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

Mucosal immune responses of gut IgM in common carp (Cyprinus carpio) following infection with spring viremia of carp virus (SVCV)

Immunoglobulin M (IgM) specifically recognizes various antigens and can activate complement, mediate cytotoxicity, opsonize and agglutinate pathogens to induce phagocytosis, all of which play an important role in immunity. However, the IgM response of common carp (Cyprinus carpio) in the intestinal mucosa after viral infection has not been thoroughly. Therefore, we successfully produced an anti-carp IgM monoclonal antibody and developed a model of viral infection to study the kinetics of immune responses after viral infection. Our results showed that the expression of IL1-β and Igs were dramatically increased, implying that common carp exhibited a significant innate and adaptive immune response to viral infection. Furthermore, we found that the IgM responses varied between the two infection strategies. At 14 days post-infection (DPI), a significant population of IgM+ B cells were observed in the gut, accompanied by a sharp rise in IgM levels. The immune response to secondary infection started at 7 DPI, suggesting that the IgM response is faster in the gut after re-infection. Importantly, we also explored the variability of different gut compartments to viral infection, and result revealed a stronger immune response in the hindgut than in the foregut and midgut. Overall, our findings indicate that IgM plays an important role in the intestinal immune response following primary and secondary viral infection, in which the hindgut plays a major immune function.

The distribution and function of teleost IgT

Given the uniquely close relationship between fish and aquatic environments, fish mucosal tissues are constantly exposed to a wide array of pathogenic microorganisms in the surrounding water. To maintain mucosal homeostasis, fish have evolved a distinct mucosal immune system known as mucosal-associated lymphoid tissues (MALTs). These MALTs consist of key effector cells and molecules from the adaptive immune system, such as B cells and immunoglobulins (Igs), which play crucial roles in maintaining mucosal homeostasis and defending against external pathogen infections. Until recently, three primary Ig isotypes, IgM, IgD, and IgT, have been identified in varying proportions within the mucosal secretions of teleost fish. Similar to the role of mucosal IgA in mammals and birds, teleost IgT plays a predominant role in mucosal immunity. Following the identification of the IgT gene in 2005, significant advances have been made in researching the origin, evolution, structure, and function of teleost IgT. Multiple IgT variants have been identified in various species of teleost fish, underscoring the remarkable complexity of IgT in fish. Therefore, this study provides a comprehensive review of the recent advances in various aspects of teleost IgT, including its genomic and structural features, the diverse distribution patterns within various fish mucosal tissues (the skin, gills, gut, nasal, buccal, pharyngeal, and swim bladder mucosa), its interaction with mucosal symbiotic microorganisms, and its immune responses towards diverse pathogens, including bacteria, viruses, and parasites. We also highlight the existing research gaps in the study of teleost IgT, suggesting the need for further investigation into the functional aspects of IgT and IgT+ B cells. This research is aimed at providing valuable insights into the immune functions of IgT and the mechanisms underlying the immune responses of fish against infections.

CD4+ T lymphocyte responses to viruses and virus-relevant stimuli in teleost fish

Fish diseases caused by viruses are a major threat to aquaculture. Development of disease protection strategies for sustainable fish aquaculture requires a better understanding of the immune mechanisms involved in antiviral defence. The innate and adaptive arms of the vertebrate immune system collaborate to mount an effective defence against viral pathogens. The T lymphocyte components of the adaptive immune system, comprising two major classes (helper T, Th or CD4+ and cytotoxic T lymphocytes, CTLs or CD8+ T cells), are responsible for cell-mediated immune responses. In particular, CD4+ T cells and their different subsets orchestrate the actions of various other immune cells during immune responses, making CD4+ T cells central drivers of responses to pathogens and vaccines. CD4+ T cells are also present in teleost fish. Here we review the literature that reported the use of antibodies against CD4 in a few teleost fish species and transcription profiling of Th cell-relevant genes in the context of viral infections and virus-relevant immunomodulation. Studies reveal massive CD4+ T cell proliferation and expression of key cytokines, transcription factors, and effector molecules that evoke mammalian Th cell responses. We also discuss gaps in the current understanding and evaluation of teleost CD4+ T cell responses and how development and application of novel tools and approaches to interrogate such responses could bridge these gaps. A greater understanding of fish Th cell responses will further illuminate the evolution of vertebrate adaptive immunity, inform strategies to address viral infections in aquaculture, and could further foster fish as model organisms.

Histomorphometric evaluation of melanomacrophage centers (MMCs) and CD3+ T cells of two morphs of brown trout (Salmo trutta) fed diets with immunostimulants

The brown trout (Salmo trutta) is a salmonid residing in riverine and coastal waters throughout the Northern Hemisphere, whose various populations evolved into distinct ecological morphs, differing in their migratory tendencies and preferred habitats. Unfortunately, due to progressing degradation of natural environment, the conservation of these populations is of growing importance and is undoubtedly a challenging task. Therefore, various means to refine the preparatory protocols for restocking using hatchery-reared fish are being pursued, some of which involve the administration of immunity-boosting substances. The current study assessed the effects of two dietary immunostimulants: Bioimmuno (4% inosine pranobex and 96% β-glucan) and Focus Plus (commercial preparation by Biomar, Denmark) on two morphs of the brown trout - the river trout (S. trutta morpha fario) and the sea trout (S. trutta morpha trutta). Tissue samples were obtained from ∼75 to 100g fish after 0, 2 and 4 weeks of experimental feeding. Multi-factorial analysis of conducted histological measurements of melanomacrophage centers (MMCs) revealed no changes of their parameters within spleens, but showed a decrease of the occupied tissue area and MMC counts in the livers, progressing with time regardless of the applied diet. Immunohistochemical analysis of CD3+ T cells showed their increased recruitment into mucosal folds of pyloric caeca in the 2-week sampling of trouts fed with the diet with 2% Bioimmuno addition, but this effect was not present in the 4-week sampling. When studying all groups jointly within each morph, there was a significant difference in terms of maintained CD3+ T cells levels, as sea trouts showed significantly higher tissue areas occupied by these cells than river trouts, both in the pyloric caeca and hepatic parenchyma. The study revealed that feeding with a diet enriched with Bioimmuno for 2 weeks may be a favorable enhancement of rearing protocols of brown trout stocks prior to their release, but more studies need to be conducted to test the possibility of an even shorter feeding period.

Regulatory roles of cytokines in T and B lymphocytes-mediated immunity in teleost fish

T and B lymphocytes (T and B cells) are immune effector cells that play critical roles in adaptive immunity and defend against external pathogens in most vertebrates, including teleost fish. In mammals, the development and immune response of T and B cells is associated with cytokines including chemokines, interferons, interleukins, lymphokines, and tumor necrosis factors during pathogenic invasion or immunization. Given that teleost fish have evolved a similar adaptive immune system to mammals with T and B cells bearing unique receptors (B-cell receptors (BCRs) and T-cell receptors (TCRs)) and that cytokines in general have been identified, whether the regulatory roles of cytokines in T and B cell-mediated immunity are evolutionarily conserved between mammalians and teleost fish is a fascinating question. Thus, the purpose of this review is to summarize the current knowledge of teleost cytokines and T and B cells as well as the regulatory roles of cytokines on these two types of lymphocytes. This may provide important information on the parallelisms and dissimilarities of the functions of cytokines in bony fish versus higher vertebrates, which may aid in the evaluation and development of adaptive immunity-based vaccines or immunostimulants.

Immunity of the intestinal mucosa in teleost fish

The paper presents the problem of intestinal mucosa immunity in teleost fish. The immunity of the intestinal mucosa in teleost fish depends on the elements and mechanisms with different organizational/structural and functional properties than in mammals. The organization of the elements of intestinal mucosal immunitya in these animals is associated with the presence of immune cells that fulfil the functions assigned to the induction and effector sites of mucosal immunity in mammals; they are located at various histological sites of the mucosa - in the lamina propria (LP) and in the surface epithelium. The presence of mucosa-associated lymphoid tissue (MALT) has not been demonstrated in teleost fish, and the terminology used in relation to the structure and function of the mucosa immunity components in teleost fish is inadequate. In this article, we review the knowledge of intestinal mucosal immunity in teleost fish, with great potential for knowledge and practical applications especially in the field of epidemiological safety. We discuss the organization and functional properties of the elements that determine this immunity, according to current data and taking into account the tissue definition and terminology adopted by the Society for Mucosal Immunology General Assembly (13th ICMI in Tokyo, 2007).

Supplementation of Dietary Crude Lentinan Improves the Intestinal Microbiota and Immune Barrier in Rainbow Trout (Oncorhynchus mykiss) Infected by Infectious Hematopoietic Necrosis Virus

The effects of crude lentinan (CLNT) on the intestinal microbiota and the immune barrier were evaluated in rainbow trout (Oncorhynchus mykiss) infected by infectious hematopoietic necrosis virus (IHNV). The results showed that supplementary CLNT declined the rainbow trout mortality caused by IHNV, which suggested that CLNT has preventive effects on IHNV infection. IHNV destroyed intestinal integrity, as well as caused the intestinal oxidative and damage in rainbow trout. Supplementary CLNT significantly strengthened the intestinal immune barrier by declining intestinal permeability, as well as enhancing intestinal antioxidant and anti-inflammatory abilities in IHNV-infected rainbow trout (P<0.05). In addition, CLNT modified the aberrant changes of intestinal microbiota induced by IHNV, mainly represented by promoting the growths of Carnobacterium and Deefgea and inhibiting Mycobacterium and Nannocystis. Especially, supplementing with CLNT significantly promoted the growth of short-chain fatty acid–producing bacteria (P<0.05) and consequently increased the production of acetic acid, butanoic acid, and hexanoic acid in the intestine of IHNV-infected rainbow trout. Furthermore, it was speculated that CLNT could regulate the self-serving metabolic pathways of intestinal microbiota induced by IHNV, such as fatty acid metabolism and amino acid metabolism. Together, CLNT played the antiviral effects on IHNV infection through strengthening the intestinal immune barrier, as well as regulating intestinal microbiota and SCFA metabolism in rainbow trout. The present data revealed that CLNT exerted a promising prebiotic role in preventing the rainbow trout from IHNV infection.

Lympho-Hematopoietic Microenvironments and Fish Immune System

Simple Summary

Teleost fish, the most abundant group of vertebrates, represent an excellent tool to establish possible correlations between the histological organization of their lymphoid organs and their immunological capacities. This approach allows us to analyze embryonic and larval lymphopoiesis, the remarkable organization of the teleost thymus, the role of the kidney as a true equivalent of the lympho-hematopoietic bone marrow of higher vertebrates, the mechanisms of antigen trapping in both ellipsoids and the so-called melano-macrophage centers (MMCs) and their relation with the generation of memory and the lack of germinal centers, and the extended development of the lymphoid tissue associated to mucosae.

Abstract

In the last 50 years information on the fish immune system has increased importantly, particularly that on species of marked commercial interest (i.e., salmonids, cods, catfish, sea breams), that occupy a key position in the vertebrate phylogenetical tree (i.e., Agnatha, Chondrichtyes, lungfish) or represent consolidated experimental models, such as zebrafish or medaka. However, most obtained information was based on genetic sequence analysis with little or no information on the cellular basis of the immune responses. Although jawed fish contain a thymus and lympho-hematopoietic organs equivalents to mammalian bone marrow, few studies have accounted for the presumptive relationships between the organization of these cell microenvironments and the known immune capabilities of the fish immune system. In the current review, we analyze this topic providing information on: (1) The origins of T and B lymphopoiesis in Agnatha and jawed fish; (2) the remarkable organization of the thymus of teleost fish; (3) the occurrence of numerous, apparently unrelated organs housing lympho-hematopoietic progenitors and, presumably, B lymphopoiesis; (4) the existence of fish immunological memory in the absence of germinal centers.

Dietary Administration of Novel Multistrain Probiotics from Healthy Grouper Intestines Promotes the Intestinal Immune Response against NNV Infection

Epinephelus lanceolatus (giant grouper) is a high-value cultured species in the Asia-Pacific region. However, nervous necrosis virus (NNV) is an infectious viral disease that affects over 120 species of marine cultured species and causes high mortality, ranging from 90-100% in the grouper industry. Probiotics isolated from the intestines of healthy individuals have provided insight into novel approaches involved in the defense against viral pathogens. In this study, we isolated three strains of bacteria as candidate probiotics from healthy grouper intestines and a 28-day feeding trial was performed. At day 21, the nervous necrosis virus (NNV) challenge test was conducted for 7 days to evaluate the antiviral effect of candidate probiotics. The results showed that candidate probiotics could improve growth conditions, such as weight gain (WG) and specific growth rate (SGR), and increase the utilization of feed. Furthermore, the candidate probiotic mixture had the ability to protect against NNV, which could decrease the mortality rate by 100% in giant grouper after NNV challenge. Subsequently, we analyzed the mechanism of the candidate probiotic mixture's defense against NNV. A volcano plot revealed 203 (control vs. NNV), 126 (NNV vs. probiotics - NNV), and 5 (control vs. probiotics - NNV) differentially expressed transcripts in intestinal tissue. Moreover, principal components analysis (PCA) and cluster analysis heatmap showed large differences among the three groups. Functional pathway analysis showed that the candidate probiotic mixture could induce the innate and adaptive immunity of the host to defend against virus pathogens. Therefore, we hope that potential candidate probiotics could be successfully applied to the industry to achieve sustainable aquaculture.

Mucosal immunoglobulins of teleost fish: A decade of advances

Immunoglobulins (Igs) are complex glycoproteins that play critical functions in innate and adaptive immunity of all jawed vertebrates. Given the unique characteristics of mucosal barriers, secretory Igs (sIgs) have specialized to maintain homeostasis and keep pathogens at bay at mucosal tissues from fish to mammals. In teleost fish, the three main IgH isotypes, IgM, IgD and IgT/Z can be found in different proportions at the mucosal secretions of the skin, gills, gut, nasal, buccal, and pharyngeal mucosae. Similar to the role of mammalian IgA, IgT plays a predominant role in fish mucosal immunity. Recent studies in IgT have illuminated the primordial role of sIgs in both microbiota homeostasis and pathogen control at mucosal sites. Ten years ago, IgT was discovered to be an immunoglobulin class specialized in mucosal immunity. Aiming at this 10-year anniversary, the goal of this review is to summarize the current status of the field of fish Igs since that discovery, while identifying knowledge gaps and future avenues that will move the field forward in both basic and applied science areas.

Current use and development of fish vaccines in China

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

Differential immune gene response in gills, skin, and spleen of rainbow trout Oncorhynchus mykiss infected by Ichthyophthirius multifiliis

Infection of rainbow trout with the parasitic ciliate Ichthyopthirius multifiliis induces differential responses in gills, skin and spleen. A controlled experimental infection was performed and expression of immune-relevant genes in skin, gills, and spleen were recorded by qPCR at day 1 and 8 after parasite exposure. Infection induced a marked reaction involving regulation of innate and adaptive immune genes in rainbow trout at day 8 post-infection. The expression level of a total of 22 out of 24 investigated genes was significantly higher in gills compared to skin reflecting the more sensitive and delicate structure of gills. Especially pro-inflammatory cytokines IL-6, IL-17 C1, regulatory cytokines IL-4/13A, IL-10, TGFβ, complement factor C5, chemokines CK10, CK12, acute phase proteins (precerebellin, hepcidin) and immunoglobulins (IgM, IgT) displayed differential expression levels. The spleen, a central immune organ with no trace of the parasite, showed elevated expression of IgM, IgT, complement factor C5 and chemokine CK10 (compared to skin and gills directly exposed to the parasite), indicating an interaction between the infected surface sites and central immune organs. This communication could be mediated by chemokines CK10 and CK12 and cytokine IL-4/13A and may at least partly explain the establishment of a systemic response in rainbow trout against the parasite.

Immunity induced by recombinant attenuated IHNV (infectious hematopoietic necrosis virus)-GN438A expresses VP2 gene-encoded IPNV (infectious pancreatic necrosis virus) against both pathogens in rainbow trout

Infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV) are important pathogens in rainbow trout farming worldwide. Their co-infection is also common, which causes great economic loss in juvenile salmon species. Development of a universal virus vaccine providing broadly cross-protective immunity will be of great importance. In this study, we generated two recombinant (r) virus (rIHNV-N438A-ΔNV-EGFP and rIHNV-N438A-ΔNV-VP2) replacing the NV gene of the backbone of rIHNV at the single point mutation at residue 438 with an efficient green fluorescent protein (EGFP) reporter gene and antigenic VP2 gene of IPNV. Meanwhile, we tested their efficacy against the wild-type (wt) IHNV HLJ-09 virus and IPNV serotype Sp virus challenge. The relative per cent survival rates of two recombinant viruses against (wt) IHNV HLJ-09 virus challenge were 84.6% and 81.5%, respectively. Simultaneously, the relative per cent survival rate of rIHNV-N438A-ΔNV-VP2 against IPNV serotype Sp virus challenge was 88.9%. It showed the two recombinant viruses had high protection rates and induced a high level of antibodies against IHNV or IPNV. Taken together, these results suggest the VP2 gene of IPNV can act as candidate gene for vaccine and attenuated multivalent live vaccines and molecular marker vaccines have potential application for viral vaccine.

Generation of virus-specific CD8+ T cells by vaccination with inactivated virus in the intestine of ginbuna crucian carp

Although a previous study using ginbuna crucian carp suggested that cell-mediated immunity can be induced by the oral administration of inactivated viruses, which are exogenous antigens, there is no direct evidence that CD8⁺ cytotoxic T cells (CTLs) in teleost fish are generated by vaccination with exogenous antigens. In the present study, we investigated whether antigen-specific CD8⁺ CTLs in ginbuna crucian carp can be elicited by intestinal immunization with an exogenous antigen without any adjuvant. The IFNγ-1 and T-bet mRNA expressions were up-regulated in intestinal leukocytes following the administration of formalin-inactivated crucian hematopoietic necrosis virus (FI-CHNV), whereas the down-regulation of these genes was observed in kidney leukocytes. Furthermore, an increase in the percentage of proliferating CD8⁺ cells was detected in the posterior portion of the hindgut, suggesting that the virus-specific CTLs are locally generated in this site. In addition, cell-mediated cytotoxicity against CHNV-infected syngeneic cells and the in vivo inhibition of viral replication were induced by immunization with FI-CHNV. Unexpectedly, intraperitoneal immunization with FI-CHNV induced a type I helper T cell (Th1)-response in the intestine, but not in the kidney; however, its effect was slightly lower than that reported after intestinal immunization. These findings suggest that the posterior portion of the intestine is an important site for generating virus-specific CTLs by vaccination with the inactivated vaccine.

Local regulation of immune genes in rainbow trout (Oncorhynchus mykiss) naturally infected with Flavobacterium psychrophilum

Flavobacterium psychrophilum is the etiological agent of bacterial cold water disease (BCWD), also referred to as rainbow trout fry syndrome (RTFS), a disease with great economic impact in salmonid aquaculture. Despite this, to date, not many studies have analyzed in depth how the immune system is regulated during the course of the disease. In the current study, we have studied the transcription of several immune genes related to T and B cell activity in the skin of rainbow trout (Oncorhynchus mykiss) naturally infected with F. psychrophilum in a farm located in Lake Titicaca (Peru). The levels of expression of these genes were tested and compared to those obtained in asymptomatic and apparently healthy rainbow trout. In the case of symptomatic fish, skin samples containing characteristic ulcerative lesions were taken, as well as skin samples with no lesions. Our results pointed to a significant local up-regulation of IgD, CD4, CD8, perforin and IFNγ within the ulcerative lesions. On the other hand, no differences between the levels of expression of these genes were visible in the spleen. To confirm these results, the distribution of IgD⁺ and CD3⁺ cells was studied through immunohistochemical techniques in the ulcerative lesions. Our results demonstrate a strong local response to F. psychrophilum in rainbow trout in which IgD and T cells seem to play a major role.

Transcriptomic analysis of immunity in rainbow trout (Oncorhynchus mykiss) gills infected by Ichthyophthirius multifiliis

The parasite Ichthyophthirius multifiliis infecting skin, fins and gills of a wide range of freshwater fish species, including rainbow trout, is known to induce a protective immune response in the host. Although a number of studies have reported activation of several immune genes in infected fish host, the immune response picture is still considered incomplete. In order to address this issue, a comparative transcriptomic analysis was performed on infected versus uninfected rainbow trout gills and it showed that a total of 3352 (7.2%) out of 46,585 identified gene sequences were significantly regulated after parasite infection. Of differentially expressed gene sequences, 1796 genes were up-regulated and 1556 genes were down-regulated. These were classified into 61 Gene Ontology (GO) terms and mapped to 282 reference canonical pathways in the Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Infection of I. multifiliis induced a clear differential expression of immune genes, related to both innate and adaptive immunity. A total of 268 (6.86%) regulated gene sequences were known to take part in 16 immune-related pathways. These involved pathways related to the innate immunity such as the Chemokine signaling pathway, Platelet activation, Toll-like receptor signaling pathway, NOD-like receptor signaling pathway, and Leukocyte transendothelial migration. Elevated transcription of genes encoding the TLR 8 gene and chemokines (CCL4, CCL19, CCL28, CXCL8, CXCL11, CXCL13, CXCL14) was recorded indicating their roles in recognition of I. multifiliis and subsequent induction of the inflammatory response, respectively. A number of upregulated genes in infected gills were associated with antigen processing/presentation and T and B cell receptor signaling (including B cell marker CD22 involved in B cell development). Overall the analysis supports the notion that I. multifiliis induces a massive and varied innate response upon which a range of adaptive immune responses are established which may contribute to the long lasting protection of immunized rainbow trout.

Pichia pastoris yeast as a vehicle for oral vaccination of larval and adult teleosts

Oral vaccination is of major interest because it can be used for mass vaccination of fish of various size and age. Given that their administration is relatively easy and stress-free, oral vaccines have both economic and animal welfare benefits. Yet, mostly due to their limited efficacy, only very few oral vaccines are available to aquaculture industry. Here we present a method for oral vaccine delivery based on the yeast Pichia pastoris. We could express a model antigen, green fluorescent protein (GFP), in this yeast and subsequently show delivery of the GFP protein to the intestine of juvenile flounder or adult carp and trout. We tested this approach in several commercially-relevant fish species, from juvenile to adult stage. To test the oral delivery of antigen to larval fish, the GFP-expressing Pichia pastoris was first fed to planktonic crustacean Daphnia or rotifers that served as 'bioencapsulation vehicles' and afterwards, fed to flounder larvae. Again, we could show delivery of intact GFP protein to the intestine. In rainbow trout, the orally-administered GFP-expressing yeast elicited a rapid local innate immune response in the intestine and a subsequent systemic response in the spleen. Our results show that Pichia pastoris is a good vehicle for oral antigen delivery and that it can be used in non-encapsulated form for older fish or in bioencapsulated form for larval fish. We discuss the immunomodulatory properties of the yeast itself, and its potential to enhance local immune responses and act as an adjuvant.

CK11, a Teleost Chemokine with a Potent Antimicrobial Activity

CK11 is a rainbow trout (Oncorhynchus mykiss) CC chemokine phylogenetically related to both mammalian CCL27 and CCL28 chemokines, strongly transcribed in skin and gills in homeostasis, for which an immune role had not been reported to date. In the current study, we have demonstrated that CK11 is not chemotactic for unstimulated leukocyte populations from central immune organs or mucosal tissues but instead exerts a potent antimicrobial activity against a wide range of rainbow trout pathogens. Our results show that CK11 strongly inhibits the growth of different rainbow trout Gram-positive and Gram-negative bacteria, namely Lactococcus garvieae, Aeromonas salmonicida subsp. salmonicida, and Yersinia ruckeri and a parasitic ciliate Ichthyophthirius multifiliis Similarly to mammalian chemokines and antimicrobial peptides, CK11 exerted its antimicrobial activity, rapidly inducing membrane permeability in the target pathogens. Further transcriptional studies confirmed the regulation of CK11 transcription in response to exposure to some of these pathogens in specific conditions. Altogether, our studies related to phylogenetic relations, tissue distribution, and biological activity point to CK11 as a potential common ancestor of mammalian CCL27 and CCL28. To our knowledge, this study constitutes the first report of a fish chemokine with antimicrobial activity, thus establishing a novel role for teleost chemokines in antimicrobial immunity that supports an evolutionary relationship between chemokines and antimicrobial peptides.

Gene expression analysis of isolated salmonid GALT leucocytes in response to PAMPs and recombinant cytokines

Increased knowledge of the immune response of the intestine, a physiologically critical organ involved in absorption, secretion and homeostasis in a non-sterile environment, is needed to better understand the mechanisms involved in the induction of long-lasting immunity and, subsequently, the development of efficacious gastrointestinal immunization approaches. To this end, analysis of isolated gut cells will give an insight into the cell types present and their immune capability. Hence, in this study we first optimised a method for salmonid gut leucocyte isolation and characterised the cells on the basis of their expression of a range of selected cell markers associated with T & B cells and dendritic cells. The GALT leucocytes were then stimulated with a variety of PAMPs, recombinant cytokines and PHA, as a means to help characterise the diversity of the immune repertoire present in such cells. The stimulants tested were designed to examine the nature of the antibacterial, antiviral and T cell type responses in the cells (at the transcript level) using a panel of genes relevant to innate and adaptive immunity. The results showed distinct responses to the stimulants, with a clear delineation seen between the stimulant used (eg viral or bacterial PAMP) and the pathway elicited. The changes in the expression patterns of the immune genes in these cells indicates that the salmonid intestine contains a good repertoire of competent immune cells able to respond to different pathogen types. Such information may aid the development of efficient priming by oral vaccination in salmonids.

Pike intestinal reaction to Acanthocephalus lucii (Acanthocephala): immunohistochemical and ultrastructural surveys

Background

The Northern pike, Esox lucius, is a large, long-lived, top-predator fish species and occupies a broad range of aquatic environments. This species is on its way to becoming an important model organism and has the potential to contribute new knowledge and a better understanding of ecology and evolutionary biology. Very few studies have been done on the intestinal pathology of pike infected with helminths. The present study details the first Italian record of adult Acanthocephalus lucii reported in the intestine of E. lucius.

Results

A total of 22 pike from Lake Piediluco (Central Italy) were examined, of which 16 (72.7%) were infected with A. lucii. The most affected areas of gastrointestinal tract were the medium and distal intestine. The intensity of infection ranged from 1 to 18 parasites per host. Acanthocephalus lucii penetrated mucosal and submucosal layers which had a high number of mast cells (MCs) with an intense degranulation. The cellular elements involved in the immune response within the intestine of pike were assessed by ultrastructural techniques and immunohistochemistry using antibodies against met-enkephalin, immunoglobulin E (IgE)-like receptor (FCεRIγ), histamine, interleukin-6, interleukin-1β, substance P, lysozyme, serotonin, inducible-nitric oxide synthase (i-NOS), tumor necrosis factor-α (TNF-α) and the antimicrobial peptide piscidin 3 (P3). In intestines of the pike, several MCs were immunopositive to 9 out of the 11 aforementioned antibodies and infected fish had a higher number of positive MCs when compared to uninfected fish.

Conclusions

Pike intestinal tissue response to A. lucii was documented. Numerous MCs were seen throughout the mucosa and submucosal layers. In infected and uninfected intestines of pike, MCs were the dominant immune cell type encountered; they are the most common granulocyte type involved in several fish-helminth systems. Immunopositivity of MCs to 9 out of 11 antibodies is of great interest and these cells could play an important key role in the host response to an enteric helminth. This is the first report of A. lucii in an Italian population of E. lucius and the first account on positivity of MCs to piscidin 3 and histamine in a non-perciform fish.

Evolution of lymphocytes. Immunoglobulin T of the teleost sea bass (Dicentrarchus labrax): Quantitation of gene expressing and immunoreactive cells

Immunoglobulin T (IgT) is one of the key effector molecules of jawed vertebrate's adaptive immune system, and in this work we describe the quantitative distribution of IgT-expressing and IgT-producing cells in tissues of the European seabass Dicentrarchus labrax by using mRNA riboprobes and a specific anti-IgT antibody. A polyclonal antiserum (pAb) was prepared by immunizing rabbits with three synthetic peptides deduced from the full length IgT cDNA sequence and located in a surface-exposed CH3 domain of IgT constant region. The obtained antiserum, named RAIgT1, was able to recognize by ELISA immunization antigens and IgT from intestinal mucus and serum. In western blots of head kidney leukocytes lysates the antiserum recognized a 180 kDa polypeptide in non-reducing, and a 75 kDa peptide in reducing conditions. Interestingly, the RAIgT1 pAb crossreacted intensely in western blots with rainbow trout IgT purified from mucus and serum. Antisense mRNA IgT oligonucleotide sequences were employed in in situ hybridization to detect IgT-expressing cells in sections from lymphoid tissues, and positive cells were observed in head kidney, spleen, intestine and gills. By employing RAIgT1 in quantitative immunohistochemistry, the highest number of IgT-producing cells was observed in the gills (9.5 ± 0.7%), followed by intestine (8.4 ± 1.2%), head kidney (6.2 ± 1.4%), and spleen (4.1 ± 0.7%). Interestingly, the number of IgT-B cells showed a regionalization in the intestine, increasing from the proximal to the terminal part. By immunofluorescence and flow cytometry of live leukocytes, the percentages of RAIgT1 stained cells were 34 ± 11% in the intestine, 22 ± 5% in head kidney, 16 ± 7% in spleen, and 9 ± 5% in gills. At the fluorescence microscope, live cells from these tissues showed a typical membrane-associated positivity and a lymphocytic morphology, and no IgT/IgM double positive cells were detected. Immunoreactive cells have been purified from head kidney using magnetic beads, and IgT-enriched cells showed by RT-PCR an enhanced expression of the IgT gene, whereas IgT-depleted cells had an highest expression of IgM and TRβ genes. These data describe for the first time a quantitative panel of IgT-expressing and IgT-immunoreactive cells in tissues of a teleost fish species.

Rainbow trout CK9, a CCL25-like ancient chemokine that attracts and regulates B cells and macrophages, the main antigen presenting cells in fish

CK9 is a rainbow trout (Oncorhynchus mykiss) CC chemokine phylogenetically related to mammalian CCL25. Although CK9 is known to be transcriptionally regulated in response to inflammation particularly in mucosal tissues, its functionality has never been revealed. In the current work, we have demonstrated that CK9 is chemoattractant for antigen presenting cells (APCs) expressing major histocompatibility complex class II (MHC II) on the cell surface. Among these APCs, CK9 has a strong chemotactic capacity for both B cells (IgM+ and IgT+) and macrophages. Along with its chemotactic capacities, CK9 modulated the MHC II turnover of B lymphocytes and up-regulated the phagocytic capacity of both IgM+ cells and macrophages. Although CK9 had no lymphoproliferative effects, it increased the survival of IgT+ lymphocytes. Furthermore, we have established that the chemoattractant capacity of CK9 is strongly increased after pre-incubation of leukocytes with a T-independent antigen, whereas B cell receptor (BCR) cross-linking strongly abrogated their capacity to migrate to CK9, indicating that CK9 preferentially attracts B cells at the steady state or under BCR-independent stimulation. These results point to CK9 being a key regulator of B lymphocyte trafficking in rainbow trout, able to modulate innate functions of teleost B lymphocytes and macrophages.

B cells and their role in the teleost gut

Mucosal surfaces are the main route of entry for pathogens in all living organisms. In the case of teleost fish, mucosal surfaces cover the vast majority of the animal. As these surfaces are in constant contact with the environment, fish are perpetually exposed to a vast number of pathogens. Despite the potential prevalence and variety of pathogens, mucosal surfaces are primarily populated by commensal non-pathogenic bacteria. Indeed, a fine balance between these two populations of microorganisms is crucial for animal survival. This equilibrium, controlled by the mucosal immune system, maintains homeostasis at mucosal tissues. Teleost fish possess a diffuse mucosa-associated immune system in the intestine, with B cells being one of the main responders. Immunoglobulins produced by these lymphocytes are a critical line of defense against pathogens and also prevent the entrance of commensal bacteria into the epithelium. In this review we will summarize recent literature regarding the role of B-lymphocytes and immunoglobulins in gut immunity in teleost fish, with specific focus on immunoglobulin isotypes and the microorganisms, pathogenic and non-pathogenic that interact with the immune system.

T cell immunity in the teleost digestive tract

Fish (along with cyclostomes) constitute the most ancient animal group in which an acquired immune system is present. As in higher vertebrates, both B and T lymphocytes cooperate in implementing an adequate response. Although there is still a debate on whether fish possess a true gut associated lymphoid tissue (GALT), the presence of diffuse B and T lymphocytes throughout all mucosal surfaces has been demonstrated in a wide variety of fish species. The lack of antibodies against T lymphocyte markers has hampered the performance of functional assays in both systemic and mucosal compartments. However, most components associated with T lymphocyte function have been identified in fish through extensive genomic research, suggesting similar functionalities for fish and mammalian T lymphocytes. Thus, the aim of this review is to briefly summarize what is known in teleost concerning the characteristics and functionalities of the different T cell subsets, to then focus on what is known to date regarding their presence and role in the gastrointestinal tract, through either direct functional assays or indirectly by conclusions drawn from transcriptomic analysis.

Teleost Chemokines and Their Receptors

Chemokines are a superfamily of cytokines that appeared about 650 million years ago, at the emergence of vertebrates, and are responsible for regulating cell migration under both inflammatory and physiological conditions. The first teleost chemokine gene was reported in rainbow trout in 1998. Since then, numerous chemokine genes have been identified in diverse fish species evidencing the great differences that exist among fish and mammalian chemokines, and within the different fish species, as a consequence of extensive intrachromosomal gene duplications and different infectious experiences. Subsequently, it has only been possible to establish clear homologies with mammalian chemokines in the case of some chemokines with well-conserved homeostatic roles, whereas the functionality of other chemokine genes will have to be independently addressed in each species. Despite this, functional studies have only been undertaken for a few of these chemokine genes. In this review, we describe the current state of knowledge of chemokine biology in teleost fish. We have mainly focused on those species for which more research efforts have been made in this subject, specifically zebrafish (Daniorerio), rainbow trout (Oncorhynchusmykiss) and catfish (Ictaluruspunctatus), outlining which genes have been identified thus far, highlighting the most important aspects of their expression regulation and addressing any known aspects of their biological role in immunity. Finally, we summarise what is known about the chemokine receptors in teleosts and provide some analysis using recently available data to help characterise them more clearly.

The Mucosal Immune System of Teleost Fish

Teleost fish possess an adaptive immune system associated with each of their mucosal body surfaces. Evidence obtained from mucosal vaccination and mucosal infection studies reveal that adaptive immune responses take place at the different mucosal surfaces of teleost. The main mucosa-associated lymphoid tissues (MALT) of teleosts are the gut-associated lymphoid tissue (GALT), skin-associated lymphoid tissue (SALT), the gill-associated lymphoid tissue (GIALT) and the recently discovered nasopharynx-associated lymphoid tissue (NALT). Teleost MALT includes diffuse B cells and T cells with specific phenotypes different from their systemic counterparts that have co-evolved to defend the microbe-rich mucosal environment. Both B and T cells respond to mucosal infection or vaccination. Specific antibody responses can be measured in the gills, gut and skin mucosal secretions of teleost fish following mucosal infection or vaccination. Rainbow trout studies have shown that IgT antibodies and IgT(+) B cells are the predominant B cell subset in all MALT and respond in a compartmentalized manner to mucosal infection. Our current knowledge on adaptive immunity in teleosts is limited compared to the mammalian literature. New research tools and in vivo models are currently being developed in order to help reveal the great intricacy of teleost mucosal adaptive immunity and help improve mucosal vaccination protocols for use in aquaculture.

Mucosal Immunity and B Cells in Teleosts: Effect of Vaccination and Stress

Fish are subjected to several insults from the environment, which may endanger animal survival. Mucosal surfaces are the first line of defense against these threats, acting as a physical barrier to protect the animal but also functioning as an active immune tissue. Thus, four mucosal-associated lymphoid tissues (MALTs), which lead the immune responses in gut, skin, gills, and nose, have been described in fish. Humoral and cellular immunity, as well as their regulation and the factors that influence the response in these mucosal lymphoid tissues, are still not well known in most fish species. Mucosal B-lymphocytes and immunoglobulins (Igs) are key players in the immune response that takes place in those MALTs. The existence of IgT as a mucosal specialized Ig gives us the opportunity of measuring specific responses after infection or vaccination, a fact that was not possible until recently in most fish species. The vaccination process is influenced by several factors, being stress one of the main stimuli determining the success of the vaccine. Thus, one of the major goals in a vaccination process is to avoid possible situations of stress, which might interfere with fish immune performance. However, interaction between immune and neuroendocrine systems at mucosal tissues is still unknown. In this review, we will summarize the latest findings about B-lymphocytes and Igs in mucosal immunity and the effect of stress and vaccination on B-cell response at mucosal sites. It is important to point out that a limited number of studies have been published regarding stress in mucosa and very few about the influence of stress over mucosal B-lymphocytes.

Early immune responses in rainbow trout liver upon viral hemorrhagic septicemia virus (VHSV) infection

Among the essential metabolic functions of the liver, in mammals, a role as mediator of systemic and local innate immunity has also been reported. Although the presence of an important leukocyte population in mammalian liver is well documented, the characterization of leukocyte populations in the teleost liver has been only scarcely addressed. In the current work, we have confirmed the presence of IgM⁺, IgD⁺, IgT⁺, CD8α⁺, CD3⁺ cells, and cells expressing major histocompatibility complex (MHC-II) in rainbow trout (Oncorhynchus mykiss) liver by flow cytometry and/or immunohistochemistry analysis. Additionally, the effect of viral hemorrhagic septicemia virus (VHSV) on the liver immune response was assessed. First, we studied the effect of viral intraperitoneal injection on the transcription of a wide selection of immune genes at days 1, 2 and 5 post-infection. These included a group of leukocyte markers genes, pattern recognition receptors (PRRs), chemokines, chemokine receptor genes, and other genes involved in the early immune response and in acute phase reaction. Our results indicate that T lymphocytes play a key role in the initial response to VHSV in the liver, since CD3, CD8, CD4, perforin, Mx and interferon (IFN) transcription levels were up-regulated in response to VHSV. Consequently, flow cytometry analysis of CD8α⁺ cells in liver and spleen at day 5 post-infection revealed a decrease in the number of CD8α⁺ cells in the spleen and an increased population in the liver. No differences were found however in the percentages of B lymphocyte (IgM⁺ or IgD⁺) populations. In addition, a strong up-regulation in the transcription levels of several PRRs and chemokines was observed from the second day of infection, indicating an important role of these factors in the response of the liver to viral infections.

Immunological characterization of the teleost adipose tissue and its modulation in response to viral infection and fat-content in the diet

The immune response of the adipose tissue (AT) has been neglected in most animal models until recently, when the observations made in human and mice linking obesity to chronic inflammation and diabetes highlighted an important immune component of this tissue. In the current study, we have immunologically characterized the AT for the first time in teleosts. We have analyzed the capacity of rainbow trout (Oncorhynchus mykiss) AT to produce different immune mediators and we have identified the presence of local populations of B lymphocytes expressing IgM, IgD or IgT, CD8α⁺ cells and cells expressing major histocompatibility complex II (MHC-II). Because trout AT retained antigens from the peritoneal cavity, we analyzed the effects of intraperitoneal infection with viral hemorrhagic septicemia virus (VHSV) on AT functionality. A wide range of secreted immune factors were modulated within the AT in response to VHSV. Furthermore, the viral infection provoked a significant decrease in the number of IgM⁺ cells which, along with an increased secretion of IgM in the tissue, suggested a differentiation of B cells into plasmablasts. The virus also increased the number of CD8α⁺ cells in the AT. Finally, when a fat-enriched diet was fed to the fish, a significant modulation of immune gene expression in the AT was also observed. Thus, we have demonstrated for the first time in teleost that the AT functions as a relevant immune tissue; responsive to peritoneal viral infections and that this immune response can be modulated by the fat-content in the diet.