MASP-1 of the complement system enhances clot formation in a microvascular whole blood flow model

Microfluidics as a Powerful Tool to Investigate Microvascular Dysfunction in Trauma Conditions: A Review of the State-of-the-Art

Skeletal muscle trauma such as fracture or crush injury can result in a life-threatening condition called acute compartment syndrome (ACS), which involves elevated compartmental pressure within a closed osteo-fascial compartment, leading to collapse of the microvasculature and resulting in necrosis of the tissue due to ischemia. Diagnosis of ACS is complex and controversial due to the lack of standardized objective methods, which results in high rates of misdiagnosis/late diagnosis, leading to permanent neuro-muscular damage. ACS pathophysiology is poorly understood at a cellular level due to the lack of physiologically relevant models. In this context, microfluidics organ-on-chip systems (OOCs) provide an exciting opportunity to investigate the cellular mechanisms of microvascular dysfunction that leads to ACS. In this article, the state-of-the-art OOCs designs and strategies used to investigate microvasculature dysfunction mechanisms is reviewed. The differential effects of hemodynamic shear stress on endothelial cell characteristics such as morphology, permeability, and inflammation, all of which are altered during microvascular dysfunction is highlighted. The article then critically reviews the importance of microfluidics to investigate closely related microvascular pathologies that cause ACS. The article concludes by discussing potential biomarkers of ACS with a special emphasis on glycocalyx and providing a future perspective.

Vascular dysfunction in hemorrhagic viral fevers: opportunities for organotypic modeling

The hemorrhagic fever viruses (HFVs) cause severe or fatal infections in humans. Named after their common symptom hemorrhage, these viruses induce significant vascular dysfunction by affecting endothelial cells, altering immunity, and disrupting the clotting system. Despite advances in treatments, such as cytokine blocking therapies, disease modifying treatment for this class of pathogen remains elusive. Improved understanding of the pathogenesis of these infections could provide new avenues to treatment. While animal models and traditional 2D cell cultures have contributed insight into the mechanisms by which these pathogens affect the vasculature, these models fall short in replicatingin vivohuman vascular dynamics. The emergence of microphysiological systems (MPSs) offers promising avenues for modeling these complex interactions. These MPS or 'organ-on-chip' models present opportunities to better mimic human vascular responses and thus aid in treatment development. In this review, we explore the impact of HFV on the vasculature by causing endothelial dysfunction, blood clotting irregularities, and immune dysregulation. We highlight how existing MPS have elucidated features of HFV pathogenesis as well as discuss existing knowledge gaps and the challenges in modeling these interactions using MPS. Understanding the intricate mechanisms of vascular dysfunction caused by HFV is crucial in developing therapies not only for these infections, but also for other vasculotropic conditions like sepsis.

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.

Plasma levels of mannan-binding lectin-associated serine proteases are increased in type 1 diabetes patients with insulin resistance

Activation of the lectin pathway of the complement system, as demonstrated by elevated levels of mannan-binding lectin proteins (MBL), contributes to vascular pathology in type 1 diabetes (T1D). Vascular complications are greatest in T1D individuals with concomitant insulin resistance (IR), however, whether IR amplifies activiation of the lectin pathway in T1D is unknown. We pooled pretreatment data from two RCTs and performed a cross-sectional analysis on 46 T1D individuals. We employed estimated glucose disposal rate (eGDR), a validated IR surrogate with cut-points of: <5.1, 5.1-8.7, and > 8.7 mg/kg/min to determine IR status, with lower eGDR values conferring higher degrees of IR. Plasma levels of MBL-associated proteases (MASP-1, MASP-2, and MASP-3) and their regulatory protein MAp44 were compared among eGDR classifications. In a subset of 14 individuals, we assessed change in MASPs and MAp44 following improvement in IR. We found that MASP-1, MASP-2, MASP-3, and MAp44 levels increased in a stepwise fashion across eGDR thresholds with elevated MASPs and MAp44 levels conferring greater degrees of IR. In a subset of 14 patients, improvement in IR was associated with significant reductions in MASPs, but not MAp44, levels. In conclusion, IR in T1D amplifies levels of MASP-1/2/3 and their regulator MAp44, and improvement of IR normalizes MASP-1/2/3 levels. Given that elevated levels of these proteins contribute to vascular pathology, amplification of the lectin pathway of the complement system may offer mechanistic insight into the relationship between IR and vascular complications in T1D.

Distinction of early complement classical and lectin pathway activation via quantification of C1s/C1-INH and MASP-1/C1-INH complexes using novel ELISAs

The most commonly used markers to assess complement activation are split products that are produced through activation of all three pathways and are located downstream of C3. In contrast, C4d derives from the cleavage of C4 and indicates either classical (CP) or lectin pathway (LP) activation. Although C4d is perfectly able to distinguish between CP/LP and alternative pathway (AP) activation, no well-established markers are available to differentiate between early CP and LP activation. Active enzymes of both pathways (C1s/C1r for the CP, MASP-1/MASP-2 for the LP) are regulated by C1 esterase inhibitor (C1-INH) through the formation of covalent complexes. Aim of this study was to develop validated immunoassays detecting C1s/C1-INH and MASP-1/C1-INH complex levels. Measurement of the complexes reveals information about the involvement of the respective pathways in complement-mediated diseases. Two sandwich ELISAs detecting C1s/C1-INH and MASP-1/C1-INH complex were developed and tested thoroughly, and it was investigated whether C1s/C1-INH and MASP-1/C1-INH complexes could serve as markers for either early CP or LP activation. In addition, a reference range for these complexes in healthy adults was defined, and the assays were clinically validated utilizing samples of 414 COVID-19 patients and 96 healthy controls. The immunoassays can reliably measure C1s/C1-INH and MASP-1/C1-INH complex concentrations in EDTA plasma from healthy and diseased individuals. Both complex levels are increased in serum when activated with zymosan, making them suitable markers for early classical and early lectin pathway activation. Furthermore, measurements of C1-INH complexes in 96 healthy adults showed normally distributed C1s/C1-INH complex levels with a physiological concentration of 1846 ± 1060 ng/mL (mean ± 2SD) and right-skewed distribution of MASP-1/C1-INH complex levels with a median concentration of 36.9 (13.18 - 87.89) ng/mL (2.5-97.5 percentile range), while levels of both complexes were increased in COVID-19 patients (p<0.0001). The newly developed assays measure C1-INH complex levels in an accurate way. C1s/C1-INH and MASP-1/C1-INH complexes are suitable markers to assess early classical and lectin pathway activation. An initial reference range was set and first studies showed that these markers have added value for investigating and unraveling complement activation in human disease.

Targeting thromboinflammation in COVID-19 - A narrative review of the potential of C1 inhibitor to prevent disease progression

Coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 is associated with a clinical spectrum ranging from asymptomatic carriers to critically ill patients with complications including thromboembolic events, myocardial injury, multisystemic inflammatory syndromes and death. Since the beginning of the pandemic several therapeutic options emerged, with a multitude of randomized trials, changing the medical landscape of COVID-19. The effect of various monoclonal antibodies, antiviral, anti-inflammatory and anticoagulation drugs have been studied, and to some extent, implemented into clinical practice. In addition, a multitude of trials improved the understanding of the disease and emerging evidence points towards a significant role of the complement system, kallikrein-kinin, and contact activation system as drivers of disease in severe COVID-19. Despite their involvement in COVID-19, treatments targeting these plasmatic cascades have neither been systematically studied nor introduced into clinical practice, and randomized studies with regards to these treatments are scarce. Given the multiple-action, multiple-target nature of C1 inhibitor (C1-INH), the natural inhibitor of these cascades, this drug may be an interesting candidate to prevent disease progression and combat thromboinflammation in COVID-19. This narrative review will discuss the current evidence with regards to the involvement of these plasmatic cascades as well as endothelial cells in COVID-19. Furthermore, we summarize the evidence of C1-INH in COVID-19 and potential benefits and pitfalls of C1-INH treatment in COVID-19.

Complement lectin pathway components MBL and MASP-1 promote haemostasis upon vessel injury in a microvascular bleeding model

Background

Complement lectin pathway components, in particular mannan-binding lectin (MBL) and MBL-associated serine proteases (MASPs) have been shown to interact with coagulation factors and contribute to clot formation. Here we investigated the role of MBL and MASP-1 in the haemostatic response following mechanical vessel injury in a human microfluidic bleeding model.

Methods

We studied haemostasis in a microvascular bleeding model in the presence of human endothelial cells and human whole blood under flow conditions. We monitored incorporation of proteins into the clot with fluorescently labelled antibodies and studied their effects on clot formation, platelet activation, and bleeding time with specific inhibitors. Platelet activation was also studied by flow cytometry.

Results

Upon vessel injury, MBL accumulated at the injury site in a well-defined wall-like structure. MBL showed partial colocalisation with fibrin, and strong colocalisation with von Willebrand factor and (activated) platelets. Flow cytometry ruled out direct binding of MBL to platelets, but confirmed a PAR4- and thrombin-dependent platelet-activating function of MASP-1. Inhibiting MBL during haemostasis reduced platelet activation, while inhibiting MASP-1 reduced platelet activation, fibrin deposition and prolonged bleeding time.

Conclusion

We show in a microvascular human bleeding model that MBL and MASP-1 have important roles in the haemostatic response triggered by mechanical vessel injury: MBL recognises the injury site, while MASP-1 increases fibrin formation, platelet activation and shortens bleeding time. While the complement lectin pathway may be harmful in the context of pathological thrombosis, it appears to be beneficial during the physiological coagulation response by supporting the crucial haemostatic system.

Renal diseases and the role of complement: Linking complement to immune effector pathways and therapeutics

The complement system is an ancient and phylogenetically conserved key danger sensing system that is critical for host defense against pathogens. Activation of the complement system is a vital component of innate immunity required for the detection and removal of pathogens. It is also a central orchestrator of adaptive immune responses and a constituent of normal tissue homeostasis. Once complement activation occurs, this system deposits indiscriminately on any cell surface in the vicinity and has the potential to cause unwanted and excessive tissue injury. Deposition of complement components is recognized as a hallmark of a variety of kidney diseases, where it is indeed associated with damage to the self. The provenance and the pathophysiological role(s) played by complement in each kidney disease is not fully understood. However, in recent years there has been a renaissance in the study of complement, with greater appreciation of its intracellular roles as a cell-intrinsic system and its interplay with immune effector pathways. This has been paired with a profusion of novel therapeutic agents antagonizing complement components, including approved inhibitors against complement components (C)1, C3, C5 and C5aR1. A number of clinical trials have investigated the use of these more targeted approaches for the management of kidney diseases. In this review we present and summarize the evidence for the roles of complement in kidney diseases and discuss the available clinical evidence for complement inhibition.

Fibrinolytic Alterations in Sepsis: Biomarkers and Future Treatment Targets

Sepsis is a life-threatening condition which develops as a dysregulated immune response in the face of infection and which is associated with profound hemostatic disturbances and in the most extreme cases disseminated intravascular coagulation (DIC). In addition, the fibrinolytic system is subject to alterations during infection and sepsis, and impaired fibrinolysis is currently considered a key player in sepsis-related microthrombus formation and DIC. However, we still lack reliable biomarkers to assess fibrinolysis in the clinical setting. Furthermore, drugs targeting the fibrinolytic system have potential value in sepsis patients with severe fibrinolytic disturbances, but these are still being tested in the preclinical stage. The present review provides an overview of key fibrinolytic changes in sepsis, reviews the current literature on potential laboratory markers of altered fibrinolysis in adult sepsis patients, and discusses future perspectives for diagnosis and treatment of fibrinolytic disturbances in sepsis patients.

MASPs at the crossroad between the complement and the coagulation cascades - the case for COVID-19

Components of the complement system and atypical parameters of coagulation were reported in COVID-19 patients, as well as the exacerbation of the inflammation and coagulation activity. Mannose binding lectin (MBL)- associated serine proteases (MASPs) play an important role in viral recognition and subsequent activation of the lectin pathway of the complement system and blood coagulation, connecting both processes. Genetic variants of MASP1 and MASP2 genes are further associated with different levels and functional efficiency of their encoded proteins, modulating susceptibility and severity to diseases. Our review highlights the possible role of MASPs in SARS-COV-2 binding and activation of the lectin pathway and blood coagulation cascades, as well as their associations with comorbidities of COVID-19. MASP-1 and/or MASP-2 present an increased expression in patients with COVID-19 risk factors: diabetes, arterial hypertension and cardiovascular disease, chronic kidney disease, chronic obstructive pulmonary disease, and cerebrovascular disease. Based also on the positive results of COVID-19 patients with anti-MASP-2 antibody, we propose the use of MASPs as a possible biomarker of the progression of COVID-19 and the investigation of new treatment strategies taking into consideration the dual role of MASPs, including MASP inhibitors as promising therapeutic targets against COVID-19.

Thromboinflammation Model-on-A-Chip by Whole Blood Microfluidics on Fixed Human Endothelium

Microfluidic devices have an established role in the study of platelets and coagulation factors in thrombosis, with potential diagnostic applications. However, few microfluidic devices have assessed the contribution of neutrophils to thrombus formation, despite increasing knowledge of neutrophils’ importance in cardiovascular thrombosis. We describe a thromboinflammation model which uses straight channels, lined with fixed human umbilical vein endothelial cells, after treatment with tumour necrosis factor-alpha. Re-calcified whole blood is perfused over the endothelium at venous and arterial shear rate. Neutrophil adhesion, platelet and fibrin thrombus formation, is measured over time by the addition of fluorescent antibodies to a whole blood sample. Fixed endothelium retains surface expression of adhesion molecules ICAM-1 and E-Selectin. Neutrophils adhere preferentially to platelet thrombi on the endothelium. Inhibitors of neutrophil adhesion and anti-inflammatory agents, such as isoquercetin, decrease neutrophil adhesion. Our model offers the advantage of the use of (1) fixed endothelium, (2) whole blood, instead of isolated neutrophils, and (3) a small amount of blood (1 mL). The characteristics of this thromboinflammation model provide the potential for further development for drug screening and point-of-care applications.

Laboratory Assessment of Coagulation Factor XIII

Laboratory diagnosis of congenital and acquired deficiencies of coagulation factor XIII (FXIII) can be challenging. Determination of FXIII function requires specific and sensitive assays which are not always available. This brief review article summarizes currently used FXIII assay methods, their principles and difficulties, and discusses the recommended diagnostic workup in case of a suspected FXIII deficiency. The article also briefly touches on experimental methods used in FXIII research.

Does Complement-Mediated Hemostatic Disturbance Occur in Traumatic Brain Injury? A Literature Review and Observational Study Protocol

Despite improvements in medical triage and tertiary care, traumatic brain injury (TBI) remains associated with significant morbidity and mortality. Almost two-thirds of patients with severe TBI develop some form of hemostatic disturbance, which contributes to poor outcome. In addition, the complement system, which is abundant in the healthy brain, undergoes significant intra- and extracranial amplification following TBI. Previously considered to be structurally similar but separate systems, evidence of an interaction between the complement and coagulation systems in non-TBI cohorts has accumulated, with the activation of one system amplifying the activation of the other, independent of their established pathways. However, it is not known whether this interaction exists in TBI. In this review we summarize the available literature on complement activation following TBI, and the crosstalk between the complement and coagulation systems. We demonstrate how the complement system interacts with the coagulation cascade by activating the intrinsic coagulation pathway and by bypassing the initial cascade and directly producing thrombin as well. This crosstalk also effects platelets, where evidence points to a relationship with the complement system on multiple levels, with complement anaphylatoxins being able to induce disproportionate platelet activation and adhesion. The complement system also stimulates thrombosis by inhibiting fibrinolysis and stimulating endothelial cells to release prothrombotic microparticles. These interactions see clinical relevance in several disorders where a deficiency in complement regulation seems to result in a prothrombotic clinical presentation. Finally, based on these observations, we present the outline of an observational cohort study that is currently under preparation and aimed at assessing how complement influences coagulation in patients with isolated TBI.

Diabetes affects endothelial cell function and alters fibrin clot formation in a microvascular flow model: A pilot study

Diabetes is a proinflammatory and prothrombotic condition that increases the risk of vascular complications. The aim of this study was to develop a diabetic microvascular flow model that allows to study the complex interactions between endothelial cells, blood cells and plasma proteins and their effects on clot formation. Primary human cardiac microvascular endothelial cells from donors without diabetes or donors with diabetes (type 1 or type 2) were grown in a microfluidic chip, perfused with non-diabetic or diabetic whole blood, and clot formation was assessed by measuring fibrin deposition in real time by confocal microscopy. Clot formation in non-diabetic whole blood was significantly increased in the presence of endothelial cells from donors with type 2 diabetes compared with cells from donors without diabetes. There was no significant difference in clot formation between non-diabetic and diabetic whole blood. We present for the first time a diabetic microvascular flow model as a new tool to study clot formation as a result of the complex interactions between endothelial cells, blood cells and plasma proteins in a diabetes setting. We show that endothelial cells affect clot formation in whole blood, attributing an important role to the endothelium in the development of atherothrombotic complications.

Platelet thrombus formation in eHUS is prevented by anti-MBL2

E. coli associated Hemolytic Uremic Syndrome (epidemic hemolytic uremic syndrome, eHUS) caused by Shiga toxin-producing bacteria is characterized by thrombocytopenia, microangiopathic hemolytic anemia, and acute kidney injury that cause acute renal failure in up to 65% of affected patients. We hypothesized that the mannose-binding lectin (MBL) pathway of complement activation plays an important role in human eHUS, as we previously demonstrated that injection of Shiga Toxin-2 (Stx-2) led to fibrin deposition in mouse glomeruli that was blocked by co-injection of the anti-MBL-2 antibody 3F8. However, the markers of platelet thrombosis in affected mouse glomeruli were not delineated. To investigate the effect of 3F8 on markers of platelet thrombosis, we used kidney sections from our mouse model (MBL-2+/+ Mbl-A/C-/-; MBL2 KI mouse). Mice in the control group received PBS, while mice in a second group received Stx-2, and those in a third group received 3F8 and Stx-2. Using double immunofluorescence (IF) followed by digital image analysis, kidney sections were stained for fibrin(ogen) and CD41 (marker for platelets), von-Willebrand factor (marker for endothelial cells and platelets), and podocin (marker for podocytes). Electron microscopy (EM) was performed on ultrathin sections from mice and human with HUS. Injection of Stx-2 resulted in an increase of both fibrin and platelets in glomeruli, while administration of 3F8 with Stx-2 reduced both platelet and fibrin to control levels. EM studies confirmed that CD41-positive objects observed by IF were platelets. The increases in platelet number and fibrin levels by injection of Stx-2 are consistent with the generation of platelet-fibrin thrombi that were prevented by 3F8.

Complement and Coagulation: Cross Talk Through Time

Two complex protein defense systems-complement and coagulation-are based on amplifying enzyme cascades triggered by specific local stimuli. Excess systemic activation of either system is pathologic and is normally prevented by a family of regulatory proteins. The 2 systems are ancient biological processes which share a common origin that predates vertebrate evolution. Recent research has uncovered multiple opportunities for cross talk between complement and coagulation including proteins traditionally viewed as coagulation factors that activate and regulate complement, and proteins traditionally seen as part of the complement system that participate in coagulation. Ten examples of cross talk between the 2 systems are described. The mutual engagement of both systems is increasingly recognized to occur in human diseases. Three conditions-paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, and the antiphospholipid syndrome-provide examples of the importance of interactions between complement and coagulation in human biology. A better understanding of the mutual engagement of these 2 ancient defense systems is expected to result in improved diagnostics and new treatments for systemic diseases.

Role of complement in diabetes

Accumulating evidence suggests a role for the complement system in the pathogenesis of diabetes and the vascular complications that characterise this condition. Complement proteins contribute to the development of type 1 diabetes (T1D) by enhancing the underlying organ-specific autoimmune processes. Complement upregulation and activation is also an important feature of insulin resistance and the development of type 2 diabetes (T2D). Moreover, animal and human studies indicate that complement proteins are involved in the pathogenic mechanisms leading to diabetic microvascular and macrovascular complications. The adverse vascular effects of complement appear to be related to enhancement of the inflammatory process and the predisposition to a thrombotic environment, eventually leading to vascular occlusion. Complement proteins have been considered as therapeutic targets to prevent or treat vascular disease but studies have been mainly conducted in animal models, while human work has been both limited and inconclusive so far. Further studies are needed to understand the potential role of complement proteins as therapeutic targets for reversal of the pathological processes leading to T1D and T2D and for the prevention/treatment of diabetic vascular complications.

Mannose-Binding Lectin Drives Platelet Inflammatory Phenotype and Vascular Damage After Cerebral Ischemia in Mice via IL (Interleukin)-1α

Supplemental Digital Content is available in the text.

Objective—

Circulating complement factors are activated by tissue damage and contribute to acute brain injury. The deposition of MBL (mannose-binding lectin), one of the initiators of the lectin complement pathway, on the cerebral endothelium activated by ischemia is a major pathogenic event leading to brain injury. The molecular mechanisms through which MBL influences outcome after ischemia are not understood yet.

Approach and Results—

Here we show that MBL-deficient (MBL^−/−) mice subjected to cerebral ischemia display better flow recovery and less plasma extravasation in the brain than wild-type mice, as assessed by in vivo 2-photon microscopy. This results in reduced vascular dysfunction as shown by the shift from a pro- to an anti-inflammatory vascular phenotype associated with MBL deficiency. We also show that platelets directly bind MBL and that platelets from MBL^−/− mice have reduced inflammatory phenotype as indicated by reduced IL-1α (interleukin-1α) content, as early as 6 hours after ischemia. Cultured human brain endothelial cells subjected to oxygen-glucose deprivation and exposed to platelets from MBL^−/− mice present less cell death and lower CXCL1 (chemokine [C-X-C motif] ligand 1) release (downstream to IL-1α) than those exposed to wild-type platelets. In turn, MBL deposition on ischemic vessels significantly decreases after ischemia in mice treated with IL-1 receptor antagonist compared with controls, indicating a reciprocal interplay between MBL and IL-1α facilitating endothelial damage.

Conclusions—

We propose MBL as a hub of pathogenic vascular events. It acts as an early trigger of platelet IL-1α release, which in turn favors MBL deposition on ischemic vessels promoting an endothelial pro-inflammatory phenotype.

MASP-1 of the complement system alters fibrinolytic behaviour of blood clots

BACKGROUND

The lectin pathway serine protease mannan-binding lectin-associated serine protease 1 (MASP-1) has been demonstrated to be a major link between complement and coagulation, yet little is known about its interactions with the fibrinolytic system. The aim of this work was to assess the effects of MASP-1 on fibrin clot lysis in different experimental settings.

METHODS

Rotational thrombelastometry was used to evaluate the effect of MASP-1 on the lysis of clots formed in whole blood under static conditions. Whole blood clots were also formed in the presence and absence of MASP-1 under flow conditions in the Chandler loop and their lysis was analysed separately by fluorescence release of incorporated labelled fibrin. Real-time observation by laser scanning confocal microscopy was used to investigate the lysis of plasma clots where MASP-1 was present either during clot formation or lysis. Cleavage of tPA or plasminogen by MASP-1 was analysed by gel electrophoresis. We performed a turbidimetric clot lysis assay in the presence and absence of the MASP-1 inhibitor SGMI-1 (Schistocerca gregaria protease inhibitor (SGPI)-based MASP inhibitor-1) to evaluate the effect of endogenous MASP-1 in normal plasma and plasma samples from sepsis patients.

RESULTS

In the thrombelastometric experiments, where MASP-1 was present during the entire clotting and lysis process, MASP-1 had a significant profibrinolytic effect and accelerated clot lysis. When clots were formed in the presence of MASP-1 under flow in the Chandler loop, the effects on fibrinolysis were heterogenous with impaired fibrinolysis in some individuals (n = 5) and no (n = 3) or even the opposite effect (n = 2) in others. In plasma clot lysis observed by confocal microscopy, lysis was prolonged when MASP-1 was added to the lysis solution, yet there was no difference in lysis time when MASP-1 was present during clot formation. When MASP-1 was incubated with tPA or plasminogen, respectively, cleavage of single-chain tPA into two-chain tPA and a slight reduction of plasminogen were observed. SGMI-1 significantly prolonged clot lysis in the turbidimetric clot lysis assay suggesting that MASP-1 accelerated lysis in plasma samples.

CONCLUSION

MASP-1 is able to alter the susceptibility of blood clots to the fibrinolytic system. MASP-1 has complex, mostly promoting effects on fibrinolysis with high inter-individual variation. Interactions of MASP-1 with the fibrinolytic system may be relevant in the development and therapy of cardiovascular and thrombotic diseases.

Polymorphisms of mannose-binding lectin-associated serine protease 1 (MASP1) and its relationship with milk performance traits and complement activity in Chinese Holstein cattle

OBJECTIVE

Mannose-binding lectin (MBL)-associated serine protease1 (MASP1) is the central enzyme in the innate immune system, which has biological functions of antibacterial and anti-inflammatory activities. Moreover, MASP1 represents a candidate gene reflecting the complement activity. This study is to investigate the entire exons of MASP1 in Chinese Holstein cattle with DNA sequencing to identify novel single nucleotide polymorphisms (SNPs).

METHODS

Novel SNPs were identified through gene sequencing and genotyped by the PCR Restriction Fragment Length Polymorphism (PCR-RFLP) and Created Restriction Site PCR (CRS-PCR). The relationship between the milk performance traits and complement activity in Chinese Holstein cattle was analyzed using the General Linear Model (GLM) procedure with the SAS software (version 8.0).

RESULTS

Two novel SNPs (i.e., g.5766A > G and g.51228A > C) were detected. The SNP g.5766A > G was located in the first intron and the SNP g.51228A > C was located in the 3'-untranslated regions of MASP1. The polymorphism at g.5766A > G was correlated with protein percentage (P < 0.05). Moreover, the polymorphism at g.51228A > C had only two genotypes, and this SNP had no significant correlation with CH50, ACH50, fat percentage, protein percentage, 305-day milk yields, or SCS scores.

CONCLUSION

MASP1, reflecting the complement activity, may not be significantly related to mastitis. However, MASP1 could be implemented in the breeding program to improve the production performance of Chinese Holstein cattle.

Novel MASP-2 inhibitors developed via directed evolution of human TFPI1 are potent lectin pathway inhibitors

The lectin pathway (LP) of the complement system is an important antimicrobial defense mechanism, but it also contributes significantly to ischemia reperfusion injury (IRI) associated with myocardial infarct, stroke, and several other clinical conditions. Mannan-binding lectin-associated serine proteinase 2 (MASP-2) is essential for LP activation, and therefore, it is a potential drug target. We have previously developed the first two generations of MASP-2 inhibitors by in vitro evolution of two unrelated canonical serine proteinase inhibitors. These inhibitors were selective LP inhibitors, but their nonhuman origin rendered them suboptimal lead molecules for drug development. Here, we present our third-generation MASP-2 inhibitors that were developed based on a human inhibitor scaffold. We subjected the second Kunitz domain of human tissue factor pathway inhibitor 1 (TFPI1 D2) to directed evolution using phage display to yield inhibitors against human and rat MASP-2. These novel TFPI1-based MASP-2 inhibitor (TFMI-2) variants are potent and selective LP inhibitors in both human and rat serum. Directed evolution of the first Kunitz domain of TFPI1 had already yielded the potent kallikrein inhibitor, Kalbitor® (ecallantide), which is an FDA-approved drug to treat acute attacks of hereditary angioedema. Like hereditary angioedema, acute IRI is also related to the uncontrolled activation of a specific plasma serine proteinase. Therefore, TFMI-2 variants are promising lead molecules for drug development against IRI.

Exploring the structural similarity yet functional distinction between coagulation factor XIII-B and complement factor H sushi domains

Coagulation factor XIII (FXIII) covalently crosslinks pre-formed fibrin clots preventing their premature fibrinolysis. In plasma, FXIII circulates as a zymogenic heterotetramer composed of catalytic FXIII-A subunits, and carrier/regulatory FXIII-B subunits. FXIII-A is a well characterized component of this complex, and has been associated with several pleiotropic roles outside coagulation as well. In comparison only protective/regulatory roles towards the FXIII-A subunit have been identified for FXIII-B. Strong homology between FXIII-B and complement regulator Complement factor H suggests a putative role of FXIII-B in complement activation. In the current study we have analyzed the similarities and yet functional divergence of these two proteins using in silico sequence alignment and structural analysis. We have evaluated complement activation post reconstitution of FXIII components into FXIII deficient and CFH deficient plasma. We have also transiently expressed FXIII-B in SH-SY5Y cell lines and evaluated its effect on the endogenous complement activation. Our investigations show no effect of FXIII-B subunit on the rate of complement activation. Therefore we conclude that at a physiological level, FXIII-B subunit plays no role in the complement system, although a vestigial function in altered pathological states might still exist.

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.

The Lectin Pathway of Complement in Myocardial Ischemia/Reperfusion Injury-Review of Its Significance and the Potential Impact of Therapeutic Interference by C1 Esterase Inhibitor

Acute myocardial infarction (AMI) remains a leading cause of morbidity and mortality in modern medicine. Early reperfusion accomplished by primary percutaneous coronary intervention is pivotal for reducing myocardial damage in ST elevation AMI. However, restoration of coronary blood flow may paradoxically trigger cardiomyocyte death secondary to a reperfusion-induced inflammatory process, which may account for a significant proportion of the final infarct size. Unfortunately, recent human trials targeting myocardial ischemia/reperfusion (I/R) injury have yielded disappointing results. In experimental models of myocardial I/R injury, the complement system, and in particular the lectin pathway, have been identified as major contributors. In line with this, C1 esterase inhibitor (C1INH), the natural inhibitor of the lectin pathway, was shown to significantly ameliorate myocardial I/R injury. However, the hypothesis of a considerable augmentation of myocardial I/R injury by activation of the lectin pathway has not yet been confirmed in humans, which questions the efficacy of a therapeutic strategy solely aimed at the inhibition of the lectin pathway after human AMI. Thus, as C1INH is a multiple-action inhibitor targeting several pathways and mediators simultaneously in addition to the lectin pathway, such as the contact and coagulation system and tissue leukocyte infiltration, this may be considered as being advantageous over exclusive inhibition of the lectin pathway. In this review, we summarize current concepts and evidence addressing the role of the lectin pathway as a potent mediator/modulator of myocardial I/R injury in animal models and in patients. In addition, we focus on the evidence and the potential advantages of using the natural inhibitor of the lectin pathway, C1INH, as a future therapeutic approach in AMI given its ability to interfere with several plasmatic cascades. Ameliorating myocardial I/R injury by targeting the complement system and other plasmatic cascades remains a valid option for future therapeutic interventions.