The Mycobacterium tuberculosis cell-surface glycoprotein apa as a potential adhesin to colonize target cells via the innate immune system pulmonary C-type lectin surfactant protein A

Tuberculosis is still a major health problem, and understanding the mechanism by which Mycobacterium tuberculosis (Mtb) invades and colonizes its host target cells remains an important issue for the control of infection. The innate immune system C-type lectins (C-TLs), including the human pulmonary surfactant protein A (PSP-A), have been recently identified as determinant players in the early recognition of the invading pathogen and in mounting the host defense response. Although the antigenic lipoglycan mannosylated lipoarabinomannan is currently considered to be the major C-TL target on the mycobacterial surface, the recognition by some C-TLs of the only mycobacterial species composing the "Mtb complex" indicates that mannosylated lipoarabinomannan cannot account alone for this specificity. Thus, we searched for the mycobacterial molecules targeted by human PSP-A, focusing our attention on the Mtb surface glycoproteins. We developed an original functional proteomic approach based on a lectin blot assay using crude human bronchoalveolar lavage fluid as a source of physiological PSP-A. Combined with selective cell-surface protein extraction and mass spectrometry peptide mapping, this strategy allowed us to identify the Apa (alanine- and proline-rich antigenic) glycoprotein as new potential target for PSP-A. This result was supported by direct binding of PSP-A to purified Apa. Moreover, EDTA addition or deglycosylation of purified Apa samples completely abolished the interaction, demonstrating that the interaction is calcium- and mannose-dependent, as expected. Finally, we provide convincing evidence that Apa, formerly considered as mainly secreted, is associated with the cell wall for a sufficiently long time to aid in the attachment of PSP-A. Because, to date, Apa seems to be restricted to the Mtb complex strains, we propose that it may account for the selective recognition of those strains by PSP-A and other immune system C-TLs containing homologous functional domains.     

Dissecting the pathophysiologic role of endogenous lectins: glycan-binding proteins with cytokine-like activity?

Several families of endogenous glycan-binding proteins have been implicated in a wide variety of immunological functions including first-line defence against pathogens, cell trafficking, and immune regulation. These include, among others, the C-type lectins (collectins, selectins, mannose receptor, and others), S-type lectins (galectins), I-type lectins (siglecs and others), P-type lectins (phosphomannosyl receptors), pentraxins, and tachylectins. This review will concentrate on the immunoregulatory roles of galectins (particularly galectin-1) and collectins (mannose-binding lectins and surfactant proteins) to illustrate the ability of endogenous glycan-binding proteins to act as cytokines, chemokines or growth factors, and thereby modulating innate and adaptive immune responses under physiological or pathological conditions. Understanding the pathophysiologic relevance of endogenous lectins in vivo will reveal novel targets for immunointervention during chronic infection, autoimmunity, transplantation and cancer.     

Animal lectins: a historical introduction and overview

Some proteins we now regard as animal lectins were discovered before plant lectins, though many were not recognised as carbohydrate-binding proteins for many years after first being reported. As recently as 1988, most animal lectins were thought to belong to one of two primary structural families, the C-type and S-type (presently known as galectins) lectins. However, it is now clear that animal lectin activity is found in association with an astonishing diversity of primary structures. At least 12 structural families are known to exist, while many other lectins have structures apparently unique amongst carbohydrate-binding proteins, although some of those "orphans" belong to recognised protein families that are otherwise not associated with sugar recognition. Furthermore, many animal lectins also bind structures other than carbohydrates via protein-protein, protein-lipid or protein-nucleic acid interactions. While animal lectins undoubtedly fulfil a variety of functions, many could be considered in general terms to be recognition molecules within the immune system. More specifically, lectins have been implicated in direct first-line defence against pathogens, cell trafficking, immune regulation and prevention of autoimmunity.     

The multiple facets of HIV attachment to dendritic cell lectins

Entry of enveloped viruses into host cells depends on the interactions of viral surface proteins with cell surface receptors. Many enveloped viruses maximize the efficiency of receptor engagement by first binding to attachment‐promoting factors, which concentrate virions on target cells and thus increase the likelihood of subsequent receptor engagement. Cellular lectins can recognize glycans on viral surface proteins and mediate viral uptake into immune cells for subsequent antigen presentation. Paradoxically, many viral and non‐viral pathogens target lectins to attach to immune cells and to subvert cellular functions to promote their spread. Thus, it has been proposed that attachment of HIV to the dendritic cell lectin DC‐SIGN enables the virus to hijack cellular transport processes to ensure its transmission to adjacent T cells. However, recent studies show that the consequences of viral capture by immune cell lectins can be diverse, and can entail negative and positive regulation of viral spread. Here, we will describe key concepts proposed for the role of lectins in HIV attachment to host cells, and we will discuss recent findings in this rapidly evolving area of research.     

Toshiaki Osawa: biochemistry of lectins and their applications in immunochemistry and cellular biology

Lectins are proteins that agglutinate cells and exhibit an antibody like, sugar-binding specificity. Professor Toshiaki Osawa has discovered, purified and characterized many plant lectins that display diverse biological activities. Using lectins as biochemical tools, he developed methods to determine the biochemical structures of glycoprotein glycans that react with lectins; separated and characterized glycoproteins and cell populations; analysed the mechanisms by which lectins activate cells; and characterized several cytokines produced by immune cells stimulated by lectins. The studies on lectins, the field he took strong leadership, developed into an essential hub of the biology of multicellular organisms.     

A novel C-type lectin secreted by a tissue-dwelling parasitic nematode.

Many parasitic nematodes live for surprisingly long periods in the tissues of their hosts, implying sophisticated mechanisms for evading the host immune system. The nematode Toxocara canis survives for years in mammalian tissues, and when cultivated in vitro, secretes antigens such as TES-32. From the peptide sequence, we cloned TES-32 cDNA, which encodes a 219 amino-acid protein that has a domain characteristic of host calcium-dependent (C-type) lectins, a family of proteins associated with immune defence. Homology modelling predicted that TES-32 bears remarkable structural similarity to mammalian immune-system lectins. Native TES-32 acted as a functional lectin in affinity chromatography. Unusually, it bound both mannose- and galactose-type monosaccharides, a pattern precluded in mammalian lectins by a constraining loop adjacent to the carbohydrate-binding site. In TES-32, this loop appeared to be less obtrusive, permitting a broader range of ligand binding. The similarity of TES-32 to host immune cell receptors suggests a hitherto unsuspected strategy for parasite immune evasion.     

C型凝集素

C型凝集素（C-type lectin）代表一个识别碳水化合物配体依赖于钙离子（Ca2＋）参与的糖原结合蛋白家族,含有一个或多个一级结构和二级结构同源的碳水化合物识别结构域。随着研究的深入,越来越多的C型凝集素能够识别体内的非糖类的配体,包括蛋白质和脂类等。这些C型凝集素在维持机体稳态、免疫防御以及免疫监视等重要生理病理过程中发挥着重要作用。就C型凝集素的结构、分类和在免疫系统中的功能作一介绍。     

糖基化与免疫

蛋白质糖基化或聚糖影响免疫细胞和免疫分子的结构与功能,影响机体对抗原的应答反应。聚糖主要有三种免疫功能：首先,糖链对其所连接的糖蛋白起一定稳定作用,保护糖蛋白免受蛋白酶的降解、以及MHC：多肽复合体的装配及折叠等;其二,聚糖及其凝集素受体的相互作用在信号转导、抗原提呈、控制细胞发育与分化中起调控作用;第三,糖链的一些区域可作为抗原识别表位,调控固有免疫和适应性免疫应答。主要介绍了聚糖在抗原提呈和稳定、信号转导、免疫白稳、自身免疫、固有免疫和适应性免疫、等中的功能,以及与免疫相关疾病,如炎症反应、自身免疫性疾病、肿瘤、器官移植排斥、感染性疾病和遗传性疾病的相关性。此外,针对聚糖和糖基转移酶的一些药物分子的治疗应用前景也将一并进行介绍。     

SPR studies of carbohydrate-lectin interactions as useful tool for screening on lectin sources

Lectins are proteins or glycoproteins from plants, animals or microorganisms, which typically bind specifically to sugar residues, e.g., located in cell walls or membranes. This reaction might change the physiology of the cell wall and influences the metabolism inside the cell. Some lectins of plants stimulate the immune system by unspecific activation of T-cells or influence cell division; others cause agglutination of cells (e.g., erythrocytes) and are therefore from therapeutic interest.

In a new approach, biomolecular interaction analysis (BIA) was utilised for a screening program on lectins. The BIA has been done by surface plasmon resonance (SPR). The system can be used either for characterisation of lectin-binding domains or for a screening on lectins from natural sources. Several lectin-binding surfaces on the basis of SPR have been established.     

Innovative analytical system for screening on lectins

Lectins are proteins or glycoproteins from plants, animals or microorganisms, which typically bind specifically to sugar residues, e.g., those located in cell walls or membranes. This reaction might change the physiology of the cell wall and influences the metabolism inside the cell. Some lectins of plants stimulate the immune system by unspecific activation of T-cells or influence cell division; others cause agglutination of cells (e.g., erythrocytes) and are therefore from therapeutic interest. In a new approach, biomolecular interaction analysis (BIA) was utilized for a screening program on lectins. The BIA has been done by a new interferometric biosensor based on spectral-phase interference (SPI). The system can be used either for characterisation of lectin binding domains or for a screening on lectins obtained from natural sources. Several lectin binding surfaces on the basis of SPI have been established.     

N-糖基化在肿瘤细胞逃逸CD8+T细胞中的意义和机制

肿瘤在发生发展过程中会产生异常糖基化结构,改变细胞功能,使其能逃逸机体细胞的免疫识别和免疫攻击,实现免疫逃逸。细胞膜表面蛋白的N-糖基化参与调控肿瘤细胞的增殖、迁移、侵袭、干细胞特性等。近年来,一系列研究进一步证实,肿瘤细胞免疫检查点分子的N-糖基化,以及识别N-糖基化的动物凝集素在肿瘤免疫逃逸CD8⁺T细胞中发挥了重要作用,如PD-L1的N-糖基化能增强其自身蛋白质的稳定性,促进与其受体PD-1的相互作用,并且靶向PD-1、PD-L1蛋白N-糖基化的抗体,能够有效抑制肿瘤免疫逃逸和肿瘤发展。     

Could plant lectins become promising anti‐tumour drugs for causing autophagic cell death?

Plant lectins, a group of highly diverse carbohydrate‐binding proteins of non‐immune origin, are ubiquitously distributed through a variety of plant species, and have recently drawn rising attention due to their remarkable ability to kill tumour cells using mechanisms implicated in autophagy. In this review, we provide a brief outline of structures of some representative plant lectins such as concanavalin A, Polygonatum cyrtonema lectin and mistletoe lectins. These can target autophagy by modulating BNIP‐3, ROS‐p38‐p53, Ras‐Raf and PI3KCI‐Akt pathways, as well as Beclin‐1, in many types of cancer cells. In addition, we further discuss how plant lectins are able to kill cancer cells by modulating autophagic death, for therapeutic purposes. Together, these findings provide a comprehensive perspective concerning plant lectins as promising new anti‐tumour drugs, with respect to autophagic cell death in future cancer therapeutics.     

How C-type lectins detect pathogens

Glycosylation of proteins has proven extremely important in a variety of cellular processes, including enzyme trafficking, tissue homing and immune functions. In the past decade, increasing interest in carbohydrate-mediated mechanisms has led to the identification of novel carbohydrate-recognizing receptors expressed on cells of the immune system. These non-enzymatic lectins contain one or more carbohydrate recognition domains (CRDs) that determine their specificity. In addition to their cell adhesion functions, lectins now also appear to play a major role in pathogen recognition. Depending on their structure and mode of action, lectins are subdivided in several groups. In this review, we focus on the calcium (Ca(2+))-dependent lectin group, known as C-type lectins, with the dendritic cell-specific ICAM-3 grabbing non-integrin (DC-SIGN) as a prototype type II C-type lectin organized in microdomains, and their role as pathogen recognition receptors in sensing microbes. Moreover, the cross-talk of C-type lectins with other receptors, such as Toll-like receptors, will be discussed, highlighting the emerging model that microbial recognition is based on a complex network of interacting receptors.     

Pattern recognition proteins in Manduca sexta plasma

Recognition of nonself is the first step in mounting immune responses. In the innate immune systems of both vertebrates and arthropods, such recognition, termed pattern recognition, is mediated by a group of proteins, known as pattern recognition proteins or receptors. Different pattern recognition proteins recognize and bind to molecules (molecular patterns) present on the surface of microorganisms but absent from animals. These molecular patterns include microbial cell wall components such as bacterial lipopolysaccharide, lipoteichoic acid and peptidoglycan, and fungal beta-1,3-glucans. Binding of pattern recognition proteins to these molecular patterns triggers responses such as phagocytosis, nodule formation, encapsulation, activation of proteinase cascades, and synthesis of antimicrobial peptides. In this article, we describe four classes of pattern recognition proteins, hemolin, peptidoglycan recognition protein, beta-1,3-glucan recognition proteins, and immulectins (C-type lectins) involved in immune responses of the tobacco hornworm, Manduca sexta.     

Evolution of the lectin-complement pathway and its role in innate immunity

Discrimination between self and non-self by lectins (carbohydrate-binding proteins) is a strategy of innate immunity that is found in both vertebrates and invertebrates. In vertebrates, immune recognition mediated by ficolins (lectins that consist of a fibrinogen-like and a collagen-like domain), as well as by mannose-binding lectins, triggers the activation of the complement system, which results in the activation of novel serine proteases. The presence of a similar lectin-based complement system in ascidians, our closest invertebrate relatives, indicates that the complement system probably had a pivotal role in innate immunity before the evolution of an adaptive immune system in jawed vertebrates.     

Glycosylation and lectins-examples of immunesurveillance and immune evasion

Cell surface proteins are posttranslationally modified by tightly regulated enzymes of glycosylation. Typical patterns of glycosylation may signal pathological situations to the immune system. Here, carbohydrate receptors on the surface of cells in the immune system are involved in regulation of effector cells. Moreover, some lectins are circulating in the plasma and take part in host defense. The code of carbohydrate modifications is impaired in malignant cells and yet they are not eliminated. In this review, we focus on recent experimental evidence for regulatory functions of lectins and carbohydrate derivatives in the immune system and tumours.     

From Tumor Necrosis Factor Receptor to RANK, from the Selectins and Link Proteins to CD44: New Molecular Models of Cell Surface Receptors and Analysis of Specificity Determinants

Modeling and structure-function studies on two cell surface proteins are presented, which are implicated in the regulation of immune responses and cell adhesion. In the first part, model building of RANK, a new member of the tumor necrosis factor receptor (TNFR) superfamily (TNFRSF), is reported. The model is analyzed in light of structural studies on the TNFR-ligand complex and molecular model-based mutagenesis analyses of CD40-ligand and Fas-ligand interactions. The study makes it possible to predict residues important for ligand binding to RANK and further rationalizes differences in specificity between TNFR-like cell surface receptors. In the second part, recent investigations on the structure and carbohydrate binding site of CD44, a member of the link protein family, are discussed. The binding site in CD44 is compared to calcium-dependent (C-type) lectins, which include the selectins, another family of cell adhesion molecules. The studies on TNFRSF members and link proteins reported herein complement a recent review article in this journal, which focused on modeling and binding site analysis of immune cell surface proteins.Electronic Supplementary Material available.     

Mushroom lectins: Current status and future perspectives

Lectins are nonimmune proteins or glycoproteins that bind specifically to cell surface carbohydrates, culminating in cell agglutination. These are known to play key roles in host defense system and also in metastasis. Many new sources have been explored for the occurrence of lectins during the last few years. Numerous novel lectins with unique specificities and exploitable properties have been discovered. Mushrooms have attracted a number of researchers in food and pharmaceuticals. Many species have long been used in traditional Chinese medicines or functional foods in Japan and other Asian countries. A number of bioactive constituents have been isolated from mushrooms including polysaccharides, polysaccharopeptides, polysaccharide–protein complexes, proteases, ribonucleases, ribosome inactivating proteins, antifungal proteins, immunomodulatory proteins, enzymes, lectins, etc. Mushroom lectins are endowed with mitogenic, antiproliferative, antitumor, antiviral, and immunestimulating potential. In this review, an attempt has been made to collate the information on mushroom lectins, their blood group and sugar specificities, with an emphasis on their biomedical potential and future perspectives.     

Pattern recognition receptors acting in innate immune system of shrimp against pathogen infections

Invertebrates, including shrimp, have developed very complicated innate immune system against pathogens. Much work has been performed on the innate immunity of shrimp, including immune recognition, signal transduction, effector molecules and antiviral responses due to its great economic value. Pattern recognition is the first step of innate immunity. Pattern recognition receptors (PRRs) sense the presence of infection and activate immune responses. The studies on shrimp PRRs revealed the recognition mechanism of shrimp at a certain degree. To date, 11 types of pattern recognition receptors (PRRs) have been identified in shrimp, namely, β-1,3-glucanase-related proteins, β-1,3-glucan-binding proteins, C-type lectins, scavenger receptors, galectins, fibrinogen-related proteins, thioester-containing protein, Down syndrome cell adhesion molecule, serine protease homologs, trans-activation response RNA-binding protein and Toll like receptors. A number of PRRs have been functionally studied and have been found to have different binding specificities and immune functions. The present review aims to summarize the current knowledge on the PRRs of shrimp.     

Endogenous lectins as mediators of tumor cell adhesion

Endogenous carbohydrate-binding proteins have been found in various normal tissues and cells. Although lectins with different sugar-binding specificities have been described, the most prevalent ones are those that bind beta-galactosides. The ability of some normal and malignant cells to bind exogenous carbohydrate-containing ligands suggested that lectinlike activity is associated with the cell surface and that carbohydrate-binding proteins might mediate intercellular recognition and adhesion. We found that extracts of various cultured murine and human tumor cells exhibit a galactoside-inhibitable hemagglutinating activity. This activity was associated with two proteins of molecular weights of 34,000 and 14,500 daltons, which were purified by affinity chromatography by using immobilized asialofetuin. That these lectins are present on the cell surface was indicated by the binding of monoclonal antilectin antibodies to the surface of various tumor cells and by the immunoprecipitation of 125I-labeled lectins from solubilized cell-surface iodinated cells by polyclonal antilectin antibodies. That these cell surface lectins are functional was demonstrated by the ability of the galactose-terminating asialofetuin to enhance cell aggregation and of asialofetuin glycopeptides to block this homotypic aggregation as well as to suppress cell attachment to substratum, and by the inhibition of both asialofetuin-induced cell aggregation and cell attachment to substratum by the binding of monoclonal antilectin antibodies to the cell surface. These findings implicate cell surface lectins as mediators of cell-cell and cell-substratum adhesion. Some of these cellular interactions might be important determinants of tumor cell growth and metastasis.