Comparative study of two single CRD C-type lectins, CgCLec-4 and CgCLec-5, from pacific oyster Crassostrea gigas

Molecular identification and functional characterization of a C-type lectin gene in Meretrix meretrix

C-type lectins (CTLs) are a kind of Ca2+-dependent immunoreactive factors, which participated in pathogens recognition and defense. The present study identified a new CTL from hard clam Meretrix meretrix (designated as MmCTL4). The full-length of MmCTL4 cDNA was 608 bp, encoding a presumed signal peptide of 19 bp and a carbohydrate recognition domain (CRD) of 131 bp. The tertiary structure of recombinant MmCTL4 protein (rMmCTL4) was the typical long double-ring structure with three conserved disulfide bonds, and the motifs in Ca2+-binding sites of MmCTL4 were QPN and WSD. The SYBR Green real-time PCR analysis indicated that MmCTL4 was widely expressed in the hemocytes, hepatopancreas and mantle of healthy clams. After Vibrio splendidus stimulation, the temporal expression profile of MmCTL4 mRNA in hemocytes and hepatopancreas increased by 7.8-fold at 6 hpi and 3.9-fold at 12 hpi, respectively. The cDNA fragments encoding MmCTL4 were recombined into pET-32a (+) vectors, and transformed into Escherichia coli BL21 (DE3). The rMmCTL4 with the presence of Ca2+ performed obvious hemagglutination activity, and could agglutinate E. coli, Bacillus subtilis, and Staphylococcus aureus, while it only weakly agglutinate Vibrio parahaemolyticus and fungi P. pastoris. The agglutination activity of rMmCTL4 were significantly inhibited by D-mannose, D-xylose, D-lactose, maltose and lipopolysaccharides. These results indicated that MmCTL4, as a class of typical pattern recognition receptors (PRRs), could protect the host against pathogen invasion in the innate immunity of clams.

MicroRNA transcriptome analysis reveals the immune regulatory mechanism of Crassostrea hongkongesis against Vibrio harveyi infection

MicroRNAs (miRNAs) are small non-coding RNA molecules that modulate target-genes expression and play crucial roles in post-transcriptional regulation and immune system regulation. The Hong Kong oyster (Crassostrea hongkongesis), as the main marine aquaculture shellfish in the South China Sea, not only has high economic and ecological value, but also is an ideal model for conducting research on pathogen host interaction. Vibrio harveyi, a Gram negative luminescent marine bacterium, is widely distributed in coastal water environments and can cause large-scale death of C. hongkongesis. However, little in formation is available on the immune regulatory mechanisms of C. hongkongesis infected with V. harveyi. Therefore, we performed microRNA transcriptome analysis for elucidating the immunoregulation mechanism of C. hongkongesis infected with V. harveyi. The results show that a total of 308468208 clean reads and 288371159 clean tags were obtained. 222 differentially expressed miRNAs were identified. A total of 388 target genes that were differentially expressed and negatively correlated with miRNA expression were predicted by 222 DEmiRs. GO enrichment analysis of 388 DETGs showed that they were mainly enriched in the immune-related term of membrane-bounded vesicle, endocytic vesicle lumen, antigen processing and presentation of exogenous peptide antigen via MHC class I, antigen processing and presentation of peptide antigen via MHC class I, and other immune-related term. KEGG enrichment analysis showed that DETGs were mainly enriched in the Complement and coagulation cascades, Herpes simplex virus 1 infection, Bacterial invasion of epithelial cells, Antigen processing and presentation and NOD-like receptor signaling pathway. The 16 key DEmiRs and their target genes form a regulatory network for seven immune-related pathways. These results suggest that V. harveyi infection induces a complex miRNA response with wide-ranging effects on immune gene expression in the C. hongkongesis. This study explored the immune response of C. hongkongesis to V. harveyi infection at the level of miRNAs, which provides new ideas for the healthy culture and selective breeding of C. hongkongesis.

LDLa containing C-type lectin mediates phagocytosis of V.anguillarum and regulates immune effector genes in shrimp

C-type lectins (CTLs) function as pattern recognition receptors (PRRs) by recognizing invading microorganisms, thereby triggering downstream immune events against infected pathogens. In this study, a novel CTL containing a low-density lipoprotein receptor class A (LDLa) domain was obtained from Litopenaeus vannamei, designed as LvLDLalec. Stimulation by the bacterial pathogen Vibrio anguillarum (V. anguillarum) resulted in remarkable up-regulation of LvLDLalec, as well as release of LvLDLalec into hemolymph. The rLvLDLalec protein possessed broad-spectrum bacterial binding and agglutinating activities, as well as hemocyte attachment ability. Importantly, LvLDLalec facilitated the bacterial clearance in shrimp hemolymph and protected shrimp from bacterial infection. Further studies revealed that LvLDLalec promoted hemocytes phagocytosis against V. anguillarum and lysosomes were involved in the process. Meanwhile, LvLDLalec participated in humoral immunity through activating and inducing nuclear translocation of Dorsal to regulate phagocytosis-related genes and antimicrobial peptides (AMPs) genes, thereby accelerated the removal of invading pathogens in vivo and improved the survival rate of L. vannamei. These results unveil that LvLDLalec serves as a PRR participate in cellular and humoral immunity exerting opsonin activity to play vital roles in the immune regulatory system of L. vannamei.

A C-type lectin with a single carbohydrate-recognition domain (CRD) containing unique QPN/WDD motifs from Tegillarca granosa is involved in the innate immune defense

C-type lectins (CTLs), a superfamily of Ca2+-dependent carbohydrate-recognition proteins, serve as pattern recognition receptors (PRRs) in the immune response of many species. However, little is currently known about the CTLs of the commercially and ecologically important bivalve species, blood clam (Tegillarca granosa). In this study, a CTL (designated as TgCTL-1) with a single carbohydrate-recognition domain (CRD) containing unique QPN/WDD motifs was identified in the blood clam through transcriptome and whole-genome searching. Multiple alignment and phylogenetic analysis strongly suggested that TgCTL-1 was a new member of the CTL superfamily. Expression analysis demonstrated that TgCTL-1 was highly expressed in the hemocytes and visceral mass of the clam under normal condition. In addition, the expression of TgCTL-1 was shown to be significantly up-regulated upon pathogen challenge. Moreover, the recombinant TgCTL-1 (rTgCTL-1) displayed agglutinating and binding activities against both the gram-positive and gram-negative bacteria tested in a Ca2+-dependent manner. Furthermore, it was found that the in vitro phagocytic activity of hemocytes was significantly enhanced by rTgCTL-1. In general, our results showed that TgCTL-1 was an inducible acute-phase secretory protein, playing crucial roles in recognizing, agglutinating, and binding to pathogenic bacteria as well as modulating phagocytic activity of hemocytes in the innate immune defense of blood clam.

A C-type lectin from Crassostrea gigas with novel EFG/FVN motif involved in recognition of various PAMPs and induction of interleukin expression

C-type lectins (CTLs) are a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins, which participate in the nonself-recognition and triggering the transduction pathways in the innate immunity. In the present study, a novel CTL (designated as CgCLEC-TM2) with a carbohydrate-recognition domain (CRD) and a transmembrane domain (TM) was identified from the Pacific oyster Crassostrea gigas. Two novel EFG and FVN motifs were found in Ca²⁺-binding site 2 of CgCLEC-TM2. The mRNA transcripts of CgCLEC-TM2 were detected in all tested tissues with the highest expression level in haemocytes, which was 94.41-fold (p < 0.01) of that in adductor muscle. The relative expression level of CgCLEC-TM2 in haemocytes significantly up-regulated at 6 h and 24 h after the stimulation of Vibrio splendidus, which was 4.94- and 12.77-fold of that in control group (p < 0.01), respectively. The recombinant CRD of CgCLEC-TM2 (rCRD) was able to bind lipopolysaccharide (LPS), mannose (MAN), peptidoglycan (PGN), and poly (I: C) in a Ca²⁺-dependent manner. The rCRD exhibited binding activity to V. anguillarum, Bacillus subtilis, V. splendidus, Escherichia coli, Pichia pastoris, Staphylococcus aureus and Micrococcus luteus in a Ca²⁺-dependent manner. The rCRD also exhibited agglutination activity to E. coli, V. splendidus, S. aureus, M. luteus and P. pastoris in a Ca²⁺-dependent manner. The phagocytosis rate of haemocytes towards V. splendidus significantly down-regulated from 27.2% to 20.9% after treatment of anti-CgCLEC-TM2-CRD antibody, while the growth of V. splendidus and E. coli was inhibited compared with the TBS and rTrx groups. After the expression of CgCLEC-TM2 was inhibited by RNAi, the expression level of phospho-extracellular regulated protein kinases (p-CgERK) in haemocytes, and the mRNA expressions of interleukin17s (CgIL17-1 and CgIL17-4) decreased significantly after V. splendidus stimulation, compared with that in EGFP-RNAi oysters, respectively. These results suggested that CgCLEC-TM2 with novel motifs served as a pattern recognition receptor (PRR) involved in the recognition of microorganisms, and induction of CgIL17s expression in the immune response of oysters.

The ancient CgPEPCK-1, not CgPECK-2, evolved into a multifunctional molecule as an intracellular enzyme and extracellular PRR

Phosphoenolpyruvate carboxykinase (PEPCK) is a well-known lyase involved in gluconeogenesis, while their evolution and function differentiation have not been fully understood. In this study, by constructing a phylogenetic tree to examine PEPCKs throughout the evolution from poriferans to vertebrates, Mollusk was highlighted as the only phylum to exhibit two distinct lineages, Mollusca_PEPCK-1 and Mollusca_PEPCK-2. Further study of two representative members from Crassostrea gigas (CgPEPCK-1 and CgPEPCK-2) showed that they both shared conserved sequences and structural characteristics of the catalytic enzyme, while CgPEPCK-2 displayed a higher expression level than CgPEPCK-1 in all tested tissues, and CgPEPCK-1 was specifically implicated in the immune defense against LPS stimulation and Vibrio splendidus infection. Functional analysis revealed that CgPEPCK-2 had stronger enzymatic activity than CgPEPCK-1, while CgPEPCK-1 exhibited stronger binding activity with various PAMPs, and only the protein of CgPEPCK-1 increased significantly in hemolymph during immune stimulation. All results supported that distinct sequence and function differentiations of the PEPCK gene family should have occurred since Mollusk. The more advanced evolutionary branch Mollusca_PEPCK-2 should preserve its essential function as a catalytic enzyme to be more specialized and efficient, while the ancient branch Mollusca_PEPCK-1 probably contained some members, such as CgPEPCK-1, that should be integrated into the immune system as an extracellular immune recognition receptor.

A Genomic and Transcriptomic Analysis of the C-Type Lectin Gene Family Reveals Highly Expanded and Diversified Repertoires in Bivalves

C-type lectins belong to a widely conserved family of lectins characterized in Metazoa. They show important functional diversity and immune implications, mainly as pathogen recognition receptors. In this work, C-type lectin-like proteins (CTLs) of a set of metazoan species were analyzed, revealing an important expansion in bivalve mollusks, which contrasted with the reduced repertoires of other mollusks, such as cephalopods. Orthology relationships demonstrated that these expanded repertoires consisted of CTL subfamilies conserved within Mollusca or Bivalvia and of lineage-specific subfamilies with orthology only between closely related species. Transcriptomic analyses revealed the importance of the bivalve subfamilies in mucosal immunity, as they were mainly expressed in the digestive gland and gills and modulated with specific stimuli. CTL domain-containing proteins that had additional domains (CTLDcps) were also studied, revealing interesting gene families with different conservation degrees of the CTL domain across orthologs from different taxa. Unique bivalve CTLDcps with specific domain architectures were revealed, corresponding to uncharacterized bivalve proteins with putative immune function according to their transcriptomic modulation, which could constitute interesting targets for functional characterization.

A single-CRD C-type lectin from Haliotis discus hannai acts as pattern recognition receptor enhancing hemocytes opsonization

C-type lectins (CTLs), as a member of the Ca²⁺-dependent carbohydrate recognition protein superfamily, play multiple roles in non-self recognition and the elimination of invading pathogens. In this study, a C-type lectin was identified and characterized from the Pacific abalone Haliotis discus hannai (designed as HdClec), and its open reading frame (ORF) encoded a polypeptide of 163 amino acids containing a typical signal peptide and only one carbohydrate-recognition domain (CRD). The deduced amino acid sequence of CRD in HdClec shared identities ranging from 22.4% to 39.8% with that of other identified CRDs of CTLs. A novel NPN motif was found in Ca²⁺-binding site 2 of HdClec. The mRNA transcripts of HdClec were detectable in all the examined tissues of non-stimulated abalones, with the highest expression in hepatopancreas (224.13-fold of that in gills). The expression of HdClec mRNA in hemocytes was significantly up-regulated after Vibrio harveyi challenge. Recombinant HdClec protein (rHdClec) could bind lipopolysaccharide (LPS) and peptidoglycan (PGN) in vitro in the presence of Ca²⁺. Coinciding with the PAMPs binding assay, rHdClec displayed broad agglutination activities towards Gram-negative bacteria V. splendidus, V. anguillarum, V. parahaemolyticus, V. harveyi, Escherichia coli, and Gram-positive bacteria Micrococcus luteus. Moreover, rHdClec could significantly elicit the chemotactic response of hemocytes in vitro. And the phagocytosis and encapsulation ability of hemocytes could be significantly enhanced by rHdClec. All these results showed that HdClec could function as pattern recognition receptors (PRRs) and further enhance the opsonization of hemocytes, which might play a crucial role in the innate immune responses of Pacific abalone.

CLec-TM1-ERK-GSK3β Pathway Regulates Vibrio splendidus-Induced IL-17 Production in Oyster

C-type lectins are a family of pattern recognition receptors that recognize microbial components and subsequently activate the signaling cascade to induce the production of proinflammatory cytokines. In the current study, the homologs of ERK (named as CgERK) and GSK3β (named as CgGSK3β) and a novel C-type lectin with a transmembrane domain (named as CgCLec-TM1) were identified from oyster Crassostrea gigas CgCLec-TM1 was able to bind Escherichia coli and Vibrio splendidus through its carbohydrate recognition domain and then activated CgERK by inducing its phosphorylation. The activated CgERK interacted with CgGSK3β to phosphorylate it at Ser⁹, which eventually induced the expressions of CgIL-17-1 and CgIL-17-5. The interaction between CgERK and CgGSK3β, as well as the phosphorylation of CgGSK3β, could be inhibited by ERK inhibitor (PD98059) to reduce the expressions of CgIL-17-1 and CgIL-17-5. CgGSK3β in oyster was proposed as a new substrate of CgERK. The results defined a CLec-TM1-ERK-GSK3β signaling pathway in oyster, which was activated by V. splendidus and then induced CgIL-17 productions.

C-type lectin with a QPN motif from swimming crab Portunus trituberculatus displays broad nonself-recognition ability and functions as an opsonin

In the immune system, C-type lectins, as pattern recognition receptors, have an important function. Carbohydrate-recognition domains (CRDs) endow C-type lectins with the function of recognizing and scavenging non-self factors. In the present study, a new C-type lectin (designated as PtCTL-9 according to the order of discovery) from swimming crab (Portunus trituberculatus) was characterized. QPN (Gln-Pro-Asn) and FHS (Phe-His-Ser) were identified as the key motifs that determine carbohydrate binding. Motif QPN was mutated to QPD (Gln-Pro-Asp) (M1) and EPN (Glu-Pro-Asn) (M2) to study its immune function and for comparative analysis. The results showed that PtCTL-9 displayed broad non-self immunity. PtCTL-9 could also function as an opsonin to promote phagocytosis and the in vitro encapsulation of hemocytes. These results indicated that PtCTL-9 has an extensive nonself-recognition ability, regulates pathogen clearance, and its QPN motif is important in PtCTL-9's immune function.

A novel C-type lectin from Crassostrea gigas involved in the innate defense against Vibrio alginolyticus

C-type lectins (CTLs) are important immune molecules that participate in invertebrate defense response. In the present work, a novel structural CTL (CgLec-4E) was identified from Crassostrea gigas, which encodes 237 amino acids (aa) with an extra long chain of aa and in the C-type CRD domain with EPA, QPG and WHD mutated motifs respectively. rCgLec-4E could agglutinate and inhibit the growth of Vibrio alginolyticus, except Chlorella, which might be relevant to three mutated motifs. CgLec-4E was mainly expressed in digestive gland, and its expression level was significantly up-regulated post V. alginolyticus challenge, indicating that the high expression of CgLec-4E could provide necessary mucosal immune protections and might involve in food particle recognition for C. gigas. Moreover, the subcellular locations indicated that CgLec-4E might play different roles in the immune response. Taken together, our results enrich our understanding of the structures and function of CTLs in invertebrates.

A C-Type Lectin Highly Expressed in Portunus trituberculatus Intestine Functions in AMP Regulation and Prophenoloxidase Activation

A C-type lectin (PtCLec2) from Portunus trituberculatus was identified for characterization of its role in defense and innate immunity. PtCLec2 contains a single carbohydrate-recognition domain (CRD) with a conserved QPD motif, which was predicted to have galactose specificity. The mRNA expression of PtCLec2 was predominantly detected in intestine and increased rapidly and significantly upon pathogen challenge. The recombinant PtCLec2 (rPtCLec2) could bind various microorganisms and PAMPs with weak binding ability to yeast and PGN. It agglutinated the tested Gram-negative bacteria (Vibrio alginolyticus and Pseudomonas aeruginosa), Gram-positive bacteria (Staphylococcus aureus and Micrococcus luteus), and rabbit erythrocytes in the presence of exogenous Ca²⁺, and these agglutination activities were suppressed by LPS, d-galactose, and d-mannose. Further, rPtCLec2 enhanced phagocytosis and clearance of V. alginolyticus, and displayed inhibitory activities against the tested bacteria. Knockdown of PtCLec2 decreased the transcription of two phagocytosis genes (PtArp and PtMyosin), three prophenoloxidase (proPO) system-related genes (PtPPAF, PtcSP1, and PtproPO), six antimicrobial peptides (AMPs) (PtALF4-7, PtCrustin1, and PtCrustin3), and PtRelish but upregulated the expression levels of PtJNK, PtPelle, and PtTLR. These results collectively indicate that PtCLec2 might perform its immune recognition function via binding and agglutination, and mediate pathogen elimination via regulating hemocyte phagocytosis, AMP synthesis, and proPO activation.

Identification of a Novel Pattern Recognition Receptor DM9 Domain Containing Protein 4 as a Marker for Pro-Hemocyte of Pacific Oyster Crassostrea gigas

DM9 refers to an uncharacterized protein domain that is originally discovered in Drosophila melanogaster. Two proteins with DM9 repeats have been recently identified from Pacific oyster Crassostrea gigas as mannose-specific binding pattern-recognition receptors (PRRs). In the present study, a novel member of DM9 domain containing protein (designated as CgDM9CP-4) was identified from C. gigas. CgDM9CP-4, about 16 kDa with only two tandem DM9 domains, was highly enriched in hemocytes and gill. The transcripts level of CgDM9CP-4 in circulating hemocytes were decreased after LPS, PGN and Vibrio splendidus stimulations. The recombinant protein of CgDM9CP-4 (rCgDM9CP-4) displayed a broad binding spectrum towards various pathogen-associated molecular patterns (PAMPs) (LPS, PGN, β-glucan and Mannose) and microorganisms (Staphylococcus aureus, Micrococcus luteus, V. splendidus, V. anguillarum, Escherichia coli, Pichia pastoris and Yarrowia lipolytica). CgDM9CP-4 was mostly expressed in gill and some of the hemocytes. Flow cytometry analysis demonstrated that the CgDM9CP-4-positive hemocytes accounted for 7.3% of the total hemocytes, and they were small in size and less in granularity. CgDM9CP-4 was highly expressed in non-phagocytes (~82% of total hemocytes). The reactive oxygen species (ROS) and the expression levels of cytokines in CgDM9CP-4-positive hemocytes were much lower than that in CgDM9CP-4-negative hemocytes. The mRNA expression level of CgDM9CP-4 in hemocytes was decreased after RNAi of hematopoietic-related factors (CgGATA, CgRunt, CgSCL, and CgNotch). In addition, CgDM9CP-4-positive cells were found to be much more abundant in hemocytes from gill than that from hemolymph, with most of them located in the gill filament. All these results suggested that CgDM9CP-4 was a novel member of PRR that expressed in undifferentiated pro-hemocytes to mediate immune recognition of pathogens.

Antimicrobial properties and immune-related gene expression of a C-type lectin isolated from Pinctada fucata martensii

C-type lectins are carbohydrate-binding proteins that play important roles in the innate immune response to pathogen infections. Here, multi-step high performance liquid chromatography (HPLC), combined with mass spectrometry (MS), was used to isolate and identify proteins with antibacterial activity from the serum of Pinctada fucata martensii. Using this method, we obtained a novel isoform of C-type lectin (PmCTL-1). PmCTL-1 strongly inhibited gram-positive bacteria. The complete cDNA sequence of PmCTL-1 was 636 bp in length, and encoded a protein 149 amino acids long, containing a typical carbohydrate recognition domain (CRD). A phylogenetic analysis based on a multiple sequence alignment indicated that PmCTL-1 was highly similar to C-type lectins from other mollusks. Fluorescent quantitative real-time PCR analysis showed that PmCTL-1 mRNA was strongly upregulated in the mantle of healthy P.f. martensii, but was expressed only at low levels in the gill, gonad, hepatopancreas, adductor muscle, and hemocytes. PmCTL-1 expression levels in the mantle and hemocytes increased significantly in response to bacterial stimulation. This study provides a valuable framework for further explorations of innate immunity and the immune response in mollusks.

Involvement of a novel Ca2+-independent C-type lectin from Sinonovacula constricta in food recognition and innate immunity

Bivalve lectins perform a crucial function in recognition of foreign particles, such as microalgae and pathogenic bacteria. In this study, a novel C-type lectin form Sinonovacula constricta (ScCL) was characterized. The full-length cDNA of ScCL was 1645 bp, encoding a predicted polypeptide of 273 amino acids with one typical carbohydrate-recognition domain. ScCL has the highest similarity and closest phylogenetic relationship with the C-type lectin from Solen grandis. Real-time PCR analysis showed that ScCL was expressed in all tested tissues, with the highest expression in the foot and the lowest expression in hemocytes. Agglutination activity of ScCL was Ca²⁺-independent. ScCL showed the strongest agglutination on Chlorella vulgaris, the modest agglutination on Platymonas subcordiformis, Nannochloropsis sp., and Thalassiosira pseudonana, the weakest agglutination on Chaetoceros sp., and no agglutination on Isochrysis zhanjiangensis. Meanwhile, agglutination tests and western blot analysis revealed that the recombinant ScCL protein could agglutinate Staphylococcus aureus and Vibrio harveyi, but could not agglutinate Vibrio anguillarum, Bacillus cereus, or Vibrio parahaemolyticus. Furthermore, ScCL had a high binding activity with LPS and mannose, a low binding activity with LTA, and no binding activity with PGN. The expression of ScCL in the gill of S. constricta fed with C. vulgaris and T. pseudonana was significantly increased at 1 and/or 3 h. After injection with S. aureus, the expression of ScCL in the gill was significantly increased at 3, 6, and 24 h. These results indicated that ScCL was involved in food particle recognition and immunity of S. constricta.

Effect of pathogenic bacteria on a novel C-type lectin, hemocyte and superoxide dismutase/ alkaline phosphatase activity in Onchidium reevesii

Bacterial infection in the marine environment is a serious problem to maintain the stability of marine ecosystems. Nevertheless, there is little report so far for the biological effects of pathogenic bacteria in coastal ecosystems. Hence, we investigated the responses of shell-less Onchidium reevesii to resist against pathogenic bacterial infection. Analysis of data here could be used as fundamental information for assessment of innate immune response of O. reevesii. The full-length OrCTL cDNA was cloned and consists of 1849 base pair (bp) encoding protein of 192 amino acids. Constructing multiple alignments suggested that OrCTL has conserved carbohydrate recognition domain (CRD) of CTLs, containing an EPS (Glu-Pro-Ser) motif that may imply the function of recognition of carbohydrates like others invertebrate. OrCTL mRNAs were mainly detected in ganglion and hepatopancreas, and expression was highly up-regulated from 2 h after Vibrio harveyi challenge, rapidly decreased at 4 h, and significantly increased at 12 h. In addition, after challenge with Vibrio parahaemolytics, OrCTL gene expression was slightly up-regulated from 2 h, peaked at 12 h. Enzyme activity (in the hepatopancreas) and cell immune (in the hemolymph) were investigated along with Superoxide dismutase (SOD) activity, alkaline phosphatase (ALP) activity and cell cycle. SOD activities were significantly higher after V. harveyi and V. parahaemolytics challenge than that in the control group, respectively. By contrast, ALP activities were significantly inhibited after challenged with bacteria than that in the control group, respectively. Enzyme activities in the hepatopancreas obviously fluctuated, and ALP activity was more sensitive to bacteria. Cell responses illustrated that there were a significant higher percentage of cells in the S and G2/M phase in hemolymph after challenged with bacteria. Our results suggested that the immune response of O. reevesii could be activated by pathogenic bacteria, and the data will provide referent for the disease prevention of systematic investigation in aquatic animal.

A novel C-type lectin with a YPD motif from Portunus trituberculatus (PtCLec1) mediating pathogen recognition and opsonization

C-type lectins are a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins that function as pattern recognition receptors (PRRs) in innate immune system. In this study, a new C-type lectin was identified from the swimming crab Portunus trituberculatus (PtCLec1). The full-length cDNA of PtCLec1 was 873 bp encoding 176 amino acids. The predicted PtCLec1 protein contained a signal peptide and a single carbohydrate-recognition domain with a special YPD motif. The PtCLec1 transcripts were mainly detected in hepatopancreas and its relative expression levels were significantly up-regulated after the challenges of Vibrio alginolyticus, Micrococcus luteus and Pichia pastoris. The recombinant PtCLec1 (rPtCLec1) could bind all the tested pathogen-associated molecular patterns (PAMPs), including lipopolysaccharides (LPS), peptidoglycan (PGN) and glucan (GLU), and microorganisms, including V. alginolyticus, V. parahaemolyticus, Pseudomonas aeruginosa, Staphylococcus aureus, M. luteus and P. pastoris. It also exhibited strong activity to agglutinate bacteria and yeast in a Ca²⁺-dependent manner, and such agglutinating activity could be inhibited by d-galactose and LPS. Moreover, rPtCLec1 revealed antimicrobial activity against the tested Gram-negative (V. alginolyticus, V. parahaemolyticus and P. aeruginosa) and Gram-positive bacteria (S. aureus and M. luteus), and promoted the clearance of V. alginolyticus in vivo and hemocyte phagocytosis in vitro. Knockdown of PtCLec1 could down-regulate the expression of phagocytosis-related genes, but enhance the expression levels of prophenoloxidase (proPO) system-related genes, mannose-binding lectin (MBL), antimicrobial peptides (AMPs), MyD88 and Relish. All these results indicate that PtCLec1 might act as a PRR in immune recognition and an opsonin in pathogen elimination.

The sensing pattern and antitoxic response of Crassostrea gigas against extracellular products of Vibrio splendidus

Serious juvenile oyster disease induced by pathogenic Vibrio splendidus has resulted in tremendous economic loss, but the molecular mechanisms underlying this killing mechanism remain unclear. The resistance of adult oyster to V. splendidus or its virulence factors might provide a possible access to cognize the interaction between pathogen and host. In the present study, the extracellular products (ECP) from less virulent V. splendidus JZ6 were injected into adult Pacific oyster Crassostrea gigas, and the cellular and humoral immune response induced by ECP were investigated. The phagocytosis rate of hemocytes was significantly up-regulated (30.57%) at 6 h after ECP injection compared with that (21%) of control groups. And significantly high level of ROS production was also observed from 3 h to 12 h in ECP-injected oysters, concomitant with increased apoptosis rate of hemocytes (16.4% in ECP-injected group, p < 0.01) compared with control group (6.7%). By RT-PCR analysis, the expression level of antioxidant CgSOD in hemocytes significantly increased to 6.41-fold of that in control groups (p < 0.01) at 12 h post ECP injection. The expression levels of anti-toxic metalloprotease inhibitors CgTIMP629 and CgTIMP628 were also significantly up-regulated at the early (3-6 h) and late (6-24 h) stage of immune response, respectively. Moreover, after the ECP were incubated with serum proteins isolated from the ECP-injected oysters in vitro, the metalloprotease activity of ECP significantly declined by 21.39%, and less degraded serum proteins were detected by SDS-PAGE. When the primarily cultured hemocytes were stimulated with heat-inactivated ECP or fragments derived from ECP-degraded serum proteins, the expressions of CgTIMP629 (13.64 and 7.03-fold of that in saline group, respectively, p < 0.01) and CgTIMP628 (5.07 and 6.08-fold of that in saline group, respectively, p < 0.01) in hemocytes were all significantly induced. All the results indicated that the adult oysters could launch phagocytosis, antioxidant and anti-toxic response to resist the virulence of ECP, possibly by sensing heterologous ECP and ECP-induced endogenous alarm signals. These results provided a possible clue for the resistance mechanism of adult oysters towards the ECP of less virulent V. splendidus, which might be valuable for exploring strategies for the control of oyster disease.

Functional conservation and division of two single-carbohydrate-recognition domain C-type lectins from the nipa palm hispid beetle Octodonta nipae (Maulik)

As an invasive pest, the complete and effective innate immune system is crucial for the nipa palm hispid beetle Octodonta nipae (Maulik) to adjust to new environments. C-type lectins (CTLs) are large families of carbohydrate-binding proteins that possess one or more characteristic carbohydrate-recognition domains (CRD) and function as pattern-recognition receptors, which play important roles in mediating humoral and cellular immunity. In the present study, for the first time, we report two CTL-Ss (single-CRD CTLs) from O. nipae (Maulik) (designated OnCTL1 and OnCTL2). The two CTL-Ss share high identity at conserved amino acids associated with conserved carbohydrate binding sites Gln-Pro-Asp (QPD) motifs and clearly show a 1:1 orthologous relationship in insects, which endow them with functional conservation and diversification. mRNA abundance analysis showed that OnCTL1 was upregulated upon Staphylococcus aureus and Escherichia coli challenge at 6 and 12 h, while OnCTL2 underwent no changes upon E. coli challenge and was even downregulated after S. aureus infection. Knockdown of OnCTL1 significantly decreased the transcripts of two key serine proteases (prophenoloxidase activating factors), OnPPAF1 and OnPPAF3, followed by the reduction of haemolymph phenoloxidase activity; it also increased the expression of Defensin2B. In contrast, silencing of OnCTL2 significantly decreased the expression of Defensin2B and Attacin3C, the encapsulation index, and the phagocytosis rate compared to the dsEGFP group. The spreading results showed that more irregularly shaped plasmatocytes and lower levels of aggregation were found in OnCTL2-silenced pupae than in the dsOnCTL1 and dsEGFP groups. We can infer from the results of this study that the two OnCTLs play important roles in the immune system and generate a functional division: OnCTL1 seems to function more in humoral immunity including mediating bacterial recognition and activating the phenoloxidase cascade, and OnCTL2 plays a greater role in enhancing cellular immunity. These observations could replenish information on the functional diversification of insect CTLs, and also provide valuable information to unravel the immunity in O. nipae.

A single-CRD C-type lectin (CgCLec-3) with novel DIN motif exhibits versatile immune functions in Crassostrea gigas

C-type lectins (CTLs), as important pattern recognition receptors (PRRs), are a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins which participate in nonself-recognition and eliminating pathogens. In the present study, a novel CTL (designated as CgCLec-3) was identified from the Pacific oyster Crassostrea gigas. There was only one carbohydrate-recognition domain (CRD) of 151 amino acid residues within the deduced amino acid sequence of CgCLec-3. The deduced amino acid sequence of CgCLec-3 CRD shared low homology with the CRDs of other CTLs in oyster with the identities ranging from 12% to 22%. A novel DIN motif was found in Ca²⁺-binding site 2 of CgCLec-3. The relative expression level of CgCLec-3 in hemocytes was up-regulated significantly after the stimulations of bacteria and Pathogen Associated Molecular Patterns (PAMPs). Immunohistochemistry assay showed that CgCLec-3 protein was mainly distributed in gill and mantle, less in gonad, and could not be detected in adductor muscle and hepatopancreas. The recombinant protein (rCgCLec-3) could bind lipopolysaccharide (LPS), mannose (MAN) and peptidoglycan (PGN), but not poly (I:C). rCgCLec-3 exihibited strong binding ability to Vibrio anguillarum and V. splendidus, moderate binding activities to Escherichia coli, Pichia pastoris and Yarrowia lipolytica, weak binding affinity to Staphylococcus aureus and Micrococcus luteus. rCgCLec-3 could agglutinate microorganisms, in a Ca²⁺-dependent manner and its activity to agglutinate V. splendidus was remarkably higher than that to agglutinate E. coli, S. aureus and P. pastoris. The phagocytic activity of oyster hemocytes was significantly enhanced after incubation with rCgCLec-3. rCgCLec-3 also exhibited antibacterial activity against E. coli and S. aureus. The results clearly suggested that CgCLec-3 in Pacific oyster C. gigas not only served as a PRR involved in the PAMPs recognition and microbes binding, but also functioned as an immune effector participating in the clearance of invaders.

Immune Tolerance in Mytilus galloprovincialis Hemocytes After Repeated Contact With Vibrio splendidus

Mediterranean mussels (Mytilus galloprovincialis) are sessile filter feeders that live in close contact with numerous marine microorganisms. As is the case in all invertebrates, mussels lack an adaptive immune system, but they respond to pathogens, injuries or environmental stress in a very efficient manner. However, it is not known if they are able to modify their immune response when they reencounter the same pathogen. In this work, we studied the transcriptomic response of mussel hemocytes before and after two consecutive sublethal challenges with Vibrio splendidus. The first exposure significantly regulated genes related to inflammation, migration and response to bacteria. However, after the second exposure, the differentially expressed genes were related to the control and inhibition of ROS production and the resolution of the inflammatory response. Our results also show that the second injection with V. splendidus led to changes at the transcriptional (control of the expression of pro-inflammatory transcripts), cellular (shift in the hemocyte population distribution), and functional levels (inhibition of ROS production). These results suggest that a modified immune response after the second challenge allowed the mussels to tolerate rather than fight the infection, which minimized tissue damage.

HcCUB-Lec, a newly identified C-type lectin that contains a distinct CUB domain and participates in the immune defense of the triangle sail mussel Hyriopsis cumingii

As pattern recognition receptors (PRRs), C-type lectins (CTLs) play crucial roles in recognizing and eliminating pathogens in innate immunity. In this study, a novel CTL (HcCUB-Lec) was identified from the triangle sail mussel Hyriopsis cumingii. The full-length of HcCUB-Lec cDNA was 1558 bp with an open reading frame of 1281 bp that encodes a putative protein of 426 amino acid residues, including an N-terminal signal peptide, a complement Uegf Bmp1 (CUB) domain, a single carbohydrate recognition domain (CRD), and a transmembrane domain. Quantitative real-time PCR analysis revealed that HcCUB-Lec transcript was distributed in all examined tissues with the highest levels in hepatopancreas and was significantly upregulated in gills and hepatopancreas after immune challenge with Staphyloccocus aureus and Vibrio parahaemolyticus. When HcCUB-Lec was silenced by RNAi, the expression levels of three antimicrobial peptides, including whey acidic protein (HcWAP), defensin (HcDef), and lysozyme (HcLyso), were dramatically decreased in gills. The recombinant HcCUB-Lec and its individual CUB and CRD domains can bind with Gram-positive bacteria (S. aureus and Bacillus subtilis), Gram-negative bacteria (V. parahaemolyticus and Aeromonas hydrophila), and polysaccharides (lipopolysaccharide and peptidoglycan). Moreover, rHcCUB-Lec and its domains could also agglutinate S. aureus and V. parahaemolyticus in the presence of Ca²⁺ and can clear V. parahaemolyticus in H. cumingii. Results of this study suggest that HcCUB-Lec acts as an antimicrobial PRR that participates in the innate immune responses of H. cumingii.

The transcriptomic expression of pattern recognition receptors: Insight into molecular recognition of various invading pathogens in Oyster Crassostrea gigas

Pattern recognition receptors (PRRs) are essential in recognizing specific pathogen-associated molecular patterns (PAMPs) on microbes and triggering responses to eliminate the invading pathogens. Previous genomic studies have revealed a great number of PRR genes in the Pacific oyster Crassostrea gigas, a sessile and filter-feeder marine bivalve belonging to the phylum Mollusca. On the survey of PRRs in the assembly oyster reference genome version 9, a total of 1084 PRRs were identified, which were composed of at least 12 gene families. Some of the gene families were significantly expanded, including C-type lectins (CTLs), fibrinogen-related proteins (FREPs), scavenger receptor cysteine-rich repeat protein (SRCRs), leucine-rich repeat (LRR)-only proteins (LRRops), and especially C1q domain-containing proteins (C1qDCs). The transcriptomic profiles of these abundant PRRs in response to PAMP treatments were investigated by RNA-Seq using the SOLiD EZ BeadTM system. Compared to the control library, there were 6,655, 7,273, 7,593, 6,830, 6687 and 8250 differentially expressed genes in the haemocytes of oysters in response to lipopolysaccharide (LPS) stimulation for 6 h, 12 h and 24 h, and peptidoglycan (PGN), glucan (GLU) and poly I:C (IC) stimulation for 12 h, respectively. After stimulation for 12 h, there were 134, 97, 114 and 159 genes up-regulated in the LPS, PGN, GLU and IC library, respectively. Most of the gene families involved in immune response towards PAMPs were C1qDCs, CTLs and FREPs, while only a few members of LRR and immunoglobin-containing proteins (LRRIGs), retinoic acid-inducible gene I [RIG-I]-like receptors (RLRs) and Toll like receptors (TLRs) were up-regulated. After LPS stimulation, the expression level of 258 non-redundant PRR genes in oyster haemocytes increased significantly with different expression pattern, and most of them were C1qDCs, CTLs, LRRops and FREPs. The transcriptomic analyses indicated that there was a dynamic and orchestrated specific expression regulation of numerous PRR genes in response to pathogen invasion. The expanded PRR gene family members were differentiated with more specific functional responses to certain PAMPs rather than the versatile ones. Based on the different expression pattern during the LPS stimulation, the oyster PRRs could be assigned into three consecutive steps in the response against pathogen invading. All the results would provide useful information for future studies of oyster PRRs and deep insight into the researches on invertebrate innate immunity.

Novel C-type lectin from razor clam Sinonovacula constricta agglutinates bacteria and erythrocytes in a Ca2+-dependent manner

Among its other physiological roles, C-type lectins functioned as pattern recognition receptors (PRR) in innate immunity received much attention. In the present study, a novel C-type lectin was identified and characterized from the invertebrate razor clam Sinonovacula constrict and designated as ScCTL. The complete cDNA sequence of ScCTL was 828 bp in length and coded a secreted polypeptide of 158 amino acids with a typical CRD domain. Multiple sequence alignments combined with phylogenetic analysis both collectively confirmed that ScCTL was a novel member belong to lectin family. Spatial expression distribution analysis revealed that ScCTL was extensively expressed in all of the examined tissues, and the highest expression was detected in the hepatopancreas. After 1 × 10⁷ CFU/mL Vibrio parahaemolyticus challenge by immersion infection, the ScCTL transcript in hepatopancreas and gill were markedly upregulated and arrived the maximum levels at 24 or 12 h after challenge, respectively. Recombinant ScCTL could agglutinate not only all tested bacteria but sheep and mouse erythrocyte in the presence of Ca²⁺. All of our studies suggested that ScCTL performed important roles in protecting cells from pathogenic infection in S. constrict.

FmLC6: An ultimate dual-CRD C-type lectin from Fenneropenaeus merguiensis mediated its roles in shrimp defense immunity towards bacteria and virus

C-type lectins are a member of pattern recognition receptors (PRRs) that can interact with pathogen-associated molecular patterns of invading microorganisms by using their conserved motifs in carbohydrate recognition domain (CRD). The binding can trigger various immune responses in both direct and indirect mechanisms. Hereby, an ultimate C-type lectin with dual CRDs each of which containing a different motif was identified from hepatopancreas of Fenneropenaeus merguiensis (mentioned as FmLC6). The full-length cDNA of FmLC6 consisted of 1148 bp comprising one 1005 bp open reading frame (ORF) encoding a signal peptide and a mature protein of 317 residues. FmLC6 was composed of two CRDs with a highly conserved QPD (Gln-Pro-Asp) motif and one variant EPQ (Glu-Pro-Gln) motif for illustrating the carbohydrate binding affinity. The transcription of FmLC6 was detected only in hepatopancreas of normal shrimp. After injection with pathogens or immunostimulants, the expression of FmLC6 was significantly up-regulated and reached the highest level at 12 h post-injection except with lipoteichoic acid challenge. The FmLC6 expression was severely suppressed by knockdown based-silencing. This gene silencing with co-injection by Vibrio parahaemolyticus caused increasing in cumulative mortality and reduction of the median lethal time. Purified recombinant proteins of an entire ORF and two individual CRDs of FmLC6 produced in Escherichia coli could induce a broad spectrum of microbial agglutination with calcium dependence. The agglutination induced by rFmLC6, rCRD1 and rCRD2 was suppressed by galactose plus mannose, galactose and mannose, respectively which this event was confirmed by the inhibition of hemagglutination. All three recombinant proteins possessed ability to inhibit the bacterial growth with a dose-response. Purified rFmLC6 could bind directly to white spot syndrome virus particles and also its recombinant proteins including VP15, VP39A and VP28 with different affinity. Altogether, these results indicate that FmLC6 acts as a PRR to recognize invading microorganisms and leads to mediating the immune response to cooperation in pathogenic elimination via the binding, agglutination and antimicrobial activity.

First characterization of two C-type lectins of the tubeworm Alaysia sp. from a deep-sea hydrothermal vent

C-type lectins (CTLs) play an important role in innate immune defense. In this study, we identified and characterized two CTLs (Lec1 and Lec2) from the tubeworm Alaysia sp. collected from a hydrothermal vent in Pacmanus. Lec1 and Lec2 possess the typical CTL domain but share low sequence identities (10.8%-20.4%) with known CTLs. Recombinant (r) of Lec1 and Lec2 bound to various PAMPs and a wide arrange of bacteria from neritic and deep-sea environments in a Ca²⁺-independent manner, but only rLec1 caused agglutination of the bound bacteria. The activities of rLec1 and rLec2 were most stable and highest at 4 °C, the ambient temperature of the hydrothermal vent, and decreased at higher temperatures. Both lectins inhibited bacterial growth in a highly selective manner and agglutinated the erythrocytes of fish, rabbit, and chicken in a Ca²⁺-dependent manner. These results provided the first insights into the functional properties of CTLs in deep-sea Alaysia sp.

Pathogen-Derived Carbohydrate Recognition in Molluscs Immune Defense

Self-nonself discrimination is a common theme for all of the organisms in different evolutionary branches, which is also the most fundamental step for host immune protection. Plenty of pattern recognition receptors (PRRs) with great diversity have been identified from different organisms to recognize various pathogen-associated molecular patterns (PAMPs) in the last two decades, depicting a complicated scene of host-pathogen interaction. However, the detailed mechanism of the complicate PAMPs–PRRs interactions at the contacting interface between pathogens and hosts is still not well understood. All of the cells are coated by glycosylation complex and thick carbohydrates layer. The different polysaccharides in extracellular matrix of pathogen-host are important for nonself recognition of most organisms. Coincidentally, massive expansion of PRRs, majority of which contain recognition domains of Ig, leucine-rich repeat (LRR), C-type lectin (CTL), C1q and scavenger receptor (SR), have been annotated and identified in invertebrates by screening the available genomic sequence. The phylum Mollusca is one of the largest groups in the animal kingdom with abundant biodiversity providing plenty of solutions about pathogen recognition and immune protection, which might offer a suitable model to figure out the common rules of immune recognition mechanism. The present review summarizes the diverse PRRs and common elements of various PAMPs, especially focusing on the structural and functional characteristics of canonical carbohydrate recognition proteins and some novel proteins functioning in molluscan immune defense system, with the objective to provide new ideas about the immune recognition mechanisms.

The oyster immunity

Oysters, the common name for a number of different bivalve molluscs, are the worldwide aquaculture species and also play vital roles in the function of ecosystem. As invertebrate, oysters have evolved an integrated, highly complex innate immune system to recognize and eliminate various invaders via an array of orchestrated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, as well as encapsulation and phagocytosis of the circulating haemocytes. The hematopoietic tissue, hematopoiesis, and the circulating haemocytes have been preliminary characterized, and the detailed annotation of the Pacific oyster Crassostrea gigas genome has revealed massive expansion and functional divergence of innate immune genes in this animal. Moreover, immune priming and maternal immune transfer are reported in oysters, suggesting the adaptability of invertebrate immunity. Apoptosis and autophagy are proved to be important immune mechanisms in oysters. This review will summarize the research progresses of immune system and the immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, neuropeptides, GABAergic and nitric oxidase system, which possibly make oysters ideal model for studying the origin and evolution of immune system and the neuroendocrine-immune regulatory network in lower invertebrates.

A GTP-dependent Phosphoenolpyruvate Carboxykinase from Crassostrea gigas Involved in Immune Recognition

Phosphoenolpyruvate carboxykinase (PEPCK) is well known as a key enzyme involved in the metabolic pathway of gluconeogenesis in organisms, but the information about its involvement in immune response is still very limited. In the present study, a novel PEPCK homolog named CgPEPCK was identified from oyster Crassostrea gigas. The deduced amino acid sequence of CgPEPCK shared 52%-74% similarities with those from other known PEPCKs. There were one conserved guanosine triphosphate (GTP) binding site, one substrate binding site, one metal binding site and one active site in CgPEPCK. The mRNA transcripts of CgPEPCK were constitutively expressed in all the tested tissues including hemolymph, mantle, gill, muscle, gonad and hepatopancreas. CgPEPCK proteins were mainly distributed in adductor muscle, gonad, gill and mantle, and rarely detected in hepatopancreas by using immunohistochemical analysis. After the stimulations with lipopolysaccharide (LPS), peptidoglycan (PGN), Vibrio splendidus and V. anguillarum, CgPEPCK transcripts in hemocytes were significantly up-regulated and peaked at 6 h (LPS, 9.62-fold, p < 0.01), 9 h (PGN, 4.25-fold, p < 0.01), 12 h (V. splendidus, 5.72-fold, p < 0.01), 3 h (V. anguillarum, 2.87-fold, p < 0.01), respectively. The recombinant CgPEPCK protein (rCgPEPCK) exhibited Mn²⁺/Mg²⁺ dependent GTP binding activity, and the activities to bind LPS and PGN, but not β-1,3-glucan (GLU), lipoteichoic acid (LTA), mannan (MAN) nor polyinosinic-polycytidylic (Poly I: C). It could also bind Escherichia coli, Staphylococcus aureus, Micrococcus luteus and significantly inhibit their growth. All these results collectively suggested that CgPEPCK could not only exert GTP binding activity involved in gluconeogenesis, but also mediate the bacteria recognition and clearance in immune response of oysters.

High-Throughput Identification of Antimicrobial Peptides from Amphibious Mudskippers

Widespread existence of antimicrobial peptides (AMPs) has been reported in various animals with comprehensive biological activities, which is consistent with the important roles of AMPs as the first line of host defense system. However, no big-data-based analysis on AMPs from any fish species is available. In this study, we identified 507 AMP transcripts on the basis of our previously reported genomes and transcriptomes of two representative amphibious mudskippers, Boleophthalmus pectinirostris (BP) and Periophthalmus magnuspinnatus (PM). The former is predominantly aquatic with less time out of water, while the latter is primarily terrestrial with extended periods of time on land. Within these identified AMPs, 449 sequences are novel; 15 were reported in BP previously; 48 are identically overlapped between BP and PM; 94 were validated by mass spectrometry. Moreover, most AMPs presented differential tissue transcription patterns in the two mudskippers. Interestingly, we discovered two AMPs, hemoglobin β1 and amylin, with high inhibitions on Micrococcus luteus. In conclusion, our high-throughput screening strategy based on genomic and transcriptomic data opens an efficient pathway to discover new antimicrobial peptides for ongoing development of marine drugs.

Functional characterization of hemocytes from Chinese mitten crab Eriocheir sinensis by flow cytometry

Hemocytes comprise a diversity of cell types with functional and structural heterogeneity, and they play key roles in the host defense of invertebrates. In the present study, the hemocytes from Chinese mitten crab Eriocheir sinensis were directly separated into two groups by flow cytometry. The hemocytes in P1 group were full of round and abundant granules with deeply staining cytoplasm, while P2 hemocytes were more diverse with a wide range of sizes and less granularity. Both P1 and P2 hemocytes exhibited phagocytic ability, but the phagocytic rate of P1 hemocytes increased which was significantly higher than that of P2 hemocytes after LPS stimulations. The levels of ROS production and intracellular Calcium as well as lysosome content were higher in P1 hemocytes than that in P2 hemocytes under both normal and immune-activated situations. The genes involved in phagocytosis, antimicrobial and antioxidant activities were mainly expressed in P1 hemocytes, while the genes involved in proPO activation system were highly expressed in P2 hemocytes. These results collectively suggested that P1 hemocytes were the main immunocompetent hemocytes in Chinese mitten crab and P2 hemocytes mainly participated in proPO activation system.

Molecular cloning and analysis of a C-type lectin from silkworm Bombyx mori

C-type lectins (CTLs) play a variety of roles in plants and animals. They are involved in animal development, pathogen recognition, and the activation of immune responses. CTLs carry one or more non-catalytic carbohydrate-recognition domains (CRDs) to bind specific carbohydrates reversibly. Here, we report the molecular cloning and functional analysis of a single-CRD CTL, named C-type lectin-S2 (BmCTL-S2) from the domesticated silkmoth Bombyx mori (Lepidoptera: Bombycidae). The ORF of CTL-S2 is 666 bp, which encodes a putative protein of 221 amino acids. BmCTL-S2 is expressed in a variety of immune-related tissues, including hemocytes and fat body among others. BmCTL-S2 mRNA level in the midgut and the fat body was significantly increased by bacterial challenges. The recombinant protein (rBmCTL-S2) bound different bacterial cell wall components and bacterial cells. rBmCTL-S2 also inhibited the growth of Bacillus subtilis and Staphylococcus aureus. Taken together, we infer that BmCTL-S2 is a pattern-recognition receptor with antibacterial activities.