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

Impacts of β-1, 3-N-acetylglucosaminyltransferases (B3GNTs) in human diseases

Glycosylation modification of proteins is a common post-translational modification that exists in various organisms and has rich biological functions. It is usually catalyzed by multiple glycosyltransferases located in the Golgi apparatus. β-1,3-N-acetylglucosaminyltransferases (B3GNTs) are members of the glycosyltransferases and have been found to be involved in the occurrence and development of a variety of diseases including autoimmunity diseases, cancers, neurodevelopment, musculoskeletal system, and metabolic diseases. The functions of B3GNTs represent the glycosylation of proteins is a crucial and frequently life-threatening step in progression of most diseases. In this review, we give an overview about the roles of B3GNTs in tumor, nervous system, musculoskeletal and metabolic diseases, describing the recent results about B3GNTs, in order to provide a research direction and exploration value for the prevention, diagnosis and treatment of these diseases.

Advances in proteomics: characterization of the innate immune system after birth and during inflammation

Proteomics is the characterization of the protein composition, the proteome, of a biological sample. It involves the large-scale identification and quantification of proteins, peptides, and post-translational modifications. This review focuses on recent developments in mass spectrometry-based proteomics and provides an overview of available methods for sample preparation to study the innate immune system. Recent advancements in the proteomics workflows, including sample preparation, have significantly improved the sensitivity and proteome coverage of biological samples including the technically difficult blood plasma. Proteomics is often applied in immunology and has been used to characterize the levels of innate immune system components after perturbations such as birth or during chronic inflammatory diseases like rheumatoid arthritis (RA) and inflammatory bowel disease (IBD). In cancers, the tumor microenvironment may generate chronic inflammation and release cytokines to the circulation. In these situations, the innate immune system undergoes profound and long-lasting changes, the large-scale characterization of which may increase our biological understanding and help identify components with translational potential for guiding diagnosis and treatment decisions. With the ongoing technical development, proteomics will likely continue to provide increasing insights into complex biological processes and their implications for health and disease. Integrating proteomics with other omics data and utilizing multi-omics approaches have been demonstrated to give additional valuable insights into biological systems.

The contribution of the alternative pathway in complement activation on cell surfaces depends on the strength of classical pathway initiation

Objectives

The complement system is an important component of innate immunity. The alternative pathway (AP) amplification loop is considered an essential feed forward mechanism for complement activation. However, the role of the AP in classical pathway (CP) activation has only been studied in ELISA settings. Here, we investigated its contribution on physiologically relevant surfaces of human cells and bacterial pathogens and in antibody-mediated complement activation, including in autoimmune haemolytic anaemia (AIHA) setting with autoantibodies against red blood cells (RBCs).

Methods

We evaluated the contribution of the AP to complement responses initiated through the CP on human RBCs by serum of AIHA patients and recombinant antibodies. Moreover, we studied complement activation on Neisseria meningitidis and Escherichia coli. The effect of the AP was examined using either AP-depleted sera or antibodies against factor B and factor D.

Results

We show that the amplification loop is redundant when efficient CP activation takes place. This is independent of the presence of membrane-bound complement regulators. The role of the AP may become significant when insufficient CP complement activation occurs, but this depends on antibody levels and (sub)class. Our data indicate that therapeutic intervention in the amplification loop will most likely not be effective to treat antibody-mediated diseases.

Conclusion

The AP can be bypassed through efficient CP activation. The AP amplification loop has a role in complement activation during conditions of modest activation via the CP, when it can allow for efficient complement-mediated killing.

Complement Inhibition in Kidney Transplantation: Where Are We Now?

Kidney transplantation is a life-saving strategy for patients with end-stage renal disease. Although progress has been made in the field of transplantation medicine in recent decades in terms of surgical techniques and immunosuppression, long-term organ survival remains a challenge. Also, for reasons of organ shortage, there is an unmet need for new therapeutic approaches to improve the long-term survival of transplants. There is increasing evidence that the complement system plays a crucial role in various pathological events after transplantation, including ischemia/reperfusion injury as well as rejection episodes. The complement system is part of the innate immune system and plays a crucial role in the defense against pathogens but is also involved in tissue homeostasis. However, the tightly regulated complement system can become dysregulated or activated by non-infectious stimuli, then targeting the organism's own cells and leading to inflammatory tissue damage that exacerbates injury. In this review, we will highlight the role of the complement system after transplantation and discuss ongoing and potential therapeutic approaches.

Effect of Polymorphisms in the FCN1, FCN2, and FCN3 Genes on the Susceptibility to Develop Rheumatoid Arthritis: A Systematic Review

Genetic association studies in rheumatoid arthritis conducted in various populations have yielded heterogeneous results. The present systematic review was conducted to synthesize the results of the studies in order to establish the impact of polymorphisms in the ficolin-coding genes FCN1, FCN2, and FCN3 on the susceptibility to develop rheumatoid arthritis. A systematic literature review was performed using the following keywords "gene (FCN1/FCN2/FCN3)", "Polymorphism/Genetic Variant", and "rheumatoid arthritis" in different databases until January 2022. Authors assessed articles by title/abstract and then assessed by full text for data extraction. The risk of bias was assessed using the Newcastle-Ottawa scale. Data synthesis was performed qualitatively and quantitatively. A total of 1519 articles were eligible for inclusion in this review, 3 were identified as relevant for the quantitative synthesis with 670 patients and 1019 controls. For the FCN1 gene, an association was found in the dominant and recessive genetic models of the variants rs2989727 (genotype TT = OR: 0.577, 95% CI: 0.430-0.769) and rs1071583 (genotype GG = OR: 1.537, 95% CI: 1.153-2.049, p = 0.0032) with the development of rheumatoid arthritis as a protective or susceptibility factor. FCN2 and FCN3 genes did not show association with disease development. The FCN1 gene variants rs2989727 and rs1071583 are associated with the risk of developing rheumatoid arthritis in populations from Brazil and Belgium, but not in FCN2 and FCN3 gene variants.

A mechanistic analysis of bacterial recognition and serine protease cascade initiation in larval hemolymph of Manduca sexta

Serine protease cascades have evolved in vertebrates and invertebrates to mediate rapid defense responses. Previous biochemical studies showed that in hemolymph of a caterpillar, Manduca sexta, recognition of fungi by β-1,3-glucan recognition proteins (βGRP1 and βGRP2) or recognition of bacteria by peptidoglycan recognition protein-1 (PGRP1) and microbe binding protein (MBP) results in autoactivation of hemolymph protease-14 precursor (proHP14). HP14 then activates downstream members of a protease cascade leading to the melanization immune response. ProHP14 has a complex domain architecture, with five low-density lipoprotein receptor class A repeats at its amino terminus, followed by a Sushi domain, a Sushi domain variant called Wonton, and a carboxyl-terminal serine protease catalytic domain. Its zymogen form is activated by specific proteolytic cleavage at the amino-terminal end of the protease domain. While a molecular mechanism for recognition and triggering the response to β-1,3-glucan has been delineated, it is unclear how bacterial recognition stimulates proHP14 activation. To fill this knowledge gap, we expressed the two domains of M. sexta MBP and found that the amino-terminal domain binds to diaminopimelic acid-peptidoglycan (DAP-PG). ProHP14 bound to both the carboxyl-terminal domain (MBP-C) and amino-terminal domain (MBP-N) of MBP. In the mixture of DAP-PG, MBP, and larval plasma, inclusion of an HP14 fragment composed of LDLa repeats 2-5 (LDLa2-5) or MBP-C significantly reduced prophenoloxidase activation, likely by competing with the interactions of the full-length proteins, and suggesting that molecular interactions involving these regions of proHP14 and MBP take part in proHP14 activation in response to peptidoglycan. Using a series of N-terminally truncated versions of proHP14, we found that autoactivation required LDLa2-5. The optimal ratio of PGRP1, MBP, and proHP14 is close to 3:2:1. In summary, proHP14 autoactivation by DAP-type peptidoglycan requires binding of DAP-PG by PGRP1 and the MBP N-terminal domain and association of the LDLa2-5 region of proHP14 with the MBP C-terminal domain. These interactions may concentrate the proHP14 zymogen at the bacterial cell wall surface and promote autoactivation.

Mannose Binding Lectin-Associated Serine Protease 2 (MASP2) Gene Polymorphism and susceptibility to Human T-lymphotropic virus type 1 (HTLV-1) Infection in Blood Donors of Mashhad, Iran

Mannose binding lectin-associated serine protease 2 (MASP2) is the effector part of mannose binding lectin (MBL) that activates the complement system in an antibody-independent manner. We aimed to investigate the role of genetic polymorphisms in the MASP2 gene and susceptibility to HTLV-1 infection. A total of 172 HTLV-1 infected individuals and 170 healthy blood donors were analyzed in this case-control study. Nine single nucleotide polymorphisms (SNPs) encompassing different regions of the MASP2 gene were genotyped with a PCR-SSP assay. The relation between SNPs genotype and susceptibility to HTLV-1 infection was investigated with a chi-squared test considering p<0.05 as statistically significant. Two out of nine tested SNPs were associated with the risk of HTLV-1 infection. The genotype TT at rs17409276 decreased the risk of HTLV-1 (p=0.005, OR=0.301, 95% CI=0.124-0.728). The genotypes CC and CT at rs2273346 were also associated with a higher risk of HTLV-1 acquisition (p=0.004, OR=2.225, 95% CI=1.277-3.877). These findings highlight the importance of MASP2 genetic polymorphisms in the lectin pathway of complement activation and susceptibility to HTLV-1 infection. This article is protected by copyright. All rights reserved.

Hormone sensitive lipase ablation promotes bone regeneration

There is an inverse relationship between the differentiation of mesenchymal stem cells (MSCs) along either an adipocyte or osteoblast lineage, with lineage differentiation known to be mediated by transcription factors PPARγ and Runx2, respectively. Endogenous ligands for PPARγ are generated during the hydrolysis of triacylglycerols to fatty acids through the actions of lipases such as hormone sensitive lipase (HSL). To examine whether reduced production of endogenous PPARγ ligands would influence bone regeneration, we examined the effects of HSL knockout on fracture repair in mice using a tibial mono-cortical defect as a model. We found an improved rate of fracture repair in HSL-ko mice documented by serial μCT and bone histomorphometry compared to wild-type (WT) mice. Similarly, accelerated rates of bone regeneration were observed with a calvarial model where implantation of bone grafts from HSL-ko mice accelerated bone regeneration at the injury site. Further analysis revealed improved MSC differentiation down osteoblast and chondrocyte lineage with inhibition of HSL. MSC recruitment to the injury site was greater in HSL-ko mice than WT. Finally, we used single cell RNAseq to understand the osteoimmunological differences between WT and HSL-ko mice and found changes in the pre-osteoclast population. Our study shows HSL-ko mice as an interesting model to study improvements to bone injury repair. Furthermore, our study highlights the potential importance of pre-osteoclasts and osteoclasts in bone repair.

Compstatins: the dawn of clinical C3-targeted complement inhibition

Despite the growing recognition of the complement system as a major contributor to a variety of clinical conditions, the therapeutic arsenal has remained scarce. The introduction of an anti-C5 antibody in 2007 raised confidence in complement-targeted therapy. However, it became apparent that inhibition of late-stage effector generation might not be sufficient in multifactorial complement disorders. Upstream intervention at the level of C3 activation has therefore been considered promising. The approval of pegcetacoplan, a C3 inhibitor of the compstatin family, in 2021 served as critical validation of C3-targeted treatment. This review delineates the evolution of the compstatin family from its academic origins to the clinic and highlights current and potential future applications of this promising drug class in complement diseases.

Complement in ischaemia-reperfusion injury and transplantation

Until recently, the only known condition in which complement could mediate transplant injury was the rare occurrence of antibody-mediated rejection, in which the original concept of antibody immunity against the transplant was supported by complementary proteins present in the serum. This has changed within the last two decades because of evidence that the processes of ischaemia–reperfusion injury followed by T cell–mediated rejection are also critically dependent on components generated by the complement system. We now have a clearer understanding of the complement triggers and effectors that mediate injury, and a detailed map of their local sites of production and activation in the kidney. This is providing helpful guidelines as to how these harmful processes that restrict transplant outcomes can be targeted for therapeutic benefit. Here we review some of the recent advances highlighting relevant therapeutic targets.

Complement cascade functions during brain development and neurodegeneration

The complement system, an essential tightly regulated innate immune system, is a key regulator of normal central nervous system (CNS) development and function. However, aberrant complement component expression and activation in the brain may culminate into marked neuroinflammatory response, neurodegenerative processes and cognitive impairment. Over the years, complement-mediated neuroinflammatory responses and complement-driven neurodegeneration have been increasingly implicated in the pathogenesis of a wide spectrum of CNS disorders. This review describes how complement system contributes to normal brain development and function. We also discuss how pathologic insults such as misfolded proteins, lipid droplet/lipid droplet-associated protein or glycosaminoglycan accumulation could trigger complement-mediated neuroinflammatory responses and neurodegenerative process in neurodegenerative proteinopathies, age-related macular degeneration and neurodegenerative lysosomal storage disorders.

Emerging role of ficolins in autoimmune diseases

Ficolins are pattern-recognition molecules (PRMs) that could form complexes with mannose-binding lectin-associated serine proteases (MASPs) to trigger complement activation via the lectin pathway, thereby mediating a series of immune responses including opsonization, phagocytosis and cytokine production. In the past few decades, accumulating evidence have suggested that ficolins play a major role in the onset and development of several autoimmune diseases (ADs), including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc), Type 1 diabetes (T1D), inflammatory bowel disease (IBD), etc. In this review, we synthesized previous literatures and recent advances to elucidate the immunological regulations of ficolins and discuss the potential diagnostic ability of ficolins in ADs, as well as giving an insight into the future therapeutic options for ficolins in ADs.

The complement lectin pathway protein MAp19 and out-of-hospital cardiac arrest: Insights from two randomized clinical trials

Aim:

Activation of the complement system is known to be a potent inducer of systemic inflammation, which is an important component of post-cardiac arrest syndrome. Mannan-binding-lectin associated protein of 19 kDa (MAp19) is suggested to be a regulatory component of the lectin pathway of complement activation. The aims of this study were to describe serial levels of MAp19 protein in comatose survivors of out-of-hospital cardiac arrest (OHCA), to evaluate the effect of two different regimes of targeted temperature management and to investigate the possible association between levels of MAp19 and mortality.

Methods:

In this post-hoc study, we analysed data from two large randomized controlled studies: ‘Targeted temperature management at 33 degrees C versus 36 degrees C after cardiac arrest’ (TTM) and ‘Targeted temperature management for 48 versus 24 h and neurological outcome after out-of-hospital cardiac arrest’ (TTH). We measured serial levels of MAp19 in 240 patients within 72 h after OHCA and in 82 healthy controls. The effect of targeted temperature management on MAp19 levels was analysed according to temperature allocation in main trials.

Results:

MAp19 levels were significantly lower in OHCA patients within 48 h after OHCA (p-values &lt;0.001) compared with healthy controls. A target temperature at 33°C compared with 36°C for 24 h was associated with significantly lower levels of MAp19 (–57 ng/mL (95% confidence interval (CI): –97 to −16 mg/mL), p=0.006). Target temperature at 33°C for 48 h compared with 24 h was not associated with a difference in MAp19 levels (–31 ng/mL (95% CI: –120 to 60 mg/mL), p=0.57). Low MAp19 levels at admission were associated with higher 30-day mortality (12% vs. 38%, plog-rank =0.0008), also in adjusted analysis (two-fold higher, hazard ratio =0.48 (95% CI: 0.31 to 0.75), p=0.001). Analysis of MAp19 levels at 24–72 h showed they were not associated with 30-day mortality.

Conclusion:

Survivors after OHCA have lower levels of MAp19 protein compared with healthy controls. A targeted temperature management at 33°C compared with 36°C was associated with significantly lower MAp19 levels, whereas target temperature at 33°C for 48 h compared with 24 h did not influence MAp19 protein levels. Low MAp19 levels at admission were independently associated with increased mortality.

Complement proteins detected through iTRAQ-based proteomics analysis of serum from black carp Mylopharyngodon piceus in response to experimentally induced Aeromonas hydrophila infection

The black carp Mylopharyngodon piceus is one of the culturally important '4 famous domestic fishes' in China. Recently, infectious diseases caused by Aeromonas hydrophila have drastically altered the operation of the black carp farming industry. In the present study, isobaric tags for relative and absolute quantitation (iTRAQ) were combined with mass spectrometry analysis to screen for differentially abundant black carp serum proteins in response to experimentally induced A. hydrophila infection. A total of 86 differentially abundant proteins were quantified at 24 h post-infection, including 78 down-regulated proteins and 8 up-regulated proteins. The down-regulated proteins included complement C1q subcomponent subunit C, complement factor B/C2A, complement factor B/C2B, complement C3-Q1, complement C3, and complement C4-2. Bioinformatic analysis indicated that the differentially abundant proteins were mainly associated with complement and coagulation cascades (27.9%). Moreover, real-time PCR (qPCR) analysis revealed changes in the gene expression of both C3 and B/C2A in blood cells, liver, kidney, gills, and intestines of the black carp infected with A. hydrophila. However, mRNA expression levels did not consistently correlate with the corresponding protein levels. A polyclonal antibody was prepared using a synthetic C3 peptide. Immunofluorescence analysis showed that the expression of C3 in the kidney was increased with A. hydrophila infection. This work provides a useful characterization of the impact of A. hydrophila infection on the complement system of the black carp.

Association of Polymorphisms of MASP1/3, COLEC10, and COLEC11 Genes with 3MC Syndrome

The Malpuech, Michels, Mingarelli, Carnevale (3MC) syndrome is a rare, autosomal recessive genetic- disorder associated with mutations in the MASP1/3, COLEC1,1 or COLEC10 genes. The number of 3MC patients with known mutations in these three genes reported so far remains very small. To date, 16 mutations in MASP-1/3, 12 mutations in COLEC11 and three in COLEC10 associated with 3MC syndrome have been identified. Their products play an essential role as factors involved in the activation of complement via the lectin or alternative (MASP-3) pathways. Recent data indicate that mannose-binding lectin-associated serine protease-1 (MASP-1), MASP-3, collectin kidney-1 (collectin-11) (CL-K1), and collectin liver-1 (collectin-10) (CL-L1) also participate in the correct migration of neural crest cells (NCC) during embryogenesis. This is supported by relationships between MASP1/3, COLEC10, and COLEC11 gene mutations and the incidence of 3MC syndrome, associated with craniofacial abnormalities such as radioulnar synostosis high-arched eyebrows, cleft lip/palate, hearing loss, and ptosis.

Components of the Lectin Pathway of Complement in Haematologic Malignancies

The complement system is activated cascadically via three distinct major routes: classical pathway (CP), alternative pathway (AP) or lectin pathway (LP). The unique factors associated with the latter are collectins (mannose-binding lectin, collectin-10, collectin-11), ficolins (ficolin-1, ficolin-2, ficolin-3) and proteins of the mannose-binding lectin-associated serine protease (MASP) family (MASP-1, MASP-2, MASP-3, MAp19, MAp44). Collectins and ficolins are both pattern-recognising molecules (PRM), reactive against pathogen-associated molecular patterns (PAMP) or danger-associated molecular patterns (DAMP). The MASP family proteins were first discovered as complexes with mannose-binding lectin (MBL) and therefore named MBL-associated serine proteases, but later, they were found to interact with ficolins, and later still, collectin-10 and collectin-11. As well as proteolytic enzymes (MASP-1, MASP-2, MASP-3), the group includes non-enzymatic factors (MAp19, MAp44). In this review, the association-specific factors of the lectin pathway with haematologic malignancies and related infections are discussed.

Host genetic variability and pneumococcal disease: a systematic review and meta-analysis

Background

Pneumonia, sepsis, meningitis, and empyema due to Streptococcus pneumoniae is a major cause of morbidity and mortality. We provide a systemic overview of genetic variants associated with susceptibility, phenotype and outcome of community acquired pneumococcal pneumonia (CAP) and invasive pneumococcal disease (IPD).

Methods

We searched PubMed for studies on the influence of host genetics on susceptibility, phenotype, and outcome of CAP and IPD between Jan 1, 1983 and Jul 4, 2018. We listed methodological characteristics and when genetic data was available we calculated effect sizes. We used fixed or random effect models to calculate pooled effect sizes in the meta-analysis.

Results

We identified 1219 studies of which 60 studies involving 15,358 patients were included. Twenty-five studies (42%) focused on susceptibility, 8 (13%) on outcome, 1 (2%) on disease phenotype, and 26 (43%) on multiple categories. We identified five studies with a hypothesis free approach of which one resulted in one genome wide significant association in a gene coding for lincRNA with pneumococcal disease susceptibility. We performed 17 meta-analyses of which two susceptibility polymorphisms had a significant overall effect size: variant alleles of MBL2 (odds ratio [OR] 1·67, 95% confidence interval [CI] 1·04–2·69) and a variant in CD14 (OR 1·77, 95% CI 1·18–2·66) and none of the outcome polymorphisms.

Conclusions

Studies have identified several host genetics factors influencing risk of pneumococcal disease, but many result in non-reproducible findings due to methodological limitations. Uniform case definitions and pooling of data is necessary to obtain more robust findings.

Electronic supplementary material

The online version of this article (10.1186/s12920-019-0572-x) contains supplementary material, which is available to authorized users.

Association between -221 X/Y polymorphism of mannose-binding lectin (MBL) gene and susceptibility to HTLV-1 infection among people from an endemic region in the Northeast of Iran

BACKGROUND

The role of (MBL) gene single nucleotide polymorphisms (SNPs) has been well documented in susceptibility to several infectious diseases. This study aimed to investigate the association between two MBL promoter variants, -550 H/L and -221 X/Y, and susceptibility to HTLV-1 infection.

METHODS

A total of 153 subjects infected with HTLV-1 and 169 healthy controls were recruited. SSP-PCR method was applied to genotype -550 H/L and -221 X/Y polymorphisms. Associations between genotypes or alleles and susceptibility to HTLV-1 infection were analyzed by Pearson's Chi-Square. p ≤ .05 was considered statistically significant.

RESULTS

Statistical analysis revealed significant differences between the two groups in the -221 position (χ2 = 19.709; p = .000). The MBL YX genotype was significantly associated with increased susceptibility to HTLV-1 (OR = 2.73, %95 CI = 1.74-4.30). Combined genotype of the two loci showed that the HYHX genotype (OR = 2.20, 95% CI = 1.95-2.48) and LYLX (OR = 1.97, 95% CI = 1.13-3.45) were associated with an increased risk of HTLV-1 infection.

CONCLUSION

Our results represent the importance of -221 X > Y variants in acquisition of HTLV-1 as this is the case for several other viral and bacterial infections.

An overview of lipodystrophy and the role of the complement system

The complement system is a major component of innate immunity playing essential roles in the destruction of pathogens, the clearance of apoptotic cells and immune complexes, the enhancement of phagocytosis, inflammation, and the modulation of adaptive immune responses. During the last decades, numerous studies have shown that the complement system has key functions in the biology of certain tissues. For example, complement contributes to normal brain and embryonic development and to the homeostasis of lipid metabolism. However, the complement system is subjected to the effective balance between activation-inactivation to maintain complement homeostasis and to prevent self-injury to cells or tissues. When this control is disrupted, serious pathologies eventually develop, such as C3 glomerulopathy, autoimmune conditions and infections. Another heterogeneous group of ultra-rare diseases in which complement abnormalities have been described are the lipodystrophy syndromes. These diseases are characterized by the loss of adipose tissue throughout the entire body or partially. Complement over-activation has been reported in most of the patients with acquired partial lipodystrophy (also called Barraquer-Simons Syndrome) and in some cases of the generalized variety of the disease (Lawrence Syndrome). Even so, the mechanism through which the complement system induces adipose tissue abnormalities remains unclear. This review focuses on describing the link between the complement system and certain forms of lipodystrophy. In addition, we present an overview regarding the clinical presentation, differential diagnosis, classification, and management of patients with lipodystrophy associated with complement abnormalities.

A Rare Genetic Defect of MBL2 Increased the Risk for Progression of IgA Nephropathy

The aim of this study was to investigate the association between lectin pathway-related genetic variations and progression in IgA nephropathy. Biopsy-proven IgAN patients with eGFR ≥15 ml/min/1.73 m2 at baseline and a minimum follow-up of 12-months were enrolled. A total of 1,007 patients and 121 healthy controls were enrolled from two Chinese renal centers. The discovery cohort consisted of 606 patients, and the validation cohort consisted of 401 patients. First, promoters, all exons and their boundary regions of MBL2 and FCN2 were sequenced in 50 patients, and then 37 variations were identified. Of these variations, 7 expression-associated variations were selected and genotyped in the whole discovery cohort. We found that rs1800450 in MBL2 and rs7851696 in FCN2 were associated with an increased risk for ESRD as well as serum MBL or L-ficolin levels. However, only rs1800450 was successively validated for its association with ESRD (HR, 15.91; 3.27-77.34; P = 0.001) in the fully adjusted model in the validation cohort. In addition, 2.7% of patients, and 2.5% of healthy controls carried rs1800450-AA. IgAN patients with rs1800450-AA lacked expression of MBL in both serum and renal tissue and had more severe tubulointerstitial damage. Furthermore, a combined effect of rs1800450-AA with a previously reported clinical risk score was observed in which patients with both a high clinical risk score (≥1%) and rs1800450-AA had a strikingly increased 10-years ESRD risk by 37.1-fold (7.17 to 192.13-fold). In summary, IgAN patients carrying MBL2 rs1800450-AA have a high risk for renal function deterioration, probably due to inactivation of the complement MBL pathway.

Association of ficolin-2 (FCN2) functional polymorphisms and protein levels with rheumatic fever and rheumatic heart disease: relationship with cardiac function

Introduction

A role for ficolin (FCN) 2 gene polymorphisms in the pathogenesis of recurrent severe streptococcal infections and rheumatic carditis has been suggested. The aim of the study was to evaluate a possible relationship between single nucleotide polymorphisms located at positions -602 and -4 of the FCN2 gene and FCN2 serum levels and risk of development of rheumatic fever (RF) and rheumatic heart disease (RHD).

Material and methods

Seventy-seven Caucasian Egyptian patients with RF were recruited with a control group of 43 healthy subjects. DNA was extracted for analysis of the FCN2 gene at positions -602 and -4 and serum protein level was measured by ELISA.

Results

FCN2 AA genotype at the -4 position was more frequently observed in RF and RHD patients, as compared to healthy subjects (p = 0.005 and p = 0.013, respectively); furthermore, the A allele was identified as a possible risk factor for the development of RF (p = 0.023, OR = 1.852, 95% CI: 1.085–3.159). The haplotype –602/–4 G/A, which was associated with low median levels of L-ficolin, was observed more frequently in the RF group when compared to the healthy subjects (74/162, 48.1% vs. 29/420, 33.7%, OR = 1.834, 95% CI: 1.034–3.252, p = 0.038). Low serum ficolin-2 level was associated with ESV and EDV increases. FCN 2 level was significantly lower with AA genotypes than GG+AG genotypes of the -4 position (56.68 ±17.90 vs. 66.05 ±18.79, p = 0.008).

Conclusions

Polymorphisms linked to low levels of L-ficolin may render an individual at risk of recurrent and/or severe streptococcal infection. The -4 AA genotype and -602/-4 G/A haplotype are possible risk factors for the development of carditis.

C1q-Mediated Complement Activation and C3 Opsonization Trigger Recognition of Stealth Poly(2-methyl-2-oxazoline)-Coated Silica Nanoparticles by Human Phagocytes

Poly(2-methyl-2-oxazoline) (PMOXA) is an alternative promising polymer to poly(ethylene glycol) (PEG) for design and engineering of macrophage-evading nanoparticles (NPs). Although PMOXA-engineered NPs have shown comparable pharmacokinetics and in vivo performance to PEGylated stealth NPs in the murine model, its interaction with elements of the human innate immune system has not been studied. From a translational angle, we studied the interaction of fully characterized PMOXA-coated vinyltriethoxysilane-derived organically modified silica NPs (PMOXA-coated NPs) of approximately 100 nm in diameter with human complement system, blood leukocytes, and macrophages and compared their performance with PEGylated and uncoated NP counterparts. Through detailed immunological and proteomic profiling, we show that PMOXA-coated NPs extensively trigger complement activation in human sera exclusively through the classical pathway. Complement activation is initiated by the sensing molecule C1q, where C1q binds with high affinity ( K_d = 11 ± 1 nM) to NP surfaces independent of immunoglobulin binding. C1q-mediated complement activation accelerates PMOXA opsonization with the third complement protein (C3) through the amplification loop of the alternative pathway. This promoted NP recognition by human blood leukocytes and monocyte-derived macrophages. The macrophage capture of PMOXA-coated NPs correlates with sera donor variability in complement activation and opsonization but not with other major corona proteins, including clusterin and a wide range of apolipoproteins. In contrast to these observations, PMOXA-coated NPs poorly activated the murine complement system and were marginally recognized by mouse macrophages. These studies provide important insights into compatibility of engineered NPs with elements of the human innate immune system for translational steps.

Identification of polymorphisms in the bovine collagenous lectins and their association with infectious diseases in cattle

Infectious diseases are a significant issue in animal production systems, including both the dairy and beef cattle industries. Understanding and defining the genetics of infectious disease susceptibility in cattle is an important step in the mitigation of their impact. Collagenous lectins are soluble pattern recognition receptors that form an important part of the innate immune system, which serves as the first line of host defense against pathogens. Polymorphisms in the collagenous lectin genes have been shown in previous studies to contribute to infectious disease susceptibility, and in cattle, mutations in two collagenous lectin genes (MBL1 and MBL2) are associated with mastitis. To further characterize the contribution of variation in the bovine collagenous lectins to infectious disease susceptibility, we used a pooled NGS approach to identify short nucleotide variants (SNVs) in the collagenous lectins (and regulatory DNA) of cattle with (n = 80) and without (n = 40) infectious disease. Allele frequency analysis identified 74 variants that were significantly (p < 5 × 10⁻⁶) associated with infectious disease, the majority of which were clustered in a 29-kb segment upstream of the collectin locus on chromosome 28. In silico analysis of the functional effects of all the variants predicted 11 SNVs with a deleterious effect on protein structure and/or function, 148 SNVs that occurred within potential transcription factor binding sites, and 31 SNVs occurring within potential miRNA binding elements. This study provides a detailed look at the genetic variation of the bovine collagenous lectins and identifies potential genetic markers for infectious disease susceptibility.

Structural and functional diversity of collectins and ficolins and their relationship to disease

Pattern recognition molecules are sensors for the innate immune system and trigger a number of pathophysiological functions after interaction with the corresponding ligands on microorganisms or altered mammalian cells. Of those pattern recognition molecules used by the complement system, collagen-like lectins (collectins) are an important subcomponent. Whereas the best known of these collectins, mannose-binding lectin, largely occurs as a circulating protein following production by hepatocytes, the most recently described collectins exhibit strong local biosynthesis. This local production and release of soluble collectin molecules appear to serve local tissue functions at extravascular sites, including a developmental function. In this article, we focus on the characteristics of collectin-11 (CL-11 or CL-K1), whose ubiquitous expression and multiple activities likely reflect a wide biological relevance. Collectin-11 appears to behave as an acute phase protein whose production associated with metabolic and physical stress results in locally targeted inflammation and tissue cell death. Early results indicate the importance of fucosylated ligand marking the injured cells targeted by collectin-11, and we suggest that further characterisation of this and related ligands will lead to better understanding of pathophysiological significance and exploitation for clinical benefit.

Inhibition of immune responses and related proteins in Rhamdia quelen exposed to diclofenac

Nonsteroidal anti-inflammatory drugs are among the most widely detected pharmaceuticals in surface water worldwide. The nonsteroidal anti-inflammatory drug diclofenac is used to treat many types of pain and inflammation. Diclofenac's potential to cause adverse effects in exposed wildlife is a growing concern. To evaluate the effects of waterborne diclofenac on the immune response in Rhamdia quelen (South American catfish), fish were exposed to 3 concentrations of diclofenac (0.2, 2.0, and 20.0 μg/L) for 14 d. Some of the exposed fish were also given an intraperitoneal injection on day 14 of 1 mg/kg of carrageenan to evaluate cell migration to the peritoneum. Total blood leukocyte count and carrageenan-induced leukocyte migration to the peritoneal cavity, particularly of polymorphonuclear cells, were significantly affected for all diclofenac exposure groups. Nitric oxide production was significantly reduced in the diclofenac-treated fish. Plasma and kidney proteins were analyzed by means of liquid chromatography-tandem mass spectrometry in a shotgun proteomic approach. In both plasma and kidney of diclofenac-exposed R. quelen, the expression of 20 proteins related to the inflammatory process, nitric oxide production, leukocyte migration, and the complement cascade was significantly altered. In addition, class I major histocompatibility complex was significantly decreased in plasma of diclofenac-treated fish. Thus, waterborne exposure to diclofenac could lead to suppression of the innate immune system in R. quelen. Environ Toxicol Chem 2017;36:2092-2107. © 2017 SETAC.

Construction of human MASP-2-CCP1/2SP, CCP2SP, SP plasmid DNA nanolipoplexes and the effects on tuberculosis in BCG-infected mice

The lectin pathway, one of the complement cascade systems, provides the primary line of defense against invading pathogens. The serine protease of MASP-2 plays an essential role in complement activation of the lectin pathway. The C-terminal segment of MASP-2 is comprised of the CCP1-CCP2-SP domains, and is the crucial catalytic segment. However, what is the effect of CCP1-CCP2-SP domains in controlling chronic infection is unknown. In order to evaluate the potential impact of CCP1-CCP2-SP domains on tuberculosis, we constructed the human MASP-2 CCP1/2SP, CCP2SP and SP recombinant plasmids, and delivered these plasmids by DNA-DOTAP:cholesterol cationic nanolipoplexes to BCG-infected mice. After 21 days post DNA-DOTAP:chol nanolipoplexes application, we analyzed bacteria loads of pulmonary, pathology of granuloma, lymphocyte subpopulations. The C3a, C4a and MASP-2 levels in serum were measured with enzyme-linked immunosorbent assays. Compared to the control group that received GFP DNA-DOTAP:chol nanolipoplexes, MASP-2 CCP1/2SP DNA-DOTAP:chol nanolipoplexes treated group showed significantly enlarged pulmonary granulomas lesion (P < 0.05) and did not reduce bacteria loads in the lung tissue (P < 0.05). Furthermore, the levels of C3a in serum were decreased (P < 0.05), the number and percentage of PD1⁺ and Tim3⁺ cells subgroups were increased in BCG-infected mice after treated with MASP-2 CCP1/2SP DNA-DOTAP:chol nanolipoplexes (P < 0.05). But, there was no statistical difference in the serum C4a and MASP-2 level among DNA nanolipoplexes treated groups (P > 0.05). These findings provided experimental evidence that MASP-2 CCP1/2SP DNA nanolipoplexes shown the negative efficacy in controlling Mycobacterium tuberculosis infection, and displayed a potential role of down-regulating T-cell-mediated immunity in tuberculosis.

Association of MBL2, TLR1, TLR2 and TLR6 Polymorphisms With Production of IFN-γ and IL-12 in BCG Osteitis Survivors R1

BACKGROUND

Interferon-gamma (IFN-γ) is a key cytokine in defense against mycobacteria, including Bacillus Calmette-Guérin (BCG). Mannose-binding lectin (MBL) and toll-like receptors (TLRs) are pattern-recognizing molecules of innate immunity. The aim of the present study was to investigate the relationship between polymorphisms in MBL, TLR1, TLR2 and TLR6 encoding genes and stimulated IFN-γ and interleukin-12 (IL-12) ex vivo production in BCG osteitis survivors.

METHODS

Data on single nucleotide polymorphisms in the MBL2 gene and TLR1, TLR2 and TLR6 genes were available from 132 former BCG osteitis patients, and data on ex vivo IFN-γ and IL-12 production were available from 115 and 118 patients, respectively. The present study is a secondary analysis of these available data. In an earlier study, we were able to characterize low IFN-γ and low IL-12 producers after BCG+IL-12 or BCG+IFN-γ stimulations, respectively.

RESULTS

Three patients had the homozygous variant MBL2 genotype, and one of them was a low IFN-γ producer (both concentration and response <5th percentile). The heterozygous variant MBL2 genotype showed no association with IFN-γ or IL-12 production. The TLR2 variant genotype was present in 14 subjects; 28.6% of them were low IFN-γ producers versus 7.8% of those 103 with the TLR2 wild genotype (P = 0.037). TLR1 or TLR6 polymorphisms had no significant associations with stimulated ex vivo IFN-γ or IL-12 production.

CONCLUSIONS

Preliminary evidence was found that variant genotypes of the MBL2 gene (if homozygous) and variant genotypes of the TLR2 gene (only heterozygotes present) are associated with low IFN-γ production.

Collectin Liver 1 and Collectin Kidney 1 of the Lectin Complement Pathway Are Associated With Mortality After Kidney Transplantation

Kidney transplanted patients still have significantly higher mortality compared with the general population. The innate immune system may play an important role during periods, with suppression of the adaptive immune system. In the present study, two soluble pattern recognition molecules of the innate immune system were investigated, collectin liver 1 (CL-L1) and collectin kidney 1 (CL-K1). Potential associations of their pretransplant levels and long-term graft and recipient survival were examined. The levels of CL-L1 and CL-K1 were measured at the time of transplantation in 382 patients (≥17 years) transplanted in 2000-2001. The cohort was subsequently followed until December 31, 2014. Data on patient and graft survival were obtained from the Norwegian Renal Registry. Both high CL-L1 (≥376 ng/mL) and high CL-K1 (≥304 ng/mL) levels were significantly associated with overall mortality in multivariate Cox analyses with hazard ration (HR) 1.50, 95% confidence interval (CI) 1.09-2.07, p = 0.013 and HR 1.43, 95% CI 1.02-1.99, p = 0.038, respectively. Moreover, high CL-K1 levels were significantly associated with cardiovascular mortality. No association between measured biomarkers and death-censored graft loss was found. Finally, there was a significant correlation between these two collectins, r = 0.83 (95% CI 0.80-0.86). In conclusion, CL-L1 and CL-K1 were significantly associated with mortality in kidney transplant recipients.

Sequences and expression of pathway-specific complement components in developing red-tailed phascogale (Phascogale calura)

Marsupials are born immunologically premature, relying on cells and molecules in maternal milk for immune protection. Both immunoglobulin and complement proteins have been identified in marsupial milk, but the expression of specific complement proteins remains largely unexplored. We report partial cDNA sequences for two complement-activating proteins, C3, C1r, CFP and MASP2, in liver tissues from red-tailed phascogale (Phascogale calura). Conservation of functionally relevant motifs were identified in the translated cDNA sequences from phascogale C3, CFP and MASP2 and their eutherian homologues. Gene expression of representative molecules from each of the major complement pathways was also investigated in whole body tissues from 1 to 18 day old animals and liver tissues from 31-day to 14-month old animals. Average complement expression in whole bodies and liver tissues of C1r, CFP, MASP2 and C3 increased significantly in juveniles compared to pouch young, presumably due to the maturation of the young's own complement system. Comparing expression in liver tissues only, we found that the average CFP expression were higher in pouch young compared to juveniles, while results were still statistically similar to the average expression of all tissues for C1r, MASP2 and C3. The average complement expression then significantly decreased as the animals aged into adulthood.

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 in disease: a defence system turning offensive

Although the complement system is primarily perceived as a host defence system, a more versatile, yet potentially more harmful side of this innate immune pathway as an inflammatory mediator also exists. The activities that define the ability of the complement system to control microbial threats and eliminate cellular debris - such as sensing molecular danger patterns, generating immediate effectors, and extensively coordinating with other defence pathways - can quickly turn complement from a defence system to an aggressor that drives immune and inflammatory diseases. These host-offensive actions become more pronounced with age and are exacerbated by a variety of genetic factors and autoimmune responses. Complement can also be activated inappropriately, for example in response to biomaterials or transplants. A wealth of research over the past two decades has led to an increasingly finely tuned understanding of complement activation, identified tipping points between physiological and pathological behaviour, and revealed avenues for therapeutic intervention. This Review summarizes our current view of the key activating, regulatory, and effector mechanisms of the complement system, highlighting important crosstalk connections, and, with an emphasis on kidney disease and transplantation, discusses the involvement of complement in clinical conditions and promising therapeutic approaches.

Complement MASP-1 enhances adhesion between endothelial cells and neutrophils by up-regulating E-selectin expression

The complement system and neutrophil granulocytes are indispensable in the immune response against extracellular pathogens such as bacteria and fungi. Endothelial cells also participate in antimicrobial immunity largely by regulating the homing of leukocytes through their cytokine production and their pattern of cell surface adhesion molecules. We have previously shown that mannan-binding lectin-associated serine protease-1 (MASP-1), a complement lectin pathway enzyme, is able to activate endothelial cells by cleaving protease activated receptors, which leads to cytokine production and enables neutrophil chemotaxis. Therefore, we aimed to investigate how recombinant MASP-1 (rMASP-1) can modify the pattern of P-selectin, E-selectin, ICAM-1, ICAM-2, and VCAM-1 adhesion molecules in human umbilical vein endothelial cells (HUVEC), and whether these changes can enhance the adherence between endothelial cells and neutrophil granulocyte model cells (differentiated PLB-985). We found that HUVECs activated by rMASP-1 decreased the expression of ICAM-2 and increased that of E-selectin, whereas ICAM-1, VCAM-1 and P-selectin expression remained unchanged. Furthermore, these changes resulted in increased adherence between differentiated PLB-985 cells and endothelial cells. Our finding suggests that complement MASP-1 can increase adhesion between neutrophils and endothelial cells in a direct fashion. This is in agreement with our previous finding that MASP-1 increases the production of pro-inflammatory cytokines (such as IL-6 and IL-8) and chemotaxis, and may thereby boost neutrophil functions. This newly described cooperation between complement lectin pathway and neutrophils via endothelial cells may be an effective tool to enhance the antimicrobial immune response.

An Ixodes ricinus Tick Salivary Lectin Pathway Inhibitor Protects Borrelia burgdorferi sensu lato from Human Complement

INTRODUCTION

We previously identified tick salivary lectin pathway inhibitor (TSLPI) in Ixodes scapularis, a vector for Borrelia burgdorferi sensu stricto (s.s.) in North America. TSLPI is a salivary protein facilitating B. burgdorferi s.s. transmission and acquisition by inhibiting the host lectin complement pathway through interference with mannose binding lectin (MBL) activity. Since Ixodes ricinus is the predominant vector for Lyme borreliosis in Europe and transmits several complement sensitive B. burgdorferi sensu lato (s.l.) strains, we aimed to identify, describe, and characterize the I. ricinus ortholog of TSLPI.

METHODS

We performed (q)PCRs on I. ricinus salivary gland cDNA to identify a TSLPI ortholog. Next, we generated recombinant (r)TSLPI in a Drosophila expression system and examined inhibition of the MBL complement pathway and complement-mediated killing of B. burgdorferi s.l. in vitro.

RESULTS

We identified a TSLPI ortholog in I. ricinus salivary glands with 93% homology at the RNA and 89% at the protein level compared to I. scapularis TSLPI, which was upregulated during tick feeding. In silico analysis revealed that TSLPI appears to be part of a larger family of Ixodes salivary proteins among which I. persulcatus basic tail salivary proteins and I. scapularis TSLPI and Salp14. I. ricinus rTSLPI inhibited the MBL complement pathway and protected B. burgdorferi s.s. and Borrelia garinii from complement-mediated killing.

CONCLUSION

We have identified a TSLPI ortholog, which protects B. burgdorferi s.l. from complement-mediated killing in I. ricinus, the major vector for tick-borne diseases in Europe.

Applying complement therapeutics to rare diseases

Around 350 million people worldwide suffer from rare diseases. These may have a genetic, infectious, or autoimmune basis, and several include an inflammatory component. Launching of effective treatments can be very challenging when there is a low disease prevalence and limited scientific insights into the disease mechanisms. As a key trigger of inflammatory processes, complement has been associated with a variety of diseases and has become an attractive therapeutic target for conditions involving inflammation. In view of the clinical experience acquired with drugs licensed for the treatment of rare diseases such as hereditary angioedema and paroxysmal nocturnal hemoglobinuria, growing evidence supports the safety and efficacy of complement therapeutics in restoring immune balance and preventing aggravation of clinical outcomes. This review provides an overview of the candidates currently in the pharmaceutical pipeline with potential to treat orphan diseases and discusses the molecular mechanisms triggered by complement involved with the disease pathogenesis.

Divalent metal ions binding properties of goat serum mannose binding lectin

Mannose binding lectin (MBL) is a collectin with C-terminus Carbohydrate Recognition Domain (CRD) which binds with pathogen and arbitrate functions like activation of complement pathway, opsonization etc. The CRD required Ca(2+) ions to recognize the sugar moieties. In the present study the binding properties of CRD with divalent ions were characterized by Electron Paramagnetic Resonance (EPR) spectroscopy. The results revealed that the metal binding site of CRD is of approximately 1 Å diameter and ions greater than the size are not able to enter.

Complement System Part I - Molecular Mechanisms of Activation and Regulation

Complement is a complex innate immune surveillance system, playing a key role in defense against pathogens and in host homeostasis. The complement system is initiated by conformational changes in recognition molecular complexes upon sensing danger signals. The subsequent cascade of enzymatic reactions is tightly regulated to assure that complement is activated only at specific locations requiring defense against pathogens, thus avoiding host tissue damage. Here, we discuss the recent advances describing the molecular and structural basis of activation and regulation of the complement pathways and their implication on physiology and pathology. This article will review the mechanisms of activation of alternative, classical, and lectin pathways, the formation of C3 and C5 convertases, the action of anaphylatoxins, and the membrane-attack-complex. We will also discuss the importance of structure-function relationships using the example of atypical hemolytic uremic syndrome. Lastly, we will discuss the development and benefits of therapies using complement inhibitors.

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

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

Interaction of lectin pathway of complement-activating pattern recognition molecules with mycobacteria

We have demonstrated that mannose-binding lectin (MBL) recognizes various slow-growing, pathogenic mycobacteria [Mycobacterium tuberculosis (MTB), M. bovis, M. kansasii, M. gordonae] as well as non-pathogenic M. smegmatis. Recognition resulted in activation of the lectin pathway (LP) of complement and an enhancement of phagocytosis (shown for M. tuberculosis). Although MBL may be considered the main factor activating the LP upon recognition of mycobacteria, involvement of ficolins has also to be considered. Interaction of ficolin-3 with M. tuberculosis, M. bovis and M. kansasii, and ficolin-1 with M. tuberculosis and M. bovis was shown for the first time. Binding of recombinant MBL or ficolin-3 to MTB H37 Rv led to the agglutination of bacteria and promoted their phagocytosis, but little effect was apparent with ficolin-1 or ficolin-2. Data from Western blots suggest mannosylated lipoarabinomannan (ManLAM) to be one of the main cell components of slow-growing mycobacteria, involved in LP activation. However, the LP was also activated by other cell fractions. Results presented here supplement considerably the data concerning the ability of complement-activating lectins to interact with mycobacteria. Ficolins (especially ficolin-3) might influence host response to infection and thus have clinical significance, at least as disease modifiers.

Association of MASP2 polymorphisms and protein levels with rheumatic fever and rheumatic heart disease

MASP-2 is a key protein of the lectin pathway of complement system. Several MASP2 polymorphisms were associated with MASP-2 serum levels or functional activity. Here we investigated a possible association between MASP2 polymorphisms and MASP-2 serum levels with the susceptibility to rheumatic fever (RF) and rheumatic heart disease (RHD). We haplotyped 11 MASP2 polymorphisms with multiplex sequence-specific PCR in 145 patients with history of RF from south Brazil (103 with RHD and 42 without cardiac lesion [RFo]) and 342 healthy controls. MASP-2 levels were determined by ELISA. The low MASP-2 producing p.377A and p.439H variants were negatively associated with RF (P=0.02, OR=0.36) and RHD (P=0.01, OR=0.25). In contrast, haplotypes that share the intron 9 - exon 12 g.1961795C, p.371D, p.377V and p.439R polymorphisms increased the susceptibility to RHD (P=0.02, OR=4.9). MASP-2 levels were associated with MASP2 haplotypes and were lower in patients (P<0.0001), which may reflect protein consumption due to complement activation. MASP2 gene polymorphisms and protein levels seem to play an important role in the development of RF and establishment of RHD.

Bacteria under stress by complement and coagulation

The complement and coagulation systems are two related protein cascades in plasma that serve important roles in host defense and hemostasis, respectively. Complement activation on bacteria supports cellular immune responses and leads to direct killing of bacteria via assembly of the Membrane Attack Complex (MAC). Recent studies have indicated that the coagulation system also contributes to mammalian innate defense since coagulation factors can entrap bacteria inside clots and generate small antibacterial peptides. In this review, we will provide detailed insights into the molecular interplay between these protein cascades and bacteria. We take a closer look at how these pathways are activated on bacterial surfaces and discuss the mechanisms by which they directly cause stress to bacterial cells. The poorly understood mechanism for bacterial killing by the MAC will be reevaluated in light of recent structural insights. Finally, we highlight the strategies used by pathogenic bacteria to modulate these protein networks. Overall, these insights will contribute to a better understanding of the host defense roles of complement and coagulation against bacteria.

Mannose-Binding Lectin (MBL) and MBL-associated serine protease-2 (MASP-2) in women with malignant and benign ovarian tumours

Mannose-Binding Lectin (MBL) is a serum pattern recognition molecule, able to activate complement in association with MASP proteases. Serum levels of MBL and MASP-2, activities of MBL-MASP complexes, single nucleotide polymorphisms of the MBL2 and MASP2 genes and/or their specific mRNA expression in ovarian sections were investigated in 128 patients suffering from primary ovarian cancer (OC) and compared with 197 controls (C), encompassing both patients with benign ovarian tumours (n = 123) and others with no ovarian pathology (n = 74). MBL deficiency-associated genotypes were more common among OC patients than among controls. The O/O group of genotypes was associated with ovarian cancer (OR 3.5, p = 0.02). In A/A homozygotes, MBL concentrations and activities were elevated in the OC group and correlated with C-reactive protein. Moreover, high MBL serum levels were associated with more advanced disease stage. No differences in distribution of the MASP2 +359 A>G (D120G) SNP or MASP-2 serum levels were found between cancer patients and their controls. However, the highest frequency of the A/G (MASP2) and LXA/O or O/O (MBL2) genotypes was found among OC patients with tumours of G1-2 grade (well/moderately differentiated). Furthermore, MBL deficiency-associated genotypes predicted prolonged survival. None of the parameters investigated correlated with CA125 antigen or patients' age. The local expression of MBL2 and MASP2 genes was higher in women with ovarian cancer compared with controls. It is concluded that the expression of MBL and MASP-2 is altered in ovarian cancer, possibly indicating involvement of the lectin pathway of complement activation in the disease.

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

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

Mannan-binding lectin in cardiovascular disease

Cardiovascular disease remains the leading cause of mortality and morbidity worldwide so research continues into underlying mechanisms. Since innate immunity and its potent component mannan-binding lectin have been proven to play an important role in the inflammatory response during infection and ischaemia-reperfusion injury, attention has been paid to its role in the development of cardiovascular complications as well. This review provides a general outline of the structure and genetic polymorphism of MBL and its role in inflammation/tissue injury with emphasis on associations with cardiovascular disease. MBL appears to be involved in the pathogenesis of atherosclerosis and, in consequence, coronary artery disease and also inflammation and tissue injury after myocardial infarction and heart transplantation. The relationship between MBL and disease is rather complex and depends on different genetic and environmental factors. That could be why the data obtained from animal and clinical studies are sometimes contradictory proving not for the first time that innate immunity is a "double-edge sword," sometimes beneficial and, at other times disastrous for the host.