Lectin-carbohydrate interactions in immunoregulation

Network Analysis Reveals the Recognition Mechanism for Dimer Formation of Bulb-type Lectins

The bulb-type lectins are proteins consist of three sequential beta-sheet subdomains that bind to specific carbohydrates to perform certain biological functions. The active states of most bulb-type lectins are dimeric and it is thus important to elucidate the short- and long-range recognition mechanism for this dimer formation. To do so, we perform comparative sequence analysis for the single- and double-domain bulb-type lectins abundant in plant genomes. In contrast to the dimer complex of two single-domain lectins formed via protein-protein interactions, the double-domain lectin fuses two single-domain proteins into one protein with a short linker and requires only short-range interactions because its two single domains are always in close proximity. Sequence analysis demonstrates that the highly variable but coevolving polar residues at the interface of dimeric bulb-type lectins are largely absent in the double-domain bulb-type lectins. Moreover, network analysis on bulb-type lectin proteins show that these same polar residues have high closeness scores and thus serve as hubs with strong connections to all other residues. Taken together, we propose a potential mechanism for this lectin complex formation where coevolving polar residues of high closeness are responsible for long-range recognition.

The Immunomodulatory Effect of Trichophyton Rubrum Exoantigens in the Treatment of Experimental Septic Arthritis

Background:

Understanding the nature and function of fungal exoantigens might lead to novel approaches in the treatment and prophylaxis of some infectious diseases. Septic arthritis represents a serious problem for medicine due to the high incidence rate and severe complications.

Objective:

The present study aimed at assessing the immunomodulatory effects of Trichophyton rubrum culture filtrate as a novel compound in experimental septic arthritis.

Method:

The septic arthritis was haematogenously induced in Sprague-Dawley rats by a single intravenous injection of 10⁹ colony forming units of the human clinical isolate Staphylococcus aureus producing toxic shock syndrome toxin-1. Trichophyton rubrum culture filtrate at two different doses 20 and 40 mg/kg was administered intraperituneally two days after bacterial inoculation in the treatment groups and concurrently with the appearance of clinical signs in the patient groups. The administration of Trichophyton rubrum solution was continued every other day for 10 injections.

Results:

The clinical evaluation showed that Trichophyton rubrum-treated rats were significantly protected from disease development compared with untreated controls. This finding was correlated with results of radiological evaluation of the involved joints. Although, the inflammatory cell infiltration, cartilage/bone destruction and synovial hypertrophy had been decreased in the treatment groups in comparison with arthritic controls however, the histological changes were not significant in these two groups.

Conclusion:

It is possible that Trichophyton rubrum antigens may play a role in modulating the immune responses and would be efficient in septic arthritis treatment.

Novel therapeutic approach by culture filtrate of Cryptococcus neoformans var. gattii (CneF) in experimental immune complex glomerulonephritis

The present study was undertaken to determine the therapeutic effect of the culture filtrate of Cryptococcus neoformans var. gattii (CneF) in experimental immune complex glomerulonephritis. Bovine serum albumin (BSA) nephritis was induced in rats by a subcutaneous immunization and daily intravenous administration of BSA. CneF solution at three different doses (36, 54, and 90 mg/kg based on carbohydrate concentration) was administered intraperitoneally at regular 72-h intervals for 4 weeks. Onset of treatment was day 65, and urinary protein was measured at different intervals. Animals were euthanized on day 107. Serum and urine determinants were measured at the time of sacrifice and kidney specimens were examined. Results of this experiment showed that CneF therapy could significantly reduce the urinary protein excretion, blood urea nitrogen (BUN), plasma concentration of triglyceride, and increase the serum HDL cholesterol in treated rats vs. nontreated controls. Moreover, there was significant difference in glomerular changes between treated and nontreated groups. These observations show that the beneficial effect of CneF may be related to decreased number of glomerular leukocytes. Our findings suggest that treatment with CneF as a new antiinflammatory compound can reduce proteinuria, suppress the development of glomerular lesions, and exert lipid-lowering property in a rat model of immune complex glomerulonephritis.

Sialic acid in hemolymph and affinity purified lectins from two marine bivalves

Sialic acids have been implicated in a variety of complex biological regulatory and signalling events and their functional importance is reflected by their presence in a wide variety of phyla. Potentially they may inhibit intermolecular and intercellular interactions. Lectins that exhibit specificity for sialic acid or sialoglycoconjugates are ubiquitous in the body fluids of invertebrates and this has supported the assumption that these lectins are involved in defense against microbes that express sialic acids on their surfaces. This biological function has also been inferred from the absence of sialic acids in lower invertebrates. However, most invertebrate lectins are heterogeneous and may also bind other ligands. The biological significance of the different carbohydrate specificities are not yet known. We have demonstrated the presence of sialic acids in hemolymph from two marine bivalves, the Pacific oyster Crassostrea gigas (approximately 15 micrograms ml-1) and the horse mussel Modiolus modiolus (48-100 micrograms ml-1) by several different assays. The sialic acid was mostly in free form. Affinity purified lectins from the horse mussel also contained bound sialic acids (2-5 mumol g-1). Oyster hemolymph stimulated the in vitro phagocytosis of bacteria by oyster hemocytes. The stimulation by hemolymph is facilitated by a dialyzable component, that apparently is active irrespective of the binding to sialic acid (BSM). Addition of sialic acid had no significant effect on the in vitro phagocytosis of bacteria by oyster hemocytes.

A heterogeneous sialic acid-binding lectin with affinity for bacterial LPS from horse mussel (Modiolus modiolus) hemolymph

A sialic acid-binding lectin that agglutinates a variety of erythrocytes and bacteria and react with sialoconjugates and purified lipopolysaccharides from marine vibrios has been affinity purified from hemolymph of the horse mussel Modiolus modiolus using Bovine submaxillary mucin conjugated to CNBr-activated Sepharose 4B. The lectin demonstrated heterogeneous activity, and at least two main entities were partially characterized, and are referred to as modiolin H and modiolin E activities for the agglutination of human and horse (equine) erythrocytes, respectively. Only modiolin E activity required calcium ions for hemagglutination. The M. modiolus lectin was mainly specific for NeuAc, although the lectin demonstrated a broader range of specificity, similarly to the Limulus polyphemus lectin. The purified lectin was a glycoprotein, and in the native state existed as aggregates with M(r) in the range of 100-1,300 kDa as observed by gradient-gel electrophoresis and gel filtration on Biogel and Superose. SDS-PAGE under reducing conditions revealed three subunits of M(r) 14, 17.5 and 20 kDa. Various marine bacteria adsorbed the hemagglutinating activities of the M. modiolus lectin. Purified LPS preparations from various pathogenic marine vibrios were also effective inhibitors, in particular for modiolin E activity. These results indicate that the lectin play a role in recognition of bacteria.

Interactions of tunicate immunomodulatory proteins with mammalian cells

The ability of a tunicate immunomodulatory protein, tunIL1, to interact with mammalian cells was investigated. TunIL1 is known to regulate inflammatory defence reactions in tunicates and to stimulate the proliferation of mammalian thymocytes. In the current study, tunIL1 was shown to enchance L929 fibroblast proliferation, induce IL-2 secretion from human mononuclear cells and enhance IL-2 receptor expression by EL-4 murine lymphoma cells. These biological activities are comparable with those of the mammalian inflammatory cytokine, IL-1. However, tunIL1 does not appear to stimulate its effects on mammalian cells by interacting with cell surface receptors in a manner analogous to mammalian IL-1. TunIL1 cannot block the binding of anti-IL-1 receptor antibodies to EL-4 cells, nor can anti-IL-1 receptor antibodies inhibit the capacity of tunIL1 to stimulate thymocyte proliferation. This indicates that tunIL1 does not induce its IL-1-like activities via structural homology to mammalian IL-1.

Use of lectins as diagnostic and therapeutic tools for cancer

Within the past few years, lectins have become a well-established means for understanding varied aspects of cancer and metastasis. Evidence is now emerging that lectins are dynamic contributors to tumor cell recognition (surface markers), cell adhesion and localization, signal transduction across membranes, mitogenic stimulation, augmentation of host immune defense, cytotoxicity, and apoptosis. To advance understanding of these lectin-dependent processes, attempts are being made to discover new lectins that have one or more of these functions and to develop lectin- (or glycoconjugate-) based tools that could be used to home in on tumor cells. This review will summarize current research on the lectins and recent advances in the development of lectin-based diagnostic and therapeutic tools for cancer. Additionally, the future potential of lectin-based diagnosis and therapy is discussed.

A new mannan-binding lectin from the serum of the eel (Anguilla anguilla L.): isolation, characterization and comparison with the fucose-specific serum lectin

A new mannan-binding lectin (MBL) and a previously described fucose-binding lectin (FBL) have been isolated from serum of Anguilla anguilla by affinity chromatography on A-peptone-Sepharose in combination with electroelution (MBL) or affinity chromatography using alpha-L-fucose-agarose (FBL). MBL has a mol. wt of approximately 246,000 and is composed of identical subunits of approximately 24,000, two of each are always covalently linked. FBL has a mol. wt of about 121,000 and consists of four subunits of 30,000, which, upon reduction are split into two identical subunits of 15,000. Upon isoelectric focusing MBL displays four bands ranging from pH 4.8 to 5.2. FBL shows 17-20 bands between pH 5.5 and 6.2. Of the inhibitors utilized, hemagglutination activity of MBL is inhibited only by mannan, whereas FBL activity is inhibited by several glycosubstances. MBL and FBL activity is constant between pH 4 and 10 and 5 and 10, respectively. Temperatures above 55 degrees C totally destroy MBL activity whereas FBL activity remains constant up to 75 degrees C.

Non-carbohydrate binding partners/domains of animal lectins

1. Protein-carbohydrate interactions are involved in a large number of biologically important recognition processes. 2. Among the participating classes of proteins lectins are defined as carbohydrate-binding proteins other than an antibody or an enzyme. 3. In addition to the essential carbohydrate-binding domain other functionally and/or structurally important sites, defined by sequence comparison or by experimental demonstration of protein-protein interactions, can be present within the lectin molecule and may be relevant for its physiological significance. 4. Sequence motifs of lectins for protein-protein interactions include amino acid structures designed for cell adhesion, growth regulatory biosignalling, intracellular routing and enzymatic activity. 5. Elucidation of the complete functional role(s) of a lectin requires accurate delineation of its carbohydrate and, if present, of its protein ligands. 6. Presence of more than one carbohydrate-binding domain in a single lectin, potential ligand properties of the glycopart of a lectin, regulatory interplay between different sites and possible interaction of complementarily shaped peptide sequences to the sugar-recognizing site should all be assessed in the quest to comprehensively explain the physiological role(s) of a lectin.

Human immune toxicity

This review examines xenobiotic toxicity to the immune system, stressing in particular those aspects of most relevance to humans. Immunotoxicity is examined especially from three points of view: by what immunological component is affected, by classes of foreign agents that adversely affect the human immune system and by critical evaluation of human case reports and epidemics. Mechanisms by which xenobiotics interrupt cytokine networks are emphasized. The concept that microbial agents, both environmental as well as infectious, may act as immunotoxicants, either alone or in synergism with conventional agents is introduced. Instances of human immunotoxicology are critically evaluated in terms of clinical relevance, i.e. whether increased susceptibility to opportunistic infections or tumor emergence takes place in the affected individuals.

Effects of endogenous soluble beta-galactoside binding lectins and protein inhibitor of fucosyltransferase on the enzymes involved in the intestinal fucosylation process

Soluble beta-galactoside binding lectins were prepared from the rat small intestinal mucosa by chromatography on asialofetuin-Sepharose. The lectin fraction exhibits 3 bands with Mr of 21,5 kDa, 19 kDa and 17 kDa on SDS-PAGE. This fraction inhibits a partially purified soluble alpha(1-2)-fucosyltransferase by interaction with the glycoprotein substrate asialofetuin, whereas the inhibition is non competitive for the donor GDP-fucose. It has no effect on other enzymes of the fucosylation system, namely glycosyl-nucleotide pyrophosphatase and the system synthesizing GDP-fucose from GDP-mannose. A different and specific soluble protein inhibitor of fucosyltransferase activity inhibits this activity by a competitive mechanism for GDP-fucose and a non competitive one for asialofetuin. Unlike the lectins, this inhibitor also inhibits the action of pyrophosphatase and the formation of GDP-fucose by different mechanisms. The possible extension of these in vitro results to the in vivo regulation of glycosylation is discussed.