Surfactant protein A differentially regulates IFN-gamma- and LPS-induced nitrite production by rat alveolar macrophages

Genomic and epigenomic adaptation in SP-R210 (Myo18A) isoform-deficient macrophages

Macrophages play an important role in maintaining tissue homeostasis, from regulating the inflammatory response to pathogens to resolving inflammation and aiding tissue repair. The surfactant protein A (SP-A) receptor SP-R210 (MYO18A) has been shown to affect basal and inflammatory macrophage states. Specifically, disruption of the longer splice isoform SP-R210_L/MYO18Aα renders macrophages hyper-inflammatory, although the mechanism by which this occurs is not well understood. We asked whether disruption of the L isoform led to the hyper-inflammatory state via alteration of global genomic responses. RNA sequencing analysis of L isoform-deficient macrophages (SP-R210_L(DN)) revealed basal and influenza-induced upregulation of genes associated with inflammatory pathways, such as TLR, RIG-I, NOD, and cytoplasmic DNA signaling, whereas knockout of both SP-R210 isoforms (L and S) only resulted in increased RIG-I and NOD signaling. Chromatin immunoprecipitation sequencing (ChIP-seq) analysis showed increased genome-wide deposition of the pioneer transcription factor PU.1 in SP-R210_L(DN) cells, with increased representation around genes relevant to inflammatory pathways. Additional ChIP-seq analysis of histone H3 methylation marks showed decreases in both repressive H3K9me3 and H3K27me3 marks with a commensurate increase in transcriptionally active (H3K4me3) histone marks in the L isoform deficient macrophages. Influenza A virus (IAV) infection, known to stimulate a wide array of anti-viral responses, caused a differential redistribution of PU.1 binding between proximal promoter and distal sites and decoupling from Toll-like receptor regulated gene promoters in SP-R210_L(DN) cells. These finding suggest that the inflammatory differences seen in SP-R210_L-deficient macrophages are a result of transcriptional differences that are mediated by epigenetic changes brought about by differential expression of the SP-R210 isoforms. This provides an avenue to explore how the signaling pathways downstream of the receptor and the ligands can modulate the macrophage inflammatory response.

In vitro inflammation inhibition model based on semi-continuous toll-like receptor biosensing

A chemical inhibition model of inflammation is proposed by semi-continuous monitoring the density of toll-like receptor 1 (TLR1) expressed on mammalian cells following bacterial infection to investigate an in vivo-mimicked drug screening system. The inflammation was induced by adding bacterial lysate (e.g., Pseudomonas aeruginosa) to a mammalian cell culture (e.g., A549 cell line). The TLR1 density on the same cells was immunochemically monitored up to three cycles under optimized cyclic bacterial stimulation-and-restoration conditions. The assay was carried out by adopting a cell-compatible immunoanalytical procedure and signal generation method. Signal intensity relative to the background control obtained without stimulation was employed to plot the standard curve for inflammation. To suppress the inflammatory response, sodium salicylate, which inhibits nuclear factor-κB activity, was used to prepare the standard curve for anti-inflammation. Such measurement of differential TLR densities was used as a biosensing approach discriminating the anti-inflammatory substance from the non-effector, which was simulated by using caffeic acid phenethyl ester and acetaminophen as the two components, respectively. As the same cells exposed to repetitive bacterial stimulation were semi-continuously monitored, the efficacy and toxicity of the inhibitors may further be determined regarding persistency against time. Therefore, this semi-continuous biosensing model could be appropriate as a substitute for animal-based experimentation during drug screening prior to pre-clinical tests.

Dipeptidyl peptidase I controls survival from Klebsiella pneumoniae lung infection by processing surfactant protein D

Prior work established that a deficiency in the cysteine protease dipeptidyl peptidase I (DPPI) improves survival following polymicrobial septic peritonitis. To test whether DPPI regulates survival from severe lung infections, DPPI(-/-) mice were studied in a Klebsiella pneumoniae lung infection model, finding that survival in DPPI(-/-) mice is significantly better than in DPPI(+/+) mice 8d after infection. DPPI(-/-) mice have significantly fewer bacteria in the lung than infected DPPI(+/+) mice, but no difference in lung histopathology, lung injury, or cytokine levels. To explore mechanisms of enhanced bacterial clearance in DPPI(-/-) mice, we examined the status of pulmonary collectins, finding that levels of surfactant protein D, but not of surfactant protein A, are higher in DPPI(-/-) than in DPPI(+/+) BAL fluid, and that DPPI(-/-) BAL fluid aggregate bacteria more effectively than control BAL fluid. Sequencing of the amino terminus of surfactant protein D revealed two or eight additional amino acids in surfactant protein D isolated from DPPI(-/-) mice, suggesting processing by DPPI. These results establish that DPPI is a major determinant of survival following Klebsiella pneumoniae lung infection and suggest that the survival disadvantage in DPPI(+/+) mice is in part due to processing of surfactant protein D by DPPI.

Antioxidant Properties of Surfactant

Surfactant treatment is one of the milestones of respiratory distress syndrome (RDS) treatment in preterm infants, but it has been also demonstrated to exert consistent antioxidant and anti-inflammatory activities. Exogenous natural surfactant contains antioxidant enzymes, such as catalase (CAT) and superoxide dismutase (SOD), and nonenzymatic antioxidant molecules, such as plasmalogens and polyunsaturated phospholipids (PUPLs). Moreover, surfactant can contribute to the modulation of intra-alveolar inflammatory processes through the regulation effect of the surfactant A (SP-A) and B (SP-B) proteins. Although less extensively investigated, these functions may contribute to the efficacy of exogenous surfactant administration in preterm neonates with RDS.

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

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

Surfactant protein-A modulates LPS-induced TLR4 localization and signaling via β-arrestin 2

The soluble C-type lectin surfactant protein (SP)-A mediates lung immune responses partially via its direct effects on alveolar macrophages (AM), the main resident leukocytes exposed to antigens. SP-A modulates the AM threshold of lipopolysaccharide (LPS) activity towards an anti-inflammatory phenotype both in vitro and in vivo through various mechanisms. LPS responses are tightly regulated via distinct pathways including subcellular TLR4 localization and thus ligand sensing. The cytosolic scaffold and signaling protein β-arrestin 2 acts as negative regulator of LPS-induced TLR4 activation. Here we show that SP-A neither increases TLR4 abundancy nor co-localizes with TLR4 in primary AM. SP-A significantly reduces the LPS-induced co-localization of TLR4 with the early endosome antigen (EEA) 1 by promoting the co-localization of TLR4 with the post-Golgi compartment marker Vti1b in freshly isolated AM from rats and wild-type (WT) mice, but not in β-arrestin 2(-/-) AM. Compared to WT mice pulmonary LPS-induced TNF-α release in β-arrestin 2(-/-) mice is accelerated and enhanced and exogenous SP-A fails to inhibit both lung LPS-induced TNF-α release and TLR4/EEA1 positioning. SP-A, but not LPS, enhances β-arrestin 2 protein expression in a time-dependent manner in primary rat AM. The constitutive expression of β-arrestin 2 in AM from SP-A(-/-) mice is significantly reduced compared to SP-A(+/+) mice and is rescued by SP-A. Prolonged endosome retention of LPS-induced TLR4 in AM from SP-A(-/-) mice is restored by exogenous SP-A, and is antagonized by β-arrestin 2 blocking peptides. LPS induces β-arrestin 2/TLR4 association in primary AM which is further enhanced by SP-A. The data demonstrate that SP-A modulates LPS-induced TLR4 trafficking and signaling in vitro and in vivo engaging β-arrestin 2.

Surfactant protein A suppresses lung cancer progression by regulating the polarization of tumor-associated macrophages

Surfactant protein A (SP-A) is a large multimeric protein found in the lungs. In addition to its immunoregulatory function in infectious respiratory diseases, SP-A is also used as a marker of lung adenocarcinoma. Despite the finding that SP-A expression levels in cancer cells has a relationship with patient prognosis, the function of SP-A in lung cancer progression is unknown. We investigated the role of SP-A in lung cancer progression by introducing the SP-A gene into human lung adenocarcinoma cell lines. SP-A gene transduction suppressed the progression of tumor in subcutaneous xenograft or lung metastasis mouse models. Immunohistochemical analysis showed that the number of M1 antitumor tumor-associated macrophages (TAMs) was increased and the number of M2 tumor-promoting TAMs was not changed in the tumor tissue produced by SP-A-expressing cells. In addition, natural killer (NK) cells were also increased and activated in the SP-A-expressing tumor. Moreover, SP-A did not inhibit tumor progression in mice depleted of NK cells. Taking into account that SP-A did not directly activate NK cells, these results suggest that SP-A inhibited lung cancer progression by recruiting and activating NK cells via controlling the polarization of TAMs.

Arginase-1 expression in granulomas of tuberculosis patients

Mycobacterium tuberculosis (Mtb) is an intracellular pathogen able to survive and multiply within macrophages. Several mechanisms allow this bacterium to escape macrophage microbicidal activity. Mtb may interfere with the ability of mouse macrophages to produce antibactericidal nitric oxide, by inducing the expression of arginase 1 (Arg1). It remains unclear whether this pathway has a role in humans infected with Mtb. In this study, we investigated the expression of Arg1 in granulomas of human lung tissues from patients with tuberculosis. We show that Arg1 is expressed not only in granuloma-associated macrophages, but also in type II pneumocytes.

Mast cell TNF receptors regulate responses to Mycoplasma pneumoniae in surfactant protein A (SP-A)-/- mice

BACKGROUND

Mycoplasma pneumoniae (Mp) frequently colonizes the airways of patients with chronic asthma and likely contributes to asthma exacerbations. We previously reported that mice lacking surfactant protein A (SP-A) have increased airway hyperresponsiveness (AHR) during M pneumoniae infection versus wild-type mice mediated by TNF-α. Mast cells (MCs) have been implicated in AHR in asthma models and produce and respond to TNF-α.

OBJECTIVE

Determine the contribution of MC/TNF interactions to AHR in airways lacking functional SP-A during Mp infection.

METHODS

Bronchoalveolar lavage fluid was collected from healthy and asthmatic subjects to examine TNF-α levels and M pneumoniae positivity. To determine how SP-A interactions with MCs regulate airway homeostasis, we generated mice lacking both SP-A and MCs (SP-A(-/-)Kit(W-sh/W-sh)) and infected them with M pneumoniae.

RESULTS

Our findings indicate that high TNF-α levels correlate with M pneumoniae positivity in human asthmatic patients and that human SP-A inhibits M pneumoniae-stimulated transcription and release of TNF-α by MCs, implicating a protective role for SP-A. MC numbers increase in M pneumoniae-infected lungs, and airway reactivity is dramatically attenuated when MCs are absent. Using SP-A(-/-)Kit(W-sh/W-sh) mice engrafted with TNF-α(-/-) or TNF receptor (TNF-R)(-/-) MCs, we found that TNF-α activation of MCs through the TNF-R, but not MC-derived TNF-α, leads to augmented AHR during M pneumoniae infection when SP-A is absent. Additionally, M pneumoniae-infected SP-A(-/-)Kit(W-sh/W-sh) mice engrafted with TNF-α(-/-) or TNF-R(-/-) MCs have decreased mucus production compared with that seen in mice engrafted with wild-type MCs, whereas burden was unaffected.

CONCLUSION

Our data highlight a previously unappreciated but vital role for MCs as secondary responders to TNF-α during the host response to pathogen infection.

Lipopolysaccharide-induced overproduction of nitric oxide and overexpression of iNOS and interleukin-1β proteins in zinc-deficient rats

Zinc deficiency leads to decreased cellular immune responses. The overproduction of nitrogen species derived from inducible nitric oxide synthase (iNOS), its enzyme, and interleukine-1 beta (IL-1β), and inflammatory cytokine have been implicated in immune responses. The goal of this study was to investigate the effects of lipopolysaccharide (LPS)-induced changes in NO metabolites, iNOS, and IL-1β protein expression in the lungs of zinc-deficient rats. Male Sprague-Dawley rats (body weight, 100 g) were divided into two groups and were fed either a zinc-deficient diet (ZnD) or a zinc-containing diet (Cont). After 4 weeks on these diets, rats received a 10-mg/kg dose of LPS injected via the tail vein and were then maintained for an additional 72 h. To determine total NO concentrations in the blood, serum zinc concentration, iNOS protein expression, IL-1β, and iNOS immunohistochemistry, blood and lung samples were obtained at pre-LPS injection, 5, 24, and 72 h after injection. Total NO levels were significantly increased at 5, at 24, and at 72 h after LPS injection compared with pre-LPS injection level in ZnD group; significant changes in total NO levels was elevated at 5 h from at pre-LPS level but not significant changes from basal level at 24 and 72 h in the control group. Based on western blot analyses and immunohistochemistry, clear bands indicating iNOS and IL-1β protein expression and iNOS antibody-stained inflammatory cells were detected at 5 and 24 h in the ZnD group and 5 h in the Cont group, not observed at 24 and 72 h in the control group. These results suggest that zinc deficiency induces overexpression of iNOS and IL-1β proteins from inflammatory cells around the alveolar blood vessels, resulting in overproduction of total NO and persisted inflammatory response in the zinc-deficient rat lung. Taken together, overexpression of LPS-induced iNOS, overproduction of iNOS-derived NO, and overexpression of IL-1β may induce nitrosative and oxidative stresses in the lung, and these stresses may be involved low immunity of zinc deficiency states.

Pulmonary Collectins in Diagnosis and Prevention of Lung Diseases

Pulmonary surfactant is a complex mixture of lipids and proteins, and is synthesized and secreted by alveolar type II epithelial cells and bronchiolar Clara cells. It acts to keep alveoli from collapsing during the expiratory phase of the respiratory cycle. After its secretion, lung surfactant forms a lattice structure on the alveolar surface, known as tubular myelin. Surfactant proteins (SP)-A, B, C and D make up to 10% of the total surfactant. SP-B and SPC are relatively small hydrophobic proteins, and are involved in the reduction of surface-tension at the air-liquid interface. SP-A and SP-D, on the other hand, are large oligomeric, hydrophilic proteins that belong to the collagenous Ca²⁺-dependent C-type lectin family (known as “Collectins”), and play an important role in host defense and in the recycling and transport of lung surfactant (Awasthi 2010) (Fig. 43.1). In particular, there is increasing evidence that surfactant-associated proteins A and -D (SP-A and SP-D, respectively) contribute to the host defense against inhaled microorganisms (see 10.1007/978-3-7091-1065_24 and 10.1007/978-3-7091-1065_25). Based on their ability to recognize pathogens and to regulate the host defense, SP-A and SP-D have been recently categorized as “Secretory Pathogen Recognition Receptors”. While SP-A and SP-D were first identified in the lung; the expression of these proteins has also been observed at other mucosal surfaces, such as lacrimal glands, gastrointestinal mucosa, genitourinary epithelium and periodontal surfaces. SP-A is the most prominent among four proteins in the pulmonary surfactant-system. 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. This chapter gives an overview on the understanding of role of SP-A and SP-D in for human pulmonary disorders and points out the importance for pathology-orientated research to further elucidate the role of these molecules in adult lung diseases. As an outlook, it will become an issue of pulmonary pathology which might provide promising perspectives for applications in research, diagnosis and therapy (Awasthi 2010).

Bacterial lipopolysaccharide promotes destabilization of lung surfactant-like films

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

Pulmonary surfactant protein A enhances endolysosomal trafficking in alveolar macrophages through regulation of Rab7

Surfactant protein A (SP-A), the most abundant pulmonary soluble collectin, modulates innate and adaptive immunity of the lung, partially via its direct effects on alveolar macrophages (AM), the most predominant intra-alveolar cells under physiological conditions. Enhanced phagocytosis and endocytosis are key functional consequences of AM/SP-A interaction, suggesting a SP-A-mediated modulation of small Rab (Ras related in brain) GTPases that are pivotal membrane organizers in both processes. In this article, we show that SP-A specifically and transiently enhances the protein expression of endogenous Rab7 and Rab7b, but not Rab5 and Rab11, in primary AM from rats and mice. SP-A-enhanced GTPases are functionally active as determined by increased interaction of Rab7 with its downstream effector Rab7 interacting lysosomal protein (RILP) and enhanced maturation of cathepsin-D, a function of Rab7b. In AM and RAW264.7 macrophages, the SP-A-enhanced lysosomal delivery of GFP-Escherichia coli is abolished by the inhibition of Rab7 and Rab7 small interfering RNA transfection, respectively. The constitutive expression of Rab7 in AM from SP-A(-/-) mice is significantly reduced compared with SP-A(+/+) mice and is restored by SP-A. Rab7 blocking peptides antagonize SP-A-rescued lysosomal delivery of GFP-E. coli in AM from SP-A(-/-) mice. Activation of Rab7, but not Rab7b, by SP-A depends on the PI3K/Akt/protein kinase Cζ (PKCζ) signal transduction pathway in AM and RAW264.7 macrophages. SP-A induces a Rab7/PKCζ interaction in these cells, and the disruption of PKCζ by small interfering RNA knockdown abolishes the effect of SP-A on Rab7. The data demonstrate a novel role for SP-A in modulating endolysosomal trafficking via Rab7 in primary AM and define biochemical pathways involved.

SP-D and regulation of the pulmonary innate immune system in allergic airway changes

The airway mucosal surfaces are constantly exposed to inhaled particles that can be potentially toxic, infectious or allergenic and should elicit inflammatory changes. The proximal and distal air spaces, however, are normally infection and inflammation free due to a specialized interplay between cellular and molecular components of the pulmonary innate immune system. Surfactant protein D (SP-D) is an epithelial-cell-derived immune modulator that belongs to the small family of structurally related Ca(2+)-dependent C-type collagen-like lectins. While collectins can be detected in mucosal surfaces of various organs, SP-A and SP-D (the 'lung collectins') are constitutively expressed in the lung at high concentrations. Both proteins are considered important players of the pulmonary immune responses. Under normal conditions however, SP-A-/- mice display no pathological features in the lung. SP-D-/- mice, on the other hand, show chronic inflammatory alterations indicating a special importance of this molecule in regulating immune homeostasis and the function of the innate immune cells. Recent studies in our laboratory and others implied significant associations between changes in SP-D levels and the presence of airway inflammation both in animal models and patients raising a potential usefulness of this molecule as a disease biomarker. Research on wild-type and mutant recombinant molecules in vivo and in vitro showed that SP-D binds carbohydrates, lipids and nucleic acids with a broad spectrum specificity and initiates phagocytosis of inhaled pathogens as well as apoptotic cells. Investigations on gene-deficient and conditional over expressor mice in addition, provided evidence that SP-D directly modulates macrophage and dendritic cell function as well as T cell-dependent inflammatory events. Thus, SP-D has a unique, dual functional capacity to induce pathogen elimination on the one hand and control of pro-inflammatory mechanisms on the other, suggesting a potential suitability for therapeutic prevention and treatment of chronic airway inflammation without compromising the host defence function of the airways. This paper will review recent findings on the mechanisms of immune-protective function of SP-D in the lung.

A FQHPSFI peptide selectively binds to LPS-activated alveolar macrophages and inhibits LPS-induced MIP-2 production

OBJECTIVE

The goal of this study was to identify peptides selectively binding to lipopolysaccharide (LPS)-activated alveolar macrophages (AMs) and to characterize their effects on the production of LPS-induced cytokines.

METHODS

A phage display library was sequentially screened by binding phages to unmanipulated AMs and then to LPS-activated AMs. Individual phage clones were identified by cell-based ELISA. Positive phage clones were characterized by DNA sequencing and bioinformatics analysis. Binding specificity of the selected phage to LPS-activated AMs was tested using immunofluorescent staining. The selected candidate peptide was chemically synthesized to determine whether it could modulate LPS-induced cytokine production in AMs.

RESULTS

Twenty-two out of 40 phage clones selected randomly after four rounds of biopanning bound selectively to LPS-activated AMs, and 12 of them displayed novel peptides. A phage clone displaying FQHPSFI peptide bound effectively to LPS-activated AMs, but not to other cells tested. Furthermore, the synthetic FQHPSFI peptide, but not seven point mutants tested, competitively inhibited the binding of the phage clone to LPS-activated AMs. Importantly, the FQHPSFI peptide significantly inhibited LPS-stimulated microphage inflammatory protein 2 (MIP-2) production in vitro.

CONCLUSIONS

Our data demonstrate that phage display technology is a powerful tool for the identification of bioactive peptides. The identified FQHPSFI peptide may be used for the modulation of LPS-stimulated MIP-2 production in AMs.

Role of Surfactant Protein A in the Innate Host Defense and Autoimmunity

Surfactant protein A (SP-A) is a lung collectin with diverse immunoregulatory activities. SP-A regulates the innate host defense by enhancing phagocytosis of pathogens and modulating the production of nitric oxide and cytokines by immune cells. Additionally, SP-A also modulates the phenotypic and functional properties of the cells of adaptive immune response such as dendritic cells (DCs) and lymphocytes. Bone marrow-derived DCs generated in the presence of SP-A fail to increase lipopolysaccharide-induced upregulation of major histocompatibility complex (MHC) class II and CD86 costimulatory molecule on DCs surface and behaves like "tolerogenic DCs". SP-A may also induce tolerance by suppressing the proliferation of activated T lymphocytes. Thus, based on immunoregulatory properties of SP-A, it may be employed as a therapeutic agent for the treatment of autoimmune disease and organ transplantation.

SP-A preserves airway homeostasis during Mycoplasma pneumoniae infection in mice

The lung is constantly challenged during normal breathing by a myriad of environmental irritants and infectious insults. Pulmonary host defense mechanisms maintain homeostasis between inhibition/clearance of pathogens and regulation of inflammatory responses that could injure the airway epithelium. One component of this defense mechanism, surfactant protein-A (SP-A), exerts multifunctional roles in mediating host responses to inflammatory and infectious agents. SP-A has a bacteriostatic effect on Mycoplasma pneumoniae (Mp), which occurs by binding surface disaturated phosphatidylglycerols. SP-A can also bind the Mp membrane protein, MPN372. In this study, we investigated the role of SP-A during acute phase pulmonary infection with Mp using mice deficient in SP-A. Biologic responses, inflammation, and cellular infiltration, were much greater in Mp infected SP-A(-/-) mice than wild-type mice. Likewise, physiologic responses (airway hyperresponsiveness and lung compliance) to Mp infection were more severely affected in SP-A(-/-) mice. Both Mp-induced biologic and physiologic changes were attenuated by pharmacologic inhibition of TNF-alpha. Our findings demonstrate that SP-A is vital to preserving lung homeostasis and host defense to this clinically relevant strain of Mp by curtailing inflammatory cell recruitment and limiting an overzealous TNF-alpha response.

Role of clathrin-mediated endocytosis of surfactant protein A by alveolar macrophages in intracellular signaling

We recently provided evidence that anti-inflammatory macrophage activation, i.e., the inhibition of constitutive and signal-induced NF-kappaB activity by the pulmonary collectin surfactant protein (SP)-A, critically involves a promoted stabilization of IkappaB-alpha, the predominant inhibitor of NF-kappaB, via posttranscriptional mechanisms comprising the activation of atypical (a)PKCzeta. SP-A uptake and degradation by alveolar macrophages (AMphi) occur in a receptor-mediated, clathrin-dependent manner. However, a mutual link between endocytosis of and signaling by SP-A remains elusive. The aim of this study was to investigate whether clathrin-mediated endocytosis (CME) of SP-A by AMphi is a prerequisite for its modulation of the IkappaB-alpha/NF-kappaB pathway. The inhibition of clathrin-coated pit (CCP) formation and clathrin-coated vesicle (CCV) formation/budding abrogates SP-A-mediated IkappaB-alpha stabilization and SP-A-mediated inhibition of LPS-induced NF-kappaB activation in freshly isolated rat AMphi, as determined by Western analysis, fluorescence-activated cell sorting, confocal microscopy, and EMSA. Actin depolymerization and inhibition of CCP formation further abolished SP-A-mediated inhibition of LPS-induced TNF-alpha release, as determined by ELISA. In addition, SP-A-induced atypical PKCzeta activation was abolished by pretreatment of AMphi with CCV inhibitors as determined by in vitro immunocomplex kinase assay. Although CME is classically considered as a means to terminate signaling, our results demonstrate that SP-A uptake via CME by AMphi has to precede the initiation of SP-A signaling.

Surfactant protein A activation of atypical protein kinase C zeta in IkappaB-alpha-dependent anti-inflammatory immune regulation

The pulmonary collectin surfactant protein (SP)-A has a pivotal role in anti-inflammatory modulation of lung immunity. The mechanisms underlying SP-A-mediated inhibition of LPS-induced NF-kappaB activation in vivo and in vitro are only partially understood. We previously demonstrated that SP-A stabilizes IkappaB-alpha, the primary regulator of NF-kappaB, in alveolar macrophages (AM) both constitutively and in the presence of LPS. In this study, we show that in AM and PBMC from IkappaB-alpha knockout/IkappaB-beta knockin mice, SP-A fails to inhibit LPS-induced TNF-alpha production and p65 nuclear translocation, confirming a critical role for IkappaB-alpha in SP-A-mediated LPS inhibition. We identify atypical (a) protein kinase C (PKC) zeta as a pivotal upstream regulator of SP-A-mediated IkappaB-alpha/NF-kappaB pathway modulation deduced from blocking experiments and confirmed by using AM from PKCzeta-/- mice. SP-A transiently triggers aPKCThr(410/403) phosphorylation, aPKC kinase activity, and translocation in primary rat AM. Coimmunoprecipitation experiments reveal that SP-A induces aPKC/p65 binding under constitutive conditions. Together the data indicate that anti-inflammatory macrophage activation via IkappaB-alpha by SP-A critically depends on PKCzeta activity, and thus attribute a novel, stimulus-specific signaling function to PKCzeta in SP-A-modulated pulmonary immune response.

Immunomodulatory roles of surfactant proteins A and D: implications in lung disease

Surfactant, a lipoprotein complex, was originally described for its essential role in reducing surface tension at the air-liquid interface of the lung; however, it is now recognized as being a critical component in lung immune host defense. Surfactant proteins (SP)-A and -D are pattern recognition molecules of the collectin family of C-type lectins. SP-A and SP-D are part of the innate immune system and regulate the functions of other innate immune cells, such as macrophages. They also modulate the adaptive immune response by interacting with antigen-presenting cells and T cells, thereby linking innate and adaptive immunity. Emerging studies suggest that SP-A and SP-D function to modulate the immunologic environment of the lung so as to protect the host and, at the same time, modulate an overzealous inflammatory response that could potentially damage the lung and impair gas exchange. Numerous polymorphisms of SPs have been identified that may potentially possess differential functional abilities and may act via different receptors to ultimately alter the susceptibility to or severity of lung diseases.

The immunoregulatory roles of lung surfactant collectins SP-A, and SP-D, in allergen-induced airway inflammation

It has become increasingly evident that pulmonary surfactant proteins, SP-A and SP-D, present in the alveolar and bronchial epithelial fluid linings, not only play significant functions in the innate defense mechanism against pathogens, but also are involved in immunomodulatory roles, which result in the protection against, and resolution of, allergen-induced airway inflammation. Studies on allergen-sensitized murine models, and asthmatic patients, show that SP-A and SP-D can: specifically bind to aero-allergens; inhibit mast cell degranulation and histamine release; and modulate the activation of alveolar macrophages and dendritic cells during the acute hypersensitive phase of allergic response. They also can alleviate chronic allergic inflammation by inhibiting T-lymphocyte proliferation as well as increasing phagocytosis of DNA fragments and clearance of apoptotic cell debris. Furthermore, it has emerged, from the studies on SP-D-deficient mice, that, when these mice are challenged with allergen, they develop increased eosinophil infiltration, and abnormal activation of lymphocytes, leading to the production of Th2 cytokines. Intranasal administration of SP-D significantly attenuated the asthmatic-like symptoms seen in allergen-sensitized wild-type, and SP-D-deficient, mice. These important findings provide a new insight of the role that surfactant proteins play in handling environmental stimuli and in their immunoregulation of airway inflammatory disease.

Generation of inhibitory NFkappaB complexes and phosphorylated cAMP response element-binding protein correlates with the anti-inflammatory activity of complement protein C1q in human monocytes

The interaction of C1q with specific cells of the immune system induces activities, such as enhancement of phagocytosis in monocytes and stimulation of superoxide production in neutrophils. In contrast to some other monocyte activators, C1q itself does not induce pro-inflammatory cytokine production, but rather inhibits the lipopolysaccharide (LPS)-stimulated induction of certain pro-inflammatory cytokines and induces expression of interleukin-10. To investigate the molecular mechanism by which C1q exerts this effect on gene expression, the influence of C1q on the activation of transcription factors of the NFkappaB family and cAMP response element-binding protein (CREB) was assessed. C1q treatment increased kappaB binding activity in freshly isolated human monocytes in a time-dependent fashion as assessed by electrophoretic mobility shift assays. In antibody supershift experiments, anti-p50 antibody supershifted the C1q-induced NFkappaB complex, whereas anti-p65 antibody had little effect, suggesting that C1q induced the translocation of NFkappaB p50p50 homodimers. This is in contrast to the dominant induction of p65 containing complexes in parallel monocyte cultures stimulated with LPS. C1q treatment also induced cAMP response element (CRE)-binding activity as demonstrated by electrophoretic mobility shift assay, increased phosphorylation of CREB, and induction of CRE driven gene expression. In contrast, CREB activation was not detected in LPS-treated monocytes. These results suggest that C1q may modulate the cytokine profile expressed in response to inflammatory stimuli (e.g. LPS), by triggering inhibitory and/or competing signals. Because C1q and other defense collagens have been shown to enhance clearance of apoptotic cells, this regulatory pathway may be beneficial in avoiding autoimmunity and/or resolving inflammation.

Complement proteins C1q and MBL are pattern recognition molecules that signal immediate and long-term protective immune functions

C1q and mannose binding lectin, members of the "defense collagen" family, are pattern recognition molecules that can trigger rapid enhanced phagocytosis resulting in efficient containment of pathogens or clearance of cellular debris, apoptotic cells and immune complexes. In addition, interaction of C1q and mannose binding lectin with the phagocyte alters subsequent phagocyte cytokine synthesis, and thus may have important implications in directing acute inflammation as well as long-term protective immunity. The importance of the role of defense collagens in phagocytosis of apoptotic cells is highlighted by studies in vivo of mice deficient in C1q, pulmonary surfactant D and mannose binding lectin in which there is delayed clearance of apoptotic cells. Indeed, deficiency of C1q is a risk factor for the development of autoimmunity in both humans and mice, consistent with the hypothesis that inefficient clearance of apoptotic cells results in release of autoantigens and contributes to the pathology associated with autoimmune diseases such as systemic lupus erythematosus. Further understanding of the importance of C1q and mannose binding lectin in the clearance of apoptotic cells and regulation of cytokine synthesis and identification of the receptors implicated in mediating these processes should provide novel targets for therapeutic intervention in the control and manipulation of the immune response in terms of both host defense against infectious disease and tissue repair and remodeling.

Lung-restricted macrophage activation in the pearl mouse model of Hermansky-Pudlak syndrome

Pulmonary inflammation, abnormalities in alveolar type II cell and macrophage morphology, and pulmonary fibrosis are features of Hermansky-Pudlak Syndrome (HPS). We used the naturally occurring "pearl" HPS2 mouse model to investigate the mechanisms of lung inflammation observed in HPS. Although baseline bronchoalveolar lavage (BAL) cell counts and differentials were similar in pearl and strain-matched wild-type (WT) mice, elevated levels of proinflammatory (MIP1gamma) and counterregulatory (IL-12p40, soluble TNFr1/2) factors, but not TNF-alpha, were detected in BAL from pearl mice. After intranasal LPS challenge, BAL levels of TNF-alpha, MIP1alpha, KC, and MCP-1 were 2- to 3-fold greater in pearl than WT mice. At baseline, cultured pearl alveolar macrophages (AMs) had markedly increased production of inflammatory cytokines. Furthermore, pearl AMs had exaggerated TNF-alpha responses to TLR4, TLR2, and TLR3 ligands, as well as increased IFN-gamma/LPS-induced NO production. After 24 h in culture, pearl AM LPS responses reverted to WT levels, and pearl AMs were appropriately refractory to continuous LPS exposure. In contrast, cultured pearl peritoneal macrophages and peripheral blood monocytes did not produce TNF-alpha at baseline and had LPS responses which were no different from WT controls. Exposure of WT AMs to heat- and protease-labile components of pearl BAL, but not WT BAL, resulted in robust TNF-alpha secretion. Similar abnormalities were identified in AMs and BAL from another HPS model, pale ear HPS1 mice. We conclude that the lungs of HPS mice exhibit hyperresponsiveness to LPS and constitutive and organ-specific macrophage activation.

Pseudomonas aeruginosa et surfactant rôle de SP-A et SP-D

Surfactant-associated proteins A and D (SP-A and SP-D) are two pulmonary collectins that bind to bacterial, fungal and viral pathogens and have multiples classes of receptors on pneumocyte and macrophage membrane. They are chemoattractant for phagocytes, enhance uptake and killing of bacteria by macrophages and neutrophils. These molecules also act as activation ligand on macrophages and neutrophils to enhance phagocytosis, resulting in an increased bacterial clearance. Depending on activation of cells by stimuli, SP-A and SP-D modulate production of antimicrobial free radicals by phagocytes and secretion of cytokines. In vivo, SP-A deficient mice infected with Pseudomonas aeruginosa (P. aeruginosa) have decreased bacterial clearance and exacerbated inflammatory response in the lungs. Serious alterations in macrophages and increased production of reactive oxygen species were found in non-infected SP-D deficient mice. Patients with cystic fibrosis are frequently colonized by P. aeruginosa. Decreased levels of SP-A and SP-D have been measured in bronchoalveolar lavage fluid of these patients, as well as patients with acute pneumonia but no chronic lung disease. P. aeruginosa secretes various proteases, among them, elastase and protease IV have been found to degrade SP-A and SP-D and abrogate their immune function. However, further investigations are necessary to examine whether these deficiencies facilitate P. aeruginosa infections or stand as consequences.

Identification of the surfactant protein A receptor 210 as the unconventional myosin 18A

Mass spectrometric characterization of the surfactant protein A (SP-A) receptor 210 (SP-R210) led to the identification of myosin (Myo) XVIIIA and nonmuscle myosin IIA. Antibodies generated against the unique C-terminal tail of MyoXVIIIA revealed that MyoXVIIIA, MyoIIA, and SP-R210 have overlapping tissue distribution, all being highly expressed in myeloid cells, bone marrow, spleen, lymph nodes, and lung. Western blot analysis of COS-1 cells stably transfected with either MyoXVIIIA or MyoIIA indicated that SP-R210 antibodies recognize MyoXVIIIA. Furthermore, MyoXVIIIA but not MyoIIA localized to the surface of COS-1 cells, and most importantly, expression of MyoXVIIIA in COS-1 cells conferred SP-A binding. Western analysis of recombinant MyoXVIIIA domains expressed in bacteria mapped the epitopes of previously derived SP-R210 antibodies to the neck region of MyoXVIIIA. Antibodies raised against the neck domain of MyoXVIIIA blocked the binding of SP-A to macrophages. Together, these findings indicate that MyoXVIIIA constitutes a novel receptor for SP-A.

Role and regulation of lung collectins in allergic airway sensitization

Inhalation of allergens in atopic patients results in a characteristic inflammatory response while in normal, healthy individuals it elicits no symptoms. The mechanisms by which the pulmonary immune system accomplishes elimination of inhaled particles and suppression of the ensuing inflammatory response are poorly understood. Based on their structural uniqueness, specific localization and functional versatility the hydrophilic surfactant proteins [surfactant protein (SP)-A and SP-D] are important candidate regulators of these processes. Recent studies in our laboratory and others indicated significant changes in levels of these molecules during the asthmatic response in animal models as well as in asthmatic patients. Because of their capability to directly inhibit T-cell activation and T-cell-dependent allergic inflammatory events, SP-A and SP-D may be significant contributors to the local control of T-helper (Th)2-type inflammation in the airways. This review will discuss their relevant structural-functional features and recent evidence supporting the hypothesis that SP-A and SP-D have a role in regulation of allergic airway sensitization.

Therapeutic effect of surfactant protein D in allergic inflammation of mite-sensitized mice

BACKGROUND

Surfactant protein D (SP-D) is involved in the innate immunity within the lung and may have important roles in modulating the inflammatory process of asthma.

OBJECTIVE

To examine the potential immunomodulating role of SP-D on the allergic response in mice, and its interaction with the alveolar macrophages (AMs) during allergic inflammation.

METHODS

A recombinant 60 kDa fragment of human SP-D (rfh SP-D), Survanta, and budesonide were administrated, respectively, to Der p-sensitive BALB/c mice before or after allergen challenge (AC). Total and differential cell counts, levels of cytokines in bronchoalveolar lavage fluids(BALFs), and levels of Der p-specific IgE and IgG1 antibodies in sera, were assayed. The production of nitric oxide (NO), and inducible NO synthase (iNOS) expression, in AMs, were determined by ELISA and RT-PCR, respectively.

RESULTS

Instillation of rfh SP-D to sensitized mice 6 h after AC (therapeutic), but not 24 h before AC (preventive), markedly reduced infiltration of eosinophils, and also reduced levels of IL-4, IL-5, eotaxin, and TNF-alpha but elevated levels of IFN-gamma in the BALF. These effects were comparable with those obtained with budesonide treatment, whereas Survanta did not have a suppressive effect, either before or after AC. There was significant inhibition of NO production in the rfh SP-D pre-treated AMs of allergen-sensitized mice, but not in naive mice.

CONCLUSIONS

These results indicate that rfh SP-D has a therapeutic effect on allergen-induced bronchial inflammation, and that this might be because of its inhibitory effect on NO and TNF-alpha production by AMs, and it thus prevents the development of T-helper type 2 cytokine response.

Surfactant protein A limits Pneumocystis murina infection in immunosuppressed C3H/HeN mice and modulates host response during infection

The development of Pneumocystis murina pneumonia and host response were characterized over time and at different levels of infection in corticosteroid immunosuppressed surfactant protein A (SP-A) knockout and wild-type (WT) mice. Infection increased over time in both strains of mice; however, significantly more cyst forms were detected in the knockout mice at intermediate and late stages of infection. In mice with heavy infections, TNF-alpha and IFN-gamma protein concentrations were significantly higher in pulmonary lavage fluid from knockout mice. There was a significant positive correlation between TNF-alpha and IFN-gamma concentrations and the level of infection in knockout mice, but not in WT mice. No significant differences were detected in IL-1 levels between the two strains of mice at any of the time points or at any level of infection. At heavier infection levels, significantly more MIP-2 protein was detected in the lungs of knockout mice, but a significant positive correlation between MIP-2 concentrations and the infection level was detected in both groups of mice. At the intermediate stage of infection, a significantly higher percentage of neutrophils was detected in the lungs of knockout mice than in WT mice. There was no difference in SP-D levels between WT and KO mice with identical levels of infection. These data support a protective role for SP-A in host defense against Pneumocystis and suggest that the effects of SP-A on the host response vary based on the intensity of the infection.

Accumulation of Inhibitory κB-α as a Mechanism Contributing to the Anti-Inflammatory Effects of Surfactant Protein–A

The collectin surfactant protein (SP)-A has been implicated in multiple immunoregulatory functions of innate pulmonary host defense via modulating immune responses both in vitro and in vivo. The aim of the present study was to investigate mechanisms responsible for the anti-inflammatory effects of human (hu) SP-A on the inhibitory kappaB (IkappaB)/nuclear factor (NF)-kappaB signaling pathway in alveolar macrophages (AMs). Initial CD25 expression analysis by flow cytometry of CD14/hu Toll-like receptor 4-transfected Chinese hamster ovary reporter cells demonstrated that SP-A alone does not induce any NF-kappaB-dependent CD25 expression in these cells. In AMs, SP-A pretreatment caused a marked inhibition of lipopolysaccharide (LPS)-induced NF-kappaB activation independent of the LPS chemotype used as determined by electrophoretic mobility shift assay. Western blot analysis revealed that SP-A by itself increased the protein expression of IkappaB-alpha, the predominant regulator for rapidly induced NF-kappaB, in a dose- and time-dependent manner without enhancing IkappaB-alpha messenger RNA as determined by reverse transcription-polymerase chain reaction. SP-A did not interfere with LPS-induced serine(32) phosphorylation of IkappaB-alpha but significantly enhanced IkappaB-alpha abundance under LPS-coupled conditions. The data suggest that anti-inflammatory effects of SP-A on LPS-challenged AMs are associated with a SP-A-mediated direct modulation of the IkappaB-alpha turnover in these cells.

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.

Surfactant protein A inhibits alveolar macrophage cytokine production by CD14-independent pathway

The lung collectin surfactant protein A (SP-A) has both anti-inflammatory and prophagocytic activities. We and others previously showed that SP-A inhibits the macrophage production of tumor necrosis factor (TNF)-alpha stimulated by the gram-negative bacterial component LPS. We propose that SP-A decreases the production of proinflammatory cytokines by alveolar macrophages via a CD14-independent mechanism. SP-A inhibited LPS-simulated TNF-alpha production in rat and mouse macrophages in the presence and absence of serum (72% and 42% inhibition, respectively). In addition, SP-A inhibited LPS-induced mRNA levels for TNF-alpha, IL-1 alpha, and IL-1 beta as well as NF-kappa B DNA binding activity. SP-A also diminished ultrapure LPS-stimulated TNF-alpha produced by wild-type and CD14-null mouse alveolar macrophages by 58% and 88%, respectively. Additionally, SP-A inhibited TNF-alpha stimulated by PMA in both wild-type and TLR4-mutant macrophages. These data suggest that SP-A inhibits inflammatory cytokine production in a CD14-independent manner and also by mechanisms independent of the LPS signaling pathway.

Human SP-A genetic variants and bleomycin-induced cytokine production by THP-1 cells: effect of ozone-induced SP-A oxidation

Surfactant protein A (SP-A) plays a role in innate host defense. Human SP-A is encoded by two functional genes (SP-A1 and SP-A2), and several alleles have been characterized for each gene. We assessed the effect of in vitro expressed human SP-A genetic variants, on TNF-alpha and IL-8 production by THP-1 cells in the presence of bleomycin, either before or after ozone-induced oxidation of the variants. The oligomerization of SP-A variants was also examined. We found 1) cytokine levels induced by SP-A2 (1A, 1A(0)) were significantly higher than those by SP-A1 (6A(2), 6A(4)) in the presence of bleomycin. 2) In the presence of bleomycin, ozone-induced oxidation significantly decreased the ability of 1A and 1A/6A(4), but not of 6A(4), to stimulate TNF-alpha production. 3) The synergistic effect of bleomycin/SP-A, either before or after oxidation, can be inhibited to the level of bleomycin alone by surfactant lipids. 4) Differences in oligomerization were also observed between SP-A1 and SP-A2. The results indicate that differences among SP-A variants may partly explain the individual variability of pulmonary complications observed during bleomycin chemotherapy and/or in an environment that may promote protein oxidation.

Surfactant protein A decreases nitric oxide production by macrophages in a tumor necrosis factor-alpha-dependent mechanism

Surfactant protein A (SP-A) modulates the lung defense system through regulation of cytokines and nitric oxide (NO) production by alveolar macrophages (AMs). Whether SP-A upregulates or downregulates production of proinflammatory cytokines and NO is controversial. This study demonstrates the molecular mechanism(s) by which SP-A suppresses NO production by activated murine AMs. NO production by interferon-gamma (IFN-gamma) and IFN-gamma plus Mycobacterium avium-stimulated AMs was mediated through tumor necrosis factor-alpha (TNF-alpha) production, as addition of neutralizing anti-TNF-alpha antibodies during AMs stimulation resulted in reduced NO production. SP-A suppressed NO production by activated AMs by inhibiting TNF-alpha production. The maximum inhibitory effect of SP-A on NO production was observed at 20 microg/ml of SP-A concentration. Furthermore, SP-A inhibited activation of nuclear factor-kappa B, a transcription factor required for induction of TNF-alpha and inducible NO synthase genes. These findings suggest that SP-A suppresses NO production by activated AMs by inhibiting TNF-alpha secretion and nuclear factor-kappa B activation. This study also highlights the importance of SP-A levels in the lung, as changes in SP-A levels may modulate the local lung defense system.

Surfactant protein A differentially regulates peripheral and inflammatory neutrophil chemotaxis

Surfactant protein A (SP-A), a pulmonary lectin, plays an important role in regulating innate immune cell function. Besides accelerating pathogen clearance by pulmonary phagocytes, SP-A also stimulates alveolar macrophage chemotaxis and directed actin polymerization. We hypothesized that SP-A would also stimulate neutrophil chemotaxis. With the use of a Boyden chamber assay, we found that SP-A (0.5-25 microg/ml) did not stimulate chemotaxis of rat peripheral neutrophils or inflammatory bronchoalveolar lavage (BAL) neutrophils isolated from LPS-treated lungs. However, SP-A affected neutrophil chemotaxis toward the bacterial peptide formyl-met-leu-phe (fMLP). Surprisingly, the effect was different for the two neutrophil populations: SP-A reduced peripheral neutrophil chemotaxis toward fMLP (49 +/- 5% fMLP alone) and enhanced inflammatory BAL neutrophil chemotaxis (277 +/- 48% fMLP alone). This differential effect was not seen for the homologous proteins mannose binding lectin and complement protein 1q but was recapitulated by type IV collagen. SP-A bound both neutrophil populations comparably and did not alter formyl peptide binding. These data support a role for SP-A in regulating neutrophil migration in pulmonary tissue.

Surfactant protein a inhibits lipopolysaccharide-induced immune cell activation by preventing the interaction of lipopolysaccharide with lipopolysaccharide-binding protein

Pulmonary surfactant protein (SP)-A, an innate immune molecule, modifies lipopolysaccharide (LPS)-induced cell responses. Because SP-A avidly binds to the deep rough (Re) mutant of LPS, we first investigated the functional consequences of this interaction and found that preincubation of Re-LPS with SP-A significantly and in a dose-dependent manner decreased the sensitivity of rat alveolar macrophages and human mononuclear cells to Re-LPS-induced activation at limited amounts of LPS-binding protein (LBP). At high LBP concentrations, the SP-A-mediated cellular inhibition of Re-LPS-induced activation was abrogated. Because LBP-catalyzed binding of LPS to CD14 is essential for low-dose LPS-induced signaling, we then hypothesized that SP-A inhibits Re-LPS-induced immune cell activation via inhibiting the binding of Re-LPS to LBP. Binding competition experiments employing a surface plasmon resonance technique showed that Re-LPS preincubated with SP-A bound to LBP to a significantly lesser extent than Re-LPS alone. For enhanced cellular association of [(3)H]LPS/SP-A complexes to occur, the expression of membrane-bound CD14 by human embryonic kidney cells 293 was not essential. Therefore, the ability of SP-A to inhibit immune cell activation by Re-LPS may be due to its ability to block the binding of Re-LPS to LBP and prevent the initiation of the LBP/CD14 pathway for inflammatory reactions in the lung.

Cutting edge: the immunostimulatory activity of the lung surfactant protein-A involves Toll-like receptor 4

The collectin surfactant protein-A (SP-A) is involved in the innate host defense and the regulation of inflammatory processes in the lung. In this work we investigated the molecular mechanisms related to the immunostimulatory activity of SP-A using macrophages from C3H/HeJ mice, which carry an inactivating mutation in the Toll-like receptor (TLR)4 gene, and TLR4-transfected Chinese hamster ovary cells. We demonstrate that SP-A-induced activation of the NF-kappaB signaling pathway and up-regulation of cytokine synthesis such as TNF-alpha and IL-10 are critically dependent on the TLR4 functional complex. These findings support the concept that TLR4 is a pattern recognition receptor that signals in response to both foreign pathogens and endogenous host mediators.

Killing of Klebsiella pneumoniae by human alveolar macrophages

We investigated putative mechanisms by which human surfactant protein A (SP-A) effects killing of Klebsiella pneumoniae by human alveolar macrophages (AMs) isolated from bronchoalveolar lavagates of patients with transplanted lungs. Coincubation of AMs with human SP-A (25 microg/ml) and Klebsiella resulted in a 68% decrease in total colony forming units by 120 min compared with AMs infected with Klebsiella in the absence of SP-A, and this SP-A-mediated effect was abolished by preincubation with N(G)-monomethyl-L-arginine. Incubation of transplant AMs with SP-A increased intracellular Ca(2+) concentration ([Ca(2+)](i)) by 70% and nitrite and nitrate (NO(x)) production by 45% (from 0.24 +/- 0.02 to 1.3 +/- 0.21 nmol small middle dot 10(6) AMs(-1).h(-1)). Preincubation with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester inhibited the increase in [Ca(2+)](i) and abrogated the SP-A-mediated Klebsiella phagocytosis and killing. In contrast, incubation of AMs from normal volunteers with SP-A decreased both [Ca(2+)](i) and NO(x) production and did not result in killing of Klebsiella. Significant killing of Klebsiella was also seen in a cell-free system by sustained production of peroxynitrite (>1 microM/min) at pH 5 but not at pH 7.4. These findings indicate that SP-A mediates pathogen killing by AMs from transplant lungs by stimulating phagocytosis and production of reactive oxygen-nitrogen intermediates.

The role of surfactant-associated protein A in pulmonary host defense

Resident alveolar macrophages play a key role in the initial defense against inhaled pathogens. Surface molecules bind opsonized as well as nonopsonized microbes and mediate their internalization by the macrophage. The recent discovery that specific C-type lectins can bind to the surface of a wide range of pathogens has led to the hypothesis that these lectins are involved in the initial phases of microbe recognition by the macrophage. Studies in our laboratory focus on the role of the lung-specific lectin surfactant associated protein A (SP-A) in host defense against pulmonary pathogens. SP-A contains a carbohdyrate recognition domain that appears to bind specifically to exposed carbohydrate residues on the surface of microorganisms. This lectin-microorganism interaction leads to entry of specific pathogens into macrophages and activation of intracellular pathways, resulting in the production of antimicrobial mediators such as nitric oxide. Many studies, including those involving SP-A-deficient mice, underscore the importance of this protein in pulmonary innate immunity. However, the intramacrophage mechanisms underlying the effects of SP-A are still unclear. This article describes our current knowledge of SP-A and its interactions with immune cells and pathogens with a focus on recent findings from our laboratory regarding SP-A interactions with mycobacteria.

Lung surfactant and reactive oxygen-nitrogen species: antimicrobial activity and host-pathogen interactions

Surfactant protein (SP) A and SP-D are members of the collectin superfamily. They are widely distributed within the lung, are capable of antigen recognition, and can discern self versus nonself. SPs recognize bacteria, fungi, and viruses by binding mannose and N-acetylglucosamine residues on microbial cell walls. SP-A has been shown to stimulate the respiratory burst as well as nitric oxide synthase expression by alveolar macrophages. Although nitric oxide (NO.) is a well-recognized microbicidal product of macrophages, the mechanism(s) by which NO. contributes to host defense remains undefined. The purpose of this symposium was to present current research pertaining to the specific role of SPs and reactive oxygen-nitrogen species in innate immunity. The symposium focused on the mechanisms of NO*-mediated toxicity for bacterial, human, and animal models of SP-A- and NO.-mediated pathogen killing, microbial defense mechanisms against reactive oxygen-nitrogen species, specific examples and signaling pathways involved in the SP-A-mediated killing of pulmonary pathogens, the structure and binding of SP-A and SP-D to bacterial targets, and the immunoregulatory functions of SP-A.

Alveolar epithelial type II cell: defender of the alveolus revisited

In 1977, Mason and Williams developed the concept of the alveolar epithelial type II (AE2) cell as a defender of the alveolus. It is well known that AE2 cells synthesise, secrete, and recycle all components of the surfactant that regulates alveolar surface tension in mammalian lungs. AE2 cells influence extracellular surfactant transformation by regulating, for example, pH and [Ca2+] of the hypophase. AE2 cells play various roles in alveolar fluid balance, coagulation/fibrinolysis, and host defence. AE2 cells proliferate, differentiate into AE1 cells, and remove apoptotic AE2 cells by phagocytosis, thus contributing to epithelial repair. AE2 cells may act as immunoregulatory cells. AE2 cells interact with resident and mobile cells, either directly by membrane contact or indirectly via cytokines/growth factors and their receptors, thus representing an integrative unit within the alveolus. Although most data support the concept, the controversy about the character of hyperplastic AE2 cells, reported to synthesise profibrotic factors, proscribes drawing a definite conclusion today.