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

Increased expression of ficolin-1 is associated with airway obstruction in asthma

BACKGROUND

The activated complement cascade is involved in asthmatic airway inflammation. Ficolins are essential for innate immunity and can activate the complement lectin pathway. Despite this, the significance of ficolins in asthma has yet to be determined. This study aimed to explore the presence of ficolins in individuals with asthma and to determine the relationship between ficolins and clinical characteristics.

METHODS

For the study, 68 asthmatic patients and 30 healthy control subjects were recruited. Enzyme-linked immunosorbent assay was used to determine plasma ficolin-1, ficolin-2, and ficolin-3 concentrations both before and after inhaled corticosteroid (ICS) therapy. Further, the associations of plasma ficolin-1 level with pulmonary function and asthma control questionnaire (ACQ) score were examined in the asthma patients.

RESULTS

Patients with asthma exhibited significantly elevated plasma ficolin-1 levels (median, 493.9 ng/mL; IQR, 330.2-717.8 ng/mL) in comparison to healthy controls (median, 330.6 ng/mL; IQR, 233.8-371.1 ng/mL). After ICS treatment, plasma ficolin-1 (median, 518.1 ng/mL; IQR, 330.2-727.0 ng/mL) in asthmatic patients was significantly reduced (median, 374.7 ng/mL; IQR, 254.8-562.5 ng/mL). Additionally, ficolin-1 expressions in plasma were significantly correlated with pulmonary function parameters and ACQ score in asthmatic patients. Asthma patients with higher plasma ficolin-1 levels demonstrated poorer lung function than those with lower plasma ficolin-1 levels.

CONCLUSIONS

The results revealed that asthmatic patients had higher plasma ficolin-1 concentrations, which decreased after ICS treatment and were linked to their lung function, implying a potential involvement of ficolin-1 in asthma pathogenesis.

The Fungal Microbiome and Asthma

Asthma is a group of inflammatory conditions that compromises the airways of a continuously increasing number of people around the globe. Its complex etiology comprises both genetic and environmental aspects, with the intestinal and lung microbiomes emerging as newly implicated factors that can drive and aggravate asthma. Longitudinal infant cohort studies combined with mechanistic studies in animal models have identified microbial signatures causally associated with subsequent asthma risk. The recent inclusion of fungi in human microbiome surveys has revealed that microbiome signatures associated with asthma risk are not limited to bacteria, and that fungi are also implicated in asthma development in susceptible individuals. In this review, we examine the unique properties of human-associated and environmental fungi, which confer them the ability to influence immune development and allergic responses. The important contribution of fungi to asthma development and exacerbations prompts for their inclusion in current and future asthma studies in humans and animal models.

Clinical Relevance of Plasma Concentrations of MBL in Accordance with IgE Levels in Children Diagnosed with Bronchial Asthma

Background and objectives: Bronchial asthma is a heterogeneous, multifactorial pulmonary disease characterized by variable airway obstruction caused by chronic inflammation. Our study investigates the clinical relevance of MBL plasma levels in accordance with IgE values in children who attended a pediatric consult for respiratory symptoms with bronchial asthma. Materials and Methods: The study population consists of patients <18-years-old and included 43 patients with bronchial asthma and 64 age-matched healthy subjects as a control group. We used the ELISA Human MBL Immunoassay kit and the electrochemiluminescence immunoassay (ECLIA) kit for IgE determination. Results: Our results show significantly different distributions of patients in the bronchial asthma group and control group. The measured values were within the normal range for most controls, while the bronchial asthma patients displayed higher values of plasma MBL and IgE levels. We observed a wider heterogeneity in MBL concentrations in bronchial asthma patients when compared to the healthy age-matched controls. Our results also suggest a potential clinical usefulness of plasma MBL concentrations in accordance with IgE and eosinophil cells levels in the diagnosis of bronchial asthma, and our results may suggest a prognostic role of MBL in the evolution of asthmatic disease; however, further studies are necessary to confirm these findings. Conclusions: We can say that plasma MBL concentrations present a relative diagnostic role for bronchial asthma in pediatric patients and may suggest a more severe disease progression; however, further studies are needed to elucidate the role played by MBL in the determination and evolution of this disease.

Soluble pattern recognition molecules: Guardians and regulators of homeostasis at airway mucosal surfaces

Maintenance of homeostasis at body barriers that are constantly challenged by microbes, toxins and potentially bioactive (macro)molecules requires complex, highly orchestrated mechanisms of protection. Recent discoveries in respiratory research have shed light on the unprecedented role of airway epithelial cells (AEC), which, besides immune cells homing to the lung, also significantly contribute to host defence by expressing membrane‐bound and soluble pattern recognition receptors (sPRR). Recent evidence suggests that distinct, evolutionary ancient, sPRR secreted by AEC might become activated by usually innocuous proteins, commonly referred to as allergens. We here provide a systematic overview on sPRR detectable in the mucus lining of AEC. Some of them become actively produced and secreted by AECs (like the pentraxins C‐reactive protein and pentraxin 3; the collectins mannose binding protein and surfactant proteins A and D; H‐ficolin; serum amyloid A; and the complement components C3 and C5). Others are elaborated by innate and adaptive immune cells such as monocytes/macrophages and T cells (like the pentraxins C‐reactive protein and pentraxin 3; L‐ficolin; serum amyloid A; and the complement components C3 and C5). Herein we discuss how sPRRs may contribute to homeostasis but sometimes also to overt disease (e.g. airway hyperreactivity and asthma) at the alveolar–air interface.

Collectins: Innate Immune Pattern Recognition Molecules

Collectins are collagen-containing C-type (calcium-dependent) lectins which are important pathogen pattern recognising innate immune molecules. Their primary structure is characterised by an N-terminal, triple-helical collagenous region made up of Gly-X-Y repeats, an a-helical coiled-coil trimerising neck region, and a C-terminal C-type lectin or carbohydrate recognition domain (CRD). Further oligomerisation of this primary structure can give rise to more complex and multimeric structures that can be seen under electron microscope. Collectins can be found in serum as well as in a range of tissues at the mucosal surfaces. Mannanbinding lectin can activate the complement system while other members of the collectin family are extremely versatile in recognising a diverse range of pathogens via their CRDs and bring about effector functions designed at the clearance of invading pathogens. These mechanisms include opsonisation, enhancement of phagocytosis, triggering superoxidative burst and nitric oxide production. Collectins can also potentiate the adaptive immune response via antigen presenting cells such as macrophages and dendritic cells through modulation of cytokines and chemokines, thus they can act as a link between innate and adaptive immunity. This chapter describes the structure-function relationships of collectins, their diverse functions, and their interaction with viruses, bacteria, fungi and parasites.

The complement system in Aspergillus fumigatus infections and its crosstalk with pentraxins

Aspergillosis is a life-threatening infection mostly affecting immunocompromised individuals and primarily caused by the saprophytic fungus Aspergillus fumigatus. At the host-pathogen interface, both cellular and humoral components of the innate immune system are increasingly acknowledged as essential players in the recognition and disposal of this opportunistic mold. Fundamental hereof is the contribution of the complement system, which deploys all three activation pathways in the battle against A. fumigatus, and functionally cooperates with other soluble pattern recognition molecules, including pentraxins. In particular, preclinical and clinical observations point to the long pentraxin PTX3 as a nonredundant and complement-dependent effector with protective functions against A. fumigatus. Based on past and current literature, here we discuss how the complement participates in the immune response to this fungal pathogen, and illustrate its crosstalk with the pentraxins, with a focus on PTX3. Emphasis is placed on the molecular mechanisms underlying such processes, the genetic evidence from human epidemiology, and the translational potential of the currently available knowledge.

C-type Lectins in Immunity to Lung Pathogens

The respiratory tract is tasked with responding to a constant and vast influx of foreign agents. It acts as an important first line of defense in the innate immune system and as such plays a crucial role in preventing the entry of invading pathogens. While physical barriers like the mucociliary escalator exert their effects through the clearance of these pathogens, diverse and dynamic cellular mechanisms exist for the activation of the innate immune response through the recognition of pathogen-associated molecular patterns (PAMPs). These PAMPs are recognized by pattern recognition receptors (PRRs) that are expressed on a number of myeloid cells such as dendritic cells, macrophages, and neutrophils found in the respiratory tract. C-type lectin receptors (CLRs) are PRRs that play a pivotal role in the innate immune response and its regulation to a variety of respiratory pathogens such as viruses, bacteria, and fungi. This chapter will describe the function of both activating and inhibiting myeloid CLRs in the recognition of a number of important respiratory pathogens as well as the signaling events initiated by these receptors.

The Fungal Microbiome and Asthma

Asthma is a group of inflammatory conditions that compromises the airways of a continuously increasing number of people around the globe. Its complex etiology comprises both genetic and environmental aspects, with the intestinal and lung microbiomes emerging as newly implicated factors that can drive and aggravate asthma. Longitudinal infant cohort studies combined with mechanistic studies in animal models have identified microbial signatures causally associated with subsequent asthma risk. The recent inclusion of fungi in human microbiome surveys has revealed that microbiome signatures associated with asthma risk are not limited to bacteria, and that fungi are also implicated in asthma development in susceptible individuals. In this review, we examine the unique properties of human-associated and environmental fungi, which confer them the ability to influence immune development and allergic responses. The important contribution of fungi to asthma development and exacerbations prompts for their inclusion in current and future asthma studies in humans and animal models.

The role of Aspergillus fumigatus polysaccharides in host-pathogen interactions

Aspergillus fumigatus is a saprophytic mold that can cause infection in patients with impaired immunity or chronic lung diseases. The polysaccharide-rich cell wall of this fungus is a key point of contact with the host immune system. The availability of purified cell wall polysaccharides and mutant strains deficient in the production of these glycans has revealed that these glycans play an important role in the pathogenesis of A. fumigatus infections. Herein, we review our current understanding of the key polysaccharides present within the A. fumigatus cell wall, and their interactions with host cells and secreted factors during infection.

Immune Recognition of Fungal Polysaccharides

The incidence of fungal infections has dramatically increased in recent years, in large part due to increased use of immunosuppressive medications, as well as aggressive medical and surgical interventions that compromise natural skin and mucosal barriers. There are relatively few currently licensed antifungal drugs, and rising resistance to these agents has led to interest in the development of novel preventative and therapeutic strategies targeting these devastating infections. One approach to combat fungal infections is to augment the host immune response towards these organisms. The polysaccharide-rich cell wall is the initial point of contact between fungi and the host immune system, and therefore, represents an important target for immunotherapeutic approaches. This review highlights the advances made in our understanding of the mechanisms by which the immune system recognizes and interacts with exopolysaccharides produced by four of the most common fungal pathogens: Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, and Histoplasma capsulatum. Work to date suggests that inner cell wall polysaccharides that play an important structural role are the most conserved across diverse members of the fungal kingdom, and elicit the strongest innate immune responses. The immune system senses these carbohydrates through receptors, such as lectins and complement proteins. In contrast, a greater diversity of polysaccharides is found within the outer cell walls of pathogenic fungi. These glycans play an important role in immune evasion, and can even induce anti-inflammatory host responses. Further study of the complex interactions between the host immune system and the fungal polysaccharides will be necessary to develop more effective therapeutic strategies, as well as to explore the use of immunosuppressive polysaccharides as therapeutic agents to modulate inflammation.

Aspergillus in chronic lung disease: Modeling what goes on in the airways

Aspergillus species cause a range of respiratory diseases in humans. While immunocompromised patients are at risk for the development of invasive infection with these opportunistic molds, patients with underlying pulmonary disease can develop chronic airway infection with Aspergillus species. These conditions span a range of inflammatory and allergic diseases including Aspergillus bronchitis, allergic bronchopulmonary aspergillosis, and severe asthma with fungal sensitization. Animal models are invaluable tools for the study of the molecular mechanism underlying the colonization of airways by Aspergillus and the host response to these non-invasive infections. In this review we summarize the state-of-the-art with respect to the available animal models of noninvasive and allergic Aspergillus airway disease; the key findings of host-pathogen interaction studies using these models; and the limitations and future directions that should guide the development and use of models for the study of these important pulmonary conditions.

DAP12 Inhibits Pulmonary Immune Responses to Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that is inhaled into the lungs and can lead to life-threatening meningoencephalitis in immunocompromised patients. Currently, the molecular mechanisms that regulate the mammalian immune response to respiratory cryptococcal challenge remain poorly defined. DAP12, a signaling adapter for multiple pattern recognition receptors in myeloid and natural killer (NK) cells, has been shown to play both activating and inhibitory roles during lung infections by different bacteria and fungi. In this study, we demonstrate that DAP12 plays an important inhibitory role in the immune response to C. neoformans Infectious outcomes in DAP12(-/-) mice, including survival and lung fungal burden, are significantly improved compared to those in C57BL/6 wild-type (WT) mice. We find that eosinophils and macrophages are decreased while NK cells are increased in the lungs of infected DAP12(-/-) mice. In contrast to WT NK cells, DAP12(-/-) NK cells are able to repress C. neoformans growth in vitro Additionally, DAP12(-/-) macrophages are more highly activated than WT macrophages, with increased production of tumor necrosis factor (TNF) and CCL5/RANTES and more efficient uptake and killing of C. neoformans These findings suggest that DAP12 acts as a brake on the pulmonary immune response to C. neoformans by promoting pulmonary eosinophilia and by inhibiting the activation and antifungal activities of effector cells, including NK cells and macrophages.

Effects of MBL2 polymorphisms in patients with diisocyanate-induced occupational asthma

Diisocyanate (DI) is the most common cause of occupational asthma (OA) in Korea. Mannose-binding lectin (MBL) initiates the lectin complement activation pathway following oxidative stress and plays an important role in the regulation of inflammatory processes. To determine whether there is a genetic association between MBL2 polymorphisms and DI-OA, 99 patients with DI-OA, 99 asymptomatic exposed controls (AECs) and 144 unexposed normal controls were enrolled in this study. Three polymorphisms (-554 G>C, -431A>C and -225 G>C) in the MBL2 promoter were genotyped, and serum MBL levels were determined by enzyme-linked immunosorbent assay. Functional variabilities in the promoter polymorphisms were analyzed by a luciferase reporter assay and electrophoretic mobility shift assay (EMSA). A significantly higher frequency of haplotype (ht) 2 [CAG] was noted in the DI-OA group compared with the AEC group (P=0.044). The patients with DI-OA carrying ht2 [CAG] had significantly lower PC20 methacholine levels (P<0.001) than the non-carriers. The serum MBL levels were significantly higher in the DI-exposed subjects (both the DI-OA patients and AECs) carrying ht1 [GAG] (P=0.028). Luciferase activity was significantly enhanced in ht1 [GAG] compared with ht2 [CAG] in human hepatocarcinoma cells (Hep3B) (P=0.002). The EMSA showed that a -554G probe produced a specific shifted band compared with the -554C probe. These findings suggest that decreased serum MBL levels due to polymorphisms of the MBL2 gene may increase susceptibility to the development of DI-OA in DI-exposed individuals.

Mannose-binding lectin 2 gene polymorphism and lung damage in primary ciliary dyskinesia

BACKGROUND

Mannose-binding lectin (MBL) plays an important role in innate immunity and has been reported to be associated with the age-related decline in lung function in cystic fibrosis.

HYPOTHESIS

MBL polymorphisms are associated with lung function decline in Primary Ciliary Dyskinesia (PCD).

METHODS

We performed sputum microbiology, spirometry pre- and post-administration of salbutamol, ciliary motion analysis, ultrastructural assessment of cilia, ciliogenesis in culture, and chest high resolution computed tomography in children with a clinical history of respiratory tract infections and/or presence of bronchiectasis suggestive of PCD or secondary ciliary dyskinesia (SCD). All subjects were evaluated for single nucleotide polymorphisms in the gene encoding MBL-2.

RESULTS

The diagnosis of PCD was established in 45 subjects, while in the remaining 53 the diagnosis was SCD. A significant bronchodilator response was observed only in PCD associated with the MBL2-3 genotype, which is known to be associated with low/undetectable MBL serum levels. Also, bronchiectasis severity was significantly greater in subjects with MBL2-3 in both PCD and SCD. No other association was found between MBL genotypes and clinical findings.

CONCLUSIONS

MBL plays a relatively minor role as a disease modifier in PCD. A similar finding in SCD supports the likely significance of this result.

Recalcitrant rhinosinusitis, innate immunity, and mannose-binding lectin

BACKGROUND

Mannose-binding lectin (MBL) is a protein produced by the liver that participates in innate immunity by tagging the surface of microbes for opsonization. Mannose-binding lectin deficiency is present in 7% of the population and has been implicated in recurrent respiratory tract infections in children. Mannose-binding lectin deficiency has not been explored in rhinosinusitis but is associated with increased mortality in adult pneumococcal infection. The purpose of this report is to describe a tertiary rhinology patient experience with MBL deficiency and recalcitrant rhinosinusitis.

METHODS

This retrospective case series report characterizes predominantly adult patients with low MBL levels from January 2010 to June 2012. Indications for MBL testing, sinus culture data, immunological testing results, and treatments used to control rhinosinusitis are described.

RESULTS

Mannose-binding lectin levels were deficient in 12 of 36 patients (33.3%) tested. IgG subclasses were abnormally low in 5 of 12 patients; IgA was normal in 11 of 12 patients; and IgM was normal in 11 of 12 patients. Staphylococcus aureus, coagulase-negative Staphylococcus species, and Pseudomonas aeruginosa, known to be "tagged" by MBL, were the most common organisms grown on culture. Treatments included culture directed systemic antimicrobial therapy and topical steroids/antibiotics.

CONCLUSION

Mannose-binding lectin, an important component of the lectin complement pathway and innate immunity, is possibly associated with recalcitrant adult rhinosinusitis. Steroid/antibiotic irrigations appear to benefit patients with recalcitrant rhinosinusitis and possibly those with MBL deficiency. Given that the prevalence of MBL deficiency in this case series is 4 times that seen in the normal population, additional investigations are warranted to further elucidate the role of MBL deficiency in rhinosinusitis.

Association of MBL2 polymorphism with asthma after bronchiolitis in infancy

BACKGROUND

Mannose-binding lectin (MBL) is a component of innate immunity and has been linked with the pathogenesis of asthma. The aim of the present study was to evaluate the association of MBL genotypes with preschool asthma and allergy in children with bronchiolitis in early infancy.

METHODS

In all, 205 infants were hospitalized for bronchiolitis at <6 months of age. Asthma and allergy were studied from a total of 166 children at 6.4 years (mean). A total of 141 (85%) frozen whole blood samples were available for MBL genotyping and MBL2 gene mutations were determined on pyrosequencing for detection of three single-nucleotide polymorphisms.

RESULTS

Ninety-five children (67.4%) had the wild-type MBL genotype A/A and 46 had A/O or O/O genotypes. Asthma was present in 16 children (11.3%) at 5-7 years of age. Nine children (19.6%) with non-AA genotype had asthma (vs 7.4% of those with genotype AA, P= 0.03). The result remained significant after adjustment for age, gender and atopy. There were no significant associations between MBL genotypes and asthma at any age before the study. Atopic dermatitis, allergic rhinitis or paternal and/or maternal asthma had no significant associations with MBL genotypes.

CONCLUSIONS

The variant non-A/A MBL genotype is associated with asthma after bronchiolitis in infancy, but not earlier than at 5-7 years of age.

Childhood exposure to secondhand smoke and functional mannose binding lectin polymorphisms are associated with increased lung cancer risk

BACKGROUND

Exposure to secondhand smoke during adulthood has detrimental health effects, including increased lung cancer risk. Compared with adults, children may be more susceptible to secondhand smoke. This susceptibility may be exacerbated by alterations in inherited genetic variants of innate immunity genes. We hypothesized a positive association between childhood secondhand smoke exposure and lung cancer risk that would be modified by genetic polymorphisms in the mannose binding lectin-2 (MBL2) gene resulting in well-known functional changes in innate immunity.

METHODS

Childhood secondhand smoke exposure and lung cancer risk was assessed among men and women in the ongoing National Cancer Institute-Maryland Lung Cancer (NCI-MD) study, which included 624 cases and 348 controls. Secondhand smoke history was collected via in-person interviews. DNA was used for genotyping the MBL2 gene. To replicate, we used an independent case-control study from Mayo Clinic consisting of 461 never smokers, made up of 172 cases and 289 controls. All statistical tests were two-sided.

RESULTS

In the NCI-MD study, secondhand smoke exposure during childhood was associated with increased lung cancer risk among never smokers [odds ratio (OR), 2.25; 95% confidence interval (95% CI), 1.04-4.90]. This was confirmed in the Mayo study (OR, 1.47; 95% CI, 1.00-2.15). A functional MBL2 haplotype associated with high circulating levels of MBL and increased MBL2 activity was associated with increased lung cancer risk among those exposed to childhood secondhand smoke in both the NCI-MD and Mayo studies (OR, 2.52; 95% CI, 1.13-5.60, and OR, 2.78; 95% CI, 1.18-3.85, respectively).

CONCLUSIONS

Secondhand smoke exposure during childhood is associated with increased lung cancer risk among never smokers, particularly among those possessing a haplotype corresponding to a known overactive complement pathway of the innate immune system.

Innate immunity to Aspergillus species

All humans are continuously exposed to inhaled Aspergillus conidia, yet healthy hosts clear the organism without developing disease and without the development of antibody- or cell-mediated acquired immunity to this organism. This suggests that for most healthy humans, innate immunity is sufficient to clear the organism. A failure of these defenses results in a uniquely diverse set of illnesses caused by Aspergillus species, which includes diseases caused by the colonization of the respiratory tract, invasive infection, and hypersensitivity. A key concept in immune responses to Aspergillus species is that the susceptibilities of the host determine the morphological form, antigenic structure, and physical location of the fungus. In this review, we summarize the current literature on the multiple layers of innate defenses against Aspergillus species that dictate the outcome of this host-microbe interaction.

Resistance of MBL gene-knockout mice to experimental systemic aspergillosis

Mannose binding lectin (MBL) is a protein of the collectin family that appears important in resistance to invasive pulmonary aspergillosis. We assessed the role of MBL in experimental systemic aspergillosis. MBL-sufficient C57BL/6 (WT) mice and B6.129S4--Mb11(tm1Kata) Mb12(tm1Kata)/J MBL A and C gene-knockout (KO) mice were infected intravenously with different inocula of Aspergillus fumigatus conidia. WT and KO mice were dose-responsively susceptible. In no instance were the KO mice more susceptible than WT. At the highest inoculum, all WT and 90% of KO mice died on day 4 (P>0.05). Reduction of the inoculum to 5.5 x 10(6) conidia was lethal, but comparison showed KO mice less susceptible to lethal infection (P<0.015). At the lowest inoculum used, deaths of KO mice were delayed, but survival was not significantly different than WT (P>0.05). These results suggest MBL may play a deleterious role in systemic aspergillosis.

Altered eosinophil profile in mice with ST6Gal-1 deficiency: an additional role for ST6Gal-1 generated by the P1 promoter in regulating allergic inflammation

Cumulative evidence indicates that the sialyltransferase ST6Gal-1 and the sialyl-glycans, which it constructs, are functionally pleiotropic. Expression of the ST6Gal-1 gene is mediated by six distinct promoter/regulatory regions, and we hypothesized that these promoters may be used differentially to produce ST6Gal-1 for different biologic purposes. To examine this hypothesis, we compared a mouse with a complete deficiency in ST6Gal-1 (Siat1 null) with another mouse that we have created previously with a disruption only in the P1 promoter (Siat1DeltaP1). We noted previously greater neutrophilic inflammation associated with ST6Gal-1 deficiency. Here, we report that ST6Gal-1-deficient mice also have significantly elevated eosinophilic responses. Upon i.p. thioglycollate elicitation, eosinophils accounted for over 20% of the total peritoneal inflammatory cell pool in ST6Gal-1-deficient animals, which was threefold greater than in corresponding wild-type animals. A principal feature of allergic respiratory inflammation is pulmonary eosinophilia, we evaluated the role of ST6Gal-1 in allergic lung inflammation. Using OVA and ABPA experimental models of allergic airways, we showed that ST6Gal-1 deficiency led to greater airway inflammation characterized by excessive airway eosinophilia. The severity of airway inflammation was similar between Siat1DeltaP1 and Siat1 null mice, indicating a role for P1-generated ST6Gal-1 in regulating eosinophilic inflammation. Colony-forming assays suggested greater IL-5-dependent eosinophil progenitor numbers in the marrow of ST6Gal-1-deficient animals. Moreover, allergen provocation of wild-type mice led to a significant reduction in P1-mediated ST6Gal-1 mRNA and accompanied decline in circulatory ST6Gal-1 levels. Taken together, the data implicate ST6Gal-1 as a participant in regulating not only Th1 but also Th2 responses, and ST6Gal-1 deficiency can lead to the development of more severe allergic inflammation with excessive eosinophil production.

Monocyte exposure to Saccharomyces cerevisiae cell wall mannan and decreased TNF-alpha production in mild asthma: a role for mannan-binding lectin?

A correlation between fungal exposure and aggravation of inflammatory symptoms in asthmatic individuals is well documented in the literature. However, the molecular mediators responsible for clinical symptoms due to fungal exposure in individuals with asthma are still not known. The fungal cell wall polysaccharide mannan stimulates production of pro-inflammatory cytokines such as tumour necrosis factor (TNF)-alpha in monocytes. Recently, a role for the plasma protein mannan-binding lectin (MBL) has been proposed in individuals with severe asthmatic disease, although little is known about its role in those with mild and untreated asthma. MBL has been reported to modulate inflammatory cytokine production, but the mechanisms are not known. We conducted a pilot study and found that the cell wall mannan preparation used stimulated lower TNF-alpha production by monocytes from asthmatic subjects compared with that from healthy subjects in the presence of autologous plasma. Lipopolysaccharide-induced TNF-alpha production was not significantly different between the groups. Further, plasma MBL levels in individuals with mild asthma were slightly increased compared with those in normal subjects, although the difference was not statistically significant. We speculate that reduced TNF-alpha production in monocytes from asthmatic subjects after fungal cell wall mannan stimulation could partly be influenced by plasma components such as MBL.

Complement and fungal pathogens: an update

Fungal infections are a serious complication in immunocompromised patients such as human immunodeficiency virus-infected individuals, patients with organ transplantations or with haematological neoplasia. The lethality of opportunistic fungal infection is high despite a growing arsenal of antimycotic drugs, implying the urgent need for supportive immunological therapies to strengthen the current inefficient antimicrobial defences of the immunocompromised host. Therefore, increasing effort has been directed to investigating the interplay between fungi and the host immunity and thus to find starting points for additional therapeutic approaches. In this article, we review the actual state of the art concerning the role of complement in the pathogenesis of fungal infections. Important aspects include the activation of the complement system by the fungal pathogen, the efficiency of the complement-associated antimicrobial functions and the arsenal of immune evasion strategies applied by the fungi. The twin functions of complement as an interactive player of the innate immunity and at the same time as a modulator of the adaptive immunity make this defence weapon a particularly interesting therapeutic candidate to mobilise a more effective immune response and to strengthen in one fell swoop a broad spectrum of different immune reactions. However, we also mention the 'Yin-Yang' nature of the complement system in fungal infections, as growing evidence assigns to complement a contributory part in the pathogenesis of fungus-induced allergic manifestations.

Host-microbe interactions: innate pattern recognition of fungal pathogens

The recognition of fungi is mediated by germline pattern recognition receptors (PRRs) such as Toll-like receptors and lectin receptors that interact with conserved structures of the microorganisms, the pathogen-associated molecular patterns (PAMPs). Subsequently, PRRs activate intracellular signals that collaborate for the efficient activation of the host defense. The specificity of these responses is achieved through the activation of a particular mosaic of PRRs, that is determined by the available fungal PAMPs and the innate immune cells involved. This will determine a divergence of the final type of reaction, and in this way the innate host defense has the capability to deliver tailored responses to each pathogen.

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.

Mannose binding lectin gene polymorphisms and asthma

BACKGROUND

Bronchial asthma is a chronic inflammatory disorder of the airways. Recently, it has been suggested that complement plays significant roles in asthma. Mannose-binding lectin (MBL) is one of the key molecules in complement activation pathways that are associated with several infectious and immune disorders.

SUBJECTS AND METHOD

To investigate whether MBL plays roles in asthma, we analysed MBL2 polymorphisms (allele B, H/L and Y/X) and plasma MBL levels in a Japanese adult population including 232 healthy controls and 579 asthmatics.

RESULTS

Although there was linkage disequilibrium among the three polymorphisms, each polymorphism significantly affects serum MBL levels independently. However, there were no significant differences between asthmatics and controls in MBL2 genotype distribution and in MBL concentrations [1.47+/-0.07(SE) mg/L for asthmatics and 1.66+/-0.14 mg/L for controls, P=0.2]. MBL levels and genotype have no significant relationship with serum IgE, pulmonary functions, and the severity of asthma.

CONCLUSION

Although plasma MBL levels depend on the MBL2 polymorphisms, these polymorphisms and plasma MBL levels are not associated with the asthma phenotype.

Single nucleotide polymorphisms in mannan-binding lectins and ficolins in various strains of mice

Mannan-binding lectin (MBL) and ficolin are collagenous lectins produced primarily by the liver and are involved in innate resistance to microbial pathogens. Mice have two MBL genes (Mbl1 and Mbl2) that encode MBL-A and MBL-C, respectively. Similarly, the murine Fcna and Fcnb genes encode ficolin-A and ficolin-B. Several single nucleotide polymorphisms (SNP) in the human MBL2 gene are responsible for various innate immune dysfunctions due to abnormal structure or expression of human MBL-C. In these studies, we identified SNPs in the expressed collagenous lectin genes Mbl1, Mbl2, Fcna, and Fcnb in 10 strains of mice designated high priority Group A strains by the Mouse Phenome Project (129S1/SvImJ, A/J, BALB/cByJ, C3H/HeJ, C57BL/6 J, DBA/2 J, FVB/NJ, SJL/J, CAST/EiJ and SPRET/EiJ) by sequencing gene exons by reverse transcription-polymerase chain reaction (RT-PCR). Sequence comparisons identified a total of 15 structural SNPs in Mbl1 in two strains, 27 SNPs in Mbl2 in five strains, and 19 and 15 SNPs in Fcna and Fcnb, respectively, in two strains. Two non-synonymous SNPs were identified in the collagen-like domain of mouse Fcnb that are similar to the coding polymorphisms in the collagen-like domain of human MBL2. Most of the non-synonymous SNPs identified in Mbl1 and Mbl2 occurred in the carbohydrate-recognition domains (CRDs), and some resulted in altered residues close to known ligand binding sites. Similarly, most non-synonymous SNPs of Fcna and Fcnb were identified in the fibrinogen-like CRD. The miscoding SNPs found in the CRD regions of mouse Mbl1, Mbl2, Fcna and Fcnb may be associated with strain differences in glycan binding avidity and disposition of microbial or host ligands. Furthermore, the non-synonymous mutations in the collagen-like domain of Fcnb may alter the structure of the mature ficolin-B protein leading to functional deficiencies. These differences may be important in the pathogenesis of susceptibility differences between inbred strains to various infectious microorganisms.

Mannose-binding lectin A-deficient mice have abrogated antigen-specific IgM responses and increased susceptibility to a nematode infection

To investigate the role of mannose-binding lectin-A (MBL-A) in protection against infectious disease, MBL-A(-/-)-deficient mice were generated. Using a well-characterized mouse model of human filarial nematode infection, nematode survival and protective immune responses were tested in vivo. Blood-borne Brugia malayi microfilariae survived for significantly longer time periods in MBL-A(-/-) than in wild-type (WT) mice. However, no differences in either splenic cytokine responses or induction of leukocytes in the blood were observed. A profound abrogation of Ag-specific IgM levels was measured in B. malayi-infected MBL-A(-/-) mice, and some IgG isotypes were higher than those observed in WT animals. To establish whether there was a defect in Ab responses per se in MBL-A(-/-) mice or the effect was specific to filarial infection, we immunized these mice with OVA or a carbohydrate-free protein. Significantly, Ag-specific IgM responses were defective to both of these Ags, and Ag-specific IgG responses were largely unaffected. Furthermore, in naive mice, total IgM levels did not differ between MBL-A(-/-) and WT mice. This article describes the first demonstration that MBL-A may function independently of MBL-C and suggests that MBL-A, like other C-type lectins and members of the complement cascade, is intimately involved in the priming of the humoral Ab response.

Protective role of mannan-binding lectin in a murine model of invasive pulmonary aspergillosis

Innate immune molecules such as lung collectins and serum pentraxins have evolved as important host defence proteins against Aspergillus fumigatus, a medically important opportunistic fungal pathogen. Mannan-binding lectin (MBL), an opsonin and lectin complement pathway activator, constitutes another vital player of innate immunity against several pathogenic organisms in the serum. Studies have reported significant binding of MBL to A. fumigatus; however, the protective role of MBL against A. fumigatus-mediated invasive disease remains elusive. Henceforth, we investigated the contribution of externally administered recombinant human (rh) MBL towards anti-fungal defence in invasive pulmonary aspergillosis (IPA) by in vivo and in vitro studies. In murine models of IPA with corticosteroid-induced immunosuppression, rhMBL-treated mice showed 80% survival compared to untreated IPA mice with no survivors. Treated IPA mice also showed a marked increase in tumour necrosis factor (TNF)-alpha and interleukin (IL)-1alpha and a significant decrease in pulmonary fungal hyphae and IL-10. In vitro, rhMBL-bound A. fumigatus conidia showed a dose-dependent increase in the deposition of C4b, the first product of the lectin pathway. There was an enhanced uptake of A. fumigatus conidia by the polymorphonuclear cells (PMNs) in the presence of rhMBL that increased further in the presence of MBL supplemented with MBL-deficient serum. However, an increase in the oxidative burst of PMNs and A. fumigatus killing were observed only when MBL was supplemented with MBL-deficient serum. The study suggests a therapeutic role of ex vivo-administered MBL in host defence against aspergillosis, possibly through MBL-mediated complement activation and other protective mechanisms aimed both directly at the pathogen, and indirectly through modulation of the host inflammatory responses.

Mannan-binding lectin in asthma and allergy

Mannan-binding lectin (MBL) is a vital and versatile component of innate immunity. It is present in serum and may bind to a plethora of microbial pathogens and mediate opsonization of these by complement-dependent and/or independent mechanisms. Low-MBL levels in serum, attributed to certain genetic polymorphisms, constitute a major factor predisposing to several infectious diseases. However, recent studies propose that MBL extends beyond its classic role as a first-line host-defense molecule to a modulator of inflammation. In this review, we summarize and explore this potential and a possible novel role of MBL in asthma and allergy.

Immunity to fungi

Innate and adaptive immune responses target pathogenic fungi and provide defense against fungal infections. Recent studies demonstrate that specific host receptors recognize ligands that are unique to fungi and activate signaling cascades that lead to phagocytosis of fungi, generation of pro-inflammatory mediators, formation of reactive oxygen species, trafficking of inflammatory cells to sites of infection, and initiation of adaptive immune responses. Greater understanding of the molecular mechanisms that underlie antifungal defense has provided a framework for the investigation of protective vaccines and strategies for therapeutic adoptive cell transfer.

Innate immunity and the lung: defense at the interface between host and environment

The lung serves as a major interface between the host and the external environment. As such, numerous lines of defense protect the host from inhaled potential pathogens. A breach in pulmonary innate immunity can lead to deleterious outcomes, such as pneumonia and disseminated infection. Pulmonary innate immunity, the first line of defense, is mediated by airway and alveolar epithelial cells as well as resident and recruited leukocytes. This article will discuss the key cellular and secreted components of the pulmonary innate immune system.

Elevated levels of mannan-binding lectin [corrected] (MBL) and eosinophilia in patients of bronchial asthma with allergic rhinitis and allergic bronchopulmonary aspergillosis associate with a novel intronic polymorphism in MBL

Mannan-binding lectin (MBL), an important component of innate immunity, binds to a range of foreign antigens and initiates the lectin complement pathway. Earlier studies have reported high plasma MBL levels in allergic patients in comparison to healthy controls. In view of varied plasma MBL levels being determined by genetic polymorphisms in its collagen region, we investigated the association of single nucleotide polymorphisms (SNPs) in the collagen region of human MBL with respiratory allergic diseases. The study groups comprised patients of bronchial asthma with allergic rhinitis (n = 49) and allergic bronchopulmonary aspergillosis (APBA) (n = 11) and unrelated age-matched healthy controls of Indian origin (n = 84). A novel intronic SNP, G1011A of MBL, showed a significant association with both the patient groups in comparison to the controls (P < 0.01). Patients homozygous for the 1011A allele showed significantly higher plasma MBL levels and activity than those homozygous for the 1011G allele (P < 0.05). The 1011A allele also showed a significant correlation with high peripheral blood eosinophilia (P < 0.05) and low forced expiratory volume in 1 s (FEV(1)) (P < 0.05) of the patients. We conclude that the 1011A allele of MBL may contribute to elevated plasma MBL levels and activity and to increased severity of the disease markers in patients of bronchial asthma with allergic rhinitis and ABPA.

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.

Dectin-1: a signalling non-TLR pattern-recognition receptor

Dectin-1 is a natural killer (NK)-cell-receptor-like C-type lectin that is thought to be involved in innate immune responses to fungal pathogens. This transmembrane signalling receptor mediates various cellular functions, from fungal binding, uptake and killing, to inducing the production of cytokines and chemokines. These activities could influence the resultant immune response and can, in certain circumstances, lead to autoimmunity and disease. As I discuss here, understanding the molecular mechanisms behind these functions has revealed new concepts, including collaborative signalling with the Toll-like receptors (TLRs) and the use of spleen tyrosine kinase (SYK), that have implications for the role of other non-TLR pattern-recognition receptors in immunity.

Mannose-binding lectin 2 (MBL2) gene polymorphism in asthma and atopy among adults

Mannose-binding lectin (MBL) insufficiency due to polymorphisms in the MBL2 gene causes an opsonization defect, which has been connected to infections and atopy. We investigated the significance of MBL2 genotypes with regard to persistent asthma and atopy among adults. The genotypes were determined in 243 adults with persistent asthma and 400 controls. Atopy was determined by skin-prick test. As a result, the carriage of -221 base pairs (bp) promoter region variant allele X (nucleotide change G-->C; alleles Y-->X, respectively) causing low MBL expression proved to be a significant risk factor for asthma in non-atopic males [odds ratio (OR) = 2.52, 95% confidence interval (CI) = 1.23-5.15; P = 0.01]. Furthermore, the X-allele carriage was associated with the decrease in lung function (forced expiratory volume at 1 s, FEV(1)) during follow-up in the patients with asthma (P = 0.033), the effect being strongest for non-atopic asthmatics (P = 0.042). The MBL2 genotype had no clear effect on the occurrence of atopy in adults. In conclusion, our results abrogate the previously suggested predisposing effect of MBL insufficiency on atopy at least in adults. However, as MBL is a complement component participating in immune defence against microbes, and as in the pathogenesis of non-atopic asthma infectious agents are probably involved, the gene-environment interactions between MBL and infections should be assessed further with regard to asthma.

Modulatory effect of mannose-binding lectin on cytokine responses: possible roles in HIV infection

BACKGROUND

Mannose-binding lectin (MBL) is a soluble receptor of the innate immune system, probably contributing to antimicrobial defence. The possible role of MBL in HIV infection is unclear.

MATERIALS AND METHODS

Peripheral blood mononuclear cells (PBMCs) from 28 HIV-infected patients and 13 healthy controls were stimulated with MBL and costimulated with HIV-1 gp120 or mannan from Saccharomyces cerevisiae before inflammatory responses in PBMC cultures were examined by enzyme-linked immunosorbent assay (ELISA) and flow cytometry. HIV-1 RNA replication in vitro was assessed by quantitative reverse transcription polymerase chain reaction (RT-PCR) in supernatants from patients with measurable HIV-1 RNA levels.

RESULTS

(i) Enhanced TNF-alpha responses were observed when PBMCs from healthy controls and HIV-infected patients were stimulated with MBL and costimulated with HIV-1 gp120 or mannan. (ii) MBL stimulation induced increased HIV RNA replication in culture supernatants when costimulated with mannan.

CONCLUSIONS

The present study suggests a modulatory role of MBL on cytokine responses, and HIV replication after stimulation with microbial products. These effects of MBL on inflammatory responses and viral replication may be clinically relevant for HIV infection.

Role and regulation of lung collectins in allergic airway sensitization

Inhalation of allergens in atopic patients results in a characteristic inflammatory response while in normal, healthy individuals it elicits no symptoms. The mechanisms by which the pulmonary immune system accomplishes elimination of inhaled particles and suppression of the ensuing inflammatory response are poorly understood. Based on their structural uniqueness, specific localization and functional versatility the hydrophilic surfactant proteins [surfactant protein (SP)-A and SP-D] are important candidate regulators of these processes. Recent studies in our laboratory and others indicated significant changes in levels of these molecules during the asthmatic response in animal models as well as in asthmatic patients. Because of their capability to directly inhibit T-cell activation and T-cell-dependent allergic inflammatory events, SP-A and SP-D may be significant contributors to the local control of T-helper (Th)2-type inflammation in the airways. This review will discuss their relevant structural-functional features and recent evidence supporting the hypothesis that SP-A and SP-D have a role in regulation of allergic airway sensitization.

Mannose-binding lectin: laying the stepping stones from clinical research to personalized medicine

As a key component of the complement system, mannose-binding lectin (MBL) is one of the linchpins of innate immunity. It is, therefore, not surprising that MBL2 genetic variants affecting the quantity and activity of the MBL protein in serum have been associated with increased susceptibility to infection and autoimmune diseases, and with poorer prognostic outcomes. This enhanced risk is particularly the case for children and immunosuppressed patients, especially when immunity is further compromised by coexistent primary or secondary immune deficiencies. In several disease areas, such as sepsis, cystic fibrosis, and recurrent childhood infections, the association between low MBL-producing allelic variants and disease risk and/or severity is particularly strong. It is here that the use of MBL testing and replacement therapy has reached the threshold of personalized medicine. The role of MBL in health and disease, advances in MBL testing methodologies and key areas for possible applications of MBL replacement therapy are reviewed.