A C-type lectin (CL11X1-like) from Nile tilapia (Oreochromis niloticus) is involved in host defense against bacterial infection

Collectin-K1 Plays a Role in the Clearance of Streptococcus agalactiae in Nile Tilapia (Oreochromis niloticus)

Collectin-K1 (CL-K1) is a multifunctional C-type lectin that has been identified as playing a crucial role in innate immunity. It can bind to carbohydrates on pathogens, leading to direct neutralization, agglutination, and/or opsonization, thereby inhibiting pathogenic infection. In this study, we investigated a homolog of CL-K1 (OnCL-K1) in Nile tilapia (Oreochromis niloticus) and its role in promoting the clearance of the pathogen Streptococcus agalactiae (S. agalactiae) and enhancing the antibacterial ability of the fish. Our analysis of bacterial load displayed that OnCL-K1 substantially reduced the amount of S. agalactiae in tissues of the liver, spleen, anterior kidney, and brain in Nile tilapia. Furthermore, examination of tissue sections revealed that OnCL-K1 effectively alleviated tissue damage and inflammatory response in the liver, anterior kidney, spleen, and brain tissue of tilapia following S. agalactiae infection. Additionally, OnCL-K1 was found to decrease the expression of the pro-inflammatory factor IL-6 and migration inhibitor MIF, while increasing the expression of anti-inflammatory factor IL-10 and chemokine IL-8 in the spleen, anterior kidney, and brain tissues of tilapia. Moreover, statistical analysis of survival rates demonstrated that OnCL-K1 significantly improved the survival rate of tilapia after infection, with a survival rate of 90%. Collectively, our findings suggest that OnCL-K1 plays a vital role in the innate immune defense of resisting bacterial infection in Nile tilapia. It promotes the removal of bacterial pathogens from the host, inhibits pathogen proliferation in vivo, reduces damage to host tissues caused by pathogens, and improves the survival rate of the host.

Leukolectin is expressed in lectophages, a distinct population of zebrafish embryonic macrophages

Leukolectins (LL) belong to the tectonin-family of proteins, with functions in innate immunity. Fish larvae compensating for loss of maternal chorionic protection post-hatching, provide a model-system for studying how lectins contribute to immunity. Atlantic salmon (Ssal) LL-proteins function after secretion in mucus from dermal lectocytes, as this mucus envelops embryos and larvae. The Ssalll-gene possesses multiple putative binding sites for diverse transcription-factors, suggestive of LL-functions in non-epithelial cells. Since zebrafish (zF) perivitelline fluid (PVF) contains LL-proteins, this study aims to characterize zF-leukolectins, their cellular origin, expression and gene structure. Extracts of (10 hpf) zF-embryos contained LL-proteins, and whole mount immuno-histochemistry revealed dispersed LL-positive cells including zF-lectocytes, accounting for exocrine LL-secretion by embryos. Lectocytes are lcp1-negative, but other zF-cells co-expressed LL-proteins and lcp1-transcripts, which (at this stage) identified such non-lectocytes as early macrophages (termed lectophages). In sections, LL-expression characterized large macrophage-progenitors and smaller colonizing macrophages. RT- and RACE-PCR yielded zF-LLcDNA including parts of untranslated regions. ORF encoded 255 AAs including (19 AA) signal peptide. Processing of a primary LL-transcript to (∼1.300 nt) LL-mRNA was suggested by Northern blots. Most zebrafish-egg lectins (zFELs) possess four TECPR-domains, while five TECPR-domains were predicted for zF-LL. Minor sequence variations suggested nearly identical zF-LL isoforms. Alignment of zFEL-proteins predicted a zFEL-tree with a separate leukolectin-branch. LL-amplification using zF-DNA, revealed five exons and four introns. Predicted structures of zF- and Ssal-leukolectins showed strong structural conservation (92% sequence-identity) with shorter zF-introns 2&4, but identical introns 1&3. Non-lectocytic LL-functions were investigated further by dual in situ hybridization, revealing that only some embryonic lcp1-expressing cells in early zF-embryos co-expressed LL-transcripts. Macrophages from erythro-myeloid progenitor (EMP) are known to colonize zebrafish tissues as resident macrophages (TRM), e.g. nervous system (CNS) and epiderm. Unlike Ssal-larvae relying on yolk for months, zF-larvae switch within days to nutrition from the digestive-tract, necessitating additional immuno-protection possibly from TRMs. EMP also gives rise to microglia, the TRM of CNS. The neural tube of zF-embryos exhibited numerous small, LL-positive cells, presumably stemming from lectophage-progenitors. Functions of these LL-positive embryonic microglia (lectoglia) appear more relevant for tissue remodelling than for pathogenic threats. Lectoglia sustaining CNS-neurons suggests physiological LL-roles relevant for adult health and disease. The data focus the need for resolving whether lectophages represent an unrecognized myelogenic lineage, or whether instead, LL-expression occurs in a subpopulation of the early macrophage-lineage.

Leukolectin-proteins are synthesized and secreted by lectocytes, a distinct category of fish embryonic mucus cells

Fish perivitelline fluid (PVF) is a vital extra-embryonic compartment. At hatching, PVF-contents dissolve into the hatching fluid (HF). Analysis of Atlantic salmon HF reveals nearly hundred distinct proteins, most of which were identified by advanced mass-spectometry. However, one entity with apparent molecular weight 26 kDa, necessitated identification from its tryptic peptides. Subsequent cloning and sequencing revealed novel leukolectin-proteins. From bioinformatic analysis, leukolectins (LL) belong in the tectonin protein-family, with recognized functions in innate immunity. This study aims to identify LL-expressing cells in diverse fish species, and to characterize the ll-gene in order to predict bio-functions of leukolectins. LL-proteins were detected in HF from several fish species and one invertebrate, using polyclonal LL-specific IgGs. Embryonic LL-immunoreactive cells were numerous in Atlantic salmon, rainbow trout, fewer in Atlantic cod, and rare in Oikopleura dioica, Atlantic halibut. LL-immunoreactive cells were termed lectocytes, which corresponded to peridermal mucus-cells stained by PAS, but unstained by eosin. Hence, lectocytes and hatching-gland cells were clearly distinguished. Northern blots revealed two salmon LL-transcripts at mid-embryogenesis. Such transcripts were detected in epithelial cells of the periderm, gills and oral cavity. LL-transcripts predominated in the periderm, while choriolysin-transcripts were dominant in the gills. No co-expression of choriolysins and LL-transcripts was detected. BAC-library screening yielded salmon LL's gene-structure with 4 introns, 5 exons, TATA-box, multiple upstream putative transcription-factor binding-sites and polyadenylation site. ll-gene location on chromosome ssa17 was identified in Ssal_v3.1, the 2021-version of the salmon genome. In conclusion, larvae from several fish species are outfitted with mucus enriched by LL-proteins. Mucus cells are present in embryos of all fishes, but embryonic lectocyte-numbers are far higher in species with near total larval survival. When (maternal) chorionic first-line immuno-defence is lost at hatching, leukolectin-enriched mucus may provide vital protection for larvae.

Identification and functional characterization of CL-11 in black rockfish (Sebastes schlegelii)

CL-11 (Collectin-11, also known as Collectin kidney-1 or CL-K1) is a member of collectin family that works as a pattern recognition molecule (PRM) and participating in lectin-complement pathway in host defense against pathogens. We identified the CL-11 homologue SsCL-11 in black rockfish (Sebastes schlegelii) and investigated the functional characteristics in this study. The SsCL-11 has conserved protein modules, i.e. an N-terminal hydrophobic region, a collagen-like region, an α-helical neck region and a carbohydrate recognition domain (CRD). SsCL-11 has varying degrees of expressions in difference tissues, among which the highest expression is observed in liver. It also shows induced expressions in immune-related tissues following Aeromonas salmonicida (A. salmonicida) infection. In addition, SsCL-11 exhibits binding abilities to different kinds of carbohydrates, pathogen-associated molecular patterns (PAMPs) and bacteria. It exhibits comparatively strong binding to l-fucose, d-mannose, and d-glucose, which is consistent with the functional EPN motif in its CRD. SsCL-11 also shows agglutinating effects on various bacteria in the presence of Ca²⁺. Furthermore, SsCL-11 is confirmed to be a secretory lectin and can form multimers. These findings collectively demonstrate that SsCL-11 can function as a recognition molecule in pathogen resistance in black rockfish, which will promote our understanding of immunological roles of fish collectins.

A novel C-type lectin (CLEC12B) from Nile tilapia (Oreochromis niloticus) is involved in host defense against bacterial infection

C-type lectin (CLEC) is a family of carbohydrate-binding protein that has high affinity for calcium and mediates multiple biological events including adhesion between cells, the turnover of serum glycoproteins, and the innate immune system's reaction to prospective invaders. However, it's ill-defined for how CLEC effects bony fish's innate immunity to bacterial infection. Therefore, CLEC12B, a member of the C-type lectin domain family, was found in Nile tilapia (Oreochromis niloticus) and its functions in bacterial infection were examined. The OnCLEC12B consist of a C-type lectin domain, a transmembrane domain, and a hypothetical protein of 308 amino acids that encoded by 927 bp basic group. Besides, the OnCLEC12B protein have a series of highly conserved amino acid sites with other CLEC12B proteins. Subcellular localization showed that OnCLEC12B located in cell membrane. Transcriptional levels investigation showed that OnCLEC12B was extensively expressed in all selected organs and has high expression in the liver. The transcriptional levels of OnCLEC12B were induced by Streptococcus agalactiae and Aeromonas hydrophila in the liver, spleen, head kidney, brain, and intestine. Afterward, invitro study revealed that several kinds of pathogens could be bound and agglutinated by recombinant protein of OnCLEC12B (rOnCLEC12B). Moreover, rOnCLEC12B could not only promote the proliferation of monocytes/macrophages but also encourage its phagocytosis on S.agalactiae and A.hydrophila, and its over-expression could significantly suppress the activation of the NF-κB pathway. Summarily, our results indicated that OnCLEC12B gets involved in fish immunization activities to pathogens infection.

Functional characterization of complement factor H in host defense against bacterial pathogen in Nile tilapia (Oreochromis niloticus)

Complement factor H (CFH), a multifunctional soluble complement regulatory protein, can bind to a variety of pathogens and play a crucial role in host innate immune defense. To explore the functional characteristics of CFH (OnCFH) in Nile tilapia (Oreochromis niloticus), we cloned and characterized the open reading frame (ORF) of OnCFH in this study. The full-length of OnCFH ORF is 1359 bp, encoding 452 aa for a 48.85 kDa peptide, and its predicted structure containing six short complement-like repeats (SCRs). The analysis of tissue distribution showed that OnCFH was constitutively expressed in all tested tissues, with the highest in the liver. Upon Streptococcus agalactiae and Aeromonas hydrophila stimuli in vivo and in vitro, OnCFH mRNA transcript was significantly upregulated in head kidney tissue as well as head kidney monocytes/macrophages. Further, the recombinant OnCFH protein ((r)OnCFH) could bind to pathogenic bacteria in a dose-dependent. Moreover, it got involved in the regulation of inflammation as well as phagocytosis of monocytes/macrophages. The knockdown of OnCFH remarkably decreased the amount of bacteria in the head kidney. In summary, our data demonstrated that OnCFH could participate in the immune response of Nile tilapia against bacterial infection.

Affinity-Driven Site-Specific High Mannose Modification Determines the Structural Polymerization and Function of Tetrameric IgM in a Primitive Vertebrate

Teleost tetramer IgM is the predominant Ig in the immune system and plays essential roles in host defense against microbial infection. Due to variable disulfide polymerization of the monomeric subunits, tetrameric IgM possesses considerable structural diversity. Previous work indicated that the teleost IgM H chain was fully occupied with complex-type N-glycans. However, after challenge with trinitrophenyl (TNP) Ag, the complex N-glycans in the Asn-509 site of Oreochromis niloticus IgM H chain transformed into high mannose. This study, therefore, was conducted to examine the functional roles of the affinity-related high-mannose modification in tilapia IgM. The TNP-specific IgM Ab affinity maturation was revealed in tilapia over the response. A positive correlation between TNP-specific IgM affinity and its disulfide polymerization level of isomeric structure was demonstrated. Mass spectrometric analysis indicated that the relationship between IgM affinity and disulfide polymerization was associated with the Asn-509 site-specific high-mannose modification. Furthermore, the increase of high mannose content promoted the combination of IgM and mannose receptor (MR) on the surface of phagocytes. Moreover, the increased interaction of IgM and MR amplified the phagocytic ability of phagocytes to Streptococcus agalactiae. To our knowledge, this study demonstrates that site-specific high-mannose modification associates with IgM Ab affinity and its structural disulfide polymerization and amplifies the phagocytosis of phagocytes by the combination of IgM and MR. The present study provides evidence for understanding the association of IgM structure and function during the evolution of the immune system.

Nile tilapia TLR3 recruits MyD88 and TRIF as adaptors and is involved in the NF-κB pathway in the immune response

TLR3 plays a crucial role in innate immunity. In the present study, OnTLR3 was identified in the Nile tilapia Oreochromis niloticus, with a conserved LRR domain and a C-terminal TIR domain. OnTLR3 was broadly expressed in all tissues tested, with the highest expression levels in the blood and the lowest in the kidney. TLR3 mRNA could be detected from pharyngula (2.5 dpf) to late larva (8.5 dpf) during embryonic and larval development. Moreover, the expression level of OnTLR3 was clearly altered in all five tissues after Streptococcus agalactiae infection in vivo and could be induced by LPS, poly(I:C), S. agalactiae WC1535 and △CPS in Nile tilapia macrophages. When OnTLR3 was overexpressed in 293 T cells, it was distributed in the cytoplasm and could significantly increase NF-κB activation. The pulldown assays showed that OnTLR3 interacted with both OnMyD88 and OnTRIF. The binding assays revealed the specificity of OnTLR3 for pathogen-associated molecular patterns (PAMPs) and bacteria that included S. agalactiae, Aeromonas hydrophila and poly(I:C), LPS and PGN. Taken together, these findings suggest that OnTLR3, as a pattern recognition receptor (PRR), might play an important role in the immune response to pathogen invasion.

Advances in the complement system of a teleost fish, Oreochromisniloticus

As the earliest known vertebrate possessing a complete immune system, teleost fish played an important role in the evolution of this system. The complement system is an ancient defense mechanism present in invertebrates and vertebrates. In teleost fish the complement system is formed by more than 35 circulating proteins, or found at the cell surface. This system is activated by three pathways: alternative, classical and lectin, generating functions such as the opsonization, lysis and modulation of the innate and adaptive immune responses. The complement system is an important immunological indicator that can be used to study and monitor the effects of environmental, nutritional, and infectious processes. The Nile tilapia (Oreochromis niloticus) is a teleost fish of great economic interest due to its characteristics of easy cultivation, high growth rates, and tolerance to adverse environmental conditions. In addition, Nile tilapia is an excellent model for ecotoxicological studies, however, there are very few studies reporting the performance of the complement system in this species after exposure to environmental pollutants. The aim of this review is to gather recent studies with to address the molecular and functional characterizations of the complement system in Nile tilapia and provide new insights about this defense mechanism. Looking to the future, we believe that the complement system analysis in Tilapia can be used as a biomarker of water quality and the general health status of fish.

TLR1 in Nile tilapia: The conserved receptor cannot interact with MyD88 and TIRAP but can activate NF-κB in vitro

Toll-like receptors (TLRs) play a critical role in the innate immune response of fish. In this study, we isolated the cDNA sequence of Nile tilapia TLR1 (OnTLR1). The deduced OnTLR1 protein contains a signal peptide, 7 leucine-rich repeats (LRRs), a C-terminal LRR (LRR-CT), a transmembrane region and a highly conserved TIR domain. In healthy Nile tilapia, the OnTLR1 transcript was broadly expressed in all examined tissues, with the highest expression levels in the spleen. After infection with Streptococcus agalactiae, the OnTLR1 transcripts were upregulated in the gill and kidney. After stimulation with polyinosinic-polycytidylic acid (poly(I:C)), the expression levels of OnTLR1 were significantly downregulated in the intestine, whereas OnTLR1 transcripts were significantly upregulated in the kidney. After challenge with lipopolysaccharide (LPS), the expression levels of OnTLR1 were significantly upregulated in the spleen and kidney. The subcellular localization showed that OnTLR1 was expressed in the cytoplasm. TLR1 significantly increased MyD88-dependent NF-κB activity. However, the results of a pull-down assay showed that OnTLR1 did not interact with MyD88 or TIRAP. Binding assays revealed the specificity of OnTLR1 for pathogen-associated molecular patterns (PAMPs) and bacteria that included S. agalactiae, Aeromonas hydrophila and poly(I:C) and LPS. Taken together, these findings suggest that OnTLR1, as a pattern recognition receptor (PRR), might play an important role in the immune response to pathogen invasion.

Expression and functional characterization of the mannose receptor (MR) from Nile tilapia (Oreochromis niloticus) in response to bacterial infection

Mannose receptor (MR) as a member of the pattern recognition receptors (PRRs) plays an important role in the immune response. In mammals, the role of MR in the regulation of phagocytosis is clarified; however, its contribution to opsonize phagocytosis remains unclear in bony fish. In this study, the expression pattern of Nile tilapia mannose receptor gene (OnMR) was investigated and its regulation of the phagocytosis of monocytes/macrophages to pathogenic bacteria was identified. The full-length of OnMR open reading frame is 4314 bp, encoding a peptide containing 1437 amino acid residues. The deduced amino acid sequence revealed that OnMR contained a cysteine-rich domain, a fibronectin type II domain, multiple C-type lectin-like domains, a transmembrane domain and a short cytoplasmic domain. Tissue distribution analysis showed the OnMR transcripts was widely distribute in the ten detected tissues, and highly expressed in head kidney, hind kidney, intestine and spleen. After S. agalactiae and A. hydrophila infection, the expression of OnMR in head kidney and spleen increased significantly. Moreover, the expression of OnMR in MO/Mø were also upregulated post the infection of bacteria and mannose solutions in vitro. This suggested that MR, as a mannose receptor on macrophage surface, could respond strongly to the stimulation of pathogenic bacteria. In addition, the (r)OnMR protein could effectively bind and agglutinate S. agalactiae and A. hydrophila, and regulate the phagocytic ability of monocytes/macrophages to pathogenic bacteria. These results suggest that OnMR is involved in response against bacterial infection in Nile tilapia, and this study will help us better understand the function of MR in teleost fish.

Identification and characterization of a C-type lectin in turbot (Scophthalmus maximus) which functioning as a pattern recognition receptor that binds and agglutinates various bacteria

C-type lectins (CTLs) are important pathogen pattern recognition receptors that recognize carbohydrate structures. In present study, a C-type lectin domain family 4 member E-like gene from turbot, which tentatively named SmCLEC4E-like (SmCLEC4EL), was identified, and the expressional and functional analyses were performed. In our results, SmCLEC4EL showed conserved synteny with CLEC4E-like genes from several fish species in genome, and possessed a typical type II transmembrane CTL architecture: an N-terminal intracellular region, a transmembrane domain and a C-terminal extracellular region which contained a predicted carbohydrate recognition domain (CRD). In addition, SmCLEC4EL exhibited the highest expression level in spleen in healthy fish, and showed significantly induced expression in mucosal tissues, intestine and skin, under bacteria challenge. Finally, the recombinant SmCLEC4EL protein combined with LPS, PGN, LTA and five different kinds of bacteria in a dose-dependent manner, and agglutinated these bacteria strains in the presence of calcium. These findings collectively demonstrated that SmCLEC4EL, a calcium-dependent CTL, could function as a pattern recognition receptor in pathogen recognition and participate in host anti-bacteria immunity.

Hemopexin as an acute phase protein regulates the inflammatory response against bacterial infection of Nile tilapia (Oreochromis niloticus)

Hemopexin, a high affinity heme-binding protein is widely involved in variety physiological and pathological processes. It is an important acute phase response protein, and is important in regulating the inflammatory response. In this study, the open reading frame of Nile tilapia hemopexin (OnHpx) gene was amplified. The expression pattern of OnHpx in natural and bacterial challenged tilapia tissues were analyzed through RT-qPCR. The results indicated the OnHpx was most abundant in liver, and increased significantly in liver, spleen, head kidney and peripheral blood after bacterial challenge. Furthermore, the OnHpx mRNA was also significantly up-regulated in monocytes/macrophages and hepatocytes under the stimulation of S. agalactiae or A. hydrophila. In addition, the recombinant OnHpx protein could effectively reduce the bacteria proliferation and alleviate the inflammatory reaction caused by bacteria. Moreover, the (r)OnHpx also regulated the respiratory burst of monocytes/macrophages and played an important role in the antioxidant process. To our knowledge, these results provide the first evidence on the antibacterial and anti-inflammatory response mechanism of Hpx in early vertebrates. This brings new insights about the understanding of the evolutionary origins and ancient roles of the Hpx in the innate immune defense.

In silico characterization and expression analysis of eight C-type lectins in obscure puffer Takifugu obscurus

C-type lectins (CTLs) are a group of carbohydrate-binding proteins that play crucial roles in innate immune defense against invading pathogens. CTLs have been extensively studied in lower vertebrates, such as fish, for their roles in eliminating pathogens; however, their homologs in pufferfish are not well known. In the present study, eight CTLs from obscure puffer Takifugu obscurus (designated as ToCTL3-10 according to the order they were discovered) were obtained. All predicted ToCTL proteins contained a single carbohydrate recognition domain (CRD). ToCTL7 also contained one calcium-binding epidermal growth factor (EGF)-like domain (EGF_CA) and a transmembrane region. ToCTL9 also contained an SCP domain, an EGF domain, and an EGF-like domain. Bioinformatics analysis revealed that ToCTL3-10 mainly clustered with the corresponding CTL homologs of other pufferfish species. Tissue distribution analysis detected ToCTL3-10 in all tissues examined, including kidneys, liver, gills, spleen, intestines, and heart. Moreover, the expressions of ToCTL3-10 were significantly induced in the kidneys of obscure puffer following challenges with three Gram-negative bacterial pathogens, namely, Vibrio harveyi, Aeromonas hydrophila, and Edwardsiella tarda, and a synthetic analog of double-stranded RNA poly(I:C). The expression patterns of ToCTL3-10 in response to different immune stimulants were different. Our results indicated that the eight ToCTLs obtained herein might be involved in host defense against bacterial and poly(I:C) infections in T. obscurus.

Characterization and functional analysis of a galectin-related protein B from Nile tilapia involved in the immune response to bacterial infection

Galectin-related protein is a kind of lectin without canonical activity that regulates cell adhesion and cell growth. In this study, a novel galectin-related protein B (OnGRPB) was identified from Nile tilapia (Oreochromis niloticus). The open reading frame of OnGRPB was 438 bp and encoded a peptide of 145 amino acids. The deduced protein sequence of OnGRPB possessed a conserved carbohydrate recognition domain (CRD) with partial sugar binding sites (N-R, V-N and W-E) and shared high identities with other fish GRPB proteins. The qRT-PCR analysis found that OnGRPB was widely distributed in various tissues and monocyte/macrophages (Mo/MΦ) of healthy tilapia. After Streptococcus agalactiae infection, OnGRPB transcripts were significantly up-regulated in liver, spleen, head kidney and Mo/MΦ. The recombinant OnGRPB protein (rOnGRPB) had the binding activity and agglutination ability to bacteria. Also, rOnGRPB could modulate antibacterial activity and inflammatory factor expression of Mo/MΦ. These data collectively indicate that OnGRPB plays roles in the immune response of Nile tilapia against bacterial infection.

A C-type lectin with antibacterial activity in weather loach, Misgurnus anguillicaudatus

C-type lectins are carbohydrate-binding proteins that play important roles in immunity by serving as pattern recognition receptors. In the present study, a novel nattectin-like C-type lectin was obtained from the weather loach, Misgurnus anguillicaudatus, designated as MaCTL. MaCTL encodes a peptide with 165 amino acids, with a signal peptide and a single C-type lectin domain (CTLD), containing a galactose-specific QPD motif and a conserved Ca²⁺ -binding site. Transcripts of MaCTL were significantly upregulated after immune challenge with its pathogen A. hydrophila. In vitro assays with recombinant MaCTL protein revealed that it exhibited hemagglutinating and bacterial agglutinating activities, in a Ca²⁺ -dependent manner. MaCTL was found to bind to a wide range of bacteria, as well as bind to bacterial polysaccharides LPS and PGN. Moreover, MaCTL displayed antimicrobial activity by inhibiting the growth of bacteria. These results collectively suggest that MaCTL is involved in the antibacterial defence of weather loach.

A tandem-repeat galectin-4 from Nile tilapia (Oreochromis niloticus) is involved in immune response to bacterial infection via mediating pathogen recognition and opsonization

Galectins are the family of carbohydrate-binding proteins that participate in host-pathogen interaction. In this study, a galectin-4 homolog (OnGal-4) from Nile tilapia (Oreochromis niloticus) was characterized. The open reading frame of OnGal-4 was 1194 bp, encoding a peptide of 397 amino including two CRD regions and two carbohydrate recognition sites. OnGal-4 mRNA was expressed in all examined tissues with the highest level in spleen. After Streptococcus agalactiae (S.agalactiae) challenge, the OnGal-4 expression was up-regulated in the spleen, head kidney, brain, and monocytes/macrophages (Mo/MΦ). The in vitro experiments showed that recombinant OnGal-4 (rOnGal-4) protein could bind and agglutinate S.agalactiae and A.hydrophila. Also, rOnGal-4 could induce cytokines expressions and increased bactericidal activity of Mo/MΦ. Further in vivo analysis indicated that OnGal-4 overexpression could protect O.niloticus from S.agalactiae infection through modulating inflammation response. Our study suggested that OnGal-4 could improve immune response against bacterial infection by mediating pathogen recognition and opsonization.

A microfibril-associated glycoprotein 4 (MFAP4) from Nile tilapia (Oreochromis niloticus) possesses agglutination and opsonization ability to bacterial pathogens

Microfibril-associated glycoprotein 4 (MFAP4), a pattern recognition-like molecule with a fibrinogen-like domain (FBG), has the ability to combine and agglutinate pathogens, playing an essential role in the first line of innate immune defense. In this study, the sequence of Nile tilapia (Oreochromis niloticus) microfibril-associated glycoprotein 4 (OnMFAP4) open reading frame (ORF) was amplified and identified. The ORF of OnMFAP4 is 720 bp of nucleotides and codes for 239 amino acids. Spatial mRNA encoding analysis indicated that OnMFAP4 was highly produced in liver, intestine and head kidney in healthy tilapia, and with the lowest expression in muscle. After challenges with Streptococcus agalactiae (S. agalactiae) and Aeromonas hydrophila (A. hydrophila), the expression of OnMFAP4 mRNA was prominently produced in the liver, spleen and head kidney. The up-regulation of OnMFAP4 expression was also presented in head kidney monocytes/macrophages (MO/MΦ) and hepatocytes. Recombinant OnMFAP4 ((r)OnMFAP4) could bind and agglutinate both bacterial pathogens. Moreover, (r)OnMFAP4 could take part in the modulation of inflammation and phagocytosis. In conclusion, this study revealed that OnMFAP4 might take effect in host defense against bacterial infection in Nile tilapia, with agglutination and opsonization capability to bacterial pathogens.

A Siglec-1-like lectin from Nile tilapia (Oreochromis niloticus) possesses functions of agglutination and mediation of macrophage phagocytic activity

Siglec-1, one of the sialic acid-binding immunoglobulin-type lectins, is closely related to the recognition of host-pathogen and cell-cell interactions in the adaptive and innate immune systems. In this communication, a Siglec-1-like gene (OnSiglec-1-like) from Nile tilapia (Oreochromis niloticus) was analyzed. Relative expression revealed that the OnSiglec-1-like was expressed in all tested tissues, and the highest expression was found in the anterior kidney. Upon Streptococcus agalactiae (S. agalactiae) infection, the expression of OnSiglec-1-like was up-regulated in anterior kidney and spleen significantly in vivo. Additionally, the same phenomenon was observed in anterior kidney leukocytes upon LPS and S. agalactiae challenges as well in vitro. Western-blotting and ELISA analyses revealed that recombinant OnSiglec-1-like protein possessed high binding activity to LTA, LPS and S. agalactiae. Further, the recombinant OnSiglec-1-like was able to agglutinate S. agalactiae. Moreover, with the digestion of specific sialidase, the phagocytic ability of macrophages to S. agalactiae was greatly enhanced. Taken together, these results indicated that the Siglec-1-like possesses conserved functions of agglutination and promotion of macrophage phagocytic activity in Nile tilapia.

Fish Galectin8-Like Exerts Positive Regulation on Immune Response Against Bacterial Infection

Galectin-8 is a member of the galectin family that is involved in immune response against pathogens. However, the roles of fish galectin-8 during pathogen infection require comprehensive studies. In this study, a galectin-8 homolog (OnGal8-like, OnGal8-L) was characterized from Nile tilapia (Oreochromis niloticus), and its roles in response to bacterial infection were analyzed. The OnGal8-L contains an open reading frame of 891 bp, encoding a peptide of 296 amino acids with two CRD regions of tandem-repeat galectin and two carbohydrate recognition sites. The OnGal8-L protein shares 46.42% identities with reported Oreochromis niloticus galectin-8 protein. Transcriptional expression analysis revealed that OnGal8-L was constitutively expressed in all examined tissues and was highly expressed in spleen. The transcript levels of OnGal8-L were up-regulated in the spleen, head kidney, and brain, following Streptococcus agalactiae (S. agalactiae) challenge. Further in vitro analysis indicated that the recombinant protein of OnGal8-L (rOnGal8L) could agglutinate erythrocyte, S. agalactiae, and A. hydrophila and bind S. agalactiae, A. hydrophila, and various PAMPs (lipopolysaccharides, lipoteichoic acid, poly I:C, peptidoglycan, galactose, mannose, and maltose). Also, rOnGal8L could regulate inflammatory-related gene expression, phagocytosis, and a respiratory burst of monocytes/macrophages. Moreover, in vivo analysis showed that OnGal8-L overexpression could protect O. niloticus from S. agalactiae infection through modulating serum antibacterial activity (AKP, ACP, and LZM), antioxidant capacity (CAT, POD, and SOD), and monocyte/macrophage proliferation and cytokine expression, as well as reducing bacterial burden and decreasing tissue damage. Our results collectively indicate that OnGal8-L plays important regulatory roles in immune response against bacterial infection.

Characterization and Functional Analysis of Two Transmembrane C-Type Lectins in Obscure Puffer (Takifugu obscurus)

C-type lectins (CTLs) have received widespread attention in animal immune responses. In the present study, two CTLs (ToCTL1 and ToCTL2) were identified from obscure puffer Takifugu obscurus. The open reading frames of ToCTL1 and ToCTL2 were 687 and 1,380 bp, respectively. The predicted ToCTL1 and ToCTL2 proteins contained a single transmembrane region and one typical carbohydrate recognition domain (CRD). Quantitative real-time polymerase chain reaction detected ToCTL1 and ToCTL2 transcripts in all examined tissues, with high levels in the intestine and kidney, and their expression levels were remarkably altered upon Vibrio harveyi and Aeromonas hydrophila infection. The recombinant proteins ToCTL1-CRD and ToCTL2-CRD agglutinated the Gram-negative and Gram-positive bacteria in a Ca²⁺-dependent manner. rToCTL1-CRD and rToCTL2-CRD exhibited evident binding activities against seven kinds of bacteria and polysaccharides (lipopolysaccharide and peptidoglycan) in a Ca²⁺-independent manner. Moreover, rToCTL1-CRD and rToCTL2-CRD could inhibit the growth of four types of bacteria in vitro. These findings collectively demonstrated that ToCTL1 and ToCTL2 could be involved in host defense against bacterial infection in T. obscurus.

CD38 play roles in T cell-dependent response and B cell differentiation in nile tilapia (Oreochromis niloticus)

CD38 is a multifunctional cell surface molecule that plays a crucial role in B cell activation, differentiation, and maturation in mammals with an increased expression in B cell maturation. In this study, a CD38-like molecule (OnCD38) was cloned and identified from Nile tilapia (Oreochromis niloticus), and its functional characterization was investigated. The open reading frame of OnCD38 is 828 bp of the nucleotide sequence, encoding a polypeptide of 275 amino acids. The deduced amino acid sequence of OnCD38 is highly homologous to other teleost fish and similar to mammals, containing extracellular, intracellular and transmembrane regions. Subcellular localization studies revealed that OnCD38 molecules were presented on the surface of B cells. Three healthy tilapia were used in each experimental group and control group. Following keyhole limpet hemocyanin (KLH) challenge in vivo, the mRNA expression of OnCD38 was significantly up-regulated in peripheral blood, spleen, and head kidney, with an earlier up-regulation in the second challenge than the first one. The up-regulation of OnCD38 expression was also detected in head kidney leukocytes after stimulation with LPS, recombinant HomoIL-10 ((r)HomoIL-10), (r)OnIL-10, and LPS plus (r)OnIL-10 in vitro. Furthermore, the OnCD38 expression increased with the differentiation of B cells, reaching a high level (10.1 fold higher than resting mature B cells) at the plasma-like B cells. Taken together, in this study, these results indicate that the OnCD38 is likely involved in the T cell-dependent response and plays roles in B cell differentiation in Nile tilapia.

Molecular characterization and complement activating functional analysis of a new collectin(TfCol-11) from Trachidermus fasciatus

The complement system is a crucial component of the innate immune system that links innate and adaptive immunity. CL-11, a protein similar to Mannose-binding lectin (MBL), plays significant role in the innate immune system in mammals and fish, serving as an initiator of the lectin pathway of complement activation. In this study, a CL-11 homolog (TfCol-11) was identified in roughskin sculpin (Trachidermus fasciatus), and its expression and role in immune responses were characterized. The open reading frame of TfCol-11 is 795 bp long, encoding a 264 amino acid polypeptide. The deduced amino acid sequence of this protein is highly homologous to sequences in other teleosts, and is similar to vertebrate CL-11, containing a canonical collagen-like region, a carbohydrate recognition domain, and a neck region. Recombinant TfCol-11 purified from Escherichia coli(E.coli) was able to bind to different microbes in a Ca²⁺-independent manner. Meanwhile, a 993 bp-long of partial MASP cDNA with a 96 bp 5' untranslated region (UTR) was also cloned from roughskin sculpin, containing 299 amino acids and consisting of three domains (CUB-EGF-CUB). qRT-PCR indicated that TfCol-11 and MASP mRNAs were predominately co-expressed in the liver. The temporal expression of TfCol-11 and MASP were both drastically up-regulated in the liver, skin, and blood by LPS challenge. Recombinant TfCol-11 purified from E.coli BL21(DE3) was able to agglutinate some bacteria in a Ca²⁺-dependent manner. In addition, an in vitro pull-down experiment demonstrated that TfCol-11 was able to bind to MASP, and in vivo experiments showed that TfCol-11 was associated with increased membrane attack complex (MAC) levels. It is therefore possible that TfCol-11 may plays a role in activating the complement system and in the defense against invading microorganisms in roughskin sculpin.

Functional characterization of galectin-3 from Nile tilapia (Oreochromis niloticus) and its regulatory role on monocytes/macrophages

Galectin-3 is a kind of β-galactoside-binding lectin involved in host defense against pathogen infection. However, the immune functions of fish galectin-3 remain poorly understood. In this study, the roles of a fish galectin-3 (OnGal-3) from Nile tilapia (Oreochromis niloticus) on the binding activity on bacterial pathogens or PAMPs, the agglutinating activity on bacterial pathogens and the regulatory effects on monocytes/macrophages activity were investigated. After in vitro challenge of Streptococcus agalactiae and Aeromonas hydrophila, OnGal-3 expressions were significantly up-regulated in monocytes/macrophages. In addition, recombinant OnGal-3(rOnGal-3) protein showed strong binding activity on bacterial pathogens or PAMPs. Also, rOnGal-3 agglutinated Gram-positive and Gram-negative bacteria. Moreover, rOnGal-3 could induce the inflammatory factors expressions in monocytes/macrophages and enhance phagocytosis and respiratory burst activity of monocytes/macrophages. These results suggest that fish galectin-3 participates in anti-bacterial immune response through recognizing pathogens and modulating monocytes/macrophages activity.

Molecular and functional characterization of a mannose-binding lectin/ficolin-associated protein (MAp44) from Nile tilapia (Oreochromis niloticus) involved in the immune response to bacterial infection

The lectin pathway of the complement system has a pivotal role in the defense against infectious organisms. Mannose-binding lectin/ficolin-associated protein (MAp44), a multifunctional complement regulator, regulates the complement activation by competing with MASP-1, MASP-2 and MASP-3 for MBL and ficolin binding sites. In this study, we described the identification and functional characterization of a MAp44 homologue (OnMAp44) from Nile tilapia (Oreochromis niloticus) at molecular, cellular and protein levels. The open reading frame (ORF) of OnMAp44 is 1140 bp of nucleotide sequence encoding a polypeptide of 379 amino acids. The deduced amino acids sequence has four characteristic structures, including two C1r/C1s-Uegf-BMP domains (CUB), one epidermal growth factor domain (EGF) and one complement control protein domains (CCP). Expression analysis revealed that the OnMAp44 was highly expressed in liver, and widely existed in other examined tissues. In addition, the OnMAp44 expression was significantly up-regulated in spleen and head kidney following challenges with Streptococcus agalactiae and Aeromonas hydrophila. The up-regulations of OnMAp44 mRNA and protein expression were also observed in hepatocytes and monocytes/macrophages in vitro stimulation with S. agalactiae and A. hydrophila. Recombinant OnMAp44 protein was able to participate in the regulation of inflammation and migration reaction. Taken together, the results indicated that OnMAp44 was likely to involve in the immune response to bacterial infection in Nile tilapia.

Comparative transcriptome analysis of the transcriptional heterogeneity in different IgM+ cell subsets from peripheral blood of Nile tilapia (Oreochromis niloticus)

In teleost fish, IgM⁺ B cells play important roles in innate and adaptive immunity. Different IgM⁺ B cells are detected in teleost, named IgM^lo and IgM^hi B cell subsets, according to the distinct expression levels of membrane IgM (mIgM). However, the study on the heterogeneity in IgM⁺ B cell subsets remains poorly understood. In this study, the comparative transcriptomic profiles of IgM^-, IgM^lo and IgM^hi from peripheral blood of Nile tilapia (Oreochromis niloticus) were carried out by using RNA-sequencing technique. A total of 6045 and 5470 differentially expressed genes (DEGs) were detected in IgM^lo and IgM^hi cells, respectively, as compared with IgM^- lymphocytes, whereas 3835 genes were differentially expressed when IgM^lo compared to IgM^hi cells. Analysis of the KEGG database indicated that the DEGs were enriched in immune system categories and signaling transduction and interaction in IgM^- vs IgM^hi, IgM^- vs IgM^lo and IgM^lo vs IgM^hi. Comparatively, in IgM^lo vs IgM^hi, GO enrichment analysis indicated that the DEGs enriched in nucleic acid binding transcription factor activity. Analysis of crucial transcription factors for B cell differentiation indicated that IgM^lo and IgM^hi cell clusters belonged to the different B cell subsets. The data generated in this study may provide insights into understanding the heterogeneity of IgM⁺ cells in teleost, and suggest that IgM⁺ B cells play a crucial role in innate immunity.

Characterization of a tandem-repeat galectin-9 from Nile tilapia (Oreochromis niloticus) involved in the immune response against bacterial infection

Galectin-9 is a <beta>-galactoside-binding lectin which could modulate a variety of biological functions including recognition, aggregation and clearance of pathogen. In this study, a galectin-9 homologue (OnGal-9) was identified from Nile tilapia (Oreochromis niloticus) and its expression model and biological effects on bacterial infection were analyzed. The open reading frame of OnGal-9 sequence was 975 bp encoding 324 amino acids. It shares 45%-92% identities with other galectin-9 proteins. The deduced mature peptide of OnGal-9 possesses two conserved carbohydrate recognition domain (CRD) that connected with a linker peptide. Expression analysis indicated that OnGal-9 was distributed in all the tested tissues of healthy tilapia. The OnGal-9 expression was significantly up-regulated in spleen, head kidney, and intestine after challenged by Streptococcus agalactiae. Meanwhile, the recombinant OnGal-9 (rOnGal-9) protein displayed strong binding and agglutination activity toward both Streptococcus agalactiae and Aeromonas hydrophila. Moreover, rOnGal-9 could promote phagocytosis of macrophages. Taken together, the results here indicate that OnGal-9 might be involved in the immune response of Nile tilapia against bacterial infection.

Blimp-1 is involved in B cell activation and maturation in Nile tilapia (Oreochromis niloticus)

B lymphocyte-induced maturation protein 1 (Blimp-1), a transcription factor containing zinc finger, is required and sufficient to trigger terminal differentiation of B cells in mammals. The Blimp-1 (OnBlimp-1) from Nile tilapia (Oreochromis niloticus) was identified and characterized its expression pattern during B cell activation and maturation. The cDNA of OnBlimp-1 open reading frame is 2547 bp encoding a protein of 848 amino acids and the predicted molecular weight is 93.36 kDa. OnBlimp-1 contains a SET domain and five ZnF_C2H2 domains, which shares high homology with that of other species. OnBlimp-1 transcription was detected in all examined tissues with high expression in the spleen (SPL). Analysis of sorted lymphocyte populations, including IgM⁺ and IgM^- cells from peripheral blood (PBL), SPL and anterior kidney (AK), indicated that the OnBlimp-1 transcription was highly expressed in the IgM⁺ B cells. Upon LPS stimulation, OnBlimp-1 expression was up-regulated in tissues of PBL, SPL and AK significantly. The expression of OnBlimp-1, as well as the secreted IgM, was significantly up-regulated in the SPL and AK leukocytes stimulated with anti-OnIgM monoclonal antibody and LPS in vitro, respectively. Above results suggest that OnBlimp-1, a cytokine regulating the terminal differentiation of activated B cells to antibody-secreting cells, is likely to play important roles in B cell activation and maturation in Nile tilapia.

Expression and functional characterization of a mannose-binding lectin-associated serine protease-1 (MASP-1) from Nile tilapia (Oreochromis niloticus) in host defense against bacterial infection

Mannose-binding lectin-associated serine protease-1 (MASP-1), a multifunctional serine protease, plays an important role in innate immunity which is capable of activating the lectin pathway of the complement system and also triggering coagulation cascade system. In this study, a MASP-1 homolog (OnMASP-1) was identified from Nile tilapia (Oreochromis niloticus) and characterized at expression and inflammation functional levels. The open reading frame (ORF) of OnMASP-1 is 2187 bp of nucleotide sequence encoding a polypeptide of 728 amino acids. The deduced amino acid sequence has 6 characteristic structures, including two C1r/C1s-Uegf-BMP domains (CUB), one epidermal growth factor domain (EGF), two complement control protein domains (CCP) and a catalytic serine protease domain (SP). Expression analysis revealed that the OnMASP-1 was highly expressed in the liver, and widely exhibited in other tissues containing intestine, spleen and kidney. In addition, the OnMASP-1 expression was significantly up-regulated in spleen and head kidney following challenges with Streptococcus agalactiae and Aeromonas hydrophila. The up-regulations of OnMASP-1 mRNA and protein expression were also demonstrated in hepatocytes and monocytes/macrophages in vitro stimulation with S. agalactiae and A. hydrophila. Recombinant OnMASP-1 protein was likely to participate in the regulation of inflammatory and migration reaction by monocytes/macrophages. These results indicated that OnMASP-1, playing an important role in innate immunity, was likely to involve in host defense against bacterial infection in Nile tilapia.

Molecular characterization and transcriptional expression of a B cell transcription factor Pax5 in Nile tilapia (Oreochromis niloticus)

Pax5 (Paired Box 5), a nuclear transcription factor expressed in B cell specifically, is a key regulator for B cell activation. In this study, we cloned and identified a Pax5 gene (OnPax5) from Nile tilapia (Oreochromis niloticus), which has an open reading frame of 1278 bp, encoding deduced amino acid sequence of 425 residues. OnPax5 contains a conserved DNA-binding domain encoding the paired box, an octapeptide, a homeobox homology region, a transactivation and a repressor domain. OnPax5 is constitutively expressed in various analyzed tissues of tilapia, with a relatively high expression in lymphoid organs, including spleen (SPL), anterior kidney (AK), and thymus. What's more, OnPax5 is highly expressed in leukocytes especially in IgM⁺ lymphocytes sorted from peripheral blood (PBL), SPL and AK. When stimulated with lipopolysaccharide (LPS) in vivo, OnPax5 expression was significantly up-regulated in PBL, SPL and AK. Upon stimulation with LPS, pokeweed mitogen and mouse anti-OnIgM monoclonal antibody in vitro, the expression of OnPax5 was also significantly up-regulated in leukocytes from SPL and AK. Taken together, Pax5, the B cell lineage specific activator factor, might get involved in B cell activation in Nile tilapia.

Characterization of Galectin-2 from Nile tilapia (Oreochromis niloticus) involved in the immune response to bacterial infection

galectin-2 plays important roles in innate and adaptive immunity. In this study, galectin-2 (OnGal-2) was identified from Nile tilapia (Oreochromis niloticus). Its tissue distribution and expression patterns following bacterial infection were also investigated. OnGal-2 is widely distributed in various tissues of healthy tilapia. After Streptococcus agalactiae challenge, OnGal-2 expressions were significantly up-regulated in all tested tissues. Meanwhile, the recombinant OnGal-2 (rOnGal-2) protein showed strong agglutinating activities against both Gram-negative bacteria and Gram-positive bacteria. Moreover, rOnGal-2 could promote phagocytosis of macrophages. Taken together, the present study indicated that OnGal-2 might play roles in the immune responses of Nile tilapia against bacterial pathogens.

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.

B cell receptor accessory molecule CD79 gets involved in response against Streptococcus agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

CD79, composed of two distinct chains called CD79a and CD79b, is a transmembrane protein that forms a B cell antigen receptor with membrane immunoglobulin, and generates a signal following antigen recognition by the B cell receptor. In this study, the CD79a (OnCD79a) and CD79b (OnCD79b) were cloned and identified from Nile tilapia (Oreochromis niloticus). The cDNA of ORF for OnCD79a and OnCD79b are 669 and 627 bp, coding 222 and 208 amino acids, respectively. The deduced protein analysis showed that both CD79a andCD79b contain an immunoreceptor tyrosine-based activation motif in their intracellular tails that used to propagate a signal in a B cell. Expression analysis revealed that both CD79a and CD79b expressed at high levels in immune tissues, such as anterior kidney and spleen, and in IgM⁺ B cells. Upon Streptococcus agalactiae (S. agalactiae) infection, the expressions of OnCD79a and OnCD79b were significantly up-regulated in anterior kidney and spleen. The significant up-regulations of OnCD79a and OnCD79b were also detected in leukocytes after in vitro challenge with S. agalactiae. Further, stimulations of LPS and anti-OnIgM monoclonal antibody induced significant up-regulations of OnCD79a and OnCD79b in leukocytes. Taken together, the results of this study indicated that CD79 molecule, playing roles in BCR signaling, was likely to get involved in host defense against bacterial infection in Nile tilapia.