Surfactant protein D enhances bacterial antigen presentation by bone marrow-derived dendritic cells

Purification, characterization and immunolocalization of porcine surfactant protein D

Surfactant protein D (SP-D) is a collectin believed to play an important role in innate immunity. SP-D is characterized by having a collagen-like domain and a carbohydrate recognition domain (CRD), which has a specific Ca(2+)-dependent specificity for saccharides and thus the ability to bind complex glycoconjugates on micro-organisms. This paper describes the tissue immunolocalization of porcine SP-D (pSP-D) in normal slaughter pigs using a monoclonal antibody raised against purified pSP-D. Porcine SP-D was purified from porcine bronchoalveolar lavage (BAL) by maltose-agarose and immunoglobulin M affinity chromatography. The purified protein appeared on sodium dodecyl sulphate-polyacrylamide gel electrophoresis as a band of approximately 53,000 MW in the reduced state and approximately 138,000 MW in the unreduced state. Porcine SP-D was sensitive to collagenase digestion and N-deglycosylation, which reduced the molecular mass to approximately 24,000 MW and approximately 48,000 MW respectively, in the reduced state. N-deglycosylation of the collagen-resistant fragment, reduced the molecular mass to approximately 21,000 MW showing the presence of an N-glycosylation site located in the CRD. Porcine SP-D bound to solid-phase mannan in a dose and Ca(2+)-dependent manner with a saccharide specificity similar to rat and human SP-D. The purified protein was used for the production of a monoclonal anti-pSP-D antibody. The antibody reacted specifically with pSP-D in the reduced and unreduced state when analysed by Western blotting. Immunohistochemical evaluation of normal porcine tissues showed pSP-D immunoreactivity predominantly in Clara cells and serous cells of the bronchial submucosal glands, and to a lesser extent in alveolar type II cells, epithelial cells of the intestinal glands (crypts of Lieberkuhn) in the duodenum, jejunum and ileum and serous cells of the dorsolateral lacrimal gland.

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

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

Differential allele expression of host defense genes, pulmonary surfactant protein-A and Osteopontin, in rat

Differential allele-specific expression has been observed in several genes involved in immunity. SP-A and OPN play a role in innate host defense. To determine whether SP-A and OPN are subject to differential allele-specific regulation, we investigated their gene or allele-specific expression in various tissues. The results showed: (1) Tissue-specific expression with high levels in lung (SP-A) and kidney (OPN). (2) Differences in allele-specific expression among individuals and tissues. SP-A showed an exclusively balanced biallelic expression (BB) in lung, but both BB and imbalanced biallelic (IB) expression in colon. Allele expression of OPN was more heterogeneous, e.g. in colon BB (22%), IB (64%), and monoallelic expression (MO) (14%). (3) Differential allele-specific expression was observed in all tissues studied (OPN) or in all extrapulmonary tissues (SP-A). (4) Family studies indicated that inheritable factor(s) may be involved in the regulation of allele-specific expression. (5) Analysis of co-expression of gene-specific alleles from double heterozygous rats revealed lack of coordinate allele expression among SP-A, SP-D, and OPN. We conclude that allele-specific expression occurs among genes of innate host defense. This may yet provide another level of regulatory complexity for molecules involved in the first line of defense.

Surfactant protein D binding to terminal alpha1-3-linked fucose residues and to Schistosoma mansoni

Pulmonary surfactant protein (SP)-D is an important component of the innate immune system of the lung, which is thought to function by binding to specific carbohydrates on the surface of viruses and unicellular pathogens. SP-D has been shown to have a relatively high affinity for the monosaccharides mannose, glucose, and fucose. However, there is limited information on SP-D binding to complex carbohydrate structures, and binding of SP-D to fucose in the context of an oligosaccharide has not yet been investigated. In this study, we used surface plasmon resonance spectroscopy to examine the potential of SP-D to bind to various synthetic fucosylated oligosaccharides, and identified Fucalpha1-3GalNAc and Fucalpha1-3GlcNAc elements as strong ligands. These types of fucosylated glycoconjugates are presented at the surface of Schistosoma mansoni, a parasitic worm that, during development, transiently resides in the lung. In line with the findings by surface plasmon resonance, we found that SP-D can bind to larval stages of S. mansoni, demonstrating for the first time that SP-D interacts with multicellular lung pathogens.

Collectins and their role in lung immunity

The collectins are a small family of secreted glycoproteins that contain C-type lectin domains and collagenous regions. They have an important function in innate immunity, recognizing and binding to microorganisms via sugar arrays on the microbial surface. Their function is to enhance adhesion and phagocytosis of microorganisms by agglutination and opsonization. In the lung, two members of the collectin family, surfactant proteins A and D, are major protein constituents of surfactant. Another collectin, mannan-binding lectin, is also present in the upper airways and buccal cavity and may protect against respiratory infections. Recent work has shown that collectins have roles in resistance to allergy and in the control of apoptosis and clearance of apoptotic macrophage in the lung.

A recombinant fragment of human SP-D reduces allergic responses in mice sensitized to house dust mite allergens

C57Bl6 mice sensitized to Dermatophagoides pteronyssinus and challenged with D. pteronyssinus allergen extract given intranasally followed by treatment with intranasal applications of a 60-kDa truncated, trimeric recombinant form of human SP-D (rfhSP-D) showed a significant reduction in serum IgE, IgG1, peripheral blood eosinophilia and airway hyperresponsiveness compared to saline or bovine serum albumin-treated controls. Intracellular cytokine staining of lung and spleen homogenates showed increases in interleukin (IL)-12 production in lung tissue and normalization of IL-12 and interferon (IFN)-gamma in spleen tissue. In previous studies we demonstrated the effectiveness of native SP-D and rfhSP-D in down-regulating allergic responses to allergens of Aspergillus fumigatus. The results reported here indicate that rfhSP-D can suppress the development of allergic symptoms in sensitized mice challenged with allergens of the common house dust mite.

Interactions of surfactant protein D with fatty acids

Surfactant Protein D (SP-D) plays important roles in antimicrobial host defense, inflammatory and immune regulation, and pulmonary surfactant homeostasis. The best-characterized endogenous ligand is phosphatidylinositol; however, this lipid interaction at least in part involves the carbohydrate moiety. In this study we observed that SP-D binds specifically to saturated, unsaturated, and hydroxylated fatty acids (FA). Binding of biotinylated-SP-D to FAs or biotinylated FA to SP-D was dose-dependent, saturable, and specifically competed by the corresponding unlabeled probe. Specific binding to FA chains was also demonstrated by solution phase competition for FA binding to acrylodan-labeled FA binding protein (ADIFAB), and by overlay of thin layer chromatograms with SP-D. Maximal binding to FA was dependent on calcium, and binding was localized to the neck and carbohydrate recognition domains (CRD) using recombinant trimeric neck+CRDs. Saccharide ligands showed complex, dose-dependent effects on FA binding, and FAs showed dose- and physical state-dependent effects on the binding of SP-D to mannan. In addition, CD spectroscopy suggested alterations in SP-D structure associated with binding to monomeric FA. Together, the findings indicate specific binding of FA to one or more sites in the CRD. We speculate that the binding of SP-D to the fatty acyl chains of surfactant lipids, microbial ligands, or other complex lipids contributes to the diverse biological functions of SP-D in vivo.

Immunophenotyping of macrophages in human pulmonary tuberculosis and sarcoidosis

Classic studies of tuberculosis (TB) revealed morphologic evidence of considerable heterogeneity of macrophages (MØs), but the functional significance of this heterogeneity remains unknown. We have used newly available specific antibodies for selected membrane and secretory molecules to examine the phenotype of MØs in situ in a range of South African patients with TB, compared with sarcoidosis. Patients were human immunodeficiency virus-negative adults and children, and the examined biopsy specimens included lung and lymph nodes. Mature pulmonary MØs (alveolar, interstitial, epithelioid and multinucleated giant cells) selectively expressed scavenger receptor type A and a novel carboxypeptidase-like antigen called carboxypeptidase-related vitellogenin-like MØ molecule (CPVL). CPVL did not display enhanced expression in sarcoidosis, vs. TB patients, as observed with angiotensin-converting enzyme (ACE), a related molecule. Immunocytochemical studies with surfactant proteins (SP)-A and -D showed that type II alveolar cells expressed these collectins, as did MØs, possibly after binding of secreted proteins. Studies with an antibody specific for the C-terminus of fractalkine, a tethered CX3C chemokine, confirmed synthesis of this molecule by bronchiolar epithelial cells and occasional endothelial cells. These studies provide new marker antigens and extend previous studies on MØ differentiation, activation and local interactions in chronic human granulomatous inflammation in the lung.

Surfactant protein D regulates airway function and allergic inflammation through modulation of macrophage function

The lung collectin surfactant protein D (SP-D) is an important component of the innate immune response but is also believed to play a role in other regulatory aspects of immune and inflammatory responses within the lung. The role of SP-D in the development of allergen-induced airway inflammation and hyperresponsiveness (AHR) is not well defined. SP-D levels progressively increased up to 48 hours after allergen challenge of sensitized mice and then subsequently decreased. The levels of SP-D paralleled the development of airway eosinophilia and AHR. To determine if this association was functionally relevant, mice were administered rat SP-D (rSP-D) intratracheally. When given to sensitized mice before challenge, AHR and eosinophilia were reduced by rSP-D in a dose-dependent manner but not by mutant rSP-D. rSP-D administration resulted in increased levels of interleukin (IL)-10, IL-12, and IFN-gamma in bronchoalveolar lavage fluid and reduced goblet cell hyperplasia. Culture of alveolar macrophages together with SP-D and allergen resulted in increased production of IL-10, IL-12, and IFN-gamma. These results indicate that SP-D can (negatively) regulate the development of AHR and airway inflammation after airway challenge of sensitized mice, at least in part, by modulating the function of alveolar macrophages.

Limited proteolysis of surfactant protein D causes a loss of its calcium-dependent lectin functions

Surfactant protein D (SP-D) is a multimeric collagenous lectin that mediates the clearance of pathogens and modulates immune cell functions via its C-terminal carbohydrate recognition domain (CRD). We hypothesized that extracellular proteolysis of SP-D may result in a loss of its functional properties. Multimeric SP-D was partially digested by human leukocyte elastase (HLE) dose- and time-dependently. Physiologic concentrations of calcium slowed, but did not protect from degradation. In solution, both native and degraded SP-D had an apparent molecular weight of 650 to >1000 kDa. Under reducing conditions, the degraded SP-D monomers run at 10 kDa less than native SP-D. Amino acid sequencing located all major cleavage sites into the CRD. Functional studies showed that degraded SP-D had lost its calcium-dependent lectin properties, i.e. neither bound to mannose nor agglutinated bacteria. These studies demonstrate that elastase results in the limited proteolysis of SP-D with loss of its CRD-dependent activities and suggest that proteases at concentrations observed in various lung diseases may impair the antimicrobial and immunomodulatory roles of SP-D.

Protective effects of a recombinant fragment of human surfactant protein D in a murine model of pulmonary hypersensitivity induced by dust mite allergens

Lung surfactant protein D (SP-D) is a carbohydrate pattern recognition immune molecule. It can interact with a range of pathogens, stimulate immune cells and manipulate cytokine profiles during host's immune response. SP-D has also been shown to interact, via its carbohydrate recognition domains, with glycoprotein allergens of house dust mite (Dermatophagoides pteronyssinus, Derp), inhibiting specific IgE isolated from mite-sensitive asthmatic patients from binding these allergens, and blocking subsequent histamine release from sensitized basophils. In the present study, we have examined the protection offered by various doses of intranasal administration of a recombinant fragment of human SP-D (rhSP-D) in a murine model of pulmonary hypersensitivity to Derp allergens which showed characteristic high levels of specific IgE antibodies, peripheral blood eosinophilia, pulmonary infiltrates and a Th2 cytokine response. Treatment of Derp mice with rhSP-D led to significant reduction in Derp-specific IgE levels, blood eosinophilia and pulmonary cellular infiltration. The levels of IL-4 and IL-5 were decreased, while those of IL-12 and IFN-gamma were raised in the supernatant of the cultured splenocytes, indicating a Th2 to Th1 polarization. These results suggest that SP-D has a protective role in the modulation of allergic sensitization and in the development of allergic reactions to Derp allergens and highlight potential of the rhSP-D as a therapeutic for pulmonary hypersensitivity.

Surfactant protein A modulates the differentiation of murine bone marrow-derived dendritic cells

Surfactant protein A (SP-A) is an innate immune molecule that regulates pathogen clearance and lung inflammation. SP-A modulates innate immune functions such as phagocytosis, cytokine production, and chemotaxis; however, little is known about regulation of adaptive immunity by SP-A. Dendritic cells (DCs) are the most potent antigen-presenting cell with the unique capacity to activate naive T cells and initiate adaptive immunity. The goal of this study was to test the hypothesis that SP-A regulates the differentiation of immature DCs into potent T cell stimulators. The data show that incubation of immature DCs for 24 h with SP-A inhibits basal- and LPS-mediated expression of major histocompatibility complex class II and CD86. Stimulation of immature DCs by SP-A also inhibits the allostimulation of T cells, enhances dextran endocytosis, and alters DC chemotaxis toward RANTES and secondary lymphoid tissue chemokine. The effects on DC phenotype and function are similar for the structurally homologous C1q, but not for SP-D. These studies demonstrate that SP-A participates in the adaptive immune response by modulating important immune functions of DCs.

Heterogeneous allele expression of pulmonary SP-D gene in rat large intestine and other tissues

Random allele expression has recently been observed for several genes including interleukins and genes of the lymphoid system. We studied the hypothesis that the surfactant protein D (SP-D) gene, an innate host defense molecule, exhibits random allele expression in a tissue-specific manner. SP-D gene expression is tissue specific in the 14 tissues studied. Study of SP-D allelic expression in several tissues revealed a balanced biallelic (BB) in lung, and, in several extrapulmonary tissues, a heterogeneous pattern: BB, imbalanced biallelic (IB), and monoallelic (MO). The results from 103 heterozygous rats showed an expression profile in large intestine of BB (22%), IB (58%), and MO (20%). Among eight families, the percent of BB in siblings varied from 0 to 41%, MO from 0 to 33%, and IB from 49 to 83%. The parent-of-origin does not play a role in SP-D allele-specific expression. However, acquired epigenetic factors, family background, or other factors may contribute to the overall pattern of expression.

The scavenger receptor, cysteine-rich domain-containing molecule gp-340 is differentially regulated in epithelial cell lines by phorbol ester

Gp-340 is a glycoprotein belonging to the scavenger receptor cysteine rich (SRCR) group B family. It binds to host immune components such as lung surfactant protein D (SP-D). Recent studies found that gp-340 interacts directly with pathogenic microorganisms and induces their aggregation, suggesting its involvement in innate immunity. In order to investigate further its potential immune functions in the appropriate cell lines, the expression of gp-340 in four conventional immune cell lines (U937, HL60, Jurkat, Raji), and two innate immune-related epithelial cell lines (A549 derived from lung and AGS from stomach), was examined by RT-PCR and immunohistochemistry. The resting immune cell lines showed weak or no gp-340 mRNA expression; while the two epithelial cell lines expressed gp-340 at much higher level, which was differentially regulated by phorbol myristate acetate (PMA) treatment. In the A549 cells, gp-340 was up-regulated along with the PMA-induced proinflammatory expression of both IL-6 and IL-8. In AGS cells, PMA down-regulation of gp-340 was seen in parallel with an up-regulation of the two mature gastric epithelial specific proteins TFF1 (trefoil factor 1) and TFF2, which are implicated as markers of terminal differentiation. Analysis of the distribution of gp-340, together with the TFFs and SP-D in normal lung and gastric mucosa, supported further our in vitro data. We conclude that the differential regulation of gp-340 in the two epithelial cell lines by PMA indicates that gp-340 s involvement in mucosal defence and growth of epithelial cells may vary at different body locations and during different stages of epithelial differentiation.

Collections and ficolins: humoral lectins of the innate immune defense

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