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Lectin–glycan interactions play a crucial role in the immune system. An important class of lectins in the innate immune system is myeloid C-type lectin receptors (CLRs). Myeloid CLRs act as pattern recognition receptors and are predominantly expressed by myeloid cells, such as macrophages, dendritic cells, and neutrophils. In innate immunity, CLRs contribute to self/non-self discrimination. While the recognition of pathogen-associated molecular patterns (PAMPs) by CLRs may contribute to a protective immune response, CLR engagement can also be exploited by pathogens for immune evasion. Since various CLRs act as endocytic receptors and trigger distinct signaling pathways in myeloid cells, CLR targeting has proven useful for drug/antigen delivery into antigen-presenting cells and the modulation of immune responses. This review covers recent discoveries of pathogen/CLR interactions and novel approaches for CLR targeting within the period of the past two years. [Display omitted] • Myeloid C-type lectin receptors (CLRs) are pathogen sensors in innate immunity. • Myeloid CLRs may be exploited by pathogens for immune evasion. • CLR targeting has proven useful for cell-specific antigen/drug delivery. • Multivalent glycan display on carrier systems facilitates CLR targeting. • This review covers recent studies (mainly in the past two years) on CLR research. [ABSTRACT FROM AUTHOR]

As important pattern recognition receptors (PRRs), C‐type lectins play crucial roles in the crustacean innate immune system. In this study, a novel C‐type lectin, designated as MnLec1, was obtained from the exoskeleton of the oriental river prawn Macrobrachium nipponense for the first time. The full‐length cDNA of MnLec1 was 1329 bp with an open reading frame of 774 bp. The predicted MnLec1 protein contains a single carbohydrate‐recognition domain with an EPN/LND motif and one Ca2+ binding site‐2. MnLec1 transcripts were widely detected in the tested tissues of M. nipponense and significantly up‐regulated after Aeromonas hydrophila challenge. The recombinant MnLec1 protein was found to have a wide spectrum of binding activities towards various microorganisms, agglutinate two kinds of Gram‐negative bacteria (Escherichia coli and A. hydrophila) in a Ca2+‐independent manner. What's more, the survivability of prawns was significantly down‐regulated after RNAi of MnLec1 when infected with A. hydrophila. Collectively, these findings suggest that MnLec1 from the exoskeleton might function as a PRR and play a crucial role in immune defense against invading pathogens in M. nipponense. [ABSTRACT FROM AUTHOR]

C-type lectins (CTLs) are glycan-binding pattern recognition receptors (PRRs) that can bind to carbohydrates on pathogen surfaces, triggering immune responses in shrimp innate immunity. In this study, a unique Ca2+-inhibited CTL named Fc Lec was identified and characterized in Chinese shrimp Fenneropenaeus chinensis. The full-length cDNA sequence of Fc Lec was 976 bp (GenBank accession number KU361826), with a 615 bp open reading frame (ORF) encoding 204 amino acids. Fc Lec possesses a C-type lectin-like domain (CTLD) containing four conserved cysteines (Cys105, Cys174, Cys192, and Cys200) and two sugar-binding site structures (QPD and LNP). The tertiary structure of Fc Lec deduced revealed three α-helices and eight β-pleated sheets. The mRNA expression levels of Fc Lec in hemocytes and the hepatopancreas were markedly elevated after stimulation with Vibrio anguillarum and white spot syndrome virus (WSSV). The recombinant Fc Lec protein exhibited Ca2+-independent hemagglutination and bacterial agglutination, but these activities were observed only in the presence of EDTA to chelate metal ions. These findings suggest that Fc Lec plays important and functionally distinct roles in the shrimp's innate immune response to bacteria and viruses, enriching the current understanding of the relationship between CTL activity and Ca2+ in invertebrates. [Display omitted] • A novel CTL, Fc Lec, were identified from F. chinensis. • Fc Lec plays distinct and significant roles in the innate immune system's reaction to microbial assaults. • Fc Lec exhibits Ca2+-independent and even Ca2+-inhibited activity. [ABSTRACT FROM AUTHOR]

Abstract Galactoside-binding lectin (galectin) is a type of pathogen recognition molecule that occupies an important position in the invertebrate innate immunity system. Our previous study has identified a galectin gene in mollusk Solen grandis (SgGal-1) and illustrated its potential roles in innate immunity. By the functional study using recombinant protein and specific antibody, here, we confirmed the pivotal roles of SgGal-1 in immune defense of S. grandis. SgGal-1 protein was expressed in many tested tissues including gill, mantle, hepatopancreas and gonad, except hemocytes and muscle. The recombinant SgGal-1 (rSgGal-1) bound PGN and β-glucan instead of LPS in vitro , and it further caused significant agglutination of five different microbes, suggesting SgGal-1 served as a pattern recognition receptor (PRR) involved in immune defense of mollusk. Furthermore, SgGal-1 recruited hemocytes to encapsulate, which was blocked by anti-rSgGal-1 serum. In the meantime, rSgGal-1 as well as promoted the phagocytosis of hemocytes against Escherichia coli in vitro. All these results suggested that SgGal-1 in S. grandis not only acted as a PRR recognizing microbes but also directly participated in the process of immune opsonization to protect the host from pathogenic infection. Highlights • Galactoside-binding lectin is a multifunctional protein in Solen grandis. • SgGal-1 is distributed in many different tissues. • SgGal-1 serves as a PRR to bind PGN and glucan. • SgGal-1 involves in the anti-microbial response through agglutination. • SgGal-1 acts as an opsonin mediating encapsulation and phagocytosis of hemocytes. [ABSTRACT FROM AUTHOR]

As a type of pattern-recognition proteins, lectins perform important functions in the innate immunity of crustaceans, including prawns. Although several reports showed that C-type lectin domain family (CLEC) importantly functions in host–pathogen interactions, limited research has focused on CLEC in Macrobrachium rosenbergii . In the present study, a new single CRD containing CLEC (designated as MrLec ) was reported in freshwater prawns, M. rosenbergii . The full-length cDNA of MrLec consisted of 1027 bp with an open reading frame of 801 bp, which encoded a peptide of 266 amino acid residues. Genomic sequence for MrLec was also obtained from the M. rosenbergii , which contain 4 exons and 3 introns. MrLec was found to contain a single carbohydrate-recognition domain with an EPN motif. MrLec was ubiquitously distributed in various tissues of a normal prawn, particularly in the hepatopancreas and gills. MrLec expression in the gills was significantly upregulated after a challenge with Vibrio parahaemolyticus and downregulated at 24 h after MrLec RNA interference ( MrLec -RNAi). The expression levels of some AMPs, including antilipopolysaccharide factor 1 ( Alf1) and lysozyme 2 ( Lyso2 ), also markedly decreased after MrLec -RNAi. Recombinant MrLec can agglutinate (Ca 2+ -dependent) and bind both Gram-negative and Gram-positive bacteria. Results suggested that MrLec participates in the recognition of invading pathogens and functions in the immune response of prawn against pathogen infections. [ABSTRACT FROM AUTHOR]

The complement system represents a crucial part of innate immunity. It contains a diverse range of soluble activators, membrane-bound receptors, and regulators. Its principal function is to eliminate pathogens via activation of three distinct pathways: classical, alternative, and lectin. In the case of viruses, the complement activation results in effector functions such as virion opsonisation by complement components, phagocytosis induction, virolysis by the membrane attack complex, and promotion of immune responses through anaphylatoxins and chemotactic factors. Recent studies have shown that the addition of individual complement components can neutralise viruses without requiring the activation of the complement cascade. While the complement-mediated effector functions can neutralise a diverse range of viruses, numerous viruses have evolved mechanisms to subvert complement recognition/activation by encoding several proteins that inhibit the complement system, contributing to viral survival and pathogenesis. This review focuses on these complement-dependent and -independent interactions of complement components (especially C1q, C4b-binding protein, properdin, factor H, Mannose-binding lectin, and Ficolins) with several viruses and their consequences. [ABSTRACT FROM AUTHOR]

• A sheep C-type lectin receptor (CTLR)-Fc fusion protein library was established. • This novel library is a tool to screen for CTLR interactions with sheep pathogens. • Several ovine CTLRs share ligand preferences with the respective murine orthologs. • The utility of the CTLR-Fc library was tested with Mycoplasma mycoides subsp. capri. C-type lectin receptors (CTLRs) are pattern recognition receptors which are important constituents of the innate immunity. However, their role has mostly been studied in humans and in mouse models. To bridge the knowledge gap concerning CTLRs of veterinary relevant species, a novel ovine CTLR hFc-fusion protein library which allows in vitro ligand identification and pathogen binding studies has been established. Its utility was tested with known ligands of corresponding murine CTLRs in ELISA- and flow cytometry based binding studies. The ovine CTLR-hFc library was subsequently used in a proof-of-principle pathogen binding study with the ruminant pathogen Mycoplasma mycoides subsp. capri. Some ovine CTLRs, such as Dendritic Cell Immunoreceptor (DCIR, Clec4a), Macrophage C-Type Lectin (MCL, Clec4d) and Myeloid Inhibitory C-Type Lectin-Like Receptor (MICL, Clec12a) were identified as possible candidate receptors whose role in Mycoplasma recognition can now be unraveled in further studies. This study thus shows the utility of this novel ovine CTLR-hFc fusion protein library to screen for CTLR/pathogen interactions. [ABSTRACT FROM AUTHOR]