Analysis of mannose-binding lectin 2 (MBL2) genotype and the serum protein levels in the Korean population.

Mannose-binding lectin (MBL) is a C-type lectin produced by the liver and involved in the innate immune response. We have analyzed six SNPs of MBL2 gene--three at promoter (-550, -435, and -221), one at 5'-untranslational region (UTR) (+4), and two at coding (Gly54Asp and Leu126Leu) regions--in the Korean population (N=129), and have correlated genotypes with the serum concentration and functional characteristics. Of those, the Asp54 allele (P<10(-15)), L allele at -550 (P<10(-7)), and P allele at +4 (P=0.012) were correlated with low MBL levels. The effect of the X allele at -221 on MBL levels in the Korean population appeared to be less profound than that of other populations. The highest MBL producing promoter haplotype in the Korean population was HYP, followed by LYQ and LYP, and then LXP. From functional analysis of MBL, low MBL levels were correlated with low mannan-binding, low C4 complement activation, and lack of high ordered oligomers. Our results support that the promoter and coding polymorphisms of MBL are correlated with its functional activity as well as circulating levels, and the association patterns are quite similar to those of other populations.

Chronic Burkholderia multivorans bronchial infection in a non-cystic fibrosis individual with mannose binding lectin deficiency.

The case history is presented of a woman with multiple respiratory infections and mannose binding lectin (MBL) deficiency but no evidence of bronchiectasis who developed a chronic Burkholderia multivorans infection. Careful microbiological assessment is needed in patients with recurrent respiratory infection and the presence of B multivorans should trigger further immunological investigation including assessment of MBL status.

The two major oligomeric forms of human mannan-binding lectin: chemical characterization, carbohydrate-binding properties, and interaction with MBL-associated serine proteases.

Mannan-binding lectin (MBL) is an oligomeric C-type lectin assembled from homotrimeric structural units that binds to neutral carbohydrates on microbial surfaces. It forms individual complexes with MBL-associated serine proteases (MASP)-1, -2, -3 and a truncated form of MASP-2 (MAp19) and triggers the lectin pathway of complement through MASP-2 activation. To characterize the oligomerization state of the two major MBL forms present in human serum, both proteins were analyzed by mass spectrometry. Mass values of 228,098 +/- 170 Da (MBL-I) and 304,899 +/- 229 Da (MBL-II) were determined for the native proteins, whereas reduction of both species yielded a single chain with an average mass of 25,340 +/- 18 Da. This demonstrates that MBL-I and -II contain 9 and 12 disulfide-linked chains, respectively, and therefore are trimers and tetramers of the structural unit. As shown by surface plasmon resonance spectroscopy, trimeric and tetrameric MBL bound to immobilized mannose-BSA and N-acetylglucosamine-BSA with comparable K(D) values (2.2 and 0.55 nM and 1.2 and 0.96 nM, respectively). However, tetrameric MBL exhibited significantly higher maximal binding capacity and lower dissociation rate constants for both carbohydrates. In contrast, no significant difference was detected for binding of the recombinant MASPs or MAp19 to immobilized trimeric or tetrameric MBL. As shown by gel filtration, both MBL species formed 1:2 complexes with MASP-3 or MAp19. These results provide the first precise analysis of the major human MBL oligomers. The oligomerization state of MBL has a direct effect on its carbohydrate-binding properties, but no influence on the interaction with the MASPs.

Impaired pulmonary status in cystic fibrosis adults with two mutated MBL-2 alleles.

Mannose-binding lectin has recently been identified as a modifier of severity in cystic fibrosis, although studies have produced differing results and the mechanism of action remains unclear. The current authors have studied large cohorts of adults (n=298) and children (n=260) to explore this apparent relationship further. Adults with two structural mutations, but not heterozygotes, had significantly reduced lung function and oxygen saturations, more frequent hospital admissions and raised systemic inflammatory markers. This was not related to increased rates of infection with Pseudomonas aeruginosa, and there was no increased susceptibility to Burkholderia cepacia. None of these findings was mirrored in the paediatric cohort. In conclusion, severe mannose-binding lectin deficiency appears to be detrimental to cystic fibrosis adults, although heterozygotes are not affected. It is suggested that this is not related to impaired complement-mediated bacterial killing, and a link with the host inflammatory response is hypothesised. If mannose-binding lectin replacement is developed as a new approach to treatment for this disease, the present study would suggest that the small group of severely deficient patients with two structural mutations may be the group to benefit.

Mannose-binding lectin gene polymorphisms are associated with gestational diabetes mellitus.

Insulin resistance is a feature of gestational diabetes mellitus (GDM). Inverse correlations between indexes of insulin sensitivity and serum markers of inflammation have been observed and, particularly, TNF-alpha has been shown to be associated with the appearance of insulin resistance in pregnancy. Mannose-binding lectin (MBL) is a protein member of the collectin family. Its deficiency is genetically determined and predisposes to recurrent infections and chronic inflammatory diseases. To test the hypothesis that a genetic predisposition to a proinflammatory state could favor the appearance of GDM during pregnancy, we studied R52C and G54D polymorphisms of MBL2 gene and plasma MBL levels from 105 consecutive GDM women and 173 healthy pregnant women. An association was found between G54D and GDM [odds ratio, 2.03 (1.18-3.49); P < 0.01], and this association remained significant when the presence of both mutated alleles was considered [odds ratio, 1.76 (1.04-2.96); P < 0.05] but not for the R52C. GDM patients who carried the G54D mutation required insulin therapy more frequently (56.4 vs. 30.4%, chi(2) =5.83; P = 0.027) and had heavier infants (3326.4 +/- 546.9 vs. 3087.5 +/- 395.5 g; P < 0.05) than GDM women homozygous for the wild-type allele. An inverse correlation in GDM patients between neonatal weight and plasma MBL levels (r = -0.320; P = 0.002) was found, remaining significant after adjustment for confounding variables. In conclusion, pregnant women bearing the G54D MBL allele have a greater risk for developing GDM and having heavier infants.

Evolution of the mannose-binding lectin gene in primates.

The mannose-binding lectin MBL2 plays an important role in the innate immune system. It binds carbohydrates surface, acts as an opsonin and activates the complement system. With the aim of studying the evolution of the MBL2 gene in primates, we sequenced its coding region in 12 non-human primate species and compared them with the human sequence. We demonstrated that nucleotide and amino-acidic sequences of the MBL2 among primates are highly homologous, underlining the importance of this molecule in the defense system against pathogen invasions. In particular, in the collagen-like domain that confers the characteristic structure to MBL2 protein, the identity among primates is really high. In the carbohydrate recognition domain, we evidenced some primates' group-specific amino-acidic mutations not resulting in changes of the structure or function of this MBL2 domain. Phylogenetic analysis did not evidence any positive selective pressure in MBL2 gene among non-human primates. Our findings indicate that MBL2 is well conserved in agreement with its important role in the immune system: in non-human primates, we did not observe the same 'plasticity' of the MBL2 human gene, where a frequency of more than 1% of nucleotide variations was described in the coding and promoter regions.

Sequence analysis of the mannose-binding lectin (MBL2) gene reveals a high degree of heterozygosity with evidence of selection.

Human mannose-binding protein (MBL) is a component of innate immunity. To capture the common genetic variants of MBL2, we resequenced a 10.0 kb region that includes MBL2 in 102 individuals representing four major US ethnic groups. In all, 87 polymorphic sites were observed, indicating a high level of heterozygosity (total pi=18.3 x 10(-4)). Estimates of linkage disequilibrium across MBL2 indicate that it is divided into two blocks, with a probable recombination hot spot in the 3' end. Three non-synonymous SNPs in exon 1 of the encoding MBL2 gene and three upstream SNPs form common 'secretor haplotypes' that can predict circulating levels. Common variants have been associated with increased susceptibility to infection and autoimmune diseases. The high frequencies of B, C and D alleles in certain populations suggest a possible selective advantage for heterozygosity. There is limited diversity of haplotype structure; the 'secretor haplotypes' lie on a restricted number of extended haplotypes, which could include additional linked SNPs, which might also have possible functional implications. There is evidence for gene conversion in the region between the two blocks, in the last exon. Our data should form the basis for conducting MBL2 candidate gene association studies using a locus-wide approach.

Increased incidence and severity of the systemic inflammatory response syndrome in patients deficient in mannose-binding lectin.

OBJECTIVE: To determine whether pediatric PICU patients with mannose-binding lectin (MBL) gene polymorphisms associated with low levels of the functional protein have an increased risk of developing sepsis and SIRS. DESIGN AND SETTING: A prospective, observational cohort study in a 22-bed PICU in a tertiary referral centre. PATIENTS: One hundred consecutive admissions to a PICU with at least one organ system failure longer than 12 h. Patients were classified into those with infectious or non-infectious insults as the primary reason for intensive care admission. Patients were followed to determine which developed sepsis or non-infection related SIRS using standard criteria. MEASUREMENTS AND RESULTS: Of the 100 patients 50 had infectious and 50 had non-infectious insults as the precipitant for admission. 42 patients had variant MBL alleles (determined by MBL-2 gene exon 1 and promoter polymorphisms) and were significantly over-represented amongst the 59 patients that developed SIRS. This effect was not explained by differences in age, sex or ethnicity and was seen in both the infection and non-infection subgroups. In patients with infection, variant MBL alleles were associated with increased systemic response (2/15 with localised infection, 10/19 with sepsis and 12/16 with septic shock). MBL serum levels showed close concordance with the genotype and indicated that MBL levels less than 1000 ng/ml are associated with a greatly increased risk of SIRS. CONCLUSIONS: MBL-2 exon 1 polymorphisms with low serum levels of functional MBL protein are associated with a greatly increased risk of developing SIRS and of progression from infection to sepsis and septic shock in paediatric ICU patients.

Mannose-binding lectin and infection following allogeneic hemopoietic stem cell transplantation.

Major infection remains a major barrier to the success of allogeneic hemopoietic stem cell transplantation (SCT). There is growing interest in the importance of innate immunity in host defense, particularly when adaptive immunity is compromised. Furthermore, many host defense genes are polymorphic, and immunogenetic factors are known to influence the risk of other transplant complications, such as graft-versus-host disease. Mannose-binding lectin (MBL) has emerged as an important innate host defense molecule. MBL binds a wide range of pathogens independently of antibody and activates complement leading to lysis and phagocytosis. Genetically determined MBL deficiency is common and results in an increased risk of infection in a variety of clinical settings, especially in individuals already immunocompromised for other reasons. We conducted a retrospective study examining associations between polymorphisms in the gene encoding MBL, MBL2 and risk of major infection post-SCT in 96 related myeloablative transplants. This showed that "low-producing" MBL2 coding alleles, when present in the donor, were significantly associated with increased risk of major infection in the recipient following neutrophil count recovery. Furthermore, a "high-producing" MBL2 haplotype, HYA, when present in the recipient, was protective against infection. As MBL is under development as a therapeutic agent, these findings suggest that administration of MBL may reduce the risk of infection post-transplant. Prior to embarking upon trials of MBL replacement therapy in SCT, further work is required to confirm these results, to examine the kinetics of MBL synthesis peri-transplant, to correlate MBL2 genotype with blood MBL levels, and to examine the role of MBL in other settings, such as transplantation using reduced intensity conditioning regimens, and unrelated donor transplants. These results are the first report of a genetic determinant of risk of infection post-SCT, and highlight the importance of non-HLA genetic factors in determining the risk of transplant complications. Further studies examining other host defence genes are warranted, and are in progress.

Mannan-binding lectin insufficiency in children with recurrent infections of the respiratory system.

Blood samples were collected over a 4-year period from 335 children (aged 1-16 years) suffering from recurrent respiratory infections and 78 controls. The patients were subdivided into four groups: I, children with no immune system defects detected (n = 101); II, children with allergies (n = 94); III, children with humoral response defects (n = 93); and IV, children with disturbances of cellular immunity (n = 66). Nineteen patients had both humoral and cellular abnormalities. All patients and controls were investigated to determine the exon 1 and promoter region variants of the mbl-2 gene. MBL serum concentrations were also determined in samples from 291 patients and 75 controls. The proportion of O (B, D or C) alleles was significantly higher in the patient group compared to controls, and this association was strongest for subgroup III. The promoter LX variant frequency was also commoner in the patients as a whole, and significantly so in subgroups II and IV. Genotypes markedly influenced MBL concentrations in all groups, and correlated with ability to activate the lectin pathway of complement activation. The strongest and most significant inverse correlations between serum MBL and respiratory disease were found in patient group III and in 17 patients with multiple humoral and/or cellular abnormalities. Among nine patients with unexpectedly low LP activity in view of their MBL concentrations, one person was found to be MASP-2 deficient. Our results indicate that mannan-binding lectin insufficiency, with or without a coexisting immune defect, is associated with the occurrence of recurrent respiratory infections in childhood, and this relationship is particularly strong and statistically significant in children with concomitant impairments of humoral immunity.

Disease-associated mutations in human mannose-binding lectin compromise oligomerization and activity of the final protein.

Deficiency of human mannose-binding lectin (MBL) caused by mutations in the coding part of the MBL2 gene is associated with increased risk and severity of infections and autoimmunity. To study the biological consequences of MBL mutations, we expressed wild type MBL and mutated MBL in Chinese hamster ovary cells. The normal MBL cDNA (WT MBL-A) was cloned, and the three known natural and two artificial variants were expressed in Chinese hamster ovary cells. When analyzed, WT MBL-A formed covalently linked higher oligomers with a molecular mass of about 300-450 kDa, corresponding to 12-18 single chains or 4-6 structural units. By contrast, all MBL variants formed a dominant band of about 50 kDa, with increasingly weaker bands at 75, 100, and 125 kDa corresponding to two, three, four, and five chains, respectively. In contrast to WT MBL-A, variant MBL formed noncovalent oligomers containing up to six chains (two structural units). MBL variants bound ligands with a markedly reduced capacity compared with WT MBL-A. Mutations in the collagenous region of human MBL compromise assembly of higher order oligomers, resulting in reduced ligand binding capacity and thus reduced capability to activate complement.

Localization of the serine protease-binding sites in the collagen-like domain of mannose-binding protein: indirect effects of naturally occurring mutations on protease binding and activation.

Mutations in the collagen-like domain of serum mannose-binding protein (MBP) interfere with the ability of the protein to initiate complement fixation through the MBP-associated serine proteases (MASPs). The resulting deficiency in the innate immune response leads to susceptibility to infections. Studies have been undertaken to define the region of MBP that interacts with MASPs and to determine how the naturally occurring mutations affect this interaction. Truncated and modified MBPs and synthetic peptides that represent segments of the collagen-like domain of MBP have been used to demonstrate that MASPs bind on the C-terminal side of the hinge region formed by an interruption in the Gly-X-Y repeat pattern of the collagen-like domain. The binding sites for MASP-2 and for MASP-1 and -3 overlap but are not identical. The two most common naturally occurring mutations in MBP result in substitution of acidic amino acids for glycine residues in Gly-X-Y triplets on the N-terminal side of the hinge. Circular dichroism analysis and differential scanning calorimetry demonstrate that the triple helical structure of the collagen-like domain is largely intact in the mutant proteins, but it is more easily unfolded than in wild-type MBP. Thus, the effect of the mutations is to destabilize the collagen-like domain, indirectly disrupting the binding sites for MASPs. In addition, at least one of the mutations has a further effect on the ability of MBP to activate MASPs.

Cost-effective genotyping of human MBL2 gene mutations using multiplex PCR.

Mannose-binding lectin (MBL) deficiency is associated with increased susceptibility to various infections and autoimmune disorders. It is caused by certain polymorphisms in the MBL2 gene promoter and mutations in the coding region of the gene. In this report, we present a novel, rapid, efficient and cost-effective method of two multiplex polymerase chain reactions (PCRs) for the assessment of three structural point mutations within exon 1 at codons 52, 54 and 57. Three additional PCR reactions for the detection of promoter polymorphisms at positions -550 and -221 were performed. MBL2 haplotypes in 359 individuals of the general Czech population were detected using this approach. The rare LYD haplotype was found in 1.1% of all alleles.

Binding of human immunodeficiency virus type 1 to immature dendritic cells can occur independently of DC-SIGN and mannose binding C-type lectin receptors via a cholesterol-dependent pathway.

Interactions of human immunodeficiency virus type 1 (HIV-1) with immature dendritic cells (DC) are believed to be multifactorial and involve binding to the CD4 antigen, DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), mannose binding C-type lectin receptors (MCLR), and heparan sulfate proteoglycans (HSPG). In this study we assessed the relative contributions of these previously defined virus attachment factors to HIV binding and accumulation in DC and the subsequent transfer of the bound virus particle to CD4(+) T cells. Using competitive inhibitors of HIV-1 attachment to DC, we have identified the existence of DC-SIGN-, MCLR-, and HSPG-independent mechanism(s) of HIV attachment and internalization. Furthermore, virus particles bound by DC independently of CD4, DC-SIGN, MCLR, and HSPG are efficiently transmitted to T cells. Treatment of virus particles with the protease subtilisin or treatment of immature DC with trypsin significantly reduced virus binding, thus demonstrating the role of HIV envelope glycoprotein interactions with unidentified DC-surface factor(s). Finally, this DC-mediated virus binding and internalization are dependent on lipid rafts. We propose that pathways to HIV-1 attachment and uptake in DC exhibit functional redundancy; that is, they are made up of multiple independent activities that can, at least in part, compensate for one another.

Rapid genotyping of MBL2 gene mutations using real-time PCR with fluorescent hybridisation probes.

In this study, we describe a real-time polymerase chain reaction (PCR) for genotyping all known polymorphisms of the human mannose-binding lectin 2 (MBL2) gene. These comprised two variations in the 5' regulatory region at positions -550 (H/L) and -221 (X/Y), one in the 5' untranslated sequence at position +4 (P/Q) and three structural mutations within exon 1 at codons 52, 54, and 57, also known as the D, B and C variants, respectively. Three reactions with two different conditions were sufficient to genotype one individual unambiguously. The three mutations in exon 1 were detected in one capillary using a sensor probe covering the three mutations, whereas amplification of the variants located upstream of the coding sequence was performed in only two reactions. Single colour detection was used for detection of the (H/L) polymorphism and multiplexing by dual colour probes was used for simultaneous genotyping of (X/Y) and (P/Q). The reliability of the system was evaluated by comparison with a conventional PCR method with sequence-specific primers (PCR-SSP). For this study, 100 individuals of Danish and 30 of African descent were analysed, and the genotypes obtained were concordant in all cases. This new method is rapid and provides reliable results without ambiguities.

Mannose-binding lectin is a disease modifier in clinical malaria and may function as opsonin for Plasmodium falciparum-infected erythrocytes.

Variant alleles in the mannose-binding lectin (MBL) gene (mbl2) causing low levels of functional MBL are associated with susceptibility to different infections and are common in areas where malaria is endemic. Therefore, we investigated whether MBL variant alleles in 551 children from Ghana were associated with the occurrence and outcome parameters of Plasmodium falciparum malaria and asked whether MBL may function as an opsonin for P. falciparum. No difference in MBL genotype frequency was observed between infected and noninfected children or between children with cerebral malaria and/or severe malarial anemia and children with uncomplicated malaria. However, patients with complicated malaria who were homozygous for MBL variant alleles had significantly higher parasite counts and lower blood glucose levels than their MBL-competent counterparts. Distinct calcium-dependent binding of MBL to the membrane of P. falciparum-infected erythrocytes, which could be inhibited by mannose, was observed. Further characterization revealed that MBL reacted with a P. falciparum glycoprotein identical to the 78-kDa glucose-regulated stress protein of P. falciparum. MBL seems to be a disease modifier in clinical malaria and to function as an opsonin for erythrocytes invaded by P. falciparum and may thus be involved in sequestration of the parasite, which in turn may explain the association between homozygosity for MBL variant alleles and high parasite counts.