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Background: Surfactant protein D (SP-D), a component of innate immunity, is expressed in the gastric mucosa and is up-regulated in the presence of Helicobacter infection. SP-D binds to Helicobacter in vitro, suggesting the involvement of SP-D in Helicobacter-induced immune responses. The aim of this study was to determine the role of SP-D in gastric epithelial defense in vivo . Methods: Specific pathogen-free SP-D-deficient mice (SP-D−/–) and C57BL/6 wild-type controls were challenged by gavage with different doses of Helicobacter felis, a mouse-adapted Helicobacter strain. Mice were assessed for colonization rates and density of infection. Inflammatory responses were measured by neutrophil counting and T-cell responses by proliferation assays on spleen cells stimulated with H. felis sonicate. The in vitro effect of SP-D on Helicobacter uptake by monocyte-derived dendritic cells was assessed by confocal microscopy and FACS analyses. Results: SP-D−/– mice were more susceptible to low-dose infectious challenge than C57BL/6 controls ( p = .02). The density of colonization was higher in the SP-D−/– infected mice. Neutrophil infiltrates were lower in the SP-D−/– mice, particularly in the acid-secreting regions of the stomach. T-cell proliferative responses to Helicobacter antigen were reduced in SP-D−/– mice ( p = .001) after 12 weeks infection. In vitro uptake of Helicobacter by dendritic cells was significantly enhanced in the presence of SP-D ( p = .001). Conclusion: In the absence of SP-D, Helicobacter uptake by dendritic cells is impaired. This provides an explanation for the diminished inflammation and immune responses in the SP-D−/– mice. [ABSTRACT FROM AUTHOR]

Rapid removal of apoptotic cells is an important mechanism for immune homeostasis and the resolution of inflammation. Delayed clearance of apoptotic alveolar macrophages may cause activation of healthy bystander macrophages and contribute to high macrophage number and emphysema in surfactant protein D (SP-D) knock-out mice. Using flow cytometry and Annexin V and propidium iodide as markers for apoptosis and necrosis, respectively, SP-D-deficient mice were found to have a 5- to 10-fold increase in the number of apoptotic and necrotic alveolar macrophages in the lungs. SP-D-deficient mice accumulate apoptotic macrophages in the lung, and this accumulation can be reduced by treatment with recombinant SP-D (but not SP-A). The recombinant SP-D binds preferentially to apoptotic cells. The data are consistent with a specific role in vivo for SP-D in promoting apoptotic cell clearance in the lungs to limit macrophage-mediated inflammation and reveal a potential new mechanism for therapeutic targeting in the prevention of emphysema. [ABSTRACT FROM AUTHOR]

Integrin a2β1 is a major cellular receptor for collagens, but the molecular basis of its function is unknown. The α2 subunit contains a von Willebrand factor A-domain (I-domain) in its N-terminal region, and it has been demonstrated recently that this domain binds specifically to collagen I. This interaction requires divalent cations (e.g., Mg2+) and native collagen conformation, as does binding of the parent integrin to collagen. The α2 A-domain therefore has a number of functional similarities to the parent integrin, α2β1. However, while sequence specificity has been demonstrated for the parent integrin, no such observations have been made for the A-domain. In particular, it is not known whether the A-domain is responsible for sequence-specific recognition of collagens or whether it binds to the generic collagenous motif. To investigate the ligand specificity of the α2 A-domain, its binding to a range of collagenous ligands has been studied, with cation dependence, collagen triple-helicity, and inhibition by function blocking antibodies as criteria for specificity. Binding of the parent integrin was examined for comparison. The α2 A-domain was found to bind specifically to collagens l, 11, IV and XI. The complement component Clq has a collagenous domain but this was unable to support specific binding of α2 A domain or α2β1. Furthermore, synthetic triple-helical collagenous peptides failed to act as specific ligands. In conclusion, the α2 A-domain binds specifically to a range of extracellular matrix collagens, but it is not a receptor for all collagenous triple helices. By inference, these findings indicate the existence of an integrin-specific sequence motif within collagenous ligands recognised by the α2 A-domain. [ABSTRACT FROM AUTHOR]

Activation of component C3 is central to the pathways of complement and leads directly to neutralization of pathogens and stimulation of adaptive immune responses. The convertases that catalyze this reaction assemble from fragments of complement components via multistep reactions. In the lectin pathway, mannose-binding lectin (MBL) and ficolins bind to pathogens and activate MBL-associated serine protease-2 (MASP-2). MASP-2 cleaves C4 releasing C4a and generating C4b, which attaches covalently to the pathogen surface upon exposure of its reactive thioester. C2 binds to C4b and is also cleaved by MASP-2 to form the C3 convertase (C4b2a). To understand how this complex process is coordinated, we have analyzed the interactions between MASP-2, C4, C2, and their activation fragments and have compared MASP-2-catalyzed cleavage of C4b2 and C2. The data show that C2 binds tightly to C4b but not to C4, implying that C4 and C2 do not circulate as preformed complexes but that C2 is recruited only after prior activation of C4. Following cleavage of C4, C4b still binds to MASP-2 (KD ∼ 0.6 μ) and dissociates relatively slowly (koff ∼ 0.06 s-1) compared with the half-life of the thioester (≤0.7 s) (Sepp, A., Dodds, A. W., Anderson, M. J., Campbell, R. D., Willis, A. C., and Law, S. K. (1993) Protein Sci. 2, 706–716). We propose that the C4b·MASP-2 interaction favors attachment of C4b near to the activating MBL·MASP complex on the bacterial surface so that, following recruitment of C2, the proximity of enzyme and substrate (C4b2) combined with more favorable reaction kinetics drive the formation of the C3 convertase, promoting complement activation. [ABSTRACT FROM AUTHOR]

Collectins are a family of innate immune proteins that contain fibrillar collagen-like regions and globular carbohydrate recognition domains (CRDs). The CRDs of these proteins recognize various microbial surface-specific carbohydrate patterns, particularly hexoses. We hypothesized that collectins, such as pulmonary surfactant proteins (SPs) SP-A and SP-D and serum protein mannose-binding lectin, could recognize nucleic acids, pentose-based anionic phosphate polymers. Here we show that collectins bind DNA from a variety of origins, including bacteria, mice, and synthetic oligonucleotides. Pentoses, such as arabinose, ribose, and deoxyribose, inhibit the interaction between SP-D and mannan, one of the well-studied hexose ligands for SP-D, and biologically relevant D-forms of the pentoses are better competitors than the L-forms. In addition, DNA and RNA polymer-related compounds, such as nucleotide diphosphates and triphosphates, also inhibit the carbohydrate binding ability of SP-D, or ∼60 kDa trimeric recombinant fragments of SP-D that are composed of the α-helical coiled-coil neck region and three CRDs (SP-D(n/ CRD)) or SP-D(n/CRD) with eight GXY repeats (SPD(GXY)8(n/CRD)). Direct binding and competition studies suggest that collectins bind nucleic acid via their CRDs as well as by their collagen-like regions, and that SP-D binds DNA more effectively than do SP-A and mannose-binding lectin at physiological salt conditions. Furthermore, the SP-D(GXY)8(n/CRD) fragments co-localize with DNA, and the protein competes the interaction between propidium iodide, a DNA-binding dye, and apoptotic cells. In conclusion, we show that collectins are a new class of proteins that bind free DNA and the DNA present on apoptotic cells by both their globular CRDs and collagen-like regions. Collectins may therefore play an important role in decreasing the inflammation caused by DNA in lungs and other tissues. [ABSTRACT FROM AUTHOR]

Lung surfactant protein D (SP-D) can directly interact with carbohydrate residues on pulmonary pathogens and allergens, stimulate immune cells, and manipulate cytokine and chemokine profiles during the immune response in the lungs. Therapeutic administration of rfhSP-D, a recombinant homotrimeric fragment of human SP-D comprising the α-helical coiled-coil neck plus three CRDs, protects mice against lung allergy and infection caused by the fungal pathogen Aspergillus fumigatus. The high resolution crystal structures of maltose-bound rfhSP-D to 1.4 A˚, and of rfhSP-D to 1.6 A˚, define the fine detail of the mode and nature of carbohydrate recognition and provide insights into how a small fragment of human SP-D can bind to allergens/antigens or whole pathogens, and at the same time recruit and engage effector cells and molecules of humoral immunity. A previously unreported calcium ion, located on the trimeric axis in a pore at the bottom of the funnel formed by the three CRDs and close to the neck–CRD interface, is coordinated by a triad of glutamate residues which are, to some extent, neutralised by their interactions with a triad of exposed lysine residues in the funnel. The spatial relationship between the neck and the CRDs is maintained internally by these lysine residues, and externally by a glutamine, which forms a pair of hydrogen-bonds within an external cleft at each neck–CRD interface. Structural links between the central pore and the cleft suggest a possible effector mechanism for immune cell surface receptor binding in the presence of bound, extended natural lipopolysaccharide and phospholipid ligands. The structural requirements for such an effector mechanism, involving both the trimeric framework for multivalent ligand binding and recognition sites formed from more than one subunit, are present in both native hSP-D and rfhSP-D, providing a possible explanation for the significant biological activity of rfhSP-D. [Copyright &y& Elsevier]