Co-complexes of MASP-1 and MASP-2 associated with the soluble pattern-recognition molecules drive lectin pathway activation in a manner inhibitable by MAp44

Serum Proteomic Signatures in Cervical Cancer: Current Status and Future Directions

In 2020, the World Health Organization (WHO) reported 604,000 new diagnoses of cervical cancer (CC) worldwide, and over 300,000 CC-related fatalities. The vast majority of CC cases are caused by persistent human papillomavirus (HPV) infections. HPV-related CC incidence and mortality rates have declined worldwide because of increased HPV vaccination and CC screening with the Papanicolaou test (PAP test). Despite these significant improvements, developing countries face difficulty implementing these programs, while developed nations are challenged with identifying HPV-independent cases. Molecular and proteomic information obtained from blood or tumor samples have a strong potential to provide information on malignancy progression and response to therapy in CC. There is a large amount of published biomarker data related to CC available but the extensive validation required by the FDA approval for clinical use is lacking. The ability of researchers to use the big data obtained from clinical studies and to draw meaningful relationships from these data are two obstacles that must be overcome for implementation into clinical practice. We report on identified multimarker panels of serum proteomic studies in CC for the past 5 years, the potential for modern computational biology efforts, and the utilization of nationwide biobanks to bridge the gap between multivariate protein signature development and the prediction of clinically relevant CC patient outcomes.

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.

Comprehensive analysis of subtypes and risk model based on complement system associated genes in ccRCC

BACKGROUND

Immune therapy is widely used in treating clear cell renal cell carcinoma (ccRCC), yet identifying patient subgroups that are expected to response remains challenging. As complement system can mediate immune effects, including the progression of tumors, a correlation between complement system and immune therapy may exist.

METHODS

Based on 11 complement system associated genes (CSAGs) identified from The Cancer Genome Atlas (TCGA), we performed unsupervised clustering and classified the tumors into two different complement system (CS) patterns. The clinical significance, tumor microenvironment (TME), functional enrichment, and immune infiltration were further analyzed. A novel scoring system named CSscore was developed based on the expression levels of the 11 CSAGs.

RESULTS

Two distinct CS patterns were identified, classified as Cluster1 and Cluster2, and Cluster1 showed poor clinical outcome. Further analysis of functional enrichment, immune cell infiltration, and genetic variation revealed that Cluster1 had high infiltration of TME immune cells, but also exhibited high immune escape. The novel prognostic model, CSscore could act as an independent prognostic factor and effectively predict patients' prognosis and distinguish the therapeutic efficacy of different immune treatment strategies. The pan-cancer analysis of the CSscore indicates its potential to be further generalized to other types of cancer.

CONCLUSIONS

Two distinct CS patterns were identified and were further analyzed in terms of infiltration of TME immune cells and immune escape, providing potential explanations for the impact on prognosis of ccRCC. Our CSscore prognostic model may offer a novel perspective in the management of ccRCC patients, and potentially other types of cancer as well.

Factor D

Complement factor D (FD) is a serine protease that plays an essential role in the activation of the alternative pathway (AP) by cleaving complement factor B (FB) and generating the C3 convertases C3(H₂ O)Bb and C3bBb. FD is produced mainly from adipose tissue and circulates in an activated form. On the contrary, the other serine proteases of the complement system are mainly synthesized in the liver. The activation mechanism of FD has long been unknown. Recently, a serendipitous discovery in the mechanism of FD activation has been provided by a generation of Masp1 gene knockout mice lacking both the serine protease MASP-1 and its alternative splicing variant MASP-3, designated MASP-1/3-deficient mice. Sera from the MASP-1/3-deficient mice had little-to-no lectin pathway (LP) and AP activity with circulating zymogen or proenzyme FD (pro-FD). Sera from patients with 3MC syndrome carrying mutations in the MASP1 gene also had circulating pro-FD, suggesting that MASP-1 and/or MASP-3 are involved in activation of FD. Here, we summarize the current knowledge of the mechanism of FD activation that was finally elucidated using the sera of mice monospecifically deficient for MASP-1 or MASP-3. Sera of the MASP-1-deficient mice lacked LP activity, but those of the MASP-3-deficient mice lacked AP activity with pro-FD. This review illustrates the pivotal role of MASP-3 in the physiological activation of the AP via activation of FD.

The complex formation of MASP-3 with pattern recognition molecules of the lectin complement pathway retains MASP-3 in the circulation

The complement system plays an important role in host defense and is activated via three different activation pathways. We have previously reported that mannose-binding lectin-associated serine protease (MASP)-3, unlike its splicing variant MASP-1, circulates in an active form and is essential for the activation of the alternative pathway (AP) via the activation of complement factor D (FD). On the other hand, like MASP-1 and MASP-2 of the lectin pathway (LP), MASP-3 forms a complex with the pattern recognition molecules (PRMs) of the LP (LP-PRMs). Both MASP-1 and MASP-2 can be activated efficiently when the LP-PRMs complexed with them bind to their ligands. On the other hand, it remains unclear how MASP-3 is activated, or whether complex formation of MASP-3 with LP-PRMs is involved in activation of MASP-3 or its efficiency in the circulation. To address these issues, we generated wild-type (WT) and four mutant recombinant mouse MASP-3 proteins fused with PA (human podoplanin dodecapeptide)-tag (rmMASP-3-PAs), the latter of which have single amino acid substitution for alanine in the CUB1 or CUB2 domain responsible for binding to LP-PRMs. The mutant rmMASP-3-PAs showed significantly reduced in-vivo complex formation with LP-PRMs when compared with WT rmMASP-3-PA. In the in-vivo kinetic analysis of MASP-3 activation, both WT and mutant rmMASP-3-PAs were cleaved into the active forms as early as 30 minutes in the circulation of mice, and no significant difference in the efficiency of MASP-3 cleavage was observed throughout an observation period of 48 hours after intravenous administration. All sera collected 3 hours after administration of each rmMASP-3-PA showed full restoration of the active FD and AP activity in MASP-3-deficient mouse sera at the same levels as WT mouse sera. Unexpectedly, all mutant rmMASP-3-PAs showed faster clearance from the circulation than the WT rmMASP-3-PA. To our knowledge, the current study is the first to show in-vivo kinetics of MASP-3 demonstrating rapid activation and clearance in the circulation. In conclusion, our results demonstrated that the complex formation of MASP-3 with LP-PRMs is not required for in-vivo activation of MASP-3 or its efficiency, but may contribute to the long-term retention of MASP-3 in the circulation.

Tinker, tailor, soldier, cell: the role of C-type lectins in the defense and promotion of disease

C-type lectins (CTLs) represent a large family of soluble and membrane-bound proteins which bind calcium dependently via carbohydrate recognition domains (CRDs) to glycan residues presented on the surface of a variety of pathogens. The deconvolution of a cell's glycan code by CTLs underpins several important physiological processes in mammals such as pathogen neutralization and opsonization, leukocyte trafficking, and the inflammatory response. However, as our knowledge of CTLs has developed it has become apparent that the role of this innate immune family of proteins can be double-edged, where some pathogens have developed approaches to subvert and exploit CTL interactions to promote infection and sustain the pathological state. Equally, CTL interactions with host glycoproteins can contribute to inflammatory diseases such as arthritis and cancer whereby, in certain contexts, they exacerbate inflammation and drive malignant progression. This review discusses the 'dual agent' roles of some of the major mammalian CTLs in both resolving and promoting infection, inflammation and inflammatory disease and highlights opportunities and emerging approaches for their therapeutic modulation.

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.

The Complement System in Spondyloarthritis: What Do We Know?

Spondyloarthritis (SpA) encompasses a group of rheumatologic diseases, including axial spondyloarthritis (axSpA), psoriatic arthritis, arthritis with associated inflammatory bowel disease (i.e. Crohn’s disease and ulcerative colitis), reactive arthritis and undifferentiated SpA, which all share certain clinical, biological and genetic features. However, the pathogenesis remains largely unexplained. Recent evidence suggests an autoinflammatory component of the disease. The complement system is a cornerstone of the innate immune system. This review aims to evaluate the current knowledge of the complement system in SpA. Animal models have shown that complement activation is associated with axSpA. Complement proteins L-ficolin and H-ficolin levels are elevated in patients with axSpA, and complement factor C3 levels decrease after the initiation of tumour necrosis factor-inhibitor therapy. Associations with disease activity are inconsistent, as one study found that the serum levels of complement factors C3 and C4 did not differ in patients with different Bath Ankylosing Spondylitis Disease Activity Index scores but, in another study, were associated with baseline Ankylosing Spondylitis Disease Activity Score with C-reactive protein and Bath Ankylosing Spondylitis Disease Activity Index improvement after treatment with a tumour necrosis factor inhibitor. Future studies should focus on the complement system in various SpA entities, involvement in pathogenesis and disease progression under clinically relevant conditions.

Advances in immunological research of amphioxus

Amphioxus, one of the most closely related invertebrates to vertebrates, is an important animal model for studying the origin and evolution of vertebrate immunity, especially the transition from innate immunity to adaptive immunity. The current research progresses of amphioxus in the field of immune organs, immune cells, complement system, cytokines, nuclear factor kappa B, immune-related lectins and enzymes are summarized, and some issues that remain to be understood or are in need of further clarification are highlighted. We hope to provide references for more in-depth study of the amphioxus immune system and lay a solid foundation for the construction of three-dimensional immune network in amphioxus from ontogeny to phylogeny.

MAp34 Regulates the Non-specific Cell Immunity of Monocytes/Macrophages and Inhibits the Lectin Pathway of Complement Activation in a Teleost Fish

The lectin pathway of the complement system is one of the main components of innate immunity, which plays a pivotal role in the defense against infectious microorganisms and maintains immune homeostasis. However, its control mechanisms remain unclear in teleost fish. In this study, we described the identification and functional characterization of a mannose-binding lectin associated protein MAp34 (OnMAp34) from Nile tilapia (Oreochromis niloticus) at molecular, cellular, and protein levels. The open reading frame (ORF) of OnMAp34 is 918 bp of nucleotide sequence encoding a polypeptide of 305 amino acids. The deduced amino acid sequence has three characteristic structures, including two C1r/C1s-Uegf-BMP domains (CUB) and one epidermal growth factor domain (EGF). Expression analysis revealed that the OnMAp34 was highly expressed in the liver and widely existed in other examined tissues. In addition, the mRNA and protein expression levels of OnMAp34 were remarkably altered upon infection with Streptococcus agalactiae and Aeromonas hydrophila in vivo and in vitro. Further, we found that the OnMAp34 could participate in the non-specific cellular immune defense, including the regulation of inflammation, migration, and enhancement of phagocytosis of monocytes/macrophages. Moreover, the OnMAp34 could compete with OnMASPs to combine OnMBL and inhibit the lectin pathway of complement activation. Overall, our results provide new insights into the understanding of MAp34 as a potent regulator in the lectin complement pathway and non-specific cell immunity in an early vertebrate.

Revisiting the complement system in systemic lupus erythematosus

Introduction: Systemic lupus erythematosus (SLE) is a multi-system autoimmune disease, characterized by the production of autoantibodies. Numerous mechanisms contribute to the pathogenesis and autoimmunity in SLE. One of the most important mechanisms is the defective function of the early complement components that are involved in clearing the immune-complexes and apoptotic debris. Major evidence supporting this hypothesis is the development of severe lupus in individuals with monogenic defects in any one of the early complement components such as C1q, C1 s, C1 r, C2, or C4.Areas covered: In this review, we discuss hereditary defects in classical complement components and their clinical manifestations, acquired defects of complements in lupus, the role of complements in the pathogenesis of antiphospholipid antibody syndrome and lupus nephritis, and laboratory assessment of complement components and their functions. Articles from the last 20 years were retrieved from PubMed for this purpose.Expert opinion: Complements have a dual role in the pathogenesis of SLE. On one hand, deficiency of complement components predisposes to lupus, while, on the other, excess complement activation plays a role in the organ damage. Understanding the intricacies of the role of complements in SLE can pave way for the development of targeted therapies.

A novel complement inhibitor sMAP-FH targeting both the lectin and alternative complement pathways

Inhibition of the complement activation has emerged as an option for treatment of a range of diseases. Activation of the lectin and alternative pathways (LP and AP, respectively) contribute to the deterioration of conditions in certain diseases such as ischemia-reperfusion injuries and age-related macular degeneration (AMD). In the current study, we generated dual complement inhibitors of the pathways MAp44-FH and sMAP-FH by fusing full-length MAp44 or small mannose-binding lectin-associated protein (sMAP), LP regulators, with the N-terminal five short consensus repeat (SCR) domains of complement factor H (SCR1/5-FH), an AP regulator. The murine forms of both fusion proteins formed a complex with endogenous mannose-binding lectin (MBL) or ficolin A in the circulation when administered in mice intraperitoneally. Multiple complement activation assays revealed that sMAP-FH had significantly higher inhibitory effects on activation of the LP and AP in vivo as well as in vitro compared to MAp44-FH. Human form of sMAP-FH also showed dual inhibitory effects on LP and AP activation in human sera. Our results indicate that the novel fusion protein sMAP-FH inhibits both the LP and AP activation in mice and in human sera, and could be an effective therapeutic agent for diseases in which both the LP and AP activation are significantly involved.

Exercise, the Gut Microbiome, and Frailty

The gut microbiome is deeply associated with both skeletal muscle and brain function. In particular, gut microbiome dysbiosis may accelerate age-related diseases by affecting these systems. Although there is increasing evidence of the correlations between the gut microbiome and skeletal muscle and brain, it remains unclear whether changes in the gut microbiome due to exercise training can lead to healthy aging. This review covers the current status of gut microbiome-related research and future directions related to aging (e.g., physical frailty and cognitive dysfunction) as well as the effect of exercise training on both. We reviewed relevant literature including original articles and reviews identified from searches of the PubMed, Google Scholar, SCOPUS, EBSCOHost, ScienceDirect, Cochrane Library, and EMBASE databases using the following terms: 'gut microbiome', 'exercise', 'physical frailty', and 'cognitive dysfunction'. We identified a strong positive correlation between cognitive dysfunction or physical frailty and the gut microbiome. Furthermore, exercise had a significant effect on the composition of the gut microbiome. These results suggest that exercise training can prevent physical frailty or cognitive dysfunction by altering the gut microbiome. However, the exact mechanism by which these effects occur is not yet clear. Further studies are needed to determine whether exercise training can prevent age-related diseases by balancing the gut microbiome.

Gut Microbiota Disorder, Gut Epithelial and Blood-Brain Barrier Dysfunctions in Etiopathogenesis of Dementia: Molecular Mechanisms and Signaling Pathways

Emerging evidences indicate a critical role of the gut microbiota in etiopathogenesis of dementia, a debilitating multifactorial disorder characterized by progressive deterioration of cognition and behavior that interferes with the social and professional functions of the sufferer. Available data suggest that gut microbiota disorder that triggers development of dementia is characterized by substantial reduction in specific species belonging to the Firmicutes and Bacteroidetes phyla and presence of pathogenic species, predominantly, pro-inflammatory bacteria of the Proteobacteria phylum. These changes in gut microbiota microecology promote the production of toxic metabolites and pro-inflammatory cytokines, and reduction in beneficial substances such as short chain fatty acids and other anti-inflammatory factors, thereby, enhancing destruction of the gut epithelial barrier with concomitant activation of local and distant immune cells as well as dysregulation of enteric neurons and glia. This subsequently leads to blood-brain barrier dysfunctions that trigger neuroinflammatory reactions and predisposes to apoptotic neuronal and glial cell death, particularly in the hippocampus and cerebral cortex, which underlie the development of dementia. However, the molecular switches that control these processes in the histo-hematic barriers of the gut and brain are not exactly known. This review integrates very recent data on the molecular mechanisms that link gut microbiota disorder to gut epithelial and blood-brain barrier dysfunctions, underlying the development of dementia. The signaling pathways that link gut microbiota disorder with impairment in cognition and behavior are also discussed. The review also highlights potential therapeutic options for dementia.

Factors involved in initiation and regulation of complement lectin pathway influence postoperative outcome after pediatric cardiac surgery involving cardiopulmonary bypass

Congenital heart disease (CHD) often requires surgical intervention, and is sometimes associated with life-threatening post-operative complications. We have investigated some factors of the innate immune system involved in the initiation or regulation of complement lectin pathway activation (MASP-1, MASP-2 MASP-3, MAp19, MAp44, ficolin-3) and related them to complications and prognosis in 190 pediatric patients undergoing CHD repair with the use of cardiopulmonary bypass (CPB). Patients with MAp44 levels ≤1.81 µg/ml more frequently experienced low cardiac output syndrome (LCOS), renal insufficiency, systemic inflammatory response syndrome (SIRS) and multiorgan dysfunction (MODS). Low MASP-3 (≤5.18 µg/ml) and high MASP-1 (≥11.7 µg/ml) levels were often associated with fatal outcome. Low ficolin-3 concentrations (≤10.1 µg/ml) were more common among patients experiencing SIRS and MODS than in those without complications. However, patients suffering from SIRS and MODS with low ficolin-3 had a much better prognosis (91% survival vs. 37% among other patients; p = 0.007). A discriminating value of 12.7 µg/ml ficolin-3 yielded 8% vs. 60% mortality (p = 0.001). Our data extend the knowledge concerning involvement of proteins of the lectin pathway in development of post-CPB complications. The potential prognostic value of low preoperative MAp44 and high preoperative ficolin-3 seems promising and warrants independent confirmation.

MASP-1 and MASP-2 Serum Levels Are Associated With Worse Prognostic in Cervical Cancer Progression

Background: MBL-associated serine proteases (MASP-1, MASP-2, MASP-3, MAp-44, and MAp-19) are key factors in the activation of the lectin pathway of complement. Serum levels of these components have been associated with recurrence and poor survival of some types of cancer, such as colorectal and ovarian cancer. In this investigation, we determined the serum levels of MASP-1, MASP-2, MASP-3, MAp-44, and MAp-19 in patients with cervical cancer and cervical intraepithelial neoplasia (CIN).

Methods:A total of 351 women who underwent screening for cervical cancer or treatment at the Erasto Gaertner Cancer Hospital in Curitiba-Brazil, were enrolled in the study. Based on their latest cervical colposcopy-guided biopsy results, they were divided into four groups: CIN-I: n = 52; CIN-II: n = 73; CIN-III: n = 141; and invasive cancer: n = 78. All the serum protein levels were determined by time-resolved immunofluorometric assay (TRIFMA).

Results:Patients with invasive cancer presented significantly higher MASP-2, MASP-1, and MAp-19 serum levels than other groups (p < 0.0001; p = 0.012; p = 0.025 respectively). No statistically significant differences in MASP-3 and MAp-44 serum levels were found between the four studied groups. In addition, high MASP-2, MASP-1, and MAp-19 serum levels were significantly associated with poor survival in patients with invasive cancer and relapse (p = 0.002, p = 0.0035 and p = 0.025, respectively).

Conclusion:High MASP-2, MASP-1, and MAp-19 serum levels were associated with cervical cancer progression and worse disease prognosis. These novel findings demonstrate the involvement of the serine proteases of the lectin pathway in the pathogenesis of cervical cancer and future investigations should clarify their role in the disease process.

Current Perspectives and Mechanisms of Relationship between Intestinal Microbiota Dysfunction and Dementia: A Review

BACKGROUND

Accumulating data suggest a crucial role of the intestinal microbiota in the development and progression of neurodegenerative diseases. More recently, emerging reports have revealed an association between intestinal microbiota dysfunctions and dementia, a debilitating multifactorial disorder, characterized by progressive deterioration of cognition and behavior that interferes with the social and professional life of the sufferer. However, the mechanisms of this association are not fully understood.

SUMMARY

In this review, I discuss recent data that suggest mechanisms of cross-talk between intestinal microbiota dysfunction and the brain that underlie the development of dementia. Potential therapeutic options for dementia are also discussed. The pleiotropic signaling of the metabolic products of the intestinal microbiota together with their specific roles in the maintenance of both the intestinal and blood-brain barriers as well as regulation of local, distant, and circulating immunocytes, and enteric, visceral, and central neural functions are integral to a healthy gut and brain.

KEY MESSAGES

Research investigating the effect of intestinal microbiota dysfunctions on brain health should focus on multiple interrelated systems involving local and central neuroendocrine, immunocyte, and neural signaling of microbial products and transmitters and neurohumoral cells that not only maintain intestinal, but also blood brain-barrier integrity. The change in intestinal microbiome/dysbiome repertoire is crucial to the development of dementia.

Be on Target: Strategies of Targeting Alternative and Lectin Pathway Components in Complement-Mediated Diseases

The complement system has moved into the focus of drug development efforts in the last decade, since its inappropriate or uncontrolled activation has been recognized in many diseases. Some of them are primarily complement-mediated rare diseases, such as paroxysmal nocturnal hemoglobinuria, C3 glomerulonephritis, and atypical hemolytic uremic syndrome. Complement also plays a role in various multifactorial diseases that affect millions of people worldwide, such as ischemia reperfusion injury (myocardial infarction, stroke), age-related macular degeneration, and several neurodegenerative disorders. In this review, we summarize the potential advantages of targeting various complement proteins with special emphasis on the components of the lectin (LP) and the alternative pathways (AP). The serine proteases (MASP-1/2/3, factor D, factor B), which are responsible for the activation of the cascade, are straightforward targets of inhibition, but the pattern recognition molecules (mannose-binding lectin, other collectins, and ficolins), the regulatory components (factor H, factor I, properdin), and C3 are also subjects of drug development. Recent discoveries about cross-talks between the LP and AP offer new approaches for clinical intervention. Mannan-binding lectin-associated serine proteases (MASPs) are not just responsible for LP activation, but they are also indispensable for efficient AP activation. Activated MASP-3 has recently been shown to be the enzyme that continuously supplies factor D (FD) for the AP by cleaving pro-factor D (pro-FD). In this aspect, MASP-3 emerges as a novel feasible target for the regulation of AP activity. MASP-1 was shown to be required for AP activity on various surfaces, first of all on LPS of Gram-negative bacteria.

Mannose Binding Lectin and Pentraxin 3 in Patients with Diabetic Retinopathy

BACKGROUND

Mannose binding lectin (MBL) is a protein of the complement system and pentraxin-3 (PTX3) is an acute phase protein both with an important role in inflammatory diseases, such as diabetic retinopathy (DR).

AIM OF THE STUDY

To evaluate whether plasma MBL and PTX3 levels are associated with the development of DR and if patients with and without DR can be distinguished.

METHODS

The patients were divided into three groups: diabetic without DR; with mild/moderate DR, and with severe/proliferative DR. PTX3 and MBL levels were measured with enzyme-linked immunosorbent assay kits.

RESULTS

A total of 74 patients were included. A significant association was observed between high levels of MBL and severe DR; 47% of patients with severe/proliferative DR had high levels of MBL, whereas 12% of the patients with diabetes but no DR had high levels of MBL (p = 0.008; odds ratio [OR]: 6.06; 95% confidence interval [CI]: 1.4-25.0). High levels of MBL were more frequent in patients with severe/proliferative disease (47%) when compared to those with mild/moderate DR (20%), p = 0.04 (OR: 3.46; 95% CI: 1.0-11.8). PTX3 levels were similar among the groups and were not related to the development or severity of DR.

CONCLUSION

We found a significant association between high plasma MBL levels and DR development as well as with severe/proliferative DR. We observed no relationship between plasma PTX3 levels and the development or severity of DR.

Substitution of Mannan-Binding Lectin (MBL)-Deficient Serum With Recombinant MBL Results in the Formation of New MBL/MBL-Associated Serine Protease Complexes

The lectin pathway (LP) of complement activation depends on the activation of the MBL-associated serine proteases (MASPs) circulating in complex with mannan-binding lectin (MBL). MBL deficiency is the most common complement deficiency and has been associated with several pathological conditions. As we had previously shown, plasma-derived MBL (pdMBL) contains pre-activated MASPs that upon in vivo pdMBL substitution results in restoration of MBL concentrations but no LP functionality due to immediate inactivation of pdMBL-MASP complexes upon infusion. In this study, we analyzed MBL-sufficient and -deficient serum by size-exclusion chromatography for complexes of LP activation. In both sera, we identified non-bound free forms of MASP-2 and to lesser extent MASP-1/3. After addition of recombinant MBL (rMBL) to MBL-deficient serum, these free MASPs were much less abundantly present, which is highly suggestive for the formation of high-molecular complexes that could still become activated upon subsequent ligand binding as shown by a restoration of C4-deposition of MBL-deficient serum. Ficolin (FCN)-associated MASPs have been described to redistribute to ligand-bound MBL, hereby forming new MBL/MASP complexes. However, reconstitution of MBL-deficient serum with rMBL did not change the relative size of the FCN molecules suggestive for a limited redistribution in fluid phase of already formed complexes. Our findings demonstrate that rMBL can associate with free non-bound MASPs in fluid phase while preserving full restoration of LP functionality. In contrast to pdMBL products containing pre-activated MASPs which become inactivated almost immediately, these current data provide a rationale for substitution studies using rMBL instead.

Extensive Basal Level Activation of Complement Mannose-Binding Lectin-Associated Serine Protease-3: Kinetic Modeling of Lectin Pathway Activation Provides Possible Mechanism

Serine proteases (SPs) are typically synthesized as precursors, termed proenzymes or zymogens, and the fully active form is produced via limited proteolysis by another protease or by autoactivation. The lectin pathway of the complement system is initiated by mannose-binding lectin (MBL)-associated SPs (MASP)-1, and MASP-2, which are known to be present as proenzymes in blood. The third SP of the lectin pathway, MASP-3, was recently shown to be the major activator, and the exclusive “resting blood” activator of profactor D, producing factor D, the initiator protease of the alternative pathway. Because only activated MASP-3 is capable of carrying out this cleavage, it was presumed that a significant fraction of MASP-3 must be present in the active form in resting blood. Here, we aimed to detect active MASP-3 in the blood by a more direct technique and to quantitate the active to zymogen ratio. First, MASPs were partially purified (enriched) from human plasma samples by affinity chromatography using immobilized MBL in the presence of inhibitors. Using this MASP pool, only the zymogen form of MASP-1 was detected by Western blot, whereas over 70% MASP-3 was in an activated form in the same samples. Furthermore, the active to zymogen ratio of MASP-3 showed little individual variation. It is enigmatic how MASP-3, which is not able to autoactivate, is present mostly as an active enzyme, whereas MASP-1, which has a potent autoactivation capability, is predominantly proenzymic in resting blood. In an attempt to explain this phenomenon, we modeled the basal level fluid-phase activation of lectin pathway proteases and their subsequent inactivation by C1 inhibitor and antithrombin using available and newly determined kinetic constants. The model can explain extensive MASP-3 activation only if we assume efficient intracomplex activation of MASP-3 by zymogen MASP-1. On the other hand, the model is in good agreement with the fact that MASP-1 and -2 are predominantly proenzymic and some of them is present in the form of inactive serpin–protease complexes. As an alternative hypothesis, MASP-3 activation by proprotein convertases is also discussed.

Dangerous liaisons: complement, coagulation, and kallikrein/kinin cross-talk act as a linchpin in the events leading to thromboinflammation

Innate immunity is fundamental to our defense against microorganisms. Physiologically, the intravascular innate immune system acts as a purging system that identifies and removes foreign substances leading to thromboinflammatory responses, tissue remodeling, and repair. It is also a key contributor to the adverse effects observed in many diseases and therapies involving biomaterials and therapeutic cells/organs. The intravascular innate immune system consists of the cascade systems of the blood (the complement, contact, coagulation, and fibrinolytic systems), the blood cells (polymorphonuclear cells, monocytes, platelets), and the endothelial cell lining of the vessels. Activation of the intravascular innate immune system in vivo leads to thromboinflammation that can be activated by several of the system's pathways and that initiates repair after tissue damage and leads to adverse reactions in several disorders and treatment modalities. In this review, we summarize the current knowledge in the field and discuss the obstacles that exist in order to study the cross-talk between the components of the intravascular innate immune system. These include the use of purified in vitro systems, animal models and various types of anticoagulants. In order to avoid some of these obstacles we have developed specialized human whole blood models that allow investigation of the cross-talk between the various cascade systems and the blood cells. We in particular stress that platelets are involved in these interactions and that the lectin pathway of the complement system is an emerging part of innate immunity that interacts with the contact/coagulation system. Understanding the resulting thromboinflammation will allow development of new therapeutic modalities.

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.

COLEC10 is mutated in 3MC patients and regulates early craniofacial development

3MC syndrome is an autosomal recessive heterogeneous disorder with features linked to developmental abnormalities. The main features include facial dysmorphism, craniosynostosis and cleft lip/palate; skeletal structures derived from cranial neural crest cells (cNCC). We previously reported that lectin complement pathway genes COLEC11 and MASP1/3 are mutated in 3MC syndrome patients. Here we define a new gene, COLEC10, also mutated in 3MC families and present novel mutations in COLEC11 and MASP1/3 genes in a further five families. The protein products of COLEC11 and COLEC10, CL-K1 and CL-L1 respectively, form heteromeric complexes. We show COLEC10 is expressed in the base membrane of the palate during murine embryo development. We demonstrate how mutations in COLEC10 (c.25C>T; p.Arg9Ter, c.226delA; p.Gly77Glufs*66 and c.528C>G p.Cys176Trp) impair the expression and/or secretion of CL-L1 highlighting their pathogenicity. Together, these findings provide further evidence linking the lectin complement pathway and complement factors COLEC11 and COLEC10 to morphogenesis of craniofacial structures and 3MC etiology.

The 3MC syndrome is a unifying term amalgamating four rare recessive genetic disorders with overlapping features namely; Mingarelli, Malpuech, Michels and Carnevale syndromes. It is characterised by facial malformations including, high-arched eyebrows, cleft lip/palate, hypertelorism, developmental delay and hearing loss. We previously reported that lectin complement pathway genes COLEC11 and MASP1/3 were mutated in 3MC syndrome patients. Here we describe a new gene from the same pathway, COLEC10, mutated in 3MC patients. Our results show that COLEC10 is expressed in craniofacial tissues during development. We demonstrate how CL-L1, the protein expressed by COLEC10, can act as a cellular chemoattractant in vitro, controlling cell movement and migration. We overexpressed constructs carrying COLEC10 non-sense mutations found in our patients, CL-L1 failed to be expressed and secreted. Moreover, when we expressed a missense COLEC10 construct, CL-L1 was expressed but failed to be secreted. In sum, we discovered a new gene, COLEC10, mutated in 3MC syndrome and we propose a pathogenic mechanism for 3MC relating to the failure of CL-L1 function and its craniofacial developmental consequences.

Endogenous Natural Complement Inhibitor Regulates Cardiac Development

Congenital heart defects are a major cause of perinatal mortality and morbidity, affecting >1% of all live births in the Western world, yet a large fraction of such defects have an unknown etiology. Recent studies demonstrated surprising dual roles for immune-related molecules and their effector mechanisms during fetal development and adult homeostasis. In this article, we describe the function of an endogenous complement inhibitor, mannan-binding lectin (MBL)-associated protein (MAp)44, in regulating the composition of a serine protease-pattern recognition receptor complex, MBL-associated serine protease (MASP)-3/collectin-L1/K1 hetero-oligomer, which impacts cardiac neural crest cell migration. We used knockdown and rescue strategies in zebrafish, a model allowing visualization and assessment of heart function, even in the presence of severe functional defects. Knockdown of embryonic expression of MAp44 caused impaired cardiogenesis, lowered heart rate, and decreased cardiac output. These defects were associated with aberrant neural crest cell behavior. We found that MAp44 competed with MASP-3 for pattern recognition molecule interaction, and knockdown of endogenous MAp44 expression could be rescued by overexpression of wild-type MAp44. Our observations provide evidence that immune molecules are centrally involved in the orchestration of cardiac tissue development.

Oligomerization of Mannan-binding Lectin Dictates Binding Properties and Complement Activation

The complement system is a part of the innate immune system and is involved in recognition and clearance of pathogens and altered-self structures. The lectin pathway of the complement system is initiated when soluble pattern recognition molecules (PRMs) with collagen-like regions bind to foreign or altered self-surfaces. Associated with the collagen-like stems of these PRMs are three mannan-binding lectin (MBL)-associated serine proteases (MASPs) and two MBL-associated proteins (MAps). The most studied of the PRMs, MBL, is present in serum mainly as trimeric and tetrameric oligomers of the structural subunit. We hypothesized that oligomerization of MBL may influence both the potential to bind to micro organisms and the interaction with the MASPs and MAps, thus influencing the ability to initiate complement activation. When testing binding at 37 °C, we found higher binding of tetrameric MBL to Staphylococcus aureus (S. aureus) than trimeric and dimeric MBL. In serum, we found that tetrameric MBL was the main oligomeric form present in complexes with the MASPs and MAp44. Such preference was confirmed using purified forms of recombinant MBL (rMBL) oligomers, where tetrameric rMBL interacted stronger with all of the MASPs and MAp44, compared to trimeric MBL. As a direct consequence of the weaker interaction with the MASPs, we found that trimeric rMBL was inferior to tetrameric rMBL in activating the complement system. Our data suggest that the oligomeric state of MBL is crucial both for the binding properties and the effector function of MBL.

Flexibility in Mannan-Binding Lectin-Associated Serine Proteases-1 and -2 Provides Insight on Lectin Pathway Activation

The lectin pathway of complement is activated by complexes comprising a recognition component (mannose-binding lectin, serum ficolins, collectin-LK or collectin-K1) and a serine protease (MASP-1 or MASP-2). MASP-1 activates MASP-2, and MASP-2 cleaves C4 and C4b-bound C2. To clarify activation, new crystal structures of Ca²⁺-bound MASP dimers were determined, together with their solution structures from X-ray scattering, analytical ultracentrifugation, and atomistic modeling. Solution structures of the CUB1-EGF-CUB2 dimer of each MASP indicate that the two CUB2 domains were tilted by as much as 90° compared with the crystal structures, indicating considerable flexibility at the EGF-CUB2 junction. Solution structures of the full-length MASP dimers in their zymogen and activated forms revealed similar structures that were much more bent than anticipated from crystal structures. We conclude that MASP-1 and MASP-2 are flexible at multiple sites and that this flexibility may permit both intra- and inter-complex activation.

CsMAP34, a teleost MAP with dual role: A promoter of MASP-assisted complement activation and a regulator of immune cell activity

In teleost fish, the immune functions of mannan-binding lectin (MBL) associated protein (MAP) and MBL associated serine protease (MASP) are scarcely investigated. In the present study, we examined the biological properties both MAP (CsMAP34) and MASP (CsMASP1) molecules from tongue sole (Cynoglossus semilaevis). We found that CsMAP34 and CsMASP1 expressions occurred in nine different tissues and were upregulated by bacterial challenge. CsMAP34 protein was detected in blood, especially during bacterial infection. Recombinant CsMAP34 (rCsMAP34) bound C. semilaevis MBL (rCsBML) when the latter was activated by bacteria, while recombinant CsMASP1 (rCsMASP1) bound activated rCsBML only in the presence of rCsMAP34. rCsMAP34 stimulated the hemolytic and bactericidal activities of serum complement, whereas anti-CsMAP34 antibody blocked complement activities. Knockdown of CsMASP1 in C. semilaevis resulted in significant inhibition of complement activities. Furthermore, rCsMAP34 interacted directly with peripheral blood leukocytes (PBL) and enhanced the respiratory burst, acid phosphatase activity, chemotactic activity, and gene expression of PBL. These results indicate for the first time that a teleost MAP acts one hand as a regulator that promotes the lectin pathway of complement activation via its ability to recruit MBL to MASP, and other hand as a modulator of immune cell activity.

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.

Low level of MAp44, an inhibitor of the lectin complement pathway, and long-term graft and patient survival; a cohort study of 382 kidney recipients

Background

Higher incidence of malignancy and infectious diseases in kidney transplant recipients is related to immunosuppressive treatment after transplantation and the recipient’s native immune system. The complement system is an essential component of the innate immunity. The aim of the present study was to investigate the association of effector molecules of the lectin complement pathway with graft and patient survival after kidney transplantation.

Methods

Two mannan-binding lectin (MBL) associated proteases, MASP-2 and MASP-3 (activators of the lectin pathway) and two MBL-associated proteins, MAp44 and MAp19 (inhibitors of the lectin pathway) were measured at the time of transplantation in 382 patients (≥17 years old) transplanted in 2000–2001. The cohort was followed until December 31, 2014. Data on patient and graft survival were obtained from the Norwegian Renal Registry. Cox proportional hazard regression models were performed for survival analyses.

Results

Low MAp44 level (1st versus 2–4 quartile) was significantly associated with overall mortality; HR 1.52, 95 % CI 1.08–2.14, p = 0.017. In the sub analyses in groups below and above median age (51.7 years), low MAp44 as a predictor of overall mortality was statistically significant only in recipients of ≤51.7 years; HR 2.57, 95 % CI 1.42–4.66, p = 0.002. Furthermore, low MAp44 was associated with mortality due to infectious diseases; HR 2.22, 95 % CI 1.11–4.41, p = 0.023. There was no association between MASP-2, MASP-3 or MAp19 levels and patient mortality. No association between any measured biomarkers and death censored graft loss was found.

Conclusions

Low MAp44 level at the time of transplantation was associated with increased overall mortality in kidney recipients of median age of 51.7 years or below and with mortality due to infectious diseases in the whole patient cohort after nearly 14-years of follow up after transplantation. No associations between other effector molecules; MASP-2, MASP-3 or MAp19 and recipient mortality were found, as well as no association of any biomarker with death censored graft loss.

Electronic supplementary material

The online version of this article (doi:10.1186/s12882-016-0373-9) contains supplementary material, which is available to authorized users.

A New Ligand-Based Method for Purifying Active Human Plasma-Derived Ficolin-3 Complexes Supports the Phenomenon of Crosstalk between Pattern-Recognition Molecules and Immunoglobulins

Despite recombinant protein technology development, proteins isolated from natural sources remain important for structure and activity determination. Ficolins represent a class of proteins that are difficult to isolate. To date, three methods for purifying ficolin-3 from plasma/serum have been proposed, defined by most critical step: (i) hydroxyapatite absorption chromatography (ii) N-acetylated human serum albumin affinity chromatography and (iii) anti-ficolin-3 monoclonal antibody-based affinity chromatography. We present a new protocol for purifying ficolin-3 complexes from human plasma that is based on an exclusive ligand: the O-specific polysaccharide of Hafnia alvei PCM 1200 LPS (O-PS 1200). The protocol includes (i) poly(ethylene glycol) precipitation; (ii) yeast and l-fucose incubation, for depletion of mannose-binding lectin; (iii) affinity chromatography using O-PS 1200-Sepharose; (iv) size-exclusion chromatography. Application of this protocol yielded average 2.2 mg of ficolin-3 preparation free of mannose-binding lectin (MBL), ficolin-1 and -2 from 500 ml of plasma. The protein was complexed with MBL-associated serine proteases (MASPs) and was able to activate the complement in vitro. In-process monitoring of MBL, ficolins, and total protein content revealed the presence of difficult-to-remove immunoglobulin G, M and A, in some extent in agreement with recent findings suggesting crosstalk between IgG and ficolin-3. We demonstrated that recombinant ficolin-3 interacts with IgG and IgM in a concentration-dependent manner. Although this association does not appear to influence ficolin-3-ligand interactions in vitro, it may have numerous consequences in vivo. Thus our purification procedure provides Ig-ficolin-3/MASP complexes that might be useful for gaining further insight into the crosstalk and biological activity of ficolin-3.

Complement activation, regulation, and molecular basis for complement-related diseases

The complement system is an essential element of the innate immune response that becomes activated upon recognition of molecular patterns associated with microorganisms, abnormal host cells, and modified molecules in the extracellular environment. The resulting proteolytic cascade tags the complement activator for elimination and elicits a pro-inflammatory response leading to recruitment and activation of immune cells from both the innate and adaptive branches of the immune system. Through these activities, complement functions in the first line of defense against pathogens but also contributes significantly to the maintenance of homeostasis and prevention of autoimmunity. Activation of complement and the subsequent biological responses occur primarily in the extracellular environment. However, recent studies have demonstrated autocrine signaling by complement activation in intracellular vesicles, while the presence of a cytoplasmic receptor serves to detect complement-opsonized intracellular pathogens. Furthermore, breakthroughs in both functional and structural studies now make it possible to describe many of the intricate molecular mechanisms underlying complement activation and the subsequent downstream events, as well as its cross talk with, for example, signaling pathways, the coagulation system, and adaptive immunity. We present an integrated and updated view of complement based on structural and functional data and describe the new roles attributed to complement. Finally, we discuss how the structural and mechanistic understanding of the complement system rationalizes the genetic defects conferring uncontrolled activation or other undesirable effects of complement.

Targeting mechanisms at sites of complement activation for imaging and therapy

The complement system plays a key role in many acute injury states as well as chronic autoimmune and inflammatory diseases. Localized complement activation and alternative pathway-mediated amplification on diverse target surfaces promote local recruitment of pro-inflammatory cells and elaboration of other mediators. Despite a general understanding of the architecture of the system, though, many of the mechanisms that underlie site-specific complement activation and amplification in vivo are incompletely understood. In addition, there is no capability yet to measure the level of local tissue site-specific complement activation in patients without performing biopsies to detect products using immunohistochemical techniques. Herein is reviewed emerging evidence obtained through clinical research studies of human rheumatoid arthritis along with translational studies of its disease models which demonstrate that several parallel mechanisms are involved in site-specific amplification of activation of the complement system in vivo. Among these processes are de-regulation of the alternative pathway, effector pathway-catalyzed amplification of proximal complement activation, recognition of injury-associated ligands by components of the lectin pathway, and engagement of pathogenic natural antibodies that recognize a limited set of injury-associated neoepitopes. Studies suggest that each of these inter-related processes can play key roles in amplification of complement-dependent injury on self-tissues in vivo. These findings, in addition to development of an imaging strategy described herein designed to quantitatively measure local complement C3 fixation, have relevance to therapeutic and diagnostic strategies targeting the complement system.

Possible mechanisms of C-reactive protein mediated acute myocardial infarction

Myocardial infarction is a relevant cardiovascular event worldwide for morbidity and mortality. It has been theorized that acute myocardial infarctions (AMIs) and other acute coronary events that are precipitated by atherosclerosis are due to arterial blockage from fat deposits. It is now known, however, that atherosclerosis involves more than just lipids. Inflammation has also been studied extensively to play a substantial role in myocardial infarction. There have been debates and conflicting reports over the past few years about the value of assessing levels of C-reactive protein and other biomarkers of inflammation for the prediction of cardiovascular events. Several studies have shown that CRP is not only an inflammatory marker, but also involved in the pathogenesis of myocardial infarction. Studies have linked atherogenesis and rupture of atherosclerotic lesion to endothelial dysfunction. CRP directly inhibits endothelial cell nitric oxide (NO) production via destabilizing endothelial NO synthase (eNOS). Decreased NO release causes CRP mediated inhibition of angiogenesis, stimulating endothelial cell apoptosis. CRP can also activate the complement system through the classical pathway. Complement activation plays an important role in mediating monocyte and neutrophil recruitment in an injured myocardium and may therefore lead to increase in infarct size. This article discusses the possible roles of CRP in complement activation, endothelial dysfunction and its impact on the development of myocardial infarction. We also reviewed the possible therapeutic approaches to myocardial infarction.

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.

Human lectins and their roles in viral infections

Innate recognition of virus proteins is an important component of the immune response to viral pathogens. A component of this immune recognition is the family of lectins; pattern recognition receptors (PRRs) that recognise viral pathogen-associated molecular patterns (PAMPs) including viral glycoproteins. In this review we discuss the contribution of soluble and membrane-associated PRRs to immunity against virus pathogens, and the potential role of these molecules in facilitating virus replication. These processes are illustrated with examples of viruses including human immunodeficiency virus (HIV), hepatitis C virus (HCV) and Ebola virus (EBOV). We focus on the structure, function and genetics of the well-characterised C-type lectin mannose-binding lectin, the ficolins, and the membrane-bound CD209 proteins expressed on dendritic cells. The potential for lectin-based antiviral therapies is also discussed.

Structural insights into the initiating complex of the lectin pathway of complement activation

The proteolytic cascade of the complement system is initiated when pattern-recognition molecules (PRMs) bind to ligands, resulting in the activation of associated proteases. In the lectin pathway of complement, the complex of mannan-binding lectin (MBL) and MBL-associated serine protease-1 (MASP-1) initiates the pathway by activating a second protease, MASP-2. Here we present a structural study of a PRM/MASP complex and derive the overall architecture of the 450 kDa MBL/MASP-1 complex using small-angle X-ray scattering and electron microscopy. The serine protease (SP) domains from the zymogen MASP-1 dimer protrude from the cone-like MBL tetramer and are separated by at least 20 nm. This suggests that intracomplex activation within a single MASP-1 dimer is unlikely and instead supports intercomplex activation, whereby the MASP SP domains are accessible to nearby PRM-bound MASPs. This activation mechanism differs fundamentally from the intracomplex initiation models previously proposed for both the lectin and the classical pathway.

Complement activation by ligand-driven juxtaposition of discrete pattern recognition complexes

Defining mechanisms governing translation of molecular binding events into immune activation is central to understanding immune function. In the lectin pathway of complement, the pattern recognition molecules (PRMs) mannan-binding lectin (MBL) and ficolins complexed with the MBL-associated serine proteases (MASP)-1 and MASP-2 cleave C4 and C2 to generate C3 convertase. MASP-1 was recently found to be the exclusive activator of MASP-2 under physiological conditions, yet the predominant oligomeric forms of MBL carry only a single MASP homodimer. This prompted us to investigate whether activation of MASP-2 by MASP-1 occurs through PRM-driven juxtaposition on ligand surfaces. We demonstrate that intercomplex activation occurs between discrete PRM/MASP complexes. PRM ligand binding does not directly escort the transition of MASP from zymogen to active enzyme in the PRM/MASP complex; rather, clustering of PRM/MASP complexes directly causes activation. Our results support a clustering-based mechanism of activation, fundamentally different from the conformational model suggested for the classical pathway of complement.

Essential role for the lectin pathway in collagen antibody-induced arthritis revealed through use of adenovirus programming complement inhibitor MAp44 expression

Previous studies using mannose-binding lectin (MBL) and complement C4-deficient mice have suggested that the lectin pathway (LP) is not required for the development of inflammatory arthritis in the collagen Ab-induced arthritis (CAIA) model. MBL, ficolins and collectin-11 are key LP pattern recognition molecules that associate with three serine proteases-MASP-1, MASP-2, and MASP-3-and with two MBL-associated proteins designated sMAP and MBL-associated protein of 44kDA (MAp44). Recent studies have shown that MAp44, an alternatively spliced product of the MASP-1/3 gene, is a competitive inhibitor of the binding of the recognition molecules to all three MASPs. In these studies, we examined the effect of treatment of mice with adenovirus (Ad) programmed to express human MAp44 (AdhMAp44) on the development of CAIA. AdhMAp44 and Ad programming GFP (AdGFP) expression were injected i.p. in C57BL/6 wild type mice prior to the induction of CAIA. AdhMAp44 significantly reduced the clinical disease activity (CDA) score by 81% compared with mice injected with AdGFP. Similarly, histopathologic injury scores for inflammation, pannus, cartilage and bone damage, as well as C3 deposition in the cartilage and synovium, were significantly reduced by AdhMAp44 pretreatment. Mice treated with AdmMAp44, programming expression of mouse MAp44, also showed significantly decreased CDA score and histopathologic injury scores. In addition, administration of AdhMAp44 significantly diminished the severity of Ross River virus-induced arthritis, an LP-dependent model. Our study provides conclusive evidence that an intact complement LP is essential to initiate CAIA, and that MAp44 may be an appropriate treatment for inflammatory arthritis.

Restoration of MBL-deficiency: redefining the safety, efficacy and viability of MBL-substitution therapy

MBL-deficiency is a commonly occurring deficiency of the innate immune system, affecting a substantial part of the population and has been extensively studied. MBL appears to function as a disease modifier. The role of MBL in different conditions is context-dependent. Many clinical studies show conflicting results, which can be partially explained by different definitions of MBL-deficiency, including phenotype- and genotype-based approaches. In this review we give an overview of literature of MBL, its role in different pathologies, diseases and patient populations. We review MBL replacement studies, and discuss the potential of MBL substitution therapy. We finally suggest that new MBL substitution trials should be conducted within a predefined patient population. MBL-deficiency should be based on serum levels and confirmed by genotyping.

Heterocomplex formation between MBL/ficolin/CL-11-associated serine protease-1 and -3 and MBL/ficolin/CL-11-associated protein-1

The activity of the complement system is tightly controlled by many fluid-phase and tissue-bound regulators. Mannose-binding lectin (MBL)/ficolin/collectin-11-associated protein-1 (MAP-1) is a recently discovered plasma protein that acts as an upstream inhibitor of the lectin complement pathway (LCP). It has previously been shown that MAP-1 can compete with the MBL/ficolin/collectin-11-associated serine proteases (MASPs) in binding to MBL and the ficolins. However, this mechanism may only partly explain the inhibitory complement effect of MAP-1. We hypothesized that MAP-1 is also involved in heterocomplex formation with the MASPs thereby breaking the stoichiometry of the activation complexes of the LCP, which could represent an alternative mechanism of MAP-1-mediated complement inhibition. We assessed the heterocomplex formation with ELISA, size-exclusion chromatography, and immunoblotting using both recombinant proteins and serum/plasma. We found that rMAP-1 can engage in heterocomplexes with rMASP-1 and rMASP-3 in a calcium-dependent manner. Moreover, we discovered that rMASP-1 and rMASP-3 also form heterocomplexes under these conditions. Complexes containing both MAP-1 and MASP-1 or -3 were detected in normal human serum and plasma, and depletion of the LCP recognition molecules from ficolin-3-deficient human serum showed that free circulating heterocomplexes also exist in the blood, although the major part appears to be associated with the LCP recognition molecules. Altogether, these findings suggest that MASPs can associate in various combinations and bring new perspectives to the complexity of lectin pathway-driven complement activation.

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.