Molecular characterization of goose-type lysozyme homologue of large yellow croaker and its involvement in immune response induced by trivalent bacterial vaccine as an acute-phase protein

The Interplay of TLR-NFκB Signalling Pathway and Functional Immune-Related Enzymes in the Inflammatory Response of Ciona robusta

The close phylogenetic relationship between ascidians (Tunicata) and vertebrates makes them a powerful model for studying the innate immune system. To better understand the nature and dynamics of immune responses and the mechanisms through which bacterial infections are detected and translated into inflammation in Ciona robusta, we applied an approach combining in vivo lipopolysaccharide (LPS) stimulation, immune-labelling techniques and functional enzymatic analyses. The immunohistochemistry showed that Toll-like receptor 4 (TLR4) and nuclear factor kappa B (NFκB) were expressed during the inflammatory pharynx response 4 h post-LPS, with the formation of nodules in pharynx vessel lumen. Also, the endothelium vessels were involved in the inflammatory response. Observations of histological sections from naive and buffer-inoculated ascidians confirmed an immuno-positive response. Enzyme immune parameters-which included the activity of phenoloxidase, glutathione peroxidase, lysozyme, alkaline phosphatase and esterase-showed up-modulation 4 h after LPS injection, confirming their participation during ascidian inflammatory response. These findings provide new insights into the mechanisms underlying the LPS-induced C. robusta response and suggest that a broad innate immune mechanism, as in vertebrates, is involved in the regulation of inflammatory responses. Further findings in this direction are needed to cover knowledge gaps regarding the organized set of molecular and cellular networks involved in universal immune interactions with pathogens.

Δfur mutant as a potential live attenuated vaccine (LAV) candidate protects American eels (Anguilla rostrata) from Vibrio harveyi infection

The eel farming industry is highly susceptible to Vibriosis. Although various types of vaccines against Vibriosis have been investigated, there is limited research on decreasing the virulence of Vibrions through gene knockout and utilizing it as live attenuated vaccines (LAV). In this study, we aim to develop a LAV candidate against Vibrio harveyi infection in American eels (Anguilla rostrata) using a ferric uptake regulator (fur) gene mutant strain of V. harveyi (Δfur mutant). After the eels were administrated with the Δfur mutant at the dose of 4 × 102 cfu/g body weight, the phagocytic activity of the leucocytes, plasma IgM antibody titers, activity of lysozyme and Superoxide Dismutase (SOD) enzyme, and gene expression levels of 18 immune related proteins were detected to evaluate the protection effect of the LAV. Preliminary findings suggest that the LAV achieved over 60% relative percent survival (RPS) after the American eels were challenged by a wild-type strain of V. harveyi infection on 28 and 42 days post the immunization (dpi). The protection was mainly attributed to increased plasma IgM antibody titers, higher levels of lysozyme, enhanced activity of SOD and some regulated genes encoded immune related proteins. Together, the Δfur mutant strain of V. harveyi, as a novel LAV vaccine, demonstrates promising protective effects against V. harveyi infection in American eels, thus presenting a potential candidate vaccine for fish farming.

Uncovering the antimicrobial activity of G-type lysozyme 2 derived from Cyprinus carpio mucus against bacterial and fungal pathogens

This study aimed to explore the antimicrobic potential of mucus samples collected from Cyprinus carpio and identify the specific antimicrobial peptides responsible for its activity. The crude extract was tested against various bacterial and fungal pathogens, and its protein content and profile were analyzed. Purification steps, including gel filtration chromatography, were employed to isolate the most active fraction (peak IV), which was further identified via liquid chromatography and mass spectroscopy. The results revealed varying degrees of antimicrobial activity of the crude extract against different bacterial and fungal strains, with Leclercia adecarboxylata, Candida glabrata, and Candida parapsilosis showing the highest susceptibility. SDS-PAGE analysis demonstrated the existence of multiple low molecular weight protein bands in the crude extract, while fraction IV obtained from gel filtration chromatography exhibited the strongest antimicrobial activity. Peak IV displayed a range of minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and minimum fungicidal concentration (MFC) values against the tested pathogens, spanning from 0.038 to 4.960 mg/mL. Further investigation identified the purified peptide derived from peak IV as G-type lysozyme 2, characterized by a molecular weight of 21 kDa. These findings shed light on the existence of a highly effective antimicrobial peptide, G-type lysozyme 2, within the mucus of Cyprinus carpio. This peptide demonstrates notable activity against diverse bacterial and fungal pathogens. The insights from this study enhance our understanding of the fish's antimicrobial defense mechanisms and hold promise for developing novel antimicrobial agents.

Evaluation of immunogenicity and protective efficacy of the outer membrane porin F (OprF) against Pseudomonas plecoglossicida in large yellow croaker (Larimichthys crocea)

Large yellow croaker (Larimichthys crocea) farming dominates the marine aquaculture industry in China. However, the epidemic outbreaks of visceral white nodules disease (VWND), caused by bacterial pathogen Pseudomonas plecoglossicida, have emerged as a significant concern within the large yellow croaker industry. Although vaccination is considered to be an effective method for preventing and controlling P. plecoglossicida infection, there is currently no commercially available vaccine targeting this bacterium. In the present study, the outer membrane porin F (OprF) of P. plecoglossicida was characterized and revealed a high sequence similarity with that of other Pseudomonas species. The recombinant OprF protein (rOprF) produced in Escherichia coli was then evaluated for its immunogenicity and protective role against P. plecoglossicida in large yellow croaker. The rOprF was identified to have immunogenicity by Western blot using large yellow croaker anti-P. plecoglossicida sera. Additionally, the indirect immunofluorescence assay (IIFA) provided evidence indicating the surface exposure of OprF in P. plecoglossicida. Fish vaccinated twice via intraperitoneal (IP) injection with the purified rOprF combined with commercial adjuvant ISA 763A VG exhibited a relative percent survival (RPS) of 70.60% after challenge with virulent P. plecoglossicida strain through immersion. The administration of rOprF resulted in a notable increase in specific serum antibody levels and serum lysozyme activity compared to the control groups. The immune-related genes in the spleen and head kidney of rOprF-vaccinated fish were remarkably upregulated compared with the PBS-vaccinated sham group after the P. plecoglossicida challenge. In summary, the findings of this study suggest that rOprF exhibits considerable potential in inducing a robust immune response, making it a viable candidate for vaccination against P. plecoglossicida infection in large yellow croaker.

Expression Profiles of Zebrafish (Danio rerio) Lysozymes and Preparation of c-Type Lysozyme with High Bacteriolytic Activity against Vibrio vulnificus

Lysozyme is a crucial component of the host's innate immune system. Due to its natural non-toxic and harmless characteristics, lysozyme is considered to be an ideal antibiotic substitute. In this study, we analyzed the expression profiles of lysozymes from zebrafish (Danio rerio) in uninfected or V. vulnificus-infected tissues using real-time quantitative PCR (qPCR). Furthermore, lysozymes that might be involved in the defense against V. vulnificus were selected for over-expression, and the antibacterial activity of over-expressed lysozyme proteins were evaluated using V. vulnificus. The results showed that three types of zebrafish lysozyme, i.e., c-type lysozyme (DrLysC), g1-type lysozyme (DrLysG1), and g2-type lysozyme (DrLysG2), were identified, and V. vulnificus infection significantly changed the expression levels of DrLysC and DrLysG1. Then, DrLysC and DrLysG1 were over-expressed in E. coli, and the purified recombinant DrLysC (rDrLysC) showed more potent antibacterial activity against V. vulnificus. This finding lays the foundation for further application of rDrLysC to treat V. vulnificus infection.

Molecular characterization and antibacterial activities of a goose-type lysozyme gene from roughskin sculpin (Trachidermus fasciatus)

Lysozymes, acting as antimicrobial molecules, play a vital role in the host's innate immune response to pathogen infections. In the present study, a g-type lysozyme gene termed Tf-LyzG from roughskin sculpin, Trachidermus fasciatus was firstly reported. The deduced amino acid sequence of Tf-LyzG contained 188 residues and possessed conserved catalytic residues (Glu⁷¹, Asp⁸⁴, and Asp⁹⁵). Gene expression analysis revealed that Tf-LyzG was widely distributed in the tested eleven tissues with the highest expression in the gill and could be significantly induced post lipopolysaccharide (LPS) challenge. The lysozyme activity of the purified recombinant protein (rTf-LyzG) was found to be most active at pH 5.5 and 37 °C. rTf-LyzG exhibited a wide spectrum of potent bacteriolytic activity against four Gram-positive bacteria and six Gram-negative bacteria. It also displayed a high affinity to polysaccharides on bacteria surfaces including LPS, lipoteichoic acid (LTA), and peptidoglycan (PGN). rTf-LyzG was capable of binding and agglutinating all nine bacteria. Flow cytometry assay further revealed that rTf-LyzG could disrupt the membrane of Micrococcus lysodeikticus which is confirmed by scanning electron microscope (SEM) analysis that reveals blebs around the bacterial cell membrane. In summary, these data indicate that Tf-LyzG is of great importance in the fish immune response against pathogens invasion.

Aeromonas veronii infection remarkably increases expression of lysozymes in grass carp (Ctenopharyngodon idellus) and injection of lysozyme expression cassette along with QCDC adjuvant significantly upregulates immune factors and decreases cumulative mortality

Aeromonas veronii AvX005 is a pathogenic bacterium with high toxicity to grass carp (Ctenopharyngodon idellus). The expression levels of g-type (goose-type lysozyme, Lys-g) and c-type lysozyme (chicken-type lysozyme, Lys-c) in the spleen of grass carp infected with AvX005 were significantly increased by approximately 4.5 times and 27 times, respectively. The recombinant proteins rLys-g and rLys-c produced in a recombinant expression system of Escherichia coli showed significant antibacterial activity against the pathogenic bacteria AvX005. A challenge test was conducted after rLys-g and rLys-c were expressed in grass carp L8824 liver cells, and compared with the survival rate of the control cells (46.3%), the survival rate of the experimental cells (77.6% for rLys-g and 68.6% for rLys-c) was significantly increased. Grass carp were infected with AvX005 on the second day after delivering pcDNA3.1-lys-g and pcDNA-lys-c with the Quil A/cholesterol/DDA/Carbopol (QCDC) adjuvant, and both pcDNA3.1-lys-g and pcDNA-lys-c provided 70% relative protection for grass carp. The activity of lysozyme and alkaline phosphatase in the serum of grass carp was significantly increased after injection of DNA. The expression of the immune factors IgM, C3 and IL8 in the kidney was upregulated to varying degrees for pcDNA3.1-lys-g and immune factors C3 and IgM was upregulated for pcDNA-lys-c. The results indicated that pcDNA3.1-lys-g and pcDNA-lys-c may be used as immunostimulants to protect grass carp from the pathogenic bacterium AvX005.

An Overview of Antimicrobial Activity of Lysozyme and Its Functionality in Cheese

Due to the concern of consumers about the presence of synthetic preservatives, researchers and food manufacturers have recently conducted extensive research on the limited use of these preservatives and the introduction and use of natural preservatives, such as herbal extracts and essential oils, bacteriocins, and antimicrobial enzymes. Lysozyme is a natural enzyme with antimicrobial activity that has attracted considerable attention to be potentially utilized in various industries. Since lysozyme is an intrinsic component of the human immune system and has low toxicity; it could be considered as a natural antimicrobial agent for use in food and pharmaceutical industries. Lysozyme exerts antimicrobial activity against microorganisms, especially Gram-positive bacteria, by hydrolyzing 1,4-beta-linkages between N-acetylmuramic acid and N-acetylglucosamine in the cell wall. In addition, increased antimicrobial activity of lysozyme against Gram-negative bacteria could be achieved by the modification of lysozyme through physical or chemical interactions. Lysozyme is presented as a natural preservative in mammalian milk and can be utilized as a bio-preservative in dairy products, such as cheese. Both bacteria and fungi can contaminate and spoil the cheese; especially the one that is made traditionally by raw milk. Furthermore, uncontrolled and improper processes and post-pasteurization contamination can participate in the cheese contamination. Therefore, besides common preservative strategies applied in cheese production, lysozyme could be utilized alone or in combination with other preservative strategies to improve the safety of cheese. Hence, this study aimed to review the antimicrobial properties of lysozyme as natural antimicrobial enzyme and its functionality in cheese.

Development of Recombinant Dihydrolipoamide Dehydrogenase Subunit Vaccine against Vibrio Infection in Large Yellow Croaker

Large yellow croaker (Larimichthys crocea), an economically important marine fish in China, has suffered from serious vibriosis, which has resulted in great economic losses for the large yellow croaker industry. Vaccination has been considered to be a safe and effective method to prevent and control vibriosis. However, due to the complex diversity and serotypes of the Vibrio genus, the progress of Vibrio vaccine development is still slow. In this study, we prepared recombinant Vibrio dihydrolipoamide dehydrogenase (rDLD) protein and investigated its potential as a candidate to be a subunit vaccine against Vibrio. The lysozyme activity and the rDLD-specific antibody level in sera of large yellow croakers immunized with rDLD were significantly higher than those in the control group, and the transcript levels of proinflammatory cytokines (IL-6, IL-8, IL-1β), MHC IIα/β, CD40, CD8α, IL-4/13A, and IL-4/13B were significantly up-regulated in the spleen and head kidney of large yellow croakers immunized with rDLD, suggesting that rDLD could induce both specific and nonspecific immune responses in this species. In addition, rDLD protein increased the survival rate of large yellow croakers against Vibrio alginolyticus and Vibrio parahaemolyticus, with the relative percent of survival (RPS) being 74.5% and 66.9%, respectively. These results will facilitate the development of a potential subunit vaccine against Vibrio in large yellow croaker aquaculture.

Lysozymes in Fish

In recent years, the deterioration of the aquaculture ecological environment has led to a high incidence of fish diseases. Lysozymes, important antimicrobial enzymes, play an important role in the innate immune system of fish. The studies of fish lysozymes benefit the control of fish infections caused by pathogens. In this review, we reviewed recent progress in fish lysozymes, including their classification, structural characteristics, biological functions and mechanisms, tissue distributions, and properties of their recombinant proteins, which will help us to systematically understand the fish lysozymes and facilitate their applications in the fields of food and agriculture.

Fish lysozyme gene family evolution and divergent function in early development

Lysozymes are an ancient group of antimicrobial enzymes of the innate immune system. Here we provide a comparative analysis of the evolution and function of lysozymes during early development in fish, the most speciose vertebrate group. In fishes, lineage and species-specific evolution of both C-type (chicken or conventional) and G-type (goose type) genes occurred. Phylogenetic analysis revealed that the teleost lysozyme G-type members group with the tetrapod homologues but the teleost C-type form three different clusters with the tetrapods. Most of the teleost C-type cluster with tetrapod Lyz but there are some that group with the mammalian Lyzl1/2 and LALBA. This suggests that early in gnathostome evolution these genes already existed and that lyzl1/2 and lalba genes are present in fish and tetrapods. Gene synteny analysis to confirm sequence orthologies failed to identify conserved genome regions between teleosts and other vertebrates lysozyme gene regions suggesting that in the ancestral bony fish genome lyz, lyzl1/2, lalba and lyg precursor genes were transposed to different chromosome regions. The homologue of the mammalian lactalbumin (LALBA) gene was identified for the first time in teleosts and was expressed in skin and during egg and larval development. Lysozyme activity was detected in teleost eggs and varied between species and in the gilthead sea bream lyg and lalba transcript abundance differed in eggs and larvae from different brood stock suggesting differences exist in maternal innate immune protection.

Molecular characterization and expression analysis of the chicken-type and goose-type lysozymes from totoaba (Totoaba macdonaldi)

Lysozymes play a key role in innate immune response to bacterial pathogens, catalyzing the hydrolysis of the peptidoglycan layer of bacterial cell walls. In this study, the genes encoding the c-type (TmLyzc) and g-type (TmLyzg) lysozymes from Totoaba macdonaldi were cloned and characterized. The cDNA sequences of TmLyzg and TmLyzc were 582 and 432 bp, encoding polypeptides of 193 and 143 amino acids, respectively. Amino acid sequences of these lysozymes shared high identity (60-90%) with their counterparts of other teleosts and showed conserved functional-structural signatures of the lysozyme superfamily. Phylogenetic analysis indicated a close relationship with their vertebrate homologues but distinct evolutionary paths for each lysozyme. Expression analysis by qRT-PCR revealed that TmLyzc was expressed in stomach and pyloric caeca, while TmLyzg was highly expressed in stomach and heart. These results suggest that both lysozymes play important roles in defense of totoaba against bacterial infections or as digestive enzyme.

Molecular cloning, expression analyses and functional characterization of a goose-type lysozyme gene from Bostrychus sinensis (family: Eleotridae)

In this study, we sequenced and characterized the goose-type lysozyme gene, termed as BsLysG, from the Chinese black sleeper (Bostrychus sinensis). The BsLysG encodes 196 amino acids and contains a soluble bacterial lytic transglycosylases domain, three catalytic residues (Glu⁷², Asp⁸⁵ and Asp¹⁰²) and the GLMQ motif (Gly⁹⁷, Leu⁹⁸, Met⁹⁹ and Gln¹⁰⁰). No signal peptide was observed in the BsLysG protein. The genomic DNA of BsLysG contains five exons and four introns. The sequence analyses showed that the BsLysG exhibits high similarity with LysG from other fishes. Phylogenetic analyses showed that the BsLysG is clustered together with its counterparts from other teleost fishes. The Real-time PCR analyses showed that the BsLysG was found to be ubiquitously expressed in ten examined organs in Chinese black sleeper, with predominant expression in spleen, followed by head kidney and peripheral blood. Expression analyses showed that the BsLysG was significantly upregulated in vivo after either pathogen Vibrio parahemolyticus infection or poly (I:C) challenge in peripheral blood, head kidney, liver and spleen organs. The purified recombinant BsLysG (rBsLysG) has optimal activity at 35 °C and pH 5.5. The rBsLysG exhibited antimicrobial activity against two Gram-positive bacteria (Micrococcus lysodeikticus and Staphylococcus aureus) and two Gram-negative bacteria (Escherichia coli and V. parahemolyticus). The Scanning electron microscope (SEM) imaging analyses showed that the rBsLysG-treated V. parahemolyticus cells displayed morphological deformation. These results indicate that the BsLysG is involved in host immune defense against bacterial infection.

Identification and functional characterization of a g-type lysozyme gene of Labeo rohita, an Indian major carp species

Lysozyme, an important secretory innate immune component, possesses antimicrobial activity against broad spectrum of bacteria and viruses. In the present study, complete CDs (558 bps) of g-type lysozyme of rohu (Labeo rohita) was amplified and translated for a putative protein of 185 amino acids. The domain architecture and tertiary structure was also predicted for the protein. Its expression profile was studied in three infection models (bacteria: Aeromonas hydrophila, poly I:C, a dsRNA viral analogue and an ectoparasite: Argulus siamensis) in liver and kidney tissues of rohu. An up-regulation of 630-fold and 420-fold of the gene was observed at 48 h in liver and anterior kidney tissues respectively, after A. hydrophila infection. Significant increase in transcript level was noticed in both liver (0.8-fold) and kidney (480-fold) after 1 h and 12 h of poly I:C induction, respectively. Similarly, expression of lysozyme g transcripts was increased 6000-fold after 7 d of A. siamensis infection in liver tissue. The recombinant protein of g-type lysozyme of rohu (rLr-lysG) of 20.19 kDa was produced in Escherichia coli system and the lysozyme activity of rLr-lysG was found to be most active at pH 6.0 and temperature 35 °C. The potential lytic activity was found to be against A. hydrophila (U_L = 0.53) followed by for E. tarda (U_L = 0.45) whereas the lytic activity was the least against S. aureus (U_L = 0.35) and M. lysodeikticus (U_L = 0.34), at pH 6.0 and temperature 35 °C. The normal serum level of protein was estimated using indirect ELISA and was found to be very low (0.12-0.15 μg/ml). These results suggested that g-type lysozyme of rohu might be a potent immunostimulant against microbial infections, with a major role in innate immunity.

Molecular cloning, expression and antibacterial activity of goose-type lysozyme gene in Microptenus salmoides

It is well known that lysozymes are key proteins to teleosts in the innate immune system and possess high bactericidal properties. In the present study, a g-type lysozyme gene was cloned from Microptenus salmoides. The g-type sequence consisted of 582 bp, which translated into a 193 amino acid (AA) protein (GenBank accession no: MH087462). The predicted molecular weight and theoretical isoelectric point were 21.36 kDa and 6.91 respectively and no signal peptide was observed. The qRT-PCR analysis showed that the g-type lysozyme gene was differentially expressed in various tissues under normal conditions and the highest g-type lysozyme level was observed in liver, gill and spleen while there seemed to be low expression in the muscle, heart and head-kidney. The expression of g-type lysozyme was differentially upregulated in the spleen, gill and intestine after stimulation with heat stress and Aeromonas hydrophila (A. hydrophila). Under heat stress and A. hydrophila injection, the g-type lysozyme mRNA levels all in spleens, gill and intestine tissues increased significantly (P < 0.05), with the maximum levels attained at 12 h, 24 h (or 12 h) and 24 h. Thereafter, they all decreased significantly (P < 0.01) and the expression in gill returned to nearly the basal value within 72 h. Those results suggested that g-type lysozyme was involved in the immune response to heat stress and bacterial challenge. The cloning and expression analysis of the g-type lysozyme provide theoretical basis to further study the mechanism of anti-adverseness in Microptenus salmoides. The g-type lysozyme gene perhaps also played an important role in the immune responses against bacterial invasion.

Molecular cloning and bioactivity of an IL-2 homologue in large yellow croaker (Larimichthys crocea)

Interleukin-2 (IL-2), an important immunomodulatory cytokine, plays a crucial role in promoting the proliferation, activation and differentiation of T cells. Here, the cDNA of an IL-2 homologue (LcIL-2) in large yellow croaker (Larimichthys crocea) was cloned by RACE-PCR techniques. The open reading frame (ORF) of LcIL-2 gene is 426 bp long and encoded a precursor protein of 141 amino acids (aa), with a 20-aa signal peptide and a 121-aa mature peptide containing two putative N-glycosylation sites at Asn⁷⁷ and Asn¹⁰¹. The LcIL-2 is preferentially expressed in lymphocytes-rich tissues, such as spleen and blood, and is increased in head kidney and spleen upon inactivated trivalent bacterial vaccine or poly(I:C) stimulation. LcIL-2 expression could also be detected in primary head kidney leukocytes (PKL), primary head kidney macrophages (PKM) and primary head kidney granulocytes (PKG), with the highest level in PKL. In addition, the expression level of LcIL-2 in PKL was slightly induced by LPS or poly(I:C), while markedly induced by PHA or Con-A. The recombinant LcIL-2 protein produced in Pichia pastoris could increase the expression of genes involved in Th1 (IL-2, IFN-γ and T-bet) and Th2 (IL-4/13A, IL-4/13B and GATA3) development and differentiation, and of the IL-2 downstream transcription factor STAT5B gene, but inhibit the expression of genes related to Th17 (IL-17A/F2 and IL-17A/F3) development and differentiation. Taken together, our results indicated that LcIL-2 possesses similar structural and functional characteristics to other vertebrate IL-2s, and may play a role in T cell development and differentiation.

Identification of two IL-4/13 homologues in large yellow croaker (Larimichthys crocea) revealed their similar roles in inducing alternative activation of monocytes/macrophages

Mammalian interleukin-4 (IL-4) and -13 (IL-13), two anti-inflammatory T helper cell type 2 (Th2) cytokines, play the central roles in mediating the alternative activation of monocytes/macrophages (MO/Mφs). However, exact functions in MO/Mφs polarization of IL-4/13 homologues in teleost fish remain largely unknown. In this study, we identified two IL-4/13 homologues from large yellow croaker Larimichthys crocea, LcIL-4/13A and LcIL-4/13B, which share low amino acid sequence identities to the known fish IL-4/13 molecules. Phylogenetic analysis showed that LcIL-4/13A is evolutionarily closely related to Dicentrarchus labrax IL-4/13A, and LcIL-4/13B to Takifugu rubripes IL-4/13B. The two LcIL-4/13 genes were constitutively expressed in all examined tissues, but with different expression levels. Both LcIL-4/13A and LcIL-4/13B were up-regulated by inactivated trivalent bacterial vaccine in the head kidney, and LcIL-4/13B appeared more responsive to bacterial vaccine than LcIL-4/13A. Recombinant LcIL-4/13A and LcIL-4/13B proteins (rLcIL-4/13A and rLcIL-4/13B) produced in Escherichia coli could significantly decrease production of reactive oxygen species (ROS) and nitrogen oxide (NO) in the head kidney MO/Mφs from large yellow croaker. Furthermore, rLcIL-4/13A and rLcIL-4/13B obviously down-regulated expression of pro-inflammatory cytokine (IL-1β and TNF-α) and inducible NO synthase (iNOS) genes in MO/Mφs, while they increased mRNA expression of anti-inflammatory cytokines (TGF-β and VEGF) and arginase-2. Additionally, the phagocytic activity of MO/Mφs was also inhibited by rLcIL-4/13A or rLcIL-4/13B. All these results therefore indicated that both LcIL-4/13A and LcIL-4/13B, although exhibiting a lower degree of sequence identity of 15.6% and differential expression pattern, have the similar roles in promoting alternative activation of head kidney MO/Mφs.

Molecular characterization and functional activity of CXCL8_L3 in large yellow croaker Larimichthys crocea

CXCL8, also called interleukin-8, is a typical CXC chemokine that plays a key role in promoting inflammation. Phylogenetically, fish CXCL8 chemokines can be divided into three subgroups, CXCL8_L1, CXCL8_L2, and CXCL8_L3, of which CXCL8_L3 is a new subgroup. The CXCL8_L3 gene sequences have been reported in many fish species, but their function remains unknown. Here, a CXCL8_L3 (LycCXCL8_L3) gene was cloned from large yellow croaker Larimichthys crocea. Its open reading frame (ORF) was 309 nucleotides long, encoding a peptide of 102 amino acids. The deduced LycCXCL8_L3 protein contains an 18-aa signal peptide and an 84-aa mature polypeptide, which has four conserved cysteine residues (C³⁰, C³², C⁵⁷, and C⁷³) as found in other known CXCL8 chemokines. Phylogenetic analysis showed LycCXCL8_L3 formed a major clade with CXCL8_L3 sequences from other fish species. The LycCXCL8_L3 transcript was constitutively expressed in all examined tissues and significantly up-regulated in the spleen and head kidney tissues by inactivated trivalent bacterial vaccine. The LycCXCL8_L3 transcript was also detected in peripheral blood leukocytes (PBLs), primary head kidney macrophages (PKM), and large yellow croaker head kidney cell line (LYCK), with the highest levels in PKM. Recombinant LycCXCL8_L3 (rLycCXCL8_L3) protein could not only chemotactically attract lymphocytes and eosinophils in PBLs, but also enhance the respiratory burst activity of PKM. These results indicate that LycCXCL8_L3 may play an important role in the inflammatory response of large yellow croaker. To our knowledge, this is the first report on functional study of the CXCL8_L3 in fish.

Immune response of interferon-γ-inducible lysosomal thiol reductase (GILT) from Chinese sturgeon (Acipenser sinensis) to microbial invasion and its antioxdative activity in lipopolysaccharides-treated mammalian dentritic cells

Interferon-γ-inducible lysosomal thiol reductase (GILT) plays an important role in the major histocompatibility complex-restricted antigen processing of endocytosed proteins via catalyzing the disulfide bond reduction in the endocytic pathway. Here, the cDNA of Chinese sturgeon (Acipenser sinensis) GILT (CsGILT) was cloned. It contained an open reading frame of 762 nucleotides encoding a protein of 254 amino acids with an estimated molecular weight of 28.1 kDa. The characteristic structural features, including a signature sequence CQHGX2ECX2NX4C, a CXXC motif, two potential N-glycosylation sites, and eight conserved cysteines were detected in the deduced amino acid sequence of CsGILT. CsGILT was widely expressed in Chinese sturgeon with the highest expression in the spleen, and CsGILT mRNA expression was significantly up-regulated when Chinese sturgeons were challenged with polyinosinic polycytidylic acid or Vibrio anguillarum. The recombinant CsGILT displayed obvious thiol reductase activity demonstrated by catalyzing the reduction of mouse IgG(H+L) by dithiothreitol into heavy chain and light chain. CsGILT also displayed significant antioxidant activity in mouse dentritic cells as indicated by its increasing GSH level and GSH/GSSG ratio, decreasing intracellular reactive oxygen species and nitric oxide levels and lipid peroxidation, as well as enhancing the activities of the antioxidative redox enzymes including catalase and superoxide dismutase. Our results suggested an important role for CsGILT in the immune response in Chinese sturgeon to pathogen invasion possibly via a conserved functional mechanism throughout vertebrate evolution, contributing to our understanding the immune biology and protection of Chinese sturgeon.

Functional activities of interferon gamma in large yellow croaker Larimichthys crocea

Interferon gamma (IFN-γ) is a T helper cell type 1 (Th1) cytokine that plays important roles in almost all phases of immune and inflammatory responses. Although IFN-γ gene in large yellow croaker Larimichthys crocea has been reported, little is known about its bioactivity. In this study, large yellow croaker IFN-γ (LycIFN-γ) gene was found to be constitutively expressed in all tissues tested, with the highest levels in blood and heart. Based on stimulation with polyinosinic-polycytidylic acid [poly (I:C)] or inactivated trivalent bacterial vaccine, LycIFN-γ mRNA was significantly increased in spleen and head kidney tissues. LycIFN-γ transcripts were also detected in head kidney granulocytes, primary head kidney macrophages (PKM), head kidney leukocytes, and large yellow croaker head kidney cell line (LYCK), and were significantly up-regulated by poly(I:C) or lipopolysaccharide (LPS) in head kidney leukocytes. Recombinant LycIFN-γ protein (rLycIFN-γ) produced in Escherichia coli could enhance respiratory burst responses in PKM. Furthermore, rLycIFN-γ not only induced the expression of iNOS gene and release of NO, but also up-regulated the expression of proinflammatory cytokines TNF-α and IL-1β in PKM. These findings therefore indicated that LycIFN-γ has a role in mediating inflammatory response. In addition, rLycIFN-γ could significantly up-regulate expression of IFN-γ receptor CRFB13, signal transduction factor STAT1, transcription factors IRF1 and T-bet, and Th1-related cytokines IFN-γ and IL-2 in head kidney leukocytes, suggesting that LycIFN-γ may have the potential to promote Th1 immune response in large yellow croaker. Taken together, our results show that LycIFN-γ may be involved in inflammatory response and promote Th1 immune response as its mammalian counterpart.

MHC class II alpha, beta and MHC class II-associated invariant chains from Chinese sturgeon (Acipenser sinensis) and their response to immune stimulation

The major histocompatibility complex class II (MHC II) molecules play a vital role in adaptive immune response through presenting antigenic peptides to CD4⁺ T lymphocytes. To accomplish this physiologic function, the MHC class II-associated invariant chain interacts with the MHC II α/β subunits and promotes their correct assembly and efficient traffic. Here, we isolated the cDNAs of MHC II α, β and MHC II-associated invariant chains (designated as CsMHC II α, CsMHC II β, and CsMHC II γ) from Chinese sturgeon (Acipenser sinensis). The CsMHC II α, β, and γ mRNAs were widely expressed in Chinese sturgeon, and the highest expression was found in spleen for CsMHC II α and β chains, while in head kidney for CsMHC II γ chain. Stimulation to Chinese sturgeon with inactivated trivalent bacterial vaccine or polyinosinic polycytidylic acid (poly(I:C)) up-regulated the expressions of CsMHC II α, and β mRNAs, and their transcripts were overall more quickly up-regulated by poly(I:C) than by bacterial vaccine. Poly(I:C) induced higher CsMHC II γ expression than bacterial vaccine in intestine and spleen, while lower than bacterial vaccine in head kidney and liver. When co-expressed in mouse dendritic cells, the CsMHC II γ chain bound to both the MHC II α and β chains. Furthermore, the over-expressed CsMHC II γ chain, not CsMHC II α or CsMHC II β chain, activated NF-κB and STAT3 in mouse dendritic cells, and induced TNF-α and IL-6 expressions as well. This activity was nearly abolished by mutation of the Ser29/Ser34 to Ala29/Ala34 in CsMHC II γ. These results suggested that CsMHC II α, β, and γ chains might play important role in immune response to pathogen microbial infection of Chinese sturgeon possibly via a conserved functional mechanism throughout vertebrate evolution, which might contribute to our understanding the immune biology of sturgeons.

Molecular, transcriptional and functional insights into duplicated goose-type lysozymes from Sebastes schlegelii and their potential immunological role

Black rockfish (Sebastes schlegelii), an important aquaculture species in Korea, has been affected by bacterial diseases leading to a drastic decline in production. Goose-type lysozyme (LysG) is a key enzyme of the innate immune system to eradicate bacterial infections. In this study, two isoforms of LysG from black rockfish, designated as RfLysG1 and RfLysG2, have been identified and characterized at the molecular, transcriptional, and functional levels. The deduced amino acid sequences had the LysG family characteristics and exhibited conserved properties, including active residues and domains. The cDNA sequences of RfLysG1 and RfLysG2 were 1514 bp and 900 bp in length, respectively. The 567-bp open reading frame (ORF) of RfLysG1 encoded a protein of 188 amino acids with molecular mass 20.11 kDa, and the 600-bp ORF of RfLysG2 encoded a polypeptide with 199 amino acids and molecular mass of 22.19 kDa. Homology studies indicated that RfLysG1 showed the highest identity (84.6%) with LysG-B of Oplegnathus fasciatus, while RfLysG2 showed the highest identity (74.4%) with LysG of Siniperca chuatsi. Both sequences possessed a soluble lytic trans-glycosylase domain. Both lacked signal peptide and they were not identified as proteins secreted by non-classical pathway by the SecretomeP server. Transcriptional analysis of the two genes showed constitutive expression, where both genes were highly expressed in blood under normal physiological conditions. In response to the immune challenges lipopolysaccharide (LPS), Streptococcus iniae, and poly I:C injection, the expression of RfLysG1 and RfLysG2 was significantly upregulated in blood and spleen tissues in a time-dependent manner. Turbidimetric assays indicated that both recombinant proteins tagged with maltose-binding protein (MBP) were reactive against several Gram-positive and Gram-negative bacteria, but MBP was inactive. Optimum temperatures for the recombinant RfLysG1 and RfLysG2 were 40 °C and 50 °C, respectively, and both were highly active at pH 3.0. The results provide evidence for the vital immunological role and bacteriolytic potential of RfLysG1 and RfLysG2.

Molecular characterization and expression analysis during embryo development of CD4-1 homologue in large yellow croaker Larimichthys crocea

CD4⁺ helper T (Th) cells are a master component of the adaptive immune response. CD4 is one of the most effective surface markers for identifying Th cells. In the present study, we cloned and characterized a CD4-1 homologue, LycCD4-1, from large yellow croaker Larimichthys crocea. The full-length cDNA of LycCD4-1 is 1695 bp long, encoding a protein of 462 amino acids. The deduced LycCD4-1 protein has a typical domain architecture as found in mammalian CD4 molecules, including a signal peptide, four extracellular immunoglobulin-like (Ig-like) domains, a transmembrane region, and a CXC signaling motif in the cytoplasmic tail. Four N-glycosylation sites and 10 cysteine residues were also found in LycCD4-1, which may be essential for its tertiary structure and succeeding function. Homology comparison showed that LycCD4-1 has 27.9-58.4% identity to other teleost fish CD4-1 molecules, and 16.4-20% identity to those of higher vertebrates. Genomic analysis revealed that the LycCD4-1 gene consisted of nine exons and eight introns and exhibited a similar exon-intron organization to other species CD4 genes except for a different intron length. Phylogenetic analysis showed that LycCD4-1 form a cluster with CD4-1 molecules in other fish species. The LycCD4-1 was constitutively expressed in all tissues tested, with a higher expression in gills and spleen. LycCD4-1 mRNA expression in the spleen and head kidney tissue was increased by poly (I:C) at 48 h, whereas its expression levels were somewhat down-regulated at 6 h and 72 h after bacterial vaccine induction in spleen. Unexpectedly, LycCD4-1 mRNA could be detected in each stage of early embryo development since fertilized eggs, with a higher level before mid-gastrula and the highest level in high blastocysts. These results will be helpful for better understanding molecular characteristics of CD4-1 and tracing origin of CD4-1⁺ cell precursors in fish.

Induction of type I interferons in response to bacterial stimuli in large yellow croaker Larimichthys crocea

In addition to the crucial roles in coordinating antiviral immune responses, type I interferons (IFNs) also play a role in the host immunity against bacterial pathogens. Our previous study identified two type I IFNs from large yellow croaker Larimichthys croaea(Lc), LcIFNd and LcIFNh, and showed their strong induction by poly(I:C) and antiviral activities. In the present study, both LcIFNd and LcIFNh were found to be rapidly induced in head kidney and spleen by mixed bacteria of Vibrio alginolyticus, Vibrio parahaemolyticus, and Aeromonas hydrophila. In the head kidney primary cells (HKCs), expression of these two LcIFN genes was increased by peptidoglycan (PGN) from Bacillus subtilis and lipopolysaccharide (LPS) from Escherichia coli. Consistently, Lc IFN-regulatory factor (LcIRF) 3 and LcIRF7, two key transcription factors of type I IFN expression, were also induced by these three bacteria, PGN, and LPS. These observations strongly suggested that large yellow croaker type I IFNs are involved in the immune response against bacterial infection. Luciferase assays showed that promoters of both LcIFNd and LcIFNh were activated by PGN, LPS, and genomic DNA of A. hydrophila, and A. hydrophila DNA was more potent than PGN and LPS in activating LcIFNd and LcIFNh promoters. Furthermore, the induction of LcIFNd promoter by these bacterial stimuli was further enhanced by the overexpression of LcIRF7 or LcIRF7 along with LcIRF3, while that of LcIFNh promoter was increased following the overexpression of LcIRF3 alone, suggesting that the induction of these two large yellow croaker IFNs by bacterial stimuli may be regulated via distinct manners. These results therefore revealed novel aspects of the functional regulation of teleost type I IFNs.

Molecular characterisation, evolution and expression analysis of g-type lysozymes in Ciona intestinalis

Lysozyme is an important defense molecule of the innate immune system. Known for its bactericidal properties, lysozyme catalyzes the hydrolysis of b-(1,4)-glycosidic bonds between the N-acetyl glucosamine and N-acetyl muramic acid in the peptidoglycan layer of bacterial cell walls. In this study, the complete coding sequence of four g-type lysozymes were identified in Ciona intestinalis. Phylogenetic analysis and modelling supported the hypothesis of a close relationship with the vertebrate g-type lysozymes suggesting that the C. intestinalis g-type lysozyme genes (CiLys-g1, Cilys-g2, CiLys-g3, CiLys-g4) share a common ancestor in the chordate lineage. Protein motif searches indicated that C. intestinalis g-type lysozymes contain a GEWL domain with a GXXQ signature, typical of goose lysozymes. Quantitative Real-Time PCR analysis results showed that transcripts are expressed in various tissues from C. intestinalis. In order to determine the involvement of C. intestinalis g-type lysozymes in immunity, their expression was analyzed in the pharynx, showing that transcripts were significantly up-regulated in response to a challenge with lipopolysaccharide (LPS). These data support the view that CiLys g-type are molecules with potential for immune defense system against bacterial infection.

Transcriptome Analysis Reveals Comprehensive Insights into the Early Immune Response of Large Yellow Croaker (Larimichthys crocea) Induced by Trivalent Bacterial Vaccine

Vaccination is an effective and safe strategy for combating bacterial diseases in fish, but the mechanisms underlying the early immune response after vaccination remain to be elucidated. In the present study, we used RNA-seq technology to perform transcriptome analysis of spleens from large yellow croaker (Larimichthys crocea) induced by inactivated trivalent bacterial vaccine (Vibrio parahaemolyticus, Vibrio alginolyticus and Aeromonas hydrophila). A total of 2,789 or 1,511 differentially expressed genes (DEGs) were obtained at 24 or 72 h after vaccination, including 1,132 or 842 remarkably up-regulated genes and 1,657 or 669 remarkably down-regulated genes, respectively. Gene ontology and Kyoto Encyclopedia of Genes and Genomes enrichments revealed that numerous DEGs belong to immune-relevant genes, involved in many immune-relevant pathways. Most of the strongly up-regulated DEGs are innate defense molecules, such as antimicrobial peptides, complement components, lectins, and transferrins. Trivalent bacterial vaccine affected the expressions of many components associated with bacterial ligand-depending Toll-like receptor signaling pathways and inflammasome formation, indicating that multiple innate immune processes were activated at the early period of vaccination in large yellow croaker. Moreover, the expression levels of genes involved in antigen processing were also up-regulated by bacterial vaccine. However, the expression levels of several T cell receptors and related CD molecules and signal transducers were down-regulated, suggesting that the T cell receptor signaling pathway was rapidly suppressed after vaccination. These results provide the comprehensive insights into the early immune response of large yellow croaker to vaccination and valuable information for developing a highly immunogenic vaccine against bacterial infection in teleosts.

Enhancing gilthead seabream immune status and protection against bacterial challenge by means of antigens derived from Vibrio parahaemolyticus

In an attempt to control the proliferation of the pathogenic bacterium Vibrio parahaemolyticus in gilthead seabream (Sparus aurata), the immunostimulant effect of lysate and ToxA from this bacterium was evaluated. Fish were intraperitoneally injected twice (first injection, day 1 of the experiment; second injection, day 7) and sampled after one week (on days 8 and 15). Afterwards, all fish specimens were experimentally infected with V. parahaemolyticus and mortality was recovered for 1 week. Fish injected with lysate, ToxA and phosphate buffer saline (control) showed 100%, 50% and 0% survival, respectively, when challenged with the pathogen. Skin mucus immune parameters and immune-related gene expression in skin and spleen were also evaluated. The results showed that mucus immune parameters were enhanced in the lysate and ToxA groups compared with the values obtained for fish from the control group. Expression of IL-1β, TNF-α, C3 and IgM genes was significantly up-regulated in the lysate and ToxA groups, principally after infection with the bacterium. Interestingly, TLR5 gene expression increased in fish immunized with lysate. The most prominent histological characteristic in gut from infected fish was the presence of a great number of intraepithelial leucocytes as well as inflammation of the submucosa, while severe hydropic degeneration and hemosiderosis were detected in liver from infected fish. Injection of lysate or ToxA had a protective effect against the deleterious consequences of subsequent infection with V. parahaemolyticus in gut and liver. The findings underline the potential of lysate and ToxA as potent preventive antigens against this kind of vibriosis.

Molecular characteristics of lysozyme G in Euryglossa orientalis; cDNA cloning, phylogenic analysis, physicochemical properties and tissue gene expression

Lysozymes are the key molecules of innate immune system against bacterial infections. In the present study, we identified the molecular characteristics, physicochemical properties, antibacterial activity, evolutionary relationship and tissue expression pattern of g-type lysozyme in Euryglossa orientalis (EuOr LysG). The full-length EuOr LysG cDNA is composed of 588 nucleotides and an open reading frame encoding a protein with 195 amino acids with more than 65 % identity to g-type lysozyme of Solea senegalensis (73 %) and Scophthalmus rhombus (64 %). Amino acid sequence alignment showed that EuOr LysG protein possessed a conserved catalytic motif (Glu71-Asp84-Asp101) and one predicted disulfide bond between Cys32 and Cys122. Phylogenetic analysis based on the g-type lysozyme sequences indicated that E. orientalis and other fish of Pleuronectiformes were diverged together in the evolutionary history. The K _m and V _max values of the recombinant EuOr LysG were 0.266 mg/ml of Micrococcus lysodeikticus as substrate and 667 U/mg of protein, respectively. The optimum temperature and pH of recombinant EuOr LysG were 45 and 6 °C, respectively. Real-time PCR analysis showed that EuOr LysG transcript was most abundant in head kidney and gill and less in muscle. We also showed that the EuOr LysG had potent lytic activity against major fish bacterial pathogens with the highest activity against Bacillus cereus and Aeromonas hydrophila. Bacterial challenge with Vibrio parahaemolyticus could upregulate LysG in immune-related tissues. Our results help to understand the molecular and physicochemical characteristics of g-type lysozyme in E. orientalis which might play an important role in host defense against the bacterial infection.

Identification of a fish specific chemokine CXCL_F2 in large yellow croaker (Larimichthys crocea) reveals its primitive chemotactic function

Chemokines are a superfamily of cytokines regulating immune cell migration under both inflammatory and normal physiological conditions. Currently, a number of fish specific CXC chemokines, named as CXCL_F1-5, have been identified in several species. However, understanding of their functional characteristics is still limited. In this study, we identified a fish specific chemokine CXCL_F2 (LycCXCL_F2) from large yellow croaker (Larimichthys crocea). The open reading frame (ORF) of LycCXCL_F2 is 348 nucleotides long, encoding a protein of 115 amino acids (aa). The deduced LycCXCL_F2 protein contains a 20-aa signal peptide and a 95-aa mature polypeptide. Phylogenetic analysis showed that LycCXCL_F2 fell into a major clade formed by CXCL_F2 sequences and was separated from CXCL_F1 and CXCL_F3-5 subgroups. LycCXCL_F2 mRNA transcript was constitutively expressed in various tissues, with the highest levels in spleen and head kidney. After stimulation with inactivated trivalent bacterial vaccines, LycCXCL_F2 mRNA transcription was significantly increased in both spleen and head kidney. Moreover, recombinant LycCXCL_F2 protein exhibited obvious chemotaxis to monocytes, lymphocytes and eosnophils of PBLs isolated from large yellow croaker, but could not induce the respiratory burst of macrophages. These results indicate that this fish specific CXC chemokine LycCXCL_F2 possesses primitive chemotactic activity and may play a role in immune response in large yellow croaker.

Molecular and functional characterization of Toll-like receptor 21 in large yellow croaker (Larimichthys crocea)

Toll-like receptor 21 (TLR21) is a non-mammalian TLR that functions similar to mammalian TLR9 in recognizing CpG DNA. In the present study, we identified a TLR21 homologue, LycTLR21, from large yellow croaker (Larimichthys crocea). The complete coding sequence of LycTLR21 is 2946 nucleotides long, encoding a protein of 981 amino acids. The deduced LycTLR21 protein has typical TLR domain architecture, including a signal peptide, 13 leucine-rich repeats (LRRs) in the extracellular region, a transmembrane region, and a cytoplasmic Toll-Interleukin-1 receptor (TIR) domain. Phylogenetic analysis showed that LycTLR21 falls into a major clade formed by all fish TLR21 sequences and is closely related to TLR21 in Epinephelus coioides and Oplegnathus fasciatus. LycTLR21 mRNA was constitutively expressed in all tissues tested, with higher levels in immune-related tissues, such as spleen, head kidney, and gills. Upon stimulation with inactivated trivalent bacterial vaccine, LycTLR21 mRNA was significantly increased in these three tissues. Overexpression of a chimeric plasmid containing the extracellular domain of human cluster of differentiation 4 (CD4) and the transmembrane and cytoplasmic domains of LycTLR21 could activate NF-κB, but not IFN-β in Chinese hamster ovary (CHO) cells, suggesting that LycTLR21 could mediate activation of NF-κB. LycTLR21 could specifically recognize three CpG-oligodeoxynucleotides (CpG-ODNs), CpG-ODN 1826, 2006, and 2007, but not other CpG-ODNs detected, poly(I:C), lipopolysaccharide (LPS), and lipoteichoic acid (LTA-SA). These three CpG-ODNs were found to significantly up-regulate the expression of LycTLR21 and downstream proinflammatory cytokines IL-1β and IL-6 of NF-κB pathway in large yellow croaker head kidney (LYCK) cells. In addition, the expression levels of LycTLR21, c-Rel subunit of NF-κB, IL-1β and IL-6 genes were quickly increased in the spleen and head kidney by bacterial infection, suggesting that LycTLR21 signaling pathway may play a role in immune response to bacterial infection.

Molecular identification and expression analysis of a goose-type lysozyme (LysG) gene in yellow catfish Pelteobagrus fulvidraco

Lysozymes, innate immunity molecules, play a vital role in immune response to pathogens. The yellow catfish Pelteobagrus fulvidraco (Siluriformes: Bagridae) is an economically important fish in China. The aim of this study was to quantify expression of the P. fulvidraco LysG gene (a g-type lysozyme) in response to pathogen-associated molecular patterns (PAMP) challenge. First, the P. fulvidraco LysG gene (PfLysG) was cloned and characterized. The full-length cDNA of PfLysG is 1323 bp, including a 5'-untranslated region (UTR) of 131 bp, a 3'-UTR of 634 bp, and an open reading frame of 558 bp encoding a polypeptide of 185 amino acids, which contains a transglycosylase SLT domain (Pfam01464). The predicted molecular weight of the protein is 20.52 kDa with a pI of 9.08. Two catalytic residues and seven N-acetyl-D-glucosamine binding sites are conserved in the sequence and there is no predicted signal peptide. The deduced PfLysG protein sequence has 84%, 76% and 69% percent identity with the LysGs from Ictalurus furcatus, Danio rerio, and Salmo salar, respectively. The predicted tertiary structure of PfLysG is very similar to that from other animals. Phylogenetic analysis showed that PfLysG is closely related to those from Teleostei. Real-time quantitative reverse transcription-PCR (qPCR) analysis showed that PfLysG was expressed in all examined tissues and most highly expressed in head kidney, spleen, and intestine. After simulated pathogen challenge with lipopolysaccharide and polyriboinosinic polyribocytidylic acid, respectively, the mRNA expression of PfLysG was upregulated significantly at different time points. The results suggest that the identified g-type lysozyme of P. fulvidraco is involved in innate immune responses.

Molecular characterization, transcriptional profiling, and antibacterial potential of G-type lysozyme from seahorse (Hippocampus abdominalis)

Lysozymes are a family of enzymes that catalyze the hydrolysis of bacterial cell wall, acting as antimicrobial effectors of the innate immune system. In the present study, an ortholog of goose-type lysozyme (ShLysG) from the big-belly seahorse (Hippocampus abdominalis) was identified and characterized structurally and functionally. The full-length cDNA sequence (1213 bp) of ShLysG is comprised of an open reading frame made up of 552 bp, encoding a polypeptide of 184 amino acid (aa) with a predicted molecular mass of 20 kDa. In silico analysis of ShLysG revealed the absence of signal peptide and the presence of a characteristic bacterial soluble lytic transglycosylase (SLT) domain bearing three catalytic residues (Glu⁷¹, Asp⁸⁴, and Asp⁹⁵) and seven N-acetyl-d-glucosamine binding sites (Glu⁷¹, Asp⁹⁵, Tyr⁹⁸, His⁹⁹, Ile¹¹⁷, Tyr¹⁴⁵, and Asn¹⁴⁶). Homology analysis demonstrated that the aa sequence of ShLysG shared 60.7-67.4% identity and 72.6-79.3% similarity with the orthologs of other teleosts. Phylogenetic analysis of ShLysG indicated a closest relationship with the ortholog from Gadus morhua. In healthy seahorse, ShLysG mRNA showed a constitutive expression in all the tissues examined, with the highest expression in kidney and the least expression in liver. The ShLysG mRNA levels were also shown significant elevation upon the bacterial and pathogen-associated molecular pattern (PAMPs) challenges. Furthermore, lytic activities of ShLysG recombinant protein were detected against several Gram-negative and Gram-positive bacterial species. Taken together, these results suggest that ShLysG might possess a potential immune defensive role against invading microbial pathogens in seahorse.

Identification of Nod like receptor C3 (NLRC3) in Asian seabass, Lates calcarifer: Characterisation, ontogeny and expression analysis after experimental infection and ligand stimulation

Nod like receptors (NLRs) are a large group of cytoplasmic PRRs believed to play an important role in bacterial recognition in higher vertebrates. In this study, a novel Nod like receptor C3 (AsNLRC3) has been identified, cloned and characterised from Asian seabass, Lates calcarifer. The full-length AsNLRC3 transcript composed of a 4142 bp nucleic acid sequence encode for a protein of 1134 deduced amino acids. Three signature domains identified are conserved NACHT-domain, C-terminal LLR domain and N-terminal CARD effector domain. From the domain architecture and phylogenetic analysis, it was quite evident that AsNLRC3 is different from the NLR subfamily C of other teleosts. AsNLRC3 expressed in all the 11 tissues tested but highly expressed in tissues facing external environment such as gill, hindgut and midgut. The ontogenic expression profile of this receptor showed constitutive expression throughout the embryonic and larval developmental stages, which could be an innate immune strategy against different marine pathogens for larval survival. Infection with Vibrio alginolyticus and poly I:C induction showed an alteration of expression pattern in different tissues but did not show significant alteration in expression with Staphylococcus aureus infection. In vitro study in Asian seabass kidney cell line (SISK) stimulated with different ligands such as LPS, PGN and poly I:C showed considerable up-regulation at some of the time-points tested. These results suggest that AsNLRC3 can be a pivotal cytosolic innate immune receptor for recognizing wide array of pathogens in a euryhaline teleost model like Asian seabass in diverse environmental conditions.

The mucosal expression signatures of g-type lysozyme in turbot (Scophthalmus maximus) following bacterial challenge

The mucosal surfaces constitute the first line of host defense against infection, and also serve as the dynamic interfaces that simultaneously mediate a diverse array of critical physiological processes, while in constantly contact with a wide range of pathogens. The lysozymes are considered as key components for innate immune response to pathogen infection with their strong antibacterial activities. But their activities in mucosal immune responses were always overlooked, especially for g-type lysozymes, whose expression patterns in mucosal tissues following bacterial challenge are still limited. Towards to this end, here, we characterized the g-type lysozymes, Lyg1 and Lyg2 in turbot, and determined their expression patterns in mucosal barriers following different bacterial infection. The phylogenetic analysis revealed the turbot g-type lysozyme genes showed the closest relationship to Cynoglossus semilaevis. The two lysozyme genes showed different expression patterns following challenge. Lyg2 was significantly up-regulated in mucosal tissues following Vibrio anguillarum and Streptococcus iniae challenge, while Lyg1 showed a general trend of down-regulation. The significant mucosal expression signatures of g-type lysozyme genes indicated their key roles to prevent pathogen attachment and entry in the first line of host defense system. Further functional studies should be carried out to better characterize the availability of utilization of g-type lysozyme to increase the disease resistance in the mucosal surfaces and facilitate the disease resistant breeding selection.

A cystatin F homologue from large yellow croaker (Larimichthys crocea) inhibits activity of multiple cysteine proteinases and Ii chain processing in vitro

Cystatin F, a member of the family II cystatins, plays important roles in immune response-related processes through inhibiting specific enzyme targets. In this study, a cystatin F homologue, LycCysF, was identified and characterized from large yellow croaker (Larimichthys crocea). The deduced LycCysF protein exhibits a typical structural feature of type II cystatins, including three evolutionally conserved motifs, Gly(35), QVVRG(79-83) and PW(130-131). Tissue expression analysis showed that LycCysF mRNA was expressed in all tissues examined, albeit at different levels. Recombinant LycCysF (rLycCysF) produced in Pichia pastoris could inhibit the activity of multiple cysteine proteases, including papain, legumain and recombinant large yellow croaker cathepsin B, L and S. Moreover, rLycCysF could inhibit the Ii chain processing by recombinant cathepsin S in vitro. These data suggest that LycCysF may participate in regulation of cathepsins and MHC-II associated Ii chain processing. In addition, mammalian cystatin F is produced as an inactive dimer, becoming activated by proteolysis in the endo/lysosome of immune cells and then exerts its function of regulating downstream proteases activity. However, the N-terminal extension and two additional cysteine residues responsible for dimer formation are absent in LycCysF and cystatin F from other fish species, reptiles and Aves, indicating that these proteins can not form dimer and may regulate the proteases activity via an alternate pathway distinct from mammalian cystatin F. To our knowledge, this is the first report on molecular characteristics of a teleost cystatin F and its role in Ii chain processing.

Molecular Characterization and Biological Effects of a C-Type Lectin-Like Receptor in Large Yellow Croaker (Larimichthys crocea)

The C-type lectin-like receptors (CTLRs) play important roles in innate immunity as one type of pattern recognition receptors. Here, we cloned and characterized a C-type lectin-like receptor (LycCTLR) from large yellow croaker Larimichthys crocea. The full-length cDNA of LycCTLR is 880 nucleotides long, encoding a protein of 215 amino acids. The deduced LycCTLR contains a C-terminal C-type lectin-like domain (CTLD), an N-terminal cytoplasmic tail, and a transmembrane region. The CTLD of LycCTLR possesses six highly conserved cysteine residues (C1–C6), a conserved WI/MGL motif, and two sugar binding motifs, EPD (Glu-Pro-Asp) and WYD (Trp-Tyr-Asp). Ca²⁺ binding site 1 and 2 were also found in the CTLD. The LycCTLR gene consists of five exons and four introns, showing the same genomic organization as tilapia (Oreochromis niloticus) and guppy (Poecilia retitculata) CTLRs. LycCTLR was constitutively expressed in various tissues tested, and its transcripts significantly increased in the head kidney and spleen after stimulation with inactivated trivalent bacterial vaccine. Recombinant LycCTLR (rLycCTLR) protein produced in Escherichia coli BL21 exhibited not only the hemagglutinating activity and a preference for galactose, but also the agglutinating activity against two food-borne pathogenic bacteria E. coli and Bacillus cereus in a Ca²⁺-dependent manner. These results indicate that LycCTLR is a potential galactose-binding C-type lectin that may play a role in the antibacterial immunity in fish.

Molecular characterization and biological effects of a CXCL8 homologue in large yellow croaker (Larimichthys crocea)

CXCL8 also called interleukin-8, is a CXC-type chemokine that plays a key role in promoting inflammation. Three subgroups of CXCL8 homologues have been reported in teleost fish, including CXCL8_L1, CXCL8_L2 and CXCL8_L3. In the present study, we identified a CXCL8 homologue belonging to CXCL8_L1 subgroup (LycCXCL8_L1) in large yellow croaker (Larimichthys crocea) that shares low identity to the previously reported large yellow croaker CXCL8 (LycCXCL8). The full-length cDNA of LycCXCL8_L1 is 716 nucleotides (nt) long and encodes a protein consisting of 99 amino acids (aa) with a putative molecular weight of 11.2 kDa. The deduced LycCXCL8_L1 protein contains a 22-aa signal peptide and a 77-aa mature polypeptide, which possesses an arrangement of four cysteines typical of other known CXC chemokines (C(34), C(36), C(60), and C(77)). Genomic analysis revealed that the LycCXCL8_L1 gene consisted of four exons and three introns and exhibited a similar exon-intron organization to LycCXCL8 and other species CXCL8 genes except for a different intron length. Phylogenetic analysis showed that both LycCXCL8_L1 and LycCXCL8 belong to CXCL8_L1 subgroup. LycCXCL8_L1 mRNA was constitutively expressed in all tissues examined although at different levels. Upon bacterial vaccine induction, LycCXCL8_L1 mRNA expression was rapidly increased in the spleen and head kidney tissues. Recombinant LycCXCL8_L1 and LycCXCL8 proteins produced in Escherichia coli both induced chemotaxis and superoxide production in peripheral blood leucocytes from large yellow croaker. These results indicate that two CXCL8_L1 molecules exist in large yellow croaker and play roles in inflammatory response.

Transcriptional Onset of Lysozyme Genes during Early Development in Olive Flounder (Paralichthys olivaceus)

The immune system in teleost fish is not completely developed during embryonic and larval stages, therefore effective innate mechanisms is very important for survival in such an environment. However, the knowledge of the development of immune system assumed to be restricted. In many species, lysozymes have been considered as important genes of the first line immune defense. The early detection of lysozyme mRNA in previous reports, led to the investigation of its presence in oocytes. As a result, c-type lysozyme mRNA transcripts were detected in unfertilized oocytes indicating maternal transfer. Therefore, we investigated the expression patterns of lysozymes in flounder, including the matured oocyte. In our results, c-type lysozyme mRNA was first detected in unfertilized oocyte stage, observed the significantly decreased until hatching stage, and was significantly increased after hatching stage. On the other hand, g-type lysozyme mRNA transcripts were first detected at late neurula stage, and the mRNA level was significantly increased after 20 dph. It may be suggest that maternally supplied mRNAs are selectively degraded prior to the activation of embryonic transcription. This study will be help in understanding the maturation and onset of humoral immunity during development of olive flounder immune system.

Transcriptional Onset of Lysozyme Genes during Early Development in Olive Flounder (Paralichthys olivaceus)

The immune system in teleost fish is not completely developed during embryonic and larval stages, therefore effective innate mechanisms is very important for survival in such an environment. However, the knowledge of the development of immune system assumed to be restricted. In many species, lysozymes have been considered as important genes of the first line immune defense. The early detection of lysozyme mRNA in previous reports, led to the investigation of its presence in oocytes. As a result, c-type lysozyme mRNA transcripts were detected in unfertilized oocytes indicating maternal transfer. Therefore, we investigated the expression patterns of lysozymes in flounder, including the matured oocyte. In our results, c-type lysozyme mRNA was first detected in unfertilized oocyte stage, observed the significantly decreased until hatching stage, and was significantly increased after hatching stage. On the other hand, g-type lysozyme mRNA transcripts were first detected at late neurula stage, and the mRNA level was significantly increased after 20 dph. It may be suggest that maternally supplied mRNAs are selectively degraded prior to the activation of embryonic transcription. This study will be help in understanding the maturation and onset of humoral immunity during development of olive flounder immune system.

Molecular cloning and characterization of a new G-type lysozyme gene (Ec-lysG) in orange-spotted grouper, Epinephelus coioides

Lysozyme acts as an innate immunity molecule against pathogen infection. In this study, a new G-type lysozyme gene with a typical G-type lysozyme domain (designated as Ec-lysG) was cloned and characterized from the orange-spotted grouper, Epinephelus coioides. The full-length Ec-lysG cDNA contains 1419 bp and encodes a 256-residue protein containing a 25-residue signal peptide at the N-terminus. BLAST analysis reveals Ec-lysG shares 64% identity with Siniperca chuatsi, but 63% to another reported G-type lysozyme from orange-spotted grouper (OSG-lysG). The genomic DNA of Ec-lysG contains four exons and three introns, with a total length of 2062 bp. An amino acid sequence alignment showed that Ec-lysG shares the fundamental structural features of G-type lysozyme, including the catalytic residues, substrate binding sites, and soluble lytic transglycosylase domain. Quantitative PCR showed that Ec-lysG transcript is most abundant in the head kidney, and less abundant in the heart. The expression of Ec-lysG was differentially upregulated in the head kidney after stimulation with lipopolysaccharide, Vibrio alginolyticus, and Singapore grouper iridovirus (SGIV). A subcellular localization analysis showed that Ec-lysG is distributed predominantly in the cytoplasm. Recombinant Ec-lysG (rEc-lysG) has optimal activity at pH 7.5 and 35°C. rEc-lysG showed lytic activities against Gram-positive bacterium Streptococcus iniae, Staphylococcus aureus, and Micrococcus lysodeikticus, and the Gram-negative bacterium V. alginolyticus. Scanning electron microscopy (SEM) showed that rEc-lysG acts on M. lysodeikticus cell walls. The overexpression of Ec-lysG in grouper cells did not significantly delay the occurrence of the cytopathic effect (CPE) induced by SGIV, and did not inhibit viral gene transcription. In conclusion, Ec-lysG might be a potent antibacterial protein, with a role in innate immunity.

Evolution of the vertebrate goose-type lysozyme gene family

BACKGROUND

Lysozyme g is an antibacterial enzyme that was first found in the eggs of some birds, but recently has been found in additional species, including non-vertebrates. Some previously characterized lysozyme g sequences are suggested to have altered secretion potential and enzymatic activity, however the distribution of these altered sequences is unknown. Duplicated copies of the lysozyme g gene exist in some species; however, the origins of the duplicates and their roles in altered function are unclear.

RESULTS

We identified 234 lysozyme g sequences from 118 vertebrate species, including 181 sequences that are full or near full length representing all vertebrate classes except cartilaginous fish. Phylogenetic analysis shows that most lysozyme g gene duplicates are recent or lineage specific events, however three amplification events are more ancient, those in an early amniote, an early mammal, and an early teleost. The older gene duplications are associated with changes in function, including changes in secretion potential and muramidase antibacterial enzymatic activity.

CONCLUSIONS

Lysozyme g is an essential muramidase enzyme that is widespread in vertebrates. Duplication of the lysozyme g gene, and the retention of non-secreted isozymes that have lost enzymatic activity indicate that lysozyme g has an activity other than the muramidase activity associated with being an antibacterial enzyme.

Molecular cloning, sequence analysis and phylogeny of first caudata g-type lysozyme in axolotl (Ambystoma mexicanum)

Lysozymes are key proteins that play important roles in innate immune defense in many animal phyla by breaking down the bacterial cell-walls. In this study, we report the molecular cloning, sequence analysis and phylogeny of the first caudate amphibian g-lysozyme: a full-length spleen cDNA library from axolotl (Ambystoma mexicanum). A goose-type (g-lysozyme) EST was identified and the full-length cDNA was obtained using RACE-PCR. The axolotl g-lysozyme sequence represents an open reading frame for a putative signal peptide and the mature protein composed of 184 amino acids. The calculated molecular mass and the theoretical isoelectric point (pl) of this mature protein are 21523.0 Da and 4.37, respectively. Expression of g-lysozyme mRNA is predominantly found in skin, with lower levels in spleen, liver, muscle, and lung. Phylogenetic analysis revealed that caudate amphibian g-lysozyme had distinct evolution pattern for being juxtaposed with not only anura amphibian, but also with the fish, bird and mammal. Although the first complete cDNA sequence for caudate amphibian g-lysozyme is reported in the present study, clones encoding axolotl's other functional immune molecules in the full-length cDNA library will have to be further sequenced to gain insight into the fundamental aspects of antibacterial mechanisms in caudate.

Analysis of two lysozyme genes and antimicrobial functions of their recombinant proteins in Asian seabass

Lysozymes are important proteins of the innate immune system for the defense against bacterial infection. We cloned and analyzed chicken-type (c-type) and goose-type (g-type) lysozymes from Asian seabass (Lates calcarifer). The deduced amino acid sequence of the c-type lysozyme contained 144 residues and possessed typical structure residues, conserved catalytic residues (Glu(50) and Asp(67)) and a "GSTDYGIFQINS" motif. The deduced g-type lysozyme contained 187 residues and possessed a goose egg white lysozyme (GEWL) domain containing three conserved catalytic residues (Glu(71), Asp(84), Asp(95)) essential for catalytic activity. Real time quantitative PCR (qRT-PCR) revealed that the two lysozyme genes were constitutively expressed in all the examined tissues. The c-type lysozyme was most abundant in liver, while the g-type lysozyme was predominantly expressed in intestine and weakly expressed in muscle. The c-type and g-type transcripts were up-regulated in the kidney, spleen and liver in response to a challenge with Vibrio harveyi. The up-regulation of the c-type lysozyme was much stronger than that of the g-type lysozyme in kidney and spleen. The recombinant proteins of the c-type and g-type lysozymes showed lytic activities against the bacterial pathogens Vibrio harveyi and Photobacterium damselae in a dosage-dependent manner. We identified single nucleotide polymorphisms (SNPs) in the two lysozyme genes. There were significant associations of these polymorphisms with resistance to the big belly disease. These results suggest that the c- and g-type genes play an important role in resistance to bacterial pathogens in fish. The SNP markers in the two genes associated with the resistance to bacterial pathogens may facilitate the selection of Asian seabass resistant to bacterial diseases.

Recombinant goose-type lysozyme in channel catfish: lysozyme activity and efficacy as plasmid DNA immunostimulant against Aeromonas hydrophila infection

The objectives of this study were: 1) to investigate whether recombinant channel catfish lysozyme-g (CC-Lys-g) produced in Escherichia coli expression system possesses any lysozyme activity; and 2) to evaluate whether channel catfish lysozyme-g plasmid DNA could be used as an immunostimulant to protect channel catfish against Aeromonas hydrophila infection. Recombinant CC-Lys-g produced in E. coli expression system exhibited significant (P < 0.05) lytic activity against Gram-positive Micrococcus lysodeikticus and Gram-negative A. hydrophila. When pcDNA3.2-vectored recombinant channel catfish lysozyme-g (pcDNA-Lys-g) was transfected in channel catfish gill cells G1B, the over-expression of pcDNA-Lys-g offered significant (P < 0.05) protection to G1B cells against A. hydrophila infection. When channel catfish were intraperitoneally injected with pcDNA-Lys-g along with an adjuvant QCDCR, the transcriptional level of Lys-g was significantly (P < 0.05) increased. When pcDNA-Lys-g injected fish was challenged with a highly virulent A. hydrophila strain AL-09-71, pcDNA-Lys-g offered 100% protection to channel catfish at two days post DNA injection. Macrophages of fish injected with pcDNA-Lys-g produced significantly (P < 0.05) higher amounts of reactive oxygen species and nitric oxide than that of fish injected with pcDNA vector alone at two days post DNA injection. Taken together, our results suggest that pcDNA-Lys-g could be used as a novel immunostimulant to offer immediate protection to channel catfish against A. hydrophila infection.

Molecular characterization and expression analysis of TLR 7 and TLR 8 homologs in large yellow croaker (Pseudosciaena crocea)

The two toll-like receptor (TLR) genes, LycTLR7 and LycTLR8, were cloned from large yellow croaker (Pseudosciaena crocea), an economically important marine fish in China. The full-length cDNAs of LycTLR7 and LycTLR8 are 3544 and 3593 bp, with an open reading frame (ORF) of 3165 and 3093 bp, encoding 1053 and 1030 amino acids, respectively. The TLR family motifs, such as leucine rich repeat (LRR) and Toll/interleukin (IL)-1 receptor (TIR) domain, are conserved in the LycTLR7 and LycTLR8, with 17 and 14 LRRs, and with a TIR domain, respectively. It is also noted that an LRR N-terminal domain (LRR-NT, residues 24-60) is present in the LycTLR7 but not in the LycTLR8. Both LycTLR7 and LycTLR8 contain a conserved extracellular CxRCxxxxxPCxxC motif, which was found in TLR7/TLR8 of other species and required for stimulus-induced signal transduction. Homology comparison shows that LycTLR7 has 79%, 71.9%, 65.9% and 65.8% identity to fugu, rainbow trout, carp and catfish TLR7, while LycTLR8 has 67.1%, 60.7%, 60.6%, 52.4%, and 51.5% identity to fugu TLR8, rainbow trout TLR8a1, rainbow trout TLR8a2, catfish TLR8-2, and catfish TLR8-1, respectively. Subcellular localization analysis revealed that both LycTLR7 and LycTLR8 are located in the endoplasmic reticulum of epithelioma papulosum cyprini (EPC) cells, which is similar to TLR7/TLR8 in mammals. The two TLRs were constitutively expressed in all tissues tested, especially in immune-related tissues such as spleen, head kidney and gills. An increased expression of LycTLR7 and LycTLR8 was observed in head kidney and spleen of large yellow croakers stimulated by poly (I:C), a viral mimic. In head kidney, their mRNA expression was up-regulated more than 10 times compared to the controls at 12 h after poly (I:C) stimulation. These results suggested that LycTLR7 and LycTLR8 may play a role in the defense against viral infection like their mammalian homologs.

Four lysozymes (one c-type and three g-type) in catfish are drastically but differentially induced after bacterial infection

Lysozyme is an important component of the innate immune system. In this study, four lysozyme genes including one c-type lysozyme and three g-type lysozymes were identified from channel catfish (Ictalurus punctatus). The lysozyme genes are highly conserved in their structural features as compared to those from other species. Phylogenetic analyses were conducted allowing annotation of these genes. Additional analyses using conserved syntenies allowed determination of orthologies for the c-type lysozyme. Phylogenetic analysis indicated that the g-type lysozyme may have gone through species-specific gene duplications leading to multiple copies in some teleost species. Channel catfish possessed three copies of the g-type lysozyme genes. Expression analysis revealed that the catfish lysozyme genes were expressed in a broad range of tissues. The highest levels of expression were found in head kidney, liver, spleen, and trunk kidney, compatible with the immune functions of these tissues/organs. The c-type and g-type lysozymes were drastically induced after bacterial infection, but exhibited large differences in the extent of induction and the tissue with the highest level of induction, with the g-type lysozyme being most highly induced in the head kidney whereas the other three lysozymes being most highly induced in the liver, suggesting their cooperative actions in the immune responses but difference in their detailed functions.

Enzyme characterisation and gene expression profiling of Atlantic salmon chicken- and goose-type lysozymes

Lysozymes represent important innate immune components against bacteria. In this study, Atlantic salmon (Salmo salar) goose (g-) and chicken (c-) types of lysozyme were subjected to protein characterisations and tissue expression analyses. Specific bacterial protein inhibitors of g- and c-type lysozymes were employed to discriminate between respective enzyme activities. Blood, gills and liver contained activities exclusive for the g-type lysozyme. Only haematopoietic organs (head kidney and spleen) contained enzyme activities of both g- and c-lysozyme enzymes and c-type activity was not found outside these organs. Gene transcript levels proportional to enzyme activity levels were detected for the g-type lysozyme but not for the c-type. In vitro studies revealed significant induction of c-type gene expression and enzyme activity in macrophages after incubation with lipopolysaccharide (LPS) while expression of the g-type lysozyme gene was unaffected. The activity of purified native c-type enzyme was profoundly reduced by divalent cations and displayed low tolerance to monovalent cations, while the native g-type lysozyme was stimulated by monovalent cations and tolerated low concentrations of divalent cations. Activities of both enzymes increased with temperature elevations up to 60°C. The native g-type lysozyme responses to temperature in particular are in apparent conflict to the ones for the recombinant salmon g-lysozyme. Our results imply separate expression regulations and different functions of c- and g-type lysozymes in salmon. LPS-induced expression of c-type lysozyme and broad constitutive tissue distribution of g-type lysozyme in salmon is different from findings in other studied fish species.