The non-specific cytotoxic cell receptor (NCCRP-1): molecular organization and signaling properties

Distribution and response strategies of non-specific cytotoxic cell receptor protein 1 in large yellow croaker

The non-specific cytotoxic cell receptor protein 1 (NCCRP1) is considered the universal marker for teleost non-specific cytotoxic cells (NCCs). However, the specific distribution characteristics and response patterns of NCCRP1, and the confirmed existence of NCCs in fish species remain debatable. In this study, we investigated the distribution of NCCRP1 in the croaker and observed the most dominant abundance in the head kidney. While most common markers of cytotoxicity were localized in the trunk kidney lymphocytes (TKLs) and spleen lymphocytes (SPLs), NCCRP1-positive cells were predominantly detected in head kidney lymphocytes (HKLs) with a positive rate of approximately 10 %, where present a huge amount of macrophages (Mϕ) as well. Furthermore, the remarkable induction evidence of NCCRP1 in HKLs was determined. Collectively, these findings contribute significantly to comprehending the immunological function of NCCRP1 in fish species 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.

Molecular characteristics and functional analysis of non-specific cytotoxic cell receptor (NCCRP1) in golden pompano (Trachinotus ovatus)

Non-specific cytotoxic cells (NCCs) are cytotoxic cell population found in innate immune system of teleost, playing crucial role in immune defense. Non-specific cytotoxic cell receptor protein 1 (NCCRP1) is responsible for recognizing target cells and activating NCCs. That said, since the studies regarding NCCs' role in fish during pathogen infection are few, it is necessary to conduct more comprehensive studies. In this study, we identified NCCRP1 from Trachinotus ovatus (ToNCCRP1). The open reading frame of ToNCCRP1 was found to be 702 bp, encoding a protein of 233 amino acids. Additionally, ToNCCRP1 contained a conserved F-box-associated domain and exhibited more than 61 % similarity to NCCRP1 in other fish species. Quantitative real-time PCR analysis showed that ToNCCRP1 mRNA was generally expressed in all tissues, with the highest level expressed in the liver. Furthermore, the expression of ToNCCRP1 was significantly upregulated following infection with Streptococcus iniae. In vitro experiments demonstrated that recombinant ToNCCRP1 possessed bacterial agglutination and binding capabilities, suggesting its antibacterial function. Additionally, we investigated the immune response of head kidney leukocytes (HKLs) to ToNCCRP1. The challenge experiments revealed that ToNCCRP1 played a role in the immune response by influencing the inflammatory response, regulating signaling pathways and apoptosis in HKLs. These findings suggest that NCCRP1 is involved in the immune defense against pathogenic infections in golden pompano, providing insights into the immune mechanisms of teleost.

Non-specific cytotoxic cell receptor protein-1 (NCCRP-1) is involved in anti-parasite innate CD8+ T cell-mediated cytotoxicity in ginbuna crucian carp

CD8⁺ cytotoxic T cells (CTLs) are a main cellular component of adaptive immunity. Our previous research has shown that CD8⁺ cells demonstrate spontaneous cytotoxic activity against the parasite Ichthyophthirius multifiliis in ginbuna crucian carp, suggesting that CD8⁺ cells play an important role in innate immunity. Herein, we investigated the molecules and cellular signal pathways involved in the cytotoxic response of ginbuna crucian carp. We considered non-specific cytotoxic receptor protein-1 (NCCRP-1) as candidate molecule for parasite recognition. We detected NCCRP-1 protein in CD8⁺ cells and the thymus as well as in other cells and tissues. CD8⁺ cells expressed mRNA for NCCRP-1, Jak2, and T cell-related molecules. In addition, treatment with a peptide containing the presumed antigen recognition site of ginbuna NCCRP-1 significantly inhibited the cytotoxic activity of CD8⁺ cells against the parasites. The cytotoxic activity of CD8⁺ cells was significantly inhibited by treatment with the JAK1/2 inhibitor baricitinib. These results suggest that teleost CTLs recognize I. multifiliis through NCCRP-1 and are activated by JAK/STAT signaling.

Expression changes of non-specific cytotoxic cell receptor (NCCRP1) and proliferation and migration of NCCs post-Nocardia seriolae infection in Northern Snakehead

Non-specific cytotoxic cells (NCCs) are essential to the cytotoxic cell-mediated immune response in teleost. The fish non-specific cytotoxic cell receptor protein 1 (NCCRP1) plays an important role as a membrane protein in the recognition of target cells and the activation of NCC. However, the roles of fish NCCs during pathogen infection require comprehensive studies. In this study, the coding sequence of northern snakehead (Channa argus) nccrp1 (Canccrp1) was cloned. Canccrp1 contains an open reading frame of 690 bp, encoding a peptide of 229 amino acids with a conserved F-box-associated domain (FBA) and proline-rich motifs (PRMs). Transcriptional expression analysis revealed that the constitutive expression of Canccrp1 was higher in the immune-related organs, such as liver, kidneys, and spleen. Moreover, mRNA and protein expression of Canccrp1 gradually increased in the spleen at 1-6 days post infection (dpi) with Nocardia seriolae, in addition to reaching peak expression in both the kidneys and liver at 2 dpi. A polyclonal antibody prepared against recombinant CaNCCRP1 effectively labeled NCCs in peripheral blood and different tissues. Then, immunofluorescence (IF) staining showed that the number of NCCs was significantly increased and showed a scattered distribution in the early stages of N. seriolae infection (2 and 4 dpi) before the forming of granulomas. At the late stages of N. seriolae infection (6 dpi), more NCCs migrated to preexisting granulomas, showing significant coaccumulation with N. seriolae. All these results clearly indicate the expression changes of CaNCCRP1, and the number and localization changes of NCCs post-N. seriolae infection, implying potential roles for fish NCCs in the antimicrobial infection process in fish.

RNA-Seq of Single Fish Cells – Seeking Out the Leukocytes Mediating Immunity in Teleost Fishes

The immune system is a complex and sophisticated biological system, spanning multiple levels of complexity, from the molecular level to that of tissue. Our current understanding of its function and complexity, of the heterogeneity of leukocytes, is a result of decades of concentrated efforts to delineate cellular markers using conventional methods of antibody screening and antigen identification. In mammalian models, this led to in-depth understanding of individual leukocyte subsets, their phenotypes, and their roles in health and disease. The field was further propelled forward by the development of single-cell (sc) RNA-seq technologies, offering an even broader and more integrated view of how cells work together to generate a particular response. Consequently, the adoption of scRNA-seq revealed the unexpected plasticity and heterogeneity of leukocyte populations and shifted several long-standing paradigms of immunology. This review article highlights the unprecedented opportunities offered by scRNA-seq technology to unveil the individual contributions of leukocyte subsets and their crosstalk in generating the overall immune responses in bony fishes. Single-cell transcriptomics allow identifying unseen relationships, and formulating novel hypotheses tailored for teleost species, without the need to rely on the limited number of fish-specific antibodies and pre-selected markers. Several recent studies on single-cell transcriptomes of fish have already identified previously unnoticed expression signatures and provided astonishing insights into the diversity of teleost leukocytes and the evolution of vertebrate immunity. Without a doubt, scRNA-seq in tandem with bioinformatics tools and state-of-the-art methods, will facilitate studying the teleost immune system by not only defining key markers, but also teaching us about lymphoid tissue organization, development/differentiation, cell-cell interactions, antigen receptor repertoires, states of health and disease, all across time and space in fishes. These advances will invite more researchers to develop the tools necessary to explore the immunology of fishes, which remain non-conventional animal models from which we have much to learn.

Severe Natural Outbreak of Cryptocaryon irritans in Gilthead Seabream Produces Leukocyte Mobilization and Innate Immunity at the Gill Tissue

The protozoan parasite Cryptocaryon irritans causes marine white spot disease in a wide range of fish hosts, including gilthead seabream, a very sensitive species with great economic importance in the Mediterranean area. Thus, we aimed to evaluate the immunity of gilthead seabream after a severe natural outbreak of C. irritans. Morphological alterations and immune cell appearance in the gills were studied by light microscopy and immunohistochemical staining. The expression of several immune-related genes in the gills and head kidney were studied by qPCR, including inflammatory and immune cell markers, antimicrobial peptides (AMP), and cell-mediated cytotoxicity (CMC) molecules. Serum humoral innate immune activities were also assayed. Fish mortality reached 100% 8 days after the appearance of the C. irritans episode. Gill filaments were engrossed and packed without any space between filaments and included parasites and large numbers of undifferentiated and immune cells, namely acidophilic granulocytes. Our data suggest leukocyte mobilization from the head kidney, while the gills show the up-regulated transcription of inflammatory, AMPs, and CMC-related molecules. Meanwhile, only serum bactericidal activity was increased upon infection. A potent local innate immune response in the gills, probably orchestrated by AMPs and CMC, is triggered by a severe natural outbreak of C. irritans.

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.

Post-translational protein deimination in cod (Gadus morhua L.) ontogeny novel roles in tissue remodelling and mucosal immune defences?

Peptidylarginine deiminases (PADs) are calcium dependent enzymes with physiological and pathophysiological roles conserved throughout phylogeny. PADs promote post-translational deimination of protein arginine to citrulline, altering the structure and function of target proteins. Deiminated proteins were detected in the early developmental stages of cod from 11 days post fertilisation to 70 days post hatching. Deiminated proteins were present in mucosal surfaces and in liver, pancreas, spleen, gut, muscle, brain and eye during early cod larval development. Deiminated protein targets identified in skin mucosa included nuclear histones; cytoskeletal proteins such as tubulin and beta-actin; metabolic and immune related proteins such as galectin, mannan-binding lectin, toll-like receptor, kininogen, Beta2-microglobulin, aldehyde dehydrogenase, bloodthirsty and preproapolipoprotein A-I. Deiminated histone H3, a marker for anti-pathogenic neutrophil extracellular traps, was particularly elevated in mucosal tissues in immunostimulated cod larvae. PAD-mediated protein deimination may facilitate protein moonlighting, allowing the same protein to exhibit a range of biological functions, in tissue remodelling and mucosal immune defences in teleost ontogeny.

Salmon louse rhabdoviruses: Impact on louse development and transcription of selected Atlantic salmon immune genes

Recently, it has been shown that the salmon louse (Lepeophtheirus salmonis) is commonly infected by one or two vertically transmitted Lepeophtheirus salmonis rhabdoviruses (LsRVs). As shown in the present study, the viruses have limited effect on louse survival, developmental rate and fecundity. Since the LsRVs were confirmed to be present in the louse salivary glands, the salmon cutaneous immune response towards LsRV positive and negative lice was analyzed. In general, L. salmonis increased the expression of IL1β, IL8 and IL4/13A at the attachment site, in addition to the non-specific cytotoxic cell receptor protein 1 (NCCRP-1). Interestingly, LsRV free lice induced a higher skin expression of IL1β, IL8, and NCCRP-1 than the LsRV infected lice. The inflammatory response is important for louse clearance, and the present results suggest that the LsRVs can be beneficial for the louse by dampening inflammation. Further research is, however; needed to ascertain whether this is a direct modulatory effect of secreted virions, or if virus replication is altering the level of louse salivary gland proteins.

Dietary administration of the probiotic Shewanella putrefaciens Pdp11 promotes transcriptional changes of genes involved in growth and immunity in Solea senegalensis larvae

Senegalese sole (Solea senegalensis) has been proposed as a high-potential species for aquaculture diversification in Southern Europe. It has been demonstrated that a proper feeding regimen during the first life stages influences larval growth and survival, as well as fry and juvenile quality. The bacterial strain Shewanella putrefaciens Pdp11 (SpPdp11) has shown very good probiotic properties in Senegalese sole, but information is scarce about its effect in the earliest stages of sole development. Thus, the aim of this study was to investigate the effect of SpPdp11, bioencapsulated in live diet, administered during metamorphosis (10-21 dph) or from the first exogenous feeding of Senegalese sole (2-21 dph). To evaluate the persistence of the probiotic effect, we sampled sole specimens from metamorphosis until the end of weaning (from 23 to 73 dph). This study demonstrated that probiotic administration from the first exogenous feeding produced beneficial effects on Senegalese sole larval development, given that specimens fed this diet exhibited higher and less dispersed weight, as well as increases in both total protein concentration and alkaline phosphatase activity, and in non-specific immune response. Moreover, real-time PCR documented changes in the expression of a set of genes involved in central metabolic functions including genes related to growth, genes coding for proteases (including several digestive enzymes), and genes implicated in the response to stress and in immunity. Overall, these results support the application of SpPdp11 in the first life stages of S. senegalensis as an effective tool with the clear potential to benefit sole aquaculture.

Transcription of immune related genes in Solea senegalensis vaccinated against Photobacterium damselae subsp. piscicida. Identification of surrogates of protection

Solea senegalensis is a flatfish with a great potential for aquaculture, but infectious diseases restrict its production, being this fish species highly susceptible to Photobacterium damselae subsp. piscicida (Phdp) infections. A better understanding of the mechanisms related to fish immune response is crucial for the development of effective approaches in disease management. In the present work, transcriptional changes of immune related genes have been evaluated in farmed S. senegalensis specimens vaccinated against Phdp by intraperitoneal injection (IP) and immersion (IM). IP fish showed higher antibody levels and increased transcription of genes encoding lysozyme C1, complement factors involved in the classical pathway and components involved in the opsonization and the limitation of free iron availability, all of them facilitating the faster elimination of the pathogen and promoting higher RPS after the infection with Phdp. The results of this study seem to support a different intensity of the specimens immune response in the head kidney. Analysis of the immune response in 15 day post-challenged fish showed up-regulation of genes involved in all stages of S. senegalensis immune response, but especially those genes encoding proteins related to the innate response such as complement, lysozyme and iron homeostasis in the head kidney. On the other hand, liver transcription was higher for genes related to inflammation, apoptosis and cell mediated cytotoxicity (CMC). Furthermore, comparison of the differential response of S. senegalensis genes in vaccinated and unvaccinated fish to Phdp infection allowed the identification of a potential biosignature, consisting in 10 genes, as a surrogate of protection and therefore, as indicator of vaccine success against fotobacteriosis after IP vaccination. These results provide important insights into the S. senegalensis protection against Phdp induced by vaccination.

Two routes of infection with Photobacterium damselae subsp. piscicida are effective in the modulation of the transcription of immune related genes in Solea senegalensis

The marine fish pathogen Photobacterium damselae subsp. piscicida (Phdp) is responsible for important disease outbreaks affecting cultured fish species including the flatfish Solea senegalensis. In the present work, transcription of iron metabolism related genes (TF, FERR-M, HP-1 and HAMP-1) as well as innate immune system components such as complement proteins (C3 and C7), lysozyme (LYS-G), TNF family (TNFα, TRAF-3), NCCRP-1 and heat shock protein encoding genes (HSP70, HSP90AA, HSP90AB and GP96) has been determined in the liver and kidney of S. senegalensis specimens after Phdp infection. Intraperitoneal injection (IP) and immersion (IM) routes have been used for infection. Fish developed specific antibodies in both cases, higher levels being detected in IP infected specimens. Both infection routes resulted in increased relative transcript levels of FERR-M, HP-1 and HAMP-1 genes and TF decreased relative transcription, conducting to lower iron availability for the pathogen. This response can be considered as a strategy to limit iron availability for Phdp, a pathogen capable to obtain iron from transferrin. Relative transcription of genes encoding lysozyme and complement factors C3 and C7 were also increased regardless the infection route; the liver was the main organ involved in the initial stages and the kidney in later stages of the infection. TNFα and TRAF-3 relative gene transcription increased 24h post-infection. TRAF-3 gene induction was detected 30 d post-infection, whilst no changes in TNFα were observed 72h or 30 d post-infection. NCCRP-1 changes were observed after IP infection in the liver and kidney; however, IM infection resulted only in slight changes in the kidney of infected fish. This different response observed maybe related to a lower number of invaded cells by the pathogen. Finally, changes in HSP90AB and GP96 have been detected after infection by both routes. Different late modulation has been observed in assayed genes depending on the route of infection. Thus, only LYS-G, TF, NCCRP-1, GP96 and HSP90AB gene transcription was modulated 30 d post-infection in the kidney of IM infected specimens; however, IP infected fish showed modulation in a higher number of genes both in liver and kidney tissues. The implications of these responses in resistance to infection by Phdp need to be elucidated.

Ovarian cavity fluid of the viviparous surfperch Neoditrema ransonnetii suppresses the spontaneous cytotoxic activity of head-kidney leucocytes against xenogeneic targets

In this study, the effect of ovarian cavity fluid (OCF) from a surfperch Neoditrema ransonnetii on the cytotoxic activity of leucocytes was investigated. In an assay targeting RTG-2, a cell line derived from rainbow trout Oncorhynchus mykiss gonads, leucocytes from both the spleen and head kidney showed spontaneous killing activity. Pre-incubation with OCF significantly suppressed the cytotoxic activity of head-kidney leucocytes towards RTG-2. This suppressive activity was due to the presence of low molecular-mass materials. These results suggest that OCF plays significant roles in pregnancy by its ability to modulate cytotoxicity with maternal leucocytes.

Molecular cloning of NCCRP-1 gene from orange-spotted grouper (Epinephelus coioides) and characterization of NCCRP-1(+) cells post Cryptocaryon irritans infection

Nonspecific cytotoxic cells (NCCs) are an important cytotoxic cell population in the innate teleost immune system. The receptor designated "NCC receptor protein 1" (NCCRP-1) has been reported to be involved in the recognition and activation of NCCs. In this study, the full-length cDNA of Epinephelus coioides NCCRP-1 (ecnccrp-1) was cloned. The open reading frame (ORF) of ecnccrp-1 is 699 bp, encoding a 232 amino acid protein that includes proline-rich motifs at the N-terminus and is related to the F-box associated family. Although a bioinformatics analysis showed that EcNCCRP-1 had no signal peptide or transmembrane helices, a polyclonal antibody directed against recombinant EcNCCRP-1 efficiently labeled a membrane protein in the head kidney, detected with Western blot analysis, which indicated that the protein localized to the cell surface. RT-PCR showed that the constitutive expression of ecnccrp-1 was higher in the lymphoid organs, such as the trunk kidney, spleen, head kidney, and thymus, and lower in brain, heart, fat, liver, muscle, and skin. After infection with Cryptocaryon irritans, the transcription of ecnccrp-1 was analyzed at the infected sites (skin and gills) and in the systemic immune organs (head kidney and spleen). At the infected sites, especially the skin, ecnccrp-1 expression was upregulated at 6h post infection, reaching peak expression on day 3 post the primary infection. However, the expression patterns differed in the systemic immune organs. In the spleen, ecnccrp-1 was gradually increased in the early infection period and decreased sharply on day 3 post the primary infection, whereas in the head kidney, the transcription of ecnccrp-1 was depressed during almost the whole course of infection. An immunohistochemical analysis showed that EcNCCRP-1(+) cells accumulated at the sites of infection with C. irritans. These results suggested that NCCs were involved in the process of C. irritans infection in E. coioides.

Endosulfan decreases cytotoxic activity of nonspecific cytotoxic cells and expression of granzyme gene in Oreochromis niloticus

The effect of the organochlorinated insecticide endosulfan, on the cytotoxic activity of Nile tilapia nonspecific cytotoxic cells (NCC) was assessed. Juvenile Nile tilapia were exposed to endosulfan (7 ppb) for 96 h and splenic NCC were isolated. Flow cytometric phenotyping of NCC was based on the detection of the NCC specific membrane signaling protein NCCRP-1 by using the monoclonal antibody Mab 5C6; granzyme expression was evaluated by quantitative RT-PCR. The cytotoxic activity of sorted NCC on HL-60 tumoral cells was assessed using propidium iodide (PI) staining of DNA in HL-60 nuclei, indicating dead cells. Nile tilapia splenic NCC had the ability to kill HL-60 tumoral cells, however, the exposure to endosulfan significantly reduced, by a 65%, their cytotoxic activity when using the effector:target ratio of 40:1. Additionally, the exposure to endosulfan tended to increase the expression of NCCRP-1, which is involved in NCC antigen recognition and signaling. Moreover, it decreased the expression of the granzyme gene in exposed group as compared with non-exposed group; however significant differences between groups were not detected. In summary, the acute exposure of Nile tilapia to sublethal concentration of endosulfan induces alteration in function of NCC: significant decrease of cytotoxic activity and a tendency to lower granzyme expression, severe enough to compromise the immunity of this species.

Immunity to betanodavirus infections of marine fish

Betanodaviruses cause viral nervous necrosis in numerous fish species, but some species are resistant to infection by these viruses. It is essential to fully characterize the immune responses that underlie this protective response. Complete characterization of the immune responses against nodaviruses may allow the development of methods that stimulate fish immunity and of an effective betanodavirus vaccine. Such strategies could include stimulation of specific immune system responses or blockage of factors that decrease the immune response. The innate immune system clearly provides a front-line defense, and this includes the production of interferons and other cytokines. Interferons that are released inside infected cells and that suppress viral replication may be the most ancient form of innate immunity. This review focuses on the immune responses of fish to betanodavirus infection.

Teleost T and NK cell immunity

The main function of the immune system is to maintain the organism's homeostasis when invaded by foreign material or organisms. Prior to successful elimination of the invader it is crucial to distinguish self from non-self. Most pathogens and altered cells can be recognized by immune cells through expressed pathogen- or danger-associated molecular patterns (PAMPS or DAMPS, respectively), through non-self (e.g. allogenic or xenogenic cells) or missing major histocompatibility (MHC) class I molecules (some virus-infected target cells), and by presenting foreign non-self peptides of intracellular (through MHC class I-e.g. virus-infected target cells) or extracellular (through MHC class II-e.g. from bacteria) origin. In order to eliminate invaders directly or by destroying their ability to replicate (e.g. virus-infected cells) specialized immune cells of the innate and adaptive responses appeared during evolution. The first line of defence is represented by the evolutionarily ancient macrophages and natural killer (NK) cells. These innate mechanisms are well developed in bony fish. Two types of NK cell homologues have been described in fish: non-specific cytotoxic cells and NK-like cells. Adaptive cell-mediated cytotoxicity (CMC) requires key molecules expressed on cytotoxic T lymphocytes (CTLs) and target cells. CTLs kill host cells harbouring intracellular pathogens by binding of their T cell receptor (TCR) and its co-receptor CD8 to a complex of MHC class I and bound peptide on the infected host cell. Alternatively, extracellular antigens are taken up by professional antigen presenting cells such as macrophages, dendritic cells and B cells to process those antigens and present the resulting peptides in association with MHC class II to CD4(+) T helper cells. During recent years, genes encoding MHC class I and II, TCR and its co-receptors CD8 and CD4 have been cloned in several fish species and antibodies have been developed to study protein expression in morphological and functional contexts. Functional assays for innate and adaptive lymphocyte responses have been developed in only a few fish species. This review summarizes and discusses recent results and developments in the field of T and NK cell responses with focus on economically important and experimental model fish species in the context of vaccination.

Characterization of non-specific cytotoxic cell receptor protein 1: a new member of the lectin-type subfamily of F-box proteins

Our previous microarray study showed that the non-specific cytotoxic cell receptor protein 1 (Nccrp1) transcript is significantly upregulated in the gastric mucosa of carbonic anhydrase IX (CA IX)-deficient (Car9^−/−) mice. In this paper, we aimed to characterize human NCCRP1 and to elucidate its relationship to CA IX. Recombinant NCCRP1 protein was expressed in Escherichia coli, and a novel polyclonal antiserum was raised against the purified full-length protein. Immunocytochemistry showed that NCCRP1 is expressed intracellularly, even though it has previously been described as a transmembrane protein. Using bioinformatic analyses, we identified orthologs of NCCRP1 in 35 vertebrate genomes, and up to five paralogs per genome. These paralogs are FBXO genes whose protein products are components of the E3 ubiquitin ligase complexes. NCCRP1 proteins have no signal peptides or transmembrane domains. NCCRP1 has mainly been studied in fish and was thought to be responsible for the cytolytic function of nonspecific cytotoxic cells (NCCs). Our analyses showed that in humans, NCCRP1 mRNA is expressed in tissues containing squamous epithelium, whereas it shows a more ubiquitous tissue expression pattern in mice. Neither human nor mouse NCCRP1 expression is specific to immune tissues. Silencing CA9 using siRNAs did not affect NCCRP1 levels, indicating that its expression is not directly regulated by CA9. Interestingly, silencing NCCRP1 caused a statistically significant decrease in the growth of HeLa cells. These studies provide ample evidence that the current name, “non-specific cytotoxic cell receptor protein 1,” is not appropriate. We therefore propose that the gene name be changed to FBXO50.

Peroxiredoxin IV regulates pro-inflammatory responses in large yellow croaker (Pseudosciaena crocea) and protects against bacterial challenge

In this study, we applied a comparative proteomic approach to the analysis of differentially expressed proteins in the spleens of large yellow croaker following treatment with an inactivated trivalent bacterial vaccine. Twenty-four altered proteins were identified by MALDI-TOF or MALDI-TOF-TOF, including immune-related proteins, antioxidant proteins, signal transducers, protein biosynthesis and catabolism modulators, and carbonic anhydrases. Three Prx family members, namely, Prx I, Prx II, and Prx IV, were upregulated after treatment with the vaccine, indicating potentially important roles for these antioxidant proteins in the antibacterial immune response. Large yellow croaker Prx IV (LycPrxIV), which has thiol-dependent peroxidase activity, was constitutively expressed in all tissues examined. Immunoelectron microscopy showed that LycPrxIV was primarily localized to the rER or peroxisome in spleen cells of healthy fish, and its synthesis on the rER increased following treatment with bacterial vaccine. Suppression of LycPrxIV by siRNA resulted in an increase in NF-kappaB activity in spleen tissues, while in vivo administration of recombinant LycPrxIV (rLycPrxIV) caused a decrease in NF-kappaB activity, indicating that LycPrxIV negatively regulates NF-kappaB activation. Likewise, siRNA-mediated knockdown of LycPrxIV increased the expression of TNF-alpha and CC chemokine, and downregulated the expression of IL-10. However, injection of fish with rLycPrxIV induced the opposite expression pattern of these cytokines, suggesting a role for LycPrxIV in regulating pro-inflammatory responses. Bacterial challenge experiments showed that suppression of LycPrxIV expression by siRNA significantly increased fish mortality as compared to controls, whereas rLycPrxIV provided a protective effect. Together, our data suggest that LycPrxIV may regulate pro-inflammatory responses to protect large yellow croaker from bacterial challenge, revealing a novel antibacterial mechanism in fish.

Solea senegalensis genes responding to lipopolysaccharide and copper sulphate challenges: large-scale identification by suppression subtractive hybridization and absolute quantification of transcriptional profiles by real-time RT-PCR

Solea senegalensis is a commercially relevant aquaculture species that remains largely unexplored at the genomic level. The aim of this study was to identify novel genomic responses to lipopolysaccharide and copper sulphate challenges using suppression subtractive hybridization (SSH) and real-time RT-PCR. Forward- and reverse-subtractive libraries were generated for the identification of genes whose transcription is altered in response to lipopolysaccharide (LPS) (immunomodulator) in head kidney (immunologically important organ) and to CuSO(4) (common algacide) in liver (central metabolic organ and important source of immune transcripts). A total of 156 genes involved in major physiological functions were identified by SSH, the identified sequences representing a significant increase in the number of sole ESTs in public databases. Fifteen genes represented in the subtracted libraries were selected for further tissue, temporal and inducible transcriptional profiling by real-time RT-PCR. A rigorous quantification of transcript copy numbers was performed for this purpose in both pooled and individual samples from two independent experiments. More than half of the investigated mRNAs encode proteins that deal with different aspects of the immune response, like NCCRP1 (non-specific cytotoxic cell receptor), C3 and C7 (complement components), and ferritin M, HP and TF (iron homeostasis), or play a crucial role in its regulation, like TRAF3. Other mRNAs studied encode proteins involved in metabolism, like TKT and NDUFA4, the response to stimulus, like CEBPB (transcription factor) and CIRBP (RNA-binding protein), and other cell processes. Highly abundant (>500 molecules/pg total RNA) and rare (< or =1 molecules/pg) mRNA species were quantified in each sole organ examined, and outstanding differences were also recorded in the comparison between the two organs, e.g. C3 and TF mRNAs were largely overexpressed in liver (>5000 molecules/pg) as compared to head kidney (<5 molecules/pg). Most investigated mRNAs displayed significant alterations in their steady-state copy number following LPS and/or CuSO(4) stimulation, i.e. they were (i) up-regulated in response to both treatments in at least one of the two organs (NCCRP1, CEBPB, SQSTM1, NDUFA4, C7 and HP), (ii) up-regulated (TF, CIRBP, TRFA, C3) or down-regulated (TKT) by LPS, their levels remaining essentially unchanged upon CuSO(4) challenge, or (iii) down-regulated by LPS, though up-regulated by CuSO(4) (ferritin M). Quantifications in individual fish were consistent with those in pooled samples with respect to both the direction and the absolute changes in transcript abundance.

Intraperitoneal vaccination of Atlantic cod, Gadus morhua with heat-killed Listonella anguillarum enhances serum antibacterial activity and expression of immune response genes

Serum-mediated reduction in bacterial count and expression of a number of immune response genes in the blood of Atlantic cod, Gadus morhua were investigated following intraperitoneal vaccination with heat-killed Listonella (Vibrio) anguillarum. Blood was collected from the caudal vein of both vaccinated and non-vaccinated (PBS-injected) fish at 0, 1, 3, 7 and 10 days post-vaccination (dpv). Serum protein concentration and antibacterial activity of the serum samples were determined. Whole blood was used for semi-quantitative RT-PCR of immune-related genes. Total serum protein was not significantly different between the vaccinated and non-vaccinated groups. Sera from the vaccinated fish significantly reduced L. anguillarum count on 3 dpv, with reductions of at least 2 log colony forming units per ml (CFU/ml) relative to the non-vaccinated fish. Expression of antibacterial genes, bactericidal/permeability-increasing protein/lipopolysaccharide-binding protein (BPI/LBP), g-type lysozyme and transferrin was significantly upregulated in the vaccinated fish, with maximum expression within 7 dpv. Cytotoxic-related and cell-mediated immunity genes such as, apolipoprotein A-I and the non-specific cytotoxic cell receptor protein (NCCRP-1) had maximum expression at 3 and 7 dpv, respectively. Significant upregulation in expression of pro-inflammatory cytokines, IL-1 beta and IL-8 was also observed in the vaccinated fish at 1 dpv. The upregulation of immune response genes following vaccination provides valuable information in the understanding of immune mechanisms against vibriosis in Atlantic cod particularly on the acute phase response during bacterial infection.

A review on the interactions between gut microbiota and innate immunity of fish

Although fish immunology has progressed in the last few years, the contribution of the normal endogenous microbiota to the overall health status has been so far underestimated. In this context, the establishment of a normal or protective microbiota constitutes a key component to maintain good health, through competitive exclusion mechanisms, and has implications for the development and maturation of the immune system. The normal microbiota influences the innate immune system, which is of vital importance for the disease resistance of fish and is divided into physical barriers, humoral and cellular components. Innate humoral parameters include antimicrobial peptides, lysozyme, complement components, transferrin, pentraxins, lectins, antiproteases and natural antibodies, whereas nonspecific cytotoxic cells and phagocytes (monocytes/macrophages and neutrophils) constitute innate cellular immune effectors. Cytokines are an integral component of the adaptive and innate immune response, particularly IL-1 beta, interferon, tumor necrosis factor-alpha, transforming growth factor-beta and several chemokines regulate innate immunity. This review covers the innate immune mechanisms of protection against pathogens, in relation with the installation and composition of the normal endogenous microbiota in fish and its role on health. Knowledge of such interaction may offer novel and useful means designing adequate therapeutic strategies for disease prevention and treatment.

The gene structure and expression of the non-specific cytotoxic cell receptor protein (NCCRP-1) in Atlantic cod (Gadus morhua L.)

The non-specific cell receptor protein (NCCRP-1) serves an important function in target cell recognition and activation of non-specific cytotoxic cells in teleosts. Atlantic cod NCCRP-1 was identified in a suppression-subtractive cDNA library and NCCRP-1 from Atlantic salmon, rainbow trout, Japanese medaka and fathead minnow was found deposited in the GenBank as EST sequences. The predicted amino acid sequences of these receptors contain the characteristic functional domains representing NCCRP-1, and phylogenetic analyses support the identification of five NCCRP-1 orthologues. Cod NCCRP-1 is shorter and has a different intron/exon organization from the common carp and channel catfish counterparts, but shows high extent of conservation in NCCRP-1 signature motives. Quantitative real-time PCR analyses showed that the gene expression of cod NCCRP-1 was higher in the lymphoid organs, head kidney (90-fold) and spleen (30-fold), compared to the organ with lowest expression. NCCRP-1 gene expression was not induced by in vitro treatment of head kidney cells with polyinosinic polycytidylic acid (poly I:C) or lipopolysaccharide (LPS), or by in vivo injections with poly I:C or formalin killed Vibrio anguillarum. These results show that the cod NCCRP-1 gene is differentially expressed in organs, and that gene expression is not induced by the tested treatments.

Generation of monoclonal antibodies to coelomocytes of the purple sea urchin Arbacia punctulata: characterization and phenotyping

Cellular cytotoxicity is a key component of animal innate immune responses that is one of the first lines of defense against invaders. There is increasing interest in the study of the cellular immune response, particularly non-specific cytotoxic cells and natural killer cells and their receptors. Studies of non-specific cytotoxic cell and natural killer cell recognition and killing (and the receptors involved) will reveal new and important insights into cellular mechanisms of host defense. Here we describe mAbs specific for coelomocyte sub-populations of the purple sea urchin, Arbacia punctulata, using highly purified coelomocyte populations as the antigen source. Monoclonal antibodies were selected using flow cytometric screening methods. Several of the mAbs were shown to bind to two sub-types of coelomocytes when assayed by fluorescence microscopy. Furthermore, these mAbs inhibited coelomocyte cytotoxicity against vertebrate target cells in a functional assay. The mAbs have been used in immunoprecipitation studies.

Modulation of juvenile brook trout (Salvelinus fontinalis) cellular immune system after Aeromonas salmonicida challenge

In fish, the first line of defence against infectious microorganisms is based on non-specific cellular immune mechanisms (innate immunity). In this study, we measured the non-specific immune parameters (natural cytotoxic cells (NCC) activity, lymphoproliferation, percentage of phagocytosis and phagocytic activity) in brook trout (Salvelinus fontinalis) infected by a virulent strain of Aeromonas salmonicida. Eight days post-infection, the mortality of infected fish reached 70%. A transient immunostimulation of the NCC activity was noticed 24h post-infection, but there was no significant difference at 48 h. Then, infection of brook trout with A. salmonicida induced a biphasic immune response. At 24h post-infection, lymphoproliferation was drastically depressed but returned to control level at 96 h. A slight increase in the percentage of phagocytosis and the phagocytic activity was noticed throughout the experiment. Conversely the cell mortality was significantly higher in infected fish compared to control. The modulation of immunological parameters might reveal important clues on how innate immunity might protect fish from bacterial infections.

The eel immune system: present knowledge and the need for research

The European eel, Anguilla anguilla, is one of the most important warm water fish species cultured in southern Europe and the Mediterranean as well as in northern countries including Germany, the Netherlands and Denmark. The Japanese eel, A. japonica, is an important cultured fish in several Asiatic countries including Japan, China and Taiwan. During recent decades, research has been performed to elucidate the immune response of these species against different pathogens (viruses, bacteria or parasites). Nevertheless, there is very limited information in terms of both cellular and humoral immune responses. This review summarizes the present knowledge relating to the eel immune system and includes new data.

Cytotoxic activities of fish leucocytes

Like mammalian leucocytes, white blood cells of fish are able to kill altered (e.g. virus-infected) and foreign (allogeneic or xenogeneic) cells. The existence of natural killer (NK)-like and specific cytotoxic cells in fish was first shown using allogeneic and xenogeneic effector/target cell systems. In addition to in vivo and ex vivo studies, very important contributions were made by in vitro analysis using a number of different long-term cytotoxic cell lines established from channel catfish. In mammals, specific cell-mediated cytotoxicity (CMC) as part of the adaptive immune response requires a number of key molecules expressed on effector leucocytes and target cells. CD8+ T lymphocytes kill infected cells only, if their antigen receptor (TCR) matches the MHC class I with bound peptide of the target cell. Expression patterns of the fish gene homologues for TCR, CD8 and MHC class I, as well as related genes, are in agreement with similar function. Convenient systems for the analysis of specific CMC have only recently become available for fish with the combination of clonal fish with syngeneic or allogeneic but MHC class I matching cell lines. It was demonstrated that both, NK- and cytotoxic T (Tc) cells are involved in the killing of virus infected MHC class I matching and mismatching target cells. Analysis of these lymphocyte subsets is only starting for fish. There is also evidence that the different viral proteins trigger different subsets of killer cells. This review further discusses findings on fish CMC with regard to temperature/seasons and ontogeny.

Comparison of select innate immune mechanisms of fish and mammals

The study of innate immunity has become increasingly popular since the discovery of homologs of many of the innate immune system components and pathways in lower organisms including invertebrates. As fish occupy a key position in the evolution of the innate and adaptive immune responses, there has been a great deal of interest regarding similarities and differences between their defense mechanisms and those of higher vertebrates. This review focuses on describing select mechanisms of the innate immune responses of fish and the implications for evolution of immunity in higher vertebrates.

Cloning and analysis of non-specific cytotoxic cell receptor (NCCRP)-1 from common carp Cyprinus carpio L

Nonspecific cytotoxic cell receptor protein (NCCRP-1) provides an important function in target cell recognition and activation of cytotoxicity. NCCRP-1 has been cloned from common carp Cyprinus carpio L. from fish barbel by EST analysis. The isolated gene is composed of 945 bp with a 79 bp 5' UTR, 714 bp open reading frame and 152 bp 3' UTR. The predicted NCCRP-1 gene is composed of 237 amino acid residues and its predicted signal peptide is 19 amino acid residues in length. This gene has conservation of all the related domains characteristic to the NCCRP-1 gene in fish. Phylogenetic and genomic analyses showed that carp NCCRP-1 was similar to other fish orthologues. The expression of NCCRP-1 gene was constitutive in both lymphoid and non-lymphoid tissues. Furthermore, by semi-quantitative RT-PCR studies, we showed that NCCRP-1 gene expression is increased in anterior kidney challenged with Aeromonas hydrophila.

Molecular characterization of the nonspecific cytotoxic cell receptor (NCCRP-1) demonstrates gilthead seabream NCC heterogeneity

Teleost fish NCCs (nonspecific cytotoxic cells) are thought to be the evolutionary precursors of the mammalian NK cells. A novel mechanism mediating the NCC-mediated cytotoxicity has been described in teleosts. Now, this NCC receptor protein-1 (NCCRP-1) was characterized in gilthead seabream. The NCCRP-1 full-length sequence contains 1036 bp with an open reading frame of 702 bp. A comparison of the predicted 233-amino acid protein with several fish orthologues indicates a highly conserved sequence containing the F-box associated (FBA) domain and proline-rich motifs (PRM) characteristics of this family. The phylogenetical tree shows that seabream NCCRP-1 belongs to the NCCRP subfamily within the FBA family of proteins. This is a single copy gene with a constitutive and ubiquitous expression as determined by RT-PCR and flow cytometry. The results show that lymphocytes, monocyte/macrophages and acidophilic granulocytes from lymphoid tissues express the receptor, both at gene and protein level. Immunofluorescence microscopic observations confirm the previous results. The implications of this receptor on seabream NCC activity are discussed.

Investigating the morphology, function and genetics of cytotoxic cells in bony fish

Bony fish (teleosts) possess multiple cytotoxic cell lineages that recognize and destroy virally infected and transformed cells. In general, these lineages parallel their functional equivalents in mammals and include neutrophilic granulocytes, macrophages, cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. These four cell types have been morphologically identified in multiple fish species but only limited information is available about their function. In contrast, much work has gone into examining the function of a fifth cytotoxic cell lineage, termed nonspecific cytotoxic cells (NCC), that has been referred to as the bony fish equivalent of NK cells. However, evidence suggesting that NCC do not represent the NK lineage has come through the development of multiple cytotoxic catfish cell lines that are morphologically and functionally similar to human NK cells and are distinct from NCC. In addition to characterizing cytotoxic cells from fish, recent work has identified the novel immune-type receptors (NITR) and cichlid killer leukocyte receptors (cKLR) that are structurally related to mammalian NK receptors and likely play a role in cytotoxic function in fish. This review summarizes the morphological and functional evidence for cytotoxic cells within bony fish and discusses future directions for examining cytotoxicity through genomics and transgenics.

Non-specific cytotoxic cell receptor (NCCRP)-1 type gene in tilapia (Oreochromis niloticus): its cloning and analysis

Non specific cytotoxic cells are natural killer cell equivalent in teleosts. These cells are known to mediate innate immune responses in fishes. In this study, from tilapia, a non-specific cytotoxic cell receptor protein (NCCRP) like cDNA composed of 960 bps with a 12 bps 5' UTR, 702 bps open reading frame and 246 bps 3' UTR has been cloned. The predicted gene product is 233 amino acid with a molecular weight of 26.6 kDa and pI of 6.07. The gene has a typical F-box associated domain, which harbors the antigen binding region, implicated in formation of conjugates with target cells necessary for killing. By semi-quantitative RT-PCR studies, the expression level of NCCRP-1 was highest in liver followed by head kidney, spleen and intestine. Low expression levels were recorded in brain, gill and heart. Lowest expression of transcripts was in the skin. As evidenced by in situ hybridisation the NCCRP-1 transcripts were expressed on lymphocyte like cells as well as neutrophils.