Activation of the classical complement pathway by mannose-binding protein in association with a novel C1s-like serine protease

Immune Recognition of Fungal Polysaccharides

The incidence of fungal infections has dramatically increased in recent years, in large part due to increased use of immunosuppressive medications, as well as aggressive medical and surgical interventions that compromise natural skin and mucosal barriers. There are relatively few currently licensed antifungal drugs, and rising resistance to these agents has led to interest in the development of novel preventative and therapeutic strategies targeting these devastating infections. One approach to combat fungal infections is to augment the host immune response towards these organisms. The polysaccharide-rich cell wall is the initial point of contact between fungi and the host immune system, and therefore, represents an important target for immunotherapeutic approaches. This review highlights the advances made in our understanding of the mechanisms by which the immune system recognizes and interacts with exopolysaccharides produced by four of the most common fungal pathogens: Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, and Histoplasma capsulatum. Work to date suggests that inner cell wall polysaccharides that play an important structural role are the most conserved across diverse members of the fungal kingdom, and elicit the strongest innate immune responses. The immune system senses these carbohydrates through receptors, such as lectins and complement proteins. In contrast, a greater diversity of polysaccharides is found within the outer cell walls of pathogenic fungi. These glycans play an important role in immune evasion, and can even induce anti-inflammatory host responses. Further study of the complex interactions between the host immune system and the fungal polysaccharides will be necessary to develop more effective therapeutic strategies, as well as to explore the use of immunosuppressive polysaccharides as therapeutic agents to modulate inflammation.

The emerging roles of mannose-binding lectin-associated serine proteases (MASPs) in the lectin pathway of complement and beyond

Mannose-binding lectin (MBL)-associated serine proteases (MASPs) are the enzymatic constituents of the lectin pathway of the complement system. They are complexed with large pattern recognition molecules (PRMs) such as MBL, other collectins, and ficolins. The main function of two of the three MASPs has crystallized lately: MASP-1 autoactivates first, then it activates MASP-2, and finally both participate in the formation of the C4b2a convertase. In addition to this, both enzymes are involved in several other processes which are subject to intense research nowadays. Notably, MASP-1, as a promiscuous enzyme, has been implicated in the coagulation cascade, in the kinin generating contact system, and in cellular activation through protease-activated receptor (PAR) cleavage on endothelial cells. The third protease MASP-3 has emerged recently as the protease responsible for pro-factor D activation in resting blood, providing a fundamental link between two complement pathways. At present all three MASPs have at least one well-defined role and several other possible functions were implicated. Defect or more likely over-activation of MASPs may culminate into diseases such as ischemia-reperfusion injury (IRI); hence, MASPs are all potential targets of drug development.

Serine Proteases in the Lectin Pathway of the Complement System

The complement system plays a crucial role in host defense against pathogen infections and in the recognition and removal of damaged or altered self-components. Complement system activation can be initiated by three different pathways—classical, alternative, and lectin pathways—resulting in a proteolytic cascade, which culminates in multiple biological processes including opsonization and phagocytosis of intruders, inflammation, cell lysis, and removal of immune complexes and apoptotic cells. Furthermore, it also functions as a link between the innate and adaptive immune responses. The lectin pathway (LP) activation is mediated by serine proteases, termed mannan-binding lectin (MBL)-associated serine proteases (MASPs), which are associated with the pattern recognition molecules (PRMs) that recognize carbohydrates or acetylated compounds on surfaces of pathogens or apoptotic cells. These result in the proteolysis of complement C2 and C4 generating C3 convertase (C4b2a), which carries forward the activation cascade of complements, culminating in the elimination of foreign molecules. This chapter presents an overview of the complement system focusing on the characterization of MASPs and its genes, as well as its functions in the immune response.

Identification and characterization of C1 inhibitor in Nile tilapia (Oreochromis niloticus) in response to pathogenic bacteria

C1 inhibitor (C1INH) is a multi-functional serine protease inhibitor in plasmatic cascades, not only inactivating various proteases, but also regulating both complement and contact system activation. In this study, we described the identification and characterization of a C1INH ortholog from Nile tilapia (Oreochromis niloticus) at molecular, protein and cellular levels. The full-length cDNA of Oreochromis niloticus C1INH (OnC1INH) consisted of 1791 bp of nucleotide sequence encoding polypeptides of 596 amino acids. The deduced protein possessed a serpin domain at the C-terminal domain, and two Ig-like domains in the N-terminal domain with significant homology to teleost. Expression analysis revealed that the OnC1INH was extremely highly expressed in the liver; however, much weakly exhibited in other tissues including spleen, kidney, blood and heart. After the in vivo challenges of the lipopolysaccharide (LPS) and Streptococcus agalactiae, the expression of OnC1INH was significantly up-regulated in liver and spleen at the late phase, which was confirmed at the protein level with immunohistochemical analysis. The up-regulation of OnC1INH expression was also demonstrated in head kidney monocytes/macrophages in vitro stimulated with LPS, Aeromonas hydrophila and Streptococcus agalactiae, which was positively correlated with the protein expression pattern in the culture media. Taken together, the results of this study indicated that OnC1INH might be involved in the immune response of Nile tilapia against to bacterial challenge.

Mannose-Binding Lectin: Biologic Characteristics and Role in the Susceptibility to Infections and Ischemia-Reperfusion Related Injury in Critically Ill Neonates

The mannose-binding lectin (MBL) is a member of the collectin family, belonging to the innate immunity system. Genetic, biologic, and clinical properties of MBL have been widely investigated throughout the last decades, although some interesting aspects of its potential clinical relevance are still poorly understood. Low circulating concentrations of MBL have been associated with increased risk of infection and poor neurologic outcome in neonates. On the other hand, an excessive and uncontrolled inflammatory response by the neonatal intestine after the exposure to luminal bacteria, leading to an increased production of MBL, may be involved in the onset of necrotizing enterocolitis. The purpose of the present review is to summarize the current knowledge about genetic and biologic characteristics of MBL and its role in the susceptibility to infections and to ischemia-reperfusion related tissue injuries to better explore its clinical relevance during the perinatal period and the possible future therapeutic applications.

Collectin Kidney 1 Plays an Important Role in Innate Immunity against Streptococcus pneumoniae Infection

Collectins are C-type lectins that are involved in innate immunity as pattern recognition molecules. Recently, collectin kidney 1 (CL-K1) has been discovered, and in vitro studies have shown that CL-K1 binds to microbes and activates the lectin complement pathway. However, in vivo functions of CL-K1 against microbes have not been elucidated. To investigate the biological functions of CL-K1, we generated CL-K1 knockout (CL-K1-/-) mice and then performed a Streptococcus pneumoniae infection analysis. First, we found that recombinant human CL-K1 bound to S. pneumoniae in a calcium-dependent manner, and induced complement activation. CL-K1-/- mice sera formed less C3 deposition on S. pneumoniae. Furthermore, immunofluorescence analysis in the wild-type (WT) mice demonstrated that CL-K1 and C3 were localized on S. pneumoniae in infected lungs. CL-K1-/- mice revealed decreased phagocytosis of S. pneumoniae. Consequently, less S. pneumoniae clearance was observed in their lungs. CL-K1-/- mice showed severe pulmonary inflammation and weight loss in comparison with WT mice. Finally, the decreased clearance and severe pulmonary inflammation caused by S. pneumoniae infection might cause higher CL-K1-/- mice lethality. Our results suggest that CL-K1 might play an important role in host protection against S. pneumoniae infection through the activation of the lectin complement pathway.

Pharmacogenetic association of MBL2 and CD95 polymorphisms with grade 3 infection following adjuvant therapy for breast cancer with doxorubicin and cyclophosphamide

Life-threatening infection as an adverse reaction to cytotoxic therapy of cancer remains a major problem, potentially limiting efficacy. Administration of colony-stimulation factors benefits only a minority of patients, and improved stratification guidelines are needed to identify those patients likely to benefit. We investigated single nucleotide polymorphisms (SNPs) in two genes related to immune function to identify associations with severe infection following treatment of breast cancer with doxorubicin and cyclophosphamide. CD95 mediates the extrinsic apoptosis pathway in haematopoietic cells and a CD95 promoter SNP (rs2234767) has been shown to result in reduced expression of the receptor. MBL2 activates the classical complement pathway in the presence of pathogens and independently of antibodies. Numerous SNPs have been described including a promoter SNP (rs7096206) which results in decreased expression of the protein. Homozygotes for the CD95 minor allele were more likely to experience a grade 3 infection than heterozygote and homozygote wild-type patients (29%, 3% and 5%, respectively p=0.048). CD95 minor allele homozygotes also had higher basal white blood cell and neutrophil counts compared with wild-type allele carriers, which was sustained throughout therapy. There was an allele-dose association between the MBL2 SNP and grade 3 infection, with 2, 8 and 17% of wild-type homozygotes, heterozygotes and minor allele homozygotes, respectively, experiencing grade 3 infection (p=0.02). These associations demonstrate the utility of a pharmacogenetic approach to identify individuals more likely to acquire a life-threatening infection during chemotherapy. The apparent association with a CD95 SNP and a mild neutrophilia merits further investigation.

A journey through the lectin pathway of complement-MBL and beyond

Mannose-binding lectin (MBL), collectin-10, collectin-11, and the ficolins (ficolin-1, ficolin-2, and ficolin-3) are soluble pattern recognition molecules in the lectin complement pathway. These proteins act as mediators of host defense and participate in maintenance of tissue homeostasis. They bind to conserved pathogen-specific structures and altered self-antigens and form complexes with the pentraxins to modulate innate immune functions. All molecules exhibit distinct expression in different tissue compartments, but all are found to a varying degree in the circulation. A common feature of these molecules is their ability to interact with a set of serine proteases named MASPs (MASP-1, MASP-2, and MASP-3). MASP-1 and -2 trigger the activation of the lectin pathway and MASP-3 may be involved in the activation of the alternative pathway of complement. Furthermore, MASPs mediate processes related to coagulation, bradykinin release, and endothelial and platelet activation. Variant alleles affecting expression and structure of the proteins have been associated with a variety of infectious and non-infectious diseases, most commonly as disease modifiers. Notably, the severe 3MC (Malpuech, Michels, Mingarelli, and Carnevale) embryonic development syndrome originates from rare mutations affecting either collectin-11 or MASP-3, indicating a broader functionality of the complement system than previously anticipated. This review summarizes the characteristics of the molecules in the lectin pathway.

Production and purification of recombinants of mouse MASP-2 and sMAP

Small mannose-binding lectin (MBL)-associated protein (sMAP) is a component of the complex consisting of MBL and MBL-associated serine proteases (MASPs) in the lectin complement pathway. sMAP is a truncated form of MASP-2, which is generated by an alternative splicing from a single structural MASP-2 gene. Upon activation of the MBL—MASPs complex, MASP-2 cleaves the complement C4, but the role of sMAP which lacks the serine protease domain is not clear. To clarify the role of sMAP in activation of the lectin pathway, we have generated sMAP-gene deficient mice which are also deficient for MASP-2. In this study, we generated and purified mouse recombinant sMAP (rsMAP) and rMASP-2 using the Drosophila expression system for the reconstitution assay of the deficient mice. In preliminary experiments, these purified recombinants were able to reconstitute the MBL—MASPs—sMAP complexes and the addition of rMASP-2 to deficient serum restored the C4 cleavage activity of the MBL—MASPs complex. From these data, rsMAP and rMASP-2 generated in this study seem to be useful in analysis of the deficient mice.

Complement inhibitors to treat IgM-mediated autoimmune hemolysis

Complement activation in autoimmune hemolytic anemia may exacerbate extravascular hemolysis and may occasionally result in intravascular hemolysis. IgM autoantibodies as characteristically found in cold autoantibody autoimmune hemolytic anemia, in cold agglutinin disease but also in a considerable percentage of patients with warm autoantibodies are very likely to activate complement in vivo. Therapy of IgM-mediated autoimmune hemolytic anemia mainly aims to decrease autoantibody production. However, most of these treatments require time to become effective and will not stop immediate ongoing complement-mediated hemolysis nor prevent hemolysis of transfused red blood cells. Therefore pharmacological inhibition of the complement system might be a suitable approach to halt or at least attenuate ongoing hemolysis and improve the recovery of red blood cell transfusion in autoimmune hemolytic anemia. In recent years, several complement inhibitors have become available in the clinic, some of them with proven efficacy in autoimmune hemolytic anemia. In the present review, we give a short introduction on the pathogenesis of autoimmune hemolytic anemia, followed by an overview on the complement system with a special focus on its regulation. Finally, we will discuss complement inhibitors with regard to their potential efficacy to halt or attenuate hemolysis in complement-mediated autoimmune hemolytic anemia.

Polymorphisms in the Mannose-Binding Lectin Gene are Associated with Defective Mannose-Binding Lectin Functional Activity in Crohn's Disease Patients

Mannose-binding lectin, together with mannose-associated serine proteases, activates the lectin pathway of the complement system and subsequent inflammatory mechanisms. An association between mannose-binding lectin deficiency and anti-Saccharomyces cerevisiae antibody levels is observed in Crohn’s disease and this deficiency is frequently associated with a severe Crohn’s disease phenotype. In the present study, we assessed the relationship between serum concentrations of mannose-binding lectin, mannose-binding lectin functional activity, MBL2 and NOD2 polymorphisms, anti-S. cerevisiae antibody levels and clinical Crohn’s disease phenotype in 69 Crohn’s disease patients and 30 age- and sex-matched healthy controls. The results show that the MBL2 variant rs5030737 at codon 52 was associated with a low level of mannose-binding lectin and impaired mannose-binding lectin–mannose-associated serine protease (MBL-MASP) functional activity in Crohn’s disease patients. This MBL2 variant was also associated with a higher level of anti-S. cerevisiae antibodies. In addition, the NOD2 variant rs2066844, which is associated with susceptibility to Crohn’s disease, was significantly correlated with an impairment in MBL-MASP functional activity. These results provide evidence that Crohn’s disease patients have an impairment in MBL-MASP functional activity and that this defect is associated with MBL2 and NOD2 variants.

Impact of Mannose-Binding Protein Gene Polymorphisms in Omani Sickle Cell Disease Patients

Objectives

Our aim was to study mannose-binding protein (MBP) polymorphisms in exonic and promoter region and correlate it with associated infections and vasoocculsive (VOC) episodes in sickle cell disease (SCD) patients since MBP plays an important role in innate immunity by activating the complement system.

Methods

We studied the genetic polymorphisms in the Exon 1 (alleles A/O) and promoter region (alleles Y/X; H/L, P/Q) of the MBL2 gene, in SCD patients as an increased incidence of infections is seen in these patients. A PCR-based, targeted genomic DNA sequencing of MBL2 was used to study 68 SCD Omani patients and 44 controls (healthy voluntary blood donors).

Results

In SCD patients, the frequency of the genotype related to the high production of MBL was 0.35 (YA/YA) and for intermediate/low production was 0.65 (YA/XA, XA/XA, YA/YO, XA/YO, YO/YO). The observed frequencies of MBL2 gene promoter polymorphism (-221, Y/X) were 44.4% and 20.5% for the heterozygous genotype Y/X and 3.2% and 2.2% for the homozygous (X/X) respectively between SCD patients and controls. MBL2 Exon1 gene mutations were 29.4% and 50% for the heterozygous genotype A/O and 5.9% and 6.8% respectively for the homozygous (O/O) genotype between SCD patients and controls. The distribution of variant MBL2 gene polymorphisms did not show any correlation in SCD patients with or without VOC attacks (p=0.16; OR −0.486; CI=0.177 −1.33), however, it was correlated with infections (p=0.0162; OR −3.55; CI 1.25–10.04).

Conclusions

Although the frequency of the genotypes and haplotypes of MBL2 in SCD patients did not differ from controls, overall in the SCD patient cohort the increased representation of variant alleles was significantly correlated with infections (p<0.05). However, these variant MBL2 polymorphisms did not seem to play a significant role in the VOC episodes in this SCD cohort.

Systemic Lupus Erythematosus and Deficiencies of Early Components of the Complement Classical Pathway

The complement system plays an important role in the innate and acquired immune response against pathogens. It consists of more than 30 proteins found in soluble form or attached to cell membranes. Most complement proteins circulate in inactive forms and can be sequentially activated by the classical, alternative, or lectin pathways. Biological functions, such as opsonization, removal of apoptotic cells, adjuvant function, activation of B lymphocytes, degranulation of mast cells and basophils, and solubilization and clearance of immune complex and cell lysis, are dependent on complement activation. Although the activation of the complement system is important to avoid infections, it also can contribute to the inflammatory response triggered by immune complex deposition in tissues in autoimmune diseases. Paradoxically, the deficiency of early complement proteins from the classical pathway (CP) is strongly associated with development of systemic lupus erythematous (SLE) - mainly C1q deficiency (93%) and C4 deficiency (75%). The aim of this review is to focus on the deficiencies of early components of the CP (C1q, C1r, C1s, C4, and C2) proteins in SLE patients.

Activated Complement Factors as Disease Markers for Sepsis

Sepsis is a leading cause of death in the United States and worldwide. Early recognition and effective management are essential for improved outcome. However, early recognition is impeded by lack of clinically utilized biomarkers. Complement factors play important roles in the mechanisms leading to sepsis and can potentially serve as early markers of sepsis and of sepsis severity and outcome. This review provides a synopsis of recent animal and clinical studies of the role of complement factors in sepsis development, together with their potential as disease markers. In addition, new results from our laboratory are presented regarding the involvement of the complement factor, mannose-binding lectin, in septic shock patients. Future clinical studies are needed to obtain the complete profiles of complement factors/their activated products during the course of sepsis development. We anticipate that the results of these studies will lead to a multipanel set of sepsis biomarkers which, along with currently used laboratory tests, will facilitate earlier diagnosis, timely treatment, and improved outcome.

Early decreased plasma levels of factor B and C5a are important biomarkers in children with Kawasaki disease

BACKGROUND

The mechanisms underpinning Kawasaki disease (KD) are incompletely understood. There is an unmet need for specific biomarkers for the early diagnosis of KD.

METHODS

Eighty-five KD patients suffering from acute-phase and subacute-phase KD, 40 healthy children, and 40 febrile children comprised the study cohort. An enzyme-linked immunosorbent assay was used to measure plasma levels of C1q, C1q-circulating immune complex (C1q-CIC), mannan-binding lectin-associated serine protease (MASP)-1, factor B, C4d, C3d, C5a, C5b-9 and CD59.

RESULTS

Plasma concentrations of factor B and C5a in the acute phase were lower than those in healthy and febrile control groups (all P < 0.05). Compared with acute-phase KD patients, plasma concentrations of C1q, factor B, and C3d in KD patients were increased significantly (P < 0.05), but those of C4d, MASP-1 and CD59 decreased significantly (P < 0.05), in patients with sub-acute KD.

CONCLUSION

These data suggest that more than one pathway in the complement system is activated in KD. Importantly, decreased plasma concentrations of factor B and C5a in the acute phase (6-10 d) could be employed as biomarkers for the early diagnosis of KD.

Gene expression associated with white syndromes in a reef building coral, Acropora hyacinthus

BACKGROUND

Corals are capable of launching diverse immune defenses at the site of direct contact with pathogens, but the molecular mechanisms of this activity and the colony-wide effects of such stressors remain poorly understood. Here we compared gene expression profiles in eight healthy Acropora hyacinthus colonies against eight colonies exhibiting tissue loss commonly associated with white syndromes, all collected from a natural reef environment near Palau. Two types of tissues were sampled from diseased corals: visibly affected and apparently healthy.

RESULTS

Tag-based RNA-Seq followed by weighted gene co-expression network analysis identified groups of co-regulated differentially expressed genes between all health states (disease lesion, apparently healthy tissues of diseased colonies, and fully healthy). Differences between healthy and diseased tissues indicate activation of several innate immunity and tissue repair pathways accompanied by reduced calcification and the switch towards metabolic reliance on stored lipids. Unaffected parts of diseased colonies, although displaying a trend towards these changes, were not significantly different from fully healthy samples. Still, network analysis identified a group of genes, suggestive of altered immunity state, that were specifically up-regulated in unaffected parts of diseased colonies.

CONCLUSIONS

Similarity of fully healthy samples to apparently healthy parts of diseased colonies indicates that systemic effects of white syndromes on A. hyacinthus are weak, which implies that the coral colony is largely able to sustain its physiological performance despite disease. The genes specifically up-regulated in unaffected parts of diseased colonies, instead of being the consequence of disease, might be related to the originally higher susceptibility of these colonies to naturally occurring white syndromes.

MBL-associated serine proteases (MASPs) and infectious diseases

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MASP-1 and MASP-2 are central players of the lectin pathway of complement.

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MASP1 and MASP2 gene polymorphisms regulate protein serum levels and activity.

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MASP deficiencies are associated with increased infection susceptibility.

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MASP polymorphisms and serum levels are associated with disease progression.

The lectin pathway of the complement system has a pivotal role in the defense against infectious organisms. After binding of mannan-binding lectin (MBL), ficolins or collectin 11 to carbohydrates or acetylated residues on pathogen surfaces, dimers of MBL-associated serine proteases 1 and 2 (MASP-1 and MASP-2) activate a proteolytic cascade, which culminates in the formation of the membrane attack complex and pathogen lysis. Alternative splicing of the pre-mRNA encoding MASP-1 results in two other products, MASP-3 and MAp44, which regulate activation of the cascade. A similar mechanism allows the gene encoding MASP-2 to produce the truncated MAp19 protein. Polymorphisms in MASP1 and MASP2 genes are associated with protein serum levels and functional activity. Since the first report of a MASP deficiency in 2003, deficiencies in lectin pathway proteins have been associated with recurrent infections and several polymorphisms were associated with the susceptibility or protection to infectious diseases. In this review, we summarize the findings on the role of MASP polymorphisms and serum levels in bacterial, viral and protozoan infectious diseases.

Role of mannose-binding lectin deficiency in HIV-1 and schistosoma infections in a rural adult population in Zimbabwe

BACKGROUND

Polymorphism in the MBL2 gene lead to MBL deficiency, which has been shown to increase susceptibility to various bacterial, viral and parasitic infections. We assessed role of MBL deficiency in HIV-1 and schistosoma infections in Zimbabwean adults enrolled in the Mupfure Schistosomiasis and HIV Cohort (MUSH Cohort).

METHODS

HIV-1, S. haematobium and S. mansoni infections were determined at baseline. Plasma MBL concentration was measured by ELISA and MBL2 genotypes determined by PCR. We calculated and compared the proportions of plasma MBL deficiency, MBL2 structural variant alleles B (codon 54A>G), C (codon 57A>G), and D (codon 52T>C) as well as MBL2 promoter variants -550(H/L), -221(X/Y) and +4(P/Q) between HIV-1 and schistosoma co-infection and control groups using Chi Square test.

RESULTS

We assessed 379 adults, 80% females, median age (IQR) 30 (17-41) years. HIV-1, S. haematobium and S. mansoni prevalence were 26%, 43% and 18% respectively in the MUSH baseline survey. Median (IQR) plasma MBL concentration was 800μg/L (192-1936μg/L). Prevalence of plasma MBL deficiency was 18% with high frequency of the C (codon 57G>A) mutant allele (20%). There was no significant difference in median plasma MBL levels between HIV negative (912μg/L) and HIV positive (688μg/L), p = 0.066. However plasma MBL levels at the assay detection limit of 20μg/L were more frequent among the HIV-1 infected (p = 0.007). S. haematobium and S. mansoni infected participants had significantly higher MBL levels than uninfected. All MBL2 variants were not associated with HIV-1 infection but promoter variants LY and LL were significantly associated with S. haematobium infection.

CONCLUSION

Our data indicate high prevalence of MBL deficiency, no evidence of association between MBL deficiency and HIV-1 infection. However, lower plasma MBL levels were protective against both S. haematobium and S. mansoni infections and MBL2 promoter and variants LY and LL increased susceptibility to S. haematobium infection.

New insights into the role of ficolins in the lectin pathway of innate immunity

In the innate immune system, a variety of recognition molecules provide the first-line host defense to prevent infection and maintain endogenous homeostasis. Ficolin is a soluble recognition molecule, which senses pathogen-associated molecular patterns on microbes and aberrant sugar structures on self-cells. It consists of a collagen-like stalk and a globular fibrinogen-like domain, the latter binding to carbohydrates such as N-acetylglucosamine. Ficolins have been widely identified in animals from higher invertebrates to mammals. In mammals, ficolins form complexes with mannose-binding lectin-associated serine proteases (MASPs), and ficolin-MASP complexes trigger complement activation via the lectin pathway. Once activated, complement mediates many immune responses including opsonization, phagocytosis, and cytokine production. Although the precise function of each ficolin is still under investigation, accumulating information suggests that ficolins have a crucial role in host defense by recognizing a variety of microorganisms and interacting with effector proteins.

Mannose-binding lectin impairs Leptospira activity through the inhibitory effect on the motility of cell

Mannose-binding lectin (MBL) plays key role in lectin pathway of innate immunity, and shows the ability of triggering opsonization intermediately. Substantial increase in the serum level of MBL has been confirmed during leptospirosis, which caused by a pathogenic spirochete, Leptospira. Leptospira has a fascinating locomotion pattern, which simultaneously gyrating and swimming forward, such motility enables that Leptospira is difficult to be captured by immune cells if without any assistance. In this study, the effect of mannose-binding lectin to Leptospira was quantitatively investigated by measuring some kinematic parameters, to discover the mechanism behind MBL-mediated immune responses during leptospiral infection. The results showed that mannose-binding lectin is capable of inhibiting the motility of Leptospira by transforming free swimming cells to tumbled rotating cells, resulted in the increase number of rotating cells. Otherwise, decrease in rotation rate of rotating cell has been observed. However, the swimming speed of swimming Leptospira cells showed no observable change under the effect of MBL. The inhibitory effect were only valid in a relatively short period, Leptospira cells regained their original motility after 2 h. This raises an interesting topic that Leptospira is somehow able to escape from the inhibitory effect of MBL by dragging such unfavorable molecules toward to the cell end and eventually throwing it out. The inhibitory effect of MBL on the motility of Leptospira is expected to provide a new insight into lectin pathway.

The Lectin Pathway of Complement and Biocompatibility

In modern health technologies the use of biomaterials in the form of stents, haemodialysis tubes, artificial implants, bypass circuits etc. is rapidly expanding. The exposure of synthetic, foreign surfaces to the blood and tissue of the host, calls for strict biocompatibility in respect to contact activation, the coagulation system and the complement system. The complement system is an important part of the initial immune response and consists of fluid phase molecules in the blood stream. Three different activation pathways can initiate the complement system, the lectin, the classical and the alternative pathway, all converging in an amplification loop of the cascade system and downstream reactions. Thus, when exposed to foreign substances complement components will be activated and lead to a powerful inflammatory response. Biosurface induced complement activation is a recognised issue that has been broadly documented. However, the specific role of lectin pathway and the pattern recognition molecules initiating the pathway has only been transiently investigated. Here we review the current data on the field.

Deficiency in mannose-binding lectin-associated serine protease-2 does not increase susceptibility to Trypanosoma cruzi infection

Trypanosoma cruzi is the causative agent of Chagas' disease, a chronic illness affecting 10 million people around the world. The complement system plays an important role in fighting microbial infections. The recognition molecules of the lectin pathway of complement activation, mannose-binding lectin (MBL), ficolins, and CL-11, bind to specific carbohydrates on pathogens, triggering complement activation through MBL-associated serine protease-2 (MASP-2). Previous in vitro work showed that human MBL and ficolins contribute to T. cruzi lysis. However, MBL-deficient mice are only moderately compromised in their defense against the parasite, as they may still activate the lectin pathway through ficolins and CL-11. Here, we assessed MASP-2-deficient mice, the only presently available mouse line with total lectin pathway deficiency, for a phenotype in T. cruzi infection. Total absence of lectin pathway functional activity did not confer higher susceptibility to T. cruzi infection, suggesting that it plays a minor role in the immune response against this parasite.

Lessons learned from mice deficient in lectin complement pathway molecules

The lectin pathway of the complement system is initiated when the pattern-recognition molecules, mannose-binding lectin (MBL), ficolins or collectin-11, bind to invading pathogens or damaged host cells. This leads to activation of MBL/ficolin/collectin-11 associated serine proteases (MASPs), which in turn activate downstream complement components, ultimately leading to elimination of the pathogen. Mice deficient in the key molecules of lectin pathway of complement have been generated in order to build knowledge of the molecular mechanisms of the lectin pathway in health and disease. Despite differences in the genetic arrangements of murine and human orthologues of lectin pathway molecules, the knockout mice have proven to be valuable models to explore the effect of deficiency states in humans. In addition, new insight and unexpected findings on the diverse roles of lectin pathway molecules in complement activation, pathogen infection, coagulation, host tissue injury and developmental biology have been revealed by in vivo investigations. This review provides an overview of the mice deficient in lectin pathway molecules and highlights some of the most important findings that have resulted from studies of these.

Trypanosoma cruzi calreticulin inhibits the complement lectin pathway activation by direct interaction with L-Ficolin

Trypanosoma cruzi, the agent of Chagas' disease, the sixth neglected tropical disease worldwide, infects 10-12 million people in Latin America. Differently from T. cruzi epimastigotes, trypomastigotes are complement-resistant and infective. CRPs, T-DAF, sialic acid and lipases explain at least part of this resistance. In vitro, T. cruzi calreticulin (TcCRT), a chaperone molecule that translocates from the ER to the parasite surface: (a) Inhibits the human classical complement activation, by interacting with C1, (b) As a consequence, an increase in infectivity is evident and, (c) It inhibits angiogenesis and tumor growth. We report here that TcCRT also binds to the L-Ficolin collagenous portion, thus inhibiting approximately between 35 and 64% of the human complement lectin pathway activation, initiated by L-Ficolin, a property not shared by H-Ficolin. While L-Ficolin binds to 60% of trypomastigotes and to 24% of epimastigotes, 50% of the former and 4% of the latter display TcCRT on their surfaces. Altogether, these data indicate that TcCRT is a parasite inhibitory receptor for Ficolins. The resulting evasive activities, together with the TcCRT capacity to inhibit C1, with a concomitant increase in infectivity, may represent T. cruzi strategies to inhibit important arms of the innate immune response.

Multiple roles of complement MASP-1 at the interface of innate immune response and coagulation

MASP-1 is a versatile serine protease that cleaves a number of substrates in human blood. In recent years it became evident that besides playing a crucial role in complement activation MASP-1 also triggers other cascade systems and even cells to mount a more powerful innate immune response. In this review we summarize the latest discoveries about the diverse functions of this multi-faceted protease. Recent studies revealed that among MBL-associated serine proteases, MASP-1 is the one responsible for triggering the lectin pathway via its ability to rapidly autoactivate then cleave MASP-2, and possibly MASP-3. The crystal structure of MASP-1 explains its more relaxed substrate specificity compared to the related complement enzymes. Due to the relaxed specificity, MASP-1 interacts with the coagulation cascade and the kinin generating system, and it can also activate endothelial cells eliciting pro-inflammatory signaling.

Serum MASP-1 in complex with MBL activates endothelial cells

The complement system plays an important role in the induction of inflammation. In this study we demonstrate that the initiation complexes of the lectin pathway, consisting of mannose-binding lectin (MBL) and associated serine proteases (MASPs) elicit Ca(2+) signaling in cultured endothelial cells (HUVECs). This is in agreement with our previous results showing that the recombinant catalytic fragment of MASP-1 activates endothelial cells by cleaving protease activated receptor 4. Two other proteases, MASP-2 and MASP-3 are also associated with MBL. Earlier we showed that recombinant catalytic fragment of MASP-2 cannot activate HUVECs, and in this study we demonstrate that the same fragment of MASP-3 has also no effect. We find the same to be the case if we use recombinant forms of the N-terminal parts of MASP-1 and MASP-2 which only contain non-enzymatic domains. Moreover, stable zymogen mutant form of MASP-1 was also ineffective to stimulate endothelial cells, which suggests that in vivo MASP-1 have the ability to activate endothelial cells directly as well as to activate the lectin pathway simultaneously. We show that among the components of the MBL-MASPs complexes only MASP-1 is able to trigger response in HUVECs and the proteolytic activity of MASP-1 is essential. Our results strengthen the view that MASP-1 plays a central role in the early innate immune response.

Purification, measurement of concentration, and functional complement assay of human ficolins

Ficolins constitute a group of lectins involved in innate immunity. L-Ficolin, H-ficolin, and M-ficolin are present in human serum. The human ficolins differ in carbohydrate-binding specificity, but they have in common the ability to recognize the acetyl group. L-Ficolin and H-ficolin are associated with serine proteases termed MASPs (MBL-associated serine proteases) and their truncated proteins, and the complexes (L/H-ficolin-MASP) activate the lectin pathway of complement upon binding to their ligands. Recombinant M-ficolin is also able to form a complex with MASP, resulting in complement activation. L-Ficolin and H-ficolin can be purified as a complex with MASP from serum by utilizing their binding specificities. These ficolin-MASP complexes have an ability to activate C4. Human ficolins are quantified by ELISA using specific antibodies or ligands.

Mannose-binding lectin 2 and ficolin-2 gene polymorphisms influence the susceptibility to bloodstream infections in kidney transplant recipients

INTRODUCTION

Both mannose-binding lectin (MBL) and ficolin (FCN) interact with carbohydrate structures on microbial surfaces. Polymorphisms at the promoter and exon 1 of the MBL2 gene, which are responsible for low serum levels of MBL, have been shown to play important roles to increase the risk of post-transplant infections. Three gene polymorphisms in the promoter region of FCN2 and 2 in exon 8 (+6424 G > T) are associated with serum levels of FCN2 or binding capacity toward N-acetylglucosamine on microbial surfaces.

METHODS

We prospectively analyzed 81 kidney transplant recipients for 6 well-known functional single-nucleotide polymorphisms in the MBL2 and 5 in the FCN2 gene of the recipients determined by gene sequencing. The bloodstream infections collected prospectively were associated with MBL2 and FCN2 genotypic variants over the first year after kidney transplantation.

RESULTS

Multivariate analyses only found an association of recipient QQ + PQ genotypes of MBL2 5'-UTR +4 (odds ratio [OR] = 3.677, 95% confidence intervals [CI] = 1.127-11.998, P = .031) and FCN2 exon 8 Thr 236 Met(+6359 C > T) (OR = 4.917, 95% CI = 1.229-19.667, P = .024) with the incidence of bacteremia.

CONCLUSION

Recipient QQ + PQ genotypes of MBL2 5'-UTR +4 and recipient FCN2 exon 8 Thr 236 Met(+6359 C > T) variants showed significant impacts on the risk of developing bloodstream infections after kidney transplantation.

Replication study in Chinese Han population and meta-analysis supports association between the MBL2 gene polymorphism and HIV-1 infection

Mannose-binding lectin (MBL) plays an important role in immunity to HIV-1 infection. The exon1 coding polymorphisms of the MBL2 gene have been implicated in the susceptibility to HIV-1 infection, but the results were controversial. In the present study, a case-control study in a Chinese population was conducted to replicate the association, and then a meta-analysis combing our new data and published data was performed to clarify these findings. In total, 15 studies consisting 2219 HIV-1 patients and 2744 controls were included. Odds ratios (ORs) with 95% confidence intervals (95% CIs) were assessed in the main analyses. By dividing the controls into two groups, healthy controls and HIV-1 exposed but seronegative (HESN) controls, we explored different genetic models and allelic model to detect the association. By using the healthy controls, we found that the MBL2 exon 1 polymorphisms were associated with hosts' susceptibility to HIV-1 infection in dominant model (p=0.01, 95% CI 1.05-1.43), recessive model (p<0.0001, 95% CI 1.35-2.28), allelic model (p<0.0001, 95% CI 1.12-1.37) and O/O vs. A/A model (p<0.00001, 95% CI 1.40-2.38). In the subgroup analysis by ethnicity, significant elevated risks were found in Caucasians (recessive model: p<0.0001, 95% CI 1.36-2.51), but not in Asians (recessive model: p=0.10, 95% CI 0.91-2.77). Collectively, our findings from our case-control replication study and meta-analysis suggested that the MBL2 gene exon 1 coding variants were associated with hosts' susceptibility to HIV-1 infection, especially in Caucasians, but not in Asians.

Ficolins in complement activation

Ficolins are a group of multimeric lectins made up of single subunits each of which is composed of a collagen-like domain and a fibrinogen-like domain. Most of the ficolins identified to date bind to acetylated compounds such as N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc). Ficolins in serum are complexed with MBL-associated serine proteases (MASPs) and their truncated proteins. These lectins play an important role in innate immunity. Binding of the ficolin-MASP complex to carbohydrates present on the surface of microbes initiates complement activation via the lectin pathway.

Structural and functional overview of the lectin complement pathway: its molecular basis and physiological implication

The complement system is an effector mechanism in immunity. It is activated in three ways, the classical, alternative and lectin pathways. The lectin pathway is initiated by the binding of mannose-binding lectin (MBL) or ficolins to carbohydrates on the surfaces of pathogens. In humans, MBL and three types of ficolins (L-ficolin, H-ficolin, and M-ficolin) are present in plasma. Of these lectins, at least, MBL, L-ficolin, and H-ficolin are complexed with three types of MBL-associated serine proteases (MASPs), MASP-1, MASP-2, and MASP-3 and their truncated proteins (MAp44 and sMAP). In the lectin pathway, the lectin-MASP complex (i.e., a complex of lectin, MASPs and their truncated proteins) binds to pathogens, resulting in the activation of C4 and C2 to generate a C3 convertase capable of activating C3. MASP-2 is involved in the activation of C4 and C2. MASP-1 activates C2 and MASP-2. The functions of MASP-3, sMAP, and MAp44 in the lectin pathway remain unknown. MASP-1 and MASP-3 also have a role in the alternative pathway. MBL and ficolins are able to bind to a variety of pathogens depending on their carbohydrate binding specificity, resulting in the activation of the lectin pathway. Deficiencies of the components of the lectin pathway are associated to susceptibility to infection, indicating an important role of the lectin pathway in innate immunity. The lectin-MASP complex is also involved in innate immunity by activating the coagulation system. Recent findings suggest a crucial role of MASP-3 in development.

Zonulin as prehaptoglobin2 regulates lung permeability and activates the complement system

Zonulin is a protein involved in the regulation of tight junctions (TJ) in epithelial or endothelial cells. Zonulin is known to affect TJ in gut epithelial cells, but little is known about its influences in other organs. Prehaptoglobin2 has been identified as zonulin and is related to serine proteases (MASPs, C1qrs) that activate the complement system. The current study focused on the role of zonulin in development of acute lung injury (ALI) in C57BL/6 male mice following intrapulmonary deposition of IgG immune complexes. A zonulin antagonist (AT-1001) and a related peptide with permeability agonist activities (AT-1002) were employed and given intratracheally or intravenously. Also, zonulin was blocked in lung with a neutralizing antibody. In a dose-dependent manner, AT-1001 or zonulin neutralizing antibody attenuated the intensity of ALI (as quantitated by albumin leak, neutrophil accumulation, and proinflammatory cytokines). A similar pattern was found using the bacterial lipopolysaccharide model of ALI. Using confocal microscopy on sections of injured lungs, staining patterns for TJ proteins were discontinuous, reduced, and fragmented. As expected, the leak of blood products into the alveolar space confirmed the passage of 3 and 20 kDa dextran, and albumin. In contrast to AT-1001, application of the zonulin agonist AT-1002 intensified ALI. Zonulin both in vitro and in vivo induced generation of complement C3a and C5a. Collectively, these data suggest that zonulin facilitates development of ALI both by enhancing albumin leak and complement activation as well as increased buildup of neutrophils and cytokines during development of ALI.

Recombinant expression of the autocatalytic complement protease MASP-1 is crucially dependent on co-expression with its inhibitor, C1 inhibitor

MASP-1 is a protease of the lectin pathway of complement. It is homologous with MASP-2, previously thought both necessary and sufficient for lectin pathway activation. Recently MASP-1 has taken centre stage with the observation that it is crucial to the activation of MASP-2 and thus central to complement activation. Numerous additional functions have been suggested for MASP-1 and its importance is obvious. Yet, thorough analyses of proteolytic activities and physiological roles in the human scenario have been hampered by difficulties in purifying or producing full-length human MASP-1. We present the successful expression of full-length recombinant human MASP-1 entirely in the zymogen form in a mammalian expression system. We found that the catalytic activity of MASP-1 suppresses its expression through rapid auto-activation and auto-degradation. This auto-degradation was not inhibited by the addition of inhibitors to the culture medium, and it was subsequently found to occur intracellularly. Numerous mutations aimed at attenuating auto-activation or preventing auto-degradation failed to rescue expression, as did also attempts at stabilizing the protease by co-expression with MBL or ficolins or expression in hepatocyte cell lines, representing the natural site of synthesis. The active protease was finally produced through co-expression with the serine protease inhibitor C1 inhibitor. We demonstrate that the expressed protease is capable of binding MBL and auto-activating, and is catalytically active. We have generalized the concept to the expression also of MASP-2 entirely in its zymogen form and with improved yields. We suggest a general advantage of expressing aggressive, autocatalytic proteases with their cognate inhibitors.

Mechanisms of complement lectin pathway activation and resistance by trypanosomatid parasites

Studies in the past decade have demonstrated a crucial role for the complement lectin pathway in host defence against protozoan microbes. Recognition of pathogen surface molecules by mannan-binding lectin and ficolins revealed new mechanisms of innate immune defence and a diversity of parasite strategies of immune evasion. In the present review, we will discuss the current knowledge of: (1) the molecular mechanism of lectin pathway activation by trypanosomes; (2) the mechanisms of complement evasion by trypanosomes; and (3) host genetic deficiencies of complement lectin pathway factors that contribute to infection susceptibility and disease progression. This review will focus on trypanosomatids, the parasites that cause Chagas disease, leishmaniasis and sleeping sickness (African trypanosomiasis).

Assembly and activation of alternative complement components on endothelial cell-anchored ultra-large von Willebrand factor links complement and hemostasis-thrombosis

Background

Vascular endothelial cells (ECs) express and release protein components of the complement pathways, as well as secreting and anchoring ultra-large von Willebrand factor (ULVWF) multimers in long string-like structures that initiate platelet adhesion during hemostasis and thrombosis. The alternative complement pathway (AP) is an important non-antibody-requiring host defense system. Thrombotic microangiopathies can be associated with defective regulation of the AP (atypical hemolytic-uremic syndrome) or with inadequate cleavage by ADAMTS-13 of ULVWF multimeric strings secreted by/anchored to ECs (thrombotic thrombocytopenic purpura). Our goal was to determine if EC-anchored ULVWF strings caused the assembly and activation of AP components, thereby linking two essential defense mechanisms.

Methodology/Principal Findings

We quantified gene expression of these complement components in cultured human umbilical vein endothelial cells (HUVECs) by real-time PCR: C3 and C5; complement factor (CF) B, CFD, CFP, CFH and CFI of the AP; and C4 of the classical and lectin (but not alternative) complement pathways. We used fluorescent microscopy, monospecific antibodies against complement components, fluorescent secondary antibodies, and the analysis of >150 images to quantify the attachment of HUVEC-released complement proteins to ULVWF strings secreted by, and anchored to, the HUVECs (under conditions of ADAMTS-13 inhibition). We found that HUVEC-released C4 did not attach to ULVWF strings, ruling out activation of the classical and lectin pathways by the strings. In contrast, C3, FB, FD, FP and C5, FH and FI attached to ULVWF strings in quantitative patterns consistent with assembly of the AP components into active complexes. This was verified when non-functional FB blocked the formation of AP C3 convertase complexes (C3bBb) on ULVWF strings.

Conclusions/Significance

AP components are assembled and activated on EC-secreted/anchored ULVWF multimeric strings. Our findings provide one possible molecular mechanism for clinical linkage between different types of thrombotic and complement-mediated disorders.

Increased titers of anti-Saccharomyces cerevisiae antibodies in Crohn's disease patients with reduced H-ficolin levels but normal MASP-2 activity

BACKGROUND AND AIMS

Mannan-binding lectin (MBL) and ficolins are microbial pattern recognition molecules that activate the lectin pathway of complement. We previously reported the association of MBL deficiency with anti-Saccharomyces cerevisiae antibodies (ASCA) in patients with Crohn's disease (CD). However, ASCA are also frequently found in MBL-proficient CD patients. Here we addressed expression/function of ficolins and MBL-associated serine protease-2 (MASP-2) regarding potential association with ASCA.

METHODS

ASCA titers and MBL, ficolin and MASP-2 concentrations were determined by ELISA in the serum of patients with CD, ulcerative colitis (UC), and in healthy controls. MASP-2 activity was determined by measuring complement C4b-fixation. Anti-MBL autoantibodies were detected by ELISA.

RESULTS

In CD and UC patients, L-ficolin concentrations were significantly higher compared to healthy controls (p<0.001 and p=0.029). In contrast, H-ficolin concentrations were slightly reduced in CD and UC compared to healthy controls (p=0.037 for UC vs. hc). CD patients with high ASCA titers had significantly lower H-ficolin concentrations compared to ASCA-low/negative CD patients (p=0.009). However, MASP-2 activity was not different in ASCA-negative and ASCA-positive CD patients upon both, ficolin- or MBL-mediated MASP-2 activation. Finally, anti-MBL autoantibodies were not over-represented in MBL-proficient ASCA-positive CD patients.

CONCLUSIONS

Our results suggest that low expression of H-ficolin may promote elevated ASCA titers in the ASCA-positive subgroup of CD patients. However, unlike MBL deficiency, we found no evidence for low expression of serum ficolins or reduced MASP-2 activity that may predispose to ASCA development.

The role of MASP-1/3 in complement activation

The complement system, which consists of more than 30 plasma and cell surface proteins, is activated by three pathways: the classical, lectin, and alternative pathways, leading to the generation of opsonins and pathogen destruction. In the lectin pathway, mannose-binding lectin (MBL) and ficolins act as pattern recognition molecules for pathogens, resulting in the activation of MBL-associated serine proteases (MASPs: MASP-1, MASP-2, and MASP-3). Among these proteases, MASP-2 is a key enzyme that cleaves C4 and C2 to assemble a C3 convertase (C4b2a). However, the physiological function of MASP-1 and MASP-3 remains unclear. To investigate the roles of MASP-1 and MASP-3, we generated a MASP-1- and MASP-3-deficient (M1/3 KO) mouse model and found that the deficient mice lacked alternative pathway activation because factor D (Df) remained as a proenzyme in the serum. MASP-1 and MASP-3 were able to convert the proenzyme of Df to an active form in vitro. In addition, MASP-1 was able to activate MASP-2 and MASP-3 as C1r activates C1s. Thus, MASP-1 and MASP-3 seem to be involved in activation of both the lectin and alternative pathways.

Mice deficient in ficolin, a lectin complement pathway recognition molecule, are susceptible to Streptococcus pneumoniae infection

Mannose-binding lectin (MBL) and ficolin are complexed with MBL-associated serine proteases, key enzymes of complement activation via the lectin pathway, and act as soluble pattern recognition molecules in the innate immune system. Although numerous reports have revealed the importance of MBL in infectious diseases and autoimmune disorders, the role of ficolin is still unclear. To define the specific role of ficolin in vivo, we generated model mice deficient in ficolins. The ficolin A (FcnA)-deficient (Fcna(-/-)) and FcnA/ficolin B double-deficient (Fcna(-/-)b(-/-)) mice lacked FcnA-mediated complement activation in the sera, because of the absence of complexes comprising FcnA and MBL-associated serine proteases. When the host defense was evaluated by transnasal infection with a Streptococcus pneumoniae strain, which was recognized by ficolins, but not by MBLs, the survival rate was significantly reduced in all three ficolin-deficient (Fcna(-/-), Fcnb(-/-), and Fcna(-/-)b(-/-)) mice compared with wild-type mice. Reconstitution of the FcnA-mediated lectin pathway in vivo improved survival rate in Fcna(-/-) but not in Fcna(-/-)b(-/-) mice, suggesting that both FcnA and ficolin B are essential in defense against S. pneumoniae. These results suggest that ficolins play a crucial role in innate immunity against pneumococcal infection through the lectin complement pathway.

The role of complement system in septic shock

Septic shock is a critical clinical condition with a high mortality rate. A better understanding of the underlying mechanisms is important to develop effective therapies. Basic and clinical studies suggest that activation of complements in the common cascade, for example, complement component 3 (C3) and C5, is involved in the development of septic shock. The involvement of three upstream complement pathways in septic shock is more complicated. Both the classical and alternative pathways appear to be activated in septic shock, but the alternative pathway may be activated earlier than the classical pathway. Activation of these two pathways is essential to clear endotoxin. Recent investigations have shed light on the role of lectin complement pathway in septic shock. Published reports suggest a protective role of mannose-binding lectin (MBL) against sepsis. Our preliminary study of MBL-associated serine protease-2 (MASP-2) in septic shock patients indicated that acute decrease of MASP-2 in the early phase of septic shock might correlate with in-hospital mortality. It is unknown whether excessive activation of these three upstream complement pathways may contribute to the detrimental effects in septic shock. This paper also discusses additional complement-related pathogenic mechanisms and intervention strategies for septic shock.

Role of endothelial N-glycan mannose residues in monocyte recruitment during atherogenesis

OBJECTIVE

Upregulated expression of endothelial adhesion molecules and subsequent binding to cognate monocytic receptors are established paradigms in atherosclerosis. However, these proteins are the scaffolds, with their posttranslational modification with sugars providing the actual ligands. We recently showed that tumor necrosis factor-α increased hypoglycosylated (mannose-rich) N-glycans on the endothelial surface. In the present study, our aim was to determine whether (1) hypoglycosylated N-glycans are upregulated by proatherogenic stimuli (oscillatory flow) in vitro and in vivo, and (2) mannose residues on hypoglycosylated endothelial N-glycans mediate monocyte rolling and adhesion.

METHODS AND RESULTS

Staining with the mannose-specific lectins concanavalin A and lens culinaris agglutinin was increased in human aortic endothelial cells exposed to oscillatory shear or tumor necrosis factor-α and at sites of plaque development and progression in both mice and human vessels. Increasing surface N-linked mannose by inhibiting N-glycan processing potentiated monocyte adhesion under flow during tumor necrosis factor-α stimulation. Conversely, enzymatic removal of high-mannose N-glycans, or masking mannose residues with lectins, significantly decreased monocyte adhesion under flow. These effects occurred without altering induced expression of adhesion molecule proteins.

CONCLUSIONS

Hypoglycosylated (high mannose) N-glycans are present on the endothelial cell surface at sites of early human lesion development and are novel effectors of monocyte adhesion during atherogenesis.