The tissue factor pathway inhibitor 1 of Sciaenops ocellatus possesses antimicrobial activity and is involved in the immune response against bacterial infection

Tissue factor pathway inhibitors disrupt structures of rhabdovirus/ranairidovirus and inhibit viral infection in Chinese perch, Siniperca chuatsi

Viral diseases have caused great economic losses to the aquaculture industry. However, there are currently no specific drugs to treat these diseases. Herein, we utilized Siniperca chuatsi as an experimental model, and successfully extracted two tissue factor pathway inhibitors (TFPIs) that were highly distributed in different tissues. We then designed four novel peptides based on the TFPIs, named TS20, TS25, TS16, and TS30. Among them, TS25 and TS30 showed good biosafety and high antiviral activity. Further studies showed that TS25 and TS30 exerted their antiviral functions by preventing viruses from invading Chinese perch brain (CPB) cells and disrupting Siniperca chuatsi rhabdovirus (SCRV)/Siniperca chuatsi ranairidovirus (SCRIV) viral structures. Additionally, compared with the control group, TS25 and TS30 could significantly reduce the mortality of Siniperca chuatsi, the relative protection rates of TS25 against SCRV and SCRIV were 71.25 % and 53.85 % respectively, and the relative protection rate of TS30 against SCRIV was 69.23 %, indicating that they also had significant antiviral activity in vivo. This study provided an approach for designing peptides with biosafety and antiviral activity based on host proteins, which had potential applications in the prevention and treatment of viral diseases.

Comparative transcriptome analysis reveals potential regulatory mechanisms of genes and immune pathways following Vibrio harveyi infection in red drum (Sciaenops ocellatus)

Red drum (Sciaenops ocellatus), as an important economical marine fish, has been affected by various bacterial diseases in recent years. Vibrio harveyi cause fatal vibriosis in S. ocellatus, leading to massive mortality and causing significant setbacks in aquaculture. However, the regulatory mechanisms of S. ocellatus response to V. harveyi infection are poorly understood. In this regard, we performed transcriptomic analysis with head kidney tissues of S. ocellatus after V. harveyi infection from 12 h to 48 h to reveal genes, gene expression profiles, and pathways involved in immune and inflammation responses. Specifically, a total of 9,599, 5,728, and 7144 differentially expressed genes (DEGs) were identified after V. harveyi infection at 12 h, 24 h, and 48 h, respectively, and 1,848 shared DEGs have been identified from the above three comparison groups. Subsequent pathway analysis revealed that the shared DEGs following V. harveyi were involved in complement and coagulation cascades (C1R, C1QC, C3, C4, C5, C7, C8A, C8B, C8G, C9, CFB, CFH, and CFI), MAPK signaling pathway, chemokine signaling pathway (CCL19, CXCL8, CXCL12, CXCL14, CCR4, CCR7, and CXCR2), PPAR signaling pathway (PPAR-α, PPAR-γ and PPAR-β), and TNF signaling pathway. Finally, the expression patterns of DEGs in head kidney tissues and S. ocellatus macrophages were validated by qRT-PCR, suggesting the reliability of RNA sequencing for gene expression analysis. This dynamic transcriptome analyses provided insights into gene expression regulation and immune related pathways involved in S. ocellatus after V. harveyi infection, and provides useful information for further study on the immune defense mechanisms in S. ocellatus as well as other teleost species.

NKHs27, a sevenband grouper NK-Lysin peptide that possesses immunoregulatory and antimicrobial activity

As an effective and broad-spectrum antimicrobial peptide, NK-Lysin is attracted more and more attention at present. However, the functions and action mechanism of NK-Lysin peptides are still not comprehensive enough at present. In this study, a sevenband grouper (Hyporthodus septemfasciatus) NK-Lysin peptide, NKHs27, was identified and synthesized, and its biological functions were studied. The results indicated that NKHs27 shares 44.44%∼88.89% overall sequence identities with other teleost NK-Lysin peptides. The following antibacterial activity assay exhibited that NKHs27 was active against both Gram-negative and Gram-positive bacteria, including Staphylococcus aureus, Listonella anguillarum, Vibrio parahaemolyticus and Vibrio vulnificus. Additionally, NKHs27 showed a synergistic effect when it was combined with rifampicin or erythromycin. In the process of interaction with the L. anguillarum cells, NKHs27 changed the cell membrane permeability and retained its morphological integrity, then penetrated into the cytoplasm to act on genomic DNA or total RNA. Then, in vitro studies showed that NKHs27 could enhance the respiratory burst ability of macrophages and upregulate immune-related genes expression in it. Moreover, NKHs27 incubation improved the proliferation of peripheral blood leukocytes significantly. Finally, in vivo studies showed that administration of NKHs27 prior to bacterial infection significantly reduced pathogen dissemination and replication in tissues. In summary, these results provide new insights into the function of NK-Lysin peptides in teleost and support that NKHs27, as a novel broad-spectrum antibacterial peptide, has potential applications in aquaculture against pathogenic infections.

Antimicrobial and Immunoregulatory Activities of TS40, a Derived Peptide of a TFPI-2 Homologue from Black Rockfish (Sebastes schlegelii)

Tissue factor pathway inhibitor-2 (TFPI-2) is a Kunitz-type serine protease inhibitor. Previous reports have shown that TFPI-2 plays an important role in innate immunity, and the C-terminal region of TFPI-2 proved to be active against a broad-spectrum of microorganisms. In this study, the TFPI-2 homologue (SsTFPI-2) of black rockfish (Sebastods schegelii) was analyzed and characterized, and the biological functions of its C-terminal derived peptide TS40 (FVSRQSCMDVCAKGAKQHTSRGNVRRARRNRKNRITYLQA, corresponding to the amino acid sequence of 187-226) was investigated. The qRT-PCR (quantitative real-time reverse transcription-PCR) analysis showed that the expression of SsTFPI-2 was higher in the spleen and liver. The expression of SsTFPI-2 increased significantly under the stimulation of Listonella anguillarum. TS40 had a strong bactericidal effect on L. anguillarum and Staphylococcus aureus. Further studies found that TS40 can destroy the cell structure and enter the cytoplasm to interact with nucleic acids to exert its antibacterial activity. The in vivo study showed that TS40 treatment could significantly reduce the transmission of L. anguillarum and the viral evasion in fish. Finally, TS40 enhanced the respiratory burst ability, reactive oxygen species production and the expression of immune-related genes in macrophages, as well as promoted the proliferation of peripheral blood leukocytes. These results provide new insights into the role of teleost TFPI-2.

Antimicrobial and immunoregulatory activities of the derived peptide of a natural killer lysin from black rockfish (Sebastes schlegelii)

Natural killer lysin (NK-lysin) is a small molecule antimicrobial peptide secreted by natural killer cells and T lymphocytes. In this study, we characterized a cDNA sequence encoding an NK-lysin homologue (SsNKL1) from black rockfish, Sebastes schlegelii. The open reading frame (ORF) of SsNKL1 encodes a putative protein of 149 amino acids and shares 44%-87% overall sequence identities with other teleost NK-lysins. SsNKL1 possesses conserved NK-lysin family features, including a signal sequence and a surfactant-associated protein B (SapB) domain, sequence analysis revealed that SsNKL1 is most closely related to false kelpfish (Sebastiscus marmoratus) NK-lysin (with 87% sequence identity). SsNKL1 transcripts were detected in all the tested tissues, with the highest level in the kidney, followed by the spleen and gills. Upon Listonella anguillarum infection, the mRNA expression of SsNKL1 in the black rockfish was significantly up-regulated in the liver and kidney. The derived peptide SsNKLP27 from SsNKL1 was synthesized, and its biological function was studied. SsNKLP27 showed direct antibacterial activity against Gram-negative and Gram-positive bacteria, including Staphylococcus aureus, Bacillus subtilis, L. anguillarum, Vibrio parahaemolyticus, Vibrio alginolyticus and Vibrio vulnificus. SsNKLP27 treatment facilitated the bactericidal process of erythromycin by enhancing the permeability of the outer membrane. In the process of interaction with the target bacterial cells, SsNKLP27 changed the permeability and retained the morphological integrity of the cell membrane, then penetrated into the cytoplasm, and induced the degradation of genomic DNA and total RNA. In vivo studies showed that administration of SsNKLP27 before bacterial and viral infection significantly reduced the transmission and replication of pathogens in tissues. In vitro analysis showed that SsNKLP27 could enhance the respiratory burst ability and regulate the expression of some immune-related genes of macrophages. In summary, these results provided new insights into the function of NK-lysins in teleost fish and support that SsNKLP27 is a new broad-spectrum antimicrobial peptide that has a potential application prospect in aquaculture against pathogenic infection.

Ly-6D of Japanese flounder (Paralichthys olivaceus) functions as a complement regulator and promotes host clearance of pathogen

The Lymphocyte antigen-6 (Ly-6) superfamily has been considered to play an important role in the innate immunity of mammals. The functions of Ly-6 proteins are diverse since their low sequence homology. Currently, the function of Ly-6D, a member of Ly-6 family proteins, is completely unknown in teleost. In the present study, we identified and characterized a Ly-6D homologue (named PoLy-6D) from the teleost fish Paralichthys olivaceus and examined its immune function. PoLy-6D possesses a hydrophobic signal peptide, a LU domain including a conserved "LXCXXC" motif in N-terminus and a "CCXXXXCN" motif in C-terminus. Under normal physiological condition, PoLy-6D expression distributes in all the examined tissues, the highest three tissues are successively spleen, head kidney, and blood. When infected by extracellular and intracellular bacterial pathogens and viral pathogen, PoLy-6D expression was induced and the patterns vary with different types of microbial pathogens infection and different immune tissues. In vitro experiment showed recombinant PoLy-6D (rPoLy-6D) inhibited the lysis of rabbit red blood cells by serum and selectively improved bacterial survival in serum. After serum were treated by antibody of rPoLy-6D, bacteriostatic effect of serum was obviously enhanced. These results indicate the importance of PoLy-6D as a complement regulator. rPoLy-6D possessed the binding activity to multiple bacteria but did not exhibit antimicrobial activities. The interaction between rPoLy-6D and bacteria suggests that PoLy-6D is involved in host clearance of pathogens probably by serving as a receptor for pathogens. Overexpression of PoLy-6D in vivo promoted the host defense against invading E. piscicida. These findings add new insights into the regulation mechanism of the complement system in teleost and emphasize the importance of Ly-6D products for the control of pathogen infection.

Macrophage colony stimulating factor (MCSF) of Japanese flounder (Paralichthys olivaceus): Immunoregulatory property, anti-infectious function, and interaction with MCSF receptor

Macrophage colony-stimulating factor (MCSF) is an essential growth factor to control the proliferation, differentiation and survival of the macrophage lineage in vertebrates. Sequences of MCSF have been identified in multiple teleost species, however, the functional investigations of MCSF were documented in only a few species. In this study, we examined the biological activity and the immunomodulatory property of a MCSF homologue, PoMCSF, from Japanese flounder (Paralichthys olivaceus). Structural analysis showed that PoMCSF possesses conserved structural characteristics of MCSF proteins, including a signal peptide, a CSF-1 domain, and a transmembrane region closed to the C-terminal. Under normal physiological condition, PoMCSF expression distributes in all the examined tissues, the highest three tissues are blood, muscle, and head kidney. When infected by extracellular and intracellular bacterial pathogens and viral pathogen, the PoMCSF expression patterns vary with different types of microbial pathogens infection and different immune tissues. In vitro experiment showed recombinant PoMCSF promoted the activity of macrophage. In vivo experiment indicated that PoMCSF overexpression boosted the defensive ability of flounder against Edwardsiella piscicida, a severe fish pathogen that infects multiple species of economically important fish, and regulated the expression of multiple immune-related genes. To explore the relationship between PoMCSF and its receptor PoMCSFR, anti-PoMCSFR antibody was prepared and PoMCSFR knockdown was conducted. The neutralization assay showed that when PoMCSFR was neutralized by its antibody, the role of PoMCSF on host defense against E. piscicida was weakened. Knockdown of PoMCSFR impaired the phagocytic capacity of macrophages. Collectively, these findings suggest that PoMCSF plays a crucial role in the immune defense system of Japanese flounder and the effect of PoMCSF is dependent on PoMCSFR. This study provides new insights into the biological activity of MCSF and the relationship between MCSF and MCSFR in teleost.

Expression analysis of tissue factor pathway inhibitors TFPI-1 and TFPI-2 in Paralichthys olivaceus and antibacterial and anticancer activity of derived peptides

Tissue factor pathway inhibitors (TFPI), including TFPI-1 and TFPI-2, are Kunitz-type serine protease inhibitors that mainly inhibit the blood coagulation induced by tissue factors. Previous reports on teleost proved TFPI play important roles in innate immunity. In this study, two TFPI (PoTFPI-1 and PoTFPI-2) molecules from Japanese flounder (Paralichthys olivaceus) were analyzed and characterized for their expression patterns, antibacterial and anticancer activities of the C-terminal derived peptides. Quantitative real time RT-PCR analysis shows that constitutive PoTFPI-1 expression occurred, in increasing order, in the brain, muscle, spleen, gills, head kidney, blood, intestine, heart, and liver; PoTFPI-2 was expressed, in increasing order, in the brain, gills, head kidney, muscle, intestine, spleen, liver, heart, and blood. Under the stimulation of fish pathogens, both PoTFPI-1 and PoTFPI-2 expressions increased significantly in a manner that depended on the pathogens, tissue type, and infection stage. Furthermore, C-terminal peptides TP25 and TP26, derived from PoTFPI-1 and PoTFPI-2, respectively, were synthesized and proved to be active against Micrococcus luteus (for TP25 and TP26) and Staphylococcus aureus (for TP25) via retardation effects on bacterial nucleic acids. In addition, TP25 and TP26 also displayed significant inhibitory effects on human colon cancer cell line HT-29. These results reveal that both PoTFPI-1 and PoTFPI-2 play important roles in host innate immunity. The antibacterial activity and anticancer cells function of TP25 and TP26 will add new insights into the roles of teleost TFPI.

Two CD9 tetraspanin family members of Japanese flounder (Paralichthys olivaceus): characterization and comparative analysis of the anti-infectious immune function

CD9 is a glycoprotein of the transmembrane 4 superfamily that is involved in various cellular processes. Studies related to the immune functions and activities of CD9 in teleost fish are limited. In this study, we characterized two CD9 homologs, PoCD9.1 and PoCD9.3, from Japanese flounder (Paralichthys olivaceus). Sequence analysis showed that PoCD9.1 and PoCD9.3 possess characteristic transmembrane 4 superfamily (TM4SF) structures. PoCD9.1 shares 70.61% sequence identity with PoCD9.3. The expression of PoCD9.1 and PoCD9.3 in the three main immune tissues was significantly induced in a time-dependent manner by extracellular and intracellular pathogen infection, which indicates that the two CD9 homologs play an important role in the response to pathogenic infection. Following infection with the extracellular pathogen Vibrio anguillarum, the expression profiles of both PoCD9.1 and PoCD9.3 were similar. After infection with the intracellular pathogen Edwardsiella piscicida, the expression levels of PoCD9.1 and PoCD9.3 were different at different stages of infection, especially in the spleen. The spleen was the most important tissue for the PoCD9.1 and PoCD9.3 responses to pathogen infection among the three examined immune tissues. Knockdown of PoCD9.1 and PoCD9.3 attenuated the ability of host cells to eliminate pathogenic bacteria, and PoCD9.1 knockdown was more lethal than PoCD9.3 knockdown for host cells with E. piscicida infection. Overexpression of PoCD9.1 and PoCD9.3 promoted host or host cell defence against E. piscicida infection. These findings suggest that PoCD9.1 and PoCD9.3 serve as immune-related factors, play an important role in the immune defence system of Japanese flounder, and display different functions in response to different pathogens at different stages of infection.

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.

The identification and determination of toxin genes of Vibrio strains causing hemorrhagic disease on red drum (Sciaenops ocellatus) using PCR

Data were collected from 30 strains of Vibrio and sampled on different organs (brain, hemorrhagic site and digestive tract) of Sciaenops ocellatus infection. The results showed that the nucleotide sequences 16S rRNA region are highly similar to those of V. alginolyticus, V. azureus, V. fluvialis, V. natriengens and V. orientalis, which were published on Genbank and other, ranging from 98.05 to 100%. The digestive tract has the most common Vibrio strains (V. alginolyticus [16] V. azureus [7] and V. fluvialis). Thereout, 25 of 30 strains of Vibrio contained 1 to 3 toxin genes, except V. parahaemolyticus. Six parameters were used to measure the DNA polymorphism of 33 homologous DNA sequences in this Vibrio bacteria population. The results indicated that number of separate polymorphic sites (S), total number of mutant sites (Eta), number of haplotype (h), haplotype diversity (Hd), average number of nucleotide differences (k), nucleotide diversity (Pi) were 98 (S), 103 (Eta), 9 (h), 0.887 ± 0.032 (Hd), 25.789 (k) and 17.980 × 10-3 ± 0.003 (Pi), respectively (P < 0,05). The G + C content above 1434 sites positions of nucleotide sequences accounted for 0.542. The phylogenetic tree showed that these strains are divided into six groups. As observed, the appearance of isolated Vibrio on 3 organs of fish (S. ocellatus) hemorrhagic are V. azureus (27,67%), V. alginolyticus (50%), V. orientalis (6,67%) and V. fluvialis (16,67%). Through this result, we found that the diversity of Vibrio species that appeared on the red drum was used in the 16S rRNA region and the presence of toxin genes in these Vibrio species.

Identification and characterization of SaRpAMP, a 60S ribosomal protein L27-derived antimicrobial peptide from amur catfish, Silurus asotus

Aquatic freshwater fish like catfish, Silurus asotus, lives in microbe-rich environments, which enable this fish to develop necessary defense mechanisms. Antimicrobial peptides, along with other innate immune factors, are regarded as an important group in this defense. An antimicrobial peptide, which was isolated from the skin of S. asotus, was identified as a C-terminal fragment of 60S ribosomal protein L27 from S. asotus. The peptide was, then, designated Silurus asotus 60S ribosomal protein L27-derived antimicrobial peptide, SaRpAMP. Primary structure analyses and cDNA cloning revealed that SaRpAMP was 4185.36 Da and composed of 33 amino acids (AAs). Its precursor had a total of 136 AAs containing a pro-sequence of 103 AAs encoded by the nucleotide sequence of 512 bp that comprises a 5' untranslated region (UTR) of 32 bp, an open reading frame (ORF) of 411 bp, and a 3' UTR of 69 bp. Secondary structure analyses showed that SaRpAMP had two α-helices with turns and coils and an amphiphilic structure, a finding consistent with the 3D model of the peptide. SaRpAMP exhibited potent antibacterial activity comparable to piscidin 1, a powerful positive control. Its antimicrobial activity against fungus C. albicans was relatively weak. The antimicrobial activity of SaRpAMP was not diminished by heat treatment and changes in pH but was abolished by proteolytic enzyme digestion. Membrane permeability assays suggested that SaRpAMP interacts with both the outer and inner bacterial membranes. This was consistent with the results of lipid titration and quenching of Trp fluorescence that demonstrated SaRpAMP's interaction with acidic liposomes. Collectively, these findings suggest that the identified peptide, SaRpAMP, was the first antimicrobial peptide reported to be derived from the C-terminal region of 60S ribosomal protein L27. The findings also suggest that the action mechanism of SaRpAMP involved the interaction of the peptide with the bacterial membranes.

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.

TC26, a teleost TFPI-1 derived antibacterial peptide that induces degradation of bacterial nucleic acids and inhibits bacterial infection in vivo

At present, several reports have indicated that the C-terminal peptides of tissue factor pathway inhibitor 1 (TFPI-1) were active antibacterial peptides. However, the functions of TFPI-1 C-terminal peptides in teleost are still very limited. In this study, a C-terminal peptide, TC26 (with 26 amino acids), derived from common carp (Cyprinus carpio) TFPI-1, was synthesized and investigated for its antibacterial spectrum, action mechanism, as well as the in vivo effects on bacterial invasion. Our results showed that TC26 was active against Gram-positive bacteria Micrococcus luteus and Staphylococcus aureus, as well as Gram-negative bacterium Vibrio vulnificus. TC26 treatment facilitated the bactericidal process of erythromycin by enhancing the out-membrane permeability of V. vulnificus. During the bactericidal process, TC26 killed the target bacterial cells Vibrio vulnificus, by destroying cell membrane integrity, penetrating into the cytoplasm and inducing degradation of genomic DNA and total RNA. In vivo study showed that administration of turbot with TC26 before bacterial infection significantly reduced pathogen dissemination and replication in tissues. These results indicated that TC26 is a novel and active antibacterial peptide and may play a vital role in fighting pathogenic infection in aquaculture.

Characterization, expression, and antimicrobial activity of histones from Japanese flounder Paralichthys olivaceus

Histone proteins are not only structurally important for chromosomal DNA packaging but also involved in the regulation of gene expression and the immune response of host against pathogens. Japanese flounder (Paralichthys olivaceus) as one of the most important marine flatfish, suffered from widespread outbreaks of diseases, and its immunological functioning remained to be elucidated. In the present study, we reported the expression patterns of four histones (H1, H2A, H3, and H3.3) and functional characterization of the histone H3.3 from flounder. Quantitative real time RT-PCR (RT-qPCR) analysis showed that expression of the four histones occurred in multiple tissues, but their levels of expression were relatively high in immune organs, and inducible in response to pathogens infection. Infection with extracellular and intracellular bacterial pathogens and viral pathogen regulated the expression of histones in a manner that depended on tissue type, pathogen, and infection stage. Specifically, H1 expression was highly induced by intracellular viral pathogens; H2AX and H3 expressions were highly induced by intracellular bacterial pathogen; dissimilarly, H3.3 expression was slightly induced by extracellular bacterial pathogen, but was inhibited by intracellular bacterial and viral pathogens. To further investigate H3.3 function, recombinant H3.3 (rH3.3) was obtained, and in vitro experiments showed rH3.3 possessed the capability of binding to both Gram-negative and Gram-positive bacteria and inhibiting the growth of some target bacteria. Consistently, In vivo results showed that overexpression of H3.3 promoted the host defense against invading pathogenic microorganism and regulated the expressions of several cytokines. These results suggested that flounder histones exhibit different expression patterns in response to the infection of different microbial pathogens, and H3.3 serves as an immune-related protein and plays an important role in antimicrobial immunity of Japanese flounder. Taken together, this study is the first report about the expression profile of different histones upon different kind of pathogens and anti-infectious immunity of H3.3 in teleost, which offered new insights into the immunological function of histones in teleost.

Major histocompatibility complex class I (MHC Iα) of Japanese flounder (Paralichthys olivaceus) plays a critical role in defense against intracellular pathogen infection

The major histocompatibility complex (MHC) is a highly polymorphic region of the vertebrate genome that plays a critical role in initiating immune responses towards invading pathogens. It is well known that MHC I molecules play a central role in the immune response to viruses. However, rare literatures were reported the role of MHC I in the resistance to intracellular bacteria. Sequences of MHC Iα were identified in multiple teleost species, including Japanese flounder (Paralichthys olivaceus), however, the immunological function of MHC Iα remain largely unknown. In this study, we examined the expression profile and biological activity of an MHC Iα homologue, PoMHC Iα, from P. olivaceus. Structural analysis showed that PoMHC Iα possesses conserved structural characteristics of MHC Iα proteins, including MHC_I domain, IGc1 domain, transmembrane region. Expression of PoMHC Iα was upregulated in a time-dependent manner by extracellular and intracellular bacterial pathogens and viral pathogen infection. Different expression patterns were exhibited in response to the infection of different types of microbial pathogens in different immune tissues. Recombinant PoMHC Iα increased the capability of host cells to defense against intracellular pathogen Edwardsiella tarda infection and enhanced the expression of immune related genes. The knockdown of PoMHC Iα attenuated the ability of cells to eliminate E. tarda, which was sustained by the in vivo results that overexpression of PoMHC Iα promoted the host defense against invading E. tarda. Antigen uptake assay indicated PoMHC Iα participated in cells antigen presentation. Collectively, this study is the first report that MHC Iα plays an important role in immune defense against intracellular bacterial pathogen in teleost. Taken together, these findings add new insights into the biological function of teleost MHC Iα and emphasize the importance of MHC I gene products for the control of E. tarda infection.

Characterization, expression, enzymatic activity, and functional identification of cathepsin S from black rockfish Sebastes schlegelii

Cathepsin S belong to the cathepsin L-like family of cysteine cathepsins. It is well known that Cathepsin S participate in various physiological processes and host immune defense in mammals. However, in teleost fish, the function of cathepsin S is less investigated. In the present study, a cathepsin S homologue (SsCTSS) from the teleost fish black rockfish (Sebastes schlegelii) were identified and examined at expression and functional levels. In silico analysis showed that three domains, including signal peptide, cathepsin propeptide inhibitor I29 domain, and functional domain Pept_C1, were existed in the cathepsin. SsCTSS possesses a peptidase domain with three catalytically essential residues (Cys25, His162, and Asn183). Phylogenetic profiling indicated that SsCTSS are evolutionally close to the cathepsin S of other teleost fish. The expression of SsCTSS in immune-related tissues was upregulated in a time-dependent manner upon bacterial pathogen infection. Purified recombinant SsCTSS (rSsCTSS) exhibited apparent peptidase activity, which was remarkably declined in the presence of the cathepsin inhibitor E-64. rSsCTSS showed strong binding ability to LPS and PGN, the major constituents of the outer membranes of Gram-negative and Gram-positive bacteria, respectively. rSsCTSS also exhibited the capability of agglutination to different bacteria. The knockdown of SsCTSS attenuated the ability of host to eliminate pathogenic bacteria. Taken together, our results suggested that SsCTSS functions as cysteine protease which might be involved in the antibacterial immunity of black rockfish.

Granulocyte colony stimulating factor (GCSF) of Japanese flounder (Paralichthys olivaceus): Immunoregulatory property and anti-infectious function

Granulocyte colony stimulating factor (GCSF) is a key regulator of neutrophil production, and plays a vital role in immune response of mammals and teleost against pathogen. Sequences of GCSF were identified in several teleost species, however, the function and activity of GCSF in teleost remain largely unknown. In this study, we examined the biological activity and the immunomodulatory property of a GCSF homologue, PoGCSF, from Japanese flounder (Paralichthys olivaceus). Structural analysis showed that PoGCSF possesses conserved structural characteristics of GCSF proteins, including a signal peptide and a typical IL-6 domain. The expression of PoGCSF was upregulated in a time-dependent manner by extracellular and intracellular bacterial pathogens and viral pathogen. Different expression patterns were exhibited in response to the infection of different types of microbial pathogens in different immune tissues. Recombinant PoGCSF increased the capability of host cells to defense against pathogen infection and enhanced the expression of immune related genes. The knockdown of PoGCSF attenuated the ability of host cells to eliminate pathogenic bacteria. In vivo results showed that overexpression of PoGCSF promoted the host defense against invading pathogenic microorganism. Collectively, this study is the first report about the immunoregulatory property and anti-infectious immunity of GCSF in teleost. These findings suggested that PoGCSF serves as an immune-related cytokine and plays an important role in the immune defense system of Japanese flounder.

A novel C1q domain containing protein in black rockfish (Sebastes schlegelii) serves as a pattern recognition receptor with immunoregulatory properties and possesses binding activity to heat-aggregated IgG

C1q-domain-containing (C1qDC) proteins, which are involved in a series of immune responses, are important pattern recognition receptors in innate immunity in vertebrates and invertebrates. Functional studies of C1qDC proteins in vertebrates are scarce. In the present study, a C1qDC protein (SsC1qDC) from the teleost black rockfish (Sebastes schlegelii) was identified and examined at expression and functional levels. The open reading frame of SsC1qDC is 636 bp, and the predicted amino acid sequence of SsC1qDC shares 62%-69% overall identity with the C1qDC proteins of several fish species. SsC1qDC possesses conserved C1qDC features, including a signal sequence and a C1q domain. SsC1qDC was expressed in different tissues and its expression was up-regulated by bacterial and viral infection. Recombinant SsC1qDC (rSsC1qDC) exhibited apparent binding activities against PAMPs including LPS and PGN. rSsC1qDC had antibacterial activity against Vibrio parahaemolyticus, and was able to enhance the phagocytic activity of macrophages towards Vibrio anguillarum. rSsC1qDC interacted with human heat-aggregated IgG. Furthermore, in the presence of rSsC1qDC, fish exhibited enhanced resistance against bacterial infection. Collectively, these results indicated that SsC1qDC serves as a pattern recognition receptor and plays a vital role in the defense system of black rockfish.

Post-transcriptional, post-translational and pharmacological regulation of tissue factor pathway inhibitor

: Tissue factor (TF) pathway inhibitor (TFPI) is an endogenous natural anticoagulant that readily inhibits the extrinsic coagulation initiation complex (TF-FVIIa-Xa) and prothrombinase (FXa, FVa and calcium ions). Alternatively, spliced TFPI isoforms (α, β and δ) are expressed by vascular and extravascular cells and regulate thrombosis and haemostasis, as well as cell signalling functions of TF complexes via protease-activated receptors (PARs). Proteolysis of TFPI plays an important role in regulating physiological roles of the TF pathway in host defense and possibly haemostasis. Elimination of TFPI inhibition has therefore been proposed as an approach to improve haemostasis in haemophilia patients. In this review, we focus on posttranscription and translational modification of TFPI and its function in thrombosis and how pharmacological inhibitors and endogenous proteases interfere with TFPI and alter haemostasis.

A teleost TFPI-2 peptide that possesses a broad antibacterial spectrum and immune-stimulatory properties

Tissue factor pathway inhibitor 2 (TFPI-2) is an analogue of TFPI-1 and a potent endogenous inhibitor of tissue factor (TF)-mediated blood coagulation. Previous reports have shown that several peptides derived from human and vertebrates TFPI-2 possess antibacterial activity against diverse bacteria. In this study, a C-terminal peptide, TO24 (with 24 amino acids), derived from red drum (Sciaenops ocellatus) TFPI-2, was synthesized and investigated for its antimicrobial spectrum, action mode, as well as the immune-stimulatory property. Our results indicated that TO24 was active against Gram-positive bacteria Micrococcus luteus and Staphylococcus aureus; Gram-negative bacteria Vibrio litoralis, Vibrio ichthyoenteri, Vibrio vulnificus and Vibrio scophthalmi, as well as fish megalocytivirus, infectious spleen and kidney necrosis virus (ISKNV). During its interaction with V. vulnificus, TO24 exerted its antibacterial activity by destroying cell membrane integrity, penetrating the cytoplasm and inducing degradation of genomic DNA and total RNA. In addition, TO24 had no hemolytic activity against red drum blood cells. In vitro, TO24 enhanced bactericidal activity of red drum macrophages. In vivo, administration of red drum with TO24 before bacterial infection significantly reduced pathogen dissemination and replication in tissues. These results indicate that TO24 is a broad-spectrum antimicrobial peptide with immune-stimulatory properties and it has the potential to be used as an antimicrobial agent in aquaculture.

Black rockfish C-type lectin, SsCTL4: A pattern recognition receptor that promotes bactericidal activity and virus escape from host immune defense

C-type lectin (CTL) is an immune receptor and is received extensive attention of its important roles in immune response and immune escape. Some CTL, such as CTL4, has been well characterized in human and several other mammals, but much less documentation exists about the immunological function of CTL4 in lower vertebrates. In the present study, a C-type lectin domain family 4 member, SsCTL4, which is also high homology with CD209 antigen-like protein, from the teleost fish black rockfish (Sebastes schlegelii) was identified and examined at expression and functional levels. The open reading frame of SsCTL4 is 765 bp, and the deduced amino acid sequence of SsCTL4 shares 78%-84% overall identities with the C-type lectin of several fish species. In silico analysis identified several conserved C-type lectin features, including a carbohydrate-recognition domain and four disulfide bond-forming cysteine residues. Expression of SsCTL4 occurred in multiple tissues and was upregulated during bacterial and viral infection. Recombinant SsCTL4 (rSsCTL4) exhibited apparent binding activities against bacteria (Edwardsiella tarda and Vibrio anguillarum) and virus (infectious spleen and kidney necrosis virus, ISKNV). rSsCTL4 was able to agglutinate the Gram-negative and Gram-positive bacteria in a Ca²⁺-dependent manner. The agglutinating ability of rSsCTL4 was abolished in the absence of calcium or presence of mannose. rSsCTL4 also increased macrophage bactericidal activity. In the presence of rSsCTL4, fish exhibited enhanced resistance against bacterial infection but increased susceptibility to viral infections. Collectively, these results indicate that SsCTL4 serves as a pattern recognition receptor that not only promotes bactericidal activity, but may also serve as targets for virus manipulation of host defense system.

Under control: The innate immunity of fish from the inhibitors' perspective

The innate immune response involves a concerted network of induced gene products, preformed immune effectors, biochemical signalling cascades and specialised cells. However, the multifaceted activation of these defensive measures can derail or overshoot and, if left unchecked, overwhelm the host. A plenty of regulatory devices therefore mediate the fragile equilibrium between pathogen defence and pathophysiological manifestations. Over the past decade in particular, an almost complete set of teleostean sequences orthologous to mammalian immunoregulatory factors has been identified in various fish species, which prove the remarkable conservation of innate immune-control concepts among vertebrates. This review will present the current knowledge on more than 50 teleostean regulatory factors (plus additional fish-specific paralogs) that are of paramount importance for controlling the clotting cascade, the complement system, pattern-recognition pathways and cytokine-signalling networks. A special focus lies on those immunoregulatory features that have emerged as potential biomarker genes in transcriptome-wide research studies. Moreover, we report on the latest progress in elucidating control elements that act directly with immune-gene-encoding nucleic acids, such as transcription factors, hormone receptors and micro- and long noncoding RNAs. Investigations into the function of teleostean inhibitory factors are still mainly based on gene-expression profiling or overexpression studies. However, in support of structural and in-vitro analyses, evidence from in-vivo trials is also available and revealed many biochemical details on piscine immune regulation. The presence of multiple gene copies in fish adds a degree of complexity, as it is so far hardly understood if they might play distinct roles during inflammation. The present review addresses this and other open questions that should be tackled by fish immunologists in future.

TO17: A teleost antimicrobial peptide that induces degradation of bacterial nucleic acids and inhibits bacterial infection in red drum, Sciaenops ocellatus

Tissue factor pathway inhibitor (TFPI)-1 is well known for its role as an inhibitor of blood coagulation. Several studies have demonstrated that the C-terminal peptides of TFPI-1 are active against a broad spectrum of microorganisms. In a previous study, we found that TO17 (with 17 amino acids), a TFPI-1 C-terminal peptide from red drum (Sciaenops ocellatus), was active against Edwardsiella tarda. In the present study, we investigated further the antimicrobial spectrum, action mode, as well as the immunostimulatory property of TO17. Our results showed that TO17 displayed antimicrobial activity against Staphylococcus aureus, Micrococcus luteus, Vibrio vulnificus, and infectious spleen and kidney necrosis virus, independent of host serum. Furthermore, the activity of TO17 was influenced by the length or type of amino acids at the N and C termini. During its interaction with V. vulnificus, TO17 exerted its antibacterial activity by destroying cell membrane integrity, penetrating the cytoplasm and inducing degradation of genomic DNA and total RNA. In addition, TO17 had no hemolytic activity against red drum blood cells. In vitro, TO17 enhanced production of nitric oxide and bactericidal activity of red drum macrophages. In vivo, administration of red drum with TO17 before bacterial infection significantly reduced pathogen dissemination and replication in tissues. These results indicate that TO17 is a broad-spectrum antimicrobial peptide with immunostimulatory properties and it has the potential to be used as an antimicrobial agent in aquaculture.

Identification and characterization of a novel calreticulin involved in the immune response of the Zhikong scallop, Chlamys farreri

Calreticulin (CRT) is a multifunctional calcium-binding chaperone shared among vertebrates and invertebrates. In this study, a novel CRT (CfCRT) was identified in the Zhikong scallop Chlamys farreri by rapid amplification of cDNA ends. The full-length cDNA was composed of 1345 bp, which included a 1158 bp open reading frame, a 25 bp 5'-untranslated region (UTR) and a 162 bp 3'-UTR. The predicted molecular mass of CfCRT was 44.8 kDa. CfCRT contained three highly conserved domains (N-, P- and C-domains) essential to the function of CRT. BLAST analysis revealed significant sequence similarity (73%-92%) with CRT proteins from other mollusks. The mRNA transcripts of CfCRT were present in all the tested tissues of Zhikong scallops, with the higher expression level in the hemocytes and mantle. After stimulation by Vibrio anguillarum, the mRNA transcript of CfCRT in hemocytes was significantly upregulated. Recombinant plasmid pBCRT was successfully expressed in Escherichia coli BL21 (DE3). The recombinant (r)CfCRT protein could bind to the surface of several bacteria including the Gram-negative bacteria V. anguillarum, E. coli, and the Gram-positive bacterium Staphylococcus aureus. Moreover, rCfCRT was able to suppress their growth significantly. These results indicate that CfCRT might act as an immune effector in Zhikong scallop innate immunity.

A TFPI-1 peptide that induces degradation of bacterial nucleic acids, and inhibits bacterial and viral infection in half-smooth tongue sole, Cynoglossus semilaevis

Tissue factor pathway inhibitor 1 (TFPI-1) is a serine protease inhibitor that inhibits tissue factor (TF)-mediated coagulation. The C-terminal region of TFPI-1 could be cleaved off and proved to be antimicrobial against a broad-spectrum of microorganism. In a previous study, a C-terminal peptide, TC24 (with 24 amino acids), derived from tongue sole (Cynoglossus semilaevis) TFPI-1, was synthesized and found antibacterial against Micrococcus luteus. In the present study, the antibacterial spectrum and the action mode of TC24 was further examined, and its in vivo function was analyzed. Our results showed that TC24 also possesses bactericidal activity against Staphylococcus aureus and Vibrio vulnificus. During its interaction with the target bacterial cells, TC24 destroyed cell membrane integrity, penetrated into the cytoplasm, and induced degradation of genomic DNA and total RNA. In vivo study showed that administration of tongue sole with TC24 before bacterial and viral infection significantly reduced pathogen dissemination and replication in tissues. These results indicated that TC24 is a novel antimicrobial peptide against bacterial and viral pathogens, and that the observed effect of TC24 on bacterial RNA adds new insights to the action mechanism of fish antimicrobial peptides. Moreover, TC24 may play an important role in fighting pathogenic infection in aquaculture.

Molecular characterization, expression analysis, and bactericidal activity of the derivative peptides of TFPI-1 and TFPI-2 in half-smooth tongue sole, Cynoglossus semilaevis

Tissue factor pathway inhibitors (TFPIs) are Kunitz-type serine protease inhibitors that reversibly regulate the blood coagulation induced by tissue factor. TFPI family contain two members, TFPI-1 and TFPI-2. Recent studies have shown TFPI-1 and TFPI-2 also play important roles in innate immunity, however, the potential function of teleost TFPI are very limited. In this study, we characterized two TFPI (CsTFPI-1 and CsTFPI-2) molecules from half-smooth tongue sole (Cynoglossus semilaevis), examined their tissue distributions and expression patterns under pathogens stimulation as well as investigated the antibacterial activity of the C-terminal peptides. Quantitative real time RT-PCR analysis showed that constitutive CsTFPI-1 expression occurred, in increasing order, in head kidney, intestine, brain, spleen, liver, skin, gills, heart, and muscle; CsTFPI-2 was expressed, in increasing order, in the gills, intestine, skin, head kidney, liver, brain, spleen, muscle, and heart. Under Vibrio anguillarum, Streptococcus agalactiae and fish megalocytivirus stimulation, both CsTFPI-1 and CsTFPI-2 expression increased significantly in a manner that depended on the pathogen, tissue type, and infection stage, which suggested CsTFPI-1 and CsTFPI-2 play important roles in anti-bacterial and anti-viral infection. Finally, C-terminal peptides of CsTFPI-1 and CsTFPI-2, were synthesized and proved to have antibacterial effect against Micrococcus luteus that were independent of host serum. Take together, these results indicate that CsTFPI-1 and CsTFPI-2 play important roles in antimicrobial immunity of this fish.

A C-type lectin that inhibits bacterial infection and facilitates viral invasion in black rockfish, Sebastes schlegelii

C-type lectins (CTLs) are important pattern recognition receptors (PRRs) that play vital roles in innate immunity. In teleosts, a number of CTLs have been reported, but their in vivo effects on host defense are still limited. In this study, a CTL homolog (SsLec1) was identified from black rockfish, Sebastes schlegelii, and its structure, expression and biological function was analyzed. The open reading frame of SsLec1 is 633 bp, with a 5'- untranslated region (UTR) of 36 bp and a 3'- UTR of 117 bp. The deduced amino acid sequence of SsLec1 shares the highest overall identity (73.20%) with the CTL of Oplegnathus fasciatus. SsLec1 possesses conserved CTL features, including a carbohydrate-recognition domain, four disulfide bond-forming cysteine residues, the mannose-type carbohydrate-binding motif, the conserved calcium binding sites and a putative signal peptide. The expression of SsLec1 was highest in liver and could be induced by experimental infection with Listonella anguillarum. Recombinant SsLec1 (rSsLec1) purified from E. coli was able to bind and agglutinate the Gram-negative fish pathogens Vibrio ichthyoenteri and Vibrio vulnificus. The agglutinating ability of rSsLec1 was abolished in the presence of mannose or ethylenediaminetetraacetic acid. Further analysis showed that rSsLec1 could enhance phagocytosis by macrophages. In vivo experiments indicated that rSsLec1 could inhibit bacterial infection and promote viral invasion. Taken together, these results suggest that SsLec1 is a novel CTL that possesses apparent immunoregulation property and plays a critical role in host defense against pathogens invasion.

A high-mobility group box 1 that binds to DNA, enhances pro-inflammatory activity, and acts as an anti-infection molecule in black rockfish, Sebastes schlegelii

High-mobility group box (HMGB) 1 is a chromosomal protein that plays critical roles in DNA transcription, replication and repair. In addition, HMGB1 functions as a pro-inflammatory molecule in many vertebrates and invertebrates. In teleosts, very limited studies of HMGB1 have been reported. In this study, we identified a HMGB1 homologue (SsHMGB1) from black rockfish (Sebastes schlegelii) and analyzed its structure, expression and biological function. The open reading frame of SsHMGB1 is 621 bp, with a 5'-untranslated region (UTR) of 62 bp and a 3'-UTR of 645 bp. SsHMGB1 contains two typical HMG boxes and an acidic C-terminal tail. The deduced amino acid sequence of SsHMGB1 shares the highest overall identity (89.4%) with the HMGB1 of Anoplopoma fimbria. The expression of SsHMGB1 occurred in multiple tissues and was highest in the brain. Moreover, the mRNA level of SsHMGB1 in head kidney (HK) macrophages could be induced by Listonella anguillarum in a time-dependent manner. Recombinant SsHMGB1 purified from Escherichia coli (i) bound DNA fragments in a dose-dependent manner; and (ii) induced the expression of cytokines in HK macrophages, including a significant increase in TNF-α activity and enhanced mRNA level of TNF13B and IL-1 β, which are known to be involved in antibacterial defense; moreover, (iii) significantly improved the macrophage bactericidal activity together with reduced pathogen dissemination and replication of bacteria in fish kidney. These results indicated that SsHMGB1 is a novel HMGB1 that possesses apparent immunoregulatory properties and is likely to be involved in fighting bacterial infection.

A single immunoglobulin-domain IgSF protein from Sciaenops ocellatus regulates pathogen-induced immune response in a negative manner

The immunoglobulin superfamily (IgSF) is a large group of cell surface proteins that include various immunoregulatory receptors such as novel immune type receptors (NITRs), which are a family of diversified proteins found exclusively in bony fish. In this study, we identified and analyzed an IgSF protein, SoIgSF1, from red drum (Sciaenops ocellatus). SoIgSF1 is composed of 225 amino acid residues and moderately related to teleost NITRs. In silico analysis indicated that SoIgSF1 is a type I transmembrane glycoprotein and contains an N-terminal signal peptide sequence, a single extracellular immunoglobulin V domain, a transmembrane region, and a cytoplasmic region. However, unlike most NITRs, the cytoplasmic region of SoIgSF1 exhibits no consensus inhibitory or stimulatory signaling sequences but has two tyrosine-containing motifs that conform to the right-half sequence of the immunoreceptor tyrosine-based inhibitory motif (ITIM). Quantitative real time RT-PCR analysis showed that SoIgSF1 expression occurred mainly in immune organs and was drastically induced by viral and bacterial infection. Immunofluorescence microscopy indicated that viral infection of head kidney (HK) leukocytes induced surface expression of SoIgSF1, which was able to interact with antibodies against recombinant SoIgSF1. Antibody cross-linking of SoIgSF1 on HK leukocytes inhibited the expression of immune relevant genes and promoted viral and bacterial infection. Taken together, these results indicate that SoIgSF1, though lacking canonical intracellular signaling motifs, is involved in regulation of host immune response during pathogen infection possibly by functioning as a negative signaling receptor through a novel mechanism.

The high mobility group box 1 protein of Sciaenops ocellatus is a secreted cytokine that stimulates macrophage activation

High mobility group box 1 protein (HMGB1) is a chromatin-associated nonhistone protein that is involved in nucleosome formation and transcriptional regulation. In addition, HMGB1 is also known as an extracellular cytokine that triggers inflammation and immune responses. HMGB1-like sequences have been identified in a number of fish species, however, the function of piscine HMGB1 remains uninvestigated. In this study, we reported the identification and analysis of SoHMGB1, an HMGB1 homologue from red drum (Sciaenops ocellatus). SoHMGB1 is 206 residues in length and contains two basic HMG boxes and a highly acidic C-terminal domain. SoHMGB1 shares 71-87% overall sequence identities with the HMGB1 counterparts from human, rat, and several fish species. Quantitative real time RT-PCR analysis showed that constitutive SoHMGB1 expression was detected in various tissues, with the lowest and highest levels found in kidney and muscle respectively. Bacterial challenge upregulated SoHMGB1 expression in head kidney (HK) and HK macrophages and induced extracellular secretion of SoHMGB1 by the activated macrophages. Recombinant SoHMGB1 (rSoHMGB1) purified from yeast exhibited no direct antimicrobial effect but was significantly stimulatory on the proliferation, activation, and bactericidal activity of HK macrophages. Taken together, these results indicate for the first time that a fish HMGB1, SoHMGB1, can function as a secreted cytokine in the event of bacterial infection and promote innate defense through the activation of macrophages.