The N- and C-terminal carbohydrate recognition domains of galectin-9 from Carassius auratus contribute differently to its immunity functions to Aeromonas hydrophila and Staphylococcus aureus

Galectin-9, an important pathogen recognition receptor (PRR), could recognize and bind pathogen-associated molecular patterns (PAMPs) on the surface of invading microorganisms, initiating the innate immune responses. A galectin-9 was identified from Qihe crucian carp Carassius auratus and designated as CaGal-9. The predicted CaGal-9 protein contained two non-identical carbohydrate recognition domains (CRDs), namely, N-CRD and C-CRD. The recombinant proteins (rCaGal-9, rN-CRD and rC-CRD) were purified from Escherichia coli BL21 (DE3) and exhibited strong agglutinating activity with erythrocytes of rabbit. The haemagglutination was inhibited by D-galactose, α-lactose and N-acetyl-D-galactose. Results of microbial agglutination assay showed that three recombinant proteins agglutinated Gram-negative bacterium Aeromonas hydrophila and Gram-positive bacterium Staphylococcus aureus. With regard to the binding activity, each recombinant protein could bind to LPS, PGN and the examined microorganisms (A. hydrophila and S. aureus) with different binding affinities. The integrated analyses suggested that CaGal-9 with two CRD domains could play an important role in immune defence against pathogenic microorganisms for C. auratus.

The Antiviral Role of Galectins toward Influenza A Virus Infection-An Alternative Strategy for Influenza Therapy

Animal lectins are proteins with carbohydrate recognition activity. Galectins, the β-galactoside binding lectins, are expressed in various cells and have been reported to regulate several immunological and physiological responses. Recently, some galectins have been reported to regulate some viral infections, including influenza A virus (IAV); however, the mechanism is still not fully understood. Thus, we aim to review systemically the roles of galectins in their antiviral functions against IAVs. The PRISMA guidelines were used to select the eligible articles. Results indicated that only Galectin-1, Galectin-3, and Galectin-9 were reported to play a regulatory role in IAV infection. These regulatory effects occur extracellularly, through their carbohydrate recognition domain (CRD) interacting with glycans expressed on the virus surface, as well as endogenously, in a cell-cell interaction manner. The inhibition effects induced by galectins on IAV infection were through blocking virus-host receptors interaction, activation of NLRP-3 inflammasome, augment expression of antiviral genes and related cytokines, as well as stimulation of Tim-3 related signaling to enhance virus-specific T cells and humoral immune response. Combined, this study concludes that currently, only three galectins have reported antiviral capabilities against IAV infection, thereby having the potential to be applied as an alternative anti-influenza therapeutic strategy.

Identification of a secondary binding site in human macrophage galactose-type lectin by microarray studies: Implications for the molecular recognition of its ligands

The human macrophage galactose-type lectin (MGL) is a C-type lectin characterized by a unique specificity for terminal GalNAc residues present in the tumor-associated Tn antigen (αGalNAc-Ser/Thr) and its sialylated form, the sialyl-Tn antigen. However, human MGL has multiple splice variants, and whether these variants have distinct ligand-binding properties is unknown. Here, using glycan microarrays, we compared the binding properties of the short MGL 6C (MGL^short) and the long MGL 6B (MGL^long) splice variants, as well as of a histidine-to-threonine mutant (MGL^short H259T). Although the MGL^short and MGL^long variants displayed similar binding properties on the glycan array, the MGL^short H259T mutant failed to interact with the sialyl-Tn epitope. As the MGL^short H259T variant could still bind a single GalNAc monosaccharide on this array, we next investigated its binding characteristics to Tn-containing glycopeptides derived from the MGL ligands mucin 1 (MUC1), MUC2, and CD45. Strikingly, in the glycopeptide microarray, the MGL^short H259T variant lost high-affinity binding toward Tn-containing glycopeptides, especially at low probing concentrations. Moreover, MGL^short H259T was unable to recognize cancer-associated Tn epitopes on tumor cell lines. Molecular dynamics simulations indicated that in WT MGL^short, His²⁵⁹ mediates H bonds directly or engages the Tn-glycopeptide backbone through water molecules. These bonds were lost in MGL^short H259T, thus explaining its lower binding affinity. Together, our results suggest that MGL not only connects to the Tn carbohydrate epitope, but also engages the underlying peptide via a secondary binding pocket within the MGL carbohydrate recognition domain containing the His²⁵⁹ residue.

The Role and Molecular Mechanism of Action of Surfactant Protein D in Innate Host Defense Against Influenza A Virus

Influenza A viruses (IAVs) continue to pose major risks of morbidity and mortality during yearly epidemics and periodic pandemics. The genomic instability of IAV allows it to evade adaptive immune responses developed during prior infection. Of particular concern are pandemics which result from wholesale incorporation of viral genome sections from animal sources. These pandemic strains are radically different from circulating human strains and pose great risk for the human population. For these reasons, innate immunity plays a strong role in the initial containment of IAV infection. Soluble inhibitors present in respiratory lining fluids and blood provide a level of early protection against IAV. In general, these inhibitors act by binding to the viral hemagglutinin (HA). Surfactant protein D (SP-D) and mannose-binding lectin (MBL) attach to mannosylated glycans on the HA in a calcium dependent manner. In contrast, surfactant protein A, ficolins, and other inhibitors present sialic acid rich ligands to which the HA can bind. Among these inhibitors, SP-D seems to be the most potent due to its specific mode of binding to viral carbohydrates and its ability to strongly aggregate viral particles. We have studied specific properties of the N-terminal and collagen domain of SP-D that enable formation of highly multimerized molecules and cooperative binding among the multiple trimeric lectin domains in the protein. In addition, we have studied in depth the lectin activity of SP-D through expression of isolated lectin domains and targeted mutations of the SP-D lectin binding site. Through modifying specific residues around the saccharide binding pocket, antiviral activity of isolated lectin domains of SP-D can be markedly increased for seasonal strains of IAV. Wild-type SP-D causes little inhibition of pandemic IAV, but mutated versions of SP-D were able to inhibit pandemic IAV through enhanced binding to the reduced number of mannosylated glycans present on the HA of these strains. Through collaborative studies involving crystallography of isolated lectin domains of SP-D, glycomics analysis of the HA, and molecular modeling, the mechanism of binding of wild type and mutant forms of SP-D have been determined. These studies could guide investigation of the interactions of SP-D with other pathogens.

Identification and Characterization of C-type Lectins in Ostrinia furnacalis (Lepidoptera: Pyralidae)

C-type lectins (CTLs) are a large family of calcium-dependent carbohydrate-binding proteins. They function primarily in cell adhesion and immunity by recognizing various glycoconjugates. We identified 14 transcripts encoding proteins with one or two CTL domains from the transcriptome from Asian corn borer, Ostrinia furnacalis (Guenée; Lepidoptera: Pyralidae). Among them, five (OfCTL-S1 through S5) only contain one CTL domain, the remaining nine (OfIML-1 through 9) have two tandem CTL domains. Five CTL-Ss and six OfIMLs have a signal peptide are likely extracellular while another two OfIMLs might be cytoplasmic. Phylogenetic analysis indicated that OfCTL-Ss had 1:1 orthologs in Lepidoptera, Diptera, Coleoptera and Hymenoptera species, but OfIMLs only clustered with immulectins (IMLs) from Lepidopteran. Structural modeling revealed that the 22 CTL domains adopt a similar double-loop fold consisting of β-sheets and α-helices. The key residues for calcium-dependent or independent binding of specific carbohydrates by CTL domains were predicted with homology modeling. Expression profiles assay showed distinct expression pattern of 14 CTLs: the expression and induction were related to the developmental stages and infected microorganisms. Overall, our work including the gene identification, sequence alignment, phylogenetic analysis, structural modeling, and expression profile assay would provide a valuable basis for the further functional studies of O. furnacalis CTLs.

Galectin-3 type-C self-association on neutrophil surfaces; The carbohydrate recognition domain regulates cell function

Galectin-3 is an endogenous β-galactoside-binding lectin comprising a carbohydrate recognition domain (CRD) linked to a collagen-like N-domain. Both domains are required for galectin-3 to induce cellular effects; a C-terminal fragment of galectin-3, galectin-3C, containing the CRD but lacking the N-domain, binds cell surface glycoconjugates but does not induce cellular effects since cross-linking promoted by the N-domain is thought to be required. Instead, galectin-3C is proposed to antagonize the effects of galectin-3 by competing for binding sites. The aim of this study was to investigate the effects of galectin-3C on galectin-3 interactions with human neutrophils. Recombinant galectin-3C inhibited galectin-3-induced production of reactive oxygen species in primed neutrophils. Surprisingly, this inhibition was not due to competitive inhibition of galectin-3 binding to the cells. In contrast, galectin-3C potentiated galectin-3 binding, in line with emerging evidence that galectin-3 can aggregate not only through the N-domain but also through the CRD. The cell surface interaction between galectin-3C and galectin-3 was corroborated by colocalization of fluorescently labeled galectin-3 and galectin-3C. Galectin-3C can be generated in vivo through cleavage of galectin-3 by proteases. Indeed, in circulation, galectin-3 and galectin-3C were both attached to the cell surface of neutrophils, which displayed great capacity to bind additional galectin-3 and galectin-3C. In conclusion, galectin-3C enhances galectin-3 binding to neutrophils by nonactivating type-C self-association, in parallel to inhibiting neutrophil activation by galectin-3 (induced by type-N self-association). This implicates type-C self-association as a termination system for galectin-3-induced cell activation, with the purpose of avoiding oxidant-dependent tissue damage.

Bispecific chimeric antigen receptors targeting the CD4 binding site and high-mannose Glycans of gp120 optimized for anti-human immunodeficiency virus potency and breadth with minimal immunogenicity

BACKGROUND AIMS

Chimeric antigen receptors (CARs) offer great potential toward a functional cure of human immunodeficiency virus (HIV) infection. To achieve the necessary long-term virus suppression, we believe that CARs must be designed for optimal potency and anti-HIV specificity, and also for minimal probability of virus escape and CAR immunogenicity. CARs containing antibody-based motifs are problematic in the latter regard due to epitope mutation and anti-idiotypic immune responses against the variable regions.

METHODS

We designed bispecific CARs, each containing a segment of human CD4 linked to the carbohydrate recognition domain of a human C-type lectin. These CARs target two independent regions on HIV-1 gp120 that presumably must be conserved on clinically significant virus variants (i.e., the primary receptor binding site and the dense oligomannose patch). Functionality and specificity of these bispecific CARs were analyzed in assays of CAR-T cell activation and spreading HIV-1 suppression.

RESULTS

T cells expressing a CD4-dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DCSIGN) CAR displayed robust stimulation upon encounter with Env-expressing targets, but negligible activity against intercellular adhesion molecule (ICAM)-2 and ICAM-3, the natural dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin ligands. Moreover, the presence of the lectin moiety prevented the CD4 from acting as an entry receptor on CCR5-expressing cells, including CD8⁺ T cells. However, in HIV suppression assays, the CD4-DCSIGN CAR and the related CD4-liver/lymph node-specific intercellular adhesion molecule-3-grabbing non-integrin CAR displayed only minimally increased potency compared with the CD4 CAR against some HIV-1 isolates and reduced potency against others. By contrast, the CD4-langerin and CD4-mannose binding lectin (MBL) CARs uniformly displayed enhanced potency compared with the CD4 CAR against all the genetically diverse HIV-1 isolates examined. Further experimental data, coupled with known biological features, suggest particular advantages of the CD4-MBL CAR.

DISCUSSION

These studies highlight features of bispecific CD4-lectin CARs that achieve potency enhancement by targeting two distinct highly conserved Env determinants while lacking immunogenicity-prone antibody-based motifs.

Targeting and Recognition of Toll-Like Receptors by Plant and Pathogen Lectins

We have reported that some lectins act as agonists of toll-like receptors (TLRs) and have immunomodulatory properties. The plant lectin ArtinM, for example, interacts with N-glycans of TLR2, whereas other lectins of microbial origin interact with TLR2 and TLR4. Expression of the receptors on the surface of antigen-presenting cells exposes N-glycans that may be targeted by lectins of different structures, specificities, and origins. In vitro, these interactions trigger cell signaling that leads to NF-κB activation and production of the Th1 polarizing cytokine IL-12. In vivo, a same sequence of events follows the administration of an active lectin to mice infected with an intracellular pathogen, conferring resistance to the pathogen. The lectins of the human pathogens Toxoplasma gondii (TgMIC1 and TgMIC4) and Paracoccidioides brasiliensis (Paracoccin), by recognition and activation of TLR2 and TLR4, induce cell events and in vivo effects comparable to the promoted by the plant lectin ArtinM. In this article, we highlight these two distinct mechanisms for activating antigen-presenting cells. On the one hand, TLRs act as sensors for the presence of conventional pathogen-associated molecular patterns, such as microbial lipids. On the other hand, we showed that TLR-mediated cell activation might be triggered by an alternative way, in which lectins bind to TLRs N-glycans and stimulate cells to increase the expression of pro-inflammatory cytokines. This process may lead to the development of new pharmaceutical tools that promote protective immune responses directed against intracellular pathogens and tumors.

Human galectin-2 interacts with carbohydrates and peptides non-classically: new insight from X-ray crystallography and hemagglutination

Galectin-2 (Gal-2) plays a role in cancer, myocardial infarction, immune response, and gastrointestinal tract diseases. The only reported crystal structure of Gal-2 shows that it is a dimer in which the monomer subunits have almost identical structures, each binding with one molecule of lactose. In this study, we crystallized Gal-2 under new conditions that produced three crystal structures. In each Gal-2 dimer structure, lactose was shown to be bound to only one of the carbohydrate recognition domain subunits. In solution studies, the thermal shift assay demonstrated that inequivalent monomer subunits in the Gal-2 dimer become equivalent upon ligand binding. In addition, galectin-mediated erythrocyte agglutination assays using lactose and larger complex polysaccharides as inhibitors showed the structural differences between Gal-1 and Gal-2. Overall, our results reveal some novel aspects to the structural differentiation in Gal-2 and expand the potential for different types of molecular interactions that may be specific to this lectin.

Acyclic tethers mimicking subunits of polysaccharide ligands: selectin antagonists

We report on the design and synthesis of molecules having E- and P-selectins blocking activity both in vitro and in vivo. The GlcNAc component of the selectin ligand sialyl Lewis(X) was replaced by an acyclic tether that links two saccharide units. The minimization of intramolecular dipole-dipole interactions and the gauche effect would be at the origin of the conformational bias imposed by this acyclic tether. The stereoselective synthesis of these molecules, their biochemical and biological evaluations using surface plasmon resonance spectroscopy (SPR), and in vivo assays are described. Because the structure of our analogues differs from the most potent E-selectin antagonists reported, our acyclic analogues offer new opportunities for chemical diversity.

Crystal structures of carbohydrate recognition domain of blood dendritic cell antigen-2 (BDCA2) reveal a common domain-swapped dimer

We report on crystal structures of a carbohydrate recognition domain (CRD) of human C-type lectin receptor blood dendritic cell antigen-2 (BDCA2). Three different crystal forms were obtained at 1.8-2.3 Å resolution. In all three, the CRD has a basic C-type lectin fold, but a long loop extends away from the core domain to form a domain-swapped dimer. The structures of the dimers from the three different crystal forms superimpose well, indicating that domain swapping and dimer formation are energetically stable. The structure of the dimer is compared with other domain-swapped proteins, and a possible regulation mechanism of BDCA2 is discussed.

Peptides derived from human galectin-3 N-terminal tail interact with its carbohydrate recognition domain in a phosphorylation-dependent manner

Galectin-3 (Gal-3) is a multi-functional effector protein that functions in the cytoplasm and the nucleus, as well as extracellularly following non-classical secretion. Structurally, Gal-3 is unique among galectins with its carbohydrate recognition domain (CRD) attached to a rather long N-terminal tail composed mostly of collagen-like repeats (nine in the human protein) and terminating in a short non-collagenous terminal peptide sequence unique in this lectin family and not yet fully explored. Although several Ser and Tyr sites within the N-terminal tail can be phosphorylated, the physiological significance of this post-translational modification remains unclear. Here, we used a series of synthetic (phospho)peptides derived from the tail to assess phosphorylation-mediated interactions with (15)N-labeled Gal-3 CRD. HSQC-derived chemical shift perturbations revealed selective interactions at the backface of the CRD that were attenuated by phosphorylation of Tyr 107 and Tyr 118, while phosphorylation of Ser 6 and Ser 12 was essential. Controls with sequence scrambling underscored inherent specificity. Our studies shed light on how phosphorylation of the N-terminal tail may impact on Gal-3 function and prompt further studies using phosphorylated full-length protein.

Characterization of the interaction between collectin 11 (CL-11, CL-K1) and nucleic acids

Collectins are a group of innate immune proteins that contain collagen-like regions and globular C-type lectin domains. Via the lectin domains, collectins recognize and bind to various microbial carbohydrate patterns. Collectin 11 (CL-11) exists in complex with the complement activating MBL-associated proteases, MASPs. In the present work, we characterize the interaction between CL-11 and DNA, and show that CL-11 binds to DNA from a variety of origins in a calcium-independent manner. CL-11 binds also to apoptotic cells presenting extracellular DNA on their surface. The binding to DNA is sensitive to changes in ionic strength and pH. Competition studies show that CL-11 binds to nucleic acids and carbohydrates via separate binding-sites and oligomericity appears crucial for binding activity. Combined interaction with DNA and mannan strongly increases binding avidity. By surface plasmon resonance we estimate the dissociation constant for the binding between CL-11 and double stranded DNA oligonucleotides to K(D)=9-20 nM. In an in vitro assay we find that CL-11 binds to DNA coated surfaces, which leads to C4b deposition via MASP-2. We propose that CL-11, e.g. via complement, may play a role in response to particles and surfaces presenting extracellular DNA, such as apopototic cells, neutrophil extracellular traps and biofilms.

A novel hypothesis for an alkaline phosphatase 'rescue' mechanism in the hepatic acute phase immune response

The liver isoform of the enzyme alkaline phosphatase (AP) has been used classically as a serum biomarker for hepatic disease states such as hepatitis, steatosis, cirrhosis, drug-induced liver injury, and hepatocellular carcinoma. Recent studies have demonstrated a more general anti-inflammatory role for AP, as it is capable of dephosphorylating potentially deleterious molecules such as nucleotide phosphates, the pathogenic endotoxin lipopolysaccharide (LPS), and the contact clotting pathway activator polyphosphate (polyP), thereby reducing inflammation and coagulopathy systemically. Yet the mechanism underlying the observed increase in liver AP levels in circulation during inflammatory insults is largely unknown. This paper hypothesizes an immunological role for AP in the liver and the potential of this system for damping generalized inflammation along with a wide range of ancillary pathologies. Based on the provided framework, a mechanism is proposed in which AP undergoes transcytosis in hepatocytes from the canalicular membrane to the sinusoidal membrane during inflammation and the enzyme's expression is upregulated as a result. Through a tightly controlled, nucleotide-stimulated negative feedback process, AP is transported in this model as an immune complex with immunoglobulin G by the asialoglycoprotein receptor through the cell and secreted into the serum, likely using the receptor's State 1 pathway. The subsequent dephosphorylation of inflammatory stimuli by AP and uptake of the circulating immune complex by endothelial cells and macrophages may lead to decreased inflammation and coagulopathy while providing an early upstream signal for the induction of a number of anti-inflammatory gene products, including AP itself.

Molecular cloning and characterization of perlucin from the freshwater pearl mussel, Hyriopsis cumingii

Perlucin is an important functional protein that regulates shell and pearl formation. In this study, we cloned the perlucin gene from the freshwater pearl mussel Hyriopsis cumingii, designated as Hcperlucin. The full-length cDNA transcribed from the Hcperlucin gene was 1460 bp long, encoding a putative signal peptide of 20 amino acids and a mature protein of 141 amino acids. The mature Hcperlucin peptide contained six conserved cysteine residues and a carbohydrate recognition domain, similar to other members of the C-type lectin families. In addition, a "QPS" and an invariant "WND" motif near the C-terminal region were also found, which are extremely important for polysaccharide recognition and calcium binding of lectins. The mRNA of Hcperlucin was constitutively expressed in all tested H. cumingii tissues, with the highest expression levels observed in the mantle, adductor, gill and hemocytes. In situ hybridization was used to detect the presence of Hcperlucin mRNA in the mantle, and the result showed that the mRNA was specifically expressed in the epithelial cells of the dorsal mantle pallial, an area known to express genes involved in the biosynthesis of the nacreous layer of the shell. The significant Hcperlucin mRNA expression was detected on day 14 post shell damage and implantation, suggesting that the Hcperlucin might be an important gene in shell nacreous layer and pearl formation. The change of perlucin expression in pearl sac also confirmed that the mantle transplantation results in a new expression pattern of perlucin genes in pearl sac cells that are required for pearl biomineralization. These findings could help better understanding the function of perlucin in the shell and pearl formation.

A new approach to explore the binding space of polysaccharide-based ligands: selectin antagonists

The discovery of molecules that interfere with the binding of a ligand to a receptor remains a topic of great interest in medicinal chemistry. Herein, we report that a monosaccharide unit of a polysaccharide ligand can be replaced advantageously by a conformationally locked acyclic molecular entity. A cyclic component of the selectin ligand Sialyl Lewis(x), GlcNAc, is replaced by an acyclic tether, tartaric esters, which link two saccharide units. The conformational bias of this acyclic tether originates from the minimization of intramolecular dipole-dipole interaction and the gauche effect. The evaluation of the binding of these derivatives to P-selectin was measured by surface plasmon resonance spectroscopy. The results obtained in our pilot study suggest that the discovery of tunable tethers could facilitate the exploration of the carbohydrate recognition domain of various receptors.

Genetic complexity of the human surfactant-associated proteins SP-A1 and SP-A2

Pulmonary surfactant protein A (SP-A) plays a key role in innate lung host defense, in surfactant-related functions, and in parturition. In the course of evolution, the genetic complexity of SP-A has increased, particularly in the regulatory regions (i.e. promoter, untranslated regions). Although most species have a single SP-A gene, two genes encode SP-A in humans and primates (SFTPA1 and SFTPA2). This may account for the multiple functions attributed to human SP-A, as well as the regulatory complexity of its expression by a relatively diverse set of protein and non-protein cellular factors. The interplay between enhancer cis-acting DNA sequences and trans-acting proteins that recognize these DNA elements is essential for gene regulation, primarily at the transcription initiation level. Furthermore, regulation at the mRNA level is essential to ensure proper physiological levels of SP-A under different conditions. To date, numerous studies have shown significant complexity of the regulation of SP-A expression at different levels, including transcription, splicing, mRNA decay, and translation. A number of trans-acting factors have also been described to play a role in the control of SP-A expression. The aim of this report is to describe the genetic complexity of the SFTPA1 and SFTPA2 genes, as well as to review regulatory mechanisms that control SP-A expression in humans and other animal species.

X-ray structures of human galectin-9 C-terminal domain in complexes with a biantennary oligosaccharide and sialyllactose

Galectin-9, a tandem-repeat-type β-galactoside-specific animal lectin with two carbohydrate recognition domains (CRDs) at the N- and C-terminal ends, is involved in chemoattraction, apoptosis, and the regulation of cell differentiation and has anti-allergic effects. Its ability to recognize carbohydrates is essential for its biological functions. Human galectin-9 (hG9) has high affinity for branched N-glycan-type oligosaccharides (dissociation constants of 0.16-0.70 μM) and linear β1-3-linked poly-N-acetyllactosamines (0.09-8.3 μM) and significant affinity for the α2-3-sialylated oligosaccharides (17-34 μM). Further, its N-terminal CRD (hG9N) and C-terminal CRD (hG9C) differ in specificity. To elucidate this unique feature of hG9, x-ray structures of hG9C in the free form and in complexes with N-acetyllactosamine, the biantennary pyridylaminated oligosaccharide, and α2-3-sialyllactose were determined. They are the first x-ray structural analysis of C-terminal CRD of the tandem-repeat-type galectin. The results clearly revealed the mechanism by which branched and α2-3-sialylated oligosaccharides are recognized and explained the difference in specificity between hG9N and hG9C. Based on structural comparisons with other galectins, we propose that the wide entrance for ligand binding and the shallow binding site of hG9C are favorable for branched oligosaccharides and that Arg(221) is responsible for recognizing sialylated oligosaccharides.