterial pneumonia, we observed the accumulation of SP-D at sites of neutrophil infiltration coinciding with the appearance of approximately 35-kDa SP-D fragments in bronchoalveolar lavage fluids. Together, our data suggest that neutrophil-derived serine proteinases cleave SP-D at sites of inflammation with potential deleterious effects on its biological functions.

Structure-function studies of the adipocyte-secreted hormone Acrp30/adiponectin. Implications fpr metabolic regulation and bioactivity

Acrp30/adiponectin is an adipocyte-specific secretory protein that has recently been implicated as a mediator of systemic insulin sensitivity with liver and muscle as target organs. Acrp30 is found as two forms in serum, as a lower molecular weight trimer-dimer and a high molecular weight complex. Little is know about the regulation and significance of these Acrp30 complexes in serum and about the events that lead to the generation of the bioactive ligand. Here, we show that there is a profound sexual dimorphism of Acrp30 levels and complex distribution in serum. Female mice display significantly higher levels of the high molecular weight complex in serum than males. In both females and males, levels of the high molecular weight complex are significantly reduced in response to a systemic increase of insulin. The ratio of the two complexes is restored upon normalization of glucose levels. Structurally, we show that oligomer formation of Acrp30 critically depends on disulfide bond formation mediated by Cys-39. Mutation of Cys-39 results in trimers that are subject to proteolytic cleavage in the collagenous domain. Surprisingly, Acrp30(C39S) or wild-type Acrp30 treated with dithiothreitol are significantly more bioactive than the higher order oligomeric forms of the protein with respect to reduction of serum glucose levels. Furthermore, treatment of primary hepatocytes with trimeric and higher order forms of Acrp30 confirms that the increased bioactivity seen in vivo is reflected in an augmented potency to reduce glucose output in the presence of gluconeogenic stimuli. Combined, these results shed new light on the regulation of this complex protein and suggest a new model for in vivo activation of the protein, implicating a serum reductase activity.

Contributions of the N- and C-terminal domains of surfactant protein d to the binding, aggregation, and phagocytic uptake of bacteria

Collectins play important roles in host defense against infectious microorganisms. We now demonstrate that the serum collectins mannose-binding lectin (MBL) and conglutinin have less ability to bind to, aggregate, and enhance neutrophil uptake of several strains of gram-negative and gram-positive bacteria than pulmonary surfactant protein D (SP-D). Collectins are composed of four major structural domains (i.e., N-terminal, collagen, and neck and carbohydrate recognition domains). To determine which domains of SP-D are responsible for its greater bacterial binding or aggregating activity, activities of chimeric collectins containing the N-terminal and collagen domains of SP-D coupled to the neck recognition domains and carbohydrate recognition domains (CRD) of MBL or conglutinin (SP-D/Cong(neck+CRD) and SP-D/MBL(neck+CRD)) were tested. The SP-D/Cong(neck+CRD) and SP-D/MBL(neck+CRD) chimeras bound to and aggregated the bacteria more strongly than did wild-type MBL or conglutinin. SP-D/MBL(neck+CRD) also enhanced neutrophil uptake of bacteria more so than MBL. Hence, the SP-D N-terminal and/or collagen domains contribute to the enhanced bacterial binding and aggregating activities of SP-D. In prior studies, SP-D/Cong(neck+CRD) and SP-D/MBL(neck+CRD) had increased ability to bind to influenza virus compared not only with that of conglutinin or MBL but with that of wild-type SP-D as well. In contrast, the chimeras had either reduced or unchanged ability to bind to or aggregate bacteria compared to that of wild-type SP-D. Hence, although replacement of the neck recognition domains and CRDs of SP-D with those of MBL and conglutinin conferred increased viral binding activity, it did not favorably affect bacterial binding activity, suggesting that requirements for optimal collectin binding to influenza virus and bacteria differ.

Surfactant protein D binds selectively to Klebsiella pneumoniae lipopolysaccharides containing mannose-rich O-antigens

Surfactant protein D (SP-D) plays important roles in the regulation of innate immune responses in the lung. We have previously shown that SP-D can agglutinate and enhance the macrophage-dependent killing of specific unencapsulated phase variants of Klebsiella pneumoniae. In the present studies, we used 16 clinical isolates of Klebsiella representing four O-serotypes and examined the interaction of SP-D with their isolated LPSs. Although SP-D bound to the core oligosaccharide of rough LPS from all isolates, it selectively bound to smooth forms of LPS expressed by O-serotypes with mannose-rich repeating units in their O-polysaccharides. SP-D was more potent in agglutinating unencapsulated phase variants of O-serotypes expressing these SP-D "reactive" O-polysaccharides, and more effectively inhibited the adhesion of these serotypes to lung epithelial cells. This novel anti-adhesion activity required the multimerization of trimeric SP-D subunits (dodecamers). Klebsiella serotypes expressing "nonreactive" LPS O-Ags were isolated at a significantly higher frequency from patients with K. pneumoniae. Our findings suggest that SP-D plays important roles in the clearance of opportunistic Gram-negative bacteria and contributes to known serotypic differences in the pathogenicity of Klebsiella through specific interactions with O-polysaccharides.

Distinctive anti-influenza properties of recombinant collectin 43

Collectins play important roles in innate defence against viral, fungal and bacterial pathogens. CL-43, a bovine serum collectin, which appears to have evolutionarily evolved from surfactant protein D (SP-D), shows unique structural and functional properties. In the present study, we describe the initial characterization of a recombinant CL-43 expressed in mammalian cells. Like natural CL-43, the recombinant is secreted as trimeric forms that show a preference for mannose and N-acetyl mannosamine. The natural and recombinant proteins have significantly higher haemagglutination-inhibiting activity against influenza A virus (IAV) than recombinant trimeric forms of SP-D. In contrast with the more highly multimerized forms of SP-D, namely conglutinin or mannose-binding lectin, CL-43 did not induce viral or bacterial aggregation and did not enhance IAV-induced neutrophil H(2)O(2) generation. Like SP-D, CL-43 also strongly enhanced neutrophil uptake of IAV. However, the mechanism of this enhanced internalization is different from that of SP-D in that it did not require viral aggregation. These studies establish that the trimeric structure of CL-43 is specified by its primary sequence and indicate that this naturally occurring trimeric collectin has unique antiviral activities. These findings could facilitate the development of recombinant collectins with novel antimicrobial properties.

Complementation of pulmonary abnormalities in SP-D(-/-) mice with an SP-D/conglutinin fusion protein

Surfactant protein D (SP-D) and serum conglutinin are closely related members of the collectin family of host defense lectins. Although normally synthesized at different anatomic sites, both proteins participate in the innate immune response to microbial challenge. To discern the roles of specific domains in the function of SP-D in vivo, a fusion protein (SP-D/Cong(neck+CRD)) consisting of the NH(2)-terminal and collagenous domains of rat SP-D (rSP-D) and the neck and carbohydrate recognition domains (CRDs) of bovine conglutinin (Cong) was expressed in the respiratory epithelium of SP-D gene-targeted (SP-D(-/-)) mice. While SP-D/Cong(neck+CRD) fusion protein did not affect lung morphology and surfactant phospholipid levels in the lungs of wild type mice, the chimeric protein substantially corrected the increased lung phospholipids in SP-D(-/-) mice. The SP-D/Cong(neck+CRD) fusion protein also completely corrected defects in influenza A clearance and inhibited the exaggerated inflammatory response that occurs following viral infection. However, the chimeric protein did not ameliorate the ongoing lung inflammation, enhanced metalloproteinase expression, and alveolar destruction that characterize this model of SP-D deficiency. By contrast, a single arm mutant (RrSP-D(Ser15,20)) partially restored antiviral activity but otherwise failed to rescue the deficient phenotype. Our findings directly implicate the CRDs of both SP-D and conglutinin in host defense in vivo. Our findings also strongly suggest that the molecular mechanisms underlying impaired pulmonary host defense and abnormal lipid metabolism are distinct from those that promote ongoing inflammation and the development of emphysema.

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.

The amino-terminal heptad repeats of the coiled-coil neck domain of pulmonary surfactant protein d are necessary for the assembly of trimeric subunits and dodecamers

Pulmonary surfactant protein D (SP-D), a lung host defense protein, is assembled as multimers of trimeric subunits. Trimerization of SP-D monomers is required for high affinity saccharide binding, and the oligomerization of trimers is required for many of its functions. A peptide containing the alpha-helical neck region can spontaneously trimerize in vitro. However, it is not known whether this sequence is necessary for the complete cellular assembly of disulfide-cross-linked, trimeric subunits and dodecamers. For the present studies, we synthesized mutant cDNAs with deletions or site-directed substitutions in the neck domain of rat SP-D, and examined the assembly of the newly synthesized proteins after transfection of CHO-K1 cells. The neck domain contains three "classical" heptad repeat motifs with leucine residues at the "d position," and a distinctive C-terminal repeat previously suggested to drive trimeric chain association. Deletion of the highly conserved core of the latter repeat (FSRYLKK) did not interfere with the secretion of dodecamers with lectin activity. By contrast, deletion of the entire neck domain or deletion of one or two amino-terminal repeats resulted in defective molecular assembly. The secreted proteins eluted in the position of monomers by gel filtration under nondenaturing conditions. In addition, the neck + carbohydrate recognition domain of SP-D was necessary and sufficient for the trimerization of a heterologous collagen sequence located amino-terminal to the trimeric coiled-coil. These studies provide strong evidence that the amino-terminal heptad repeats of the neck domain are necessary for the intracellular, trimeric association of SP-D monomers and for the assembly and secretion of functional dodecamers.

Activity of pulmonary surfactant protein-D (SP-D) in vivo is dependent on oligomeric structure

Pulmonary surfactant protein-D (SP-D) is a member of the collectin family of C-type lectins that is synthesized in many tissues including respiratory epithelial cells in the lung. SP-D is assembled predominantly as dodecamers consisting of four homotrimeric subunits each. Association of these subunits is stabilized by interchain disulfide bonds involving two conserved amino-terminal cysteine residues (Cys-15 and Cys-20). Mutant recombinant rat SP-D lacking these residues (RrSP-Dser15/20) is secreted in cell culture as trimeric subunits rather than as dodecamers. In this study, transgenic mice that express this mutant were generated to elucidate the functional importance of SP-D oligomerization in vivo. Expression of RrSP-Dser15/20 failed to correct the pulmonary phospholipid accumulation and emphysema characteristic of SP-D null (mSP-D-/-) mice. Expression of high concentrations of the mutant protein in wild-type mice reduced the abundance of disulfide cross-linked oligomers of endogenous SP-D in the bronchoalveolar lavage fluid and demonstrated a phenotype that partially overlapped with that of the SP-D-/- mice; the animals developed emphysema and foamy macrophages without the associated abnormalities in alveolar phospholipids typical of SP-D-/- mice. Development of foamy macrophages in SP-D-deficient mice is not secondary to the increased abundance of surfactant phospholipids. Disulfide cross-linked SP-D oligomers are required for the regulation of surfactant phospholipid homeostasis and the prevention of emphysema and foamy macrophages in vivo.

Surfactant protein-D and pulmonary host defense

Surfactant protein-D (SP-D) participates in the innate response to inhaled microorganisms and organic antigens, and contributes to immune and inflammatory regulation within the lung. SP-D is synthesized and secreted by alveolar and bronchiolar epithelial cells, but is also expressed by epithelial cells lining various exocrine ducts and the mucosa of the gastrointestinal and genitourinary tracts. SP-D, a collagenous calcium-dependent lectin (or collectin), binds to surface glycoconjugates expressed by a wide variety of microorganisms, and to oligosaccharides associated with the surface of various complex organic antigens. SP-D also specifically interacts with glycoconjugates and other molecules expressed on the surface of macrophages, neutrophils, and lymphocytes. In addition, SP-D binds to specific surfactant-associated lipids and can influence the organization of lipid mixtures containing phosphatidylinositol in vitro. Consistent with these diverse in vitro activities is the observation that SP-D-deficient transgenic mice show abnormal accumulations of surfactant lipids, and respond abnormally to challenge with respiratory viruses and bacterial lipopolysaccharides. The phenotype of macrophages isolated from the lungs of SP-D-deficient mice is altered, and there is circumstantial evidence that abnormal oxidant metabolism and/or increased metalloproteinase expression contributes to the development of emphysema. The expression of SP-D is increased in response to many forms of lung injury, and deficient accumulation of appropriately oligomerized SP-D might contribute to the pathogenesis of a variety of human lung diseases.

Hypoxic induction of prolyl 4-hydroxylase alpha (I) in cultured cells

Accumulated evidence indicates that hypoxia activates collagen synthesis in tissues. To explore the molecular mechanism of activation, we screened genes that are up-regulated or down-regulated by hypoxia. Fibroblasts isolated from fetal rat lung were cultured under hypoxia. Differential display technique showed that the mRNA level of prolyl 4-hydroxylase (PH) alpha(I), an active subunit that catalyzes the oxygen-dependent hydroxylation of proline residue in procollagen, increased 2-3-fold after an 8-h exposure to hypoxia. This elevated level was maintained over 40 h and returned to the basal level after reoxygenation. The transcription rate, protein level, and hydroxyproline content (an indicator of the prolyl hydroxylation) were all elevated by hypoxic culture. Analysis of the promotor region of PHalpha(I) gene indicated that a motif similar to hypoxia-responsive element (HRE) of hypoxia-inducible genes such as erythropoietin, was identified within a 120-base pair sequence upstream of the transcription start site. Luciferase reporter assay and mutational analysis showed that a site similar to the HRE in this motif is functionally essential to hypoxic response. Electrophoretic mobility shift assay revealed that hypoxia-inducible factor-1 was stimulated and bound to the PHalpha(I) HRE upon hypoxic challenge. Our results indicate that PHalpha(I), an essential enzyme for collagen synthesis, is a target gene for hypoxia-inducible factor-1.

Porcine lung surfactant protein D: complementary DNA cloning, chromosomal localization, and tissue distribution

Porcine organs and lung surfactant have medically important applications in both xenotransplantation and therapy. We have started to characterize porcine lung surfactant by cloning the cDNA of porcine surfactant protein D (SP-D). SP-D and SP-A are important mediators in innate immune defense for the lung and possibly other mucosal surfaces. Porcine SP-D will also be an important reagent for use in existing porcine animal models for human lung infections. The complete cDNA sequence of porcine SP-D, including the 5' and 3' untranslated regions, was determined from two overlapping bacteriophage clones and by PCR cloning. Three unique features were revealed from the porcine sequence in comparison to SP-D from other previously characterized species, making porcine SP-D an intriguing species addition to the SP-D/collectin family. The collagen region contains an extra cysteine residue, which may have important structural consequences. The other two differences, a potential glycosylation site and an insertion of three amino acids, lie in the loop regions of the carbohydrate recognition domain, close to the carbohydrate binding region and thus may have functional implications. These variations were ruled out as polymorphisms or mutations by confirming the sequence at the genomic level in four different pig breeds. Porcine SP-D was shown to localize primarily to the lung and with less abundance to the duodenum, jejunum, and ileum. The genes for SP-D and SP-A were also shown to colocalize to a region of porcine chromosome 14 that is syntenic with the human and murine collectin loci.

Enhanced anti-influenza activity of a surfactant protein D and serum conglutinin fusion protein

We previously demonstrated that bovine serum conglutinin has markedly greater ability to inhibit influenza A virus (IAV) infectivity than other collectins. We now show that recombinant conglutinin and a chimeric protein containing the NH(2) terminus and collagen domain of rat pulmonary surfactant protein D (rSP-D) fused to the neck region and carbohydrate recognition domain (CRD) of conglutinin (termed SP-D/Cong(neck+CRD)) have markedly greater ability to inhibit infectivity of IAV than wild-type recombinant rSP-D, confirming that the potent IAV-neutralizing activity of conglutinin resides in its neck region and CRD. Furthermore, by virtue of incorporation of the NH(2) terminus and collagen domain of SP-D, SP-D/Cong(neck+CRD) caused substantially greater aggregation of IAV particles and enhancement of neutrophil binding of, and H(2)O(2) responses to, IAV than recombinant conglutinin or recombinant rSP-D. Hence, SP-D/Cong(neck+CRD) combined favorable antiviral and opsonic properties of conglutinin and SP-D. This study demonstrates an association of specific structural domains of SP-D and conglutinin with specific functional properties and illustrates that antimicrobial activities of wild-type collectins can be enhanced through recombinant strategies.

Microfibril-associated protein 4 is present in lung washings and binds to the collagen region of lung surfactant protein D

We have purified a glycoprotein from bovine lung washings using affinity chromatography on a maltose-affinity column. On SDS-polyacrylamide gel electrophoresis the protein showed a molecular mass of 36 kDa in the reduced state and 66 kDa in the unreduced state. On gel permeation chromatography the apparent molecular mass was 250 kDa. N-terminal sequencing showed homology to the human matrix protein microfibril-associated protein (hMFAP4), and the glycoprotein was designated bovine MFAP4 (bMFAP4). Lung surfactant protein D (SP-D) was also purified from lung washings, and calcium-dependent binding was demonstrated between bMFAP4 and SP-D. hMFAP4 was cloned, and recombinant hMFAP4 showed the same binding pattern to SP-D as bMFAP4. No binding was seen to recombinant SP-D composed of the neck region and carbohydrate recognition domain of SP-D, indicating that the interaction between MFAP4 and SP-D is mediated via the collagen region of SP-D. MFAP4 also showed calcium-dependent binding to mannan, which was partially inhibited by maltose. Our findings indicate that MFAP4 has two binding specificities, one for collagen and one for carbohydrate, and we suggest that MFAP4 may fix the collectins in the extracellular compartment during inflammation.

Genomic organization of the mouse gene for lung surfactant protein D

Lung surfactant protein (SP)-D belongs to the family of soluble collagenous C-type lectins, named collectins. SP-D participates in the local innate immune defense of the lung, eliciting various effector functions by acting as a pattern recognition receptor for the carbohydrate structures on inhaled microorganisms and particulate matter. This work describes the isolation and characterization of the mouse SP-D gene (Sftpd), which spans 8 exons over 14 kb of sequence and shows an overall organization similar to other collectin genes. The complete 5' untranslated region of the messenger RNA, absent from the published complementary DNA for mouse SP-D, was also cloned and is shown to be encoded by a single exon. Analysis of 3.5 kb of 5' flanking nucleotide sequence for Sftpd is described and reveals positional conservation of a number of transcription factor binding sites on comparison of Sftpd with the human SP-D gene and the bovine conglutinin gene. In addition, a single copy SP-D-like gene has been shown to be present in mammals, birds, and amphibians but is absent in fish. An atypical, rodent-specific, long terminal repeat of retroviral origin containing a minisatellite that has become inserted in Sftpd is described. Three new polymorphic microsatellites are also described, one of which is just 160 base pairs upstream of Sftpd. This microsatellite was used to map the gene to the central region of chromosome 14; fine-scale mapping indicates that it lies in a 5. 64-centimorgan area between D14Mit45 and D14Mit60. This will allow the easy identification of the collectin gene cluster and aid in the construction of a physical map over this region.

Deletion mapping of N-terminal domains of surfactant protein A. The N-terminal segment is required for phospholipid aggregation and specific inhibition of surfactant secretion

The objective of the current study was to examine the functional importance of the N-terminal domains of surfactant protein A (SP-A) including the N-terminal segment from Asn1 to Ala7 (denoted domain 1), the N-terminal portion of the collagen domain from Gly8 to Gly44 (domain 2), and the C-terminal portion of the collagen-like domain from Gly45 to Pro80 (domain 3). Wild type recombinant SP-A (SP-Ahyp; where hyp indicates hydroxyproline-deficient) and truncated mutant (TM) SP-As containing deletions of domain(s) 1 (TM1), 2 (TM2), 1 and 2 (TM1-2), and 1, 2, and 3 (TM1-2-3) were synthesized in insect cells and purified by mannose-Sepharose affinity chromatography. N-terminal disulfide-dependent dimerization was preserved at near wild type levels in the TM1-2 (at Cys-1) and TM2 proteins (at Cys-1 and Cys6), and to a lesser extent in TM1 (at Cys-1), but not in TM1-2-3. Cross-linking analyses demonstrated that the neck + CRD was sufficient for assembly of monomers into noncovalent trimers and that the N-terminal segment was required for the association of trimers to form higher oligomers. All TM proteins except TM1-2-3 bound to phospholipid, but only the N-terminal segment containing TM proteins aggregated phospholipid vesicles. The TM1, TM1-2, and TM2 but not the TM1-2-3 inhibited the secretion of surfactant from type II cells as effectively as SP-Ahyp, but the inhibitory activity of each mutant was blocked by excess alpha-methylmannoside and therefore nonspecific. TM1 and TM1-2-3 did not enhance the uptake of phospholipids by isolated type II cells, but the TM1-2 and TM2 had activities that were 72 and 83% of SP-Ahyp, respectively. We conclude the following for SP-A: 1) trimerization does not require the collagen-like region or interchain disulfide linkage; 2) the N-terminal portion of the collagen-like domain is required for specific inhibition of surfactant secretion but not for binding to liposomes or for enhanced uptake of phospholipids into type II cells; 3) N-terminal interchain disulfide linkage can functionally replace the N-terminal segment for lipid binding, receptor binding, and enhancement of lipid uptake; 4) the N-terminal segment is required for the association of trimeric subunits into higher oligomers, for phospholipid aggregation, and for specific inhibition of surfactant secretion and cannot be functionally replaced by disulfide linkage alone for these activities.

Structure, biologic properties, and expression of surfactant protein D (SP-D)

Surfactant protein D (SP-D) is a member of the family of collagenous host defense lectins, designated collectins. There is increasing evidence that SP-D, like SP-A, is an important component of the innate immune response to microbial challenge, and that it may participate in other aspects of immune and inflammatory regulation within the lung. SP-D binds to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms and certain other organic particles, in vitro. Although binding may facilitate microbial clearance through aggregation or other direct effects on the organism, SP-D also has the capacity to modulate leukocyte function, and in some circumstances, to enhance their killing of microorganisms.

Structure, processing and properties of surfactant protein A

Surfactant protein A (SP-A) is a highly ordered, oligomeric glycoprotein that is secreted into the airspaces of the lung by the pulmonary epithelium. The in vitro activities of protein suggest diverse roles in pulmonary host defense and surfactant homeostasis, structure and surface activity. Functional mapping of SP-A using directed mutagenesis has identified domains which interact with surfactant phospholipids, alveolar type II cells and microbes. Recently developed genetically manipulated animal models are beginning to clarify the critical physiological roles for SP-A in the normal lung, and in the pathophysiology of pulmonary disease.

Recombinant Rat Surfactant-Associated Protein D Inhibits Human T Lymphocyte Proliferation and IL-2 Production

Components of the airspace-lining material may contribute to the local regulation of immune function within the lung. We report here that recombinant rat pulmonary surfactant-associated protein D (SP-D) inhibits the lectin- and anti-CD3-stimulated proliferation of human PBMCs. Inhibition was associated with a decreased production of IL-2, and the addition of human rIL-2 blocked the inhibitory action of SP-D. These effects were not inhibited by maltose, indicating that the inhibitory activity was not dependent upon the lectin activity of SP-D. Studies employing mutant SP-D lacking N-linked sugars or defective in multimerization further indicated that inhibition was not dependent upon cellular interactions with the N-linked oligosaccharide on SP-D or the oligomerization of trimeric SP-D subunits. Although a peptide containing an inverted DGR showed similar IL-2-dependent effects on anti-CD3-stimulated proliferation, deletion of the conserved DGRDGR sequence near the amino-terminal end of the collagen domain did not decrease the suppressive activity of SP-D. We hypothesize that SP-D can dampen lymphocyte responses to exogenous stimuli and protect the lung against collateral immune-mediated damage.

Structural aspects of collectins and receptors for collectins

The collectins are oligomeric molecules composed of C-type lectin domains attached to collagen regions via alpha-coiled neck regions. Five members of the collectins have been characterized. Mannan-binding lectin (MBL), conglutinin and collectin-43 (CL-43) are serum proteins produced by the liver. Lung surfactant protein A (SP-A) and lung surfactant protein D (SP-D) are mainly found in the lung, where they are synthesized by alveolar type II cells and secreted to the alveolar surface. The collectins are believed to play an important role in innate immunity. They bind oligosaccharides on the surface of a variety of microbial pathogens. After binding of the collectins to the microbial surface effector mechanisms such as agglutination, neutralizing or opsonization of the microorganisms for phagocytosis are initiated. SP-A and SP-D stimulate chemotaxis of phagocytes and once bound to the phagocytes, the production of oxygen radicals can be induced. In the case of MBL the opsonization can be further enhanced by complement activation via the MBLectin pathway while conglutinin interacts with the complement system by binding to the complement degradation product iC3b. A number of receptors and binding molecules interacting with the collectins are found on the membrane or in association with the membrane of various cells responsible for phagocytosis and clearance of microorganisms. This paper focus on the structural aspects of the collectins and the receptors for collectins.

Collectins and pulmonary host defense

The surfactant-associated proteins SP-A and SP-D are members of a family of collagenous host defense lectins, designated collectins. There is increasing evidence that these pulmonary epithelial-derived proteins are important components of the innate immune response to microbial challenge, and that they participate in other aspects of immune and inflammatory regulation within the lung. The collectins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms and certain other organic particles in vitro. Although binding may facilitate microbial clearance through aggregation or other direct effects on the organism, SP-A and SP-D also have the capacity to modulate leukocyte function and, in some circumstances, to enhance their killing of microorganisms. The biologic activity of cell wall components, such as gram-negative bacterial polysaccharides, may be altered by interactions with collectins. Complementary or cooperative interactions between SP-A and SP-D could contribute to the efficiency of this defense system. Collectins may play particularly important roles in settings of inadequate or impaired specific immunity. Acquired or genetic alterations in the levels of active proteins within the airspaces and distal airways may increase susceptibility to infection.

Pulmonary surfactant proteins A and D enhance neutrophil uptake of bacteria

The collectins are a class of collagenous lectin proteins present in serum and pulmonary secretions [pulmonary surfactant protein (SP) A and SP-D] that are believed to participate in innate immune responses to various pathogens. With the use of flow cytometric and fluorescent-microscopic assays, SP-A and SP-D were shown to increase calcium-dependent neutrophil uptake of Escherichia coli, Streptococcus pneumoniae, and Staphylococcus aureus. Evidence is provided that the collectins enhanced bacterial uptake through a mechanism that involved both bacterial aggregation and direct actions on neutrophils. The degree of multimerization of SP-D preparations was a critical determinant of both aggregating activity and potency in enhancing bacterial uptake. The mechanisms of opsonizing activity of SP-D and SP-A differed in important respects from those of opsonizing antibodies. These results provide the first evidence that surfactant collectins may promote neutrophil-mediated clearance of bacteria in the lung independently of opsonizing antibody.

Mechanisms of anti-influenza activity of surfactant proteins A and D: comparison with serum collectins

The present study provides the first direct comparison of anti-influenza A virus (IAV) activities of the collectins surfactant protein (SP) A and SP-D, mannose-binding lectin (MBL), and conglutinin. SP-D, MBL, and conglutinin inhibited IAV hemagglutination activity with a greater potency than and by a distinct mechanism from SP-A. Although isolated trimeric SP-D carbohydrate recognition domains inhibited hemagglutination activity, preparations of SP-D also containing the collagen domain and NH2 terminus caused greater inhibition. In contrast to SP-A (or nonmultimerized SP-D), absence of the N-linked attachment did not effect interactions of multimerized SP-D with IAV. SP-D, SP-A, and conglutinin caused viral precipitation through formation of massive viral aggregates, whereas MBL formed aggregates of smaller size that did not precipitate. All of the collectins enhanced IAV binding to neutrophils; however, in the case of MBL, this effect was modest compared with the binding enhancement induced by SP-D or conglutinin. These studies clarify the structural requirements for viral inhibition by SP-D and reveal significant differences in the mechanisms of anti-IAV activity among the collectins.