Opsonic function and concentration of human serum ficolin/P35

The Role of Glycosylation in Infectious Diseases

Glycosylation plays an important role in infectious diseases. Many important interactions between pathogens and hosts involve their carbohydrate structures (glycans). Glycan interactions can mediate adhesion, recognition, invasion, and immune evasion of pathogens. To date, changes in many protein N/O-linked glycosylation have been identified as biomarkers for the development of infectious diseases and cancers. In this review, we will discuss the principal findings and the roles of glycosylation of both pathogens and host cells in the context of human important infectious diseases. Understanding the role and mechanism of glycan-lectin interaction between pathogens and hosts may create a new paradigm for discovering novel glycan-based therapies that can lead to eradication or functional cure of pathogens infection.

The serum opsonin L-ficolin is detected in lungs of human transplant recipients following fungal infections and modulates inflammation and killing of Aspergillus fumigatus

BACKGROUND

Invasive aspergillosis (IA) is a life-threatening systemic fungal infection in immunocompromised individuals that is caused by Aspergillus fumigatus. The human serum opsonin, L-ficolin, has been observed to recognize A. fumigatus and could participate in fungal defense.

METHODS

Using lung epithelial cells, primary human monocyte-derived macrophages (MDMs), and neutrophils from healthy donors, we assessed phagocytosis and killing of L-ficolin-opsonized live A. fumigatus conidia by flow cytometry and microscopy. Additionally, cytokines were measured by cytometric bead array, and L-ficolin was measured in bronchoalveolar lavage (BAL) fluid from lung transplant recipients by enzyme-linked immunosorbent assay.

RESULTS

L-ficolin opsonization increased conidial uptake and enhanced killing of A. fumigatus by MDMs and neutrophils. Opsonization was also shown to manifest an increase in interleukin 8 release from A549 lung epithelial cells but decreased interleukin 1β, interleukin 6, interleukin 8, interleukin 10, and tumor necrosis factor α release from MDMs and neutrophils 24 hours after infection. The concentration of L-ficolin in BAL fluid from patients with fungal infection was significantly higher than that for control subjects (P = .00087), and receiving operating characteristic curve analysis highlighted the diagnostic potential of L-ficolin for lung infection (area under the curve, 0.842; P < .0001).

CONCLUSIONS

L-ficolin modulates the immune response to A. fumigatus. Additionally, for the first time, L-ficolin has been demonstrated to be present in human lungs.

Ficolins and infectious diseases

Ficolins are serum complement lectins, with a structure similar to mannose-binding lectin (MBL) and lung surfactant protein (SP)-A and SP-D. Ficolins activate the lectin complement system and play important roles in host innate immunity. Ficolins are members of the collectin family of proteins, which act as pattern recognition receptors (PRRs). They are soluble oligomeric defense proteins with lectin-like activity, and are able to recognize pathogen-associated molecular patterns (PAMPs), which are carbohydrate molecules on the surface of pathogens, and of apoptotic, necrotic, and malignant cells. Upon binding to their specific PAMPs, ficolins may trigger activation of the immune system either (1) by initiating activation of complement via the lectin pathway, (2) by a primitive type of opsonophagocytosis, or (3) by stimulating secretion of the inflammatory cytokines interferon (IFN)-Γ, interleukin (IL)-17, IL-6, and tumor necrosis factor (TNF)-α, and production of nitric oxide (NO) by macrophages, thus limiting the infection and concurrently orchestrating the subsequent adaptive immune response. Recently, a number of reports have shown that dysfunction or abnormal expression of ficolins may play crucial roles in viral and bacterial diseases and in inflammation. This review summarizes the reports on the roles of ficolins in the infectious diseases, and provides insight into ficolins as novel innate immune therapeutic options to treat these diseases.

First evidence of protein G-binding protein in the most primitive vertebrate: serum lectin from lamprey (Lampetra japonica)

The intelectins, a recently identified subgroup of extracellular animal lectins, are glycan-binding receptors that recognize glycan epitopes on foreign pathogens in host systems. Here, we have described NPGBP (novel protein G-binding protein), a novel serum lectin found in the lamprey, Lampetra japonica. RT-PCR yielded a 1005 bp cDNA sequence from the lamprey liver encoding a 334 amino acid secretory protein with homology to mammalian and aquatic organism intelectins. Gene expression analyses showed that the NPGBP gene was expressed in the blood, intestines, kidney, heart, gill, liver, adipose tissue and gonads. NPGBP was isolated by protein G-conjugated agarose immunoprecipitation, and SDS-PAGE analyses showed that NPGBP migrated as a specific band (∼35 and ∼124 kDa under reducing and non-reducing conditions, respectively). These results suggested that NPGBP forms monomers and tetramers. NPGBP gene expression was induced by in vivo bacterial stimulation, and NPGBP showed different agglutination activities against pathogenic Gram-positive bacteria, Gram-negative bacteria and fungi. The induction of NPGBP suggested that it plays an important role in defense against microorganisms in the internal circulation system of the lamprey. When incubated with an unrelated antibody, the specific binding between NPGBP and protein G was competitively inhibited, indicating that NPGBP and the Fc region of Ig bind to the same site on protein G. We thus assume that the tertiary structure of NPGBP is similar to that of the Fc region of Ig. Additionally, NPGBP can effectively promote endothelial cell mitosis. These findings suggest that NPGBP plays a role in the immune defense against microorganisms, and this study represents one of the few examples of the characterization and functional analysis of an aquatic organism intelectin.

Ficolin-2 defends against virulent Mycobacteria tuberculosis infection in vivo, and its insufficiency is associated with infection in humans

Human ficolin-2 (ficolin-2/P35) is a lectin complement pathway activator that is present in normal human plasma and is associated with infectious diseases; however, little is known regarding the roles and mechanisms of ficolin-2 during Mycobacterium tuberculosis (Mtb) infection. Here, we describe our novel findings that the ficolin-2 serum levels of 107 pulmonary tuberculosis (TB) patients were much lower compared with 107 healthy controls. In vitro analysis showed that ficolin-2 bound to the virulent Mtb H37Rv strain much more strongly than to the non-virulent M. bovis BCG and M. smegmatis. Ficolin-2 bound to the surface glycolipid portion of H37Rv and blocked H37Rv infection in human lung A549 cells. Opsonophagocytosis was also promoted by ficolin-2. Importantly, we found that administration of exogenous ficolin-2 had a remarkable protective effect against virulent Mtb H37Rv infection in both C57BL/6J and BALB/c mice. Ficolin-A (a ficolin-2-like molecule in mouse) knockout mice exhibited increased susceptibility to H37Rv infection. We further demonstrated that ficolin-2 could defend against virulent Mtb H37Rv infection at least partially by activating JNK phosphorylation and stimulating the secretion of interferon (IFN)-γ, interleukin (IL)-17, IL-6, tumor necrosis factor (TNF)-α, and nitric oxide (NO) production by macrophages. Our data provide a new immunotherapeutic strategy against TB based on the innate immune molecule ficolin-2 and indicate that ficolin-2 insufficiency is associated with higher susceptibility to infection in humans.

Circulating ficolin-2 and ficolin-3 in normal pregnancy and pre-eclampsia

Ficolins are soluble molecules of the innate immune system that recognize carbohydrate molecules on microbial pathogens, apoptotic and necrotic cells. They act through two distinct routes: initiating the lectin pathway of complement activation and mediating a primitive opsonophagocytosis. In this study, we measured plasma levels of ficolin-2 and ficolin-3 in 60 pre-eclamptic patients, 60 healthy pregnant women and 59 healthy non-pregnant women by enzyme-linked immunosorbent assay (ELISA). Circulating levels of complement activation products (C4d, C3a, SC5b9), angiogenic factors (soluble fms-like tyrosine kinase-1, placental growth factor) and markers of endothelial activation (von Willebrand factor antigen), endothelial injury (fibronectin) and trophoblast debris (cell-free fetal DNA) were also determined. Plasma levels of ficolin-2 were significantly lower in healthy pregnant than in healthy non-pregnant women, while ficolin-3 levels did not differ significantly between the two groups. Furthermore, pre-eclamptic patients had significantly lower ficolin-2 and ficolin-3 concentrations than healthy non-pregnant and pregnant women. In the pre-eclamptic group, plasma ficolin-2 levels showed a significant positive correlation with serum placental growth factor (PlGF) concentrations and significant inverse correlations with serum levels of soluble fms-like tyrosine kinase-1 (sFlt-1), blood urea nitrogen and creatinine, serum lactate dehydrogenase activities, as well as with plasma VWF:antigen, fibronectin and cell-free fetal DNA concentrations. In conclusion, circulating levels of ficolin-2 are decreased in the third trimester of normal pregnancy. There is a further decrease in plasma ficolin-2 concentrations in pre-eclampsia, which might contribute to the development of the maternal syndrome of the disease through impaired removal of the trophoblast-derived material released into the maternal circulation by the hypoxic and oxidatively stressed pre-eclamptic placenta.

L-ficolin binds to the glycoproteins hemagglutinin and neuraminidase and inhibits influenza A virus infection both in vitro and in vivo

L-ficolin, one of the complement lectins found in human serum, is a novel pattern recognition molecule that can specifically bind to microbial carbohydrates, thereby activating the lectin complement pathway and mounting a protective innate immune response. However, little is known about the role of L-ficolin during viral infections in vivo. In the present study, we used a mouse model of influenza A virus infection to demonstrate that the administration of exogenous L-ficolin or ficolin A (FCNA - an L-ficolin-like molecule in the mouse) is protective against the virus. Furthermore, FCNA-null mice have a greatly increased susceptibility to infection with the influenza A virus. Moreover, we found recombinant human L-ficolin inhibited influenza A virus entry into Madin-Darby canine kidney cells. More importantly, L-ficolin can recognize and bind hemagglutinin (HA) and neuraminidase (NA) glycoproteins and different subtypes of influenza A virus, and these interactions can be competitively inhibited by N-acetyl-D-glucosamine. In addition, the binding of L-ficolin and FCNA may lead to the activation of the lectin complement pathway. To our knowledge, this is the first report demonstrating that L-ficolin can block influenza virus infections both in vitro and in vivo using FCNA-knockout mice, possibly by interacting with the carbohydrates of HA and NA. Therefore, these data may provide new immunotherapeutic strategies based on the innate immune molecule L-ficolin against the influenza A virus.

Ficolins and FIBCD1: soluble and membrane bound pattern recognition molecules with acetyl group selectivity

A network of molecules, which recognizes pathogens, work together to establish a quick and efficient immune response to infectious agents. Molecules containing a fibrinogen related domain in invertebrates and vertebrates have been implicated in immune responses against pathogens, and characterized as pattern recognition molecules. Ficolins are soluble oligomeric proteins composed of trimeric collagen-like regions linked to fibrinogen-related domains (FReDs) that have the ability to sense molecular patterns on both pathogens and apoptotic cell surfaces and activate the complement system. The ficolins have acetyl-binding properties, which have been localized to different binding sites in the FReD-region. A newly discovered tetrameric transmembrane protein, FIBCD1, likewise binds acetylated structures via the highly conserved FReD. This review presents current knowledge on acetyl binding FReD-containing molecules, and discusses structural resemblance but also diversity in recognition of acetylated ligands.

Multiple routes of complement activation by Mycobacterium bovis BCG

Mycobacterium tuberculosis is the leading cause of infectious disease in humans in the world. It evades the host immune system by being phagocytosed by macrophages and residing intracellularly. Complement-dependent opsonisation of extracellular mycobacteria may assist them to enter macrophages. This work examines in detail the mechanisms of complement activation by whole mycobacteria using Mycobacterium bovis BCG as a model organism. M. bovis BCG directly activates the classical, lectin and alternative pathways, resulting in fixation of C3b onto macromolecules of the mycobacterial surface. Investigation into the classical pathway has shown direct binding of human C1q to whole mycobacteria in the absence of antibodies. Most human sera contain IgG and IgM-anti-(M. bovis BCG), and pre-incubation with human immunoglobulin enhances C1q binding to the bacteria. Therefore classical pathway activation is both antibody-independent and dependent. The bacteria also activate the alternative pathway in an antibody-independent manner, but Factor H also binds, suggesting some regulation of amplification by this pathway. For the lectin pathway we have demonstrated direct binding of both MBL and L-ficolin from human serum to whole mycobacteria and subsequent MASP2 activation. H-ficolin binding was not observed. No M. bovis BCG cell surface or secreted protease appears likely to influence complement activation. Together, these data provide a more detailed analysis of the mechanisms by which M. bovis BCG interacts with the complement system.

Identification, cloning and tissue localization of a rainbow trout (Oncorhynchus mykiss) intelectin-like protein that binds bacteria and chitin

Intelectins are a recently identified group of animal lectins involved in innate immune surveillance. This paper describes the primary structure, expression and immunohistochemical localization of a rainbow trout plasma intelectin (RTInt). RTInt exhibited calcium-dependent binding to N-acetylglucosamine (GlcNAc) and mannose conjugated Toyopearl Amino 650 M matrices. When GlcNAc eluates from chromatography matrices were analyzed by reducing 1D PAGE and Western blots, the lectin appeared as approximately 37 kDa and approximately 72 kDa bands. Similar analysis of plasma revealed a single 72 kDa band under reducing conditions. MALDI-TOF MS demonstrated five, approximately 37 kDa isoforms (pI 5.3-6.1) separated by 2D-PAGE. A 975 bp cDNA sequence obtained by RT-PCR from liver and spleen tissue encoded a 325 amino acid secretory protein with homology to human and murine intelectins, which bind bacterial components and are induced during parasitic infections. Gene expression and immunohistochemistry detected RTInt in gill, spleen, hepatic sinusoid, renal interstitium, intestine, skin, swim bladder and within leukocytes. Direct binding assays demonstrated the ability of RTInt to bind relevant bacterial and chitinous targets. These findings suggest that RTInt plays a role in innate immune defense against bacterial and chitinous microbial organisms.

Comparative study of the human ficolins reveals unique features of Ficolin-3 (Hakata antigen)

The ficolins and mannose-binding lectin (MBL) are collagen-like defence proteins that serve as recognition molecules in lectin complement pathway. Differential features that may indicate diverse functions of these proteins are poorly understood. In this study we compared important biological features of the ficolins and MBL. We investigated the tissue distribution of the FCN1-3 and the MBL2 genes encoding the ficolins and MBL by quantitative PCR. Recombinant proteins were produced and structural and biological characteristics were investigated and compared. Our main findings were that FCN3 mRNA was highly expressed in the liver and lung compared with the other genes revealing the lung as the tissue with the highest FCN3 expression pattern. Ficolin-3 revealed higher complement activating capacity compared with Ficolin-2, MBL and Ficolin-1 and was highly resistant to bacterial collagenase treatment, which is different from the other ficolins and MBL. We discovered several unique properties of Ficolin-3 showing that FCN3 is the most highly expressed gene in liver and lung among the lectin complement pathway initiators. Moreover, Ficolin-3 has a high complement activating potential and is the only collagenase proteolytic resistant molecule among the lectin complement pathway initiators.

Porcine plasma ficolin binds and reduces infectivity of porcine reproductive and respiratory syndrome virus (PRRSV) in vitro

Ficolins are collagenous lectins that bind N-acetylated glycans and participate in innate immune responses, including phagocytosis and complement activation. Related collagenous lectins such as mannan binding lectin (MBL) and surfactant proteins A and D possess antiviral activity, but this activity has not been demonstrated for ficolins. In these studies, we used purified porcine plasma ficolin alpha and recombinant ficolin alpha to assess their ability to bind and neutralize porcine reproductive and respiratory virus (PRRSV) in various assays. Recombinant ficolin alpha was designed with a C-terminal 6-histidine tag using a pcDNA3.1 expression vector system in CHO K1 cells. Plasma-purified and recombinant ficolin alpha reduced cytopathic effect of PRRSV-infected Marc-145 cells in neutralization assays and inhibited replication of infectious viral particles in a GlcNAc-dependent manner. In vitro replication determined by plaque assay was inhibited in the presence of plasma-purified ficolin alpha and recombinant ficolin. Immunoreactive plasma ficolin alpha and recombinant ficolin alpha also bound PRRSV-coated wells in a GlcNAc-dependent manner. These studies indicate that porcine ficolin can bind and neutralize a common arterivirus that is a major pathogen of swine.

Stat3 is involved in control of MASP2 gene expression

Little is known about determinants regulating expression of Mannan-binding lectin associated serine protease-2 (MASP-2), the effector component of the lectin pathway of complement activation. Comparative bioinformatic analysis of the MASP2 promoter regions in human, mouse, and rat, revealed conservation of two putative Stat binding sites, termed StatA and StatB. Site directed mutagenesis specific for these sites was performed. Transcription activity was decreased 5-fold when StatB site was mutated in the wildtype reporter gene construct. Gel retardation and competition assays demonstrated that proteins contained in the nuclear extract prepared from HepG2 specifically bound double-stranded StatB oligonucleotides. Supershift analysis revealed Stat3 to be the major specific binding protein. We conclude that Stat3 binding is important for MASP2 promoter activity.

Ficolin-2 recognizes DNA and participates in the clearance of dying host cells

Ficolin-2 is a serum opsonin, which has been shown to be a pattern recognition molecule in the lectin complement activation pathway. Because innate immune mechanisms are involved in maintaining tissue homeostasis we hypothesized that Ficolin-2 also participate in the clearance of dying host cells. We found that Ficolin-2 binds to late apoptotic cells, as well as to apoptotic bodies and necrotic cells, but not to early apoptotic cells. We demonstrated that Ficolin-2 binds DNA in a calcium dependent manner and that DNA inhibits the binding to late apoptotic and necrotic cells, suggesting that DNA on permeable dying cells is a plausible ligand. Reconstituting serum deficient of Ficolin-2, C1q and mannose-binding lectin with Ficolin-2 augmented deposition of complement C4 on necrotic cells. Opsonization leads to an enhanced attachment/uptake of necrotic cells by macrophages. In conclusion dying host cells expose ligands with the capacity of binding Ficolin-2, which in turn leads to increased attachment and engulfment. Binding of Ficolin-2 to DNA points at nucleic acid exposed by permeable late apoptotic and necrotic cells as one of the ligands for Ficolin-2. Ficolin-2 may therefore be a scavenger molecule participating in the removal of host cells and maintenance of tissue homeostasis.

Coding and non-coding polymorphisms in the lectin pathway activator L-ficolin gene in 188 Dutch blood bank donors

Human L-ficolin (FCN) is a serum lectin characterized by a collagen-like and a fibrinogen-like domain that can activate the lectin pathway of complement. Structural and functional similarities to mannose-binding lectin (MBL) suggest a role for L-ficolin in innate immunity. Structural polymorphisms in the MBL2 gene lead to functional deficiency of MBL. Polymorphisms in the FCN2 gene have not been studied previously. We developed 10 denaturing gradient gel electrophoresis (DGGE) assays to screen a total of 188 Dutch Caucasians for polymorphisms in FCN2. Total gene screening in this large cohort revealed 10 single nucleotide polymorphisms (SNPs). Interestingly, two conserved coding SNPs were found in exon 8, leading to amino acid substitutions within the fibrinogen-like domain. Fibrinogen-like domains are highly conserved among several proteins in many species. As this domain is responsible for binding of L-ficolin, these newly found coding polymorphisms could alter the affinity of the protein for its substrates and possibly alter the ability of L-ficolin to recognize invading microorganisms.

Comparative genetics and innate immune functions of collagenous lectins in animals

Collagenous lectins such as mannan-binding lectins (MBLs), ficolins (FCNs), surfactant proteins A and D (SP-A, SP-D), conglutinin (CG), and related ruminant lectins are multimeric proteins with carbohydrate-binding domains aligned in a manner that facilitates binding to microbial surface polysaccharides. MBLs and FCNs are structurally related to C1q, but activate the lectin complement pathway via interaction with MBL-associated serine proteases (MASPs). MBLs, FCNs, and other collagenous lectins also bind to some host macromolecules and contribute to their removal. While there is evidence that some lectins and the lectin complement pathway are conserved in vertebrates, many differences in collagenous lectins have been observed among humans, rodents, and other vertebrates. For example, humans have only one MBL but three FCNs, whereas most other species express two FCNs and two MBLs. Bovidae express CG and other SP-D-related collectins that are not found in monogastric species. Some dysfunctions of human MBL are due to single nucleotide polymorphisms (SNPs) that affect its expression or structure and thereby increase susceptibility to some infections. Collagenous lectins have well-established roles in innate immunity to various microorganisms, so it is possible that some lectin genotypes or induced phenotypes influence resistance to some infectious or inflammatory diseases in animals.

Ficolin A and ficolin B are expressed in distinct ontogenic patterns and cell types in the mouse

Ficolins are a group of proteins characterized by the presence of collagen-like and fibrinogen-like domains. Two of three human ficolins, L-ficolin and H-ficolin, are serum lectins that form complexes with mannose-binding lectin-associated serine proteases (MASPs) and play important roles in the lectin complement pathway. The other human ficolin, M-ficolin, is a non-serum-type ficolin that is expressed in monocytes. Little is known about the physiological roles of ficolins. In this study, we delineated the ontogeny and cell types that express ficolins in mice. RT-PCR analysis showed that the expression pattern of ficolin A expression was closely similar to that of Masps, suggesting that these molecules may function in coordination as components of the lectin complement pathway. The cell types that express ficolin A mRNA in both adult liver and spleen were identified as macrophages by in situ hybridization. Ficolin B exhibited a distinct ontogeny pattern that switched from embryonic liver to postnatal bone marrow and spleen. The cells that express ficolin B mRNA were identified as belonging to the myeloid cell lineage by magnetic sorting and by subsequent RT-PCR in bone marrow cells. Thus, the different spatial-temporal expression patterns of ficolins A and B suggest that these molecules play distinct roles in the prenatal and postnatal stages.

Polymorphisms in the FCN2 gene determine serum variation and function of Ficolin-2

The ficolin 1, 2 and 3 (derived from the FCN1, 2 and 3 genes, respectively) are homologous soluble pattern recognition molecules of importance for innate immunity, comprising collagen-like and fibrinogen-like domains, binding to sugar groups on different types of microorganisms. Serum concentration of Ficolin-2 varies considerably in healthy individuals. Thus, we speculated whether this could be due to variations in the FCN2 gene. We sequenced the promoter region and the exons and intron-exon boundaries of FCN2 in Danish Caucasians. For comparison, FCN1 and FCN3 were also investigated. Ficolin-2 concentrations were measured in serum and the functional relevance of amino acid substituting polymorphisms in FCN2 was investigated by binding to and recovery from N-acetylglucosamine (GlcNAc). Both FCN1 and FCN2 contained polymorphisms in the promoters and structural parts of the genes, but only polymorphisms in FCN2 resulted in amino acid exchanges. FCN2 promoter polymorphisms were associated with marked changes in the Ficolin-2 serum concentration, whereas two polymorphisms clustered in the exon encoding the fibrinogen-like domain were associated with increased and decreased GlcNAc binding, respectively. In FCN3, only a single frame-shift deletion in exon 5 was detected. These results show that the FCN genes are polymorphic and that particularly FCN2 harbors functional polymorphic sites that regulate both the expression as well as the function of Ficolin-2, which may have pathophysiological implications for innate immunity.

Effect of capsulation of opportunistic pathogenic bacteria on binding of the pattern recognition molecules mannan-binding lectin, L-ficolin, and H-ficolin

Mannan-binding lectin (MBL), L-ficolin, and H-ficolin are pattern recognition molecules of the innate immune system. We investigated their ability to bind to different serotypes and noncapsulated variants of two gram-positive bacterial species, Streptococcus pneumoniae and Staphylococcus aureus. MBL did not bind to capsulated S. aureus or capsulated S. pneumoniae but did bind to a noncapsulated S. aureus variant (Wood). L-ficolin bound to some capsulated S. aureus serotypes (serotypes 1, 8, 9, 11, and 12) and capsulated S. pneumoniae serotypes (11A, 11D, and 11F) but not to noncapsulated strains. H-ficolin did not bind to any of the S. pneumoniae and S. aureus serotypes included in this study but did bind to one strain of Aerococcus viridans. The concentrations of the three proteins in 97 plasma samples were estimated. The median concentrations were 0.8 mug per ml for MBL, 3.3 mug per ml for L-ficolin, and 18.4 mug per ml for H-ficolin.

L-ficolin is a pattern recognition molecule specific for acetyl groups

L-ficolin and H-ficolin are molecules of the innate immune system. Upon recognition of a suitable target they activate the complement system. The ligand recognition structure of ficolins is contained within a fibrinogen-like domain. We examined the selectivity of the ficolins through inhibiting the binding to bacteria or to beads coupled with N-acetylglucosamine. The binding of L-ficolin to Streptococcus pneumoniae 11F and the beads was inhibited by N-acetylated sugars and not by non-acetylated sugars. However, it was also inhibited by other acetylated compounds. Based on this selectivity L-ficolin is not easily defined as a lectin. The binding of H-ficolin to Aerococcus viridans was not inhibited by any of the sugars or other compounds examined. Based on the selectivity of L-ficolin we developed a new purification procedure involving affinity chromatography on N-acetylcysteine-derivatized Sepharose. The column was loaded in the presence of EDTA and high salt, and L-ficolin was eluted by decreasing the salt concentration. Further purification was achieved by ion exchange chromatography.

Expression and secretion of ficolin beta by porcine neutrophils

Ficolins are collagenous lectins that bind N-acetylglucosamine (GlcNAc) as well as some bacterial surfaces, and may have opsonic and complement-activating functions. Ficolin alpha in porcine plasma binds Actinobacillus pleuropneumoniae serotype 5 (APP) in a GlcNAc-dependent manner. In the present study, we discovered that porcine neutrophils, but not platelets or mononuclear cells, contained a different ficolin that migrated as a 39-kDa band on SDS-PAGE and resembled a minor component of plasma ficolins that binds APP. However, neutrophil ficolins (pI range 6.4-7.4) were readily distinguished from plasma ficolin alpha (pI 5.2-5.8) by 2D PAGE. Neutrophil ficolin was consistent with ficolin beta by pI and peptide mass fingerprinting with matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Porcine neutrophils expressed ficolin beta, but not ficolin alpha, as determined by RT-PCR. Ficolin beta was present in the membrane and cytoplasmic fractions of nonactivated neutrophils, but the majority of ficolin beta was secreted upon activation with PMA. Ficolin alpha readily bound to intact APP, but ficolin beta did not under the same conditions. These studies demonstrate that neutrophils express ficolin beta and secrete it when activated; however, ficolin beta may have different binding functions than ficolin alpha.

Characterization of the interaction between L-ficolin/p35 and mannan-binding lectin-associated serine proteases-1 and -2

Ficolins are oligomeric lectins comprising a collagen-like and a fibrinogen-like domain, with a binding specificity for N-acetylglucosamine. It has been reported recently that L-ficolin/P35 associates with mannan-binding lectin (MBL)-associated serine proteases (MASP-1 and -2) and MBL-associated protein 19 (MAp19) in serum and forms complexes able to activate complement. Using surface plasmon resonance spectroscopy we have shown that recombinant MASP-1 and -2, their N-terminal CUB1 (module originally found in complement proteins C1r/C1s, Uegf, and bone morphogenetic protein-1)-epidermal growth factor (EGF)-CUB2 and CUB1-EGF segments, and MAp19 bind to immobilized L-ficolin/P35 in the presence of Ca(2+) ions. Comparable K(d) values were obtained for the full-length proteases and their CUB1-EGF-CUB2 segments (9.2 and 10 nM for MASP-1 and 4.6 and 5.4 nM for MASP-2, respectively), whereas higher values were obtained for the CUB1-EGF segments (26.7, 15.6, and 14.3 nM for MASP-1, MASP-2, and MAp19). These values are in the same range as those determined for the interaction of these proteins with MBL. Binding was Ca(2+) dependent and was only partly sensitive to EDTA for MASP-1, MASP-2, and MASP-2 CUB1-EGF-CUB2. Half-maximal binding was obtained at comparable Ca(2+) concentrations for MASP-1 and MASP-2 (0.45 and 0.47 micro M, respectively), their CUB1-EGF-CUB2 segments (0.37 and 0.72 micro M), and their CUB1-EGF segments (0.31 and 0.79 micro M). These values are lower than those determined in the case of MBL, indicating a difference between MBL and L-ficolin/P35 with respect to the Ca(2+) dependence of their interaction with the MASPs. Preincubation of the MASPs with soluble MBL inhibited subsequent binding to immobilized L-ficolin/P35 and, conversely, suggesting that these lectins compete with each other for binding to the MASPs in vivo.