SP-A、SP-D与肺部天然免疫防御

肺表面活性物质脱辅基蛋白A、D(SP-A、SP-D)系C型凝集素超家族中胶凝素家族成员,是肺部重要的天然免疫防御分子,不仅能清除病原体,还参与免疫、炎症及过敏反应的调节.     

Mannan-binding protein levels in human amniotic fluid during gestation and its interaction with collectin receptor from amnion cells.

Collectins are a group of soluble proteins, each of which has collagenous and non-collagenous globular domains, and may be complement associated and/or have lectin activity. The complement protein C1q is structurally and functionally related to the collectins. Members of the collectin family are mannan-binding protein (MBP), lung surfactant protein A, CL-43, conglutinin and lung surfactant protein D. All except the last of these proteins have been shown to interact with a single, widely distributed receptor, named collectin receptor (also called C1q receptor). It has been proposed that collectins play an important role as opsonins in innate immunity. MBP is a serum protein and has been shown to activate the classical complement pathway. MBP has also been shown to opsonize bacteria and enhance clearance of bacteria by phagocytosis. In this article we demonstrate the presence of MBP in amniotic fluid and the localization of collectin receptor on the surface of amniotic epithelium. MBP levels in amniotic fluid were found to increase sharply from about 32 weeks of gestation. Collectin receptor was purified from a cell line, FL, derived from amniotic epithelium and was shown to interact with MBP. These results suggest that MBP may play a role in the antibody-independent recognition and clearance of pathogens in the amniotic cavity, towards term.     

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

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

Surfactant proteins SP-A and SP-D as modulators of the allergic inflammation in asthma.

Surfactant proteins A and D (SP-A and SP-D) are members of the collectin family and bind to various motifs of microorganisms, particles and allergens. They play an important role in the first-line defense within the lung. Recent research has highlighted that these proteins not only augment innate immune responses to invading microorganisms but also act on adaptive immune functions like dendritic cell maturation and T cell proliferation. Both SP-A and SP-D downregulate the eosinophilic inflammation in murine asthma models and shift the cytokine profile towards a T helper cell type 1 response. In addition, they are effective at alleviating bronchial hyperresponsiveness. Although our knowledge about surfactant proteins as modulators of the allergic inflammatory reaction in asthma is still limited, the idea that surfactant proteins play a role in asthma has attracted increasing attention. In this review, the impact of the lung collectins SP-A and SP-D on asthmatic allergic inflammation and vice versa will be discussed.     

Characterization and expression sites of newly identified chicken collectins

Collectins are members of the family of vertebrate C-type lectins. They have been found almost exclusively in mammals, with the exception of chicken MBL. Because of their important role in innate immunity, we sought to identify other collectins in chicken. Using the amino acid sequences of known collectins, the EST database was searched and related to the chicken genome. Three chicken collectins were found and designated chicken Collectin 1 (cCL-1), chicken Collectin 2 (cCL-2), and chicken Collectin 3 (cCL-3), which resemble the mammalian proteins Collectin Liver 1, Collectin 11 and Collectin Placenta 1, respectively. Additionally, a lectin was found which resembled Surfactant Protein A, but lacked the collagen domain. Therefore, it was named chicken Lung Lectin (cLL). Tissue distribution analysis showed cCL-1, cCL-2 and cCL-3 are expressed in a wide range of tissues throughout the digestive, the reproductive and the lymphatic system. Similar to SP-A, cLL is mainly localized in lung tissue. Phylogenetic analysis indicates that cCL-1, cCL-2 and cCL-3 represent new subgroups within the collectin family. The newly found collectins may have an important function in avian host defence. Elucidation of the role of these pattern-recognition molecules could lead to strategies that thwart infectious diseases in poultry, which could also be beneficial for public health.     

Binding of host collectins to the pathogenic yeast Cryptococcus neoformans: human surfactant protein D acts as an agglutinin for acapsular yeast cells.

Cryptococcus neoformans is an opportunistic pathogen in AIDS patients causing disseminated disease and lethal meningitis after inhalation of acapsular or sparsely encapsulated yeast cells. In this study we have investigated whether a recently described family of primitive opsonins, termed collectins, contribute to innate resistance against C. neoformans. The pulmonary surfactant proteins SP-A and SP-D as well as the serum collectins mannose-binding protein and CL-43 bound in a calcium-dependent manner to acapsular C. neoformans in vitro. Binding was concentration dependent and abolished by competition with defined mono- and oligosaccharides. In contrast, no binding of the collectins was observed with the encapsulated form of the yeast. Furthermore, binding of purified collectin SP-D, but not SP-A, mannose-binding protein, or CL-43, led to a concentration-dependent agglutination of acapsular C. neoformans. These data indicate that collectins recognize carbohydrate structures in the cell wall of an initial infectious form of C. neoformans and may play a role in early antifungal defenses in the lung.     

Surfactant protein A and surfactant protein D variation in pulmonary disease

Surfactant proteins A (SP-A) and D (SP-D) have been implicated in pulmonary innate immunity. The proteins are host defense lectins, belonging to the collectin family which also includes mannan-binding lectin (MBL). SP-A and SP-D are pattern-recognition molecules with the lectin domains binding preferentially to sugars on a broad spectrum of pathogen surfaces and thereby facilitating immune functions including viral neutralization, clearance of bacteria, fungi and apoptotic and necrotic cells, modulation of allergic reactions, and resolution of inflammation. SP-A and SP-D can interact with receptor molecules present on immune cells leading to enhanced microbial clearance and modulation of inflammation. SP-A and SP-D also modulate the functions of cells of the adaptive immune system including dendritic cells and T cells. Studies on SP-A and SP-D polymorphisms and protein levels in bronchoalveolar lavage and blood have indicated associations with a multitude of pulmonary inflammatory diseases. In addition, accumulating evidence in mouse models of infection and inflammation indicates that recombinant forms of the surfactant proteins are biologically active in vivo and may have therapeutic potential in controlling pulmonary inflammatory disease. The presence of the surfactant collectins, especially SP-D, in non-pulmonary tissues, such as the gastrointestinal tract and genital organs, suggest additional actions located to other mucosal surfaces. The aim of this review is to summarize studies on genetic polymorphisms, structural variants, and serum levels of human SP-A and SP-D and their associations with human pulmonary disease.     

Developmental regulation of chicken surfactant protein A and its localization in lung

Surfactant Protein A (SP-A) is a collagenous C-type lectin (collectin) that plays an important role in the early stage of the host immune response. In chicken, SP-A (cSP-A) is expressed as a 26 kDa glycosylated protein in the lung. Using immunohistochemistry, cSP-A protein was detected mainly in the lung lining fluid covering the parabronchial epithelia. Specific cSP-A producing epithelial cells, resembling mammalian type II cells, were identified in the parabronchi. Gene expression of cSP-A markedly increased from embryonic day 14 onwards until the time of hatch, comparable to the SP-A homologue chicken lung lectin, while mannan binding lectin and collectins CL-L1 and CL-K1 only showed slightly changed expression during development. cSP-A protein could be detected as early as ED 18 in lung tissue using Western blotting, and expression increased steadily until day 28 post-hatch. Our observations are a first step towards understanding the role of this protein in vivo.     

Collectin-43 is a serum lectin with a distinct pattern of carbohydrate recognition.

Collectin-43 (CL-43) is a C-type serum lectin and a member of the collectin family of soluble proteins that are effector molecules in innate immunity. We have investigated the binding specificity of CL-43 using as model systems a panel of structurally defined oligosaccharides in the form of neoglycolipids, and several glycoproteins derived from the complement glycoprotein C3 during activation of the complement cascade. A specificity is revealed towards fucose as part of the Lea oligosaccharide sequence, and towards mannose as found on high mannose-type chains. These are features shared with other serum collectins, conglutinin and mannan-binding proteins; a major difference is the lack of detectable binding by CL-43 to N-glycosidic oligosaccharides terminating in N-acetylglucosamine. CL-43 has a unique pattern of reactivity towards high mannose-type oligosaccharides on the two glycosylation sites of C3 and derived glycoproteins: it binds to C3c (not bound by conglutinin and mannan-binding protein) but not to hydrolysed C3 [C3(H2O)], C3b or iC3b immobilized on microwells (all bound by conglutinin but not by mannan-binding protein). When these glycoproteins are sodium dodecyl sulphate (SDS)-treated and immobilized on nitrocellulose, CL-43 (but not conglutinin nor mannan-binding protein) binds strongly to C3(H2O), iC3b and C3c. The salient conclusions are, first, that there are remarkable positive or negative effects of carrier protein on oligosaccharide presentation and these differ for the individual collectins. Second, the different though partially overlapping binding patterns among the collectins may be important for their function as circulating effector molecules with broad surveillance capabilities.     

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

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

Collectins and pulmonary innate immunity

Summary: The surfactant-associated proteins SP-A and SP-D are members of a family of host defense lectins, designated collectins. There is increasing evidence that these pulmonary, epithelial-derived proteins are important components of the innate immune response to microbial challenge and participate in other aspects of immune and inflammatory regulation within the lung. Both proteins bind to glycoconjugates and/or lipid moieties expressed by a wide variety of microorganisms, and to certain organic particles, such as pollens. SP-A and SP-D have the capacity to modulate leukocyte function and, in some circumstances, to opsonize and enhance the killing of microorganisms. The biologic activity of cell wall components, such as Gram-negative bacterial polysaccharides, or viral glycoproteins, such as the hemagglutinin of influenza viruses, may be altered by interactions with collectins. In addition, complementary or cooperative interactions between SP-A, SP-D and other host defense lectins could contribute to the efficiency of this defense system. Collectins could play particularly important roles in settings of inadequate or impaired specific immunity, and acquired alterations in the levels of active collectins within the airspaces and distal airways may increase susceptibility to infection.     

The role of mannose-binding lectin in health and disease

Mannose-binding lectin (MBL) is a pattern recognition molecule of the innate immune system. It belongs to the collectin family of proteins in which lectin (carbohydrate-recognition) domains are found in association with collagenous structures. In man, these proteins include serum MBL, lung surfactant protein A (SP-A) and lung surfactant protein D (SP-D). MBL binds to a range of sugars including N-acetyl-D-glucosamine, mannose, N-acetyl-mannosamine, fucose and glucose. This permits the protein to interact with a wide selection of viruses, bacteria, yeasts, fungi and protozoa decorated with such sugars. Unlike the other collectins, MBL bound to microbial surfaces is able to activate the complement system in an antibody and C1-independent manner. This activation is mediated by complexes of MBL with a serine protease called MBL-associated serine protease 2 (MASP-2), which specifically cleaves C4 and C2 to create a C3 convertase enzyme. MBL may also interact directly with cell surface receptors and thereby promote opsonophagocytosis by a complement-independent pathway. It has been suggested that MBL plays an important role in the first hours/days of any primary immune response to a sugar decorated pathogen. This provides the host with a first-line of defence before the adaptive immune system becomes operative and in humans may be particularly important between 6 and 18 months of age when the adaptive system is still immature. MBL deficiency is one of the most common human immunodeficiencies and arises primarily from three single point mutations in exon 1 of the MBL-2 gene. These mutations result in a failure to assemble fully functional multimeric protein. Several studies have shown that deficiency of MBL increases the overall susceptibility of an individual to infectious disease. The most striking example of this is the association of acute respiratory tract infections with MBL deficiency in early childhood. In contrast, there is evidence that for some intracellular parasites MBL deficiency may be protective and this might explain the high frequency of MBL mutations in sub-Saharan Africa and South America. Increasingly, there is evidence that the association between MBL levels and disease is complex. For example, the protein appears to influence the severity of several diseases. The mechanism whereby MBL exerts such effects is unclear but one possibility is through a dose-dependent modulation of pro-inflammatory cytokines.     

Lung surfactant proteins (SP-A and SP-D) in non-adaptive host responses to infection

The lung surfactant proteins A and D (SP-A and SP-D) are collectins composed of C-type lectin domains attached to collagen regions. SP-A and SP-D are mainly found in the surfactant covering the pulmonary epithelial cells, but are also produced by cells lining the gastrointestinal tract. The main role of SP-A and SP-D is to interact directly with carbohydrate on the surface of microbial pathogens, thereby initiating a variety of effector mechanisms. This review focuses on the non-adaptive host responses of SP-A and SP-D to infection. Interaction of SP-A and SP-D with phagocytes is discussed and the structure and function of the putative receptors for SP-A and SP-D is presented. SP-A and SP-D seem to be regulated in a way similar to acute-phase proteins in the course of inflammation and evidence for the involvement of SP-A and SP-D as immunomodulators as well as their role in clearing allergens and modulating effector mechanisms in allergic reactions is discussed.     

Secretion of a collectin-like protein in tunicates is enhanced during inflammatory responses

The sub-cellular and humoral concentrations of a collectin-like protein from the solitary tunicate, Styela plicata, were measured after in vivo challenge with the inflammatory agent, zymosan. Tunicates were injected with zymosan before hemocytes and serum were harvested, subjected to western blotting and immunostained with an anti-S. plicata collectin antibody to determine the relative titers of collectin-like proteins. Concentrations of the predominant 43kDa collectin polypeptide were found to decrease in hemocytes immediately after zymosan injection, before rising to levels that were six times higher than controls within 96h. Similarly, immunohistochemistry showed that the frequency of collectin-positive hemocytes in the circulating hemolymph increased significantly within 96h of injection. Levels of the 43kDa polypeptide in serum mirrored those of hemocytes. Humoral collectin concentrations decreased immediately after zymosan injection before rising, within 96h post-injection, to levels three times higher than controls. This response to an inflammatory stimulus resembles that of mammalian collectins like mannose-binding lectin. The data suggest that, like its mammalian counterparts, the tunicate collectin acts as an acute phase antigen recognition protein.     

The SARS coronavirus spike glycoprotein is selectively recognized by lung surfactant protein D and activates macrophages

The severe acute respiratory syndrome coronavirus (SARS-CoV) infects host cells with its surface glycosylated spike-protein (S-protein). Here we expressed the SARS-CoV S-protein to investigate its interactions with innate immune mechanisms in the lung. The purified S-protein was detected as a 210 kDa glycosylated protein. It was not secreted in the presence of tunicamycin and was detected as a 130 kDa protein in the cell lysate. The purified S-protein bound to Vero but not 293T cells and was itself recognized by lung surfactant protein D (SP-D), a collectin found in the lung alveoli. The binding required Ca(2+) and was inhibited by maltose. The serum collectin, mannan-binding lectin (MBL), exhibited no detectable binding to the purified S-protein. S-protein binds and activates macrophages but not dendritic cells (DCs). It suggests that SARS-CoV interacts with innate immune mechanisms in the lung through its S-protein and regulates pulmonary inflammation.     

Ficolins: novel pattern recognition molecules of the innate immune response

Ficolins are members of the collectin family of proteins which are able to recognize pathogen-associated molecular pattern (PAMP) on microbial surfaces. Upon binding to their specific PAMP, ficolins may trigger activation of the immune system by either binding to cellular receptors for collectins or by initiating activation of complement via the lectin pathway. For the latter, the human ficolins (i.e. L-, H- and M-ficolin) and murine ficolin-A were shown to associate with the lectin pathway-specific serine protease MBL-associated serine protease-2 (MASP-2) and catalyse its activation which in turn activates C4 and C4b-bound C2 to generate the C3 convertase C4b2a. There is mounting evidence underlining the lectin nature of ficolins with a wide range of carbohydrate moieties recognized on microbial surfaces. However, not all members of the ficolin family appear to act as lectin pathway recognition components. For example, murine ficolin-B does not associate with MASP-2 and appears to be absent in plasma and other humoral fluids. Its stringent cellular localization points to other functions within the immune response, possibly acting as an intracellular scavenger to target and facilitate clearance of PAMP-bearing debris. When comparing ficolin orthologues from different species, it appears evident that human, murine, and porcine ficolins differ in many aspects, a specific point that we aim to address in this review.     

L-ficolin in children with recurrent respiratory infections

The lectin pathway of complement activation is used by a collectin, mannan-binding lectin (MBL), and two ficolins, L-ficolin and H-ficolin, to opsonize microorganisms for phagocytosis. We published evidence recently that MBL insufficiency is associated with recurrent respiratory infections in childhood. We have now measured serum L-ficolin in 313 respiratory infection patients and 74 healthy control children. L-ficolin concentrations below the lower limit of the control group were found in 6% of the patients (P <0.02) and were associated most strongly with children having co-existing atopic disorders (11%; P=0.002). We suggest that L-ficolin may have a role in protection from microorganisms complicating allergic disease.     

Lung surfactant proteins involved in innate immunity

The two lung surfactant collectins, surfactant protein (SP)-A and SP-D, are involved in host defence against infectious and allergenic agents via enhancement of killing and clearance by macrophages and neutrophils. Recent gene-knockout, protein engineering and physiological studies have emphasised the roles that SP-A and SP-D play in acute inflammatory responses.     

Surfactant proteins SP-A and SP-D in human health and disease

Surfactant proteins A (SP-A) and D (SP-D) are lung surfactant-associated hydrophilic proteins that have been implicated in surfactant homeostasis and pulmonary innate immunity.They are collagen-containing C-type (calcium-dependent) lectins, called collectins, and are structurally similar to mannose-binding protein of the lectin pathway of the complement system. Being carbohydrate pattern-recognition molecules, they recognize a broad spectrum of pathogens and allergens via the lectin domain, with subsequent activation of immune cells via the collagen region, thus offering protection against infection and allergenic challenge. SP-A and SP-D have been shown to be involved in viral neutralization, clearance of bacteria, fungi, and apoptotic and necrotic cells, down-regulation of allergic reaction, and resolution of inflammation. Studies on single-nucleotide polymorphism, protein levels in broncho-alveolar lavage, and gene knock-out mice have clearly indicated an association between SP-A and SP-D and a range of pulmonary diseases. In addition, recent studies using murine models of allergy and infection have raised the possibility that the recombinant forms of SP-A and SP-D may have therapeutic potential in controlling pulmonary infection, inflammation, and allergies in humans.