Teleost intestinal immunology

An oral DNA vaccine against infectious haematopoietic necrosis virus (IHNV) encapsulated in alginate microspheres induces dose-dependent immune responses and significant protection in rainbow trout (Oncorrhynchus mykiss)

Administered by intramuscular injection, a DNA vaccine (pIRF1A-G) containing the promoter regions upstream of the rainbow trout interferon regulatory factor 1A gene (IRF1A) driven the expression of the infectious hematopoietic necrosis virus (IHNV) glycoprotein (G) elicited protective immune responses in rainbow trout (Oncorhynchus mykiss). However, less laborious and cost-effective routes of DNA vaccine delivery are required to vaccinate large numbers of susceptible farmed fish. In this study, the pIRF1A-G vaccine was encapsulated into alginate microspheres and orally administered to rainbow trout. At 1, 3, 5, and 7 d post-vaccination, IHNV G transcripts were detected by quantitative real-time PCR in gills, spleen, kidney and intestinal tissues of vaccinated fish. This result suggested that the encapsulation of pIRF1A-G in alginate microparticles protected the DNA vaccine from degradation in the fish stomach and ensured vaccine early delivery to the hindgut, vaccine passage through the intestinal mucosa and its distribution thought internal and external organs of vaccinated fish. We also observed that the oral route required approximately 20-fold more plasmid DNA than the injection route to induce the expression of significant levels of IHNV G transcripts in kidney and spleen of vaccinated fish. Despite this limitation, increased IFN-1, TLR-7 and IgM gene expression was detected by qRT-PCR in kidney of vaccinated fish when a 10 μg dose of the oral pIRF1A-G vaccine was administered. In contrast, significant Mx-1, Vig-1, Vig-2, TLR-3 and TLR-8 gene expression was only detected when higher doses of pIRF1A-G (50 and 100 μg) were orally administered. The pIRF1A-G vaccine also induced the expression of several markers of the adaptive immune response (CD4, CD8, IgM and IgT) in kidney and spleen of immunized fish in a dose-dependent manner. When vaccinated fish were challenged by immersion with live IHNV, evidence of a dose-response effect of the oral vaccine could also be observed. Although the protective effects of the oral pIRF1A-G vaccine after a challenge with IHNV were partial, significant differences in cumulative percent mortalities among the orally vaccinated fish and the unvaccinated or empty-plasmid vaccinated fish were observed. Similar levels of protection were obtained after the intramuscular administration of 5 μg of pIRF1A-G or after the oral administration of a high dose of pIRF1A-G vaccine (100 μg); with 70 and 56 relative percent survival values, respectively. When fish were vaccinated with alginate microspheres containing high doses of the pIRF1A-G vaccine (50 or 100 μg), a significant increase in the production of anti-IHNV antibodies was detected in serum samples of the vaccinated fish compared with that in unvaccinated fish. At 10 days post-challenge, IHNV N gene expression was nearly undetectable in kidney and spleen of orally vaccinated fish which suggested that the vaccine effectively reduced the amount of virus in tissues of vaccinated fish that survived the challenge. In conclusion, our results demonstrated a significant increase in fish immune responses and resistance to an IHNV infection after the oral administration of increasing concentrations of a DNA vaccine against IHNV encapsulated into alginate microspheres.

Modulation of immunity and gut microbiota after dietary administration of alginate encapsulated Shewanella putrefaciens Pdp11 to gilthead seabream (Sparus aurata L.)

The potential benefits of probiotics when administering to fish could improve aquaculture production. The objective of this study was to examine the modulation of immune status and gut microbiota of gilthead seabream (Sparus aurata L.) specimens by a probiotic when administered encapsulated. Commercial diet was enriched with Shewanella putrefaciens Pdp11 (SpPdp11, at a concentration of 10(8) cfu g(-1)) before being encapsulated in calcium alginate beads. Fish were fed non-supplemented (control) or supplemented diet for 4 weeks. After 1, 2 and 4 weeks the main humoral and cellular immune parameters were determined. Furthermore, gene expression profile of five immune relevant genes (il1β, bd, mhcIIα, ighm and tcrβ) was studied by qPCR in head kidney. On the other hand, intestinal microbiota of fish was analysed at 7 and 30 days by DGGE. Results demonstrated that administration of alginate encapsulated SpPdp11 has immunostimulant properties on humoral parameters (IgM level and serum peroxidase activity). Although no immunostimulant effects were detected on leucocyte activities, significant increases were detected in the level of mRNA of head-kidney leucocytes for mhcIIα and tcrβ after 4 weeks of feeding the encapsulated-probiotic diet. The administration of SpPdp11 encapsulated in alginate beads produced important changes in the DGGE patterns corresponding to the intestinal microbiota. Predominant bands related to lactic acid bacteria, such as Lactococcus and Lactobacillus strains, were sequenced from the DGGE patterns of fish fed the probiotic diet, whereas they were not sequenced from fish receiving the control diet. The convenience or not of probiotic encapsulation is discussed.

Transcription Factor T-Bet in Atlantic Salmon: Characterization and Gene Expression in Mucosal Tissues during Aeromonas Salmonicida Infection

The T-box transcription factor T-bet is expressed in a number of hematopoietic cell types in mammals and plays an essential role in the lineage determination of Th1 T-helper cells and is considered as an essential feature for both innate and adaptive immune responses in higher vertebrates. In the present study, we have identified and characterized the full-length Atlantic salmon T-bet cDNA (3502 bp). The putative primary structure of the polypeptide deduced from the cDNA sequence contained 612 aa, which possessed a T-box DNA binding domain. Phylogenetic study and gene synteny revealed it is as a homolog to mammalian T-bet. Quantitative PCR analysis of different tissues in healthy fish showed that salmon T-bet gene was highly expressed in spleen, followed by head kidney, and was expressed in intestine, skin, and liver at lower levels. Moreover, the time-dependent expression profile of T-bet, interferon gamma (IFNγ), interleukin-22 (IL-22), and natural killer enhancement factor in mucosal tissues during water-borne infection with live Aeromonas salmonicida, indicated the involvement of T-bet in mucosal immune response in Atlantic salmon.

First Demonstration of Antigen Induced Cytokine Expression by CD4-1+ Lymphocytes in a Poikilotherm: Studies in Zebrafish (Danio rerio)

Adaptive immunity in homeotherms depends greatly on CD4+ Th cells which release cytokines in response to specific antigen stimulation. Whilst bony fish and poikilothermic tetrapods possess cells that express TcR and CD4-related genes (that exist in two forms in teleost fish; termed CD4-1 and CD4-2), to date there is no unequivocal demonstration that cells equivalent to Th exist. Thus, in this study we determined whether CD4-1⁺ lymphocytes can express cytokines typical of Th cells following antigen specific stimulation, using the zebrafish (Danio rerio). Initially, we analyzed the CD4 locus in zebrafish and found three CD4 homologues, a CD4-1 molecule and two CD4-2 molecules. The zfCD4-1 and zfCD4-2 transcripts were detected in immune organs and were most highly expressed in lymphocytes. A polyclonal antibody to zfCD4-1 was developed and used with an antibody to ZAP70 and revealed double positive cells by immunohistochemistry, and in the Mycobacterium marinum disease model CD4-1⁺ cells were apparent surrounding the granulomas typical of the infection. Next a prime-boost experiment, using human gamma globulin as antigen, was performed and revealed for the first time in fish that zfCD4-1⁺ lymphocytes increase the expression of cytokines and master transcription factors relevant to Th1/Th2-type responses as a consequence of boosting with specific antigen.

Metabolism, health and fillet nutritional quality in Atlantic salmon (Salmo salar) fed diets containing n-3-rich microalgae

Microalgae, as primary producers of EPA and DHA, are among the most prominent alternative sources to fish oil for n-3 long-chain PUFA in animal and human nutrition. The present study aimed to assess technical, nutritional and fish health aspects of producing n-3-rich Atlantic salmon (Salmo salar) fish fillets by dietary supplementation of increasing levels of a DHA-producing Schizochytrium sp. and reduced or without use of supplemental fish oil. Atlantic salmon smolt were fed diets with graded levels of microalgae for 12 weeks, during which all fish showed high feed intake rates with postprandial plasma leptin levels inversely correlating with final mean fish body weights. Fish performance was optimal in all experimental treatments (thermal growth coefficient about 4·0 and feed conversion ratio 0·8-0·9), protein digestibility was equal in all diets, whereas dietary lipid digestibility inversely correlated with the dietary levels of the SFA 16 : 0. Fillet quality was good and similar to the control in all treatments in terms of n-3 long-chain PUFA content, gaping, texture and liquid losses during thawing. Histological fluorescence staining and immunofluorescence analysis of salmon intestines (midgut: base of intestine and villi) revealed significant effects on slime, goblet cell production and inducible nitric oxide synthase (iNOS) activity with increasing levels of dietary Schizochytrium sp. supplementation. Microarray analysis did not reveal any signs of toxicity, stress, inflammation or any other negative effects from Schizochytrium sp. supplementation in diets for Atlantic salmon.

Effect of copper nanoparticles and copper sulphate on oxidation stress, cell apoptosis and immune responses in the intestines of juvenile Epinephelus coioides

Copper nanoparticles (Cu-NPs) are widely used in various industrial and commercial applications, but little is known about their potential hazard in the intestines of marine teleosts. In this study we investigated the effects of Cu-NPs and soluble Cu in the intestines of juvenile Epinephelus coioides. The fish were exposed in triplicate to control, 20 or 100 μg Cu L(-1) as either copper sulphate (CuSO₄) or Cu-NPs for 25 days. With an increase in Cu-NPs or CuSO₄ dose, the concentration of malonaldehyde in the intestines significantly increased, whereas the activities of total superoxide dismutase and catalase as well as glutathione concentration decreased compared to the control. Statistical analysis of an apoptosis index of intestinal cells showed that general dose-dependent apoptosis was induced by Cu-NPs or CuSO₄, with Cu-NPs inducing the significantly higher apoptosis index than CuSOv. Caspase-3 and caspase-9 activities were increased with an increase in Cu-NPs or CuSO₄ dose, more so in the Cu-NPs than CuSO₄ treatment. With an increase in Cu-NPs or CuSOv dose, succinate dehydrogenase and Na(+)-K(+)-ATPase activity and cytochrome c concentration in mitochondria decreased, accompanied by increased cytochrome c concentration in the cytosol. Concentration of heat shock proteins 70 and 90 in the intestines and expression of corresponding genes were enhanced with an increase in the Cu-NPs or CuSOv dose, but the concentrations and expressions of immunoglobulin M and lysozyme decreased (more in the Cu-NPs than CuSO₄ treatment) compared to the control. Expression of interleukin-1beta and tumor necrosis factor-alpha showed a dose-dependent increase with the increased Cu-NPs or CuSO₄ dose, with the highest expression found in the Cu-NPs treatment. In conclusion, Cu-NPs had similar toxic effects as CuSOv in the intestines of juvenile E. coioides, but toxicity of Cu-NPs was more severe than that of CuSO₄.

IPNV Antigen Uptake and Distribution in Atlantic Salmon Following Oral Administration

One impediment to the successful oral vaccination in fish is the hostile stomach environment that antigens must cross. Furthermore, uptake of antigens from the gut to systemic distribution is required for induction of systemic immunity, the dynamics of which are poorly understood. In the present study, groups of Atlantic salmon parr were intubated with live or inactivated infectious pancreatic necrosis virus (IPNV), either orally or anally. At 1, 24 and 72 h post infection (p.i.), the fish were sacrificed. Serum was used for assessing IPNV by ELISA, while formalin-fixed head-kidney, spleen, liver and intestine tissues were used for the demonstration of antigens by immunohistochemistry. Both live and inactivated IPNV antigens were observed in enterocytes of the intestines and in immune cells of the head-kidneys and spleens of all groups. In the liver, no antigens were observed in any of the groups. Significantly higher serum antigen OD values (p < 0.04) were observed in orally- compared to anally-intubated fish. By contrast, no difference (p = 0.05) was observed in tissue antigens between these groups by immunohistochemistry. No significant difference (p = 0.05) in serum antigens was observed between groups intubated with live and inactivated IPNV, while in tissues, significantly more antigens (p < 0.03) were observe in the latter compared to the former. These findings demonstrate that both live and inactivated IPNV are taken up by enterocytes in the intestines of Atlantic salmon, likely by receptor-mediated mechanisms. Higher IPNV uptake by the oral compared to anal route suggests that both the anterior and posterior intestines are important for the uptake of the virus and that IPNV is resistant to gastric degradation of the Atlantic salmon stomach.

Immunoglobulin isotypes in Atlantic salmon, Salmo salar

There are three major immunoglobulin (Ig) isotypes in salmonid fish: IgM, IgD and IgT, defined by the heavy chains μ, δ and τ, respectively. As a result of whole genome duplication in the ancestor of the salmonid fish family, Atlantic salmon (Salmo salar) possess two highly similar Ig heavy chain gene complexes (A and B), comprising two μ genes, two δ genes, three intact τ genes and five τ pseudogenes. The μA and μB genes correspond to two distinct sub-populations of serum IgM. The IgM-B sub-variant has a characteristic extra cysteine near the C-terminal part of the heavy chain and exhibits a higher degree of polymer disulfide cross-linking compared to IgM-A. The IgM-B:IgM-A ratio in serum is typically 60:40, but skewed ratios are also observed. The IgT isotype appears to be specialized to mucosal immune responses in salmonid fish. The concentration of IgT in serum is 100 to 1000 times lower than IgM. Secreted forms of IgD have been detected in rainbow trout, but not yet in Atlantic salmon.

A deficiency or an excess of dietary threonine level affects weight gain, enzyme activity, immune response and immune-related gene expression in juvenile blunt snout bream (Megalobrama amblycephala)

A feeding trial was conducted to investigate the impacts of deficient and excess dietary threonine levels on weight gain, plasma enzymes activities, immune responses and expressions of immune-related genes in the intestine of juvenile blunt snout bream. Triplicate groups of fish (initial weight 3.01 ± 0.01 g, 30 fish per tank) were fed with deficient (0.58%), optimum (1.58%) and excess (2.58%) threonine level diets to near satiation four times a day for 9 weeks. A mixture of l-amino acids was supplemented to simulate the whole body amino acid pattern of blunt snout bream, except for threonine. The results showed that both deficiency and excess threonine level diets significantly (P < 0.05) reduced the weight gain of blunt snout bream. Excess dietary threonine level triggered plasma aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities (P < 0.05); whereas superoxide dismutase (SOD) activity was not significantly influenced by imbalanced-dietary threonine level (P > 0.05). Plasma complement component 3 (C3) and component 4 (C4) concentrations were significantly depressed by the deficiency of dietary threonine (P < 0.05). Dietary threonine regulated the target of rapamycin (TOR), eukaryotic translation initiation factor 4E-binding protein 2 (4E-BP2), tumour necrosis factor alpha (TNF-α) and copper-zinc superoxide dismutase (Cu/Zn-SOD) gene expressions in the intestine of blunt snout bream, which may go further to explain the adverse effects of a deficient and/or an excess dietary threonine level on growth, immunity and health of fish. Furthermore, the present study also suggests that an optimum dietary threonine could play an important role in improving growth, enhancing immune function and maintaining health of fish.

Humoral immune response and TLR9 gene expression in Pacific red snapper (Lutjanus peru) experimentally exposed to Aeromonas veronii

Aquaculture production of Pacific red snapper Lutjanus peru is growing rapidly in Mexico, especially in Gulf of California. As it is a relatively new aquaculture species there are few reports evaluating its immune response to pathogens. The Gram-negative bacteria Aeromonas veronii is a heterogeneous organism that causes the disease known as motile aeromonad septicemia, which is responsible for serious economic loss in seabream culture due to bacterial infections. For the purpose of this study, juvenile Pacific red snapper specimens were intraperitoneally injected with low doses of A. veronii (1 × 10(6) CFU ml(-1)). Changes in humoral immune parameters (total protein, myeloperoxidase, lisozyme and antiprotease activities and IgM levels), as well as superoxide dismutase and catalase activities, and TLR9 gene expression were evaluated 24 and 48 h after injection. Overall, the results showed an enhanced in humoral immune parameters and SOD and CAT activities in fish infected with A. veronii compared with control group at 24 or 48 h. By real time PCR assays, the basal mRNA transcripts of TLR9 showed that were highly expressed in intestine and leucocytes compared to skin, head kidney, liver and gill. Then, the mRNA expression levels of TLR9 in head kidney, skin, liver and intestine were analyzed in non-infected and experimentally infected fish 24 and 48 h after injection. A. veronii up-regulated the expression of TLR9 at 24 or 48 h of exposure in all samples analyzed except in liver. Interestingly, intestine produced the greatest increase in transcript levels upon exposure (48 h) to A. veronii. Taken together, our results suggest that low doses of A. veronii infection inducing humoral immune system and TLR9 immune gene in Pacific red snapper that can be useful in the health control of this species.

Unique Features of Fish Immune Repertoires: Particularities of Adaptive Immunity Within the Largest Group of Vertebrates

Fishes (i.e., teleost fishes) are the largest group of vertebrates. Although their immune system is based on the fundamental receptors, pathways, and cell types found in all groups of vertebrates, fishes show a diversity of particular features that challenge some classical concepts of immunology. In this chapter, we discuss the particularities of fish immune repertoires from a comparative perspective. We examine how allelic exclusion can be achieved when multiple Ig loci are present, how isotypic diversity and functional specificity impact clonal complexity, how loss of the MHC class II molecules affects the cooperation between T and B cells, and how deep sequencing technologies bring new insights about somatic hypermutation in the absence of germinal centers. The unique coexistence of two distinct B-cell lineages respectively specialized in systemic and mucosal responses is also discussed. Finally, we try to show that the diverse adaptations of immune repertoires in teleosts can help in understanding how somatic adaptive mechanisms of immunity evolved in parallel in different lineages across vertebrates.

Differences in IgY gut absorption in gastric rainbow trout (Oncorhynchus mykiss) and agastric common carp (Cyprinus carpio) assessed in vivo and in vitro

Oral IgY antibodies offer promising potential for passive immunization strategies. To evaluate barriers for successful IgY plasma recovery after oral application in vivo, gastric rainbow trout and agastric common carp were comparatively assessed. A positive control that received a low dose of unspecific IgY antibodies by intraperitoneal injection (0.0076 mg IgY g BW(-1) d(-1); BW=body mass) was compared with an oral administration of 75 and 150 fold in rainbow trout (corresponding to 0.57 and 1.14 mg g BW(-1)) and in carp (0.57 mg g BW(-1)). Dietary antibodies were delivered with the antacid sodium bicarbonate and three different putative uptake enhancers (Tween 20, vitamin E TPGS, sodium deoxycholate). IgY concentrations in the plasma were determined 1d (rainbow trout) or 5d after last feeding (both species). Irrespective of the enhancer used, ELISA revealed IgY absorption after feeding in carp, whereas IgY concentration in rainbow trout remained below the detection threshold. Intraperitoneal injections revealed IgY in plasma of both species. In vitro Ussing chamber experiments with posterior intestine tissue of carp and trout were carried out to determine whether species-specific differences in IgY translocation were due to acidic stomach passage or species-specific differences in transepithelial IgY passage. Significantly higher IgY translocation was measured in carp at high application dosage compared to all other groups, indicating that species-specific differences in IgY uptake after oral administration are not only related to peptic IgY degradation in the stomach, but also likely a result of differences in IgY transcytosis in the posterior intestine.

Molecular cloning, characterisation and expression analysis of melanoma differentiation associated gene 5 (MDA5) of green chromide, Etroplus suratensis

Innate immune system recognises pathogen-associated molecular patterns (PAMPs) by limited number of germline encoded and non-clonally developed pathogen recognition receptors (PRRs). Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are important cytosolic PRRs for sensing viral RNAs. The receptor encoded by melanoma differentiation associated gene 5 (MDA5), an RLR, recognises viral RNA and enhances antiviral response in host cells. The full-length MDA5 cDNA in Etroplus suratensis was cloned and found to have 3673 nucleotides encoding a polypeptide of 978 amino acids. The deduced amino acid sequence contains four main structural domains: two CARD domains in the N-terminal region, a DExDc (DEAH/DEAD box helicase domain), HELICc (C-terminal helicase) domain and a C-terminal regulatory domain (RD). Phylogenetic analysis revealed a close relationship of E. suratensis MDA5 (EsMDA5) with MDA5 of Neolamprologus brichardi and Oreochromis niloticus, both belonging to Cichlidae family. EsMDA5 transcripts were ubiquitously expressed in all the 12 tissues tested in healthy fish. Although, transcript level was found to be the highest in muscle, high expression was also detected in the spleen, head kidney and hindgut. In poly I:C-injected fish, EsMDA5 transcripts showed peak expression in the spleen, intestine and heart at 12h post-injection (hpi). However, in gill and kidney tissues, maximum up-regulation of EsMDA5 was observed at 6 and 48 hpi, respectively. Further, liver tissue showed an increasing trend in expression profile from 6 to 48 hpi. Interferon promoter stimulator-1 (IPS-1) gene, an adaptor triggering RIG-I- and MDA5-mediated type I interferon induction, also showed up-regulated expression at initial time-points in poly I:C-injected E. suratensis. The constitutive expression and up-regulation of EsMDA5 and the IPS-1 genes in different tissues indicate that EsMDA5 may play an important role in sensing viral PAMPs in conjunction with IPS-1.

T cell transcripts and T cell activities in the gills of the teleost fish sea bass (Dicentrarchus labrax)

The gills of fish are a mucosal tissue that contains T cells involved in the recognition of non-self and pathogens, and in this work we describe some features of gill-associated T cells of European sea bass, a marine model species. A whole transcriptome was obtained by deep sequencing of RNA from unstimulated gills that has been analyzed for the presence of T cell-related transcripts. Of the putative expressed sequences identified in the transcriptome, around 30 were related to main functions related to T cells including Th1/Th2/Th17/Treg cell subpopulations, thus suggesting their possible presence in the branchial epithelium. The number of T cells in the gills of sea bass, measured with the specific T cell mAb DLT15 range from 10% to 20%, and IHC analysis shows their abundance and distribution in the epithelium. Leukocytes from gills are able to proliferate in the presence of lectins ConA and PHA, as measured by flow cytometry using CFSE fluorescence incorporation, and during proliferation the number of T cells counted by immunofluorescence increased. In lectin-proliferating cells the expression of T cell-related genes TRβ, TRγ, CD4, CD8α, CD45 and IL-10 increased dramatically. Our data represent a first analysis on T cell genes and on basic T cell activities of fish gills, and suggest the presence of functionally active subpopulations of T lymphocytes in this tissue.

Probiotic supplementation influences the diversity of the intestinal microbiota during early stages of farmed senegalese sole (Solea Senegalensis, Kaup 1858)

Ingestion of bacteria at early stages results in establishment of a primary intestinal microbiota which likely undergoes several stages along fish life. The role of this intestinal microbiota regulating body functions is crucial for larval development. Probiotics have been proved to modulate this microbiota and exert antagonistic effects against fish pathogens. In the present study, we aimed to determine bacterial diversity along different developmental stages of farmed Senegalese sole (Solea senegalensis) after feeding probiotic (Shewanella putrefaciens Pdp11) supplemented diet for a short period (10-30 days after hatching, DAH). Intestinal lumen contents of sole larvae fed control and probiotic diets were collected at 23, 56, 87, and 119 DAH and DNA was amplified using 16S rDNA bacterial domain-specific primers. Amplicons obtained were separated by denaturing gradient gel electrophoresis (DGGE), cloned, and resulting sequences compared to sequences in GenBank. Results suggest that Shewanella putrefaciens Pdp11 induces a modulation of the dominant bacterial taxa of the intestinal microbiota from 23 DAH. DGGE patterns of larvae fed the probiotic diet showed a core of bands related to Lactobacillus helveticus, Pseudomonas acephalitica, Vibrio parahaemolyticus, and Shewanella genus, together with increased Vibrio genus presence. In addition, decreased number of clones related to Photobacterium damselae subsp piscicida at 23 and 56 DAH was observed in probiotic-fed larvae. A band corresponding to Shewanella putrefaciens Pdp11 was sequenced as predominant from 23 to 119 DAH samples, confirming the colonization by the probiotics. Microbiota modulation obtained via probiotics addition emerges as an effective tool to improve Solea senegalensis larviculture.

Effect of dietary isoleucine on the immunity, antioxidant status, tight junctions and microflora in the intestine of juvenile Jian carp (Cyprinus carpio var. Jian)

This study was conducted to investigate the effects of dietary isoleucine (Ile) on the immune response, antioxidant status, tight junctions, and microbial population in the intestine of juvenile Jian carp (Cyprinus carpio var. Jian). A total of 1200 juvenile Jian carp with average initial weight 6.9 ± 0.03 g were fed semi-purified isonitrogenous diets containing 4.2 (unsupplemented control group), 7.0, 9.5, 11.9, 13.9 and 16.9 g Ile kg(-1) diet for 60 days. Results indicated that Ile supplementation decreased malondialdehyde (MDA) and protein carbonyl content, and the amounts of Escherichia coli and Aeromonas in the intestine (P < 0.05), and increased the activities of superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR), glutathione content and the amounts of Lactobacillus and Bacillus in the intestine (P < 0.05). Furthermore, real time polymerase chain reaction revealed that relative mRNA expression of copper/zinc superoxide dismutase (Cu-ZnSOD), manganese superoxide dismutase (MnSOD), CAT, NF-E2-related factor 2 (Nrf2), p38 mitogen-activated protein kinases (p38MAPK) in the intestine were increased with increasing of dietary Ile up to a certain point (P < 0.05). Conversely, the relative mRNA expression of occludin, claudin-3, claudin-7, TNF-α, IL-10, Kelch-like-ECH- associated protein 1 (Keap1), extracellular signal-regulated kinase 1 (ERK1) in the intestine showed a downward trend (P < 0.05). In conclusion, dietary Ile improves intestinal immune function, antioxidant capacity and microbial population, and regulates gene expression of antioxidant enzyme, tight junctions, Nrf2, Keap1, p38 and ERK1 in the intestine of Jian carp.

Immune parameters in the intestine of wild and reared unvaccinated and vaccinated Atlantic salmon (Salmo salar L.)

Forming a barrier to the outside world, the gut mucosa faces the challenge of absorbing nutrients and fluids while initiating immune reactions towards potential pathogens. As a continuation to our previous publication focusing on the regional intestinal morphology in wild caught post smolt and spawning Atlantic salmon, we here investigate selected immune parameters and compare wild, reared unvaccinated and vaccinated post smolts. We observed highest transcript levels for most immune-related genes in vaccinated post smolts followed by reared unvaccinated and finally wild post smolts, indicating that farming conditions like commercial feed and vaccination might contribute to a more alerted immune system in the gut. In all groups, higher levels of immune transcripts were observed in the second segment of mid-intestine and in the posterior segment. In the life stages and conditions investigated here, we found no indication of a previously suggested population of intestinal T cells expressing MHC class II nor RAG1 expression.

Uptake of yeast cells in the Atlantic salmon (Salmo salar L.) intestine

The intestinal mucosa is an important port of entry for many pathogens. Information of antigen uptake mechanisms is essential to understand and to possibly prevent infections. In teleosts, several studies have aimed at investigating particulate uptake in the gastrointestinal system that seems to vary dependent on fish species and antigen. In the present study, particulate uptake in the Atlantic salmon intestine by anal intubation of yeast cells has been investigated. In the anal intubated fish, yeast were found in the epithelium close to nuclei of macrophage-like cells and inside large mononuclear cells in the intestinal lumen, indicating uptake and possible transport of large antigen particles over the epithelium by macrophage-like cells.

Adaptive immune responses at mucosal surfaces of teleost fish

This review describes the extant knowledge on the teleostean mucosal adaptive immune mechanisms, which is relevant for the development of oral or mucosal vaccines. In the last decade, a number of studies have shed light on the presence of new key components of mucosal immunity: a distinct immunoglobulin class (IgT or IgZ) and the polymeric Ig receptor (pIgR). In addition, intestinal T cells and their putative functions, antigen uptake mechanisms at mucosal surfaces and new mucosal vaccination strategies have been reported. New information on pIgR of Atlantic cod and common carp and comparison of natural and specific cell-mediated cytotoxicity in the gut of common carp and European seabass, is also included in this review. Based on the known facts about intestinal immunology and mucosal vaccination, suggestions are made for the advancement of fish vaccines.

Functional analysis of an orange-spotted grouper (Epinephelus coioides) interferon gene and characterisation of its expression in response to nodavirus infection

We cloned and sequenced 2C I-IFN, a two-cysteine containing type I interferon (I-IFN) gene, in orange-spotted grouper (Epinephelus coioides). The cDNA has 769 base pairs, the protein has 172 amino acids, and the predicted signal peptide has 18 amino acids with two cysteines. This gene is similar to I-FNs from sea bass and other teleosts. 2C I-IFN has 5 exons and 4 introns, also similar to other teleost I-IFNs. Immunohistochemical (IHC) analysis indicated that expression is predominantly membrane-localized in healthy grouper, but has a zonal distribution in nodavirus-infected grouper. Grouper infected with nodavirus had elevated levels of 2C I-IFN at 72 h and Mx at days 6-7. Recombinant 2C I-IFN activated grouper Mx, leading to upregulated antiviral activity. The grouper Mx promoter was highly induced after treatment with recombinant 2C I-IFN. The present results suggest that expression of grouper 2C I-IFN may participate in the immunologic barrier function against nodavirus.

Probiotic Bacillus pumilus SE5 shapes the intestinal microbiota and mucosal immunity in grouper Epinephelus coioides

The health benefits of probiotics are thought to occur, at least in part, through an improved intestinal microbial balance in fish, although the molecular mechanisms whereby probiotics modulate the intestinal microbiota by means of activation of mucosal immunity are rarely explored. In this study, the effects of viable and heat-inactivated probiotic Bacillus pumilus SE5 on the intestinal dominant microbial community and mucosal immune gene expression were evaluated. The fish were fed for 60 d with 3 different diets: control (without probiotic), and diets T1 and T2 supplemented with 1.0 × 10⁸ cells g⁻¹ viable and heat-inactivated B. pumilus SE5, respectively. Upregulated expression of TLR1, TLR2 and IL-8, but not MyD88 was observed in fish fed the viable probiotic, while elevated expression of TLR2, IL-8 and TGF-β1, but not MyD88 was observed in fish fed the heat-inactivated B. pumilus SE5. The induced activation of intestinal mucosal immunity, especially the enhanced expression of antibacterial epinecidin-1, was consistent with the microbial data showing that several potentially pathogenic bacterial species such as Psychroserpens burtonensis and Pantoea agglomerans were suppressed by both the viable and heat-inactivated probiotic B. pumilus SE5. These results lay the foundation for future studies on the molecular interactions between probiotics, intestinal microbiota and mucosal immunity in fish.

Notable mucosal immune responses induced in the intestine of zebrafish (Danio rerio) bath-vaccinated with a live attenuated Vibrio anguillarum vaccine

Live attenuated vaccine is one of the efficient vaccine candidates in aquaculture, which can be easily delivered to fish via bath-vaccination. An outstanding advantage of bath-vaccination is that vaccine delivery is through the same route as that utilized by many fish pathogens, generating specific mucosal immune responses. In this work, we investigated the mucosal immune responses induced by a live attenuated Vibrio anguillarum vaccine in zebrafish via bath-vaccination. Bacteria proliferated rapidly in 3 h after vaccination and maintained at a high level until 6 h in the intestine. Besides, bacteria persisted in the intestine for a longer time whereas decreased rapidly in the skin and gills. Moreover, a significant up-regulation of TLR5 triggering a MyD88-dependent signaling pathway was observed in the intestine, which implied that flagella were the crucial antigenic component of the live attenuated vaccine. And macrophages and neutrophils showed active responses participating in antigen recognition and sampling after vaccination. Furthermore, an inflammation was observed with plenty of lymphocytes in the intestine at 24 h post vaccination but eliminated within 7 days. In conclusion, the live attenuated V. anguillarum vaccine induced notable mucosal immune responses in the intestine which could be used as a mucosal vaccine vector in the future.

Dietary tryptophan modulates intestinal immune response, barrier function, antioxidant status and gene expression of TOR and Nrf2 in young grass carp (Ctenopharyngodon idella)

The present research evaluated the effects of dietary tryptophan (Trp) on growth performance, intestinal mucosal immune, barrier function and antioxidant capacity and gene expression of young grass carp (Ctenopharyngodon idella). Fish were fed six different experimental diets containing graded levels of Trp at 0.7(control), 1.7, 3.1, 4.0, 5.2 and 6.1 g kg(-1) diet for 8 weeks. The results showed that Trp supplementation significantly enhanced the percent weight gain (PWG), feed intake and feed efficiency (P < 0.05), and decreased the plasma ammonia content (PAC) (P < 0.05). After the 8-week feeding trail, an environmental copper exposure trail was conducted for 4 days. Results from the copper exposure trail showed that dietary Trp enhanced the lysozyme, acid phosphatase activities and complement 3 contents in the intestine of young grass carp (P < 0.05). In addition, Trp supplementation increased the copper/zinc superoxide dismutase (SOD1), glutathione peroxidase (GPx) activities and glutathione contents (P < 0.05), and decreased the protein carbonyl and malondialdehyde contents (P < 0.05). Furthermore, the relative gene expression levels of interleukin 10, transforming growth factor-β1, occludin, zonula occludens 1, claudin-b, -c, and -3, SOD1, GPx and NF-E2-related factor 2 in the intestine were significantly up-regulated with increasing of dietary Trp up to a certain level (P < 0.05). Conversely, the mRNA levels of tumor necrosis factor α, interleukin 8, target of rapamycin, Kelch-like-ECH-associated protein 1, claudin-12 and -15a in the intestine were significantly down-regulated by Trp (P < 0.05). Collectively, appropriate dietary Trp level improves fish growth, intestinal immune response, barrier function and antioxidant status, and regulated the mRNA levels of related signal molecules of young grass carp. Based on the quadratic regression analysis of the PWG and PAC, the dietary Trp requirement of young grass carp (287-699 g) was estimated to be 3.81 g kg(-1) diet (12.7 g kg(-1) protein) and 3.89 g kg(-1) diet (13.0 g kg(-1) protein), respectively.

Fish Immunology. The modification and manipulation of the innate immune system: Brazilian studies

The understanding of fish immune system structure and function is essential for the development of new technologies and products to improve productivity. This is the first review on immune system of fish with Brazilian studies. Aquaculture in Brazil has shown massive growth in recent years due to methods of culture intensification. However, these procedures led to disease outbreaks, as well as the chemotherapy and the misuse of antibiotics. A viable alternative to avoid the use of chemicals and prevent economic losses is the administration of immunostimulants and prebiotcs, which act by increasing the innate immune system. In Brazil there is a lack of studies on fish immune system, except by some groups that have studied the effects of the immunostimulants administration in various species.

Mucosal immunity and probiotics in fish

Teleost mucosal immunity has become the subject of unprecedented research studies in recent years because of its diversity and defining characteristics. Its immune repertoire is governed by the mucosa-associated lymphoid tissues (MALT) which are divided into gut-associated lymphoid tissues (GALT), skin-associated lymphoid tissues (SALT), and gill-associated lymphoid tissues (GIALT). The direct contact with its immediate environment makes the mucosal surfaces of fish susceptible to a wide variety of pathogens. The inherent immunocompetent cells and factors in the mucosal surfaces together with the commensal microbiota have pivotal role against pathogens. Immunomodulation is a popular prophylactic strategy in teleost and probiotics possess this beneficial feature. Most of the studies on the immunomodulatory properties of probiotics in fish mainly discussed their impacts on systemic immunity. In contrast, few of these studies discussed the immunomodulatory features of probiotics in mucosal surfaces and are concentrated on the influences in the gut. Significant attention should be devoted in understanding the relationship of mucosal immunity and probiotics as the present knowledge is limited and are mostly based on extrapolations of studies in humans and terrestrial vertebrates. In the course of the advancement of mucosal immunity and probiotics, new perspectives in probiotics research, e.g., probiogenomics have emerged. This review affirms the relevance of probiotics in the mucosal immunity of fish by revisiting and bridging the current knowledge on teleost mucosal immunity, mucosal microbiota and immunomodulation of mucosal surfaces by probiotics. Expanding the knowledge of immunomodulatory properties of probiotics especially on mucosal immunity is essential in advancing the use of probiotics as a sustainable and viable strategy for successful fish husbandry.

Transcriptional response of immune genes in gills and the interbranchial lymphoid tissue of Atlantic salmon challenged with infectious salmon anaemia virus

Previously, it has been assumed that fish lack organized mucosa-associated lymphoid structures. Recently, an interbranchial lymphoid tissue (ILT) was described in salmonid gills at a site with substantial exposure to antigen. In this study, immune responses were examined in gills, mid-kidney and the laser-dissected ILT of Atlantic salmon (Salmo salar L.) infected with infectious salmon anaemia virus (ISAV). A strong innate response was observed in gills and mid-kidney and even in the laser-dissected ILT, despite the fact that no virus could be traced in this tissue. A small delayed increase in IgT transcripts, exclusively in the ILT, could indicate that this tissue has a role as a secondary lymphoid organ with clonal expansion of IgT expressing B-cells. Compared to the other examined tissues, gills displayed the earliest replication of the virus, further supporting this tissue as the main entry route for infection with ISAV.

The effect of TLR9 agonist CpG oligodeoxynucleotides on the intestinal immune response of cobia (Rachycentron canadum)

Cytosine-guanine oligodeoxynucleotide (CpG ODN) motifs of bacterial DNA are recognized through toll-like receptor 9 (TLR9) and are potent activators of innate immunity. However, the interaction between TLR9 and CpG ODN in aquatic species has not been well characterized. Hence, cobia TLR9 isoform B (RCTLR9B) was cloned and its expression and induction in intestine were investigated. RCTLR9B cDNA consists of 3113bp encoding 1009 amino acids containing three regions, leucine rich repeats, transmembrane domain, and toll/interleukin-1 receptor (TIR) domain. Intraperitoneal injection of CpG ODN 2395 upregulated RCTLR9 A and B and MyD88 and also induced the expressions of Mx, chemokine CC, and interleukin IL-1β. Cobia intraperitoneally injected with CpG ODN 1668 and 2395 had increased survival rates after challenge with Photobacterium damselae subsp. piscicida. In addition, formulation of CpG ODN with formalin-killed bacteria (FKB) and aluminum hydroxide gel significantly increased expressions of RCTLR9 A (50 folds) and B (30 folds) isoforms at 10 dpi (CpG ODN 1668) and MyD88 (21 folds) at 6 dpv (CpG ODN 2395). Subsequently, IL-1β increased at 6 dpv in 1668 group. No histopathological damage and inflammatory responses were observed in the injected cobia. Altogether, these results facilitate CpG ODNs as an adjuvant to increase bacterial disease resistance and efficacy of vaccines in cobia.

Modulation of genes related to the recruitment of immune cells in the digestive tract of trout experimentally infected with infectious pancreatic necrosis virus (IPNV) or orally vaccinated

There are still many details of how intestinal immunity is regulated that remain unsolved in teleost. Although leukocytes are present all along the digestive tract, most immunological studies have focused on the posterior segments and the importance of each gut segment in terms of immunity has barely been addressed. In the current work, we have studied the regulation of several immune genes along five segments of the rainbow trout (Oncorhynchus mykiss) digestive tract, comparing the effects observed in response to an infectious pancreatic necrosis virus (IPNV) infection to those elicited by oral vaccination with a plasmid coding for viral VP2. We have focused on the regulation of several mucosal chemokines, chemokine receptors, the major histocompatibility complex II (MHC-II) and tumor necrosis factor α (TNF-α). Furthermore, the recruitment of IgM(+) cells and CD3(+) cells was evaluated along the different segments in response to IPNV by immunohistochemical techniques. Our results provide evidences that there is a differential regulation of these immune genes in response to both stimuli along the gut segments. Along with this chemokine and chemokine receptor induction, IPNV provoked a mobilization of IgM(+) and IgT(+) cells to the foregut and pyloric caeca region, and CD3(+) cells to the pyloric caeca and midgut/hindgut regions. Our results will contribute to a better understanding of how mucosal immunity is orchestrated in the different gut segments of teleost.

Antiviral functions of CD8(+) cytotoxic T cells in teleost fish

Cytotoxic T-cells (CTLs) play a pivotal role in eliminating viruses in mammalian adaptive immune system. Many recent studies on T-cell immunity of fish have suggested that teleost CTLs are also important for antiviral immunity. Cellular functional studies using clonal ginbuan crucian carp and rainbow trout have provided in vivo and in vitro evidence that in many respects, virus-specific CTLs of fish have functions similar to those of mammalian CTLs. In addition, mRNA expression profiles of CTL-related molecules, such as CD8, TCR and MHC class I, have shown that in a wide range of fish species, CTLs are involved in antiviral adaptive immunity. These findings are a basis to formulate possible vaccination strategies to trigger effective antiviral CTL responses in teleost fish. This review describes recent advances in our understanding of antiviral CTL functions in teleost fish and discusses vaccination strategies for efficiently inducing CTL activities.

Food pellets as an effective delivery method for a DNA vaccine against infectious pancreatic necrosis virus in rainbow trout (Oncorhynchus mykiss, Walbaum)

A DNA vaccine based on the VP2 gene of infectious pancreatic necrosis virus (IPNV) was incorporated into feed to evaluate the effectiveness of this oral delivery method in rainbow trout. Lyophilized alginate-plasmid complexes were added to feed dissolved in water and the mixture was then lyophilized again. We compared rainbow trout that were fed for 3 consecutive days with vaccine pellets with fish that received the empty plasmid or a commercial pellet. VP2 gene expression could be detected in tissues of different organs in the rainbow trout that received the pcDNA-VP2 coated feed (kidney, spleen, gut and gill) throughout the 15 day time-course of the experiments. This pcDNA-VP2 vaccine clearly induced an innate and specific immune-response, significantly up-regulating IFN-1, IFN-γ, Mx-1, IL8, IL12, IgM and IgT expression. Strong protection, with relative survival rates of 78%-85.9% were recorded in the vaccinated trout, which produced detectable levels of anti-IPNV neutralizing antibodies during 90 days at least. Indeed, IPNV replication was significantly down-regulated in the vaccinated fish 45 days pi.

Th17-like immune response in fish mucosal tissues after administration of live attenuated Vibrio anguillarum via different vaccination routes

This study aimed to investigate the protective mucosal immunity elicited by live attenuated Vibrio anguillarum in fish. Zebrafish were immunized by bath or injection way, and undertook bath challenge at 28 days post vaccination. The results implied that bath vaccination was the better delivery route for inducing the protective immunity against bath challenge in zebrafish. The expressions of genes related to Th1, Th2 and Th17 cells were measured in the mucosal tissues of vaccinated and challenged zebrafish. Gene expression profiles showed that Th17-like responses were induced in mucosal immune system by vaccination via bath and injection routes while Th1 and Th2-like responses were not remarkable. Compared to injection vaccination, bath vaccination elicited the intense Th17-like immune responses in the gut tissue of zebrafish. Additionally, in gills and skin, Th17-like mucosal immunity elicited by injection vaccination occurred later than that by bath vaccination. Our results proved the immunological importance of gut in bath vaccination and the presence of two-compartmental model for immune response in zebrafish. In conclusion, bath vaccination more efficiently elicited protective Th17-like immunity than injection vaccination in mucosal tissues of vaccinated zebrafish. In turbot, effective immune protection against wild-type V. anguillarum was obtained by bath-vaccinated and the Th17-like responses were found in mucosal and systemic tissues.

A short history of research on immunity to infectious diseases in fish

This review describes the history of research on immunity to infectious diseases of fish in the period between 1965 and today. Special attention is paid to those studies, which are dealing with the interaction between immune system and invading pathogens in bony fish. Moreover, additional biographic information will be provided of people involved. In the 1960s and 1970s the focus of most studies was on humoral (Ig, B-cell) responses. Thorough studies on specific cellular (T-cell) responses and innate immunity (lectins, lysozyme, interferon, phagocytic cells) became available later. In the period between 1980 and today an overwhelming amount of data on regulation (e.g. cell cooperation, cytokines) and cell surface receptors (e.g. T-cell receptor; MHC) was published. It became also clear, that innate responses were often interacting with the acquired immune responses. Fish turned out to be vertebrates like all others with a sophisticated immune system showing specificity and memory. These basic data on the immune system could be applied in vaccination or in selection of disease resistant fish. Successful vaccines against bacterial diseases became available in the 1970s and 1980s. Effective anti-viral vaccines appeared from the 1980s onwards. There is no doubt, that Fish Immunology has become a flourishing science by the end of the 20th century and has contributed to our understanding of fish diseases as well as the success of aquaculture.

Interleukin gene expression is strongly modulated at the local level in a fish-parasite model

The goal of this work was to identify interleukin (IL)-related genes in the gilthead sea bream (GSB) (Sparus aurata L.) and how they are modulated by the parasite Enteromyxum leei, a myxozoan that causes severe enteritis with a strong inflammatory response. A Blast-X search of our transcriptomic GSB database (www.nutrigroup-iats.org/seabreamdb) identified 16 new sequences encompassing seven ILs (IL-7, IL-8, IL-10, IL-12β, IL-15, IL-18, and IL-34), the interleukin enhancer-binding factor 2 (ILF2), and eight IL receptors (IL-R); IL-R1, IL-6RA, IL-6RB, IL-8RA, IL-10RA, IL-10RB, IL-18R1, and IL-22R. Except for ILF2, their expression, plus that of IL-1β, IL-1R2, IL-6, and TNF-α (from public repositories), were analysed by 96-well PCR array of samples of blood, spleen, head kidney, and intestine of GSB that were anally intubated with E. leei (recipient group, RCPT). Only the expression profile of the intestine of RCPT fish showed significant difference as compared to samples from PBS-inoculated fish. At 17 days post intubation (dpi), the expression of key pro-inflammatory ILs, such as IL-8, IL-8R, IL-12β, and TNFα was significantly up-regulated, whereas at 64 dpi, anti-inflammatory IL expression (IL-6, IL-6RB, IL-7, IL-10, IL-10RA, and IL-15) was predominant. These results indicate a modification of the IL expression at late times post infection, probably to protect the fish intestine from the parasite and damage inflicted by an excessive inflammatory response. Furthermore, the response is mainly mediated at the local level as no significant changes were detected in blood, spleen and head kidney.

Substitution of dietary fish oil with plant oils is associated with shortened mid intestinal folds in Atlantic salmon (Salmo salar)

Background

Fish meal and fish oil are increasingly replaced by ingredients from terrestrial sources in the feeds for farmed salmonids due to expanding production and reduced availability of marine feed raw material. Fish oil that is rich in n-3 polyunsaturated fatty acids is considered beneficial to human health in general and to prevent intestinal inflammation and carcinogenesis in particular. In contrast, n-6 fatty acids that are present in many vegetable oils have been associated with increased risk of colitis and colon cancer in rodents and humans, as well as lowered transcription levels of certain stress and antioxidant-related genes in Atlantic salmon.

The aim of the present study was to investigate the intestinal health in Atlantic salmon fed with different vegetable oils as partial substitutes of fish oil in the diet. A feed trial lasting for 28 weeks included one reference diet containing fish oil as the sole lipid source and three diets where 80% of the fish oil was replaced by a plant oil blend with either olive oil, rapeseed oil or soybean oil as the main lipid source. These plant oils have intermediate or low n-3/n-6-ratios compared to fish oil having a high n-3/n-6-ratio. The protein and carbohydrate fractions were identical in all the feeds.

Results

Morphometric measurements showed significantly shorter folds in the mid intestine in all groups fed vegetable oils compared to the group fed fish oil. In the distal intestine, the complex folds were significantly shorter in the fish fed soybean oil compared to the fish fed rapeseed oil. Histological and immunohistochemical examination did not show clear difference in the degree of inflammation or proliferation of epithelial cells related to dietary groups, which was further confirmed by real-time RT-PCR which revealed only moderate alterations in the mRNA transcript levels of selected immune-related genes.

Conclusions

Shortened intestinal folds might be associated with reduced intestinal surface and impaired nutrient absorption and growth, but our results suggest that partial substitution of dietary fish oil with vegetable oils does not have any major negative impact on the intestinal health of Atlantic salmon.

The influence of probiotics on zebrafish Danio rerio innate immunity and hepatic stress

In this study, the effects of probiotic administration on zebrafish Danio rerio intestinal innate immunity and hepatic stress were evaluated. Zebrafish adults were treated for 10 days with the probiotic Lactobacillus rhamnosus IMC 501(®). To assess the effects at the molecular level, the mRNA levels of genes involved in the innate immune system, stress response, oxidative stress, and apoptosis were quantified by real-time polymerase chain reaction. An increase of biomarkers related to innate immune responses was observed in intestinal tissue from the probiotic-treated fish compared with the control fish. In addition, a decrease in the abundance of stress and apoptotic-related genes was observed in the liver of the probiotic-fed fish. Finally, imaging Fourier transform infrared analysis was conducted on liver sections and the data obtained confirmed that probiotic administration decreased oxidative stress levels, decreased DNA damage, and increased lipid saturation levels. Overall, the results show that probiotic administration may enhance zebrafish welfare by modulating the innate immune response and improving hepatic stress tolerance.

Comparative skin mucus and serum humoral defence mechanisms in the teleost gilthead seabream (Sparus aurata)

Mucosal surfaces of fish, including skin, gill and gut, contain numerous immune substances poorly studied that act as the first line of defence against a broad spectrum of pathogens. This study aimed to identify and characterize for the first time different constitutive humoral defence mechanisms of the skin mucus of gilthead seabream (Sparus aurata). To do this, the levels of total immunoglobulin M, several enzymes and proteins (peroxidase, lysozyme, alkaline phosphatase, esterases, proteases and antiproteases), as well as the bactericidal activity against opportunist fish pathogens (Vibrio harveyi, Vibrio angillarum, Photobacterium damselae) and non-pathogenic bacteria (Escherichia coli, Bacillus subtilis) were measured in the skin mucus and compared with those found in the serum. This study demonstrates that gilthead seabream skin mucus contains lower levels of IgM, similar levels of lysozyme, alkaline phosphatase and proteases, and higher esterase, peroxidase and antiprotease activities than serum. In addition, skin mucus revealed stronger bactericidal activity against tested fish pathogen bacteria compared to the serum activity, while human bacteria can even grow more in the presence of mucus. The results could be useful for better understanding the role of the skin mucus as a key component of the innate immune system with potential application for the aquaculture.

Effect of the dietary inclusion of soybean components on the innate immune system in zebrafish

Some components of plant-based meals, such as saponins and vegetal proteins, have been proposed as inducers of intestinal inflammation in some fish. However, the molecular and cellular bases for this phenomenon have not been reported. In this work, zebrafish were used as a model to evaluate the effects of individual soybean meal components, such as saponins and soy proteins. Zebrafish larvae fed a fish meal feed containing soy components were assessed according to low and high inclusion levels. The granulocytes associated with the digestive tract and the induction of genes related to the immune system were quantitated as markers of the effects of the dietary components. A significant increase in the number of granulocytes was observed after feeding fish diets containing high saponin or soy protein contents. These dietary components also induced the expression of genes related to the innate immune system, including myeloid-specific peroxidase, as well as the complement protein and cytokines. These results reveal the influence of dietary components on the stimulation of the immune system. These observations could be significant to understanding the contributions of saponin and soy protein to the onset of enteritis in aqua-cultured fish, and this knowledge may aid in defining the role of the innate immune system in other inflammatory diseases involving dietary components in mammals.

Transcriptional heterogeneity of IgM+ cells in rainbow trout (Oncorhynchus mykiss) tissues

Two major classes of B lymphocytes have been described to date in rainbow trout: IgM⁺ and IgT⁺ cells. IgM⁺ cells are mainly localized in the spleen, peripheral blood and kidney but are also found in other tissues. However, differences among IgM⁺ cell populations attending to its location are poorly defined in fish. Thus, the aim of this work was to characterize the expression of different immune molecules such as chemokine receptors, Toll-like receptors (TLRs) and transcription factors on sorted IgM⁺ lymphocytes from different rainbow trout tissues. IgM⁺ populations from blood, spleen, kidney, gills, intestine and liver were isolated by cell sorting and the constitutive levels of transcription of these genes evaluated by real-time PCR. To further characterize B cells, we identified an MS4A sequence. In humans, the MS4A family includes several genes with immune functions, such as the B cell marker CD20 or FcRβ. Subsequently, we have also evaluated the mRNA levels of this MS4A gene in the different IgM⁺ populations. The relevant differences in transcriptional patterns observed for each of these IgM⁺ populations analyzed, point to the presence of functionally different tissue-specific B cell populations in rainbow trout. The data shown provides a pattern of genes transcribed in IgM⁺ B cells not previously revealed in teleost fish. Furthermore, the constitutive expression of all the TLR genes analyzed in IgM⁺ cells suggests an important role for these cells in innate immunity.

The mucosal immune system of fish: the evolution of tolerating commensals while fighting pathogens

The field of mucosal immunology research has grown fast over the past few years, and our understanding on how mucosal surfaces respond to complex antigenic cocktails is expanding tremendously. With the advent of new molecular sequencing techniques, it is easier to understand how the immune system of vertebrates is, to a great extent, orchestrated by the complex microbial communities that live in symbiosis with their hosts. The commensal microbiota is now seen as the "extended self" by many scientists. Similarly, fish immunologist are devoting important research efforts to the field of mucosal immunity and commensals. Recent breakthroughs on our understanding of mucosal immune responses in teleost fish open up the potential of teleosts as animal research models for the study of human mucosal diseases. Additionally, this new knowledge places immunologists in a better position to specifically target the fish mucosal immune system while rationally designing mucosal vaccines and other immunotherapies. In this review, an updated view on how teleost skin, gills and gut immune cells and molecules, function in response to pathogens and commensals is provided. Finally, some of the future avenues that the field of fish mucosal immunity may follow in the next years are highlighted.

Modulation of leukocytic populations of gilthead sea bream (Sparus aurata) by the intestinal parasite Enteromyxum leei (Myxozoa: Myxosporea)

The cellular mucosal and systemic effectors of gilthead sea bream (GSB) (Sparus aurata) involved in the acute immune response to the intestinal parasite Enteromyxum leei were studied in fish experimentally infected by the anal route. In the intestinal inflammatory infiltrates and in lymphohaematopoietic organs (head kidney and spleen) of parasitized fish, the number of plasma cells, B cells (IgM immunoreactive) and mast cells (histamine immunoreactive) were significantly higher, whereas the number of acidophilic granulocytes (G7 immunoreactive) decreased, compared with non-parasitized and unexposed fish. These differences were stronger at the posterior intestine, the main target of the parasite, and no differences were found in the thymus. In non-parasitized GSB, the percentage of splenic surface occupied by melanomacrophage centres was significantly higher. These results suggest that the cellular response of GSB to E. leei includes proliferation of leukocytes in lymphohaematopoietic organs and recruitment into intestines via blood circulation involving elements of innate and adaptive immunity. Acidophilic granulocytes and mast cells presented opposite patterns of response to the parasite infection, with an overall depletion of the former and an increased amount of the latter. Some differences between both cell types were also detected in regard to their granule density and cell morphology.

Piscidins in the intestine of European perch, Perca fluviatilis, naturally infected with an enteric worm

This study set out to determine how an enteric parasite, the thorny-headed worm Acanthocephalus lucii, affected the expression of antimicrobial peptides (piscidins) in its host population, the European perch (Perca fluviatilis) collected from Lake Piediluco in Central Italy. A total of 87 perch were examined; 44 (50.5%) were infected with A. lucii (1-18 worms fish(-1)). Pathological changes and immune response were assessed using histological, ultrastructural and immunohistochemical techniques. The acanthocephalans only penetrated the surficial zone of the intestinal wall and induced only slight inflammation. The main damage was destruction of the mucosal epithelium covering the villi adjacent to the parasite's attachment site, and included necrosis and degeneration. Infected intestine had numerous mast cells (MCs), often in close proximity to, and within, the capillaries, and were associated with fibroblasts of the submucosal layer. Mast cells were irregular in shape with a cytoplasm filled by numerous electron-dense, membrane-bounded granules. Immunostaining of intestine with antibodies against the antimicrobial peptides piscidin 3 and piscidin 4 showed subpopulations of MCs that were positive. Piscidin-positive MCs were mainly observed among the epithelial cells of the intestine, but also within the submucosa. In both uninfected and parasite-infected perch, the number of MCs positive for piscidin 4 was higher than those immunoreactive with piscidin 3 (p < 0.05). For both piscidins, there was no significant difference in the number of positive MCs between parasite-infected and uninfected intestine (p > 0.05). However, uninfected fish showed higher immunostaining intensity for piscidin 3 than infected conspecifics (p < 0.05).

Functional aspects of fish lymphocytes

After almost 40 years of studies in comparative immunology, some light has been shed on the evolutive immunobiology of vertebrates, and experimental evidences have shown that acquired immunity, defined by somatic recombination of antigen-binding molecules and memory, is an achievement as ancient as jawless vertebrates. However, the molecular processes generating antigen receptors evolved independently between jawless and jawed fishes, and produced lymphocytic cells with similar functions but employing different sets of genes. In recent years, data have been provided describing some in vitro and in vivo functional responses of fish lymphocytes. After a long gap, the number of specific markers for fish lymphocytes is increasing, thus allowing a first characterisation of lymphocyte subsets. Overall, in the near future it will be possible to open a new chapter in fish immunology and investigate functional immunity of lymphocyte responses by combining the extensive knowledge on immune gene products with markers for molecules and cells. The present review summarizes current knowledge on functional features of fish lymphocytes.

Short-term feed deprivation alters immune status of surface mucosa in channel catfish (Ictalurus punctatus)

Short-term feed deprivation (or fasting) is a common occurrence in aquacultured fish species whether due to season, production strategies, or disease. In channel catfish (Ictalurus punctatus) fasting impacts susceptibility to several bacterial pathogens including Flavobacterium columnare, the causative agent of columnaris disease. As columnaris gains entry through the gills and skin of fish, we examined here changes in transcriptional regulation induced in these surface mucosal tissues due to short-term (7 day) fasting. RNA-seq expression analysis revealed a total of 1,545 genes perturbed by fasting. Fasting significantly altered expression of critical innate immune factors in a manner consistent with lower immune fitness as well as dysregulating key genes involved in energy metabolism and cell cycling/proliferation. Downregulation of innate immune actors such as iNOS2b, Lysozyme C, and peptidoglycan recognition protein 6 is predicted to impact the delicate recognition/tolerance balance for commensal and pathogenic bacteria on the skin and gill. The highlighted expression profiles reveal potential mechanistic similarities between gut and surface mucosa and underscore the complex interrelationships between nutrition, mucosal integrity, and immunity in teleost fish.

Basal polarization of the mucosal compartment in Flavobacterium columnare susceptible and resistant channel catfish (Ictalurus punctatus)

The freshwater bacterial pathogen, Flavobacterium columnare, infects a variety of ornamental and farmed fish species worldwide through mucosal attachment points on the gill and skin. While previous studies have demonstrated a chemotactic response of F. columnare to fish mucus, little is known about how host gill mucosal molecular and cellular constituents may impact rates of adhesion, tissue invasion, and ultimately, mortality. Here, we describe the use of RNA-seq to profile gill expression differences between channel catfish (Ictalurus punctatus) differing in their susceptibility to F. columnare both basally (before infection) and at three early timepoints post-infection (1 h, 2 h, and 8 h). After sequencing and de novo assembly of over 350 million 100 base-pair transcript reads, between group comparisons revealed 1714 unique genes differentially expressed greater than 1.5-fold at one or more timepoints. In the large dataset, we focused our analysis on basal differential expression between resistant and susceptible catfish as these genes could potentially reveal genetic and/or environmental factors linked with differential rates of infection. A number of critical innate immune components including iNOS2b, lysozyme C, IL-8, and TNF-alpha were constitutively higher in resistant catfish gill, while susceptible fish showed high expression levels of secreted mucin forms, a rhamnose-binding lectin previously linked to susceptibility, and mucosal immune factors such as CD103 and IL-17. Taken together, the immune and mucin profiles obtained by RNA-seq suggest a basal polarization in the gill mucosa, with susceptible fish possessing a putative mucosecretory, toleragenic phenotype which may predispose them to F. columnare infection.