Immunoglobulin producing cells in the spotted wolffish (Anarhichas minor Olafsen): localization in adults and during juvenile development

Sea water acclimation of rainbow trout (Oncorhynchus mykiss) modulates the mucosal transcript immune response induced by Vibrio anguillarum and Aeromonas salmonicida vaccine, and prevents further transcription of stress-immune genes in response to acute stress

Mucosal tissues appear to be more important in fish than in mammals due to living in a microbial-rich aquatic milieu, yet the complex interaction between the immune and the neuroendocrine system in these tissues remains elusive. The aim of this work was to investigate the mucosal immune response in immunized rainbow trout vaccinated with Alpha ject vaccine (bivalent), kept in fresh water (FW) or transferred to seawater (SW), and to evaluate their response to acute stress (chasing). Acute stress resulted in higher levels of plasma cortisol (Sham + Stress and Vaccine + Stress). A similar response was observed in skin mucus, but it was lower in Vaccine + Stress compared with stressed fish. With a few exceptions, minimal alterations were detected in the transcriptomic profile of stress-immune gene in the skin of vaccinated and stressed fish in both FW and SW. In the gills, the stress elicited activation of key stress-immune components (gr1, mr, β-ar, hsp70, c3, lysozyme, α-enolase, nadph oxidase, il1β, il6, tnfα, il10 and tgfβ1) in FW, but fewer immune changes were induced by the vaccine (nadph oxidase, il6, tnfα, il10 and igt) in both SW and FW. In the intestine, an array of immune genes was activated by the vaccine particularly those related with B cells (igm, igt) and T cells (cd8α) in FW with no stimulation observed in SW. Therefore, our survey on the transcriptomic mucosal response demonstrates that the immune protection conferred by the vaccine to the intestine is modulated in SW. Overall, our results showed: i) plasma and skin mucus cortisol showed no additional stress effect induced by prolonged SW acclimation, ii) the stress and immune response were different among mucosal tissues which indicates a tissue-specific response to specific antigens/stressor. Further, the results suggest that the systemic immune organs may be more implicated in infectious events in SW (as few changes were observed in the mucosal barriers of immunized fish in SW) than in FW.

Ontogeny of myeloperoxidase (MPO) positive cells in flounder (Paralichthys olivaceus)

Neutrophils represent an important asset of innate immunity. Neutrophils express myeloperoxidase (MPO) which is a heme-containing peroxidase involved in microbial killing. In this study, by using real-time quantitative PCR and Western blot analysis, the flounder MPO (PoMPO) was observed to be highly expressed in the head kidney, followed by spleen, gill, and intestine during ontogeny - during developmental stages from larvae to adults. Furthermore, PoMPO positive cells were present in major immune organs of flounder at all developmental stages, and the number of neutrophils was generally higher as the fish grew to a juvenile stage. In addition, flow cytometry analysis revealed that the proportion of PoMPO positive cells relative to leukocytes, in the peritoneal cavity, head kidney, and peripheral blood of flounder juvenile stage was 18.3 %, 34.8 %, and 6.0 %, respectively, which is similar to the adult stage in flounder as previously reported. The presence and tissue distribution of PoMPO during ontogeny suggests that PoMPO positive cells are indeed a player of the innate immunity at all developmental stages of flounder.

Increased water temperature contributes to a chondrogenesis response in the eyes of spotted wolffish

Adult vertebrate cartilage is usually quiescent. Some vertebrates possess ocular scleral skeletons composed of cartilage or bone. The morphological characteristics of the spotted wolffish (Anarhichas minor) scleral skeleton have not been described. Here we assessed the scleral skeletons of cultured spotted wolffish, a globally threatened marine species. The healthy spotted wolffish we assessed had scleral skeletons with a low percentage of cells staining for the chondrogenesis marker sex-determining region Y-box (Sox) 9, but harboured a population of intraocular cells that co-express immunoglobulin M (IgM) and Sox9. Scleral skeletons of spotted wolffish with grossly observable eye abnormalities displayed a high degree of perochondrial activation as evidenced by cellular morphology and expression of proliferating cell nuclear antigen (PCNA) and phosphotyrosine. Cells staining for cluster of differentiation (CD) 45 and IgM accumulated around sites of active chondrogenesis, which contained cells that strongly expressed Sox9. The level of scleral chondrogenesis and the numbers of scleral cartilage PCNA positive cells increased with the temperature of the water in which spotted wolffish were cultured. Our results provide new knowledge of differing Sox9 spatial tissue expression patterns during chondrogenesis in normal control and ocular insult paradigms. Our work also provides evidence that spotted wolffish possess an inherent scleral chondrogenesis response that may be sensitive to temperature. This work also advances the fundamental knowledge of teleost ocular skeletal systems.

Diversity and structure of sparids external microbiota (Teleostei) and its link with monogenean ectoparasites

Background

Animal-associated microbial communities appear to be key factors in host physiology, ecology, evolution and its interactions with the surrounding environment. Teleost fish have received relatively little attention in the study of surface-associated microbiota. Besides the important role of microbiota in homeostasis and infection prevention, a few recent studies have shown that fish mucus microbiota may interact with and attract some specific parasitic species. However, our understanding of external microbial assemblages, in particular regarding the factors that determine their composition and potential interactions with parasites, is still limited. This is the objective of the present study that focuses on a well-known fish-parasite interaction, involving the Sparidae (Teleostei), and their specific monogenean ectoparasites of the Lamellodiscus genus. We characterized the skin and gill mucus bacterial communities using a 16S rRNA amplicon sequencing, tested how fish ecological traits and host evolutionary history are related to external microbiota, and assessed if some microbial taxa are related to some Lamellodiscus species.

Results

Our results revealed significant differences between skin and gill microbiota in terms of diversity and structure, and that sparids establish and maintain tissue and species-specific bacterial communities despite continuous exposure to water. No phylosymbiosis pattern was detected for either gill or skin microbiota, suggesting that other host-related and environmental factors are a better regulator of host-microbiota interactions. Diversity and structure of external microbiota were explained by host traits: host species, diet and body part. Numerous correlations between the abundance of given bacterial genera and the abundance of given Lamellodiscus species have been found in gill mucus, including species-specific associations. We also found that the external microbiota of the only unparasitized sparid species in this study, Boops boops, harbored significantly more Fusobacteria and three genera, Shewenella, Cetobacterium and Vibrio, compared to the other sparid species, suggesting their potential involvement in preventing monogenean infection.

Conclusions

This study is the first to explore the diversity and structure of skin and gill microbiota from a wild fish family and present novel evidence on the links between gill microbiota and monogenean species in diversity and abundance, paving the way for further studies on understanding host-microbiota-parasite interactions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-022-00180-1.

Study on Immune Response of Organs of Epinephelus coioides and Carassius auratus After Immersion Vaccination With Inactivated Vibrio harveyi Vaccine

Immersion vaccination relies on the response of fish mucosa-associated lymphoid tissues, the Crucian carp (Carassius auratus) and Grouper (Epinephelus coioides) were researched in this paper to examine local mucosal immune responses and associated humoral system responses following immersion vaccination. We administered 1.5 × 10⁷ CFU/ml formalin-inactivated Vibrio harveyi cells and measured mucus and serum antibody titers as well as IgM, MHC II mRNA levels in immune organs. The mucosal antibody response preceded the serum response indicating a role for local mucosal immunity in immersion vaccination. IgM and MHC II mRNA levels were relatively greater for the spleen and head kidney indicating the importance and central position of systemic immunity. Expression levels were also high for the gills while skin levels were the lowest. IgM and MHC II mRNA levels were altered over time following vaccination and the hindgut, liver and spleen were similar indicating a close relationship, so the absolute value of r is used to analyze the correlation among different organs immunized. It can be inferred the existence of an internal immune molecular mechanism for Immune synergy hindgut-liver-spleen, from the peak time (14^th day), the relative ratio of genes expression in the same tissues between the immunized grouper and the control group (26 times), and Pearson correlation coefficient (0.8<|r|<1). Injection challenges with live V. harveyi indicated that the relative protection rates for the crucian carp and Grouper was basically the same at 44.4% and 47.4%, respectively. It is believe that crucian carp may be used as a substitute for the valuable grouper in immunity experiment, just from aspect of the relative percent survival (RPS) and how it changes with time. But they were not consistent about the IgM mRNA expression between that of crucian carp and grouper after immersion the Vibrio vaccine.

Blimp-1 is involved in B cell activation and maturation in Nile tilapia (Oreochromis niloticus)

B lymphocyte-induced maturation protein 1 (Blimp-1), a transcription factor containing zinc finger, is required and sufficient to trigger terminal differentiation of B cells in mammals. The Blimp-1 (OnBlimp-1) from Nile tilapia (Oreochromis niloticus) was identified and characterized its expression pattern during B cell activation and maturation. The cDNA of OnBlimp-1 open reading frame is 2547 bp encoding a protein of 848 amino acids and the predicted molecular weight is 93.36 kDa. OnBlimp-1 contains a SET domain and five ZnF_C2H2 domains, which shares high homology with that of other species. OnBlimp-1 transcription was detected in all examined tissues with high expression in the spleen (SPL). Analysis of sorted lymphocyte populations, including IgM⁺ and IgM^- cells from peripheral blood (PBL), SPL and anterior kidney (AK), indicated that the OnBlimp-1 transcription was highly expressed in the IgM⁺ B cells. Upon LPS stimulation, OnBlimp-1 expression was up-regulated in tissues of PBL, SPL and AK significantly. The expression of OnBlimp-1, as well as the secreted IgM, was significantly up-regulated in the SPL and AK leukocytes stimulated with anti-OnIgM monoclonal antibody and LPS in vitro, respectively. Above results suggest that OnBlimp-1, a cytokine regulating the terminal differentiation of activated B cells to antibody-secreting cells, is likely to play important roles in B cell activation and maturation in Nile tilapia.

Molecular characterization and transcriptional expression of a B cell transcription factor Pax5 in Nile tilapia (Oreochromis niloticus)

Pax5 (Paired Box 5), a nuclear transcription factor expressed in B cell specifically, is a key regulator for B cell activation. In this study, we cloned and identified a Pax5 gene (OnPax5) from Nile tilapia (Oreochromis niloticus), which has an open reading frame of 1278 bp, encoding deduced amino acid sequence of 425 residues. OnPax5 contains a conserved DNA-binding domain encoding the paired box, an octapeptide, a homeobox homology region, a transactivation and a repressor domain. OnPax5 is constitutively expressed in various analyzed tissues of tilapia, with a relatively high expression in lymphoid organs, including spleen (SPL), anterior kidney (AK), and thymus. What's more, OnPax5 is highly expressed in leukocytes especially in IgM⁺ lymphocytes sorted from peripheral blood (PBL), SPL and AK. When stimulated with lipopolysaccharide (LPS) in vivo, OnPax5 expression was significantly up-regulated in PBL, SPL and AK. Upon stimulation with LPS, pokeweed mitogen and mouse anti-OnIgM monoclonal antibody in vitro, the expression of OnPax5 was also significantly up-regulated in leukocytes from SPL and AK. Taken together, Pax5, the B cell lineage specific activator factor, might get involved in B cell activation in Nile tilapia.

Detection and quantification of Aeromonas schubertii in Channa maculata by TaqMan MGB probe fluorescence real-time quantitative PCR

Aeromonas schubertii is a major epidemiological agent that threatens cultured snakeheads (Channidae) and has caused great economic losses in fish-farming industries in China in recent years. In present study, a specific TaqMan minor groove binder (MGB) probe fluorescence real-time quantitative PCR (qPCR) assay was developed to rapidly detect and quantify A. schubertii. A pair of qPCR primers and a TaqMan MGB probe were selected from the rpoD gene, which were shown to be specific for A. schubertii. A high correlation coefficient (R²  = 0.9998) in a standard curve with a 103% efficiency was obtained. Moreover, the qPCR method's detection limit was as low as 18 copies/μl, which was 100 times more sensitive than that of conventional PCR. The detection results for the A. schubertii in pond water and fish tissue were consistent with those of the viable counts. Bacterial load changes detected by qPCR in different tissues of snakeheads infected with A. schubertii showed that the gills and intestines may be the entry for A. schubertii, and the spleen and kidney are major sites for A. schubertii replication. The established method in present study should be a useful tool for the early surveillance and quantitation of A. schubertii.

Skin microbiome of coral reef fish is highly variable and driven by host phylogeny and diet

BACKGROUND

The surface of marine animals is covered by abundant and diversified microbial communities, which have major roles for the health of their host. While such microbiomes have been deeply examined in marine invertebrates such as corals and sponges, the microbiomes living on marine vertebrates have received less attention. Specifically, the diversity of these microbiomes, their variability among species, and their drivers are still mostly unknown, especially among the fish species living on coral reefs that contribute to key ecosystem services while they are increasingly affected by human activities. Here, we investigated these knowledge gaps analyzing the skin microbiome of 138 fish individuals belonging to 44 coral reef fish species living in the same area.

RESULTS

Prokaryotic communities living on the skin of coral reef fishes are highly diverse, with on average more than 600 OTUs per fish, and differ from planktonic microbes. Skin microbiomes varied between fish individual and species, and interspecific differences were slightly coupled to the phylogenetic affiliation of the host and its ecological traits.

CONCLUSIONS

These results highlight that coral reef biodiversity is greater than previously appreciated, since the high diversity of macro-organisms supports a highly diversified microbial community. This suggest that beyond the loss of coral reefs-associated macroscopic species, anthropic activities on coral reefs could also lead to a loss of still unexplored host-associated microbial diversity, which urgently needs to be assessed.

Production-level risk factors for syncytial hepatitis in farmed tilapia (Oreochromis niloticus L)

Syncytial hepatitis (SHT) is an emerging viral disease of tilapia characterized by significant morbidity and mortality. This study aimed to establish the production-level risk factors associated with presence and severity of SHT. Production factors were analysed during multiple outbreaks of SHT that occurred between 2011 and 2013 on a single tilapia farm in Ecuador and compared with the year 2010 before the SHT outbreaks. Relative risks, t tests, modified Poisson and forward stepwise linear regression analyses were performed using EPIINFO™. Compared to other strains, Chitralada had an elevated risk of SHT [RR = 2.1 (95%CI 1.8-2.4)]. Excessive mortality associated with the presence (and severity) of SHT increased by 611 (365), 6,814 (5,768) and 388 (340) deaths per 100,000 fry when stocking density, dissolved oxygen and pond production cycles were raised by 1 fish/m² , 1 mg/L and 1 cycle, respectively. Excessive mortality associated with the presence (and severity) of SHT decreased by 337 (258) and 1,354 (1,025) deaths per 100,000 when stocking weight and water temperature increased by 1 g and 1°C, respectively. Time (season and stocking year) was not significantly associated with SHT. This study shows that some production factors increase the risk incidence and severity of SHTon a farm.

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.

Transcriptional changes in three immunoglobulin isotypes of rohu, Labeo rohita in response to Argulus siamensis infection

Immunoglobulin heavy chains of three isotypes viz., IgM, IgD and IgT/IgZ are described in teleosts. In this study, a challenge experiment with an ectoparasite Argulus siamensis was conducted to evaluate the changes in adaptive immune response by quantitation of expression of Ig heavy chains in skin, head kidney and mucus of infected rohu, Labeo rohita. Rohu were challenged with 100 metanauplii of A. siamensis/fish. Head kidney, skin and mucus samples were collected at 0 h, 12 h, 24 h, 3 d, 7 d, 15 d and 30 d by sacrificing four fish each from infected and control groups at each time point. The expression of IgM, IgD and IgZ in these tissues were measured by reverse transcription real time quantitative PCR. IgM level was found to reach its peak significantly 30 d post-infection in head kidney tissue, while IgM transcripts were below detectable range in skin and mucus at all time points. IgZ and IgD levels were significantly up-regulated post-infection in all the three tissue samples. Early up-regulation of IgD was observed in skin and mucus, compared to head kidney. This study showed that parasitic invasion can trigger varied expressions of immunoglobulin types to provide systemic as well as local protection in the host. In particular, the appearance of high level of expression of IgZ and IgD in skin and mucus will pave the way for vaccine development against A. siamensis which feeds on those tissues.

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.

Molecular cloning and expression analysis of immunoglobulin M heavy chain gene of blunt snout bream (Megalobrama amblycephala)

Immunoglobulins (Igs), which bind antigens with high specificity, are essential molecules in adaptive immune system of jawed vertebrates. In this study, cDNA encoding the secreted form of the immunoglobulin heavy chain of IgM (sIgM) was cloned from the mesonephros of blunt snout bream (Megalabrama amblycephala) using RT-PCR and rapid amplification of cDNA ends (RACE). The full-length cDNA of sIgM heavy chain gene has 1961 nucleotides encoding a putative protein of 569 amino acids, constant region shares high amino acid identity with that of Ctenopharyngodon idella (80%), Carassius auratus langsdorfii (65%) and Danio rerio (59%). Multiple protein sequence alignment revealed that blunt snout bream sIgM was clustered with the homologues of cyprinid fish and constructed one clade. Using quantitative real-time PCR (qRT-PCR) analysis, the level of sIgM mRNA was determined, with a V-shape change pattern: decreased initially from unfertilized egg stage to 4 cells stage and increased from 16 cells stage to prelarva. This sharp drop indicates that sIgM mRNA is maternally transferred, and was continuously degraded until 16 cells stage. The drastic rising in sIgM level from blastula stage to prelarva might be attributed to embryonic stem cell differentiation procedure. Compared with juvenile fish, the expression of sIgM was significantly higher in pronephros, liver, spleen, gill and muscle of adult fish. After the injection of Aeromonas hydrophila, the expression pattern of sIgM was found first down-regulated at 4 h, then up-regulated and reached the peak at 7 d and 21 d in mesonephros, spleen, liver and gill, respectively.

Mast cells in common wolffish Anarhichas lupus L.: ontogeny, distribution and association with lymphatic vessels

The morphology, ontogeny and tissue distribution of mast cells were studied in common wolffish(Anarhichas lupus L.) at the larval, juvenile and adult life stages using light and electron-microscopy and immunohistochemistry. Fish were sampled at 1 day, 1, 2, 3, 4, 8 and 12 weeks post-hatching in addition to 6 and 9 months and 2 years and older. From 8 weeks post-hatching, mast cells in common wolffish mainly appeared as oval or rounded cells 8-15 mm in diameter with an eccentrically placed, ovoid nucleus and filled with cytoplasmic granules up to 1.2 mm in diameter. Granules were refractile and eosinophilic to slightly basophilic in H&E and stained bright red with Martius-scarlet-blue and purple with pinacyanol erythrosinate in formalin-fixed tissues. Mast cells stained positive for piscidin 4 and Fc ε RI by immunohistochemistry. From 1 day to 4 weeks post-hatching, immature mast cell containing only a few irregularly sized cytoplasmic granules were observed by light and electron-microscopy in loose connective tissue of cranial areas. From 1 day post-hatching, these cells stained positive for piscidin 4 and Fc ε RI by immunohistochemistry. From 12 weeks post-hatching, mast cells showed a primarily perivascular distribution and were particularly closely associated with lymphatic vessels and sinuses. Mast cells were mainly located at the peripheral border of the adventitia of arteries and veins, while they were in intimate contact with the endothelium of the lymphatic vessels. Numerous mast cells were observed in the intestine. A stratum compactum, as described in salmonids, was not observed in wolffish intestine,nor were mast cells confined to a separate layer, a stratum granulosum. Lymphatic vessels consisting of endothelium, intimal connective tissue and a poorly developed basal lamina were observed in the intestine. Scanning electron microscopy was used to compare the structure and localization of intestinal mast cells of common wolffish and rainbow trout. Scanning electron microscopy also revealed endothelial surface features and confirmed the existence of three distinctly different types of vessels in the wolffish intestine. Rainbow trout mast cell granules appeared as intact globular structures while empty vacuoles were observed in common wolffish. Mast cells were closely associated with lymphatic vessels in common wolffish, but not in rainbow trout.

B cell memory following infection and challenge of channel catfish with Ichthyophthirius multifiliis

B cell responses in channel catfish to infection with the parasitic ciliate Ichthyophthirius multifiliis were followed for 3 years. High titers of serum IgM antibodies recognizing I. multifiliis immobilization antigens were present 5weeks after immunizing infection, but by 1 year titers were at low or undetectable levels. Two to three years after infection the numbers of antibody secreting cells recognizing immobilization antigens in skin and head kidney of immune fish had decreased to the level found in uninfected controls. Challenge of immune fish showed they remained immune and that the numbers of antibody secreting cells recognizing immobilization antigens increased in skin but not head kidney. This suggests that antigen-specific memory B cells persisted for 3 years after infection and upon challenge differentiated into antibody secreting cells that localized in skin. Our results suggest that humoral immunity in channel catfish is maintained through IgM(+) memory B cells.

An Overview of the Immunological Defenses in Fish Skin

The vertebrate immune system is comprised of numerous distinct and interdependent components. Every component has its own inherent protective value, and the final combination of them is likely to be related to an animal’s immunological history and evolutionary development. Vertebrate immune system consists of both systemic and mucosal immune compartments, but it is the mucosal immune system which protects the body from the first encounter of pathogens. According to anatomical location, the mucosa-associated lymphoid tissue, in teleost fish is subdivided into gut-, skin-, and gill-associated lymphoid tissue and most available studies focus on gut. The purpose of this paper is to summarise the current knowledge of the immunological defences present in skin mucosa as a very important part of the fish immune system, serving as an anatomical and physiological barrier against external hazards. Interest in defence mechanism of fish arises from a need to develop health management tools to support a growing finfish aquaculture industry, while at the same time addressing questions concerning origins and evolution of immunity in vertebrates. Increased knowledge of fish mucosal immune system will facilitate the development of novel vaccination strategies in fish.

Mucosal immunoglobulins and B cells of teleost fish

As physical barriers that separate teleost fish from the external environment, mucosae are also active immunological sites that protect them against exposure to microbes and stressors. In mammals, the sites where antigens are sampled from mucosal surfaces and where stimulation of naïve T and B lymphocytes occurs are known as inductive sites and are constituted by mucosa-associated lymphoid tissue (MALT). According to anatomical location, the MALT in teleost fish is subdivided into gut-associated lymphoid tissue (GALT), skin-associated lymphoid tissue (SALT), and gill-associated lymphoid tissue (GIALT). All MALT contain a variety of leukocytes, including, but not limited to, T cells, B cells, plasma cells, macrophages and granulocytes. Secretory immunoglobulins are produced mainly by plasmablasts and plasma cells, and play key roles in the maintenance of mucosal homeostasis. Until recently, teleost fish B cells were thought to express only two classes of immunoglobulins, IgM and IgD, in which IgM was thought to be the only one responding to pathogens both in systemic and mucosal compartments. However, a third teleost immunoglobulin class, IgT/IgZ, was discovered in 2005, and it has recently been shown to behave as the prevalent immunoglobulin in gut mucosal immune responses. The purpose of this review is to summarise the current knowledge of mucosal immunoglobulins and B cells of fish MALT. Moreover, we attempt to integrate the existing knowledge on both basic and applied research findings on fish mucosal immune responses, with the goal to provide new directions that may facilitate the development of novel vaccination strategies that stimulate not only systemic, but also mucosal immunity.

Plasmablasts and plasma cells: reconsidering teleost immune system organization

Comparative immunologists have expended extensive efforts in the characterization of early fish B cell development; however, analysis of the post-antigen induction stages of antibody secreting cell (ASC) differentiation has been limited. In contrast, work with murine ASCs has resolved the physically and functionally distinct cells known as plasmablasts, the short-lived plasma cells and long-lived plasma cells. Teleost ASCs are now known to also possess comparable subpopulations, which can greatly differ in such basic functions as lifespan, antigen sensitivity, antibody secretion rate, differentiative potential, and distribution within the body. Understanding the mechanisms by which these subpopulations are produced and distributed is essential for both basic understanding in comparative immunology and practical vaccine engineering.

Teleost intestinal immunology

Teleosts clearly have a more diffuse gut associated lymphoid system, which is morphological and functional clearly different from the mammalian GALT. All immune cells necessary for a local immune response are abundantly present in the gut mucosa of the species studied and local immune responses can be monitored after intestinal immunization. Fish do not produce IgA, but a special mucosal IgM isotype seems to be secreted and may (partly) be the recently described IgZ/IgT. Fish produce a pIgR in their mucosal tissues but it is smaller (2 ILD) than the 4-5 ILD pIgR of higher vertebrates. Whether teleost pIgR is transcytosed and cleaved off in the same way needs further investigation, especially because a secretory component (SC) is only reported in one species. Teleosts also have high numbers of IEL, most of them are CD3-ɛ+/CD8-α+ and have cytotoxic and/or regulatory function. Possibly many of these cells are TCRγδ cells and they may be involved in the oral tolerance induction observed in fish. Innate immune cells can be observed in the teleost gut from first feeding onwards, but B cells appear much later in mucosal compartments compared to systemic sites. Conspicuous is the very early presence of putative T cells or their precursors in the fish gut, which together with the rag-1 expression of intestinal lymphoid cells may be an indication for an extra-thymic development of certain T cells. Teleosts can develop enteritis in their antigen transporting second gut segment and epithelial cells, IEL and eosinophils/basophils seem to play a crucial role in this intestinal inflammation model. Teleost intestine can be exploited for oral vaccination strategies and probiotic immune stimulation. A variety of encapsulation methods, to protect vaccines against degradation in the foregut, are reported with promising results but in most cases they appear not to be cost effective yet. Microbiota in fish are clearly different from terrestrial animals. In the past decade a fast increasing number of papers is dedicated to the oral administration of a variety of probiotics that can have a strong health beneficial effect, but much more attention has to be paid to the immune mechanisms behind these effects. The recent development of gnotobiotic fish models may be very helpful to study the immune effects of microbiota and probiotics in teleosts.

Identification and characterization of a novel intraepithelial lymphoid tissue in the gills of Atlantic salmon

In addition to being the respiratory organ in fish, the gills form a barrier against the external milieu. Innate and adaptive immune system components have been detected in the gills, but lymphoid cell accumulations similar to that seen in the mammalian mucosa have not been described. The present investigations revealed cell accumulations on the caudal edge of interbranchial septum at the base of the gill filaments in the Atlantic salmon. Cytokeratin immunohistochemical staining and identification of a basal membrane and desmosome cell junctions by electron microscopy showed that the cell accumulation was located intraepithelially. Major histocompatibility complex (MHC) class II+ cells were detected by immunohistochemistry, and laser capture micro-dissection and subsequent RT-PCR analysis revealed expression of T-cell receptor transcripts in the investigated tissue, suggesting the presence of T cells. The intraepithelial tissue reported here may be a suitable location for immune surveillance of gill infections, as well as a target site for new vaccine approaches and investigations of epithelial immunity. This is the first description of a lymphocyte cell aggregation within a teleostian gill epithelium network, illustrating a phylogenetically early form of leukocyte accumulations in a respiratory organ.

Cutaneous antibody-secreting cells and B cells in a teleost fish

Antibodies in cutaneous mucus and skin of teleosts play a critical role in the protective immune response against infection. We demonstrate by ELISPOT that antibody-secreting cells (ASC), which include LPS-inducible B cells (plasmablasts) and non-replicating plasma cells, reside in low numbers in the skin of channel catfish. Following immunization against the protozoan parasite Ichthyophthirius multifiliis, which infects skin and gills, the number of ASC in skin increased 20-fold, indicating that the number of ASC in skin is dynamic and increases in response to parasite infection. The number of ASC in skin remained elevated for at least 17 weeks after the last parasite exposure. Cutaneous ASC included I. multifiliis-specific ASC, which undoubtedly serve as the primary source of cutaneous antibodies that confer long-term humoral immunity against reinfection. Our demonstration that skin contains B cells and plasma cells suggests that it is an integral component of the teleost immune system.

Cytokine expression in the intestine of rainbow trout (Oncorhynchus mykiss) during infection with Aeromonas salmonicida

Gene expression of a number of cytokines in the intestine of rainbow trout (Oncorhynchus mykiss) was investigated after challenge with a pathogenic strain of Aeromonas salmonicida. Fish were exposed to A. salmonicida by immersion in a bacterial suspension (bath challenge) and tissue samples of the distal and proximal intestine were collected at days 0, 2, 4, 6 and 8 post-exposure. Head kidney tissue was also collected to assess the effect in a systemic immune tissue. A classic profile of pro-inflammatory cytokine upregulation was observed in the proximal intestine of fish infected by bath challenge, as determined by semi-quantitative RT-PCR. Expression of IL-1beta, IL-8, TNF-alpha and IFN-gamma was increased in the proximal intestine. TGF-beta was significantly decreased in the distal intestine. In the head kidney, infection with A. salmonicida by bath challenge caused decreased expression levels of IL-1beta, IL-8, TNF-alpha and TGF-beta. The results are discussed in the context of potential immune mechanisms in the gut to prevent infection.

Quantitative investigation of antigen and immune response in nervous and lymphoid tissues of Atlantic halibut (Hippoglossus hippoglossus) challenged with nodavirus

The present study reports the quantitative analysis of the spatio-temporal development of nodavirus infection and corresponding immune response in juvenile Atlantic halibut (Hippoglossus hippoglossus) challenged by intramuscular injection of nodavirus. Novel quantitative real-time RT-PCR protocols were applied to evaluate the absolute copy numbers of nodavirus RNA2 (RNA2) and secretory-IgM mRNA (sec-igmicro) in the eye, brain, mid/posterior kidney and spleen sampled over a period of 81 days. In the eye and brain, levels of both RNA2 and sec-igmicro increased significantly early in the infection. In the spleen and mid/posterior kidney, both RNA2 and sec-igmicro were detected but the levels remained unchanged during the experimental period. The levels of RNA2 and sec-igmicro in the eye and brain were strongly correlated (P<0.0001). Nodavirus antigen was demonstrated by immunohistochemistry (IHC) in the retina of eyes from a relatively few fish from day 34 post challenge (brain not examined), but not at any time in the spleen and anterior kidney. By IHC, IgM+ cells were observed in conjunction with nodavirus positive IHC labelling in the retina. In both the spleen and anterior kidney, the number of IgM+ cells increased from day 3 post challenge. By conventional real-time RT-PCR, RNA2 was only sporadically demonstrated in the posterior intestine, heart and gills. ELISA analysis revealed a nodavirus specific antibody response in serum that was significant from day 18 post challenge. No clinical signs or mortality related to nodavirus infection were observed in the challenged halibut. The results suggest that the nodavirus infection induced a significant antibody response through activation of B-cells in the kidney and spleen, and involved a substantial migration of antibody-secreting cells to infected peripheral tissues.

Influence of high-M alginate on the growth and survival of Atlantic cod (Gadus morhua L.) and spotted wolffish (Anarhichas minor Olafsen) fry

Atlantic cod and spotted wolffish fry were fed high-M alginate containing feed for 59 and 55 days, respectively. During this period the fry showed a higher specific growth rate compared to controls. Uptake and distribution of alginate was studied by inclusion of the (125)I-labelled molecule in the feed. The stomach and intestine contained the highest amount while the kidney, liver and spleen contained some, indicating that the alginate was taken up by the gut and transported to internal organs. Cod fry fed 0.06% and 0.1% high-M alginate showed a death rate of 51.4% and 53.3%, respectively. The lowest mortality, 48.1%, was found in fry fed 0.01% high-M alginate. Controls showed a mortality rate of 49.0%. Differences were, however, not statistically significant. Challenge of the immunostimulated fry (fed 0.02% and 0.06% alginate for 62 days) with atypical Aeromonas salmonicida bacteria resulted in accumulated mortalities of 56% and 49%, respectively, 47 days after infection. The group that received 0.06% alginate for a shorter period (47 days) and then control feed until challenged, and the group that received alginate by bath reached a cumulative mortality of 59% and 60%, respectively. Lowest mortality (44%) was seen in the control group. Numerous microabscesses were found in both immunostimulated and control fish in secondary lamellae of the gills, haematopoietic tissues of the kidneys, the submucosa and mucosa of the intestine, the spleen, the liver and the myocardium of the heart.

Antibody repertoire development in teleosts--a review with emphasis on salmonids and Gadus morhua L

The group of teleosts is highly diverse, comprising more than 23000 extant species. Studies of the teleost antibody repertoire have been conducted in many different species within different orders, though some species and families have been better characterised than others. The Atlantic cod (Gadus morhua L.) and several species within the Salmoninae (e.g. Salmo salar and Oncorynchus mykiss) are among the best-studied teleosts in terms of the antibody repertoire. The estimated size of the repertoire, the organisation of immunoglobulin (IG) gene segments, the expressed IG repertoire, the IgM serum concentration, and the serum antibody responses reveal some fundamental differences between these species. The serum IgM concentration of G. morhua is some ten times higher than that of S. salar, though G. morhua is characterised as a 'low' (or 'non') responder in terms of specific antibody production. In contrast, an antibody response is readily induced in S. salar, although the response is strongly regulated by antigen induced suppression. The IGHD gene of G. morhua has a unique structure, while the IGHM and IGHD genes of S. salar have a characteristic genomic organisation in two parallel loci. In addition, salmonids, express a broad repertoire of IGH and IGI V-region gene segments, while a single V gene family dominates the expressed heavy and light chain repertoire of G. morhua. Little is known about the developing antibody repertoire during ontogeny, in different stages of B-cell maturation, or in separate B-cell subsets. Information on the establishment of the preimmune repertoire, and the possible role of environmental antigens is also sparse.

The ontogeny of mucosal immune cells in common carp (Cyprinus carpio L.)

The ontogeny of carp (Cyprinus carpio L.) immune cells was studied in mucosal organs (intestine, gills and skin) using the monoclonal antibodies WCL38 (intraepithelial lymphocytes), WCL15 (monocytes/macrophages) and WCI12 (B cells). In addition, recombination activating gene 1 expression was examined in the intestine with real time quantitative PCR and in situ hybridization to investigate extrathymic generation of lymphocytes. WCL38(+) intraepithelial lymphocytes (putative T cells) appeared in the intestine at 3 days post-fertilization (dpf), which is shortly after hatching but before feeding, implying an important function at early age. These lymphoid cells appear in the intestine before the observation of the first thymocytes at 3-4 dpf, and together with the expression of recombination activating gene 1 in the intestine, suggests that similar to mammals at least part of these cells are generated in the intestine. WCL15(+)monocytes/macrophages appeared in the lamina propria of the intestine at 7 dpf, but considerably later in the epithelium, while WCI12(+) (B) cells appeared in intestine and gills at 6-7 weeks. From these results it can be concluded that putative T cells occur much earlier than B cells, and that B cells appear much later in the mucosae than in other internal lymphoid organs (2 wpf).

Analysis and characterization of the expression of the secretory and membrane forms of IgM heavy chains in the pufferfish, Takifugu rubripes

We investigated the structure and expression of immunoglobulin genes in the pufferfish, Takifugu rubripes, a highly prized and economically important fish species. The cDNA fragment that partially encodes the constant region of the IgM heavy chain was isolated in these animals by RACE using degenerate primers after which it was used as a probe for screening IgM heavy chains in a fugu splenic cDNA library. The structural feature of the constant region of fugu sIgM was found to consist of four constant domains (CH1 to CH4), while mIgM was shown to contain a deletion of the CH4 domain, and its transmembrane domain was directly spliced to the CH3 domain as found in other teleosts. This feature may be common to all teleosts. In addition, five VH genes isolated in this study fell into two families based on their variability. Analysis of genomic sequences from the fugu genomic database also showed that there are only two VH families in the genome. The IgM gene was preferentially expressed in presumptive lymphoid tissues. Moreover, in situ hybridization revealed that large numbers of IgM positive cells were widely distributed throughout the spleen, head kidney, kidney, and thymus, confirming that these tissues were major sites of antibody production in fish. The expressions of IgM in the mucosal organs such as the skin, gills, and intestine suggest that they, too, contribute to humoral immunity in aquatic animals. The expression of IgM mRNA in the early development stages of this fish suggests that its larval form possesses a protective defense mechanism against foreign invaders.