Mannose-binding lectin and its roles in immune responses in grass carp (Ctenopharyngodon idella) against Aeromonas hydrophila

Towards a comprehensive understanding of the muscle proteome in Schizothorax labiatus: Insights from seasonal variations, metabolic responses, and reproductive signatures in the River Jhelum

Proteomics is a very advanced technique used for defining correlations, compositions and activities of hundreds of proteins from organisms as well as effectively used in identifying particular proteins with varying peptide lengths and amino acid counts. In the present study, an endeavour has been put forth to create muscle proteome expression of snow trout, Schizothorax labiatus. Liquid chromatography-mass spectrometry (LC-MS) using label free quantification (LFQ) technique has extensively been carried out to explore changes in protein metabolism and its composition to discriminate across species, clarify functions and pinpoint protein biomarkers from organisms. In LFQ technique, the abundances of proteins are determined based on the signal intensities of their corresponding peptides in mass spectrometry. The main benefit of using this method is that it doesn't require pre-labelling proteins with isotopic tags, which streamlines the experimental procedure and gets rid of any bias that might have been caused by the labelling process. LFQ techniques frequently offer a wider dynamic range, making it possible to detect and quantify proteins over a broad range of abundances obtained from the complex biological materials including fish muscle. The results of proteomic analysis could provide an insight in understanding about how various proteins are expressed in response to environmental challenges. For proteomic study, two different weight groups of S. labiatus were taken from River Jhelum based on biological, physiological and logistical factors. These groups corresponded to different life stages, such as younger size and adults/brooders in order to capture potential variations in the muscle proteome related to growth and development. The proteomic analysis of S. labiatus depicted that an overall of 220 proteins in male and 228 in female fish of group 1 were noted. However, when male and female S. labiatus were examined based on spectral count and peptide abundance using ProteinLynx Global Software, a total of 10 downregulated and 32 upregulated proteins were found. In group 2 of S. labiatus, a total of 249 proteins in male and 301 in female fish were documented. When the two genders of S. labiatus were likened to one another by LFQ technique, a total of 41 downregulated and 06 upregulated proteins were identified. The variability in the protein numbers between two fish weight groups reflected biological differences, influenced by factors such as age, developmental stages, physiological condition and reproductive activities. During the study, it was observed that S. labiatus exhibited downregulated levels of proteins that were involved in feeding and growth. The contributing factors to this manifestation could be explained by lower feeding and metabolic activity of fish and decreased food availability during winter in River Jhelum. Contrarily, the fish immune response proteins were found to be significantly over-expressed in S. labiatus, indicating that the environment was more likely to undergo increased microbial infection, pollution load and anthropogenic activities. In addition, it was also discovered that there was an upregulated expression of the reproductive proteins in S. labiatus, which could be linked to the fish's pre-spawning time as the fish used in this study was collected in the winter season which is the pre-spawning period of the fish. Therefore, the present study would be useful in obtaining new insights regarding the molecular makeup of species, methods of adaptation and reactions to environmental stresses. This information contributes to our understanding of basic science and may have applications in environmental monitoring, conservation and preservation of fish species.

Purification, biochemical characterization of a lectin from marine sponge Ircinia strobilina and its effect on the inhibition of bacterial biofilms

A new lectin from marine sponge Ircinia strobilina, denominated IsL, was isolated by combination of affinity chromatography in Guar gum matrix followed by size exclusion chromatography. IsL was able to agglutinate native and enzymatically treated rabbit erythrocytes, being inhibited by galactosides, such as α-methyl-D-galactopyranoside, β-methyl-D-galactopyranoside and α-lactose. IsL hemagglutinating activity was stable at neutral to alkaline pH, however the lectin loses its activity at 40° C. The molecular mass determinated by mass spectrometry was 13.655 ± 5 Da. Approximately 40% of the primary structure of IsL was determined by mass spectrometry, but no similarity was observed with any protein. The secondary structure of IsL consists of 28% α-helix, 26% β-sheet, and 46% random region, as determined by dichroism circular. IsL was a calcium-dependent lectin, but no significant variations were observed by circular dichroism when IsL was incubated in presence of calcium and EDTA. IsL was not toxic against Artemia nauplii and did not have antimicrobial activity against bacterial cells. However, the IsL was able to significantly inhibit the biofilm formation of Staphylococcus aureus and Staphylococcus epidermidis.

Proteins journey-from marine to freshwater ecosystem: blood plasma proteomic profiles of pink salmon Oncorhynchus gorbuscha Walbaum, 1792 during spawning migration

The pink salmon (Oncorhynchus gorbuscha) is a commercial anadromous fish species of the family Salmonidae. This species has a 2-year life cycle that distinguishes it from other salmonids. It includes the spawning migration from marine to freshwater environments, accompanied by significant physiological and biochemical adaptive changes in the body. This study reveals and describes variability in the blood plasma proteomes of female and male pink salmon collected from three biotopes-marine, estuarine and riverine-that the fish pass through in spawning migration. Identification and comparative analysis of blood plasma protein profiles were performed using proteomics and bioinformatic approaches. The blood proteomes of female and male spawners collected from different biotopes were qualitatively and quantitatively distinguished. Females differed primarily in proteins associated with reproductive system development (certain vitellogenin and choriogenin), lipid transport (fatty acid binding protein) and energy production (fructose 1,6-bisphosphatase), and males in proteins involved in blood coagulation (fibrinogen), immune response (lectins) and reproductive processes (vitellogenin). Differentially expressed sex-specific proteins were implicated in proteolysis (aminopeptidases), platelet activation (β- and γ-chain fibrinogen), cell growth and differentiation (a protein containing the TGF_BETA_2 domain) and lipid transport processes (vitellogenin and apolipoprotein). The results are of both fundamental and practical importance, adding to existing knowledge of the biochemical adaptations to spawning of pink salmon, a representative of economically important migratory fish species.

Mannose-binding lectin suppresses macrophage proliferation through TGF-β1 signaling pathway in Nile tilapia

Mannose-binding lectin (MBL) is a multifunctional pattern recognition molecule, which not only mediates the recognition of pathogenic microorganisms and their products, playing an important role in innate immune defense, but also participates in adaptive immune responses of mammalian. However, it's related immune mechanism remains limited, especially the regulation of cell proliferation in early vertebrates. In this study, OnMBL was found to bind to kidney macrophages (MФ) from Nile tilapia (Oreochromis niloticus). Interestingly, OnMBL was able to reduce the proliferation of activated-MФ by regulating the cell cycle, arresting a large number of cells in the G0/G1 phase, and increasing the probability of apoptosis. More importantly, we found that the inhibition of cell proliferation by OnMBL was closely related to the evolutionarily conserved canonical transforming growth factor-beta 1 (TGF-β1) signaling pathway. Mechanistically, OnMBL could significantly increase the expression of TGF-β1, activate and regulate the downstream Smad-dependent pathway to reduce the MФ proliferation, thereby maintaining cellular homeostasis in the body's internal environment. This study represents the first description regarding the regulatory mechanisms of the MBL on cell proliferation in teleost fish, which provides a novel perspective on the understanding of the multiple function and evolutionary origins of C-type lectins in the immune system.

Genome-wide identification of the traf gene family in yellow catfish (Pelteobagrus fulvidraco) and analysis of their expression in response to bacterial challenge

Tumour necrosis factor (TNF) receptor-associated factor (TRAF) is a receptor protein that has important functions in the immune system. Nonetheless, there have been few reports of traf genes in teleost fishes. The present study aimed to identify the traf genes from the genomic information of yellow catfish (Pelteobagrus fulvidraco). Eight traf genes were identified and named, which are distributed on different chromosomes but have similar conserved protein domains. Phylogenetic and syntenic analyses demonstrated conservation of traf genes during evolution. In addition, yellow catfish has the relatively rare traf1 and traf5 genes. Gene structure and motif analysis revealed the homology and distribution diversity of the traf genes. Quantitative real-time reverse transcription PCR was used to study the expression patterns of traf genes in healthy fish tissues and after infection by Aeromonas hydrophila. The results demonstrated significant changes in traf gene expression, indicating a potential role in innate immunity.

Hepcidin Gene Co-Option Balancing Paternal Immune Protection and Male Pregnancy

Viviparity has originated independently more than 150 times in vertebrates, while the male pregnancy only emerged in Syngnathidae fishes, such as seahorses. The typical male pregnancy seahorses have closed sophisticated brood pouch that act as both uterus and placenta, representing an excellent model system for studying the evolutionary process of paternal immune protection. Phylogenetic analysis indicated that the hampII gene family has multiple tandem duplicated genes and shows independent lineage-specific expansion in seahorses, and they had the highest ratio of nonsynonymous substitutions to synonymous substitutions (dN/dS) in the seahorse phylogenetic branch. The expression levels of hampIIs in the brood pouch placenta were significantly higher during pregnancy than non-pregnancy. Both LPS stimulation test in vivo and cytotoxicity test in vitro proved the immunological protection function of hampIIs against pathogen infection in seahorse. Besides, seahorse hampII peptides exhibit weaker antibacterial function, but stronger agglutination and free endotoxin inhibition. We assumed that the modified immunological function seemed to be a trade-off between the resistance to microbial attack and offspring protection. In brief, this study suggests that the rapid co-option of hampIIs contributes to the evolutionary adaption to paternal immune care during male pregnancy.

Expression and functional analysis of the BCL2-Associated agonist of cell death (BAD) gene in grass carp (Ctenopharyngodon idella) during bacterial infection

The BCL2-associated agonist of cell death protein is a key participant in apoptosis dependent on mitochondria and in disease progression that involves the regulation of cell death, such as tumorigenesis, diabetes, sepsis shock, and epilepsy. Nevertheless, the mechanisms underlying the immune responses to teleost BAD bacterial infection and mitochondrial-dependent apoptosis remains unclear. In order to elucidate the mechanisms involved, in this study, a Ctenopharyngodon idella (grass carp) BAD gene named GcBAD1 was firstly cloned and characterized. The results indicated that the ORF (open reading frame) of GcBAD1 was 438 bp in length, encoding a 145-amino acid putative protein of 16.66 kDa. This deduced amino acid sequence has a better identity than another teleost species according to a phylogenetic analysis, and contains a Bcl2-BAD-1 domain. In healthy grass carp fish, the mRNA transcripts of GcBAD1 were widely present in the studied tissues, which could be ranked as follows; spleen > brain > middle-kidney > head-kidney > liver > gills > intestines > heart and muscle. In addition, during infection by Aeromonas hydrophila and Staphylococcus aureus, the mRNA transcription and protein levels expression of GcBAD1 in the head-kidney, spleen, and liver tissues of the fish were significantly up-regulated. Moreover, when the C. idellus kidney cell line (CIK) cells were incubated with Lipopolysaccharide (LPS) and lipoteichoic acid (LTA), the GcBAD1 expression transcripts were also significantly up-regulated. Additionally, overexpression of GcBAD1 in CIK cells was able to activate apoptosis-related genes, including those encoding p53, Cytochrome C (CytoC), caspase-3, and caspase-9. Besides, in the TUNEL assays, when pEGFP-BAD1 was over-expressed, the number of red signals associated with apoptosis were significantly increased in the CIK cells infected with LPS or LTA at 12 h. This study demonstrates that GcBAD1 has a significant role in the mitochondrial apoptosis pathway of grass carp's innate immunity. Our findings provide new insight into the potential mechanisms of teleost antibacterial immunity.

Expanded collectin family in bullfrog (Rana catesbeiana): Identification and characterization of plasma collectins

BLAST searches against databases for the bullfrog (Rana catesbeiana), using the collectin sequence previously identified in tadpoles, revealed the presence of at least 20 members of the collectin gene family. Phylogenetic analysis demonstrated that the bullfrog possesses expanded gene subfamilies encoding mannose-binding lectin (MBL) and pulmonary surfactant-associated protein D (PSAPD). Two collectins, of 20 kDa (PSAPD1) and 25 kDa (PSAPD6), were purified as a mixture from adult bullfrog plasma using affinity chromatography. These collectins were present as an oligomer of ~400 kDa in their native state, and showed Ca²⁺-dependent carbohydrate binding with different sugar preferences. Affinity-purified collectins showed weak E. coli agglutination and bactericidal activities, compared with those of plasma. Although both PSAPD1 and PSAPD6 genes were predominantly expressed in the liver, PSAPD1 transcripts were abundant in adults whereas PSAPD6 transcripts were abundant in tadpoles. The findings indicate that two gene subfamilies in the collectin family have diverged structurally, functionally and transcriptionally in the bullfrog. Rapid expansion of the collectin family in bullfrogs may reflect the onset of sub-functionalization of the prototype MBL gene towards tetrapod MBL and PSAPDs, and may be one means of natural adaptation in the innate immune system to various pathogens in both aquatic and terrestrial environments.

The immune function of heme oxygenase-1 from grass carp (Ctenopharyngodon idellus) in response to bacterial infection

Heme oxygenase (HO)-1, a rate-limiting enzyme in heme catabolism, results in the formation of equivalent amounts of biliverdin (BV), carbon monoxide (CO) and ferrous iron (Fe²⁺). Previous studies have revealed that HO-1 plays an important role in immune responses. However, the mechanism underlying the immune responses against bacterial infection of teleost HO-1 remains enigmatic. To decipher the mechanisms, we have cloned and characterized the HO-1 gene of grass carp (designated as GcHO-1) in this research. The results showed that the open reading frame (ORF) of GcHO-1 was 819 bp, which encoded a putative protein of 272 amino acids. The deduced amino acid sequence phylogenetically shared the highest identity with other teleosts, and contained two domains of heme-oxygenase and a single-pass transmembrane domain. The mRNA expressions of GcHO-1 in healthy grass carp have widely existed in examined tissues in the following order of spleen > head-kidney > middle head-kidney > intestines > liver > gills > heart > muscle > brain. Besides, the mRNA and protein transcription of GcHO-1 were both significantly up-regulated in the liver and head-kidney tissues after Staphylococcus aureus and Aeromonas hydrophila infection. In addition, overexpression of GcHO-1 in kidney cell line (CIK) cells of grass carp could reduce the expression of inflammatory cytokines (IL-1β, IL-8, TNFα, CCL1 and IL-6). Herein, we demonstrate that GcHO-1 plays an anti-inflammatory role in innate immunity. Our results shed new light on the mechanisms underlying the antibacterial immunity of teleost.

Identification, characterization, and immunological analysis of complement component 4 from grass carp (Ctenopharyngodon idella)

Complement component 4 (C4) has critical immunological functions in vertebrates. In the current study, a C4 homolog (gcC4) was identified in grass carp (Ctenopharyngodon idella). The full-length 5458 bp gcC4 cDNA contained a 5148 bp open reading frame (ORF) encoding a protein of 1715 amino acids with a signal peptide and eight conservative domains. The gcC4 protein has a high level of identity with other fish C4 counterparts and is phylogenetically clustered with cyprinid fish C4. The gcC4 transcript shows wide tissue distribution and is inducible by Aeromonas hydrophila in vivo and in vitro. Furthermore, its expression also fluctuates upon lipopolysaccharide or flagellin stimulation in vitro. During infection, the gcC4 protein level decreases or increases to varying degrees, and the intrahepatic C4 expression location changes. With gcC4 overexpression, interleukin 1 beta, tumor necrosis factor alpha, and interferon transcripts are all upregulated by A. hydrophila infection. Meanwhile, overexpression of gcC4 reduces bacterial invasion or proliferation. Moreover, gcC4 may activate the NF-κB signaling pathway. These findings demonstrate the vital role of gcC4 in the innate immunity of grass carp.

Antimicrobial activity of mannose binding lectin in grass carp (Ctenopharyngodon idella) in vivo and in vitro

Mannose-binding lectin (MBL) is a crucial pattern recognition receptor in the host innate immune system. Previously, we reported the biological function of Ctenopharyngodon idella MBL (CiMBL) in initiating the lectin pathway of the complement system. In the present study, we further explored its biological function including the agglutinating ability, binding capacity and protective role in vitro and in vivo. After Aeromonas hydrophila infection, western blot analysis revealed that the CiMBL were fluctuated and expressed in the serum and major immune-related tissues. The result of quantitative PCR (qPCR) showed that the recombinant CiMBL (rCiMBL) significantly inhibited the mRNA expression of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in liver, spleen and hepatic cells. Due to rCiMBL bound to d-mannose, d-galactose, d-glucose, N-acetyl-d-glucosamine (GlcNAc), lipopolysaccharide (LPS), peptidoglycan (PGN) and Agar in the presence of Ca²⁺, herein gram-positive (Staphylococcus aureus and Micrococcus luteus) and gram-negative (A. hydrophila and Vibrio anguillarum) bacteria were agglutinated by rCiMBL in a Ca²⁺-dependent manner. More importantly, rCiMBL enhanced the survival rate of grass carp following bacterial infection. Overall, the results provide an evidence that CiMBL can protect grass carp against A. hydrophila infection in aquaculture.

Molecular and functional characterisation of a mannose-binding lectin-like gene from Japanese sea bass (Lateolabrax japonicus)

Mannose-binding lectin (MBL) plays an important role in host immune responses against pathogens. LjMBL-like-1 was identified from Japanese sea bass (Lateolabrax japonicas), which has selectivity for galactose. Herein, this lectin might be better designated as galactose-binding lectin (LjGalBL-1). LjGalBL-1 transcripts were detected in all tested tissues, with highest expression in liver. Upon Vibrio harveyi infection, LjGalBL-1 mRNA expression was increased in major immune-related tissues, and protein levels in serum were also upregulated. Recombinant LjGalBL-1 (rLjGalBL-1) bound to monosaccharides and polysaccharides, and both rLjGalBL-1 and native LjGalBL-1 (nLjGalBL-1) agglutinated three Gram-positive bacteria (Staphylococcus aureus, Streptococcus iniae and Micrococcus luteus) and four Gram-negative bacteria (Aeromonas hydrophila, Edwardsiella tarda, Vibrio anguillarum and V. harveyi) in a Ca²⁺-dependent manner in vitro. Moreover, rLjGalBL-1 increased the survival rate of V. harveyi-infected fish and decreased bacterial load in liver, spleen, kidney and blood. Thus, LjGalBL-1 protects L. japonicas against V. harveyi infection.

Identification and characterization of two mannan-binding lectin associated proteins in lectin complement pathway of grass carp

The lectin pathway of complement activation is an important component of the innate immune response, which must be tightly controlled to maintain immune homeostasis. However, its control mechanisms have not been investigated in detail in bony fish. In this study, we identified and characterized two novel, phylogenetically conserved mannan-binding lectin (MBL)-associated proteins (MAps) of grass carp (Ctenopharyngodon idella), CiMAp27 and CiMAp39, which were truncated, alternatively-spliced forms of grass carp MBL-associated serine proteases (MASPs), CiMASP1 and CiMASP2, respectively. Gene expression profiling showed that both CiMAp27 and CiMAp39 were upregulated by low doses of Aeromonas hydrophila, and inhibited by high doses, which lead to the inference that these genes acted as immune factors in antibacterial defense. Sequence analysis showed that CiMAp27 lack a catalytic domain but retains two domains (CUB1-EGF) involved in the association with MBL, while CiMAp39 retained four domains (CUB1-EGF-CUB2-CCP1). Not only the two CiMASPs but also the CiMAps were detected in grass carp serum. Furthermore, both recombinant CiMASPs (rCiMASPs) and recombinant rCiMAps (rCiMAps) interacted with recombinant MBL and the two CiMAps competed with CiMASPs for binding to MBL, and hence inhibited downstream C4 binding. These results indicated that CiMAps acted as competitive inhibitors in the lectin complement pathway of grass carp.

Role of mannose-binding lectin in regulating monocytes/macrophages functions during Aeromonas hydrophila infection in grass carp, Ctenopharyngodon idella

Mannose-binding lectin (MBL) is a vital component in host's innate immune system and the initiator of the lectin pathway of complement cascade. However, its opsonic role has rarely been reported. In this study, we revealed the biological function of Ctenopharyngodon idella MBL (CiMBL) in regulating monocytes/macrophages (MO/MФ) in the grass carp (C. idella). Flow cytometry results indicated that recombinant CiMBL (rCiMBL) significantly enhanced the phagocytotic activity of MO/MФ. Recombinant CiMBL also enhanced bactericidal activity and respiratory burst capacity in Aeromonas hydrophila-infected MO/MФ, regulated A. hydrophila-induced polarization of MO/MФ including down- and up-regulated pro- and anti-inflammatory cytokines, respectively, suppressed the inducible nitric oxide synthase activity, and enhanced the arginase activity. In addition, rCiMBL suppressed the bacteria burden in tissues and blood in vivo and enhanced the survival rate of juvenile A. hydrophila-infected grass carp. We provide evidence that CiMBL was synthesized by MO/MФ, regulating the biological function of MO/MФ against A. hydrophila infection.

Characterization and functional analysis of a novel mannose-binding lectin from the swimming crab Portunus trituberculatus

Mannose-binding lectin (MBL) is a pattern recognition receptor (PRR) that plays an important role in the innate immune response. In this study, a novel mannose-binding lectin was cloned from the swimmimg crab Portunus trituberculatus (designated as PtMBL). The complete cDNA of PtMBL gene was 1208 bp in length with an open reading frame (ORF) of 732 bp that encoded 244 amino acid proteins. PtMBL shared lower amino acid similarity with other MBLs, yet it contained the conserved carbohydrate-recognition domain (CRD) with QPD motif and was clearly member of the collectin family. PtMBL transcripts were mainly detected in eyestalk and gill with sexually dimorphic expression. The temporal expression of PtMBL in hemocytes showed different activation times after challenged with Vibrio alginolyticus, Micrococcus luteus and Pichia pastoris. The recombinant PtMBL protein revealed antimicrobial activity against the tested Gram-negative and Gram-positive bacteria. It could also bind and agglutinate (Ca²⁺-dependent) both bacteria and yeast. Furthermore, the agglutinating activity could be inhibited by both d-galactose and d-mannose, suggesting the broader pathogen-associated molecular patterns (PAMPs) recognition spectrum of PtMBL. These results together indicate that PtMBL could serve as not only a PRR in immune recognition but also a potential antibacterial protein in the innate immune response of crab.

Molecular cloning, expression analyses of polymeric immunoglobulin receptor gene and its variants in grass carp (Ctenopharyngodon idellus) and binding assay of the recombinant immunoglobulin-like domains

The Polymeric Immunoglobulin Receptor (pIgR) gene has been proved to play an important role in transporting polymeric immunoglobulin (Ig) in the mucosal tissues of mammals. pIgR gene also exists in teleost, but the genetic diversity and functions of this gene still need to be further explored. We obtained seven grass carp pIgR splicing transcripts, a full-length pIgR (CipIgR-1) and six truncated variants (CipIgR-2 to CipIgR-7). The full-length pIgR contained two immunoglobulin-like domains (ILD), a transmembrane domain (TMD) and a cytoplasmic domain (CyD). The CipIgR-2 lacked a small part in CyD, and CipIgR-3 lost TMD and CyD. Partial cDNA sequences of the other four grass carp pIgR variants (CipIgR-4 to CipIgR-7) were also cloned. The total expression levels of CipIgR and its variants in different tissues were detected by real-time quantitative PCR. The highest expression was found in the intestine, followed by the spleen and the skin. The function of the two extracellular ILDs of CipIgR was investigated based on its combining capacity with grass carp immunoglobulin M (IgM) and aquatic pathogenic bacteria. The cDNA sequences of two ILDs were cloned and expressed in Escherichia coli BL21 (DE3). Recombinant ILDs protein was purified and incubated with different bacteria respectively. Results of Western blot showed the recombinant protein could combine Bacillus subtilis, Vibrio parahaemolyticus, and Escherichia coli. In addition, binding activity of rILDs with grass carp IgM was detected. Collectively, these results indicated that multiple variants of pIgR gene in grass carp might be involved in the antibacterial immunity.

Complement component 3 (C3): An important role in grass carp (Ctenopharyngodon idella) experimentally exposed to Aeromonas hydrophila

Complement is traditionally recognized as part of the innate immune system, defending the host against the invasion of foreign pathogens. In complement system, C3 (complement component 3) is a central component. Therefore, research into C3 can help us better understand the functions of fish complement system. In this study, we detected the grass carp C3 (gcC3) mRNA expression in all sample tissues from healthy grass carp, which was highest in the liver, followed by the heart and the spleen, and lowest in the muscle, head kidney, trunk kidney, blood and intestine. After infection with Aeromonas hydrophila, gcC3 mRNA expression levels were significantly upregulated in the gill, liver, spleen, intestine, trunk kidney and head kidney. Interestingly, C3 protein levels were downregulated and subsequently upregulated in the liver and serum. Histologically, C3 protein at 24 h pi was over expressed in necrotic liver sites, and the liver index (LI) at this point was significantly higher than that of the control. These findings are indicated that C3 plays an important role in the immune response of grass carp after A. hydrophila infection, and C3 protein may play an assistant role in repairing liver tissues from A. hydrophila injury.

Functional characterization of a mannose-binding lectin (MBL) from Nile tilapia (Oreochromis niloticus) in non-specific cell immunity and apoptosis in monocytes/macrophages

Mannose-binding lectin (MBL), a soluble pattern recognition receptor, is able to recognize antigen and participate in non-specific cell immunity, such as regulation of inflammation, migration, opsonization, phagocytosis and killing, which plays an important role in innate immunity. In this study, we have investigated the contributing mechanisms and effects of MBL on the cell immunity of Nile tilapia (Oreochromis niloticus) monocytes/macrophages. The mRNA expression level of OnMBL was significantly up-regulated in monocytes/macrophages after in vitro bacterial infection (Streptococcus agalactiae and Aeromonas hydrophila). Recombinant OnMBL ((r)OnMBL) protein could participate in the regulation of inflammation, migration, and enhancement of phagocytosis and respiratory burst activity in monocytes/macrophages. Moreover, the (r)OnMBL could induce the apoptosis of monocytes/macrophages. Taken together, the results of this study indicated that OnMBL is likely to involve in immune regulation, which may play an important role in host defense of innate immunity in Nile tilapia.

A mannose receptor is involved in the anti-Vibrio defense of red swamp crayfish

Mannose receptor (MR), a member of pattern-recognition receptors (PRRs), is the first MR family member to be discovered that plays a critical role in immunity. The function of MRs has been reported in mammals and teleosts while none in invertebrates. In the present study, we identified a MR-like gene (designated as PcMR) from red swamp crayfish, Procambarus clarkii. The PcMR cDNA is 6848 bp long with a 6288 bp open reading frame that encodes a polypeptide with 2095 amino acid residues. PcMR transcripts were mainly detected in hepatopancreas and hemocytes, and upregulated by Vibrio anguillarum challenge. The PcMR protein contained 14 C-type lectin domains (CTLDs) and they were divided into four fragments (CTLD 1-3, CTLD 4-6, CTLD 7-10, CTLD 11-14). The four recombinant proteins encoded by the four fragments were all expressed and purified. Microorganism-binding and sugar-binding assay showed that CTLD 1-3, CTLD 4-6, CTLD 7-10, CTLD 11-14 could bind to a variety of bacteria, as well as glycoconjugates on the bacterial surface. Moreover, they agglutinated bacteria in a calcium-dependent manner. Bacteria clearance experiment manifested that the mixed proteins facilitated the clearance of injected bacteria in crayfish. PcMR silencing by siRNA interference impaired the bacterial clearance ability. These results suggest PcMR is involved in the antibacterial defense of crayfish, and this study will help us better understand the functions of invertebrate MRs.