Mannan-binding protein and bovine conglutinin mediate enhancement of herpes simplex virus type 2 infection in mice

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.

Meta-analysis of functional MBL polymorphisms. Associations with rheumatoid arthritis and primary Sjögren's syndrome

OBJECTIVE

The aim of this study was to determine whether functional mannose-binding lectin gene (MBL) polymorphisms are associated with the susceptibility to rheumatoid arthritis (RA) or primary Sjögren's syndrome (pSS).

METHODS

A meta-analysis was conducted to investigate the potential association of RA or pSS with MBL polymorphisms, including the codon 54 (allele B), codon 57 (allele C), and codon 52 (allele D) variants of exon 1, and the - 550 (allele L) and - 221 (allele X) promoter variants.

RESULTS

A total of 12 comparative studies, including eight RA (1623 patients and 1671 controls) and four pSS (280 patients and 516 controls) studies, were included in the meta-analysis. The meta-analysis revealed no association between the MBL B allele and RA in the overall study population (odds ratio [OR] 0.991, 95 % confidence interval [CI] 0.726-1.355, p = 0.957). However, the meta-analysis showed significant associations between the MBL D, H, and X alleles and RA in the overall population (OR 1.708, 95 % CI 1.077-2.707, p = 0.023; OR 1.936, 95 % CI 1.218-3.078, p = 0.005; OR 1.582, 95 % CI 1.216-2.057, p = 0.001, respectively). An association was found between the MBL B allele and pSS in the overall study population (OR 0.691, 95 % CI 0.541-0.917, p = 0.010). Stratification by ethnicity indicated a trend toward an association between the B allele and pSS in European populations, but no association in Asian populations (OR 0.689, 95 % CI 0.465-1.021, p = 0.063; OR 0.896, 95 % CI 0.311-2.562, p = 0.838, respectively).

CONCLUSION

This meta-analysis demonstrated an association between the MBL D, L, and X alleles and the risk of RA. It also demonstrated an association between the MBL B allele and the susceptibility to pSS, suggesting a protective role of the MBL B allele against the development of pSS.

Limited effect of recombinant human mannose-binding lectin on the infection of novel influenza A (H7N9) virus in vitro

Mannose-binding lectin (MBL), a pattern-recognition molecule in serum, recognizes specific hexose sugars rich in mannose and N-acetylglucosamine on bacterium, yeasts, viruses as well as apoptotic cells. It has been well-identified that MBL has antiviral effects via binding to seasonal influenza H1 and H3 subtype viruses. Influenza A (H7N9) virus, a novel reassortant virus to human population, possesses the surface hemagglutinin (HA) and neuraminidase (NA) genes from duck and wild-bird influenza viruses and internal genes from poultry H9N2 viruses. As of Dec 7th, 2014, a total of 467 human infections and 183 fatal cases have been identified. Here, recombinant human (rh) MBL was tested for its binding and effects on hemagglutination inhibition (HI) and NA activity inhibition (NAI) of avian H7N9, H9N2 and human H3N2 viruses. We discovered that rhMBL exhibited a strong binding to H7N9 virus as human H3N2 did at high virus titers. However, it performed a significantly weaker HI activity effect on H7N9 comparing to those of H3N2 and H9N2, even at a much higher concentration (3.67 ± 0.33 vs. 0.026 ± 0.001 and 0.083 ± 0.02 μg/mL, respectively). Similarly, minor NAI effect of rhMBL, even at up to 10 μg/mL, was found on H7N9 virus while it displayed significant effects on both H3N2 and H9N2 at a lowest concentration of 0.0807 ± 0.009 and 0.0625 μg/mL, respectively. The HI and NAI effects of rhMBL were calcium-dependent and mediated by lectin domain. Our findings suggest that MBL, the host innate molecule, has differential interference effects with human and avian influenza virus and limited antiviral effect against H7N9 virus.

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Comparable binding of rhMBL to avian H7N9 virus as human H3N2 virus.

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Limited antiviral effect of rhMBL against avian H7N9 virus.

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Insensitivity of H7N9 virus to rhMBL may confer its infection severity.

Immunologic findings in young children with early onset of acute otitis media

CONCLUSION

No significant differences in the number of immune aberrations were seen between children with or without severe recurrent acute otitis media (rAOM); however, subnormal values of immunological markers were found more often than expected, and 4 of the 60 children had treatment-requiring immune deficiencies.

OBJECTIVE

Minor immunologic aberrations have been reported to be more frequent in children with rAOM. Immune investigation is recommended in children with severe rAOM, defined as six or more AOM episodes per year. The purpose of this study was to describe immunological findings in young children at high risk of developing rAOM, and to relate these to the number of expected aberrations and to the presence of severe rAOM.

METHODS

A total of 109 children at risk of developing rAOM were offered immune investigation including complement function, immunoglobulins with subclasses and cellular immunity.

RESULTS

Sixty patients were tested, 31 of whom had severe rAOM and 12 of whom did not develop rAOM. Low levels of IgG2 (27%), C1q (31%) and mannan-binding lectin (21%) were found up to eight times as often as expected. Although subnormal values were more frequent among children with severe rAOM, the study was too small to provide reliable evidence of any difference. Four children were diagnosed with immune deficiencies that required treatment.

Mannose-binding lectin genotypes and susceptibility to epstein-barr virus infection in infancy

In a cohort study of children < 4 years of age in Greenland, mannose-binding lectin (MBL2) genotypes and Epstein-Barr virus (EBV) antibody levels were determined. EBV seropositivity was significantly lower and time to seroconversion increased in MBL-insufficient compared with MBL-sufficient children, indicating that MBL may be involved in primary EBV infection in infancy.

Conglutinin - an Important Element of Natural Immunity in Ruminants (a Review)

Conglutinin represents a collectin appearing only in some mammals, including cattle. It plays an important role in resistance of hosts against microbes and parasites. It exhibits affinity toward saccharides present on the surface of bacteria, viruses, fungi and parasites and it mobilizes the mechanisms leading to their inactivation. It manifests also the unique ability of agglutinating complement- and antibody-coated erythrocytes. Moreover, it shows the ability to activate immune system cells, including the process of phagocytosis. In this review also the factors that determine the amount and activity of conglutinin in mammalian serum were presented, including environmental conditions and infectious agents, such as various types of bacteria and viruses. The broad scope of conglutinin is genetically controlled and, therefore, this collectin may constitute a useful and crucial factor in cattle husbandry.

Mannan-binding lectin deficiency modulates the humoral immune response dependent on the genetic environment

Mannan-binding lectin (MBL) is a plasma protein implicated in innate immune defence against a broad range of microorganisms, including viruses. It is also thought that MBL plays a role in the recruitment of the specific clonal immune response. This was studied by injecting soluble hepatitis B surface antigen (HBsAg) intravenously into mice deficient in both MBL-A and MBL-C (MBL DKO mice). The MBL DKO animals on mixed genetic background (SV129EvSv x C57BL/6) produced higher antibody titres than the wild-type littermates. After primary challenge with the antigen the immunoglobulin M anti-HBsAg antibody titres were threefold higher in the MBL DKO mice than in the wild-type mice. Following the boost, the immunoglobulin G anti-HBsAg antibody titres were 10-fold higher in the MBL DKO mice, suggesting that MBL plays a role in a negative feedback regulation of adaptive immunity. However, the modulating effect of MBL was dependent on the genetic environment. The MBL DKO mice backcrossed on a C57BL/6 background showed the opposite response with the MBL DKO mice now producing fewer antibodies than the wild-type animals, whereas MBL deficiency in mice with the SV129EvSv background did not show any effect in antibody production. These findings indicate that the modifying effect of MBL on the humoral immune response is influenced by the genetic environment.

Mannose-binding lectin 2 gene polymorphism in recurrent herpes simplex virus 2 infection

Mannose-binding lectin (MBL) is a complement component and an opsonic factor recognizing and binding to herpes simplex virus 2 (HSV-2). In this study, we assessed the effect of MBL2 genotypes on the risk of recurring HSV-2 infection. The MBL2 structural variant genotype (A/O or O/O) was more common among the patients with recurrent HSV-2 infection compared with healthy controls (47% vs 26%, respectively; odds ratio [OR] = 2.6, 95% confidence interval [CI] 1.3-4.9; p = 0.005) or controls positive for HSV-2 antibodies (15%; OR 4.9, 95% CI 1.5-16; p = 0.007). Thus, we conclude that the MBL2 structural variant genotype, possibly through impaired recognition of HSV-2, seems to predispose individuals to the development of recurring HSV-2 infection.

Impact of the complement lectin pathway on cytomegalovirus disease early after kidney transplantation

BACKGROUND

This study retrospectively investigated the association between pre-transplant levels of mannose-binding lectin (MBL) plus the associated serine protease (MASP)-2 and the occurrence of cytomegalovirus (CMV) infection and symptomatic CMV disease during the first 12 weeks after kidney transplantation. Materials and methods. Altogether 159 consecutive single kidney transplant recipients were included. The patients were screened for CMV pp65 antigenaemia every second week. No CMV prophylaxis or pre-emptive treatment was given. MBL and MASP-2 were measured in samples taken at transplantation and 10 weeks later.

RESULTS

CMV infection, defined as at least one positive test, was found in 95 patients (59.8%). MBL and MASP-2 measured at transplantation were similar in patients with and without CMV infection. The incidence of CMV infection was also similar in 36 patients (58.3%) with pre-transplant MBL levels below the reference level (500 microg/L) and in patients with higher MBL levels (60.2%). Symptomatic CMV disease was diagnosed in 35 patients (22%), and MASP-2 levels at transplantation in the lower quartile range (<or=148 microg/L) was significantly associated with CMV disease during the first 12 weeks, P = 0.028. MBL levels decreased significantly from transplantation to 10 weeks later, and median (interquartile range) fell from 2597 (526-4939) microug/L to 1520 (270-3069) microg/L (P < 0.001). In contrast, MASP-2 levels increased significantly from 252 (148-382) microg/L to 380 (302-492) microg/L (P < 0.001).

CONCLUSION

Pre-transplant MBL levels do not influence the incidence of any CMV infection or symptomatic CMV disease during the first 12 weeks after kidney transplantation. However, low MASP-2 levels may play a role in the development of symptomatic CMV disease.

Mannose-binding lectin in innate immunity: past, present and future

The human collectin, mannose‐binding lectin (MBL), is an important protein of the humoral innate immune system. With multiple carbohydrate‐recognition domains, it is able to bind to sugar groups displayed on the surfaces of a wide range of microorganisms and thereby provide first‐line defence. Importantly, it also activates the complement system through a distinctive third pathway, independent of both antibody and the C1 complex. Three single point mutations in exon 1 of the expressed human MBL‐2 gene appear to impair the generation of functional oligomers. Such deficiencies of functional protein are common in certain populations, e.g. in sub‐Saharan Africa, but virtually absent in others, e.g. indigenous Australians. MBL disease association studies have been a fruitful area of research and implicate a role for MBL in infective, inflammatory and autoimmune disease processes. Overall, there appears to be a genetic balance in which individuals generally benefit from high levels of the protein. However, in certain situations, reduced levels of circulating MBL may be beneficial to the host and this may explain the persistence of the deleterious gene polymorphisms in many population groups.

Mannan-binding lectin modulates the response to HSV-2 infection

Viruses have developed numerous strategies to escape recognition by the immune system. However, some viruses such as herpes simplex virus-2 (HSV-2) are recognized by initiators of the complement system, e.g. mannan-binding lectin (MBL). To study the effects of MBL deficiency during viral infection we have chosen a model of generalized HSV-2 infection. We infected MBL-A and MBL-C double knock-out mice (DKO) with HSV-2 via the intraperitoneal (i.p.) route. DKO mice cleared HSV-2 from the liver less efficiently than the comparable wild-type animals. The impairment to effectively neutralize HSV-2 correlated with compromised liver function as measured by increased plasma levels of alanine-amino transferase. No differences in the viral burden were found in other organs such as spleen or brain. Thus, MBL-mediated protection was limited to the effects of preservation of liver homeostasis. Reconstitution with recombinant human MBL before and during the HSV-2 infection dramatically lowered the viral titres in the liver. Taken together, the data show that MBL modulates the response to HSV-2 in mice by affecting neutralization of the virus. To analyse if MBL plays a role in establishment and progression of human HSV-2 infection we analysed MBL levels in the serum samples from asymptomatic (virus-exposed people who have never displayed symptoms of HSV-2 infection) and symptomatic HSV-2 patients (people with recurrent HSV-2 infections). We found that the frequency of the MBL deficiency (<100 ng/ml) was higher in the symptomatic group and significantly different from that in the asymptomatic group (P = 0.0369). This suggests that lack of MBL-mediated complement activation increases susceptibility to viral infection.

Collectins

Collectins are a family of collagenous calcium-dependent defense lectins in animals. Their polypeptide chains consist of four regions: a cysteine-rich N-terminal domain, a collagen-like region, an alpha-helical coiled-coil neck domain and a C-terminal lectin or carbohydrate-recognition domain. These polypeptide chains form trimers that may assemble into larger oligomers. The best studied family members are the mannan-binding lectin, which is secreted into the blood by the liver, and the surfactant proteins A and D, which are secreted into the pulmonary alveolar and airway lining fluid. The collectins represent an important group of pattern recognition molecules, which bind to oligosaccharide structures and/or lipid moities on the surface of microorganisms. They bind preferentially to monosaccharide units of the mannose type, which present two vicinal hydroxyl groups in an equatorial position. High-affinity interactions between collectins and microorganisms depend, on the one hand, on the high density of the carbohydrate ligands on the microbial surface, and on the other, on the degree of oligomerization of the collectin. Apart from binding to microorganisms, the collectins can interact with receptors on host cells. Binding of collectins to microorganisms may facilitate microbial clearance through aggregation, complement activation, opsonization and activation of phagocytosis, and inhibition of microbial growth. In addition, the collectins can modulate inflammatory and allergic responses, affect apoptotic cell clearance and modulate the adaptive immune system.

Mannan-binding lectin and its role in innate immunity

Mannan-binding lectin (MBL) is a plasma collectin (C-type lectin with a collagen-like domain) and is considered an important component of innate immunity. Circulating MBL is genetically determined for the major part, but plasma concentration is also markedly influenced by nongenetic factors. The carbohydrate-binding ability of MBL can be inhibited by simple sugars like mannose, fucose and N-acetylglucosamine, but its greatest avidity appears to be for repeating mannose-based structural patterns typical of microbial surfaces. By this means, MBL can bind to a wide variety of bacteria and other microbes, neutralizing them and/or opsonizing them by activating complement using the recently discovered lectin pathway of complement activation. Individual humans differ 1000-fold in MBL concentration, and individuals with low circulating MBL appear to be more vulnerable to infections in a number of clinical settings, especially when combined with secondary immune deficiency. The best evidence that MBL deficiency or insufficiency is physiologically relevant comes from a rapidly expanding literature of clinical studies. MBL insufficiency appears to be a significant risk factor for infections in infants, and for individuals of any age undergoing chemotherapy or post-transplant immunosuppression. Moreover, MBL appears to have a significant influence on the course of certain chronic diseases like rheumatoid arthritis and cystic fibrosis. Replacement therapy with a plasma-derived product is safe and seems promising, while recombinant MBL provides hope for large-scale therapeutic applications. Randomized clinical trials of MBL therapy, which are now on the horizon, should provide unambiguous evidence for the physiological significance of MBL in innate immunity.

Contributions of the N- and C-terminal domains of surfactant protein d to the binding, aggregation, and phagocytic uptake of bacteria

Collectins play important roles in host defense against infectious microorganisms. We now demonstrate that the serum collectins mannose-binding lectin (MBL) and conglutinin have less ability to bind to, aggregate, and enhance neutrophil uptake of several strains of gram-negative and gram-positive bacteria than pulmonary surfactant protein D (SP-D). Collectins are composed of four major structural domains (i.e., N-terminal, collagen, and neck and carbohydrate recognition domains). To determine which domains of SP-D are responsible for its greater bacterial binding or aggregating activity, activities of chimeric collectins containing the N-terminal and collagen domains of SP-D coupled to the neck recognition domains and carbohydrate recognition domains (CRD) of MBL or conglutinin (SP-D/Cong(neck+CRD) and SP-D/MBL(neck+CRD)) were tested. The SP-D/Cong(neck+CRD) and SP-D/MBL(neck+CRD) chimeras bound to and aggregated the bacteria more strongly than did wild-type MBL or conglutinin. SP-D/MBL(neck+CRD) also enhanced neutrophil uptake of bacteria more so than MBL. Hence, the SP-D N-terminal and/or collagen domains contribute to the enhanced bacterial binding and aggregating activities of SP-D. In prior studies, SP-D/Cong(neck+CRD) and SP-D/MBL(neck+CRD) had increased ability to bind to influenza virus compared not only with that of conglutinin or MBL but with that of wild-type SP-D as well. In contrast, the chimeras had either reduced or unchanged ability to bind to or aggregate bacteria compared to that of wild-type SP-D. Hence, although replacement of the neck recognition domains and CRDs of SP-D with those of MBL and conglutinin conferred increased viral binding activity, it did not favorably affect bacterial binding activity, suggesting that requirements for optimal collectin binding to influenza virus and bacteria differ.

Polymorphisms of the mannose binding lectin gene in patients with Sjögren's syndrome

OBJECTIVE

To investigate polymorphisms of both codon 54 allele and promoter variants of the mannose binding lectin (MBL) gene in patients with primary Sjögren's syndrome (SS).

METHODS

Polymorphisms of codon 54 allele and promoter variants of the MBL gene in 104 patients with SS and 143 healthy controls were determined by polymerase chain reaction-restriction fragment length polymorphism and allele specific polymerase chain reaction respectively.

RESULTS

The allele frequency of the wild type of MBL codon 54 was significantly higher in patients with SS than in controls (0.836 v 0.741; p=0.011), and the frequency of the homozygous wild type of MBL codon 54 was significantly higher in patients with SS than in controls (0.692 v 0.539; p=0.024). On the other hand, the allele frequencies of the MBL promoter gene did not differ between patients and controls (chi(2)=4.01, df=2, p=0.135).

CONCLUSION

The polymorphism of the MBL gene may be one of the genetic factors that determines susceptibility to SS.

Mannose-binding lectin promoter and structural gene variants in sarcoidosis

BACKGROUND

Sarcoidosis is a chronic granulomatous disease of unknown aetiology. Studies have suggested that the causative agent may be an infectious micro-organism. The mannose binding lectin (MBL) is involved in innate immunity to a wide range of micro-organisms. Mutations in the promoter region and exon 1 of the MBL gene occur with high frequency and are associated with reduced serum levels of MBL and increased susceptibility to microbial diseases. This study investigated whether MBL variants predispose to sarcoidosis by increasing their susceptibility to micro-organisms.

METHODS

MBL gene promoter and exon 1 variants were detected by sequence specific primer polymerase chain reaction (SSP-PCR) in 167 UK Caucasian sarcoidosis patients and 164 control subjects. Severity of pulmonary disease outcome among patients was assessed by radiography after a minimum of 4 years from disease onset and classified as mild, moderate, and severe disease categories, accordingly.

RESULTS

MBL variant frequencies were similar in patients and controls studied. Among sarcoidosis patients, the frequencies of variants were similar regardless of severity of disease outcome. The average patient ages at time of diagnosis were similar for all MBL genotypes.

CONCLUSIONS

MBL gene variants do not appear to influence susceptibility to sarcoidosis, age of disease onset, or severity of disease.

Lack of association between mannose-binding lectin, acute otitis media and early Epstein-Barr virus infection among children in Greenland

Low serum levels of mannose-binding lectin (MBL) have been associated with recurrent infections in early childhood. Otitis media (OM) is frequent in Greenlandic children and the first episode of acute OM (AOM) occurs early, as is the case also with Epstein-Barr virus (EBV) infection. We have therefore investigated the association between MBL genotypes, episodes of AOM, and early EBV infection in 82 community-based, unselected children in Greenland. Nasopharyngeal aspirations for EBV and MBL genotype examination, nasopharyngeal bacterial cultures, and history of AOM episodes were obtained. MBL genotypes were established in 73 specimens: 68% of these were homozygous for normal wildtype (AA), and 32% were homozygous or heterozygous for variant alleles that are associated with absence or low MBL serum level. The allele frequencies were: A = 0.88, B = 0.08 (codon 54) and D = 0.04 (codon 52). EBV was found in 41 specimens, more often with increasing age, and significantly related to ethnicity. Presence of variant MBL alleles or EBV infection was not associated with AOM, recurrent AOM (rAOM) or age at first AOM episode and EBV positive children with homozygosity for the normal MBL genotype did not have significantly more episodes of AOM, rAOM or earlier age at the first AOM episode. MBL genotypes and EBV infection alone or in interplay are not associated with the high prevalence of OM in Greenlandic children. The study suggests that low MBL level does not by itself predispose to AOM in community-based, unselected children.

C-type lectins and galectins mediate innate and adaptive immune functions: their roles in the complement activation pathway

In recent years, a 'new' pathway for complement activation mediated by the mannose-binding lectin (MBL) has been described as a key mechanism for the mammalian acute phase response to infection. This complement activation pathway is initiated by a non-self recognition step: the binding of a humoral C-type lectin [mannose-binding lectin (MBL)] to microbial surfaces bearing 'foreign' carbohydrate determinants. The recognition factor, MBL, is associated with a serine protease [MBL-associated serine protease (MASP)] which, upon MBL binding to the microbial ligand, activates the complement component C3, leading to either (a) phagocytosis of the opsonized target via the complement receptor, or (b) humoral cell killing via assembly of the membrane attack complex. Galectins (formerly known as S-type lectins) modulate activity of the complement receptor 3 (CR3), the macrophage membrane receptor for complement components C3b and iC3b, downstream products of the MBL pathway which are covalently bound to 'target cells. Galectins also mediate macrophage- and dendrocyte-adhesion to lymphocytes activated by signaling through another C-type lectin, the L-selectin, leading to immunoglobulin-mediated responses. Thus, the functional interplay of MBL, galectins and L-selectin in the acute phase response neutralizes the microbial challenge, and lead to further adaptive immunity. Although the observation of various components of the lectin pathway in different invertebrate species demonstrates the high conservation and ancient roots of the components of innate immunity, there has previously been no evidence supporting the possibility that the integral lectin-mediated complement activation pathway is present in invertebrates. We now have evidence for the coexistence of homologs of all the pathway's key components (MBL, MASP, C3, and galectin) in the protochordate Clavelina picta, suggesting the lectin-mediated pathway of complement activation preceded the immunoglobulin pathway in evolution. Therefore, despite being 'new' to the textbooks, experimental evidence indicates that this pathway is ancient, and has been conserved intact throughout its evolution.

Reconstitution of opsonizing activity by infusion of mannan-binding lectin (MBL) to MBL-deficient humans

Mannan-binding lectin (MBL, previously named mannan-binding protein, MBP) is a serum collectin, which activates complement upon binding to microbial carbohydrates. This results in opsonization of the microorganisms as well as direct complement-mediated killing. Clinical evidence indicates that MBL has an important role in the innate immune defence against various pathogens. Genetically determined MBL deficiency is associated with increased susceptibility to infections. We have infused two MBL-deficient individuals with clinical grade MBL, purified from pooled donor plasma, in doses sufficient to attain normal concentration of MBL in serum. This resulted in normalization of complement-mediated opsonization. An initial rapid decrease in the serum concentration of MBL was followed by a slower decline with an estimated half-life of about 6 days. No adverse effects were observed and anti-MBL antibodies could not be detected following several MBL infusions. One of the two MBL recipient, a two-year-old girl, who had been suffering from repeated infections from the age of 4 months, was given a total of six MBL infusions. She has subsequently remained healthy for more than three years.

Immunohistochemical investigation of the tissue distribution of mannan-binding lectin in non-infected and virus-infected chickens

This paper describes the results of immuno-histochemical staining for chicken mannan-binding lectin (MBL) in formalin-fixed tissue sections from non-infected chickens, and from chickens infected with infectious laryngotracheitis virus (ILTV) or infectious bursal disease virus (IBDV). In the non-infected chickens, MBL was detected in the cytoplasm of a few hepatocytes and in the germinal centres of the caecal tonsils, whereas sections of kidney, heart muscle, spleen, cerebrum, thymus, adrenal gland, bursa of Fabricius, bone marrow and trachea were without staining. In the ILTV-infected chickens, an intense staining reaction for MBL was detected in the cytoplasm of all hepatocytes and on the surface of, and inside, ILTV-infected cells. Also in the IBDV-infected chickens, an intense staining reaction for MBL was detected in the cytoplasm of all hepatocytes. No staining was seen in the follicles of the bursa of Fabricius, but MBL was present in non-identified cells in the interstitium, and in the cytoplasm of macrophage-like cells, located peripheral to the ellipsoid of the spleen. These findings indicate the liver as the primary site of MBL synthesis, and points to up-regulation as a result of the viral infections. The location outside the liver could indicate a role of MBL in the immune defence.

Mechanisms of anti-influenza activity of surfactant proteins A and D: comparison with serum collectins

The present study provides the first direct comparison of anti-influenza A virus (IAV) activities of the collectins surfactant protein (SP) A and SP-D, mannose-binding lectin (MBL), and conglutinin. SP-D, MBL, and conglutinin inhibited IAV hemagglutination activity with a greater potency than and by a distinct mechanism from SP-A. Although isolated trimeric SP-D carbohydrate recognition domains inhibited hemagglutination activity, preparations of SP-D also containing the collagen domain and NH2 terminus caused greater inhibition. In contrast to SP-A (or nonmultimerized SP-D), absence of the N-linked attachment did not effect interactions of multimerized SP-D with IAV. SP-D, SP-A, and conglutinin caused viral precipitation through formation of massive viral aggregates, whereas MBL formed aggregates of smaller size that did not precipitate. All of the collectins enhanced IAV binding to neutrophils; however, in the case of MBL, this effect was modest compared with the binding enhancement induced by SP-D or conglutinin. These studies clarify the structural requirements for viral inhibition by SP-D and reveal significant differences in the mechanisms of anti-IAV activity among the collectins.

Collectin-mediated antiviral host defense of the lung: evidence from influenza virus infection of mice

Collagenous lectins (collectins) present in mammalian serum and pulmonary fluids bind to influenza virus and display antiviral activity in vitro, but their role in vivo has yet to be determined. We have used early and late isolates of H3N2 subtype influenza viruses that differ in their degree of glycosylation to examine the relationship between sensitivity to murine serum and pulmonary lectins in vitro and the ability of a virus to replicate in the respiratory tract of mice. A marked inverse correlation was found between these two parameters. Early H3 isolates (1968 to 1972) bear 7 potential glycosylation sites on hemagglutinin (HA), whereas later strains carry 9 or 10. Late isolates were shown to be much more sensitive than early strains to neutralization by the mouse serum mannose-binding lectin (MBL) and rat lung surfactant protein D (SP-D) and bound greater levels of these lectins in enzyme-linked immunosorbent assays and Western blot analyses. They also replicated very poorly in mouse lungs compared to the earlier strains. Growth in the lungs was greatly enhanced, however, if saccharide inhibitors of the collectins were included in the virus inoculum. The level of SP-D in bronchoalveolar lavage fluids increased on influenza virus infection. MBL was absent from lavage fluids of normal mice but could be detected in fluids from mice 3 days after infection with the virulent strain A/PR/8/34 (H1N1). The results implicate SP-D and possibly MBL as important components of the innate defense of the respiratory tract against influenza virus and indicate that the degree or pattern of glycosylation of a virus can be an important factor in its virulence.

Animal lectins

Protein and lipid glycosylation is no longer considered as a topic whose appeal is restricted to a limited number of analytical experts perseveringly pursuing the comprehensive cataloguing of structural variants. It is in fact arousing curiosity in various areas of basic and applied bioscience. Well founded by the conspicuous coding potential of the sugar part of cellular glycoconjugates which surpasses the storage capacity of oligonucleotide- or oligopeptide-based code systems, recognition of distinct oligosaccharide ligands by endogenous receptors, i.e. lectins and sugar-binding enzymes or antibodies, is increasingly being discovered to play salient roles in animal physiology. Having inevitably started with a descriptive stage, research on animal lectins has now undubitably reached maturity. Besides listing the current categories for lectin classification and providing presentations of the individual families and their presently delineated physiological significance, this review places special emphasis on tracing common structural and functional themes which appear to reverberate in nominally separated lectin and animal categories as well as lines of research which may come to fruition for medical sciences.

The collectins in innate immunity

The collectins are proteins with collagen tails and globular lectin domains that appear to play an important role in mammalian first line host defense. Recent insights have clarified the structural basis of ligand recognition, the interactions of collectins with complement cascades, and the association with disease susceptibility.