Complete genome sequence of Streptococcus agalactiae ZQ0910, a pathogen causing meningoencephalitis in the GIFT strain of Nile tilapia (Oreochromis niloticus)

Siglec7 functions as an inhibitory receptor of non-specific cytotoxic cells and can regulate the innate immune responses in a primitive vertebrate (Oreochromis niloticus)

In mammals, siglec7, an integral component of the siglecs, is principally found on the surface of natural killer (NK) cells, macrophages, and monocytes, where it interacts with various pathogens to perform immunological regulatory activities. Nonetheless, the immune defense and mechanism of siglec7 in early vertebrates remain unknown. In this study, we identified siglec7 from Oreochromis niloticus (OnSiglec7) and revealed its immune functions. Specifically, OnSiglec7 was abundantly expressed in immune-related tissues of healthy tilapia and its transcription level was strongly activated after being challenged with A. hydrophila, S. agalactiae, and Poly: IC. Meanwhile, OnSiglec7 protein was purified and analyzed, which could recognize multiple pathogens through binding and agglutinating activity. Moreover, OnSiglec7-positive cells were mainly distributed in non-specific cytotoxic cells (NCC) of tilapia HKLs and showed cell membrane localization. Furthermore, OnSiglec7 blockage affected multiple innate immune responses (inflammation, apoptosis, and pyroptosis process) by regulating the activation of MAPK, NF-κB, TLR, and JAK-STAT pathways. Finally, OnSiglec7 blockage also greatly enhanced the cytotoxic effect of tilapia NCC. Summarily, this study uncovers immune functions and mechanisms of siglec7 in primitive vertebrates, thereby enhancing our understanding of the systemic evolution and ancient functions of other siglecs within the host's innate immune system (to our knowledge).

The role of HMGB2 in the immune response of Nile tilapia (Oreochromis niloticus) to streptococcal infection

High mobility group protein B2 (HMGB2) is an abundant chromatin-associated protein with pivotal roles in transcription, cell proliferation, differentiation, inflammation, and tumorigenesis. However, its immune function in Nile tilapia (Oreochromis niloticus) remains unclear. In this study, we identified a homologue of HMGB2 from Nile tilapia (On-HMGB2) and investigated its functions in the immune response against streptococcus infection. The open reading frame (ORF) of On-HMGB2 spans 642 bp, encoding 213 amino acids, and contains two conserved HMG domains. On-HMGB2 shares over 80 % homology with other fish species and 74%-76 % homology with mammals. On-HMGB2 was widely distributed in various tissues, with its highest transcript levels in the liver and the lowest in the intestine. Knockdown of On-HMGB2 promoted the inflammatory response in Nile tilapia, increased the bacterial load in the tissues, and led to elevated mortality in Nile tilapia following Streptococcus agalactiae infection. Taken together, On-HMGB2 significantly influences the immune system of Nile tilapia in response to streptococcus infection.

Medulla oblongata and NCCs are central defenders against Streptococcus agalactiae infection of the tilapia brain

Various types of professional immune cells first emerge in fish and likely represent the primordial form and functions. Recent advancements revealed the direct connection between the central nervous system and the immune system in the mammalian brain. However, the specifics of brain-immune networks in the fish and the underlying mechanisms of teleost's brain against pathogen infection have not been fully elucidated. In this study, we investigated the distribution of markers representing cerebral cells associated with protection and professional lymphocytes in the seven major components of the Nile tilapia brain through RNA-Seq assay and observed the most dominant abundance in the medulla oblongata. The subsequent challenge test revealed the non-specific cytotoxic cells (NCCs) exhibited the strongest response against streptococcal infection of the brain. The presence of NCCs in the brain was then confirmed using immunofluorescence and the cytotoxic effects usually induced by NCCs under infection were determined as well. Collectively, these findings contribute significantly to comprehending the mechanism of fish neuroimmune interaction and enhancing our understanding of its evolutionary development.

Novel C-type lectin mediated non-specific cytotoxic cells killing activity through NCCRP-1 in nile tilapia (Oreochromis niloticus)

Non-specific cytotoxic cells (NCCs) are vital immune cells involved in teleost's non-specific immunity. As a receptor molecule on the NCCs' surface, the non-specific cytotoxic cell receptor protein 1 (NCCRP-1) is known to play a crucial role in mediating their activity. Nevertheless, there have been limited studies on the signal molecule that transmits signals via NCCRP-1. In this study, a yeast two-hybrid (Y2H) library of tilapia liver and head kidney was constructed and subsequently screened with the bait vector NCCRP-1 of Oreochromis niloticus (On-NCCRP-1) to obtain a C-type lectin (On-CTL) with an interacting protein sequence. Consequently, the full-length sequence of On-CTL was cloned and analyzed. The expression analysis revealed that On-CTL is highly expressed in the liver and is widely distributed in other tissues. Furthermore, On-CTL expression was significantly up-regulated in the brain, intestine, and head kidney following a challenge with Streptococcus agalactiae. A point-to-point Y2H method was also used to confirm the binding between On-NCCRP-1 and On-CTL. The recombinant On-CTL (rOn-CTL) protein was purified. In vitro experiments demonstrated that rOn-CTL can up-regulate the expression of killer effector molecules in NCCs via its interaction with On-NCCRP-1. Moreover, activation of NCCs by rOn-CTL resulted in a remarkable enhancement in their ability to eliminate fathead minnow cells, indicating that rOn-CTL effectively modulates the killing activity of NCCs through the NCC receptor molecule On-NCCRP-1. These findings significantly contribute to our comprehension of the regulatory mechanisms governing NCC activity, paving the way for future research in this field.

A CD6 homolog of Nile tilapia (Oreochromis niloticus) conserved binding bacteria involved in the regulation of Streptococcus agalactiae induced inflammation

As a lymphocyte-specific surface receptor belonging to the cysteine-rich superfamily of scavenger receptors, CD6 acts as a pattern recognition receptor for microbial components and is involved in the regulation of inflammatory responses. However, the characteristics and functions of CD6 molecules in lower vertebrates represented by teleost fish are unknown. In this study, a CD6 homolog (designated OnCD6) was characterized from Nile tilapia (Oreochromis niloticus), and establishing its role as a PRRs that participates in immune recognition. OnCD6 contains an open reading frame of 1872 bp that encodes a peptide of 623 amino acids, and contains two conserved SR domain. Multiple sequence alignment revealed that OnCD6 shares a relatively high level of identity with those of other species. Transcriptional expression analysis revealed that OnCD6 was constitutively expressed in immunes tissues such as head kidney and thymus. The expression level of OnCD6 in mainly immune tissues were found significantly upregulated after the injection of Streptococcus agalactiae (S. agalactiae). Moreover, OnCD6 protein was located in the head kidney and brain, mainly over the plasma membrane of lymphocytes in these immune tissues. In vitro experiments showed that CD6 extracellular protein bound to and aggregated several Gram-positive and -negative bacterial strains through the recognition of bacterial surface conserved components LPS and LTA etc. In vivo experiments demonstrated that overexpression OnCD6 before S. agalactiae challenge significantly improved tilapia survival, and this was concomitant with reduced bacterial load and pro-inflammatory cytokines (IL-1β and TNF-α). Taken together, our results illustrated the function of CD6 molecular pattern recognition receptors (PRRs) is conserved and plays an important role in antibacterial infection.

Immune response and protective efficacy of Streptococcus agalactiae vaccine coated with chitosan oligosaccharide for different immunization strategy in nile tilapia (Oreochromis niloticus)

In the past decade, the outbreak of Streptococcus agalactiae has caused significant economic losses in tilapia farming. Vaccine immunization methods and strategies have gradually evolved from single-mode to multi-mode overall prevention and control strategies. In this study, an inactivated vaccine of S. agalactiae with a chitosan oligosaccharide (COS) adjuvant was constructed using different administration methods: intraperitoneal injection (Ip), immersion combined with intraperitoneal injection (Im + Ip), immersion combined with oral administration (Im + Or), and oral administration (Or). Safety analysis revealed no adverse effects on tilapia, and the vaccine significantly promoted fish growth and development when administered through Im + Or or Or immunization. Following vaccination, innate immunity parameters including SOD, ACP and CAT activities were all significantly enhanced. Additionally, specific serum IgM antibodies reached their highest level at the 6th week post vaccination. Skin and intestinal mucus IgT antibodies reached peaked at the 6th and 7th week post vaccination, respectively. The relative peak expression values for IL-8, IL-12, MHC-I, MHC-II, IgM, IgT, CD4, CD8, TNFα, IFNγ from Im + Ip group were significantly higher than those in Ip group, Im + Or group and Or group in most cases (p < 0.05). Importantly, the relative protection survival of Im + Ip group was the highest (78.6%), followed by the Ip group (71.4%), the Or group (64.3%) and the Im + Or group (57.1%). In summary, this study encourages further research on multi-channel immunization strategies of other kinds of vaccines in other aquatic economic animals to improve their disease resistance.

Apolipoprotein Eb (On-ApoEb) protects Oreochromis niloticus against Streptococcus agalactiae infection

Apolipoprotein E (ApoE), a critical targeting protein, has been found to play an essential role in the protection against infection and inflammation. However, the immune functions of ApoE against bacterial infection in fish have not been investigated. In this study, a full-length cDNA for ApoE, named On-ApoEb was cloned from Oreochromis niloticus. The predicted cDNA sequence was 831bp in length and coded for a protein of 276 amino acid residues, which shared 63.87%-98.55% identity with ApoEb from other fishes, and about 22% identity with ApoEb from mammals. On-ApoEb from O. niloticus was highly expressed in the liver and could be activated in the tissues (liver, spleen, brain, and intestine) after infection with Streptococcus agalactiae. Moreover, the results revealed that On-ApoEb could decrease the expression levels of pro-inflammatory factors, immune-related pathways, and apoptosis, while increasing the expression levels of anti-inflammatory factors. Furthermore, On-ApoEb was noted to improve the survival rate and reduce the bacterial load in the liver and spleen. These results suggested that On-ApoEb was connected with immune response and had anti-inflammation and anti-apoptosis activities.

Interleukin-8 involved in Nile Tilapia (Oreochromis niloticus) against bacterial infection

Interleukin 8 (IL8) is vital in promoting inflammation and is a crucial mediator in various physiopathological processes while influencing immunological function. The effect of IL8 on the immunological response to acute bacterial infections in Nile tilapia (Oreochromis niloticus) remains unknown. This work found an IL8 gene from Nile tilapia (On-IL8). It includes a 285 bp open reading frame and codes for 94 amino acids. The transcript levels of On-IL8 were highest in the head-kidney tissue and sharply induced by Streptococcus agalactiae and Aeromonas hydrophila. Besides, in vitro experiments revealed that On-IL8 regulated a variety of immunological processes and promoted inflammatory responses. Moreover, On-IL8 suppressed the NF-κB signaling pathway, consistent with in vitro results. These significant findings serve as the basis for further investigation into how IL8 confers protection to bony fish in opposition to bacterial infections.

Apolipoprotein A-I (ApoA-I) protects Nile tilapia (Oreochromis niloticus) against bacterial infection

Apolipoprotein A-I (ApoA-I) is a lipoprotein involved in a variety of physiological and pathological processes. However, the immunomodulatory functions of ApoA-I in fish are not well understood. In this study, ApoA-I from Nile tilapia (Oreochromis niloticus) (On-ApoA-I) was identified, and its function in bacterial infection was investigated. The open reading frame of On-ApoA-I is 792 bp, which codes for a protein containing 263 amino acids. On-ApoA-I shared over 60% sequence similarity with other teleost fish and more than 20% with mammalian ApoA-I. On-ApoA-I was found to be highly expressed in the liver and significantly induced during Streptococcus agalactiae infection by qRT‒PCR analysis. Furthermore, in vivo studies revealed that recombinant On-ApoA-I protein could suppress inflammation and apoptosis and improve the likelihood of surviving bacterial infection. Additionally, On-ApoA-I showed in vitro antimicrobial properties against Gram-positive and Gram-negative bacteria. These findings offer a theoretical basis for further investigations into the role of ApoA-I in fish immunology.

α-MSH is partially involved in the immunomodulation of Nile tilapia (Oreochromis niloticus) antibacterial immunity

α-Melanocyte-stimulating hormone (α-MSH) is a well-studied neuropeptide controlling skin and hair color. Besides, numerous immunomodulation roles of α-MSH were recorded in humans and mice. However, the regulatory effects of α-MSH in teleost immunity haven't been well elucidated. In this study, several precursor molecules of α-MSH (POMCs) and its receptors (MCRs) in Nile tilapia (Oreochromis niloticus) were characterized, and their expression characteristics and specific functions on antibacterial immunity were determined. Overall, POMCs and MCRs were principally detected in the brain, skin, and liver, and were remarkably promoted post Streptococcus agalactiae infection. However, tiny POMCs and MCRs were observed in tilapia immune organs (head kidney and spleen) or lymphocytes, and no evident immunomodulation effect was detected in vitro. Moreover, the in vivo challenge experiments revealed that α-MSH protects tilapia from bacterial infection by regulating responses in the brain and intestine. This study lays theoretical data for a deeper comprehension of the immunomodulation mechanisms of teleost α-MSH and the evolutional process of the vertebrate melanocortin system.

Characterization of S100A12 from nile tilapia (Oreochromis niloticus) and its roles on inflammatory responses

S100A12 is a member of S100 proteins family that induces pro-inflammatory response via ligating with the receptor for advanced glycation end products (RAGE) and subsequent activation of intracellular signal transduction pathways. But information about fish S100A12 remain largely unclear. In this study, the S100A12 homolog (On-S100A12) was identified from Nile tilapia (Oreochromis niloticus). On-S100A12 was mainly expressed in liver and intestine. After Streptococcus agalactiae infection in vivo, S100A12 significantly increased in brain, intestine, liver and head kidney, suggesting S100A12 might played roles in immune response. The further in vitro experiments found that recombinant protein of S100A12 (rOn-S100A12) upregulated the expression of IL1-β, TLR2, TNF-α and inhibited the expression of IL-10, indicating On-S100A12 promoted inflammatory response and activation of M1 macrophages. The present data lay a foundation to further explore the roles of fish S100 during immune defense and will also be beneficial for better understanding of fish immune-regulatory network.

Molecular cloning, expression analysis and functional characterization of NEDD4 from Nile tilapia (Oreochromis niloticus)

Neural precursor cell-expressed developmentally downregulated gene 4 (NEDD4) was a member of HECT E3 ubiquitin ligases, which participated in various biological processes. In this study, a NEDD4 was identified and analyzed in Nile tilapia, Oreochromis niloticus (OnNEDD4) and its open reading frame was 2781 bp, encoding 926 amino acids. Three conserved structure features were found in OnNEDD4, including C2 domain, WW domains and HECT domain. OnNEDD4 was constitutively expressed in all examined tissues and the highest expression level was observed in thymus. After Streptococcus agalactiae stimulation, OnNEDD4 was significantly induced in several tissues, including thymus, intestine, blood and gill. Moreover, yeast two-hybrid assay shown OnNEDD4 could interact with extracellular region of OnCD40, but this interaction didn't affect the phagocytosis of monocytes/macrophages (MO/MΦ) to S. agalactiae and A. hydrophila. Taken together, the present study suggested that OnNEDD4 participate in CD40-mediated immune response excluding phagocytosis.

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

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

Vasoactive Intestinal Peptide (VIP) Protects Nile Tilapia (Oreochromis niloticus) against Streptococcus agalatiae Infection

Vasoactive intestinal peptide (VIP), a member of secretin/glucagon family, is involved in a variety of biological activities such as gut motility, immune responses, and carcinogenesis. In this study, the VIP precursor gene (On-VIP) and its receptor gene VIPR1 (On-VIPR1) were identified from Nile tilapia (Oreochromis niloticus), and the functions of On-VIP in the immunomodulation of Nile tilapia against bacterial infection were investigated and characterized. On-VIP and On-VIPR1 contain a 450 bp and a 1326 bp open reading frame encoding deduced protein of 149 and 441 amino acids, respectively. Simultaneously, the transcript of both On-VIP and On-VIPR1 were highly expressed in the intestine and sharply induced by Streptococcus agalatiae. Moreover, the positive signals of On-VIP and On-VIPR1 were detected in the longitudinal muscle layer and mucosal epithelium of intestine, respectively. Furthermore, both in vitro and in vivo experiments indicated several immune functions of On-VIP, including reduction of P65, P38, MyD88, STAT3, and AP1, upregulation of CREB and CBP, and suppression of inflammation. Additionally, in vivo experiments proved that On-VIP could protect Nile tilapia from bacterial infection and promote apoptosis and pyroptosis. These data lay a theoretical basis for further understanding of the mechanism of VIP guarding bony fish against bacterial infection.

Involvement and characterization of NLRCs and pyroptosis-related genes in Nile tilapia (Oreochromis niloticus) immune response

Pyroptosis is an inflammatory and programmed cell death initiated by the formation of the inflammasome, which consists of NLR, ASC, and Caspase. Pyroptosis has received growing attention due to its association with innate immunity and various diseases. However, the involvement and induction of the NLRCs and pyroptosis-related genes in fish immunity remain poorly studied. In this study, several NLRCs and pyroptosis-related genes in Nile tilapia (Oreochromis niloticus) were identified and characterized. Their involvement in bacterial infection and expression profiles in Nile tilapia lymphocyte responses were also assessed. Overall, three NLRC members (NOD1, NOD2, and NLRC3) and five pyroptosis-related genes (ASC1, Caspase1, Gsdme, NLRP3, and NLRP14) in Nile tilapia were cloned and characterized. The transcript levels of these molecules were broadly distributed in various tissues with comparatively high expression in the gills, intestine, and spleen. Their transcripts were also induced during Streptococcus agalactiae or Aeromonas hydrophila infection. Moreover, they were primarily expressed in T cells, NCCs, and Mo/Mφ and showed antibacterial and partially antiviral responses. The present study lays a theoretical foundation for further investigation of the pyroptosis mechanisms in fish as well as the evolution of the antiviral roles of pyroptosis in vertebrates.

Trial Evaluation of Protection and Immunogenicity of Piscine Bivalent Streptococcal Vaccine: From the Lab to the Farms

Streptococcosis is one of the major diseases that causes devastation to farmed fish, leading to significant economic losses all around the world. Currently, two serotypes of Streptococcus agalactiae, serotype Ia and III, have been identified as virulent strains and major causative agents of the disease in farmed Nile tilapia (Oreochromis niloticus Linn.) in Thailand. Upon inactivated vaccine development, monovalent inactivated whole-cell vaccines demonstrated high specific antibody production against homologous serotypes and limited production with heterologous serotypes. However, for higher efficacy, a bivalent streptococcal vaccine was designed to maximize protective immunity to both serotypes. Interestingly, our bivalent vaccine could successfully induce specific antibody production against both serotypes with similar levels, and the response could extend over the 8 weeks of the experimental period. Evaluation of vaccines in the laboratory scale revealed relative percent survival (RPS) of vaccinated tilapia to serotype Ia (81.2 ± 9.4%) and serotype III (72.2 ± 4.8%), respectively. The efficacy of the bivalent vaccine showed significant RPS higher than the monovalent vaccine (p < 0.05) at 30 days, and the protection of all those vaccines was reduced thereafter. Evaluation of the vaccine in a farm trial in different locations in Thailand revealed the efficacy of the bivalent vaccine in increasing the production yield by greater than 80% in all tested farms in 2015 and 2021. Taken together, this study affirms the efficacy of the bivalent streptococcal vaccine in the prevention of streptococcus disease in Nile tilapia, which could be used in different areas. This vaccine development could be effectively applied in the tilapia culture industry.

CRP Involved in Nile Tilapia (Oreochromis niloticus) against Bacterial Infection

Simple Summary

C-reactive protein (CRP) is an acute-phase protein that can be used as an early diagnostic marker for inflammation. Few CRPs have been isolated from teleost, and the specific immunological functions and mechanism of fish CRP have not been well-studied. Therefore, in this research, a CRP gene from Nile tilapia was identified, and its roles during bacterial infection were investigated. The current results revealed that CRP participated in anti-bacterial immune response through agglutinating bacterial, regulating phagocytosis and inflammation. Hopefully, our data might be beneficial in further study to understand the protective mechanism of fish CRP against bacterial infection.

Abstract

C-reactive protein (CRP) is an acute-phase protein that can be used as an early diagnostic marker for inflammation, which is also an evolutionarily conserved protein and has been identified from arthropods to mammals. However, the roles of CRP during the immune response of Nile tilapia (Oreochromis niloticus) remain unclear. In this study, a CRP gene from Nile tilapia (On-CRP) was identified, and its roles in response to bacterial infection were investigated in vivo or in vitro. On-CRP was found to contain an open reading frame of 675 bp, encoding a polypeptide of 224 amino acids with the conservative pentraxin domain. On-CRP shares more than 50% of its identity with other fish species, and 30% of its identity with mammals. The transcriptional level of On-CRP was most abundant in the liver and its transcripts can be remarkably induced following Streptococcus agalactiae and Aeromonas hydrophila infection. Furthermore, in vitro analysis indicated that the recombinant protein of On-CRP improved phagocytic activity of monocytes/macrophages, and possessed a bacterial agglutination activity in a calcium-dependent manner. Both in vivo and in vitro experiments indicated that On-CRP could promote inflammation and activate the complement pathway. However, a direct relationship between CRP and several immune pathways could not be confirmed. The present data lays a theoretical foundation to further explore the mechanism of how CRP protects fish against bacterial infection.

Serotonin system is partially involved in immunomodulation of Nile tilapia (Oreochromis niloticus) immune cells

Serotonin (5-hydroxytryptamine) is a well-known neurotransmitter affecting emotion, behavior, and cognition. Additionally, numerous immunomodulatory functions of serotonin have been discovered in mammals. However, the regulatory role of the serotonin system in fish immunity remains unclear. In this study, various serotonergic markers in Nile tilapia (Oreochromis niloticus) were identified and characterized. The involvement of the serotonin system during bacterial infection was investigated. Moreover, the expression characteristics and specific functions of serotonergic markers within Nile tilapia immune cells were also assessed. Overall, 22 evolutionarily conserved serotonergic marker genes in Nile tilapia were cloned and characterized. Transcriptional levels of these molecules were most abundant in the brain, and their transcripts were induced during Streptococcus agalactiae infection. Nevertheless, few serotonergic markers exist on Nile tilapia immune cells, and no distinct immunomodulation effect was observed during an immune response. The present study lays a theoretical foundation for further investigation of the immunological mechanisms in fish as well as the evolution of the serotonin system in animals.

SP protects Nile tilapia (Oreochromis niloticus) against acute Streptococcus agalatiae infection

Substance P (SP) is a neuropeptide that involves in a wide variety of physiological and pathological events, mainly exerts its roles by neurokinin 1 receptor (NK1R), also modulates immune function. However, the roles of SP during immune response to acute bacterial infection of Nile tilapia (Oreochromis niloticus) remain unclear. In this study, the gene of SP precursor (tachykinin precursor 1, TAC1) and the gene of SP receptor (NK1R) from Nile tilapia were identified, and the roles of SP during an acute bacterial infection in a warm water environment were investigated. On-TAC1(Oreochromis niloticus-TAC1) contains conservative SP & NKA peptide sequences and On-NK1R contains seven conservative transmembrane domains. Their transcriptional levels were most abundant in brain and the On-TAC1 transcripts can be induced in the tilapia challenged with Streptococcus agalactiae. Furthermore, the experimental results revealed that On-SP could promote pyroptosis, suppress inflammation, and improve survival rate during acute bacterial infection. The present data lays a theoretical foundation to further elucidate the mechanism of SP protecting fish against pathogens.

HMG20A from Nile tilapia (Oreochromis niloticus) involved in the immune response to bacterial infection

High-mobility group 20 A (HMG20A) has important biological functions, such as inhibiting the differentiation of red blood cells and nerve cells, promoting the proliferation and metastasis of cancer cells, and regulating inflammatory reaction. However, the role of HMG20A in the response to bacterial infection in the economic fish Nile tilapia (Oreochromis niloticus) remains unclear. In this study, a HMG20A homolog was successfully identified and characterized from Nile tilapia (On-HMG20A), and its expression model and biological effects on bacterial infection were analyzed. The open reading frame (ORF) of On-HMG20A was 876 bp in length, which encoded 291 amino acids and possessed a HMG domain (High mobility group domains) and coiled coil region. Results of the expression model showed that On-HMG20A was widely distributed in immune-related tissues of healthy tilapia and upregulated in a time-dependent manner after being challenged by Streptococcus agalactiae. Meanwhile, knocking down the expression of On-HMG20A can reduce the inflammatory response of tilapia and the degree of tissue damage caused by S. agalactiae. Moreover, knocking down the expression of On-HMG20A can reduce the bacterial load of tilapia tissues after being challenged by S. agalactiae and improve the survival rate. Collectively, these results showed that On-HMG20A may be related to the immune response of Nile tilapia against bacterial infection.

First comprehensive proteome analysis of lysine crotonylation in Streptococcus agalactiae, a pathogen causing meningoencephalitis in teleosts

Backgroud

Streptococcus agalactiae is a common colonizer of the rectovaginal tract and lead to infectious diseases of neonatal and non-pregnant adults, which also causes infectious disease in fish and a zoonotic risk as well. Lysine crotonylation (Kcr) is a kind of histone post-translational modifications discovered in 2011. In yeast and mammals, Kcr function as potential enhancers and promote gene expression. However, lysine crotonylation in S. agalactiae has not been studied yet.

Methods

In this study, the crotonylation profiling of fish pathogen, S. agalactiae was investigated by combining affinity enrichment with LC MS/MS. The Kcr modification of several selected proteins were further validated by Western blotting.

Results

In the present study, we conducted the proteome-wide profiling of Kcr in S. agalactiae and identified 241 Kcr sites from 675 screened proteins for the first time. Bioinformatics analysis showed that 164 sequences were matched to a total of six definitively conserved motifs, and many of them were significantly enriched in metabolic processes, cellular process, and single-organism processes. Moreover, four crotonylation modified proteins were predicted as virulence factors or to being part of the quorum sensing system PTMs on bacteria. The data are available via ProteomeXchange with identifier PXD026445.

Conclusions

These data provide a promising starting point for further functional research of crotonylation in bacterial virulence in S. agalactiae.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12953-021-00182-y.

Conservation of structural and interactional features of CD226 and Necl5 molecules from Nile tilapia (Oreochromis niloticus)

CD226 interacts with its ligand Necl5 as a costimulatory signal. In this study, we cloned a CD226 from Nile tilapia (Oreochromis niloticus, named OnCD226) and a Necl5 (named OnNecl5). The open reading frame of OnCD226 was 1071 bp, encoding a protein of 356 amino acids. Sequence alignment analysis indicated that OnCD226 contained two Ig-like domains in ectodomain. The open reading frame of OnNecl5 was 1155 bp, encoding a protein of 384 amino acids, and there are three lg-like domains in the extracellular domain. In healthy tilapia, OnCD226 was distributed in all tested tissues and relatively higher in the brain, while OnNecl5 was relatively higher in the skin. After Streptococcus agalactiae infection, OnCD226 has the same up-regulated expression pattern as OnNecl5 in different tissues. After HKLs stimulation with S. agalactiae and Poly I:C, respectively. OnCD226 was significantly up-regulated (0.01 < p < 0.05) at 12 h and extremely significant up-regulation was observed (p < 0.01) at 48 h and 96 h, the peak was observed at 96 h after stimulation by S. agalactiae. After stimulation by Poly I:C, OnCD226 expression was extremely significant (p < 0.01) at 72 h and 96 h, the peak was observed at 96 h. After stimulation by Keyhole limpet hemocyanin (KLH), a classical T cell-dependent antigen, the expression of OnCD226 was significantly up-regulated in blood, head kidney, spleen, and thymus. Moreover, when compared with the first challenge, the gene expression of OnCD226 which response to the second challenge was up-regulated earlier. Subcellular co-localization studies showed that OnCD226 and OnNecl5 were distributed mainly in the cytomembrane. Yeast two-hybrid results, indicated a strong interaction between OnCD226 and OnNecl5. These results suggested that OnCD226 plays an important role during pathogens infection, and the interaction between CD226 and Necl5 is conserved in Nile tilapia.

LECT2 Protects Nile Tilapia (Oreochromis niloticus) Against Streptococcus agalatiae Infection

Leukocyte cell-derived chemotaxin 2 (LECT2) is a multifunctional cytokine that especially plays an important role in innate immune. However, the roles of LECT2 in the immune response of the economically important fish Nile tilapia (Oreochromis niloticus) against bacterial infection remains unclear. In this study, a lect2 gene from Nile tilapia (On-lect2) was identified, and its roles in the fish’s immune response against bacterial infection were determined and characterised. On-lect2 contains an open reading frame of 456 bp that encodes a peptide of 151 amino acids, as well as the conservative peptidase M23 domain. On-LECT2 is 62%–84% identical to other fish species and about 50% identical to mammals. The highest transcriptional level of On-lect2 was detected in the liver, whereas the lowest levels were detected in the other tissues. Moreover, the On-LECT2 protein is located mainly in the brain and head kidney. The transcriptional levels of On-lect2 substantially increased in the head kidney, brain, liver and spleen after Streptococcus agalactiae infection. Knockdown On-lect2 led to higher mortality due to liver necrosis or haemorrhage and splenomegaly. In vitro analysis indicated that the recombinant protein of On-LECT2 improved phagocytic activity of head kidney-derived macrophages. In vivo challenge experiments revealed several functions of On-LECT2 in the immune response of Nile tilapia against bacterial infection, including promotion of inflammation, reduction of tissue damages and improvement of survival rate.

Fish natural killer enhancing factor-A (NKEF-A) enhance cytotoxicity of nonspecific cytotoxic cells against bacterial infection

Natural killer enhancing factor (NKEF)-A/B is a member of Peroxiredoxin (Prxs) family, which is named for the function of enhancing NK cells activity. NKEF also plays essential roles in multiple physiology/pathology processes including inflammation regulation, cancer development and redox reactions. However, the regulatory effects of fish NKEF on immune cells remain largely unknown. In this study, the full-length cDNA of NKEF-A (Accession No. MK584553, designated as On-NKEF-A) was identified from Nile tilapia (Oreochromis niloticus). On-NKEF-A encoded a 198 amino acid peptide with molecular mass of 22.085 kDa. On-NKEF-A protein contained a typical 2-Cys family domain, two active sites (51aa and 172aa) that were conserved in mammals, birds, amphibians and fish. Phylogenetic analysis showed that On-NKEF-A had the closest relationship with Zebra mbuna (Maylandia zebra) NKEF. The On-NKEF-A transcription was present in all examined tissues or organs. And the relative high expression levels of On-NKEF-A was found in head kidney leucocytes and nonspecific cytotoxic cells (NCC). After Streptococcus agalactiae stimulation, On-NKEF-A was significantly up-regulated in head kidney, spleen, gill and skin. Also, On-NKEF-A was markedly induced post S. agalactiae, LPS and poly I:C stimulation in head kidney-derived NCC. Moreover, On-NKEF-A was mainly distributed in cytoplasm of fathead minnow cells (FHM cells). The further in vitro analysis found that the recombinant protein of On-NKEF-A (rOn-NKEF-A) could induce the expression of various molecular markers of B cells, macrophages and NCC, enhanced the cytotoxicity of NCC via increasing the effectors expression. The present data collectively indicate that On-NKEF-A participates in anti-bacterial immune response via regulating NCC activity, which will provide new ideas to further explore the defense mechanism of fish against bacteria.

Nile tilapia CXCR4, the receptor of chemokine CXCL12, is involved in host defense against bacterial infection and chemotactic activity

CXC chemokine receptor 4 (CXCR4), a member of seven-transmembrane (7-TM) G-protein-coupled receptor superfamily, is the receptor of the CXC chemokine ligand 12 (CXCL12), and plays important roles in host defense and inflammation. In the current study, we cloned and identified a homolog of CXCR4 from Nile tilapia (Oreochromis niloticus), designated as OnCXCR4. The open reading frame of OnCXCR4 is 1149 bp encoding a peptide of 382 amino acids, and the predicted molecular weight is 42.65 kDa OnCXCR4 shares common features of CXCR4 family, including a 7-TM domain and a characteristic CXC motif (containing CYC). Expression analysis showed that OnCXCR4 constitutively expresses in various tested tissues of Nile tilapia, with the highest level in the anterior kidney. When stimulated with Streptococcus agalactiae, Aeromonas hydrophila, Poly(I:C), or LPS in vivo and in vitro, the expression of OnCXCR4 was significantly regulated. AMD3100, a CXCR4 antagonist, could not only inhibit the chemotactic activity of the recombinant OnCXCL12 protein on the leukocytes from anterior kidney, but also reduce the expression of OnCXCR4 significantly. Taken together, these results of our study above indicate that OnCXCR4 may play important roles in host defense against bacterial infectionin in Nile tilapia, and being a receptor of OnCXCL12 to exert functions.

Biological characterisation, expression and functional analysis of non-specific cytotoxic cell receptor protein 1 in Nile tilapia (Oreochromis niloticus)

Non-specific cytotoxic cell receptor protein 1 (NCCRP-1) plays a role in recognition of target cell and activation of non-specific cytotoxic cell (NCC). In this study, the full length of Nile tilapia NCCRP-1 (On-NCCRP-1) was cloned. cDNA is composed of 1045 bp with a 90 bp of 5'-Untranslated Regions (UTR), 702 bp open reading frame (ORF) and 253 bp 3'-UTR, encoding 233 amino acids (GenBank accession no: MF162296). The On-NCCRP-1 genomic sequence is 4471 bp in length and contains six exons and five introns. On-NCCRP-1 possesses some inherent conservative domains, such as proline-rich motifs, antigen recognition site, and F-box-related domain. Subcellular localisation and Western blot analysis indicated that On-NCCRP-1 is located in the cell membrane. The transcript of On-NCCRP-1 was detected in all the examined tissues of healthy Nile tilapia by using qRT-PCR, with the highest expression levels in the liver. Following Streptococcus agalactiae challenged in vivo, the On-NCCRP-1 expression was up-regulated significantly in brain, intestines, head kidney and spleen. In the in vitro analysis, the On-NCCRP-1 expression in NCCs was up-regulated significantly from 8 h to 12 h after LPS challenge, and up-regulated significantly at 12 h after challenged with polyI:C. After NCCs were challenged with inactivated S. agalactiae, the On-NCCRP-1 expression was down-regulated significantly after 24 h. NF-кB pathway was strongly activated by the over-expression of On-NCCRP-1 in HEK-293T cells. These results indicate that On-NCCRP-1, as a membrane surface receptor of NCCs, may play an important role in immune response to pathogenic infection in Nile tilapia.

Molecular characterization and expression analysis of chemokine (CXCL12) from Nile tilapia (Oreochromis niloticus)

Chemokines are a class of small molecular weight cytokines of 6-14 kDa, exerting important roles in the regulation of various inflammatory diseases and immune cell migration. In this study, we have identified the CXCL12 gene from Nile tilapia (Oreochromis niloticus), including CXCL12a (OnCXCL12a) and CXCL12b (OnCXCL12b). The open reading frames of OnCXCL12a and OnCXCL12b are 309 and 297 bp, encoding 102 and 98 amino acids, respectively. Multiple alignment showed that OnCXCL12a and OnCXCL12b have characteristics of CXC chemokines and share high identity with CXCL12 amino acid sequences from the known species. Tissue distribution in the healthy fish indicated that OnCXCL12a and OnCXCL12b expressed in all examined tissues, with the highest expression in muscle and anterior kidney, respectively. After challenged by Streptococcus agalactiae, Poly(I:C) and LPS in vivo and in vitro, OnCXCL12 is transcriptionally up-regulated in immune tissues and cells significantly. The recombinant OnCXCL12 proteins, (r)OnCXCL12a and (r)OnCXCL12b, enhance the release of nitric oxide and increase the expression of inflammatory cytokines (TNF-α, IL-6, and IL-10) in anterior kidney leukocytes, as well as exhibit chemotactic activity for leukocytes from anterior kidney. Summarizing, these results indicate that OnCXCL12 is involved in the immune response of Nile tilapia against pathogen infection and may play an important role in mediating inflammatory response.

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

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

Molecular characterization and expression of CD48 in Nile tilapia (Oreochromis niloticus) in response to different stimulus

CD48 is a glycosylphosphatidylinositol-anchored protein involved in lymphocyte adhesion, activation, and costimulation. In this study, the CD48 gene of tilapia (Oreochromis niloticus, named On-CD48), was cloned from the head kidney of tilapia. The coding sequences is 654 bp and encoding 217 amino acids. The deduced amino acid sequence of On-CD48 with an estimated molecular weight of 24.4 kDa and a theoretical pI of 5.03. Amino acid alignment indicated that it had two immunoglobulin-like domain conserved region. In healthy tilapia, the On-CD48 could be detected in all the examined tissues and the highest expression level in the spleen. The expression of On-CD48 in the spleen and head kidney was decreased after immunized by formalin-inactivated Streptococcus agalactiae, and the peak was observed in the spleen at 24 h and appeared again at 96 h, and in the head kidney gradual decline before 48 h then gradually increased to the original level. qPCR analysis of inactivated S. agalactiae, LPS and Poly I:C stimulated at the whole lymphocyte level showed that the stimulation of the Poly I:C was more sensitive. Prokaryotic expression results showed that efficient expression of On-CD48 protein could be realized after induced with 0.5 mmol L^-1 IPTG in E. coli BL21 (DE3) for 10 h at 18 °C. The result of subcellular localization showed that On-CD48 were evenly distributed in the whole cell of HEK-293T. Western Blot confirmed that the molecular weight of the recombinant On-CD48 was about 21 kDa, consistent with the predicted result. The results of this study will lay a strong foundation for the further study of On-CD48 molecular function in tilapia.

Current use and development of fish vaccines in China

In the past decades, the aquaculture industry made great progress in China, which contributes more than 70% yield of the world's farmed fish. Along with the rapid growth of fish production, increased emergence and outbreak of numbers of diseases pose harm to the aquaculture industry and food safety. From the efficient, safe, environmental and ethical aspects, vaccines is definitely the most appropriate and focused method to control different kinds of fish diseases. In China, researchers have done huge works on the fish vaccines, and so far six domestic aquatic vaccine products along with one imported aquatic vaccine have obtained the national veterinary medicine certificate. More critically, some new vaccines have also entered the field experiment stage and showed broad market prospects. In the present review, authors summarize seven aquatic vaccines, including the live vaccine against grass carp hemorrhagic disease, the inactivated vaccine against Aeromonas hydrophila sepsis in fish, the live vaccine against Edwardsiella tarda in turbot, the anti-idiotypic antibody vaccine against Vibrio alginolyticus, V. parahaemolyticus, and E. tarda in Japanese flounder, the cell-cultured inactivated vaccine against grass carp hemorrhagic disease, the inactivated vaccine against fish infectious spleen and kidney necrosis virus (ISKNV), and the genetically engineered live vaccine against V. anguillarum in turbot. Moreover, different delivery routes of fish vaccines are also compared in this review, along with differential fish immune response after vaccination. All these efforts will ultimately benefit the healthy and sustainable development of aquaculture industry in China.

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

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

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

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

Lyn is involved in host defense against S. agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

Lyn, a member of Src protein kinase family, plays a crucial role in immune reactions against pathogenic infection. In this study, Lyn from Nile tilapia (Oreochromis niloticus) (OnLyn) was identified and characterized at expression pattern against bacterial infection, and regulation function in BCR signaling. The open reading frame of OnLyn contained 1536 bp of nucleotide sequence encoded a protein of 511 amino acids. The OnLyn protein was highly conversed to other species Lyn, including SH3, SH2 and a catalytic Tyr kinase (TyrKc) domain. Transcriptional expression analysis revealed that OnLyn was detected in all examined tissues and was highly expressed in the head kidney. The up-regulation OnLyn expression was observed in the head kidney and spleen following challenge with Streptococcus agalactiae (S. agalactiae) in vivo, and was also displayed in head kidney leukocytes challenge with S. agalactiae and LPS in vitro. In addition, after induction with mouse anti-OnIgM mAb in vitro, the OnLyn expression and phosphorylation of OnLyn (Y⁵⁰⁷) were significantly up-regulated in the head kidney leukocytes. Moreover, after treatment with AZD0530 and mouse anti-OnIgM monoclonal antibody, the down-regulation of cytoplasmic free-Ca²⁺ concentration was detected in the head kidney leukocytes in vitro. Taken together, the findings of this study revealed that OnLyn might play potential roles in BCR signaling and get involved in host defense against bacterial infection in Nile tilapia.

Genomic Insights Into the Distribution and Evolution of Group B Streptococcus

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is a bacteria with truly protean biology. It infects a variety of hosts, among which the most commonly studied are humans, cattle, and fish. GBS holds a singular position in the history of bacterial genomics, as it was the substrate used to describe one of the first major conceptual advances of comparative genomics, the idea of the pan-genome. In this review, I describe a brief history of GBS and the major contributions of genomics to understanding its genome plasticity and evolution as well as its molecular epidemiology, focusing on the three hosts mentioned above. I also discuss one of the major recent paradigm shifts in our understanding of GBS evolution and disease burden: foodborne GBS can cause invasive infections in humans.

A comprehensive profile of the tilapia (Oreochromis niloticus) circular RNA and circRNA-miRNA network in the pathogenesis of meningoencephalitis of teleosts

The pathogenesis of tilapia meningoencephalitis is still unclear, where the involvement of circRNA is considered for its active role as a "miRNA sponge". Therefore, we aimed to investigate the profile of circRNA in tilapia meningoencephalitis further by constructing the circRNA-miRNA network for in-depth mechanism exploration. Briefly, a nile tilapia model of meningitis was established by injecting Streptococcus agalactiae (1.0 × 10⁷ cfu per mL) and we evaluated the infected tilapia brain for the expression profile of circRNAs, their potential functions and their correlation with genes involved in inflammatory pathways. A total of 11 263 circRNAs were identified from RNA sequencing (RNA-seq) data in nile tilapia (Oreochromis niloticus), a commercially important fish in China and East Asia. GO and KEGG analyses revealed that the biological functions of genes hosting the circRNAs were enriched in the progression of metabolism and binding. Notably, we found that 99% circRNAs in tilapia had abundant miRNA-binding sites and a total of 2136 of the identified circRNAs had two to six miRNA-binding sites. Six circRNAs were validated by qRT-PCR and the final circRNA-miRNA network was constructed. This is the first report of comprehensive identification of O. niloticus circRNAs being differentially regulated in the brain in normal conditions relating to S. agalactiae infection. This work will shed novel light on gene expression mechanisms under disease conditions and may identify circRNAs as new biomarkers for meningoencephalitis and neurodegenerative disorders.

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

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

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

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

Identification and expression profiling analysis of microRNAs in Nile tilapia (Oreochromis niloticus) in response to Streptococcus agalactiae infection

MicroRNAs (miRNAs) play vital regulatory roles in various biological processes, including in immune responses. Nile tilapia (Oreochromis niloticus) is an important commercial fish species in China. To identify immune-related miRNAs of O. niloticus, 4 libraries from liver during S. agalactiae infection (0 h, 5 h, 50 h, and 7 d) were sequenced by high-throughput sequencing technology in tilapia. We obtained 10,703,531, 11,507,163, 11,180,179 and 13,408,414 clean reads per library, respectively. In our results, a total of 482 miRNAs were identified through bioinformatic analysis, including 220 conserved miRNAs and 262 putative novel miRNAs. Moreover, 21 (4.36%), 50 (10.37%), and 46 (9.54%) miRNAs were significantly differentially expressed at 5 h, 50 h and 7 d, respectively. In addition, 6939 target genes regulated by these differentially expressed miRNAs were predicted, and their functional annotations were predicted by Gene Ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, which revealed that a majority of differentially expressed miRNAs were involved in apoptotic process, metabolic process, and immune responses. Finally, Real-time quantitative PCR experiments were performed for 7 miRNAs by stem-loop RT-PCR, and a general agreement was confirmed between the sequencing and RT-qPCR data. To our understanding, this is the first report of comprehensive identification of O. niloticus miRNAs being differentially regulated in liver related to S. agalactiae infection. This work provides an opportunity for further understanding of the molecular mechanisms of miRNA regulation in O. niloticus host-pathogen interactions, and genetic resources for molecular assistant selection for disease resistant breeding program.

Identification and characterization of a B-type mannose-binding lectin from Nile tilapia (Oreochromis niloticus) in response to bacterial infection

Lectins are a group of carbohydrate-binding proteins, which play an important role in innate immune system against pathogen infection. In this study, a B-type mannose-binding lectin (OnBML) was identified from Nile tilapia (Oreochromis niloticus), and characterized at expression patterns against bacterial infection and capability to promote phagocytosis by macrophages. The open reading frame of OnBML is 354 bp of nucleotide sequence encoding polypeptides of 117 amino acids. The deduced protein is highly homologous to other teleost BMLs, containing two repeats of the conserved mannose-binding motif QXDXNXVXY. Expression of OnBML was widely exhibited in all examined tissues, with the most abundance in spleen and following gill, peripheral blood, and head kidney. The OnBML expressions were significantly up-regulated following two major bacterial infections including a Gram-positive bacterium (Streptococcus agalactiae) and a Gram-negative bacterium (Aeromonas hydrophila) in vivo and in vitro. Recombinant OnBML protein possessed capacities of mannose-binding and calcium-dependent agglutination to S. agalactiae and A. hydrophila, and promoted the phagocytosis by macrophages. Taken together, the present study indicated that OnBML is likely to get involved in host defense against bacterial infection in Nile tilapia.

Interleukin-6 gets involved in response to bacterial infection and promotes antibody production in Nile tilapia (Oreochromis niloticus)

Interleukin 6 (IL-6), a pleiotropic cytokine, plays an important role in humoral immune response, not only inducing the differentiation of B cells into plasma cells, but also promoting antibody-secreting cells (ASCs) to produce antibodies. In this study, Nile tilapia (Oreochromis niloticus) IL-6 (OnIL-6) was identified and characterized at expression level in response to bacterial infection and promotion of antibody production. The open reading frame of OnIL-6 ORF is consisted of 663 bp encoding a polypeptide of 220 amino acids. The deduced OnIL-6 protein contained an IL-6/G-CSF family signature, two conserved cysteine, and four α-helix bundles, which was highly homologous to other species. Spatial mRNA expression analysis revealed that the highest expression of OnIL-6 was observed in the thymus. After in vivo challenges of lipopolysaccharide (LPS) and Streptococcus agalactia (S. agalactiae), OnIL-6 expressions were significantly up-regulated in head kidney and spleen. The similar up-regulation of OnIL-6 was observed in the head kidney and spleen leukocytes in vitro stimulation with LPS and S. agalactiae. In addition, inducement with the recombinant OnIL-6 ((r)OnIL-6) in vitro caused significant increases in expressions of both sIgM and mIgM. Moreover, the (r)OnIL-6 stimulation enhanced the secretion of sIgM (more especially in P50 plasma-like B cells) and the production of mIgM in P60 and P70 B cell subsets (resting B cells, activated B cells and plasmablast-like B cells) in vitro. Taken together, this study indicated that OnIL-6 might be involved in host defense against bacterial infection and promote the production of antibody in Nile tilapia.

Comparative genomics inferred two distinct populations of piscine pathogenic Streptococcus agalactiae, serotype Ia ST7 and serotype III ST283, in Thailand and Vietnam

The genomes of Streptococcus agalactiae (group B streptococcus; GBS) collected from diseased fish in Thailand and Vietnam over a nine-year period (2008-2016) were sequenced and compared (n = 21). Based on capsular serotype and multilocus sequence typing (MLST), GBS isolates are divided into 2 groups comprised of i) serotype Ia; sequence type (ST)7 and ii) serotype III; ST283. Population structure inferred by core genome (cg)MLST and Bayesian clustering analysis also strongly indicated distribution of two GBS populations in both Thailand and Vietnam. Deep phylogenetic analysis implied by CRISPR array's spacer diversity was able to cluster GBS isolates according to their temporal and geographic origins, though ST7 has varying CRISPR1-spacer profiles when compared to ST283 strains. Based on overall genotypic features, Thai ST283 strains were closely related to the Singaporean ST283 strain causing foodborne illness in humans in 2015, thus, signifying zoonotic potential of this GBS population in the country.

Spleen tyrosine kinase from Nile tilapia (Oreochromis niloticus): Molecular characterization, expression pattern upon bacterial infection and the potential role in BCR signaling and inflammatory response

Spleen tyrosine kinase (SYK), a member of non-receptor tyrosine kinase family, plays an important role in immune responses against pathogen infection, which is capable of activating B cells signaling pathway and regulating inflammatory response. In this study, Nile tilapia (Oreochromis niloticus) ortholog (OnSYK) was identified and characterized at expression pattern against bacterial infection, function in B cells activation pathway and inflammatory response. The cDNA of OnSYK ORF contained 1851 bp of nucleotide sequence encoding polypeptides of 616 amino acids. The deduced OnSYK protein was highly homologous to other species SYK, containing two SH2 domains and a TyrKc domain. Spatial mRNA expression analysis revealed that OnSYK had wide tissue distribution and was highly expressed in the liver. After challenge of Streptococcus agalactiae (S. agalactiae) in vivo, mRNA expression of OnSYK was significantly up-regulated in the head kidney, spleen and liver. The up-regulation of OnSYK transcript was also displayed in the head kidney and spleen leukocytes stimulation with S. agalactiae and LPS in vitro, which was confirmed at protein level in the head kidney leukocytes by FACS analysis. In addition, after induction with mouse anti-OnIgM monoclonal antibody in vitro, the expressions of OnSYK and its downstream molecules (OnLYN, OnBLNK and OnAP-1) were significantly up-regulated in the head kidney leukocytes, and pharmacological inhibition of SYK activity with inhibitor (P505-15) significantly attenuated the expressions of OnLYN, OnBLNK and OnAP-1. Moreover, upon LPS challenge, the expressions of OnSYK, OnTNF-α, OnIL-6 and OnAP-1 were also up-regulated in the head kidney monocytes/macrophages. After treatment with SYK inhibitor (BAY 61-3606), the expressions of OnTNF-α, OnIL-6 and OnAP-1 were inhibited in the LPS-challenged head kidney monocytes/macrophages. Taken together, the results of this study indicated that OnSYK, playing potential roles in BCR signaling and inflammatory response, was likely to get involved in host defense against bacterial infection in Nile tilapia.

The complement component 1 q (C1q) in Nile tilapia (Oreochromis niloticus): Functional characterization in host defense against bacterial infection and effect on cytokine response in macrophages

Complement component 1q (C1q) is the initial protein of the classical complement pathway and plays an important role in immune response against bacterial infection. In this study, the full-length of C1q subcomponent A, B and C chain genes (C1qA, C1qB and C1qC) were identified and characterized from Nile tilapia (Oreochromis niloticus). Molecular characterization of these three C1q subcomponents (OnC1qs) harbored conserved amino acids through analyses of multiple sequence alignment and phylogenetic tree, which were homologous to other teleost species. Expression analysis revealed that the OnC1qs were highly expressed in liver. After the in vivo challenges of Streptococcus agalactiae and LPS, the mRNA expressions of OnC1qs were significantly up-regulated in liver. Meanwhile, the concentration variation of OnC1qs at the protein level from tilapia serum after challenge with S. agalactiae were measured by a competitive-inhibition ELISA. In addition, the up-regulation expressions of OnC1qs were also demonstrated in head kidney adherent leukocytes and the cell culture medium in vitro stimulation with S. agalactiae, Aeromonas hydrophila and LPS, respectively. Moreover, the recombinant OnC1qs enhanced expression of cytokines IL-6, IL-8 and IL-10 in head kidney adherent leukocytes, and were able to bind both heat-aggregated mouse IgG and IgM. Taken together, the results of this study indicated that OnC1qs might be involved in host defense against bacterial infection in Nile tilapia.

Expression and functional characterization of collection-K1 from Nile tilapia (Oreochromis niloticus) in host innate immune defense

Collectin-K1 (CL-K1), a multifunctional Ca²⁺-dependent lectin, is able to bind carbohydrates on pathogens and inhibit infection by direct neutralization, agglutination, opsonization and killing, which plays an important role in innate immunity. In this study, a CL-K1 homolog (OnCL-K1) was identified from Nile tilapia (Oreochromis niloticus) and characterized at expression and agglutination functional levels. The open reading frame of OnCL-K1 is 720 bp of nucleotide sequence encoding a polypeptide of 239 amino acids. The deduced amino acid sequence has two characteristic structures, containing a collagen-like region and a carbohydrate recognition domain. Expression analysis revealed that the OnCL-K1 was highly expressed in the liver, and widely exhibited in other tissues including kidney, intestine and spleen. In addition, the OnCL-K1 expression was significantly up-regulated in spleen and anterior kidney following challenges with a Gram-positive bacterial pathogen (Streptococcus agalactiae) and a Gram-negative bacterial pathogen (Aeromonas hydrophila). The up-regulation of OnCL-K1 expression was also demonstrated in hepatocytes and monocytes/macrophages in vitro stimulation with S. agalactiae and A. hydrophila. Recombinant OnCL-K1 protein was able to agglutinate both S. agalactiae and A. hydrophila in vitro, and participate in the regulation of inflammatory, migration reaction and promote the phagocytosis by monocytes/macrophages. Taken together, the results of this study indicated that OnCL-K1, possessing apparent agglutination, opsonization and killing ability to bacterial pathogens and participating in the regulation mechanisms of the non-specific cellular immune, might be involved in host defense of innate immunity against bacterial infection in Nile tilapia.

Biological characterization, expression, and functional analysis of tumor necrosis factor receptor-associated factor 6 in Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is a pivotal economic fish that has been plagued by Streptococcus agalactiae infections for many years. Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a crucial adaptor molecule of the interleukin-1 receptor/Toll-like receptor (IL-1/TLR) superfamily, which can trigger downstream signaling cascades involved in innate immunity. In this study, the full-length cDNA of TRAF6 was cloned from O. niloticus (named On-TRAF6), which has an open reading frame of 1716 bp, and encode a polypeptide of 571 amino acids. The predicted amino acid sequence of On-TRAF6 contained the characteristic motifs of TRAF proteins, including a Zinc finger of RING-type, two Zinc fingers of TRAF-type, and a MATH (meprin and TRAF homology) domain. Multiple sequence alignment revealed that On-TRAF6 shares a relatively high level of identity with those of other fishes (64-98%). In healthy tilapia, mRNA expression of On-TRAF6 could be detected in all the examined tissues and the highest expression occurred in the spleen. Moreover, we found that On-TRAF6 was involved immune response of Nile tilapia following the stimulation with Streptococcus agalactiae and polyinosinic: polycytidylic acid (Poly I:C) when determined by using qPCR. The result of subcellular localization showed that On-TRAF6 distributed in the cytoplasm, and over-expression of On-TRAF6 could strongly activated NF-кB pathway in HEK293T cells. These findings indicated that On-TRAF6 may play an important role in the immune response to intracellular bacteria in Nile tilapia.

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

Collectins, a subfamily of the C-type lectins, are able to bind non-self glycoconjugates on the surface of microorganisms and inhibit infection by direct neutralization, agglutination and/or opsonization, which play important roles in innate immunity. In this study, a CL11X1-like collectin (OnCL11X1) was identified from Nile tilapia (Oreochromis niloticus) and characterized at expression and agglutination functional levels. The open reading frame of OnCL11X1 is 840 bp of nucleotide sequence encoding polypeptides of 279 amino acids. The deduced amino acid sequence is highly homology to teleost and similar to mammalian CL11X1, containing a canonical collagen-like region, a carbohydrate recognition domain and a neck region. Expression analysis revealed that the OnCL11X1 was highly expressed in the liver, and widely exhibited in other tissues including kidney, intestines and spleen. In addition, the OnCL11X1 expression was significantly up-regulated in spleen and anterior kidney following challenges with a Gram-positive bacterial pathogen (Streptococcus agalactiae) and a Gram-negative bacterial pathogen (Aeromonas hydrophila). The up-regulation of OnCL11X1 expression was also demonstrated in hepatocytes and macrophages in vitro stimulation with S. agalactiae and A. hydrophila. Recombinant OnCL11X1 protein was able to agglutinate both S. agalactiae and A. hydrophila in vitro and promote the phagocytosis by macrophages. Taken together, the results of this study indicated that OnCL11X1, possessing apparent agglutination and opsonization ability to bacterial pathogens, might be involved in host defense against bacterial infection in Nile tilapia.

Expression and functional characterization of transferrin in Nile tilapia (Oreochromis niloticus) in response to bacterial infection

Transferrin (TF), an iron-binding glycoprotein, plays an important role in host defense against pathogenic infection, which inhibits the growth and proliferation of pathogens, deprives iron from invading pathogens, and activates anti-microbial responses in macrophages. In this study, a TF homologue (OnTF) was identified from Nile tilapia (Oreochromis niloticus) and characterized at expression pattern against bacterial infection and capability binding bacterial pathogens. The open reading frame of OnTF is 2118 bp of nucleotide sequence encoding polypeptides of 705 amino acids. The deduced protein is highly homology to the other species, containing two conserved iron binding lobes: N-lobe and C-lobe. Expression analysis revealed that the OnTF was extremely highly expressed in liver tissue; however, much weakly exhibited in other examined tissues including spleen and head kidney. The OnTF expression was significantly up-regulated in the liver, spleen and head kidney following infection of a Gram-positive bacterial pathogen (Streptococcus agalactiae) and a Gram-negative bacterial pathogen (Aeromonas hydrophila). The up-regulation of OnTF expression was also demonstrated in hepatocytes and macrophages in vitro stimulated with S. agalactiae and A. hydrophila. In addition, recombinant OnTF ((r)OnTF) protein possessed capability to bind both S. agalactiae and A. hydrophila in vitro. Taken together, the present study indicated that OnTF might be involved in host defense against bacterial infection in Nile tilapia.

NK-lysin from Oreochromis niloticus improves antimicrobial defence against bacterial pathogens

NK-lysin, an effector of cytotoxic T cells and natural killer cells, is a potent antimicrobial peptide widely distributed in mammals. Homologues of NK-lysin have been discovered in several teleost species, but only several of their natural functions was recorded so far. Here we identified an NK-lysin from Nile tilapia (Oreochromis niloticus), On-NKL, and analysed its expression model and biological effects on pathogen infection. The open reading frame of On-NKL sequence spans 432 bp, codes for 143 amino acids and shares 27%-62% overall sequence identities with NK-lysin of other species. The deduced mature peptide of On-NKL possesses a saposin B domain and six well-conserved cysteine residues that essential for antimicrobial activity by forming three intrachain disulphide bonds. The results of qRT-PCR showed that On-NKL expression was observed in multiple tissues and head kidney leucocytes and nonspecific cytotoxic cells (NCCs) and is most abundant in gills. After bacterial challenge, On-NKL expression significantly varied in different tissues and NCCs. Following bacterial infection, On-NKL-overexpressing fish featured significantly lower pathogen loads in tissues than control fish. On-NKL-overexpressing fish also exhibited 33.3% relative percent survival compare with control groups. Findings suggested that On-NKL could be the potential effector of NCCs and act as immune-related gene that enhances antimicrobial defence.

Conservation of structural and interactional features of CD28 and CD80/86 molecules from Nile tilapia (Oreochromis niloticus)

Interaction of CD28 with CD80 or CD86 molecules provides a costimulatory signals required in T cell activation. In this study, we cloned and analyzed a CD28 gene (On-CD28) and a CD80/86 gene (On-CD80/86) from Nile tilapia (Oreochromis niloticus). Sequence analysis revealed the typical characteristics of On-CD28 protein; for instance, the proline-based motif (¹¹⁷TYPPPL¹²²) is essential in binding of CD28 to CD80/86 ligands. Moreover, an extracellular Ig domain was found in On-CD80/86; this domain is responsible in binding of CD28 to CD80/86 receptors. Subcellular localization analysis showed that both On-CD28 and On-CD80/86 were distributed predominantly in the cytomembrane. Yeast two-hybrid assay showed that On-CD28 directly interacted with On-CD80/86. On-CD28 and On-CD80/86 transcripts were detected in all the examined tissues of healthy Nile tilapia, and the highest expression levels of On-CD28 and On-CD80/86 were detected in the brain and heart, respectively. Following a bacterial challenge using Streptococcus agalactiae in vivo, On-CD28 and On-CD80/86 were upregulated in head kidney, spleen, intestines, and brain. However, they showed different expression profiles in response to stimulation with inactivated S. agalactiae in vitro. These findings indicated that the interaction of On-CD28 with On-CD80/86 provides a costimulatory signals that possibly play an important role in T cell activation during S. agalactiae infection.

C1r and C1s from Nile tilapia (Oreochromis niloticus): Molecular characterization, transcriptional profiling upon bacterial and IFN-γ inductions and potential role in response to bacterial infection

The complement components C1r and C1s play a vital role in immunity with the activation of C1 complex in the classical complement pathway against pathogen infection. In this study, Nile tilapia (Oreochromis niloticus) C1r and C1s orthologs (OnC1r and OnC1s) were identified and characterized. The cDNA of OnC1r and OnC1s ORFs consisted of 1902 bp and 2100 bp of nucleotide sequence encoding polypeptides of 633 and 699 amino acids, respectively. The deduced OnC1r and OnC1s proteins both possessed CUB, EGF, CCP and SP domains, which were significantly homology to teleost. Spatial mRNA expression analysis revealed that the OnC1r and OnC1s were highly expressed in liver. After the in vivo challenges of Streptococcus agalactiae (S. agalactiae) and lipopolysaccharide (LPS), the mRNA expressions of OnC1r and OnC1s were significantly up-regulated in liver and spleen, which were consistent with immunohistochemical detection at the protein level. The up-regulation of OnC1r and OnC1s expressions were also demonstrated in head kidney monocytes/macrophages in vitro stimulated with LPS, S. agalactiae, and recombinant OnIFN-γ. Taken together, the results of this study indicated that OnC1r and OnC1s were likely to get involved in the immune response of Nile tilapia against bacterial infection.