Immature dendritic cells possess a sugar-sensitive receptor for human mannan-binding lectin

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

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

RNA sequencing-based analysis of the spleen transcriptome following infectious bronchitis virus infection of chickens selected for different mannose-binding lectin serum concentrations

BACKGROUND

Avian infectious bronchitis is a highly contagious disease of the upper-respiratory tract caused by infectious bronchitis virus (IBV). Understanding the molecular mechanisms involved in the interaction between innate and adaptive immune responses to IBV infection is a crucial element for further improvements in strategies to control IB. To this end, two chicken lines, selected for high (L10H line) and low (L10L line) serum concentration of mannose-binding lectin (MBL) were studied. In total, 32 birds from each line were used. Sixteen birds from each line were infected with IBV and sixteen were left uninfected. Eight uninfected and infected birds from each line were euthanized at 1 and 3 weeks post infection. RNA sequencing was performed on spleen samples from all 64 birds and differential gene expression analysis was performed for four comparisons: L10L line versus L10H line for uninfected birds at weeks 1 and 3, respectively, and in the same way for infected birds. Functional analysis was performed using Gene Ontology (GO) Immune System Process terms specific for Gallus gallus.

RESULTS

Comparing uninfected L10H and L10L birds, we identified 1698 and 1424 differentially expressed (DE) genes at weeks 1 and 3, respectively. For the IBV-infected birds, 1934 and 866 DE genes were identified between the two lines at weeks 1 and 3, respectively. The two most enriched GO terms emerging from the comparison of uninfected birds between the two lines were "Lymphocyte activation involved in immune response" and "Somatic recombination of immunoglobulin genes involved in immune response" at weeks 1 and 3, respectively. When comparing IBV-infected birds between the two lines, the most enriched GO terms were "Alpha-beta T cell activation" and "Positive regulation of leukocyte activation" at weeks 1 and 3, respectively.

CONCLUSIONS

Healthy birds from the two lines showed significant differences in expression profiles for subsets of adaptive and innate immunity-related genes, whereas comparison of the IBV-infected birds from the two lines showed differences in expression of immunity-related genes involved in T cell activation and proliferation. The observed transcriptome differences between the two lines indicate that selection for MBL had influenced innate as well as adaptive immunity.

Activation of the lectin pathway of complement in experimental human keratitis with Pseudomonas aeruginosa

PURPOSE

Pseudomonas aeruginosa (P. aeruginosa) microbial keratitis (MK) is a sight-threatening disease. Previous animal studies have identified an important contribution of the complement system to the clearance of P. aeruginosa infection of the cornea. Mannose-binding lectin (MBL), a pattern recognition receptor of the lectin pathway of complement, has been implicated in the host defense against P. aeruginosa. However, studies addressing the role of the lectin pathway in P. aeruginosa MK are lacking. Hence, we sought to determine the activity of the lectin pathway in human MK caused by P. aeruginosa.

METHODS

Primary human corneal epithelial cells (HCECs) from cadaveric donors were exposed to two different P. aeruginosa strains. Gene expression of interleukin (IL)-6, IL-8, MBL, and other complement proteins was determined by reverse transcription-polymerase chain reaction (RT-PCR) and MBL synthesis by enzyme-linked immunosorbent assay and intracellular flow cytometry.

RESULTS

MBL gene expression was not detected in unchallenged HCECs. Exposure of HCECs to P. aeruginosa resulted in rapid induction of the transcriptional expression of MBL, IL-6, and IL-8. In addition, expression of several complement proteins of the classical and lectin pathways, but not the alternative pathway, were upregulated after 5 h of challenge, including MBL-associated serine protease 1. However, MBL protein secretion was not detectable 18 h after challenge with P. aeruginosa.

CONCLUSIONS

MK due to P. aeruginosa triggers activation of MBL and the lectin pathway of complement. However, the physiologic relevance of this finding is unclear, as corresponding MBL oligomer production was not observed.

Birds, babies and blood

This is an autobiographical review describing the author's career in immunology research and summarizing his current understanding of the areas involved. Contributions to autoimmunity, immune deficiency, transfusion immunology, HLA-disease associations, reproductive immunology, cellular therapies, and innate immunity are included; also discussion of medical research ethics and various research-related activities.

Increased plasma mannose binding lectin levels are associated with bronchiolitis obliterans after lung transplantation

Background

Long-term lung allograft survival is limited by bronchiolitis obliterans syndrome (BOS). Mannose binding lectin (MBL) belongs to the innate immune system, participates in complement activation, and may predispose to graft rejection. We investigated mannose binding (MBL) during cold ischemia and in tissue samples from explanted lungs with BOS, and assessed MBL and complement proteins in plasma post-lung transplantation relative to BOS staging.

Methods

MBL was detected by immunohistochemistry lung tissue at the time of cold ischemia and in samples with BOS. MBL was assayed in the peripheral blood of 66 lung transplant patients transplanted between 1990–2007.

Results

MBL localized to vasculature and basement membrane during cold ischemia and BOS. Patients further out post-lung transplant > 5 years (n = 33), had significantly lower levels of MBL in the blood compared to lung transplant patients < 5 years with BOS Op-3 (n = 17), 1738 ± 250 ng/ml vs 3198 ± 370 ng/ml, p = 0.027, and similar levels to lung transplant patients < 5 years with BOS 0 (n = 16), 1738 ± 250 ng/ml vs 1808 ± 345 ng/ml. MBL levels in all BOS 0 (n = 30) vs. all BOS Op-3 (n = 36) were 1378 ± 275 ng/ml vs. 2578 ± 390 ng/ml, p = 0.001, respectively. C3 plasma levels in BOS 0 (n = 30) vs. BOS Op-3 (n = 36) were 101 ± 19.8 mg/ml vs. 114 ± 25.2 mg/ml, p = 0.024, respectively.

Conclusions

MBL localizes within the lung during graft ischemia and BOS, higher levels of plasma MBL are associated with BOS Op-3 and < 5 years post-transplant, and higher level of plasma complement protein C3 was associated with BOS Op-3 clinical status. MBL may serve as a biomarker for poorer outcome post-lung transplantation.

MBL Deficiency as Risk of Infection and Autoimmunity

In pathogen recognition by C-type lectins, several levels of complexity can be distinguished; these might modulate the immune response in different ways. Firstly, the pathogen-associated molecular pattern repertoire expressed at the microbial surface determines the interactions with specific receptors (Fig. 42.1). Secondly, each immune cell type possesses a specific set of pathogen-recognition receptors. Thirdly, changes in the cell-surface distribution of C-type lectins regulate carbohydrate binding by modulating receptor affinity for different ligands. Crosstalk between these receptors results in a network of multimolecular complexes, adding a further level of complexity in pathogen recognition (Cambi and Figdor 2005; Thiel et al. 2006) (see 10.1007/978-3-7091-1065-2_23). MBL deficiency is genetically determined and predisposes to recurrent infections and chronic inflammatory diseases. MBL deficiency has been implicated in susceptibility and course of viral, bacterial, fungal, and protozoan infection. More than 10% of the general population may, depending on definition, be classified as MBL deficient, underlining the redundancy of the immune system. MBL-disease association studies have been a fruitful area of research, which implicates a role for MBL in infective, inflammatory and autoimmune disease processes. MBL deficiency predisposes both to infection by extra-cellular pathogens and to autoimmune disease.

Mannan-binding lectin directly interacts with Toll-like receptor 4 and suppresses lipopolysaccharide-induced inflammatory cytokine secretion from THP-1 cells

Mannan-binding lectin (MBL) plays a key role in the lectin pathway of complement activation and can influence cytokine expression. Toll-like receptor 4 (TLR4) is expressed extensively and has been demonstrated to be involved in lipopolysaccharide (LPS)-induced signaling. We first sought to determine whether MBL exposure could modulate LPS-induced inflammatory cytokine secretion and nuclear factor-κB (NF-κB) activity by using the monocytoid cell line THP-1. We then investigated the possible mechanisms underlying any observed regulatory effect. Using ELISA and reverse transcriptase polymerase chain reaction (RT-PCR) analysis, we found that at both the protein and mRNA levels, treatment with MBL suppresses LPS-induced tumor-necrosis factor (TNF)-α and IL-12 production in THP-1 cells. An electrophoretic mobility shift assay and western blot analysis revealed that MBL treatment can inhibit LPS-induced NF-κB DNA binding and translocation in THP-1 cells. While the binding of MBL to THP-1 cells was evident at physiological calcium concentrations, this binding occurred optimally in response to supraphysiological calcium concentrations. This binding can be partly inhibited by treatment with either a soluble form of recombinant TLR4 extracellular domain or anti-TLR4 monoclonal antibody (HTA125). Activation of THP-1 cells by LPS treatment resulted in increased MBL binding. We also observed that MBL could directly bind to the extracellular domain of TLR4 in a dose-dependent manner, and this interaction could attenuate the binding of LPS to cell surfaces. Taken together, these data suggest that MBL may affect cytokine expression through modulation of LPS-/TLR-signaling pathways. These findings suggest that MBL may play an important role in both immune regulation and the signaling pathways involved in cytokine networks.

Poly(ADP-ribosyl)ation of mannose-binding lectin out of human kidney cells

Mannose-binding lectin was identified as a substrate of tankyrase 2, an enzyme that catalyzes poly(ADP-ribosyl)ation. The endogenous tankyrase 2 was isolated out of cytoplasm of human embryonic kidney cells. It was bound to a soluble complex of at least two other proteins; they were identified using specific antibodies and other approaches as keratin 1 and mannose-binding lectin. Using immunoblot analysis and radioactive labeling, we detected tankyrase-2-dependent poly(ADP-ribosyl)ation of mannose-binding lectin. In the presence of NAD(+), the complex of keratin 1 and lectin was dissociated, what was recorded during elution of its separate components out of affinity columns and by decrease of their apparent molecular masses during gel-filtration. Tankyrase 2 also inhibited the carbohydrate-binding function of the lectin. The latter effect was observed using mannose-binding lectin out of human serum, which is free from keratin 1. As a result of tankyrase-2 activity, the lectin lost its affinity to mannan-agarose. The discovery of this new biochemical mechanism justifies further analysis of its physiological and medical significance.

Mannose-binding lectin deficiency confers risk for bacterial infections in a large Hungarian cohort of patients with liver cirrhosis

BACKGROUND & AIMS

Mannose-binding lectin (MBL) is a serum lectin synthesized by the liver and involved in innate host defense. MBL deficiency increases the risk of various infectious diseases mostly in immune-deficient conditions. Bacterial infections are a significant cause of morbidity and mortality in liver cirrhosis due to the relative immuncompromised state.

METHODS

Sera of 929 patients with various chronic liver diseases [autoimmune liver diseases (ALD), 406; viral hepatitis C (HCV), 185; and liver cirrhosis (LC) with various etiologies, 338] and 296 healthy controls (HC) were assayed for MBL concentration. Furthermore, a follow-up, observational study was conducted to assess MBL level as a risk factor for clinically significant bacterial infections in cirrhotic patients.

RESULTS

MBL level and the prevalence of absolute MBL deficiency (<100 ng/ml) was not significantly different between patients and controls (ALD: 14.5%, HCV: 11.9%, LC: 10.7%, HC: 15.6%). In cirrhotic patients, the risk for infection was significantly higher among MBL deficient subjects as compared to non-deficient ones (50.0% vs. 31.8%, p=0.039). In a logistic regression analysis, MBL deficiency was an independent risk factor for infections (OR: 2.14 95% CI: 1.03-4.45, p=0.04) after adjusting for Child-Pugh score, co-morbidities, gender, and age. In a Kaplan-Meier analysis, MBL deficiency was associated with a shorter time to develop the first infectious complication (median days: 579 vs. 944, pBreslow=0.016, pLogRank=0.027) and was identified as an independent predictor in a multivariate Cox-regression analysis (p=0.003, OR: 2.33, 95% CI: 1.34-4.03).

CONCLUSIONS

MBL deficiency is associated with a higher probability and shorter time of developing infections in liver cirrhosis, further supporting the impact of the MBL molecule on the host defense.

Mannose-binding lectin is produced by vaginal epithelial cells and its level in the vaginal fluid is influenced by progesterone

Mannose-binding lectin (MBL) is a recognition molecule of the complement (C) system and binds to carbohydrate ligands present on a wide range of pathogenic bacteria, viruses, fungi, and parasites. MBL has been detected in the cervico-vaginal cavity where it can provide a first-line defence against infectious agents colonizing the lower tract of the reproductive system. Analysis of the cervico-vaginal lavage (CVL) obtained from 11 normal cycling women at different phases of the menstrual cycle revealed increased levels of MBL in the secretive phase. Part of this MBL derives from the circulation as indicated by the presence of transferrin in CVL tested as a marker of vascular and tissue permeability. The local synthesis of MBL is suggested by the finding that its level is substantially higher than that of transferrin in the secretive phase. The contribution of endometrium is negligible since the MBL level did not change before and after hysterectomy. RT-PCR and in situ RT-PCR analysis showed that the vaginal tissue, and in particular the basal layer of the epithelium, is a source of MBL which binds to the basal membrane and to cells of the outer layers of the epithelium. In conclusion, we have shown that MBL detected in CVL derives both from plasma as result of transudation and from local synthesis and its level is progesterone dependent increasing in the secretive phase of the menstrual cycle.

Mannose-binding lectin level and deficiency is not associated with inflammatory bowel diseases, disease phenotype, serology profile, and NOD2/CARD15 genotype in a large Hungarian cohort

Mannose-binding lectin (MBL) is a major, soluble, pattern-recognition molecule and an important component of the innate host defense. The role of MBL in inflammatory bowel diseases (IBDs) is controversial. We determined the prevalence of MBL deficiency in a Hungarian IBD patients' cohort, and whether it is associated with the antimicrobial antibody formation or particular clinical manifestations. Nine hundred ninety IBD patients and 225 healthy subjects were investigated. Sera were assayed for MBL and a panel of antimicrobial antibodies (anti-OMP, anti-Saccharomyces cerevisiae antibodies, and antiglycans) by ELISA. TLR4 and NOD2/CARD15 variants were tested by polymerase chain reaction/restriction fragment length polymorphism. Median MBL level was not significantly different between IBDs (Crohn's disease [CD]: 929; ulcerative colitis [UC]: 810 ng/ml) and the control group (1027 ng/ml), as well as the prevalence of absolute MBL deficiency (<100 ng/ml) (CD: 15.0%, UC: 18.4%, controls: 15.6%). The presence of a low MBL level (<500 ng/ml) was not associated with any of the examined serologic markers, or their combinations. In addition, there was no association with the clinical presentation, disease course, or response to treatment. TLR4 variant genotype was more common in CD patients without MBL deficiency (11% vs. 1.7%, OR: 7.29, 95% CI: 1.08-53.9, p = 0.02). We failed to confirm any association between MBL deficiency and serologic marker positivity. MBL deficiency was not predictive for clinical phenotype or disease activity in IBDs.

Exploration of factors affecting the onset and maturation course of follicular lymphoma through simulations of the germinal center

Genetic mutations frequently observed in human follicular lymphoma (FL) B-cells result in aberrant expression of the anti-apoptotic protein bcl-2 and surface immunoglobulins (Igs) which display one or more novel variable (V) region N-glycosylation motifs. In the present study, we develop a simulation model of the germinal center (GC) to explore how these mutations might influence the emergence and clonal expansion of key mutants which provoke FL development. The simulations employ a stochastic method for calculating the cellular dynamics, which incorporates actual IgV region sequences and a simplified hypermutation scheme. We first bring our simulations into agreement with experimental data for well-characterized normal and bcl-2(+) anti-hapten GC responses in mice to provide a model for understanding how bcl-2 expression leads to permissive selection and memory cell differentiation of weakly competitive B-cells. However, as bcl-2 expression in the GC alone is thought to be insufficient for FL development, we next monitor simulated IgV region mutations to determine the emergence times of key mutants displaying aberrant N-glycosylation motifs recurrently observed in human FL IgV regions. Simulations of 26 germline V(H) gene segments indicate that particular IgV regions have a dynamical selective advantage by virtue of the speed with which one or more of their key sites can generate N-glycosylation motifs upon hypermutation. Separate calculations attribute the high occurrence frequency of such IgV regions in FL to an ability to produce key mutants at a fast enough rate to overcome stochastic processes in the GC that hinder clonal expansion. Altogether, these simulations characterize three pathways for FL maturation through positively selected N-glycosylations, namely, via one of two key sites within germline V(H) region gene segments, or via a site in the third heavy chain complementarity-determining region (CDR-H3) that is generated from VDJ recombination.

Structural basis of pattern recognition by innate immune molecules

The importance of the innate immune system as a first line defence against pathogenic challenge has long been recognised. Over the last decade the identity of many of the key molecules mediating innate host defence have been clarified and a model of self/ nonself discrimination by families of pattern recognition receptors (PRRs) has emerged. Although a large amount of information is now available concerning the action of these innate immune molecules at the level of the cell and organism, little is known about the molecular interface between pathogens and innate immune recognition molecules. In this chapter the molecular basis for innate immune discrimination of a wide variety of pathogen derived molecules is discussed in the context of the emerging literature.

Is mannan-binding lectin (MBL) detectable on monocytes and monocyte-derived immature dendritic cells?

MBL (mannan-binding lectin; also called mannose-binding lectin) is a circulating C-type lectin with a collagen-like region synthesized mainly by the liver. MBL may influence susceptibility to infection in recipients of stem cell transplants, and it has even been suggested that the MBL status of a donor can influence the recipient's susceptibility to post-transplant infections. We have previously reported that MBL can be detected on human monocytes and monocyte-derived dendritic cells, based on detection using biotinylated anti-MBL, suggesting that those cells could synthesize MBL. If true, permanent MBL replacement therapy could be achieved by stem cell infusions. However, two other groups independently failed to find mbl-2-derived mRNA in monocytes. Therefore, to confirm or refute our previous observations, we used an alternative experimental strategy. Instead of using biotinylated antibody and labelled streptavidin, detection of surface MBL was attempted using MBL-specific primary antibodies (131-1, 131-10 and 131-11) followed by fluorescein-labelled anti-IgG, and controlled by the use of non-specific IgG as primary antibody. Monocytes were counterstained with anti-CD14-PE before FACS analysis. Adherent monocytes were also cultured for 48 h in serum-free medium or converted into immature dendritic cells by culture with IL-4 (interleukin-4) and GM-CSF (granulocyte/monocyte colony-stimulating factor). During FACS analysis, the dendritic cells were gated after counter-staining with anti-CD1a-PE. MBL was readily detected on the surface of fresh monocytes using all three specific anti-MBL monoclonal antibodies, but specific anti-MBL binding was greatly diminished after monocytes had been cultured for 2 days in serum-free medium. Moreover, we could not detect any MBL present on the surface of monocyte-derived dendritic cells. We therefore conclude that MBL is indeed present on the surface of fresh human monocytes. However, in view of the mRNA findings of others and our own previous observation that no secretion of MBL took place in culture, we presume that the surface-bound MBL is derived from autologous plasma and not synthesized by the cells. This conclusion is consistent with our in vivo findings in stem cell transplant patients which provided evidence against significant extra-hepatic production of serum MBL. It provides no ready explanation for the remarkable observation of Mullighan, Heatley, Doherty, Szabo, Grigg, Hughes, Schwarer, Szer, Tait, Bik To and Bardy [(2002) Blood 99, 3524–3529] that the presence of variant alleles of mbl-2 in stem cell donors can influence susceptibility to serious infections in their recipients.

Human Follicular Lymphoma Cells Contain Oligomannose Glycans in the Antigen-binding Site of the B-cell Receptor

Expression of surface immunoglobulin appears critical for the growth and survival of B-cell lymphomas. In follicular lymphoma, we found previously that the Ig variable (V) regions in the B-cell receptor express a strikingly high incidence of N-glycosylation sequons, NX(S/T). These potential glycosylation sites are introduced by somatic mutation and are lymphoma-specific, pointing to their involvement in tumor pathogenesis. Analysis of the V region sugars from lymphoma-derived IgG/IgM reveals that they are mostly oligomannose and, remarkably, are located in the antigen-binding site, possibly precluding conventional antigen binding. The Fc region contains complex glycans, confirming that the normal glycan processing pathway is intact. Binding studies indicate that the oligomannose glycans occupying the V regions are accessible to mannose-binding lectin. These findings suggest a potential contribution to lymphoma pathogenesis involving antigen-independent interaction of surface immunoglobulin of the B-cell receptor with mannose-binding molecules of innate immunity in the germinal center.

Hormonal regulation of mannan-binding lectin synthesis in hepatocytes

Activation of the complement system via the plasma protein mannan-binding lectin (MBL) provides a first line of defence against infections. The plasma level of MBL is, in part, determined genetically, but may also be influenced by different hormones in vivo. Here we study the hormonal regulation of MBL synthesis from the human hepatocyte cell line HuH-7. Cells were exposed to medium with growth hormone (GH), hydrocortisone, insulin-like growth factor (IGF)-1, insulin, interleukin (IL)-6 or thyroid hormones (T3 or T4). After 3 days the concentration of MBL in the culture supernatants was determined and the amount of mRNA for MBL was measured, relative to mRNA for beta2 microglobulin. GH, IL-6, T3 and T4 significantly increased MBL synthesis in a dose-dependent manner, while hydrocortisone, insulin and IGF-1 had no effect. T3 caused a fourfold increase at 1 nM of T3 (P < 0.001) and at 100 nM of T3 the production was increased more than eightfold. The effect of T4 was less potent, reaching an eightfold increase at 1 microM of T4 (P < 0.001). GH augmented the production of MBL threefold at a concentration of 100 ng/ml (P = 0.018) with no further effect up to 10 microg/ml, whereas IL-6 caused only a very weak increase in MBL production. MBL mRNA levels were stable during the first 24 h of T3 stimulation but increased significantly between 24 and 48 h. The results suggest that MBL synthesis in humans may be increased by thyroid hormone and GH, whereas it does not exhibit a classical IL-6-dependent response.

Human serum amyloid P component binds to peripheral blood monocytes

The binding of human serum amyloid P component (SAP) to blood cells and monocyte-derived dendritic cells was investigated by flow cytometry. Monocytes bound biotinylated SAP with avidity in a dose-dependent and saturable manner. By contrast, the binding of SAP to monocyte-derived dendritic cells was weak. No binding to erythrocytes, NK cells, T lymphocytes or B lymphocytes could be detected. The SAP-monocyte interaction was calcium-independent and readily inhibited by C1q. SAP was nonmitogenic for human mononuclear cells and had no apparent influence on lymphocyte proliferation induced by mitogenic lectins. We speculate that binding of SAP by monocytes could be of physiological relevance at extravascular sites by influencing complement regulation.

Mannan-binding lectin in children with chronic gastritis

The involvement of mannan-binding lectin (MBL) insufficiency in the pathogenesis of chronic gastritis (CG) in children was investigated. Blood samples were collected from 78 paediatric patients suffering from CG associated with Helicobacter pylori infection (group Hp(+)) and from 41 with the disease not associated with such an infection (group Hp(-)). Control group consisted of 77 children. The frequency of mbl-2 gene mutations and serum protein concentrations did not differ significantly in both groups as compared with controls. An expression of mbl-2 gene in gastric biopsies of CG patients was demonstrated. It was found to be stronger in H. pylori-infected children. The results presented in this paper suggest that MBL deficit/dysfunction probably does not contribute to an increased risk of CG (both associated and not associated with H. pylori infection) in children. However, MBL opsonic effect and/or the lectin pathway of complement activation may be taken into account as possible host defence mechanisms in gastric patients.

Mannose-binding lectin: biology and clinical implications

The innate host defence molecule mannose-binding lectin (MBL) has attracted great interest as a potential candidate for passive immunotherapy to prevent infection. MBL is a multimeric lectin that recognizes a wide array of pathogens independently of specific antibody, and initiates the lectin pathway of complement activation. The basic structural unit is a triple helix of MBL peptides, which aggregate into complement-fixing higher-order structures (tetramers, pentamers and hexamers). The gene encoding MBL, MBL2, contains several common polymorphisms that influence transcription and assembly of the molecule into multimers. MBL2 coding alleles associated with low blood levels are present in up to 40% of Caucasoids, with up to 8% having genotypes associated with profound reduction in circulating MBL levels. Low-producing MBL2 variants and low MBL levels are associated with increased susceptibility to and severity of a variety of infective illnesses, particularly when immunity is already compromised--for example, in infants and young children, patients with cystic fibrosis, and after chemotherapy and transplantation. These observations suggest that administration of recombinant or purified MBL may be of benefit in clinical settings where MBL deficiency is associated with a high burden of infection. This review provides a background to MBL biology and disease associations, and identifies the exciting therapeutic possibilities of MBL replacement.

Detection of an Autologous Ligand for Mannan-Binding Lectin on Human B Lymphocytes

Mannan-binding lectin (MBL) is a collectin and a major soluble pattern-recognition protein. MBL can distinguish self from nonself and altered self using its C-type carbohydrate recognition domain and may also interact via its collagen-like region with autologous cells. Recently, it was found that MBL could bind to adherent cells (monocytes) and dendritic cells in a specific and sugar-sensitive manner. We have now investigated the interaction of MBL with fresh human peripheral blood cells and report binding to B lymphocytes and natural killer cells. The binding to B lymphocytes was studied in detail and was compared with the binding of MBL to monocytes and dendritic cells. Binding of MBL to B cells was evident at physiological MBL and calcium concentrations but was optimal at supraphysiological MBL concentrations. It was readily inhibited by autologous serum, mannan, mannose, GlcNAc and (to a lesser extent) galactose but not by C1q. A similar, but not identical, inhibition profile was observed with dendritic cells, but monocytes were not sensitive to mannose or mannan. We conclude that MBL is capable of binding to differently glycosylated ligands on several autologous cell types via its carbohydrate-recognition domain. We speculate that this could have functional significance at extravascular sites, but perhaps only in individuals possessing MBL genotypes conferring MBL sufficiency.

Extra-hepatic transcription of the human mannose-binding lectin gene (mbl2) and the MBL-associated serine protease 1-3 genes

Mannose-binding lectin (MBL) is a part of the innate immune defense and activates complement via MBL associated serine proteases (MASPs). Human MBL is expressed by hepatocytes, but recent evidence in mice indicates a substantial extra-hepatic transcription. Therefore, we investigated whether mRNA transcribed from the human mbl2 gene as well as the masp genes was present in different tissues. The transcription of human mbl2 is regulated by two alternative promoters (named 0 and 1) where promoter 0 derived transcripts include an additional 5' untranslated part encoded by an extra exon (exon 0). Low extra-hepatic levels of mbl2 mRNA were predominantly found in small intestine and testis tissue, and were quantitatively dominated by promoter 1 transcripts. Moreover, these transcripts varied due to the use of alternative acceptor splice sites positioned inside exon 1. The mRNA distribution of masp1 and masp2 were found very similar to that of mbl2, while masp3 mRNA seemed ubiquitous present at quite high levels when compared to liver. These results indicate that the regulation of mbl2 gene expression in man is more complex than previously anticipated and that local expression of MBL may be relevant in local immune defense, particularly in restricted areas of the gastrointestinal and reproductive tracts.

Ultraviolet-B recruits mannose-binding lectin into skin from non-cutaneous sources

Mannose-binding lectin (MBL) is an integral part of the innate immune system and functions as an opsonin by binding to pathogens and certain apoptotic cells to promote their uptake by phagocytes. We recently identified an association of low-producing MBL polymorphisms with adult dermatomyositis (DM). Our model is that MBL deficiency leads to a defect in the clearance of apoptotic debris in the skin, thereby predisposing to photosensitive autoimmune disease. In this study, we sought to determine whether MBL binds within the epidermis, and to determine its source, and potential function of this binding. We demonstrated that the MBL is present in irradiated, but not in non-irradiated skin, and in irradiated skin it is bound to apoptotic keratinocytes (KC). We found that MBL is not made by KC, showing indirectly that it comes from an exogenous source, despite the fact that other complement components are made by KC and upregulated by ultraviolet irradiation. Finally, we demonstrated that non-KC-derived MBL bound to apoptotic KC in vitro and increased the uptake of these cells by dendritic cells. We hypothesize that MBL may facilitate non-inflammatory clearance of apoptotic debris in patients with photosensitive forms of DM.

Successful haemopoietic stem cell transplantation does not correct mannan-binding lectin deficiency

It has been reported that in allogeneic stem cell transplantation, the mannan-binding lectin (MBL) status of the donor has prognostic value for the recipient. Two MBL-deficient patients, with coexisting haematological malignancy, were identified who were treated with bone marrow from donors with normal MBL concentrations. Although both patients engrafted successfully and remain in complete remission, neither seroconverted to the MBL sufficiency status of his donor over a follow-up period exceeding 2 years. This does not support the concept of MBL replacement by stem cell therapy, and does not provide an explanation for high MBL concentrations in stem cell donors protecting recipients from post transplant infections.

The potential role of mannan-binding lectin in the clearance of self-components including immune complexes

Mannan-binding lectin (MBL) is a pattern recognition receptor in the innate immune system. It recognizes certain sugar residues arranged in a pattern that enables MBL to bind with sufficient strength. Such sugar patterns are common on the surface of many microorganisms, and MBL has therefore been considered to be an agent that can discriminate between self and nonself. There is, however, increasing evidence supporting that MBL, like many membrane-bound C-type lectin-like receptors, also helps to dispose of various outworn or abnormal body components. Most self-components are protected with sialic acid or galactose that disrupt the pattern of the sugars that MBL can bind, but MBL may be significantly involved in the elimination of self-components that have lost these protective terminal residues. The role of MBL in the clearance of invading pathogens has previously been thoroughly reviewed. Here, we review some findings that support the notion that MBL may contribute to noninflammatory removal of immune complexes and abnormal cells by the reticuloendothelial system. Defects in this clearance mechanism may cause an accumulation of potentially dangerous self-components, thereby increasing the likelihood of chronic inflammation and autoimmunity.

DC-SIGN-specific liposomal targeting and selective intracellular compound delivery to human myeloid dendritic cells: implications for HIV disease

Myeloid dendritic cells (MyDCs), prime stimulators of antigen-specific immunity, can serve as one of the major reservoirs for human immunodeficiency virus type-1 (HIV-1). Utilizing mature monocyte-derived MyDCs generated with granulocyte/macrophage colony-stimulating factor, interleukin-4, and tumour necrosis factor-alpha as an in vitro model, we here present the first proof of concept for liposomal compound delivery to these cells by specifically addressing CD209, i.e. DC-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN), a MyDC-associated C-type lectin implicated in the transmission of HIV-1 to T helper cells. By employing a liposomally entrapped tracer, calcein, we demonstrate by flow cytometry and mathematics a superior targeting efficacy for DC-SIGN, as compared with select other MyDC markers (CD1a, CD4, CD45R0, and CD83). Fluorescence microscopy reveals time-dependent surface binding and intracellular uptake of DC-SIGN-specific liposomes by both immature and mature MyDCs. This pilot study implies that liposomal targeting to CD209 and related C-type lectins may afford therapeutic intracellular drug delivery to MyDCs and other reservoir and nonreservoir cells susceptible to infection with HIV-1.