Mannose-binding lectin (MBL) in health and disease

Partial overlap of anti-mycobacterial, and anti-Saccharomyces cerevisiae mannan antibodies in Crohn’s disease

AIM: To test whether humoral immune reaction against mycobacteria may play a role in anti-Saccharomyces cerevisiae antibodies (ASCA) generation in Crohn's disease (CD) and/or whether it correlates with clinical subtypes.

METHODS: The dominant ASCA epitope was detected by Galanthus nivalis lectin (GNL)-binding assay. ASCA and IgG against mycobacterial lysates [M avium, M smegmatis, M chelonae, M bovis BCG, M avium ssp. paratuberculosis (MAP)] or purified lipoarabinomannans (LAM) were detected by ELISA. ASCA and anti-mycobacterial antibodies were affinity purified to assess cross-reactivities. Anti-mycobacterial IgG were induced by BCG-infection of mice.

RESULTS: GNL bound to different extents to mycobacterial lysates, abundantly to purified mannose-capped (Man) LAM from M tuberculosis, but not to uncapped LAM from M smegmatis. Fifteen to 45% of CD patients but only 0%-6% of controls were seropositive against different mycobacterial antigens. Anti-mycobacterial IgG correlated with ASCA (r = 0.37-0.64; P = 0.003-P < 0.001). ASCA-positivity and deficiency for mannan-binding lectin synergistically associated with anti-mycobacterial IgG. In some patients, anti-mycobacterial antibodies represent cross-reactive ASCA. Vice-versa, the predominant fraction of ASCA did not cross-react with mycobacteria. Finally, fistulizing disease associated with antibodies against M avium, M smegmatis and MAP (P = 0.024, 0.004 and 0.045, respectively).

CONCLUSION: Similar to ASCA, seroreactivity against mycobacteria may define CD patients with complicated disease and a predisposition for immune responses against ubiquitous antigens. While in some patients anti-mycobacterial antibodies strongly cross-react with yeast mannan; these cross-reactive antibodies only represent a minor fraction of total ASCA. Thus, mycobacterial infection unlikely plays a role in ASCA induction.

Mannose-binding lectin (MBL) facilitates opsonophagocytosis of yeasts but not of bacteria despite MBL binding

Mannose-binding lectin (MBL) is a serum protein of the innate immune system. After binding to a microorganism, MBL in complex with MBL-associated serine proteases activates the complement system, resulting in cleavage of complement factor C3. Cleaved C3 on the surface of the microorganism mediates opsonization for clearance, but the impact of MBL on subsequent phagocytosis has not been widely studied. We investigated the role of MBL in complement activation and phagocytosis of various bacteria and yeast species by flow cytometry. We measured both the C3 deposition during serum opsonization of fluorescent-labeled microorganisms as well as subsequent uptake of these microorganisms by human neutrophils. In MBL-deficient sera, a consistently decreased C3 deposition on both zymosan and Candida albicans was found and a reduced phagocytosis by neutrophils that was restored by exogenous MBL. This indicates that the lectin pathway of complement activation is important for the opsonophagocytosis of yeasts. In contrast, the C1q-dependent classical pathway dominated in the opsonization and phagocytosis of Staphylococcus aureus, Streptococcus pneumoniae, and Escherichia coli, whereas no effect of MBL was found. Both the lectin and the classical pathway of complement activation were highly amplified by the alternative route for opsonophagocytosis by neutrophils of yeast as well as microbial species. In summary, our data demonstrate that yeast species are preferentially opsonized and subsequently phagocytosed via activation of the lectin pathway of complement, whereas the uptake of bacterial strains was found to be largely MBL independent.

Mannose-binding lectin in term newborns and their mothers: genotypic and phenotypic relationship

Functional mannose-binding lectin (f-MBL) plays an important role in the innate neonatal immune system. We studied the origin of f-MBL in umbilical cord blood (UCB) by measuring maternal MBL (n=47), collected before elective cesarean section, and neonatal MBL (n=43) in arterial umbilical cord blood. In a subgroup, arterial and venous UCB MBL levels were measured. In addition, MBL expression was correlated with genetic mutations. The f-MBL levels in term infants were lower than in their mothers (0.70 microg/ml vs 1.11 microg/ml, p<0.01) and maternal and neonatal MBL levels were only weakly correlated (R=0.32, p<0.001), which suggests a fetal origin of f-MBL. Arterial and venous UCB median MBL levels did not differ (0.98 microg/ml vs. 1.40 microg/ml, p=0.20). No homozygous mutations were found. MBL was lower in mothers and infants with a (compound) heterozygous mutation than in those with a wild type. One new (HYPB) and two rare haplotypes (HXPA, LYPD) were reported in our population. Levels of MBL differed depending on the genotype of the mother or the infant. Because the role of MBL in host defense is still unclear, both f-MBL and haplotype should be measured to determine the clinical implications of MBL deficiency in infants.

Baseline mannose binding lectin levels may not predict infection among children with leukemia

We measured baseline serum mannose binding lectin (MBL) levels in 91 patients with childhood leukemia to determine their predictive value for the development of febrile neutropenia or specific infections. Median MBL levels did not differ significantly between patients who developed febrile neutropenia, bacterial infection, or disseminated fungal infection and those who did not. In addition, low MBL levels were not associated with an increased cumulative incidence of infection or with a shorter time to first infection. This preliminary study suggests that baseline MBL levels may not be clinically useful to identify pediatric leukemia patients who are at increased risk of infection. Additional studies are required to determine whether serial MBL measurements may be valuable for this purpose.

Possible disease-modifying factors: the mannan-binding lectin pathway and infections in hereditary angioedema of children and adults

Introduction

Hereditary angioedema (HAE) is caused by mutations in the C1inh gene, leading to dysfunction of the C1-esterase inhibitor (C1-INH). C1-INH interacts with MASP-1 and MASP-2 proteases, participating in the mannan-binding lectin (MBL) pathway of complement activation. The aim of the study was to investigate the contribution of possible changes in MBL/MASP-2 complex activity and Helicobacter pylori, hepatitis B virus (HBV), and hepatitis C virus (HCV) infections to the severity and frequency of clinical symptoms of HAE.

Materials and Methods

The study was performed in 65 patients with HAE and 113 healthy persons. The parameters measured were C1-INH, C4, MBL concentration and MBL/MASP-2 complex activity, and serological markers of H. pylori, HBV, and HCV infection. Scores for the frequency and severity of HAE symptoms were determined.

Results

HAE scores were significantly higher in patients whose C1-INH activity did not exceed 10% than in patients with activity of 10-52% (p=0.016). No significant differences were found in the median levels of MBL concentration and MBL/MASP-2 complex activity between patients and the control group. There was a slight association between contact with H. pylori in patients and HAE symptom score (p=0.052, not significant). Adult patients showed a 2.6-times higher frequency of anti-HBc than the general population. HBV DNA was negative in anti-HBc(+) patients.

Conclusions

These results suggest that the MBL complement activation pathway itself does not contribute to the frequency of angioedema attacks. Infections with H. pylori and HBV may slightly influence the disease score (not significant).

MBL2 variants in relation to common childhood infections and atopy-related phenotypes in a large German birth cohort

Mannose-binding lectin (MBL) is considered an important component of innate immunity. Four functional MBL2 alterations in codons 52, 54, 57 and in the promoter at position c.1-290 are correlated with significantly lowered MBL serum levels. These variants have been associated with susceptibility to a variety of infectious agents as well as with various immunologic disorders including asthma. To reassess these observations, we analysed the four above mentioned MBL2 variants in 749 children, who were recruited by the German Multicenter Allergy Study and were prospectively evaluated for common respiratory childhood infections and atopy-related phenotypes from birth up to the age of 11 yr. We performed genotyping by melting curve analysis using fluorescence resonance energy transfer probes and the LightCycler. In contrast to previous studies, we found an association of MBL2 variants neither with the frequency of common respiratory childhood infections at any age nor with asthma or other atopy-related phenotypes. Our data suggest that MBL deficiency does not represent a pre-disposing factor for respiratory infections or atopic disorders in infants and children.

Involvement of complement pathways in patients with bacterial septicemia

The complement system is a major humoral portion of the innate immune system, playing a significant role in host defence against microorganisms. The biological importance of this system is underlined by the fact that at least three different pathways for its activation exist, the classical, the MBL and the alternative pathway. To elucidate the involvement of the classical and/or the MBL pathway during bacterial septicemia, 32 patients with gram-positive and 30 patients with gram-negative bacterial infections were investigated. In patients with gram-positive bacteria, a significant consumption of C1q (p=0.005) but not of mannose-binding lectin (MBL) (p=0.2) was found during the acute phase of infection. In contrast, in patients with gram-negative bacterial infections, a significant reduction of MBL (p=0.002) and only a moderate, less significant reduction of C1q (p=0.03) were observed. As a model for the binding of MBL to gram-negative bacteria, Salmonella strains with defined mutations in their lipopolysaccharide (LPS) structure were used. The comparison of the binding of MBL to these Salmonella strains with that of the corresponding isolated LPS forms bound to microtiter plates revealed a similar binding pattern, supporting the interpretation that LPS on the surface of gram-negative bacteria is the major acceptor molecule for MBL on these bacteria, which according to our results obviously also takes place during gram-negative bacterial septicaemia. Furthermore, we were able to demonstrate that MBL bound to LPS was able to initiate activation of the complement cascade as measured by the occurrence of the cleavage product C4c.

Coexistence of (partial) immune defects and risk of recurrent respiratory infections

BACKGROUND

Respiratory infections are major causes of morbidity and mortality, but determinants of susceptibility are poorly defined. We studied whether and to what extent immunologic and genetic factors are associated with increased susceptibility to respiratory infections.

METHODS

We evaluated the prevalence of IgA, IgM, IgG, and IgG subclass deficiencies, impairment in the antibody response against pneumococcal polysaccharides, G2m(n) allotypes, Fc gamma RIIa polymorphisms, partial C2 and partial C4 deficiency, promoter polymorphisms in MBL2, and lymphocyte subset deficiencies in a control population and in consecutive children with recurrent respiratory infections.

RESULTS

IgA and/or IgG subclass deficiency was found in 27 of 55 patients (49%) and 6 of 43 controls (14%) (P = 0.0006). An impaired antibody response to polysaccharides was found in 7 patients (19%) and in 0 of 37 controls (P = 0.002). The Gm(n)marker was absent in 25 of 55 patients (45%) and 6 of 42 controls (14%) (P = 0.009). The MBL2 variants O/O, A/O, and A/A occurred in 9, 14, and 32 of the 55 patients, respectively, and in 1, 19, and 23 of the 43 controls, respectively (P = 0.05). There was no increase in the prevalence of partial C4 deficiency, C2 deficiency, lymphocyte subset deficiency, or Fc gamma RIIa polymorphism in the patients compared to the controls. A combination of at least 2 immune defects was found in 31 of 55 patients (56%) and in 4 of 42 controls (11.6%) (P <0.0001).

CONCLUSION

Specific antipolysaccharide antibody deficiency, IgA and/or IgG subclass deficiency, Gm(n) allotype, and MBL2 genotype are susceptibility factors for recurrent respiratory infections, and coexistence of several immune defects is the strongest risk factor in this study.

Synergy between mannose-binding lectin gene polymorphisms and supplementation with vitamin A influences susceptibility to HIV infection in infants born to HIV-positive mothers

BACKGROUND

Mannose-binding lectin (MBL-2) allele variants are associated with deficiencies in innate immunity and have been found to be correlated with HIV infection in adults and children.

OBJECTIVE

We tested whether MBL-2 variants among infants born to HIV-positive mothers have an increased susceptibility to HIV.

DESIGN

MBL-2 allele variants were measured among 225 infants born to HIV-positive mothers enrolled in a trial in Durban, South Africa. Mothers of 108 infants were randomly assigned to receive vitamin A and beta-carotene supplementation and 117 to receive placebo. Infants were followed with regular HIV tests to determine rates of mother-to-child HIV transmission.

RESULTS

A high proportion of infants were either homozygous (10.7%) or heterozygous (32.4%) for MBL-2 variants. MBL-2 variants within the placebo arm were associated with an increased risk of HIV transmission (odds ratio: 3.09; 95% CI: 1.21, 7.86); however, MBL-2 variants within the supplementation arm were not associated with an increased risk of transmission (P = 0.04; test of interaction). Among infants with MBL-2 variants, supplementation was associated with a decreased risk of HIV transmission (odds ratio: 0.37; 95% CI: 0.15, 0.91).

CONCLUSION

We observed what appears to be a gene-environment interaction between MBL-2 variants and an intervention with vitamin A plus beta-carotene that is relevant to mother-to-child HIV transmission.

Mannose-Binding Lectin in Susceptibility and Progression of HIV-1 Infection in Children

Background

Mannose-binding lectin (MBL; encoded by MBL-2) is a circulating pattern-recognition molecule that recognizes microbial carbohydrate motifs, leading to complement activation and cell lysis. Mutations in the MBL-2 promoter and of the MBL-2 gene exon 1 result in reduced protein levels and increased susceptibility to infection. We have investigated the effect of MBL-2 polymorphisms on susceptibility and progression of HIV-1 infection in children.

Patients and methods

One-hundred and twenty-eight children, aged 2–16 years were recruited. MBL-2 genotypes were determined by PCR and heteroduplex analyses. Serum MBL levels were measured by ELISA. Comparison of genotypes (A=wild type, 0=variant alleles) and protein levels between groups was performed using χ2, Mann–Whitney U or Kruskal–Wallis tests.

Results

Children were classified according to the Centers for Disease Control and Prevention clinical classification: A, B or C (mildly symptomatic [ n=39], moderately symptomatic [ n=58] or severely symptomatic AIDS [ n=31]) or immune category 1 ( n=77), 2 ( n=46) or 3 ( n=5). Analysis of MBL-2 genotypes with respect to clinical classification yielded minimal differences. However, patients in immunological categories 2 and 3 (<25% CD4+ T cells) were more likely to have MBL-2 variant alleles ( P=0.01). We further explored MBL status with respect to disease progression. Only 1/10 long-term non-progressors (LTNPs) had an MBL-2 mutation (A/D) with a corresponding protein level of 611 ng/ml.

Conclusions

MBL deficiency was more frequent in patients with severe disease as assessed by CD4+ T-cell status. MBL-2 variants may be less frequent in children classified as LTNPs. MBL analysis could be useful in identifying children with slow disease progression and, consequently, may not require immediate antiretroviral treatement.

Mannan-binding lectin (MBL)-mediated opsonization is enhanced by the alternative pathway amplification loop

The complement system is a humoral effector in the innate immune system. Three activation pathways exist in the complement system, known as the classical pathway, the lectin pathway and the alternative pathway. Dysfunction of lectin pathway activation is caused by MBL deficiency. MBL deficiency in a cohort of healthy Caucasian blood bank donors was investigated with MBL genotyping and MBL plasma concentration. Recognition of the yeast-derived zymosan by MBL was investigated with Western blot. The involvement of the alternative pathway amplification loop in enhancing MBL-mediated opsonization of zymosan was investigated in a novel opsonophagocytosis assay for flow cytometry. Sera deficient for MBL, factor D or properdin were tested, and purified MBL, factor D or properdin were used to recover opsonization. The optimal receiver-operator characteristic (ROC) cut-off value for dividing the Caucasian cohort in MBL-sufficient and MBL-deficient was calculated at 0.7 microg/ml. Thirty-eight percent of the group had concentrations below 0.7 microg/ml. Zymosan eluates opsonized with MBL-sufficient sera contain high oligomers of MBL, while eluates from MBL-deficient donors contained hardly any MBL. The MBL-, factor D- and properdin-deficient sera showed reduced opsonophagocytosis by human control neutrophils, as compared to normal MBL-sufficient sera. This reduction in opsonization was restored to normal levels by addition of purified MBL, factor D and properdin. The absence of opsonization in the factor D- and properdin-deficient sera, but presence in normal serum after blocking with anti-C1q-F(ab)2 and anti-MBL-F(ab)2, demonstrates the involvement of the amplification loop in MBL-initiated zymosan opsonization, even at very low serum concentrations (up to 3%, v/v). In conclusion, our data demonstrate that the MBL-mediated route of complement activation depends on the alternative pathway amplification loop for optimal opsonization of zymosan.

Mannose-binding lectin gene polymorphism and its impact on human immunodeficiency virus 1 infection

Mannose-binding lectin (MBL) is a serum protein whose low concentration is associated with the occurrence of allele variants named MBL*B, MBL*C and MBL*D. The present study investigated the association between MBL gene polymorphism and the susceptibility to HIV-1 infection. The study of 145 HIV-1-infected subjects and 99 healthy controls showed the presence of alleles MBL*A, MBL*B and MBL*D, whose frequencies were 69%, 22% and 09% among patients and 71%, 13% and 16% among healthy controls, respectively. The presence of the variant MBL*B was associated with higher plasma viral load levels, suggesting the importance of the MBL gene polymorphism in the clinical evolution of HIV-1-infected patients.

The mannose-binding lectin gene polymorphisms and systemic lupus erythematosus: two case-control studies and a meta-analysis

OBJECTIVE

Mannose-binding lectin (MBL) enhances opsonization and activates complement. Dysfunctional alleles of MBL have been associated with low plasma concentrations of MBL and increased risk of systemic lupus erythematosus (SLE), but genotyping studies have shown inconsistent results. We performed case-control studies of the MBL polymorphisms in 2 Caucasian cohorts and a meta-analysis incorporating all published results of MBL genotyping in SLE to explore whether the MBL functional variants are associated with SLE.

METHODS

MBL genotypes at 7 single-nucleotide polymorphisms were sequenced in 96 European American patients with SLE and 96 age-, race-, and sex-matched controls. MBL codons 52, 54, and 57 were genotyped in 285 German patients with SLE and 200 race-matched controls. Allele frequencies of all known variants were tallied for meta-analysis.

RESULTS

Although there was a trend toward association with MBL polymorphisms in both patient cohorts evaluated, none of them was significantly associated with SLE on its own. Seventeen comparisons from 15 studies were included in the meta-analysis. Publication bias was excluded by Egger's regression test (P = 0.14). The overall odds ratio for MBL codon 54 variant B was 1.406 (95% confidence interval 1.221-1.608; P < 0.001). Stratification by ethnicity showed significantly increased odds ratios for association of the MBL codon 54 B variant with SLE in African, Asian, and Caucasian cohorts.

CONCLUSION

Meta-analysis of all available studies on MBL polymorphisms and SLE shows that MBL variant alleles such as MBL exon 1 codon 54 B, promoter -550 L, and promoter -221 X are SLE risk factors. This association is robust and persists after incorporation of data from our 2 cohorts in which the association failed to reach significance.

Influence of FcgammaRIIA and MBL polymorphisms on severe acute respiratory syndrome

Polymorphisms of human Fc γ‐receptor IIA (FcγRIIA) and mannose‐binding lectin (MBL) genes have been associated with susceptibility to or severity of some infectious diseases. In order to investigate whether these genetic factors might influence susceptibility to infection with the severe acute respiratory syndrome‐associated coronavirus (SARS‐Cov) as well as the course and severity of the infection, we evaluated polymorphisms of FcγRIIA and MBL genes in DNA samples from a group of approximately 180 people from Hong Kong who were infected with SARS‐Cov. These included 132 patients who had moderate course of SARS infection (home subgroup), 26 patients with a severe course requiring treatment in an intensive care ward (ICU subgroup) and a subgroup of 22 patients who died from SARS (deceased subgroup). A total of 200 normal blood donors from the same region were used as controls. A significant association was found between the FcγRIIA‐R/R131 genotype and a severe course of SARS, with higher frequency of homozygosity for FcγRIIA‐R/R131 in the ICU subgroup of SARS patients when compared with controls (P = 0.03; odds ratio: 3.2; 95% confidence interval: 1.1–9.1). In comparison with controls, a significant difference in linear trend distribution of FcγRIIA genotypes was seen among the severe SARS patients (ICU and deceased subgroups) without co‐morbidity, and the incidence of FcγRIIA‐H/H131 was lower in these patients as well. There were no significant differences in MBL genotypes and allele frequencies among SARS patients and controls. The study reveals that in addition to age and co‐morbidity, FcγRIIA polymorphism of individuals may also influence outcome after infection with the SARS‐Cov.

Distinct functions of DC-SIGN and its homologues L-SIGN (DC-SIGNR) and mSIGNR1 in pathogen recognition and immune regulation

Antigen presenting cells express C-type lectins that are involved in pathogen capture, processing and antigen presentation to induce immune responses against these pathogens. However, it is becoming clear that pathogens have evolved to subvert the function of some C-type lectins to escape immune surveillance. An important C-type lectin family is represented by DC-SIGN and its homologues in human and mouse. Here we discuss the structure in relation to the pathogen binding specificity of the SIGN receptors and the function of these receptors in mouse and humans.

Mannose Binding Lectin Acute Phase Activity in Patients with Severe Infection

Mannose Binding Lectin (MBL) is a liver derived, circulating plasma protein that plays a pivotal role in innate immunity. MBL functions as a pathogen recognition molecule, opsonising organisms and initiating the complement cascade. MBL deficiency arising from mutations and promoter polymorphisms in the MBL2 gene is common and has been associated with risk, severity, and frequency of infection in a number of clinical settings. With MBL therapy on the horizon, the usefulness of replacement MBL therapy has been challenged by the notion, that as an acute phase protein, MBL levels may rise under stress to sufficient levels, in individuals who are usually deficient. This report demonstrates that in patients with sepsis and septic shock, the majority of patients do not display an MBL acute phase response: 41.4% of individuals maintained consistent MBL levels throughout hospital stay, 31.3% of individuals demonstrated a positive acute phase response, and a negative acute phase response was observed in 27.3% of individuals studied. Importantly, a positive acute phase response was generally observed in individuals with wild-type MBL2 genes. When a positive acute phase response was observed in individuals with coding mutation, these individuals demonstrated a normal MBL level on admission to hospital. Furthermore, no individual, regardless of genotype who was MBL deficient at admission was able to demonstrate a positive acute phase response into the normal MBL range. These findings indicate MBL demonstrates a variable acute phase response in the clinical setting of sepsis and septic shock.

The mannose-binding lectin: an infection susceptibility gene

A critical but unanswered question is what defines each individual’s pre-morbid susceptibility to infection? w e propose that individuals must have an “immune haplotype” that shapes their response to infectious agents. Infection is a balance between the intrinsic virulence of the infectious agent and the host defenses. Recent viral outbreaks of SARS and influenza serve to illustrate this point as these viruses cause severe disease in certain individuals, yet there are others in whom the same infectious challenge results in minimal symptoms. On the other hand it might be that those self same people who are resistance to one particular viral infection might be susceptible to other infection challenges. Similar rules can apply to susceptibility to bacterial infections.

Genetic polymorphisms in sepsis

CONTEXT

Wide variability exists in the susceptibility to and outcome from sepsis even within similar intensive care unit populations. Some of this variability in the host may be due to genetic variation in genes coding for components of the innate immune response.

OBJECTIVE

To review the evidence for a genetic influence on the susceptibility to and outcome from sepsis.

DESIGN

Literature review.

PATIENTS

Variety of adult and pediatric patients with various critical illnesses and infections.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Susceptibility to clinical symptoms of sepsis and outcome as measured by severity of disease and mortality.

RESULTS

Polymorphisms in genes coding for proteins involved in the recognition of bacterial pathogens (Toll-like receptor 4, CD14, Fc(gamma)RIIa, and mannose-binding lectin) and the response to bacterial pathogens (tumor necrosis factor-alpha, interleukin (IL)-1alpha, IL-1beta, IL-1 receptor agonist, IL-6, IL-10, heat shock proteins, angiotensin I converting enzyme, plasminogen activator inhibitor-1) can influence the amount or function of the protein produced in response to bacterial stimuli. Evidence is discussed suggesting that some of these genetic polymorphisms influence the susceptibility to and outcome from sepsis.

CONCLUSION

Host genetic variability in the regulatory and coding regions of genes for components of the innate immune system may influence the susceptibility to and/or outcome from sepsis. The disparate results observed in many studies of polymorphisms in sepsis emphasize the need for future studies to be larger, to include the analysis of multiple polymorphisms, and to be better designed with respect to control populations to identify the degree of influence that genetic variability has on sepsis.

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.

Association of MBL gene polymorphisms with major bacterial infection in patients treated with high-dose chemotherapy and autologous PBSCT

A growing body of evidence indicates that genetic factors are involved in an increased risk of infection. We investigated whether mannose-binding lectin (MBL) gene polymorphisms that cause low levels of MBL are associated with the occurrence of major infections in patients, mainly bearing hematological malignancies, after high-dose chemotherapy (HDT) rescued by autologous peripheral blood stem cell transplantation (auto-PBSCT). A retrospective evaluation of 113 patients treated with HDT and auto-PBSCT revealed that the low-producing genotypes, B/B and B/LXA, were associated with major bacterial infection (P=0.0016, OR 7.9). We next performed a nation-wide large-scale study to assess the allele frequency of the MBL coding mutation in a total of 2623 healthy individuals in Japan. The frequency of allele B was estimated to be approximately 0.2, almost the same in seven different areas of Japan. This common occurrence suggests that MBL deficiency may play an important role in the clinical settings of immunosuppression.

Cell surface activation of the alternative complement pathway by the fusion protein of measles virus

Measles virus (MV)-infected cells are activators of the alternative human complement pathway, resulting in high deposition of C3b on the cell surface. Activation was observed independent of whether CD46 was used as a cellular receptor and did not correlate with CD46 down-regulation. The virus itself was an activator of the alternative pathway and was covered by C3b/C3bi, resulting in some loss in infectivity without loss of virus binding to target cells. The cell surface expression of MV fusion (F), but not haemagglutinin, envelope protein resulted in complement activation of the Factor B-dependent alternative pathway in a dose-dependent manner and F-C3b complexes were formed. The underlying activation mechanism was not related to any decrease in cell surface expression of the complement regulators CD46 and CD55. The C3b/C3bi coating of MV-infected cells and virus should ensure enhanced targeting of MV antigens to the immune system, through binding to complement receptors.

Interaction of C1q with the receptor calreticulin requires a conformational change in C1q

The interaction between C1q and the chaperone calreticulin was studied under various conditions. When both proteins were present in equal amounts in solution, no interaction could be demonstrated. However, C1q immobilized on a hydrophobic surface, exposed to heat-treatment or bound to immunoglobulins (Igs) showed a strong, rapid and specific binding of calreticulin. The interaction appeared to be a two-step process, and the initial phase of interaction was sensitive to high concentrations of salt but not to a physiological salt concentration. The following strong binding was insensitive to salt and extremes of pH but sensitive to strongly denaturing agents (urea and guanidine). The sensitivity to salt during the initial phase of interaction was practically identical to that observed when calreticulin was bound to type V collagen. Binding between C1q and calreticulin could be inhibited by serum amyloid P component and by proteinase K-digested ovalbumin, and the binding of calreticulin to proteinase K-digested ovalbumin was shown to be inhibited by C1q. The data indicate that C1q binds stably to the peptide-binding site of calreticulin and that the initial binding of calreticulin to C1q involves the collagen-like domain of the C1q molecule. In conclusion, our results suggest calreticulin as a potential receptor for an altered conformation of C1q as occurs during binding to Igs. Thus, the chaperone and protein-scavenging function of calreticulin may extend from the endoplasmic reticulum to the topologically equivalent cell surface, where it may contribute to the elimination of immune complexes and apoptotic cells.

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.

Mannose-binding lectin-deficient mice are susceptible to infection with Staphylococcus aureus

Gram-positive organisms like Staphylococcus aureus are a major cause of morbidity and mortality worldwide. Humoral response molecules together with phagocytes play a role in host responses to S. aureus. The mannose-binding lectin (MBL, also known as mannose-binding protein) is an oligomeric serum molecule that recognizes carbohydrates decorating a broad range of infectious agents including S. aureus. Circumstantial evidence in vitro and in vivo suggests that MBL plays a key role in first line host defense. We tested this contention directly in vivo by generating mice that were devoid of all MBL activity. We found that 100% of MBL-null mice died 48 h after exposure to an intravenous inoculation of S. aureus compared with 45% mortality in wild-type mice. Furthermore, we demonstrated that neutrophils and MBL are required to limit intraperitoneal infection with S. aureus. Our study provides direct evidence that MBL plays a key role in restricting the complications associated with S. aureus infection in mice and raises the idea that the MBL gene may act as a disease susceptibility gene against staphylococci infections in humans.

Genomic polymorphisms in sepsis

OBJECTIVE

This article aims to review all relevant genetic polymorphism studies that may contribute to the pathogenesis of sepsis with emphasis on polymorphisms of the innate immunity, pro- and anti-inflammatory cytokines, and coagulation mediators.

DATA SOURCE

Published articles reporting on studies of associations between genetic polymorphisms, sepsis, septic shock, and other relevant infectious disease models.

DATA ANALYSIS

Research into the pathogenesis of sepsis has led to the development of many potential therapeutic strategies. Several therapeutic agents and treatment modalities have been shown to decrease mortality rates in large, prospective, and randomized clinical trials. However, although these advances have resulted in improved survival for certain patient populations, the overall mortality rate for septic patients remains high. With the rapid development of molecular and genetic techniques, substantial interests have developed in using genomic information to define disease-mediating genetic variants in sepsis. Combined with microarray technology, it is anticipated in the near future that one will be able to tailor drug selection and dosage and predict outcome by correlating genetic profile with disease presentation. Numerous genetic association studies in sepsis have already been reported and more are likely to be published.

CONCLUSIONS

Although studies examined in this review are of small heterogeneous populations, the identification of strong associations between certain genetic polymorphisms and increased mortality rate or susceptibility to severe sepsis is intriguing and supports further research using this approach. The establishment of these associations does not equal causation, and further research is required in both genetic and molecular aspect of sepsis.

Mannose-binding lectin deficiency alters the development of fungal asthma: effects on airway response, inflammation, and cytokine profile

Aspergillus fumigatus is a major fungal pathogen that may be fatal to immunocompromised individuals and causes airway hyperreactivity and remodeling in sensitized individuals. Herein, we examined the role of mannose-binding lectin (MBL), a complement-activating plasma protein, during pulmonary innate and allergic immune responses directed against A. fumigatus spores or conidia. Neither group of nonsensitized MBL-A-sufficient (MBL-A+/+) nor -deficient (MBL-A-/-) mice challenged with an intravenous or intratracheal (i.t.) bolus of A. fumigatus spores experienced fungus-induced mortality, but marked airway remodeling was observed in MBL-A-/- mice challenged i.t. with conidia. In a model of chronic fungal asthma, MBL-A+/+ and MBL-A-/- A. fumigatus-sensitized mice were examined at days 4 and 28 after an i.t. challenge with A. fumigatus conidia. Airway hyperresponsiveness in sensitized MBL-A-/- mice was significantly decreased at both times after conidia challenge compared with the sensitized MBL-A+/+ group. In the sensitized MBL-A-/- mice, whole lung T helper cell type 2 cytokine levels were significantly decreased at day 4 after conidia, and whole lung interferon-gamma levels were significantly increased at day 28 after conidia when compared with controls. However, histological evidence showed similar airway remodeling at day 28 after conidia (i.e., subepithelial fibrosis and goblet cell metaplasia) in the two groups of mice. Thus, these findings show that MBL-A is not required for mouse survival following exposure to A. fumigatus conidia, and this murine collectin isoform contributes to the development and maintenance of airway hyperresponsiveness but not chronic airway remodeling during chronic fungal asthma.

Autoantibodies against mannose-binding lectin in systemic lupus erythematosus

In systemic lupus erythematosus (SLE), autoantibodies directed against complement components of the classical pathway, especially against C1q, are associated with severe disease and are of prognostic value for flares of lupus nephritis. Mannose-binding lectin (MBL), the recognition unit of the MBL pathway of complement activation, has structural similarities to C1q. Deficiencies of MBL have been shown to predispose to the development of SLE and to influence the course of the disease. We hypothesized that the presence of autoantibodies to MBL, analogous to autoantibodies to C1q in patients with SLE, may contribute to disease development. The occurrence of anti-MBL autoantibodies was assessed by enzyme-linked immunosorbent assay (ELISA) of 68 serum samples from 20 patients with SLE and in serum from 70 healthy controls. Levels of antibodies directed against MBL were significantly higher in patients with SLE compared to healthy subjects. No significant difference was found between patients with active disease compared to those with inactive disease. While the occurrence of anti-C1q autoantibodies was associated with renal involvement, no such relationship was found for anti-MBL autoantibodies. A significant correlation was found between anti-MBL and anti-C1q antibody levels. The level of anti-MBL antibodies was negatively correlated with MBL-complex activity of circulating MBL. Anti-MBL autoantibodies were of the immunoglobulin G (IgG) isotype and the binding site of IgG anti-MBL was located in the F(ab')2 portion. We conclude that anti-MBL are present in sera from SLE patients and influence the functional activity of MBL.

Inherited mannose-binding lectin deficiency as evidenced by genetic and immunologic analyses: association with severe recurrent infections

BACKGROUND

Mannose-binding lectin (MBL), an acute-phase serum protein of hepatic origin, plays an essential protective role in host innate immunity in targeting microbial pathogens for destruction via opsonization, enhancement of phagocytosis and complement activation. MBL deficiency, characterized by low serum MBL, has been attributed to genetic mutations in both structural and promoter regions of the gene coding for the protein. Concomitant MBL deficiency in patients with chronic immunologic disease has been associated with increased susceptibility to complicating infections that may hasten disease progression.

OBJECTIVE

Few cases of inherited MBL deficiency in adults and possible associations with recurrent infection have been reported. To address this issue, we investigated the MBL profile of four generations within a single adult family whose members have experienced a variety of persistent infections.

METHODS

MBL serum levels and MBL genotypes of each participating family member were ascertained by enzyme-linked immunoadsorbent assay and reverse transcriptase-polymerase chain reaction, respectively. MBL complement activation, as measured by C4b deposition against mannan-coated wells, was assayed using an enzyme-linked immunoadsorbent assay. Routine immunologic and cellular tests were carried out to evaluate the immunologic status of each family member.

RESULTS

Six of the 7 family members screened carried one or more of the variant MBL alleles in their genotype and had correspondingly low serum MBL and reduced ability to affect C4b opsonization. Medical histories of the participating family members revealed an array of mild to severe recurrent infections despite no apparent immunodeficiency.

CONCLUSIONS

Our studies show that MBL deficiency is an inherited characteristic and may be a crucial factor in maintaining immunologic health.

Genetic Evidence for Involvement of Classical Complement Pathway in Induction of Experimental Autoimmune Myasthenia Gravis

Abs to acetylcholine receptor (AChR) and complement are the major constituents of pathogenic events causing neuromuscular junction destruction in both myasthenia gravis (MG) and experimental autoimmune MG (EAMG). To analyze the differential roles of the classical vs alternative complement pathways in EAMG induction, we immunized C3(-/-), C4(-/-), C3(+/-), and C4(+/-) mice and their control littermates (C3(+/+) and C4(+/+) mice) with AChR in CFA. C3(-/-) and C4(-/-) mice were resistant to disease, whereas mice heterozygous for C3 or C4 displayed intermediate susceptibility. Although C3(-/-) and C4(-/-) mice had anti-AChR Abs in their sera, anti-AChR IgG production by C3(-/-) mice was significantly suppressed. Both C3(-/-) and C4(-/-) mice had reduced levels of B cells and increased expression of apoptotis inducers (Fas ligand, CD69) and apoptotic cells in lymph nodes. Immunofluorescence studies showed that the neuromuscular junction of C3(-/-) and C4(-/-) mice lacked C3 or membrane attack complex deposits, despite having IgG deposits, thus providing in vivo evidence for the incapacity of anti-AChR IgGs to induce full-blown EAMG without the aid of complements. The data provide the first direct genetic evidence for the classical complement pathway in the induction of EAMG induced by AChR immunization. Accordingly, severe MG and other Ab- and complement-mediated diseases could be effectively treated by inhibiting C4, thus leaving the alternative complement pathway intact.

Mannan-binding lectin in children with Escherichia coli O157:H7 haemmorrhagic colitis and haemolytic uraemic syndrome

Mannan-binding lectin (MBL) triggers complement activation upon binding to microbial surfaces. MBL deficiency has been associated with increased susceptibility to severe bacterial infections. We hypothesized that MBL deficiency may predispose children to Shiga toxin-producing Escherichia coli (STEC) O157:H7 infections and the associated haemolytic uraemic syndrome (HUS). We compared circulating levels of MBL among children with uncomplicated O157:H7 haemorrhagic colitis (HC), patients with O157:H7 HUS, normal and diseases control groups. Circulating MBL concentrations on admission were as follows: 3.22 +/- 2.43 micro g/ml among normal controls (n = 23); 2.90 +/- 2.44 micro g/ml in patients with rotavirus enteritis (n = 10); 2.78 +/- 1.65 micro g/ml in children with HC due to non-STEC bacterial pathogen (n = 15); 2.67 +/- 2.44 micro g/ml in patients with uncomplicated O157:H7 HC (n = 27); 2.80 +/- 2.97 micro g/ml in children with O157:H7 HUS (n = 15); 6.70 +/- 4.49 micro g/ml in patients with chronic renal failure unrelated to O157:H7 infection (n = 6). Higher MBL levels were found in patients with chronic renal failure compared to O157:H7 HC (P < 0.047). However, MBL concentrations <0.5 micro g/ml, which have been associated with MBL deficiency in relation to increased susceptibility to infections, were noted at comparable rates between the different groups (P = NS). Our data does not support that MBL deficiency may predispose to O157:H7 infections nor than the development of diarrhoea associated HUS.

Role of the mannose-binding lectin in innate immunity

The ability to recognize infectious agents from self is intrinsic to innate immunity. One basic tenet of innate immunity is the evolution of classes of molecules that are termed "pattern-recognition" receptors and molecules. Many pattern-recognition molecules conspire together to protect the host in the first minutes and hours after exposure to an infectious challenge. The mannose-binding lectin (MBL; also termed "mannose-binding protein") is a prototypic pattern-recognition molecule that appears to play a role as an "ante-antibody" in first line host defense. The serum levels of the human MBL are regulated in serum so that any one person will display a phenotype of low, intermediate, or high levels. There appears to be a relationship between circulating MBL and susceptibility and resistance to infection. MBL levels also appear to be regulated by distinct haplotypes. Thus, the question to be answered is what constitutes the innate immunity haplotype in any individual and how does this impact on the relationship between the host and infectious agents?

Mannose-binding lectin gene polymorphism predicts hospital admissions for COPD infections

Infection frequently causes exacerbations of chronic obstructive pulmonary disease (COPD). Mannose-binding lectin (MBL) is a pattern-recognition receptor that assists in clearing microorganisms. Polymorphisms in the MBL2 gene reduce serum MBL levels and are associated with risk of infection. We studied whether the MBL2 codon 54 B allele affected serum MBL levels, admissions for infective exacerbation in COPD and disease susceptibility. Polymorphism frequency was determined by PCR-RFLP in 200 COPD patients and 104 smokers with normal lung function. Serum MBL was measured as mannan-binding activity in a subgroup of 82 stable COPD patients. Frequency of COPD admissions for infective exacerbation was ascertained for a 2-year period. The MBL2 codon 54 B allele reduced serum MBL in COPD patients. In keeping, patients carrying the low MBL-producing B allele had increased risk of admission for infective exacerbation (OR 4.9, P(corrected)=0.011). No association of MBL2 genotype with susceptibility to COPD was detected. In COPD, serum MBL is regulated by polymorphism at codon 54 in its encoding gene. Low MBL-producing genotypes were associated with more frequent admissions to hospital with respiratory infection, suggesting that the MBL2 gene is disease-modifying in COPD. MBL2 genotype should be explored prospectively as a prognostic marker for infection risk in COPD.

Pancreatic elastase is proven to be a mannose-binding protein--implications for the systemic response to pancreatitis

BACKGROUND

Mannose-binding proteins (MBPs) have been isolated from serum, liver, lung, and kidney and are believed to play an important role in first-line host defense during acute phase inflammatory response. Because of the inflammatory nature of pancreatitis, we postulate that the pancreas produces endogenous MBP.

METHODS

Pancreatic juice, from both human and rat, was collected by pancreatic duct cannulation and subjected to mannose-Sepharose affinity chromatography to isolate pancreatic MBP (pMBP). Protein eluates from the mannose-Sepharose column were analyzed using reverse-phase high-performance liquid chromatography, sodium dodeclysulfate-polyacrylamide gel electrophoresis, and, subsequently, by N-terminal protein sequencing. Western blot analysis was used to identify the pMBP, and reverse transcriptionase-polymerase chain reaction was used to examine its mRNA expression. Complement lysis was measured using red blood cells coated with yeast mannan. Tumor necrosis factor (TNF)-alpha mRNA expression in macrophages was measured using RNase protection assay.

RESULTS

A 30-kd MBP was isolated from both human and rat pancreatic juice and a rat acinar cell line. Genetic analysis (using RT-PCR with known MBP primers) and protein analysis (using Western blot with a known anti-MBP antibody) suggest that the pMBP is different from any previously described MBP. Protein sequencing analysis of pMBP generated an N-terminus sequence of 12 residues, indicating that pMBP is human pancreatic elastase III. Western blot analysis using an anti-elastase antibody confirms that the pMBP is a pancreatic elastase. Exposure of macrophages to pancreatic elastase resulted in an increased mRNA level of TNF-alpha, a potent proinflammatory cytokine in acute-phase response. Addition of mannan to pancreatic elastase further upregulated the TNF-alpha response.

CONCLUSION

We isolated an MBP from the pancreas and identified it as pancreatic elastase. We characterized it as having properties different from that of any previously known MBP. We showed that pMBP or pancreatic elastase is involved in the activation of macrophages, and that this activation is potentiated by mannan. We postulate that the mannose-binding properties of pancreatic elastase identify this enzyme as a candidate catalyst for both pancreatic and systemic inflammation.

Mannose-binding lectin: comparison of two assays for the quantification of MBL in the serum of pediatric patients

Individuals with mannose-binding lectin (MBL)-deficiency are at an increased risk from infections with mannose-bearing microorganisms. We have investigated two quantitative research assays for measuring MBL protein in serum for routine diagnosis. The evaluation of 817 serum samples with a nephelometric assay revealed two deficiencies, a number far below the postulated 5-10% of the population. Reevaluation of 102 serum samples with an MBL-ELISA detected low levels in 27 cases (26.4%) and clear deficiencies in 21 samples (20.4%). In our hands, the MBL-ELISA permitted the detection of decreased levels of MBL in serum, as occurs in individuals with homozygous or heterozygous MBL gene mutations; in contrast, the nephelometric assay appeared to be unsuitable for the detection of MBL deficiencies. We support the routine measurement of MBL in serum, especially in children with frequent infections.

Umbilical cord mannan-binding lectin and infections in early childhood

To investigate whether mannan-binding lectin (MBL) in umbilical cord blood (UCB) is associated with the incidence rate of hospitalizations for infections during early childhood. A follow-up study from birth to 31 months of age, with endpoint data from the Danish National Hospital Discharge Registry. The concentration of MBL and immunoglobulin A (IgA) in UCB was measured in a time-resolved immunofluorometric assay. Information on possible confounding factors was obtained from questionnaires completed by mothers during their pregnancy, and characteristics of the child at birth were registered by a midwife on a structured coding sheet. A total of 2104 infants born in the period between 1 February 1990 and 25 May 1991 to mothers living in the municipality of Aarhus. Concentration of MBL and IgA in UCB, gestational age, birth weight, number of siblings less than 3 years old, mother's smoking habits, alcohol and coffee consumption, educational and marital status and previous spontaneous abortions and stillbirths. Of 2104 children, 626 were hospitalized at least once, and 346 of these were hospitalized with infection. The hazard ratio (HR) of hospitalization with infections in children with low levels of MBL (<120 ng/ml) was 1.4 (95% confidence interval (CI), 1.0-1.8), which was similar to our findings on hospitalization because of other diseases. The HR of hospitalization for viral infections was 2.8 (CI, 1.3-5.9). Low levels of MBL in UCB were associated with hospitalization in general and particularly with hospitalization for viral infections.

Therapeutic potential of targeting the complement cascade in critical care medicine

Caring for the critical care patient involves many different areas of clinical expertise and serves a diverse patient population. Novel therapeutics for the critically ill must be approached with caution, because the underlying molecular mechanisms of the disease process for several commonly seen types of patients (i.e., sepsis, shock, ischemia/reperfusion) are not fully understood. A potentially new and advancing area of therapeutics that may hold promise for the critically ill is inhibition of the complement system. Various novel complement inhibitors are being developed and several are in clinical trials. The advancement of this novel area of therapeutics may one day aid the clinician by providing several different complement inhibitors/antagonists for controlling complement activation or its biologically active mediators.

L-MBP is expressed in epithelial cells of mouse small intestine

The mannan-binding proteins (L-MBP and S-MBP, also denoted MBL-C and MBL-A), mainly produced in liver and existing in liver and serum, play important roles in the innate immunity against a variety of pathogens. Total RNA from mouse tissues were screened for MBP mRNA by RT-PCR. In addition to liver, S-MBP mRNA was detected in lung, kidney, and testis, and L-MBP mRNA was detected in kidney, thymus, and small intestine. Quantitative RT-PCR revealed that the small intestine is a predominant site of extrahepatic expression of L-MBP. Western blotting with polyclonal Abs against rat L-MBP demonstrated this protein in Triton X-100 extracts of the small intestine obtained from mice that had undergone systemic perfusion. Immunohistochemical staining with an mAb against mouse L-MBP and in situ hybridization revealed that L-MBP is selectively expressed in some villous epithelial cells of the small intestine. These findings suggest that L-MBP plays a role in mucosal innate immunity.

Mannose binding lectin (MBL) polymorphisms associated with low MBL production in patients with dermatomyositis

One theory for the pathophysiology of photosensitive autoimmune skin diseases is that photoinduction of tumor necrosis factor alpha (TNFalpha) secretion leads to keratinocyte apoptosis and translocation of previously sequestered cellular antigens that then activate the immune system. We previously found an association of the overproducing TNFalpha-308 A variant with adult dermatomyositis and with subacute cutaneous lupus erythematosus. Here we focused on mannose binding lectin (MBL), which is one of several proteins involved in clearance of apoptotic cells and could thereby lessen photosensitive autoimmunity. We examined three variant MBL polymorphisms associated with decreased MBL protein (Asp54, Glu57, and the LX promoter polymorphism) in adult dermatomyositis, subacute cutaneous lupus erythematosus, and discoid lupus, and controls. The variant Asp54 allele was positively associated with adult dermatomyositis in a dose-responsive fashion (p=0.0004), as was the Glu57 allele (p=0.004). None of the three variant MBL alleles considered individually was significantly associated with either subacute cutaneous lupus erythematosus or discoid lupus. In adult dermatomyositis patients homozygous for the wild-type TNFalpha-308G allele (GG), i.e., presumably without elevated TNFalpha production, 69% had at least two of the MBL polymorphisms, versus 20% of healthy GG controls (p=0.0011). Combinations of low-producing MBL variants were over-represented in adult dermatomyositis in a dose-responsive fashion (p=0.0002). In adult dermatomyositis patients with one variant TNFalpha-308 A allele (GA), 46% had at least two MBL polymorphisms, versus 7% of GA controls (p=0.0077). Thus, low-producing MBL genes are very strongly associated with adult dermatomyositis. Our model is that genetic polymorphisms leading to overproduction of apoptotic keratinocytes and then impaired clearance of these cells contribute to the pathogenesis of adult dermatomyositis, a photoinduced autoimmune skin disease.

Analysis of the relationship between mannose-binding lectin (MBL) genotype, MBL levels and function in an Australian blood donor population

The mannose-binding lectin (MBL) pathway of complement activation is an important component of innate host defence. Numerous studies have described associations between the MBL genotype, MBL levels and disease susceptibility. However, genotyping and quantitative assays used in these studies have frequently been limited, and comprehensive data examining the interaction between structural and coding MBL genetic variants, MBL antigenic levels and MBL functional activity are lacking. Such data may be important for accurate planning and interpretation of studies of MBL and disease. This study has examined MBL in a cohort of 236 Australian blood donors. Five MBL promoter and coding single nucleotide polymorphisms were genotyped using polymerase chain reaction-sequence-specific priming (PCR-SSP). Plasma levels of MBL antigen were quantified using a double-antibody enzyme-linked immunosorbent assay (ELISA), and functional MBL levels were quantified using a mannan-binding assay. Activation of the complement pathway by MBL was measured in a C4-deposition assay. Significant associations were found between both coding and promoter polymorphisms and MBL antigenic and functional levels. There was significant correlation between the results of MBL double-antibody, mannan-binding and C4-deposition assays. Comprehensive MBL genotyping and functional MBL quantitation using mannan-binding and C4-deposition assays have the potential to be highly informative in MBL disease association studies.

Lack of mannose-binding lectin-A enhances survival in a mouse model of acute septic peritonitis

The mannose-binding lectin (MBL) (also known as the mannose-binding protein) is a serum protein that plays a role as an "ante-antibody" in innate immunity. In man, MBL is encoded by a single gene, whereas in mice there are two homologous proteins, MBL-A and MBL-C. In order to evaluate the relative roles of these two forms of MBL, we created MBL-A null mice that were MBL-C sufficient. We found MBL-A null mice had enhanced survival in a septic peritonitis model compared to wild-type mice and complement 3 null mice at 24 h, 48 h and 10 d (P < 0.05). Reconstitution of these mice with human MBL reversed the phenotype. Surviving mice had significantly decreased TNF-alpha and IL-6 levels in the blood and peritoneal cavity (P < 0.01). In vitro studies indicate that bacteria opsonized with MBL-A-deficient serum induced significantly less cytokine by peritoneal macrophages compared to those with wild-type serum. Our results indicate that MBL-A is a modulator of inflammation in vivo and in vitro in the mouse and that the role of MBL may extend beyond its role as an opsonin.

Fungi, dendritic cells and receptors: a host perspective of fungal virulence

An association between morphogenesis and virulence has long been presumed for dimorphic fungi that are pathogenic to humans, as one morphotype exists in the environment or during commensalism, and another within the host during the disease process. For Candida albicans, putative virulence factors include the ability to switch between saprophytic yeast and pathogenic, filamentous forms of the fungus. Dendritic cells sense either form in a specific way, resulting in distinct, T-helper-cell-dependent protective and non-protective immunities. Recent evidence suggests that the use of distinct recognition receptors contributes to the disparate patterns of reactivity observed locally in response to challenge with C. albicans. These findings offer new interpretive clues to the mechanisms of fungal virulence: rather than dimorphism per se, the engagement of different recognition receptors on dendritic cells might select the mode of fungal internalization and antigen presentation, condition the nature of the T-helper response and, ultimately, favor saprophytism or infection.

Evaluation and clinical interest of mannan binding lectin function in human plasma

The mannan binding lectin (MBL) plays a major role in innate immunity through its ability to activate complement upon binding to carbohydrate arrays on the surface of various microorganisms. The question of a possible association of the MBL structural gene polymorphism and the oligomeric state of MBL was poorly documented. For these reasons, it appears difficult to evaluate MBL in blood patients on the only basis of protein contents, even in combination with MBL genotyping. This study reports a method to calculate a specific activity for circulating MBL, that relies on: (i) the availability of purified MBL; and (ii) a simplified MBL activity assay based on complement activation. The three-step MBL purification from human plasma reported here is characterized by a highly purified MBL, that occurs in two different oligomeric forms. The results on the specific activity of these forms show that the higher oligomeric forms of MBL have the ability to induce C4 cleavage more efficiently than the corresponding lower oligomers. The usefulness of this approach is illustrated by its potential interest in the biological exploration of certain pathology, for example in the follow-up of chronic hepatitis C. Further investigation is needed to establish whether MBL specific activity (MBLsa) is correlated to the polymorphic state of the molecule. The relative simplicity of the test described here allows better investigation on the relationship between MBL biological activity and its genotype.

Enhancement of complement activation and opsonophagocytosis by complexes of mannose-binding lectin with mannose-binding lectin-associated serine protease after binding to Staphylococcus aureus

Human mannose-binding lectin (MBL) is a serum protein of the innate immune system that circulates as a complex with a group of so-called MBL-associated serine proteases (MASP-1, MASP-2, and MASP-3). Complexes of MBL-MASP2 are able to activate the complement system in an Ab and C1-independent fashion after binding of the lectin to appropriate microbial sugar arrays. We have evaluated the additive effect of the lectin pathway relative to other complement activation pathways and the subsequent effect on neutrophil phagocytosis. Complement activation in the sera of MBL-deficient individuals was studied with and without the addition of exogenous MBL-MASP. Flow cytometry was used to measure the deposition of C4, factor B, C3b, and iC3b on Staphylococcus aureus. Deposition of the first cleavage product of the lectin pathway, C4b, was increased using the sera of three different MBL-deficient individuals when exogenous MBL-MASP was added. Factor B was deposited in association with C4, but there was no evidence of independent alternative pathway activation. Similar enhancement of C3b deposition was also observed, with evidence of elevated amounts of C3b processed to iC3b. The increase in opsonic C3 fragments mediated by MBL was associated with a significant increase in the uptake of organisms by neutrophils. We also observed significant increases in phagocytosis with MBL-MASPs that were independent of complement activation. We conclude that MBL-MASP makes a major contribution to complement-mediated host defense mechanisms.