Transient Binding Dynamics of Complement System Pattern Recognition Molecules on Pathogens

Previous studies of pattern recognition molecules (PRMs) of the complement system have revealed difficulties in observing binding on pathogens such as Aspergillus fumigatus and Escherichia coli, despite complement deposition indicative of classical and lectin pathway activation. Thus, we investigated the binding dynamics of PRMs of the complement system, specifically C1q of the classical pathway and mannose-binding lectin (MBL) of the lectin pathway. We observed consistently increasing deposition of essential complement components such as C4b, C3b, and the terminal complement complex on A. fumigatus and E. coli. However, C1q and MBL binding to the surface rapidly declined during incubation after just 2-4 min in 10% plasma. The detachment of C1q and MBL can be linked to complement cascade activation, as the PRMs remain bound in the absence of plasma. The dissociation and the fate of C1q and MBL seem to have different mechanistic functions. Notably, C1q dynamics were associated with local C1 complex activation. When C1s was inhibited in plasma, C1q binding not only remained high but further increased over time. In contrast, MBL binding was inversely correlated with total and early complement activation due to MBL binding being partially retained by complement inhibition. Results indicate that detached MBL might be able to functionally rebind to A. fumigatus. In conclusion, these results reveal a (to our knowledge) novel "hit-and-run" complement-dependent PRM dynamic mechanism on pathogens. These dynamics may have profound implications for host defense and may help increase the functionality and longevity of complement-dependent PRMs in circulation.

The Lectin Pathway of the Complement System-Activation, Regulation, Disease Connections and Interplay with Other (Proteolytic) Systems

The complement system is the other major proteolytic cascade in the blood of vertebrates besides the coagulation-fibrinolytic system. Among the three main activation routes of complement, the lectin pathway (LP) has been discovered the latest, and it is still the subject of intense research. Mannose-binding lectin (MBL), other collectins, and ficolins are collectively termed as the pattern recognition molecules (PRMs) of the LP, and they are responsible for targeting LP activation to molecular patterns, e.g., on bacteria. MBL-associated serine proteases (MASPs) are the effectors, while MBL-associated proteins (MAps) have regulatory functions. Two serine protease components, MASP-1 and MASP-2, trigger the LP activation, while the third component, MASP-3, is involved in the function of the alternative pathway (AP) of complement. Besides their functions within the complement system, certain LP components have secondary ("moonlighting") functions, e.g., in embryonic development. They also contribute to blood coagulation, and some might have tumor suppressing roles. Uncontrolled complement activation can contribute to the progression of many diseases (e.g., stroke, kidney diseases, thrombotic complications, and COVID-19). In most cases, the lectin pathway has also been implicated. In this review, we summarize the history of the lectin pathway, introduce their components, describe its activation and regulation, its roles within the complement cascade, its connections to blood coagulation, and its direct cellular effects. Special emphasis is placed on disease connections and the non-canonical functions of LP components.

Increased expression of ficolin-1 is associated with airway obstruction in asthma

BACKGROUND

The activated complement cascade is involved in asthmatic airway inflammation. Ficolins are essential for innate immunity and can activate the complement lectin pathway. Despite this, the significance of ficolins in asthma has yet to be determined. This study aimed to explore the presence of ficolins in individuals with asthma and to determine the relationship between ficolins and clinical characteristics.

METHODS

For the study, 68 asthmatic patients and 30 healthy control subjects were recruited. Enzyme-linked immunosorbent assay was used to determine plasma ficolin-1, ficolin-2, and ficolin-3 concentrations both before and after inhaled corticosteroid (ICS) therapy. Further, the associations of plasma ficolin-1 level with pulmonary function and asthma control questionnaire (ACQ) score were examined in the asthma patients.

RESULTS

Patients with asthma exhibited significantly elevated plasma ficolin-1 levels (median, 493.9 ng/mL; IQR, 330.2-717.8 ng/mL) in comparison to healthy controls (median, 330.6 ng/mL; IQR, 233.8-371.1 ng/mL). After ICS treatment, plasma ficolin-1 (median, 518.1 ng/mL; IQR, 330.2-727.0 ng/mL) in asthmatic patients was significantly reduced (median, 374.7 ng/mL; IQR, 254.8-562.5 ng/mL). Additionally, ficolin-1 expressions in plasma were significantly correlated with pulmonary function parameters and ACQ score in asthmatic patients. Asthma patients with higher plasma ficolin-1 levels demonstrated poorer lung function than those with lower plasma ficolin-1 levels.

CONCLUSIONS

The results revealed that asthmatic patients had higher plasma ficolin-1 concentrations, which decreased after ICS treatment and were linked to their lung function, implying a potential involvement of ficolin-1 in asthma pathogenesis.

Diverse Functions of C4b-Binding Protein in Health and Disease

C4b-binding protein (C4BP) is a fluid-phase complement inhibitor that prevents uncontrolled activation of the classical and lectin complement pathways. As a complement inhibitor, C4BP also promotes apoptotic cell death and is hijacked by microbes and tumors for complement evasion. Although initially characterized for its role in complement inhibition, there is an emerging recognition that C4BP functions in a complement-independent manner to promote cell survival, protect against autoimmune damage, and modulate the virulence of microbial pathogens. In this Brief Review, we summarize the structure and functions of human C4BP, with a special focus on activities that extend beyond the canonical role of C4BP in complement inhibition.

Lectin complement pathway and diabetes mellitus in the pathogenesis of membranous nephropathy

INTRODUCTION

Membranous nephropathy (MN) is a glomerulonephritis with growing incidence and its pathogenesis still remains unclear, despite discoveries of many new antigens. The understanding of MN pathogenesis has moved from antigen-antibody arena to the complement activation through the lectin pathway.

Lectin Pathway Enzyme MASP-2 and Downstream Complement Activation in COVID-19

Mannose-binding lectin-associated serine protease 2 (MASP-2) is the main activator of the lectin complement pathway and has been suggested to be involved in the pathophysiology of coronavirus disease 2019 (COVID-19). To study a possible association between MASP-2 and COVID-19, we aimed at developing a sensitive and reliable MASP-2 ELISA. From an array of novel mouse-monoclonal antibodies using recombinant MASP-2 as antigen, two clones were selected to create a sandwich ELISA. Plasma samples were obtained from 216 healthy controls, 347 convalescent COVID-19 patients, and 147 prospectively followed COVID-19 patients. The assay was specific towards MASP-2 and did not recognize the truncated MASP2 splice variant MAP-2 (MAp19). The limit of quantification was shown to be 0.1 ng/mL. MASP-2 concentration was found to be stable after multiple freeze-thaw cycles. In healthy controls, the mean MASP-2 concentration was 524 ng/mL (95% CI: 496.5-551.6). No significant difference was found in the MASP-2 concentrations between COVID-19 convalescent samples and controls. However, a significant increase was observed in prospectively followed COVID-19 patients (mean: 834 ng/mL [95% CI: 765.3-902.7, p < 0.0001]). In these patients, MASP-2 concentration correlated significantly with the concentrations of the terminal complement complex (ρ = 0.3596, p < 0.0001), with the lectin pathway pattern recognition molecules ficolin-2 (ρ = 0.2906, p = 0.0004) and ficolin-3 (ρ = 0.3952, p < 0.0001) and with C-reactive protein (ρ = 0.3292, p = 0.0002). Overall, we developed a specific quantitative MASP-2 sandwich ELISA. MASP-2 correlated with complement activation and inflammatory markers in COVID-19 patients, underscoring a possible role of MASP-2 in COVID-19 pathophysiology.

Ficolin-2 Lectin Complement Pathway Mediates Capsule-Specific Innate Immunity Against Invasive Pneumococcal Disease

Reports conflict regarding which lectin-microbial ligand interactions elicit a protective response from the lectin pathway (LP) of complement. Using fluorescent microscopy, we demonstrate the human lectin ficolin-2 binds to Streptococcus pneumoniae serotype 11A capsule polysaccharide dependent on the O-acetyltransferase gene wcjE. This triggers complement deposition and promotes opsonophagocytosis of encapsulated pneumococci. Even partial loss of ficolin-2 ligand expression through wcjE mutation abrogated bacterial killing. Ficolin-2 did not interact with any pneumococcal non-capsule structures, including teichoic acid. We describe multiple 11A clonal derivatives expressing varying degrees of wcjE-dependent epitopes co-isolated from single blood specimens, likely representing microevolutionary shifts towards wcjE-deficient populations during invasive pneumococcal disease (IPD). We find epidemiological evidence of wcjE impairing pneumococcal invasiveness, supporting that the LP's ficolin-2 axis provides innate, serotype-specific serological protection against IPD. The fact that the LP is triggered by only a few discrete carbohydrate ligands emphasizes the need to reevaluate its impact in a glycopolymer-specific manner.

Hemodialysis leads to plasma depletion of lectin complement pathway initiator molecule ficolin-2

INTRODUCTION

This study aimed to investigate changes in complement system-related molecules in patients undergoing hemodialysis.

METHODS

Patients >18 years of age on maintenance hemodialysis were included. Using enzyme-linked immunosorbent assays (ELISA) methods complement related molecules ficolin-1, ficolin-2, ficolin-3 mannose-binding lectin, long pentraxin 3, complement activation products C3c, and complement activation potentials were measured before and after a single hemodialysis treatment. All patients were dialyzed with synthetic high flux filters >1.6 m2 , respectively, Polyamix and Polysulfone, and the Kt/V was maintained >1.3.

FINDINGS

Three hundred and four patients were included. There was a modest decrease in plasma level of ficolin-1 (p < 0.001). Ficolin-2 was virtually depleted with median 3.9 (interquartile range [IQR]: 2.6-6.1, range 0.3-13.5) μg/ml before dialysis to median 0.0 (IQR: 0.0-0.5, range 0.0-5.5) μg/ml after dialysis (p < 0.001). No significant difference before and after hemodialysis was seen for mannose-binding lectin and long pentraxin 3 (p > 0.05). In a random subgroup of 160 patients ficolin-2-binding, ficolin-3-mediated lectin pathway capacity and classical pathway capacity were significantly decreased due to hemodialysis. The complement capacity of the alternative pathway was increased after hemodialysis (p = 0.0101), while mannose-binding lectin-mediated lectin pathway capacity was unaltered (p = 0.79). There was an increase in the complement activation product C3c (p < 0.0001), while the concentration of total C4 and C3 did not change (p > 0.158). Multivariate Cox proportional hazard analyses showed an increased risk for all-cause mortality with increasing ficolin-2 (p = 0.002) after hemodialysis.

DISCUSSION

Plasma ficolin-2 was virtually depleted from the circulation after hemodialysis. However, elevated plasma ficolin-2 levels after hemodialysis was independently associated with increased mortality.

Profile of Immunoglobulin G N-Glycome in COVID-19 Patients: A Case-Control Study

Coronavirus disease 2019 (COVID-19) remains a major health challenge globally. Previous studies have suggested that changes in the glycosylation of IgG are closely associated with the severity of COVID-19. This study aimed to compare the profiles of IgG N-glycome between COVID-19 patients and healthy controls. A case-control study was conducted, in which 104 COVID-19 patients and 104 age- and sex-matched healthy individuals were recruited. Serum IgG N-glycome composition was analyzed by hydrophilic interaction liquid chromatography with the ultra-high-performance liquid chromatography (HILIC-UPLC) approach. COVID-19 patients have a decreased level of IgG fucosylation, which upregulates antibody-dependent cell cytotoxicity (ADCC) in acute immune responses. In severe cases, a low level of IgG sialylation contributes to the ADCC-regulated enhancement of inflammatory cytokines. The decreases in sialylation and galactosylation play a role in COVID-19 pathogenesis via the activation of the lectin-initiated alternative complement pathway. IgG N-glycosylation underlines the complex clinical phenotypes of SARS-CoV-2 infection.

Complement Alternative and Mannose-Binding Lectin Pathway Activation Is Associated With COVID-19 Mortality

Background

The SARS-CoV-2 infection triggers excessive immune response resulting in increased levels of pro-inflammatory cytokines, endothelial injury, and intravascular coagulopathy. The complement system (CS) activation participates to this hyperinflammatory response. However, it is still unclear which activation pathways (classical, alternative, or lectin pathway) pilots the effector mechanisms that contribute to critical illness. To better understand the immune correlates of disease severity, we performed an analysis of CS activation pathways and components in samples collected from COVID-19 patients hospitalized in Grenoble Alpes University Hospital between 1 and 30 April 2020 and of their relationship with the clinical outcomes.

Methods

We conducted a retrospective, single-center study cohort in 74 hospitalized patients with RT-PCR-proven COVID-19. The functional activities of classical, alternative, and mannose-binding lectin (MBL) pathways and the antigenic levels of the individual components C1q, C4, C3, C5, Factor B, and MBL were measured in patients' samples during hospital admission. Hierarchical clustering with the Ward method was performed in order to identify clusters of patients with similar characteristics of complement markers. Age was included in the model. Then, the clusters were compared with the patient clinical features: rate of intensive care unit (ICU) admission, corticoid treatment, oxygen requirement, and mortality.

Results

Four clusters were identified according to complement parameters. Among them, two clusters revealed remarkable profiles: in one cluster (n = 15), patients exhibited activation of alternative and lectin pathways and low antigenic levels of MBL, C4, C3, Factor B, and C5 compared to all the other clusters; this cluster had the higher proportion of patients who died (27%) and required oxygen support (80%) or ICU care (53%). In contrast, the second cluster (n = 19) presented inflammatory profile with high classical pathway activity and antigenic levels of complement components; a low proportion of patients required ICU care (26%) and no patient died in this group.

Conclusion

These findings argue in favor of prominent activation of the alternative and MBL complement pathways in severe COVID-19, but the spectrum of complement involvement seems to be heterogeneous requiring larger studies.

Plasma Lectin Pathway Complement Proteins in Patients With COVID-19 and Renal Disease

We do not understand why non-white ethnicity and chronic kidney disease increase susceptibility to COVID-19. The lectin pathway of complement activation is a key contributor to innate immunity and inflammation. Concentrations of plasma lectin pathway proteins influence pathway activity and vary with ethnicity. We measured circulating lectin proteins in a multi-ethnic cohort of chronic kidney disease patients with and without COVID19 infection to determine if lectin pathway activation was contributing to COVID19 severity. We measured 11 lectin proteins in serial samples from a cohort of 33 patients with chronic kidney impairment and COVID19. Controls were single plasma samples from 32 patients on dialysis and 32 healthy individuals. We demonstrated multiple associations between recognition molecules and associated proteases of the lectin pathway and COVID-19, including COVID-19 severity. Some of these associations were unique to patients of Asian and White ethnicity. Our novel findings demonstrate that COVID19 infection alters the concentration of plasma lectin proteins and some of these changes were linked to ethnicity. This suggests a role for the lectin pathway in the host response to COVID-19 and suggest that variability within this pathway may contribute to ethnicity-associated differences in susceptibility to severe COVID-19.

Complement component C1q plays a critical role in VLRA/VLRC-mediated immune response

In jawless vertebrates, the lamprey complement component C1q (LC1q) acts as a lectin and activates lamprey complement component C3 (LC3) in association with mannose-binding lectin (MBL)-associated serine protease (MASP) via the lectin pathway. Furthermore, LC1q may interact with variable lymphocyte receptor B (VLRB) in a complex with antigens and mediate the activation of LC3, leading to cytolysis. In the present study, we found, for the first time, that LC1q plays a critical role in VLRA/VLRC-mediated immune response. Escherichia coli, Shigella flexneri, Aeromonas hydrophila, Pseudomonas plecoglossicida, Aeromonas allosaccharophila, P. luteola, Brevundimonas diminuta, and Bacillus cereus were isolated from infected Lampetra morii in our laboratory and identified using the 16s rRNA method. A. hydrophila was confirmed as a rapidly spreading lethal pathogen in the larvae of L. morii and was used in subsequent immune stimulation experiments. The results of real-time quantitative polymerase chain reaction (Q-PCR) and immunofluorescence analyses indicated that the RNA and protein expression levels of LC1q were upregulated following exposure to 10⁷ cfu/mL of A. hydrophila, compared to the levels of the naïve group. We obtained LC1q morphants (LC1q MO) of lamprey larvae by morpholino-mediated knockdowns. We found that LC1q played key roles in the embryonic development of lamprey. The median lethal time (LT50) of LC1q MO larvae was 2 d after being exposed to the pathogens, whereas the LT50 of control MO was 5 d. The drastic decrease in LT50 values after LC1q knockdown implies that LC1q plays a critical role in lamprey immune response. Gene expression profiles of LC1q-deficient A. hydrophila, control MO A. hydrophila, wild type A. hydrophila, and naive 1-month-old ammocoetes larvae were compared by examining the expression levels of a selected panel of orthologous genes. It is worth mentioning that LC1q MO affected the VLRA+/VLRC + population genes but did not affect the VLRB + populations. Immunohistochemical data indicated that LC1q deficiency also affected VLRA and VLRC but not VLRB. Thus, LC1q plays a critical role in VLRA/VLRC-mediated immune response in lamprey.

Human brain trauma severity is associated with lectin complement pathway activation

We explored the involvement of the lectin pathway of complement in post-traumatic brain injury (TBI) pathophysiology in humans. Brain samples were obtained from 28 patients who had undergone therapeutic contusion removal, within 12 h (early) or from >12 h until five days (late) from injury, and from five non-TBI patients. Imaging analysis indicated that lectin pathway initiator molecules (MBL, ficolin-1, ficolin-2 and ficolin-3), the key enzymes MASP-2 and MASP-3, and the downstream complement components (C3 fragments and TCC) were present inside and outside brain vessels in all contusions. Only ficolin-1 was found in the parenchyma of non-TBI tissues. Immunoassays in brain homogenates showed that MBL, ficolin-2 and ficolin-3 increased in TBI compared to non-TBI (2.0, 2.2 and 6.0-times) samples. MASP-2 increased with subarachnoid hemorrhage and abnormal pupil reactivity, two indicators of structural and functional damage. C3 fragments and TCC increased, respectively, by 3.5 - and 4.0-fold in TBI compared to non-TBI tissue and significantly correlated with MBL, ficolin-2, ficolin-3, MASP-2 and MASP-3 levels in the homogenates. In conclusion, we show for the first time the direct presence of lectin pathway components in human cerebral contusions and their association with injury severity, suggesting a central role for the lectin pathway in the post-traumatic pathophysiology of human TBI.

Lectin pathway proteins of the complement system in normotensive pregnancy and pre-eclampsia

PROBLEM

The lectin pathway of the complement system may be involved in the pathogenesis of pre-eclampsia. We aimed to investigate changes in serum concentrations of a broad range of lectin pathway proteins during normal pregnancy and their association with pre-eclampsia, placental infarctions and intrauterine growth restriction (IUGR).

METHOD OF STUDY

We included 51 women with normotensive pregnancies and 54 women with pregnancies complicated by pre-eclampsia. Blood samples were obtained at gestational weeks 16, 33, 37, and after delivery for the normotensive pregnant women and before and after delivery for women with pre-eclampsia. Mannose-binding lectin (MBL), H- and M-ficolin, collectin liver-1 (CL-L1), MBL-associated serine protease (MASP)-1, MASP-2 and MASP-3 and MBL-associated proteins of 19 (MAp19) and 44 (MAp44) kDa were analysed. Clinical information was obtained from medical records. The placentae were examined by two experienced perinatal pathologists.

RESULTS

Lectin pathway protein concentrations generally increased during normal pregnancy and decreased after delivery in both normotensive pregnant women and women with pre-eclampsia. Exceptions were MASP-3 which increased after delivery in both groups (P < 0.0001) and H-ficolin which increased after delivery in pre-eclampsia (P < 0.0001). H-ficolin (P < 0.0001), M-ficolin (P = 0.005) and MASP-3 (P = 0.03) concentrations were lower in women with pre-eclampsia than in normotensive pregnant women. Low MASP-3 concentrations were associated with placental infarction (P = 0.03) and IUGR (P = 0.04). Low H-ficolin concentrations were associated with IUGR (P < 0.01).

CONCLUSION

In general, lectin pathway protein serum concentrations increased during normal pregnancy. H-ficolin and MASP-3 may be involved in the pathophysiology of pre-eclampsia and IUGR and could be potential future pre-eclampsia biomarkers.

Deconstructing the Lectin Pathway in the Pathogenesis of Experimental Inflammatory Arthritis: Essential Role of the Lectin Ficolin B and Mannose-Binding Protein-Associated Serine Protease 2

Complement plays an important role in the pathogenesis of rheumatoid arthritis. Although the alternative pathway (AP) is known to play a key pathogenic role in models of rheumatoid arthritis, the importance of the lectin pathway (LP) pattern recognition molecules such as ficolin (FCN) A, FCN B, and collectin (CL)-11, as well as the activating enzyme mannose-binding lectin-associated serine protease-2 (MASP-2), are less well understood. We show in this article that FCN A-/- and CL-11-/- mice are fully susceptible to collagen Ab-induced arthritis (CAIA). In contrast, FCN B-/- and MASP-2-/-/sMAp-/- mice are substantially protected, with clinical disease activity decreased significantly (p < 0.05) by 47 and 70%, respectively. Histopathology scores, C3, factor D, FCN B deposition, and infiltration of synovial macrophages and neutrophils were similarly decreased in FCN B-/- and MASP-2-/-/sMAp-/- mice. Our data support that FCN B plays an important role in the development of CAIA, likely through ligand recognition in the joint and MASP activation, and that MASP-2 also contributes to the development of CAIA, likely in a C4-independent manner. Decreased AP activity in the sera from FCN B-/- and MASP-2-/-/sMAp-/- mice with arthritis on adherent anti-collagen Abs also support the hypothesis that pathogenic Abs, as well as additional inflammation-related ligands, are recognized by the LP and operate in vivo to activate complement. Finally, we also speculate that the residual disease seen in our studies is driven by the AP and/or the C2/C4 bypass pathway via the direct cleavage of C3 through an LP-dependent mechanism.

Pentraxin 3, ficolin-2 and lectin pathway associated serine protease MASP-3 as early predictors of myocardial infarction - the HUNT2 study

The lectin complement pathway is suggested to play a role in atherogenesis. Pentraxin-3 (PTX3), ficolin-1, ficolin-2, ficolin-3, MBL/ficolin/collectin-associated serine protease-3 (MASP-3) and MBL/ficolin/collectin-associated protein-1 (MAP-1) are molecules related to activation of the lectin complement pathway. We hypothesized that serum levels of these molecules may be associated with the incidence of myocardial infarction (MI). In a Norwegian population-based cohort (HUNT2) where young to middle-aged relatively healthy Caucasians were followed up for a first-time MI from 1995-1997 through 2008, the 370 youngest MI patients were matched by age (range 29-62 years) and gender to 370 controls. After adjustments for traditional risk factors, the two highest tertiles of PTX3 and the highest tertiles of ficolin-2 and MASP-3 were associated with MI, with odds ratios (95% confidence interval) of 1.65 (1.10-2.47) and 2.79 (1.83-4.24) for PTX3, 1.55 (1.04-2.30) for ficolin-2, and 0.63 (0.043-0.94) for MASP-3. Ficolin-1, ficolin-3 and MAP-1 were not associated with MI. In a multimarker analysis of all associated biomarkers, only PTX3 and MASP-3 remained significant. PTX-3 and MASP-3 enhanced prediction of MI compared to the traditional Framingham risk score alone (AUC increased from 0.64 to 0.68, p = 0.006). These results support the role of complement-dependent inflammation in the pathophysiology of cardiovascular disease.

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.

The complement system of elasmobranches revealed by liver transcriptome analysis of a hammerhead shark, Sphyrna zygaena

Comprehensive studies of the complement genes in basal vertebrates have revealed that cyclostomes have apparently primitive complement systems whereas bony fish have well-developed complement systems comparable to those of mammals. Here we have performed liver transcriptome analysis of a hammerhead shark, Sphyrna zygaeana, to elucidate the early history of vertebrate complement evolution. Identified genes were; one C1qB, one C1r, one C1s, one MASP-1/-3, one MASP-2, two factor B/C2, one C3, three C4, one C5, one C6, one C7, one C8A, three C8B, one C8G, one C9, two factor I and one S protein. No MBL, ficolin, C1qA or C1qC were found. These results indicate that the lectin, classical, alternative and lytic pathways were established in the common ancestor of jawed vertebrates. In addition to the absence of MBL and ficolin, the MASP transcripts lacked the serine protease domain, suggesting that the lectin pathway was lost in the hammerhead shark lineage.

Ficolins Promote Fungal Clearance in vivo and Modulate the Inflammatory Cytokine Response in Host Defense against Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic fungal pathogen that causes severe invasive infections in immunocompromised patients. Innate immunity plays a major role in protection against A. fumigatus. The ficolins are a family of soluble pattern recognition receptors that are capable of activating the lectin pathway of complement. Previous in vitro studies reported that ficolins bind to A. fumigatus, but their part in host defense against fungal infections in vivo is unknown. In this study, we used ficolin-deficient mice to investigate the role of ficolins during lung infection with A. fumigatus. Ficolin knockout mice showed significantly higher fungal loads in the lungs 24 h postinfection compared to wild-type mice. The delayed clearance of A. fumigatus in ficolin knockout mice could not be attributed to a compromised recruitment of inflammatory cells. However, it was revealed that ficolin knockout mice exhibited a decreased production of proinflammatory cytokines in the lungs compared to wild-type mice following A. fumigatus infection. The impaired clearance and cytokine production in ficolin knockout mice was independent of complement, as shown by equivalent levels of A. fumigatus-mediated complement activation in ficolin knockout mice and wild-type mice. In conclusion, this study demonstrates that ficolins are important in initial innate host defense against A. fumigatus infections in vivo.

The Lectin Complement Pathway in Patients with Necrotizing Soft Tissue Infection

BACKGROUND

Mannose-binding lectin (MBL) and ficolins are pattern recognition molecules (PRMs) that play an important role during infection through activation of the lectin complement pathway. We assessed whether plasma PRM levels were associated with mortality in patients with necrotizing soft tissue infection (NSTI).

METHODS

We conducted a prospective, observational study over 25 months involving 135 NSTI patients with a maximum follow-up of 2.7 years. Blood samples were taken upon admission. Non-infected patients served as controls.

RESULTS

PRM levels were significantly lower compared with controls. A baseline Ficolin-2 level below the median was associated with mortality at the end of follow-up (p = 0.007). No significant association was found for MBL, Ficolin-1 and Ficolin-3. A Ficolin-2 level below the median had a negative predictive value of 0.94 for 28-day mortality, and a level below the optimal cut-off was independently associated with 28-day mortality when adjusted for age, sex and chronicity [hazard ratio 6.27 (95% confidence interval 2.28-17.21), p < 0.0001], also when Simplified Acute Physiology Score II was included in the analysis [hazard ratio 3.16 (95% confidence interval 1.03-9.73), p = 0.045].

CONCLUSIONS

All PRMs were significantly lower in patients with NSTI than in controls. Only baseline Ficolin-2 was associated with short- and long-term mortality. A high baseline Ficolin-2 level indicated a 94% chance of surviving the first 28 days after admission.

Polymorphisms in the lectin pathway of complement activation influence the incidence of acute rejection and graft outcome after kidney transplantation

There are conflicting data on the role of the lectin pathway of complement activation and its recognition molecules in acute rejection and outcome after transplantation. To help resolve this we analyzed polymorphisms and serum levels of lectin pathway components in 710 consecutive kidney transplant recipients enrolled in the nationwide Swiss Transplant Cohort Study, together with all biopsy-proven rejection episodes and 1-year graft and patient survival. Functional mannose-binding lectin (MBL) levels were determined in serum samples, and previously described MBL2, ficolin 2, and MBL-associated serine protease 2 polymorphisms were genotyped. Low MBL serum levels and deficient MBL2 diplotypes were associated with a higher incidence of acute cellular rejection during the first year, in particular in recipients of deceased-donor kidneys. This association remained significant (hazard ratio 1.75, 95% confidence interval 1.18-2.60) in a Cox regression model after adjustment for relevant covariates. In contrast, there was no significant association with rates of antibody-mediated rejection, patient death, early graft dysfunction or loss. Thus, results in a prospective multicenter contemporary cohort suggest that MBL2 polymorphisms result in low MBL serum levels and are associated with acute cellular rejection after kidney transplantation. Since MBL deficiency is a relatively frequent trait in the normal population, our findings may lead to individual risk stratification and customized immunosuppression.

The lectin complement pathway serine proteases (MASPs) represent a possible crossroad between the coagulation and complement systems in thromboinflammation

ESSENTIALS: The lectin pathway's MASP-1/2 activates coagulation factors but the trigger of the activation is unknown. MASP-1/2 activation was assessed by quantifying complexes between MASPs and antithrombin/C1-inhibitor. Activated platelets and fibrin were demonstrated to activate MASP-1 and MASP-2 both in vitro and in vivo. These findings may represent a crossroad between the complement and the coagulation systems.

SUMMARY

BACKGROUND

The activated forms of the complement lectin pathway (LP) proteases MASP-1 and MASP-2 are able to cleave the coagulation factors prothrombin, fibrinogen, factor XIII and thrombin-activatable fibrinolysis inhibitor in vitro. In vivo studies also show that MASP-1 is involved in thrombogenesis.

OBJECTIVES

To clarify the not yet identified mechanisms involved in triggering activation of the LP during thrombotic reactions.

METHODS

Novel sandwich-ELISAs for detection of complexes between MASP-1 or MASP-2 and the serpins C1 inhibitor (C1-INH) or antithrombin (AT), were used to specifically detect and quantify the activated forms of MASP-1 and MASP-2.

RESULTS

Activated platelets were shown by flow cytometry to bind Ficolin-1, -2 and -3 but not MBL, which was associated with activation of MASP-1 and MASP-2. We also demonstrated that fibrin and the plasmin-generated fibrin fragment DD in plasma, bind and activate MASP-1 and MASP-2. As demonstrated by the ELISA and SDS-PAGE/Western blotting, the fibrin-associated activation was reflected in a specific inactivation by AT during clotting without the assistance of heparin. In all other cases the MASPs were, as previously reported, inactivated by C1-INH. In systemic lupus erythematosus patients with thrombotic disease and in polytrauma patients, the levels of activated MASP-1 and MASP-2 in complex with both AT and C1-INH were associated with markers of thrombotic disease and contact/coagulation system activation.

CONCLUSIONS

MASP-1 and MASP-2 are activated during blood clotting. This activation is triggered by activated platelets and by the generation of fibrin during thrombotic reactions in vitro and in vivo, and may represent a novel activation/amplification mechanism in thromboinflammation.

Increased Autoreactivity of the Complement-Activating Molecule Mannan-Binding Lectin in a Type 1 Diabetes Model

BACKGROUND

Diabetic kidney disease is the leading cause of end-stage renal failure despite intensive treatment of modifiable risk factors. Identification of new drug targets is therefore of paramount importance. The complement system is emerging as a potential new target. The lectin pathway of the complement system, initiated by the carbohydrate-recognition molecule mannan-binding lectin (MBL), is linked to poor kidney prognosis in diabetes. We hypothesized that MBL activates complement upon binding within the diabetic glomerulus.

METHODS

We investigated this by comparing complement deposition and activation in kidneys from streptozotocin-induced diabetic mice and healthy control mice.

RESULTS

After 20 weeks of diabetes, glomerular deposition of MBL was significantly increased. Diabetic animals had 2.0-fold higher (95% CI 1.6-2.5) immunofluorescence intensity from anti-MBL antibodies compared with controls (P < 0.001). Diabetes and control groups did not differ in glomerular immunofluorescence intensity obtained by antibodies against complement factors C4, C3, and C9. However, the circulating complement activation product C3a was increased in diabetes as compared to control mice (P = 0.04).

CONCLUSION

20 weeks of diabetes increased MBL autoreactivity in the kidney and circulating C3a concentration. Together with previous findings, these results indicate direct effects of MBL within the kidney in diabetes.

Heparin-coated cardiopulmonary bypass circuits selectively deplete the pattern recognition molecule ficolin-2 of the lectin complement pathway in vivo

The complement system can be activated via the lectin pathway by the recognition molecules mannose-binding lectin (MBL) and the ficolins. Ficolin-2 exhibits binding against a broad range of ligands, including biomaterials in vitro, and low ficolin-2 levels are associated with increased risk of infections. Thus, we investigated the biocompatibility of the recognition molecules of the lectin pathway in two different types of cardiopulmonary bypass circuits. Bloods were drawn at five time-points before, during and postoperatively from 30 patients undergoing elective cardiac surgery. Patients were randomized into two groups using different coatings of cardiopulmonary bypass circuits, Phisio® (phosphorylcholine polymer coating) and Bioline® (albumin-heparin coating). Concentrations of MBL, ficolin-1, -2 and -3 and soluble C3a and terminal complement complex (TCC) in plasma samples were measured. Ficolin-3-mediated complement activation potential was evaluated with C4, C3 and TCC as output. There was no significant difference between the two circuit materials regarding MBL, ficolin-1 and -3. In the Bioline® group the ficolin-2 levels decreased significantly after initiation of surgery (P < 0.0001) and remained reduced throughout the sampling period. This was not seen for Phisio®-coated circuits. Ficolin-3-mediated complement activation potential was reduced significantly in both groups after start of operation (P < 0.0001), whereas soluble C3a and TCC in the samples were increased (P < 0.0001). Ficolin-2 was depleted from plasma during cardiac surgery when using heparin-coated bypass circuits and did not reach baseline level 24 h postoperation. These findings may have implications for the postoperative susceptibility to infections in patients undergoing extracorporeal circulation procedures.

Ficolins and the lectin pathway of complement in patients with systemic lupus erythematosus

The complement system plays a pathophysiological role in systemic lupus erythematosus (SLE). This study aims to investigate whether an association exists between the ficolins that are part of the lectin complement pathway and SLE. EDTA plasma samples from 68 Danish SLE patients and 29 healthy donors were included in the study. Plasma concentrations of Ficolin-1, -2, and -3 were determined in specific sandwich ELISAs. Lectin pathway activity via Ficolin-3 was measured in ELISA on acetylated bovine serum albumin (acBSA) and measured as Ficolin-3 binding and deposition of C4, C3 and the terminal complement complex (TCC). SLE patients had increased levels of Ficolin-3, 21.6μg/ml as compared to 17.0μg/ml in healthy controls (P=0.0098). The Ficolin-1 plasma concentration was negatively correlated with SLE Disease Activity Index (SLEDAI) (Rho=-0.29, P=0.015) and positively correlated to the [Systemic Lupus International Collaborating Clinics (SLICC)/American College of Rheumatology (ACR) Damage Index] (SDI) (Rho=0.27, P=0.026). The Ficolin-1 concentration was also associated with the occurrence of arterial (P=0.0053) but not venous thrombosis (P=0.42). Finally, deposition of C4, C3 and TCC in the Ficolin-3 pathway were all correlated to SLEDAI, respectively (P<0.0076). The Ficolin-1 association to SLEDAI and SDI as well as arterial thrombosis shown in this study suggests that Ficolin-1 may be a potential new biomarker for patients with SLE. Furthermore, Ficolin-3 mediated complement activation may be valuable in monitoring disease activity in SLE patients due to the high sensitivity for complement consumption in the assay independent of the Ficolin-3 concentration.

[Cardiovascular diseases and mannose-binding lectin]

The role of innate immunity factors in the pathogens of ACS is not well studied, although there is evidence in the literature about their impact on the course of cardiovascular diseases. Mannose-binding lectin (MBL)--one of the key factors of the humoral innate immune system that activates one of complement activation pathways. The literature suggests an ambiguous, complex role of MBL, which can in different clinical situations either improve the prognosis of patients, or be a risk factor for complications. MBL could potentially be relevant to all main links in the pathogenesis of coronary artery disease and myocardial infarction: inflammation, thrombosis, apoptosis, and so on. At different stages of atherogenesis, including the formation and destabilization of the atherosclerotic plaque, thrombosis, MBL may have a significant impact. The review analyzes currently available literature on the impact of MBL on atherosclerosis, ischemic heart disease and acute coronary syndrome. Moreover, in the review there is data on the role of MBL in physiological reactions in innate immunity, gene structure of MBL2 and possible mutations leading to deficiency of MBL in blood, and the role of MBL in the pathogenesis of various diseases.

Recombinant human L-ficolin directly neutralizes hepatitis C virus entry

L-ficolin is a soluble pattern recognition molecule expressed by the liver that contributes to innate immune defense against microorganisms. It is well described that binding of L-ficolin to specific pathogen-associated molecular patterns activates the lectin complement pathway, resulting in opsonization and lysis of pathogens. In this study, we demonstrated that in addition to this indirect effect, L-ficolin has a direct neutralizing effect against hepatitis C virus (HCV) entry. Specific, dose-dependent binding of recombinant L-ficolin to HCV glycoproteins E1 and E2 was observed. This interaction was inhibited by soluble L-ficolin ligands. Interaction of L-ficolin with E1 and E2 potently inhibited entry of retroviral pseudoparticles bearing these glycoproteins. L-ficolin also inhibited entry of cell-cultured HCV in a calcium-dependent manner. Neutralizing concentrations of L-ficolin were found to be circulating in the serum of HCV-infected individuals. This is the first description of direct neutralization of HCV entry by a ficolin and highlights a novel role for L-ficolin as a virus entry inhibitor.

Dissociation and re-association studies on the interaction domains of mannan-binding lectin (MBL)-associated serine proteases, MASP-1 and MASP-2, provide evidence for heterodimer formation

Activation of the lectin pathway of complement begins with the activation of mannan-binding lectin (MBL)-associated serine proteases, MASP-1 and MASP-2, which are bound to the recognition molecules, MBL and ficolins. MASPs are Ca(2+)-dependent dimers. Dimerization and Ca(2+)-dependent association with the recognition molecules occurs via the first 3 domains, the CUB1-EGF-CUB2 region. The CUB1-EGF-CUB2 (D1-3) regions of MASP-1 and MASP-2, and also their tagged versions, were expressed in E. coli, refolded and purified. The first three domains of MASP-1 are identical with the respective regions of MASP-3 and MAp44, which are also associated with MBL and ficolins. The functionality of the fragments was checked by inhibition of C3 deposition from human serum. Time-course of the dissociation and re-association was examined by size exclusion chromatography. Both refolded proteins are tight Ca(2+)-dependent dimers, as expected. In buffer containing EDTA MASP-1_D1-3 dissociated to monomers, however it took about 1h to reach an equilibrium. Upon re-calcification dimers were re-formed, but this process was even slower; only after overnight incubation was the dimerization completed. MASP-2_D1-3 showed a somewhat different behavior: dissociation by EDTA was even slower, less complete, and higher MW aggregates also appeared. Heterodimer formation was detected by native PAGE. As modeled by the D1-3 fragments, MASP-1 and MASP-2 can readily form heterodimers after dissociation and re-association, however, in the presence of Ca(2+) exchange of subunits is slow between the homodimers. MASP-1:MASP-3 heterodimer formation was modeled by the tagged and untagged D1-3 fragments, and data indicate that subunits of these proteins are readily exchanged even in the presence of Ca(2+). The existence of heterodimers influences the current view on the composition of lectin pathway complexes and their activation.

Assay for estimation of the functional activity of the mannan-binding lectin pathway of the complement system

Mannan-binding lectin (MBL) is a soluble pattern recognition molecule of the innate immune system. It is found in plasma in complex with MBL-associated serine proteases (MASPs). When MBL recognizes foreign, e.g., the surface of some microorganisms, or altered host surfaces the MASPs are activated and this will in turn lead to the initiation of the complement system, i.e., activation of complement by the MBL pathway. This will end up in increased phagocytosis of the microorganism and killing by insertion of pore structures in the membrane of the microorganisms. Lack of MBL seems significant in specific situations, e.g., in immunocompromised individuals were MBL is important in battling infections and, e.g., in ischemia/reperfusion injuries were MBL can have a negative inflammatory generating, and thus tissue destructive role, as it recognizes epitopes emerging in the ischemic tissue. It may thus be relevant in several situations to test for the presence of the MBL pathway in human sera. Here we describe a functional assay for estimation of MBL pathway activity by detection of complement factor deposition onto microtiter plate wells coated with a physiological relevant ligand for MBL.

Human L-ficolin, a recognition molecule of the lectin activation pathway of complement, activates complement by binding to pneumolysin, the major toxin of Streptococcus pneumoniae

The complement system is an essential component of the immune response, providing a critical line of defense against different pathogens including S. pneumoniae. Complement is activated via three distinct pathways: the classical (CP), the alternative (AP) and the lectin pathway (LP). The role of Pneumolysin (PLY), a bacterial toxin released by S. pneumoniae, in triggering complement activation has been studied in vitro. Our results demonstrate that in both human and mouse sera complement was activated via the CP, initiated by direct binding of even non-specific IgM and IgG3 to PLY. Absence of CP activity in C1q(-/-) mouse serum completely abolished any C3 deposition. However, C1q depleted human serum strongly opsonized PLY through abundant deposition of C3 activation products, indicating that the LP may have a vital role in activating the human complement system on PLY. We identified that human L-ficolin is the critical LP recognition molecule that drives LP activation on PLY, while all of the murine LP recognition components fail to bind and activate complement on PLY. This work elucidates the detailed interactions between PLY and complement and shows for the first time a specific role of the LP in PLY-mediated complement activation in human serum.

Ficolin-2 reveals different analytical and biological properties dependent on different sample handling procedures

Ficolin-2 (L-ficolin) is a germ line encoded pattern recognition molecule circulating in the blood, and functions as a recognition molecule in the lectin complement pathway. However, consistent and reliable measurements of Ficolin-2 concentration and activity have been difficult to achieve. After recurrent observations of deviations in Ficolin-2 properties between different blood sample procedures, we decided to investigate this closer. Blood samples from ten healthy donors were collected in various serum and plasma tubes and Ficolin-2 properties were evaluated by different ELISA setups. We found that serum prepared from tubes containing the clot activator silica used as a standard technique in many routine laboratories held a significantly lower concentration of Ficolin-2 as compared to the other sample types. Furthermore, Ficolin-2 binding and complement activation potential in this type of serum was impaired when using an acetylated compound as matrix. On the other hand, Ficolin-2 in serum made without clot activator and in plasma irrespective of additive used, had the same concentration and was capable of initiating the lectin pathway measured as C4 and C3 deposition on the ligand. No Ficolin-2 mediated formation of the terminal complement complex was observed under the applied assay conditions. In conclusion, our results show that Ficolin-2 is a promiscuous molecule and that care should be taken during sampling, handling and matrix chosen for measurement of Ficolin-2 levels and activity.

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.

The role of ficolins and MASPs in hereditary angioedema due to C1-inhibitor deficiency

BACKGROUND AND OBJECTIVE

Hereditary angioedema due to C1-inhibitor deficiency (HAE-C1-INH) causes disturbances in the complement system. However, the influence of HAE-C1-INH on the lectin pathway of complement is unresolved. Thus, we studied the main initiator molecules, enzymes and regulators in the lectin pathway in patients with HAE-C1-INH.

METHODS

The serum concentrations of ficolin-2, ficolin-3, MBL, MASP-2, MASP-3, and MAP-1 were measured during symptom-free periods in 91 patients with HAE-C1-INH, and in 100 healthy controls using sandwich ELISAs.

RESULTS

Compared with controls, the levels of ficolin-2 (p<0.0001) and MASP-2 (p=0.0238) were reduced, while the levels of MBL and MASP-3 were elevated (p=0.0028 and p<0.0001, respectively) in HAE-C1-INH patients. Ficolin-3 and MAP-1 levels did not differ significantly between the two groups. Ficolin-2 correlated with MASP-3 in patients (r=0.3443, p=0.0008), while these parameters showed an opposite relationship in controls (r=-0.4625, p<0.0001). In the patients, ficolin-3 correlated with MASP-2 (r=0.3698, p=0.001). Ficolin-2, -3, and MAP-1 correlated negatively with the annual requirement of plasma derived C1-INH concentrate (r=-0.2863, p=0.0059; r=-0.2654, p=0.0110 and r=-0.2501, p=0.0168, respectively). Ficolin-3 showed a negative correlation with the annual number of attacks (r=-0.2478, p=0.0179).

CONCLUSIONS

We found significant differences between patients and controls in the levels of some of the molecules belonging to the lectin complement pathway. Low concentrations of particularly ficolin-2 and -3 were inversely correlated with the severity of HAE-C1-INH, while this was not observed for MBL. This suggests a previously unrecognized involvement of the ficolin-dependent lectin complement pathway in the pathophysiology of HAE-C1-INH.

Association of ficolin-3 with severity and outcome of chronic heart failure

BACKGROUND

Inflammatory mechanisms involving complement activation has been shown to take part in the pathophysiology of congestive heart failure, but the initiating mechanisms are unknown. We hypothesized that the main initiator molecules of the lectin complement pathway mannose-binding lectin (MBL), ficolin-2 and ficolin-3 were related to disease severity and outcome in chronic heart failure.

METHODS AND RESULTS

MBL, ficolin-2 and ficolin-3 plasma concentrations were determined in two consecutive cohorts comprising 190 patients from Hungary and 183 patients from Norway as well as controls. Disease severity and clinical parameters were determined at baseline, and all-cause mortality was registered after 5-years follow-up. In univariate analysis a low level of ficolin-3, but not that of MBL or ficolin-2, was significantly associated with advanced heart failure (New York Heart Association Class IV, p<0.001 for both cohorts) and showed inverse correlation with B- type natriuretic peptide (BNP) levels (r = -0.609, p<0.001 and r = -0.467, p<0.001, respectively). In multivariable Cox regression analysis, adjusted for age, gender and BNP, decreased plasma ficolin-3 was a significant predictor of mortality (HR 1.368, 95% CI 1.052-6.210; and HR 1.426, 95% CI 1.013-2.008, respectively). Low ficolin-3 levels were associated with increased complement activation product C3a and correspondingly decreased concentrations of complement factor C3.

CONCLUSIONS

This study provides evidence for an association of low ficolin-3 levels with advanced heart failure. Concordant results from two cohorts show that low levels of ficolin-3 are associated with advanced heart failure and outcome. The decrease of ficolin-3 was associated with increased complement activation.

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.

Mitochondria and the lectin pathway of complement

Mitochondria, the powerhouses of our cells, are remnants of a eubacterial endosymbiont. Notwithstanding the evolutionary time that has passed since the initial endosymbiotic event, mitochondria have retained many hallmarks of their eubacterial origin. Recent studies have indicated that during perturbations of normal homeostasis, such as following acute trauma leading to massive necrosis and release of mitochondria, the immune system might mistake symbiont for enemy and initiate an inappropriate immune response. The innate immune system is the first line of defense against invading microbial pathogens, and as such is the primary suspect in the recognition of mitochondria-derived danger-associated molecular patterns and initiation of an aberrant response. Conversely, innate immune mechanisms are also central to noninflammatory clearance of innocuous agents. Here we investigated the role of a central humoral component of innate immunity, the lectin pathway of complement, in recognition of mitochondria in vitro and in vivo. We found that the soluble pattern recognition molecules, mannan-binding lectin (MBL), L-ficolin, and M-ficolin, were able to recognize mitochondria. Furthermore, MBL in complex with MBL-associated serine protease 2 (MASP-2) was able to activate the lectin pathway and deposit C4 onto mitochondria, suggesting that these molecules are involved either in homeostatic clearance of mitochondria or in induction of untoward inflammatory reactions. We found that following mitochondrial challenge, C3 was consumed in vivo in the absence of overt inflammation, indicating a potential role of complement in noninflammatory clearance of mitochondria. Thus, we report here the first indication of involvement of the lectin pathway in mitochondrial immune handling.

The potential role of the lectin pathway of complement in the host defence of full-term intrauterine growth restricted neonates at birth

OBJECTIVE

To prospectively investigate the potential role of the lectin pathway of complement in intrauterine-growth-restriction (IUGR, associated with impaired immunocompetence and increased risk for neonatal infections), by determining cord blood concentrations of mannose-binding lectin (MBL), H-ficolin and L-ficolin (important mediators of neonatal innate immunity) in IUGR and appropriate for gestational age (AGA) pregnancies. Furthermore, we aimed to describe correlations among cord blood MBL, H- and L-ficolin concentrations and with several demographic parameters of the infants at birth.

METHODS

Serum MBL, H- and L-ficolin concentrations were determined by ELISA in 154 mixed arteriovenous cord blood samples from IUGR (n = 50) and AGA (n = 104) singleton full-term infants.

RESULTS

Cord blood MBL concentrations were significantly lower in IUGR cases than AGA controls (p = 0.029). No differences in cord blood H- and L-ficolin concentrations were observed between groups. In the IUGR group, cord blood MBL concentrations negatively correlated with respective L-ficolin ones (r = -0.442, p = 0.001).

CONCLUSIONS

The relatively decreased MBL expression in IUGR fetuses at term could possibly contribute to IUGR-associated neonatal immunodeficiency, predisposing to increased susceptibility to infections. The negative correlation between MBL and L-ficolin concentrations in the IUGR group might suggest an underlying immune variation and needs to be further investigated.

A tick mannose-binding lectin inhibitor interferes with the vertebrate complement cascade to enhance transmission of the lyme disease agent

The Lyme disease agent Borrelia burgdorferi is primarily transmitted to vertebrates by Ixodes ticks. The classical and alternative complement pathways are important in Borrelia eradication by the vertebrate host. We recently identified a tick salivary protein, designated P8, which reduced complement-mediated killing of Borrelia. We now discover that P8 interferes with the human lectin complement cascade, resulting in impaired neutrophil phagocytosis and chemotaxis and diminished Borrelia lysis. Therefore, P8 was renamed the tick salivary lectin pathway inhibitor (TSLPI). TSLPI-silenced ticks, or ticks exposed to TSLPI-immune mice, were hampered in Borrelia transmission. Moreover, Borrelia acquisition and persistence in tick midguts was impaired in ticks feeding on TSLPI-immunized, B. burgdorferi-infected mice. Together, our findings suggest an essential role for the lectin complement cascade in Borrelia eradication and demonstrate how a vector-borne pathogen co-opts a vector protein to facilitate early mammalian infection and vector colonization.

Serum concentrations of lectin-pathway components in healthy neonates, children and adults: mannan-binding lectin (MBL), M-, L-, and H-ficolin, and MBL-associated serine protease-2 (MASP-2)

This study aimed to measure serum concentrations of five lectin-pathway components, mannan-binding lectin (MBL), M-ficolin, L-ficolin, H-ficolin, and MBL-associated serine protease-2 (MASP-2), in healthy neonates and children, to determine if they change with age and to compare them with serum concentrations in healthy adults. Concentrations were measured in 141 preterm and 30 term neonates, in 120 children including infants and adolescents, and in 350 adults (97 for L-ficolin) by inhouse time-resolved immunofluorometric assays or commercially available enzyme-linked immunosorbent assays. The adjacent categories method applying Wilcoxon-Mann-Whitney tests was used to determine age categories where concentrations differed significantly. Displaying serum concentration vs. age, an inverted-U shape (higher concentrations in children than in neonates and adults) was found for MBL and the ficolins, and an S-shape for MASP-2. Serum concentrations of all five lectin-pathway components were significantly lower in preterm neonates <32-wk gestational age compared to older neonates, infants, and children. Only M-ficolin in children >1 yr and H-ficolin in term neonates and in children were found to be comparable with adult values. MBL, M-, L-, and H-ficolin, and MASP-2 serum concentrations show important changes with age. The respective normal ranges for adults should not be used in the pediatric population. The age-specific pediatric ranges established here may be used instead.

M-ficolin in children with cancer

OBJECTIVES

M-ficolin (ficolin-1) is a complement-activating pattern-recognition molecule structurally related to mannan-binding lectin. It is produced by monocytes and neutrophils, and is found in serum. Its biological role is largely unknown. We assessed M-ficolin concentration in serum from pediatric cancer patients. The aim of this study was to explore association of M-ficolin with clinical and hematological parameters, and to investigate whether the risk of chemotherapy-related infections was related to M-ficolin concentrations in serum.

METHODS

M-ficolin was measured by time-resolved immunofluorometric assay in serum taken at cancer diagnosis and was correlated with peripheral blood counts and bone marrow examinations performed at the same time.

RESULTS

Median M-ficolin concentration in 94 children with cancer was 1.6 μg/mL (interquartile range, 0.57-2.7; range, 0.055-25.8), and was not different from age-matched controls (median, 1.7 μg/mL; p=0.92). M-ficolin was strongly associated with absolute counts of neutrophils (Spearman's rho, 0.45; 95%-CI, 0.26-0.65; p<0.001), monocytes (0.34; 0.12-0.55; p<0.001), and thus phagocytes (0.42; 0.20-0.63; p<0.001) in peripheral blood. Similarly, M-ficolin correlated strongly with neutrophils (0.36; 0.14-0.59; p=0.002) and phagocytes (0.31; 0.08-0.54; p=0.009) in bone marrow. Low serum M-ficolin (≤0.5 μg/mL) was not associated with an increased incidence of fever in neutropenia during chemotherapy (multivariate Poisson rate ratio, 1.04; 95%-CI, 0.68-1.60; p=0.85).

CONCLUSIONS

The concentration of M-ficolin in serum from children with cancer was strongly associated with neutrophil and monocyte counts in blood and bone marrow. These results suggest that M-ficolin concentrations in serum reflect the pool of phagocytes.

Serum concentration and interaction properties of MBL/ficolin associated protein-1

Recently, a novel protein named MBL/ficolin associated protein-1 (MAP-1) derived from the MASP1 gene through differential splicing was identified. In the present study, we established biochemical characteristics, determined the serum level and assessed the interactions between the lectin complement pathway (LCP) recognition molecules and MAP-1. We expressed recombinant MAP-1 in CHO DG44 cells, developed a quantitative ELISA assay based on a MAP-1 specific monoclonal capture antibody and measured the serum levels in 100 Danish blood donors. In addition we assessed the association properties between MAP-1 and Ficolin-2, -3 and MBL in serum using ELISA and density gradient ultra centrifugation. When recombinant MAP-1 was subjected to N-glycosidase F treatment the molecular mass decreased from ∼45 kDa to ∼40 kDa equivalent with the calculated molecular mass from the deduced amino acid sequence without the signal peptide. We found that serum MAP-1 was very stable when subjected to repeated freeze and thaw cycles. The mean serum concentration of MAP-1 was found to be 240 ng/ml (range: 115-466 ng/ml). MAP-1 was predominantly found in complex with Ficolin-3 and to a lesser degree with Ficolin-2 and MBL and by use of density gradient ultra centrifugation we could show that the major part of serum MAP-1 circulates in complex with the LCP molecules. In conclusion, these results show that MAP-1 is a highly stable glycosylated human serum protein found in complex with Ficolin-3, Ficolin-2 and MBL.

Proteomic profiling of blood-dialyzer interactome reveals involvement of lectin complement pathway in hemodialysis-induced inflammatory response

PURPOSE

dialysis-induced inflammatory response including leukocyte and complement activation is considered a significant cofactor of chronic morbidity in long-term hemodialysis (HD) patients. The aim of this study was to provide better insight into its molecular background.

EXPERIMENTAL DESIGN

in 16 patients, basic biocompatibility markers, i.e. leukocyte counts and C5a levels, were monitored during HD on a polysulfone membrane. Proteins adsorbed to dialyzers were eluted and separated by 2-DE. Selected proteins were identified by MS; ficolin-2 plasma levels were assessed. Data are given as medians (quartile ranges).

RESULTS

in total, 7.2 (34.7) mg proteins were retrieved from dialyzer eluates and were resolved into 217 protein spots. The proteins most enriched in eluates (and hence selectively adsorbed) were those involved in complement activation (C3c, ficolin-2, mannan-binding lectin serine proteases, properdin) and cell adhesion (actin, caldesmon, tropomyosin, vitronectin, vinculin). A significant decrease of plasma ficolin-2 (41% [4.7], p<0.001) was evidenced during one HD session, associated with leukopenia (r=0.73, p=0.001) and C5a production (r=-0.62, p=0.01) at 15 min.

CONCLUSIONS AND CLINICAL RELEVANCE

ficolin-2 adsorption to polysulfone dialyzer initiates the lectin pathway of complement activation, mediates dialysis-induced leukopenia, and results in a significant depletion of ficolin-2, an essential component of innate immunity.

Involvement of lectin pathway activation in the complement killing of Giardia intestinalis

Giardia intestinalis (syn. G. lamblia, G. duodenalis) is a flagellated unicellular eukaryotic microorganism that commonly causes diarrheal disease throughout the world. In humans, the clinical effects of Giardia infection range from the asymptomatic carrier state to a severe malabsorption syndrome possibly due to different virulence of the Giardia strain, the number of cysts ingested, the age of the host, and the state of the host immune system at the time of infection. The question about how G. intestinalis is controlled by the organism remains unanswered. Here, we investigated the role of the complement system and in particular, the lectin pathway during Giardia infections. We present the first evidence that G. intestinalis activate the complement lectin pathway and in doing so participate in eradication of the parasite. We detected rapid binding of mannan-binding lectin, H-ficolin and L-ficolin to the surface of G. intestinalis trophozoites and normal human serum depleted of these molecules failed to kill the parasites. Our finding provides insight into the role of lectin pathway in the control of G. intestinalis and about the nature of surface components of parasite.

Mechanisms of complement activation, C4d deposition, and their contribution to the pathogenesis of antibody-mediated rejection

Complement split products have emerged as useful markers of antibody-mediated rejection in solid organ transplants. One split product, C4d, is now widely accepted as a marker for antibody-mediated rejection in renal and cardiac allografts. This review summarizes the rationale for the use of C4d as a marker of antibody-mediated rejection, along with the clinical evidence supporting its use in the clinical diagnosis of antibody-mediated rejection. Antibody-independent mechanisms by which C4d can be activated by the classical and lectin pathways of complement activation are also identified. Finally, mechanisms by which complement activation stimulates effector cells (neutrophils, monocytes, macrophages, platelets, and B and T lymphocytes) as well as target cells (endothelial cells) are discussed in relation to antibody-mediated allograft rejection.

Local inflammation induces complement crosstalk which amplifies the antimicrobial response

By eliciting inflammatory responses, the human immunosurveillance system notably combats invading pathogens, during which acute phase proteins (CRP and cytokines) are elevated markedly. However, the Pseudomonas aeruginosa is a persistent opportunistic pathogen prevalent at the site of local inflammation, and its acquisition of multiple antibiotic-resistance factors poses grave challenges to patient healthcare management. Using blood samples from infected patients, we demonstrate that P. aeruginosa is effectively killed in the plasma under defined local infection-inflammation condition, where slight acidosis and reduced calcium levels (pH 6.5, 2 mM calcium) typically prevail. We showed that this powerful antimicrobial activity is provoked by crosstalk between two plasma proteins; CRPratioL-ficolin interaction led to communication between the complement classical and lectin pathways from which two amplification events emerged. Assays for C4 deposition, phagocytosis, and protein competition consistently proved the functional significance of the amplification pathways in boosting complement-mediated antimicrobial activity. The infection-inflammation condition induced a 100-fold increase in CRPratioL-ficolin interaction in a pH- and calcium-sensitive manner. We conclude that the infection-induced local inflammatory conditions trigger a strong interaction between CRPratioL-ficolin, eliciting complement-amplification pathways which are autonomous and which co-exist with and reinforce the classical and lectin pathways. Our findings provide new insights into the host immune response to P. aeruginosa infection under pathological conditions and the potential development of new therapeutic strategies against bacterial infection.

The complement system in schizophrenia

Several lines of evidence suggest that immunological factors contribute to schizophrenia. Since 1989, the role of complement, a major effector of innate immunity and an adjuvant of adaptive immunity, has been explored in schizophrenia. Increased activity of C1, C3, C4 complement components in schizophrenia has been reported by two or more groups. Two studies on different subject cohorts showed increased MBL-MASP-2 activity in patients versus controls. More then one report indicated a significant high frequency of FB*F allotype and low prevalence of the FS phenotype of complement factor B in schizophrenia. From the data reported, it is likely that the disorder is accompanied by alterations of the complement classical and lectin pathways, which undergo dynamic changes, depending on the illness course and the state of neuro-immune crosstalk. Recent findings, implicating complement in neurogenesis, synapse remodeling and pruning during brain development, suggest a reexamination of the potential role of complement in neurodevelopmental processes contributing to schizophrenia susceptibility. It is plausible that the multicomponent complement system has more than one dimensional association with schizophrenia susceptibility, pathopsychology and illness course, understanding of which will bring a new perspective for possible immunomodulation and immunocorrection of the disease.

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.

Severe fibrosis in hepatitis C virus-infected patients is associated with increased activity of the mannan-binding lectin (MBL)/MBL-associated serine protease 1 (MASP-1) complex

Mannan-binding lectin (MBL) binds microorganisms via interactions with glycans on the target surface. Bound MBL subsequently activates MBL-associated serine protease proenzymes (MASPs). A role for MBL in hepatitis C virus (HCV) infection had been indicated by previous studies examining MBL levels and polymorphisms in relation to disease progression and response to treatment. We undertook this study to investigate a possible relationship between disease progression and functional MBL/MASP-1 complex activity. A functional assay for MBL/MASP-1 complex activity was employed to examine serum samples from patients with chronic HCV infection, non-HCV liver disease and healthy controls. Intrapatient consistency of MBL/MASP-1 complex activity levels was assessed in sequential samples from a subgroup of patients. Median values of MBL/MASP-1 complex activity were higher in sera from patients with liver disease compared with healthy controls. MBL/MASP-1 complex activity levels correlate with severity of fibrosis after adjusting for confounding factors (P = 0.003). MBL/MASP-1 complex activity was associated more significantly with fibrosis than was MBL concentration. The potential role of MBL/MASP-1 complex activity in disease progression is worthy of further study to investigate possible mechanistic links.

Increased complement classical and mannan-binding lectin pathway activities in schizophrenia

Schizophrenia is a severe mental disorder, with worldwide prevalence of 1-1.5%. Immunological research in schizophrenia indicates that infectious or autoimmune processes might play a role in the etiopathogenesis. The complement system is a major mediator of innate immune defence against infection and contributes to many functions of the immune system including inflammation, opsonization and cell lysis. Mannan-binding lectin (MBL) activates the complement system via the lectin pathway. Inherited MBL deficiency, common in most human populations, predisposes to infectious and autoimmune diseases. We measured total complement activity (CH50), C4 activity (C4 CH50), MBL level and the activities of MBL-associated serine proteases, MASP-1 and MASP-2 in sera of 45 schizophrenic patients and in 62 healthy volunteers. We found that schizophrenic patients and healthy volunteers have statistically similar MBL levels and MASP-1 activity. However, MBL-bound MASP-2 activity and therefore MBL and MASP-2-mediated complement activation capacity is increased in schizophrenic patients compared with healthy volunteers (P<0.01). The increase was accompanied by increased CH50 (P<0.02) and C4 CH50 (P<0.02). Our results support the idea that complement system alterations may be involved in schizophrenia.