Characterization and expression analysis of an intelectin gene from Megalobrama amblycephala with excellent bacterial binding and agglutination activity

Potential Involvement of the South American Lungfish Intelectin-2 in Innate-Associated Immune Modulation

Intelectins belong to a family of lectins with specific and transitory carbohydrate interaction capabilities. These interactions are related to the activity of agglutinating pathogens, as intelectins play a significant role in immunity. Despite the prominent immune defense function of intelectins, limited information about its structural characteristics and carbohydrate interaction properties is available. This study investigated an intelectin transcript identified in RNA-seq data obtained from the South American lungfish (Lepidosiren paradoxa), namely LpITLN2-B. The structural analyses predicted LpITLN2-B to be a homo-trimeric globular protein with the fibrinogen-like functional domain (FReD), exhibiting a molecular mass of 57 kDa. The quaternary structure is subdivided into three monomers, A, B, and C, and each domain comprises 11 β-sheets: an anti-parallel β-sheet, a β-hairpin, and a disordered β-sheet structure. Molecular docking demonstrates a significant interaction with disaccharides rather than monosaccharides. The preferential interaction with disaccharides highlights the potential interaction with pathogen molecules, such as LPS and Poly(I:C). The hemagglutination assay inhibited lectins activity, especially maltose and sucrose, highlighting lectin activity in L. paradoxa samples. Overall, our results show the potential relevance of LpITLN2-B in L. paradoxa immune defense against pathogens.

Innate immune and chronic heat stress responses in sturgeons: Advances and insights from studies on Russian sturgeons

Chronic stress deteriorates the immune function of fish, thereby increasing their vulnerability to infections. However, the molecular and cellular mechanisms underlying stress-mediated immunosuppression and infection susceptibility in fish remain largely unknown. Understanding these mechanisms will contribute to improving fish welfare and their farm production. Herein, we review the challenges of sturgeon aquaculture in subtropical countries, where current climate change has giving rise to significant temperature increments during summer. This leads to the exposure of fish to stressful conditions during these months. Chronic heat stress deserves attention considering the rapid warming rate of the planet. It is already affecting wild fish populations, with disastrous consequences for sturgeons, which are one of the most endangered fish species in the world. In this context, we discuss the most recent advances through the studies on the effects of chronic heat stress on the innate immune components of sturgeons. To this end, we summarise the findings of studies focusing on the aquaculture of Russian sturgeons and observations made on other Acipenser species. Special attention is given to acute-phase proteins, as they might be valuable biomarkers of heat stress and infection, with applicability in monitoring the fish health status in farms.

Muscle Transcriptome Sequencing Revealed Thermal Stress-Responsive Regulatory Genes in Farmed Rohu, Labeo rohita (Hamilton, 1822)

Rohu, Labeo rohita, is one of the most important aquaculture species in the Indian subcontinent. Understanding the molecular-level physiological responses to thermal stress or climate change is essential. In the present work, transcriptome sequencing was carried out in the muscle tissue of the rohu in response to heat stress (35 °C) in comparison with the control (28 °C). A total of 125 Gb of sequence data was generated, and the raw-reads were filtered and trimmed, which resulted in 484 million quality reads. Reference-based assembly of reads was performed using L. rohita genome, and a total of 90.17% of reads were successfully mapped. A total of 37,462 contigs were assembled with an N50 value of 1854. The differential expression analysis revealed a total of 107 differentially expressed genes (DEGs) (15 up-, 37 down-, and 55 neutrally regulated) as compared to the control group (Log2FC > 2, P < 0.05). Gene enrichment analysis of DEGs indicates that transcripts were associated with molecular, biological, and cellular activities. The randomly selected differentially expressed transcripts were validated by RT-qPCR and found consistent expression patterns in line with the RNA-seq data. Several transcripts such as SERPINE1(HSP47), HSP70, HSP90alpha, Rano class II histocompatibility A beta, PGC-1 and ERR-induced regulator, proto-oncogene c-Fos, myozenin2, alpha-crystallin B chain-like protein, angiopoietin-like protein 8, and acetyl-CoA carboxylases have been identified in muscle tissue of rohu that are associated with stress/immunity. This study identified the key biomarker SERPINE1 (HSP47), which showed significant upregulation (~ 2- to threefold) in muscle tissue of rohu exposed to high temperature. This study can pave a path for the identification of stress-responsive biomarkers linked with thermal adaptations in the farmed carps.

Janus kinase 1 in Megalobrama amblycephala: Identification, phylogenetic analysis and expression profiling after Aeromonas hydrophila infection

Janus kinase 1 (JAK1), a member of the JAK family, plays an essential and non-redundant role in the mammalian immune system. However, the potential role of JAK1 in fish immune response remains largely unclear. In the present study, the JAK1 gene of Megalobrama amblycephala (MamJAK1) was identified and characterized. The open reading frame (ORF) of MamJAK1 was 3462 bp, encoding 1153 amino acids. MamJAK1 consists of four common domains of the JAK family, including B41, SH2, STyrKc (a pseudo kinase domain), and TyrKc (a kinase domain). Phylogenetic analysis showed that JAK1s are divided into two evolutionary clades, one containing fish JAK1s, and the other containing JAK1s from other vertebrates. The results of quantitative real-time PCR (qPCR) showed that in healthy M. amblycephala, MamJAK1 mRNA was highest expressed in blood, followed by spleen, intestine and mid-kidney, and lowly expressed in other tissues including gill, liver, head kidney, muscle, brain and heart. After Aeromonas hydrophila infection, the expression of MamJAK1 mRNA was significantly induced in four selected tissues including spleen, mid-kidney, liver and intestine, reaching a peak at 24 hpi (hour post infection) in spleen and mid-kidney, at 12 hpi in liver and at 4 hpi in intestine, and then the expression level was restricted to control levels at 72 or 120 hpi. In addition, the results of Western blot showed that the phosphorylation level of MamJAK1 protein in spleen and mid-kidney increased significantly after A. hydrophila infection, although MamJAK1 protein did not change obviously. Further, the JAK1 phosphorylation in Ctenopharyngodon idellus kidney (CIK) cells was found to be significantly induced by LPS stimulation and IL-6R over-expression. The results above suggest that MamJAK1 may play an essential role in the immune response against bacterial infection through the IL-6R mediated JAK1/STAT signaling pathway, which further deepen our understanding of JAK1 and provides a potential target for the treatment and prevention of bacterial diseases in teleost.

Intelectin enhances the phagocytosis of macrophages via CDC42-WASF2-ARPC2 signaling axis in Megalobrama amblycephala

Intelectin has been identified in various vertebrates and plays an important role in the host immune system. In our previous studies, recombinant Megalobrama amblycephala intelectin (rMaINTL) protein with excellent bacterial binding and agglutination activities enhances the phagocytic and killing activities of macrophages in M. amblycephala; however, the underlying regulatory mechanisms remain unclear. The present study showed that treatment with Aeromonas hydrophila and LPS induced the expression of rMaINTL in macrophages, and its level and distribution in macrophages or kidney tissue markedly increased after incubation or injection with rMaINTL. The cellular structure of macrophages was significantly affected after incubation with rMaINTL, resulting in an increased surface area and pseudopodia extension, which might contribute to enhancing the phagocytic ability of macrophages. Then, digital gene expression profiling analysis of the kidneys from rMaINTL-treated juvenile M. amblycephala identified some phagocytosis-related signaling factors that were enriched in pathways involved in the regulation of the actin cytoskeleton. In addition, qRT-PCR and western blotting verified that rMaINTL upregulated the expression of CDC42, WASF2, and ARPC2 in vitro and in vivo; however, the expression of these proteins was inhibited by a CDC42 inhibitor in macrophages. Moreover, CDC42 mediated the promotion of rMaINTL on actin polymerization by increasing the F-actin/G-actin ratio, which led to the extension of pseudopodia and remodeling of the macrophage cytoskeleton. Furthermore, the enhancement of macrophage phagocytosis by rMaINTL was blocked by the CDC42 inhibitor. These results suggested that rMaINTL induced the expression of CDC42 as well as the downstream signaling molecules WASF2 and ARPC2, thereby facilitating actin polymerization to promote cytoskeletal remodeling and phagocytosis. Overall, MaINTL enhanced the phagocytosis activity of macrophages in M. amblycephala via activation of the CDC42-WASF2-ARPC2 signaling axis.

Comparative Expression Profiling Reveals the Regulatory Effects of Dietary Mannan Oligosaccharides on the Intestinal Immune Response of Juvenile Megalobrama amblycephala against Aeromonas hydrophila Infection

Mannan oligosaccharides (MOS) are functional oligosaccharides with beneficial effects on the non-specific immunity of Megalobrama amblycephala, but systematic studies on the immunomodulatory mechanisms of MOS are still lacking. To investigate the protective mechanisms of three different levels of dietary MOS supplementation on the intestinal immunity of juvenile M. amblycephala, comparative digital gene expression (DGE) profiling was performed. In this study, 622 differentially expressed genes (DEGs) were identified, while the similar expression tendency of 34 genes by qRT-PCR validated the accuracy of the DGE analyses. Gene Ontology (GO) enrichment revealed that the DEGs were mainly enriched in two functional categories of biological process and molecular function. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that the DEGs were mainly related to complement and coagulation cascades, coagulation cascades, platelet activation, natural killer cell mediated cytotoxicity, Fc gamma R-mediated phagocytosis and antigen processing and presentation. In addition, the pro-inflammatory, apoptosis and tight junction-related genes were more significantly up-regulated upon infection in the dietary MOS groups to enhance host immune functions and maintain the stability of the intestinal barrier. These results will be helpful to clarify the regulatory mechanism of MOS on the intestinal immunity of M. amblycephala and lay the theoretical foundation for the prevention and protection of fish bacterial diseases.

Characterization of the innate immune response to Streptococcus pneumoniae infection in zebrafish

Streptococcus pneumoniae (pneumococcus) is one of the most frequent causes of pneumonia, sepsis and meningitis in humans, and an important cause of mortality among children and the elderly. We have previously reported the suitability of the zebrafish (Danio rerio) larval model for the study of the host-pathogen interactions in pneumococcal infection. In the present study, we characterized the zebrafish innate immune response to pneumococcus in detail through a whole-genome level transcriptome analysis and revealed a well-conserved response to this human pathogen in challenged larvae. In addition, to gain understanding of the genetic factors associated with the increased risk for severe pneumococcal infection in humans, we carried out a medium-scale forward genetic screen in zebrafish. In the screen, we identified a mutant fish line which showed compromised resistance to pneumococcus in the septic larval infection model. The transcriptome analysis of the mutant zebrafish larvae revealed deficient expression of a gene homologous for human C-reactive protein (CRP). Furthermore, knockout of one of the six zebrafish crp genes by CRISPR-Cas9 mutagenesis predisposed zebrafish larvae to a more severe pneumococcal infection, and the phenotype was further augmented by concomitant knockdown of a gene for another Crp isoform. This suggests a conserved function of C-reactive protein in anti-pneumococcal immunity in zebrafish. Altogether, this study highlights the similarity of the host response to pneumococcus in zebrafish and humans, gives evidence of the conserved role of C-reactive protein in the defense against pneumococcus, and suggests novel host genes associated with pneumococcal infection.

Molecular Characterization, Expression, Evolutionary Selection, and Biological Activity Analysis of CD68 Gene from Megalobrama amblycephala

CD68 is a highly glycosylated transmembrane glycoprotein that belongs to the lysosome-associated membrane glycoprotein family and is involved in various immune processes. In this study, Megalobrama amblycephala CD68 (MaCD68) was cloned and characterized, and its expression patterns and evolutionary characteristics were analyzed. The coding region of MaCD68 was 987 bp, encoding 328 amino acids, and the predicted protein molecular weight was 34.9 kDa. MaCD68 contained two transmembrane helical structures and 18 predicted N-glycosylation sites. Multiple sequence alignments showed that the MaCD68 protein had high homology with other fish, and their functional sites were also highly conserved. Phylogenetic analysis revealed that MaCD68 and other cypriniformes fish clustered into one branch. Adaptive evolution analysis identified several positively selected sites of teleost CD68 using site and branch-site models, indicating that it was under positive selection pressure during evolution. Quantitative real-time reverse transcription polymerase chain reaction analysis showed that MaCD68 was highly expressed in the head kidney, spleen, and heart. After Aeromonas hydrophila infection, MaCD68 was significantly upregulated in all tested tissues, peaking at 12 h post-infection (hpi) in the kidney and head kidney and at 120 hpi in the liver and spleen, suggesting that MaCD68 participated in the innate immune response of the host against bacterial infection. Immunohistochemical and immunofluorescence analyses also showed that positive signals derived from the MaCD68 protein were further enhanced after bacterial and lipopolysaccharide treatment, which suggested that MaCD68 is involved in the immune response and could be used as a macrophage marker. Biological activity analysis indicated that recombinant MaCD68 (rMaCD68) protein had no agglutination or bactericidal effects on A. hydrophila but did have these effects on Escherichia coli. In conclusion, these results suggest that MaCD68 plays a vital role in the immune response against pathogens, which is helpful in understanding the immune responses and mechanisms of M. amblycephala.

Identification, expression patterns, evolutionary characteristics and recombinant protein activities analysis of CD209 gene from Megalobrama amblycephala

CD209 is a type II transmembrane protein in the C-type lectin family, which is involved in the regulation of innate and adaptive immune system. Although it has been widely studied in mammals, but little has been reported about fish CD209 genes. In the present study, Megalobrama amblycephala CD209 (MaCD209) gene was cloned and characterized, its expression patterns, evolutionary characteristics, agglutinative and bacteriostatic activities were analyzed. These results showed that the open reading frame (ORF) of MaCD209 gene was 795 bp, encoding 264 aa, and the calculated molecular weight of the encoded protein was 29.7 kDa. MaCD209 was predicted to contain 2 N-glycosylation sites, 1 functional domain (C-LECT-DC-SIGN-like) and 1 transmembrane domain. Multiple sequence alignment showed that the amino acid sequence of MaCD209 was highly homologous with that of partial fishes, especially the highly conserved C-LECT-DC-SIGN-like domain and functional sites of CD209. Phylogenetic analysis showed that the CD209 genes from M. amblycephala and other cypriniformes fishes were clustered into one group, which was reliable and could be used for evolutionary analysis. Then, adaptive evolutionary analysis of teleost CD209 was conducted, and several positively selected sites were identified using site and branch-site models. Quantitative real-time PCR analysis showed that MaCD209 gene was highly expressed in the liver and heart. Moreover, the expression of MaCD209 was significantly induced upon Aeromonas hydrophila infection, with the peak levels at 4 h or 12 h post infection. The immunohistochemical analysis also revealed increased distribution of MaCD209 protein post bacterial infection. In addition, recombinant MaCD209 (rMaCD209) protein was prepared using a pET32a expression system, which showed excellent bacterial binding and agglutinative activities in a Ca²⁺-independent manner. However, rMaCD209 could only inhibit the proliferation of Escherichia coli rather than A. hydrophila. In conclusion, this study identified the MaCD209 gene, detected its expression and evolutionary characteristics, and evaluated the biological activities of rMaCD209 protein, which would provide a theoretical basis for understanding the evolution and functions of fish CD209 genes.

Dietary Mannan Oligosaccharides Enhance the Non-Specific Immunity, Intestinal Health, and Resistance Capacity of Juvenile Blunt Snout Bream (Megalobrama amblycephala) Against Aeromonas hydrophila

Mannan oligosaccharides (MOS) have been studied and applied as a feed additive, whereas their regulation on the growth performance and immunity of aquatic animals lacks consensus. Furthermore, their immunoprotective effects on the freshwater fish Megalobrama amblycephala have not been sufficiently studied. Thus, we investigated the effects of dietary MOS of 0, 200, and 400 mg/kg on the growth performance, non-specific immunity, intestinal health, and resistance to Aeromonas hydrophila infection in juvenile M. amblycephala. The results showed that the weight gain rate of juvenile M. amblycephala was not significantly different after 8 weeks of feeding, whereas the feed conversion ratio decreased in the MOS group of 400 mg/kg. Moreover, dietary MOS increased the survival rate of juvenile M. amblycephala upon infection, which may be attributed to enhanced host immunity. For instance, dietary MOS increase host bactericidal and antioxidative abilities by regulating the activities of hepatic antimicrobial and antioxidant enzymes. In addition, MOS supplementation increased the number of intestinal goblet cells, and the intestine was protected from necrosis of the intestinal folds and disruption of the microvilli and junctional complexes, thus maintaining the stability of the intestinal epithelial barrier. The expression levels of M. amblycephala immune and tight junction-related genes increased after feeding dietary MOS for 8 weeks. However, the upregulated expression of immune and tight junction-related genes in the MOS supplemental groups was not as notable as that in the control group postinfection. Therefore, MOS supplementation might suppress the damage caused by excessive intestinal inflammation. Furthermore, dietary MOS affected the richness and composition of the gut microbiota, which improved the gut health of juvenile M. amblycephala by increasing the relative abundance of beneficial gut microbiota. Briefly, dietary MOS exhibited significant immune protective effects to juvenile M. amblycephala, which is a functional feed additive and immunostimulant.

Novel insights into the immune regulatory effects of Megalobrama amblycephala intelectin on the phagocytosis and killing activity of macrophages

Previous studies have found that the expression level of Megalobrama amblycephala intelectin (MaINTL) increased significantly post Aeromonas hydrophila infection, and recombinant MaINTL (rMaINTL) protein could activate macrophages and enhance the phagocytosis and killing activity of macrophages. In order to reveal the immune regulatory mechanisms of MaINTL, primary M. amblycephala macrophages were treated with endotoxin-removed rMaINTL and GST-tag proteins, then total RNA were extracted and used for comparative Digital Gene Expression Profiling (DGE). 1247 differentially expressed genes were identified by comparing rMaINTL and GST-tag treated macrophage groups, including 482 up-regulated unigenes and 765 down-regulated unigenes. In addition, eleven randomly selected differentially expressed genes were verified by qRT-PCR, and most of them shared the similar expression patterns as that of DGE results. GO enrichment revealed that the differentially expressed genes were mainly concentrated in the membrane part and cytoskeleton of cellular component, the binding and signal transducer activity of molecular function, the cellular process, regulation of biological process, signaling and localization of biological process, most of which might related with the phagocytosis and killing activity of macrophages. KEGG analysis revealed the activation and involvement of differentially expressed genes in immune related pathways, such as Tumor necrosis factor (TNF) signaling pathway, Interleukin 17 (IL-17) signaling pathway, Toll-like receptor signaling pathway, and NOD like receptor signaling pathway, etc. In these pathways, TNF-ɑ, Activator protein-1 (AP-1), Myeloid differentiation primary response protein MyD88 (MyD88), NF-kappa-B inhibitor alpha (ikBɑ) and other key signaling factors were significantly up-regulated. These results will be helpful to clarify the immune regulatory mechanisms of fish intelectin on macrophages, thus providing a theoretical basis for the prevention and control of fish bacterial diseases.

Characterization of a common carp intelectin gene with bacterial binding and agglutination activity

Intelectin (ITLN) is a type of glycan-binding lectin involved in many physiological processes and some human diseases. Here we report a common carp intelectin (cITLN). Like other orthologs, cITLN also contains a conserved fibrinogen-related domain (FReD) and a unique intelectin domain, expresses in all the tissues tested with the highest level in the hindgut, and responds to bacterial challenge in the acute phase. We also expressed cITLN in Escherichia coli (E. coli) system, and the purified recombinant cITLN could neither affect the surface of bacteria nor inhibit the growth of bacteria, but it can agglutinate both gram-positive and gram-negative bacteria in a calcium-dependent manner. The cITLN's ability of agglutination of gram-positive bacteria is stronger than that of gram-negative bacteria. This is probably because recombinant cITLN could binding peptidoglycan (PGN) with a higher degree to lipopolysaccharide (LPS). Our results of cITLN provided new insight into the function of intelectin in the intestinal mucosal immunity.

ITLN in diploid hybrid fish (Carassius auratus cuvieri ♀ × Carassius auratus red var ♂) is involved in host defense against bacterial infection

Hybrid genotypes in fish may be less susceptible to pathogenic infection. ITLN, a novel lectin, not only exhibits a regulatory role in pathogenic infection, but also participates in the regulation of teleostean immunity. In this study, ORF sequence of WR-ITLN was 945 bp and encoded 314 amino acid residues. Tissue-specific analysis revealed that the highest expression of WR-ITLN was observed in liver. Aeromonas hydrophila challenge can sharply increased WR-ITLN mRNA expression in liver, kidney and spleen. The purified WR-ITLN protein can directly bind to A. hydrophila and S. agalactiae, reduce their relative bacterial activity and limit bacterial growth in vitro in the presence of Ca²⁺. In addition, the treatment of WR-ITLN + Ca²⁺ can restrict bacterial dissemination in vivo and attenuate production of pro-inflammatory cytokines. These results indicated that WR-ITLN can confer protection against bacteria-stimulated MyD88-dependent pro-inflammatory cytokines activation in a Ca²⁺-dependent manner.

Novel insights into the immune regulatory effects of ferritins from blunt snout bream, Megalobrama amblycephala

Ferritins play vital roles in maintenance of iron homeostasis as iron storage proteins. Recently, the immune function of ferritins have attracted increasing attention, especially their roles in defense against pathogenic infections. However, the immune regulatory mechanism of fish ferritins are not well known. In the present study, comparative digital gene expression (DGE) profiling was performed to explore the regulatory effects of the Megalobrama amblycephala ferritins (MamFers) using MamFers overexpressed and control L8824 cells (Ctenopharyngodon idella hepatic cell line). Clean reads were aligned to the C. idella genome and differential expression analysis was conducted with representative differentially expressed genes pointed out. On that basis, further studies were performed to verify two pivotal regulated pathways in L8824 and EPC (Epithelioma Papulosum Cyprini cell line) cells, respectively. The results showed that NLRC5 (NOD-like Receptor Family CARD Domain Containing 5) mediated the regulation of MamFers on expression of MHC I (Major Histocompatibility Complex Class I) and its chaperone β2M (Beta-2-Microglobulin) in L8824 cells. Then, β2M further mediated the regulation of MamFers on hepcidin expression, indicating that MamFers regulated the expression of hepcidin via NLRC5/MHC I/β2M axis. In addition, MamFers regulated the adhesion of Aeromonas hydrophila to EPC cells by regulating the expression of two extracellular matrix proteins Intgβ1 (integrin β1) and FN (fibronectin). In a word, the present study provided novel insights into the immune regulatory functions of fish ferritins.

Intelectin mediated phagocytosis and killing activity of macrophages in blunt snout bream (Megalobrama amblycephala)

Intelectin, a lectin discovered recently, has been identified in various vertebrate species, such as fish, amphibians, and mammals. In one of our previous studies, the efficient bacteria binding and agglutinating activity of the recombinant Megalobrama amblycephala intelectin protein (rMamINTL) and the enhanced immunopositive localization have been observed in the hepatic macrophage-like cells (kupffer cells) post Aeromonas hydrophila infection. Thus, the present study primarily focuses on the regulatory effects of rMamINTL on M. amblycephala macrophages. This study revealed a prominent LPS-binding activity of rMamINTL and a significantly increased phagocytosis of rMamINTL-treated A. hydrophila by M. amblycephala macrophages. However, the rMamINTL-treated M. amblycephala macrophages exhibited no evident regulatory effect on phagocytosis, whereas the enhanced killing activity of the rMamINTL-treated macrophages was observed, which may be attributed to the induced respiratory burst activity and the expression of inflammatory cytokines. In addition, the anti-proliferation effect of rMamINTL on two tumor cells was observed. However, its mechanism remains to be further studied. In short, these results show that MamINTL is a multifunctional immune protein with effective immunomodulatory activity.

Intelectin 3 is dispensable for resistance against a mycobacterial infection in zebrafish (Danio rerio)

Tuberculosis is a multifactorial bacterial disease, which can be modeled in the zebrafish (Danio rerio). Abdominal cavity infection with Mycobacterium marinum, a close relative of Mycobacterium tuberculosis, leads to a granulomatous disease in adult zebrafish, which replicates the different phases of human tuberculosis, including primary infection, latency and spontaneous reactivation. Here, we have carried out a transcriptional analysis of zebrafish challenged with low-dose of M. marinum, and identified intelectin 3 (itln3) among the highly up-regulated genes. In order to clarify the in vivo significance of Itln3 in immunity, we created nonsense itln3 mutant zebrafish by CRISPR/Cas9 mutagenesis and analyzed the outcome of M. marinum infection in both zebrafish embryos and adult fish. The lack of functional itln3 did not affect survival or the mycobacterial burden in the zebrafish. Furthermore, embryonic survival was not affected when another mycobacterial challenge responsive intelectin, itln1, was silenced using morpholinos either in the WT or itln3 mutant fish. In addition, M. marinum infection in dexamethasone-treated adult zebrafish, which have lowered lymphocyte counts, resulted in similar bacterial burden in both WT fish and homozygous itln3 mutants. Collectively, although itln3 expression is induced upon M. marinum infection in zebrafish, it is dispensable for protective mycobacterial immune response.

Zebrafish intelectin 1 (zITLN1) plays a role in the innate immune response

Intelectin displays carbohydrate binding capacity and has been demonstrated to agglutinate bacteria, suggesting its role in innate immunity. It has also been linked to many pathogenic conditions in human. After reporting two amphioxus orthologs and the zebrafish intelectin 2 (zITLN2), here we cloned and characterized zebrafish intelectin 1 (zITLN1). Like zITLN2, zITLN1 also contains a conserved fibrinogen-related domain (FReD) and a unique intelectin domain (ITLN-D), expresses in all the tissues tested, with the highest level in intestine, and responds to bacterial challenge in acute phase. We also expressed zITLN1 in E. coli system, and purified recombinant zITLN1 could agglutinate both Gram-positive and Gram-negative bacteria in a calcium dependent manner. Its ability to agglutinate Gram-positive bacteria is stronger than that to Gram-negative bacteria whereas zITLN2 did not show such preference. This is probably due to the fact that recombinant zITLN1 could bind peptidoglycan (PGN) with a higher degree to lipopolysaccharide (LPS). Our results of zITLN1 provided new insight into the evolution and function of the intelectin family.