Molecular cloning of IgT from Atlantic salmon, and analysis of the relative expression of τ, μ and δ in different tissues

Characterization of IgD and IgT with their expressional analysis following subtype II megalocytivirus vaccination and infection in rock bream (Oplegnathus fasciatus)

In this study, heavy chain genes of IgD and IgT were sequenced and characterized their gene expression in rock bream (Oplegnathus fasciatus). Rock bream (RB)-IgD cDNA is 3319 bp in length and encodes a leader region, variable domains, a μ1 domain, and seven constant domains (CH1-CH7). A membrane-bound (mIgT) and secretory form (sIgT) of RB-IgT cDNAs are 1902 bp and 1689 bp in length, respectively, and encode a leader region, variable domains, four constant domains (CH1-CH4) and C-terminus. Their predicted 3D-structure and phylogenetic relation were similar to those of other teleost. In healthy fish, RB-IgD and mIgT gene expressions were higher in major lymphoid organs and blood, while RB-sIgT gene was more highly expressed in midgut. IgT expressing cells were detected in melano-macrophage centers (MMC) of head kidney in immunohistochemistry analysis. Under immune stimulation in vitro, RB-IgD and IgT gene expressions were upregulated in head kidney and spleen cells by bovine serum albumin or a rock bream iridovirus (RBIV) vaccine. In vivo, their expressions were significantly upregulated in head kidney, blood, and gill upon vaccination. Especially, RB-mIgT gene expression in head kidney and blood was upregulated at day 3 after vaccination while upregulated at earlier time point of day 1 by challenge with RBIV. This may suggest that memory cells might be produced during the primary response by vaccination and rapidly proliferated by secondary immune response by viral infection. RB-sIgT gene expression was highly upregulated in peripheral blood in vaccinated fish after viral infection, indicating that IgT plays an important role in systemic immune response as well as mucosal immune system. Our findings provide information on the role of RB-IgT in adaptive immunity during vaccination and viral infection in the vaccinated fish.

Organisation of the Atlantic salmon (Salmo salar) thymus and its content of Ig-expressing cells

The thymus of fishes is located as a dual organ in a rostrodorsal projection within the gill chamber and is covered by the operculum. The histological organization of the teleost fish thymus displays considerable diversity, particularly in salmonids where a clear distinction between the thymus cortex and medulla is yet to be defined. Recent interest has focused on the role of B cells in thymic function, but the presence of these cells within the salmon thymus remains poorly understood. In this morphological study, we applied in situ hybridization to investigate developing Atlantic salmon thymi for the expression of recombination activating (Rag) genes 1 and 2. We identified the location of the cortex, aligning with the previously described inner zone. Expression of IgM and IgD transcripts was predominantly observed in cells within the outer and subcapsular zones, with lesser expression in the cortex and inner zone. IgT expression was confined to a limited number of cells in the inner zone and capsule. The location of the thymus medulla could not be established. Our results are discussed in the context of the recently identified lymphoid organs, namely the intrabranchial lymphoid tissue (ILT) and the salmon bursa.

Possible transport routes of IgM to the gut of teleost fish

Fish rely on mucosal surfaces as their first defence barrier against pathogens. Maintaining mucosal homeostasis is therefore crucial for their overall well-being, and it is likely that secreted immunoglobulins (sIg) play a pivotal role in sustaining this balance. In mammals, the poly-Ig receptor (pIgR) is an essential component responsible for transporting polymeric Igs across mucosal epithelia. In teleost fish, a counterpart of pIgR has been identified and characterized, exhibiting structural differences and broader mRNA expression patterns compared to mammals. Despite supporting evidence for the binding of Igs to recombinant pIgR proteins, the absence of a joining chain (J-chain) in teleosts challenges the conventional understanding of Ig transport mechanisms. The transport of IgM to the intestine via the hepatobiliary route is observed in vertebrates and has been proposed in a few teleosts. Investigations on the stomachless fish, ballan wrasse, revealed a significant role of the hepatobiliary route and interesting possibilities for alternative IgM transport routes that might include pancreatic tissue. These findings highlight the importance of gaining a thorough understanding of the mechanisms behind Ig transport to the gut in various teleosts. This review aims to gather existing information on pIgR-mediated transport across epithelial cells and immunoglobulin transport pathways to the gut lumen in teleost fish. It provides comparative insights into the hepatobiliary transport of Igs to the gut, emphasizing the current understanding in teleost fish while exploring potential alternative pathways for Ig transport to the gut lumen. Despite significant progress in understanding various aspects, there is still much to uncover, especially concerning the diversity of mechanisms across different teleost species.

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.

A comprehensive review on the dynamic role of toll-like receptors (TLRs) in frontier aquaculture research and as a promising avenue for fish disease management

Toll-like receptors (TLRs) represent a conserved group of germline-encoded pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and play a crucial role in inducing the broadly acting innate immune response against pathogens. In recent years, the detection of 21 different TLR types in various fish species has sparked interest in exploring the potential of TLRs as targets for boosting immunity and disease resistance in fish. This comprehensive review offers the latest insights into the diverse facets of fish TLRs, highlighting their history, classification, architectural insights through 3D modelling, ligands recognition, signalling pathways, crosstalk, and expression patterns at various developmental stages. It provides an exhaustive account of the distinct TLRs induced during the invasion of specific pathogens in various fish species and delves into the disparities between fish TLRs and their mammalian counterparts, highlighting the specific contribution of TLRs to the immune response in fish. Although various facets of TLRs in some fish, shellfish, and molluscs have been described, the role of TLRs in several other aquatic organisms still remained as potential gaps. Overall, this article outlines frontier aquaculture research in advancing the knowledge of fish immune systems for the proper management of piscine maladies.

Differentiation and traffic of IgM+ B cells between focal dark spots in skeletal muscle of Atlantic salmon, lymphoid and adipose tissues

Focal dark spots (DS) in farmed Atlantic salmon fillets contain a significant number of B cells as revealed by the high abundance of immunoglobulin (Ig) transcripts in transcriptome data. The immune response in DS remains unknown while they represent a major problem in commercial aquaculture. Here, we characterized the diversity and clonal composition of B cells in DS. Sixteen gene markers of immune cells and antigen presentation were analyzed with RT-qPCR. All genes expression showed a positive correlation with DS area and intensity. The flatter the DS, the higher the expression of cd28, csfr, ctla, igt, and sigm, the lower expression of cd83 and btla, and the larger the cumulative frequency within DS. The expression of most of the analyzed immune genes, including three Ig types and markers of B cells was lower in DS than in the lymphatic organs, head kidney and spleen, but significantly higher compared to skeletal muscle. High levels of ctla4 and cd28 in DS might indicate the recruitment of T cells. Sequencing of IgM repertoire (Ig-seq) assessed migration of B cells by co-occurrence of identical CDR3 sequences in different tissues. The combination of gene expression and Ig-seq revealed the presence of several stages of B cell differentiation in DS. B cells at the earliest stage, with high ratio of membrane to secretory IgM (migm and sigm), showed minor Ig repertoire overlap with other tissues. Further differentiation stage (increased sigm to migm ratio and high expression of pax5 and cd79) was associated with active movement of B cells from DS towards lymphatic organs and visceral fat. Traffic and expression of immune genes decreased at later stages. These B cells could be involved in a response directed against viruses, pathogenic or opportunistic bacteria in DS. Seven of eight fish were positive for salmon alphavirus, and levels were higher in DS than in unstained muscle. PCR with universal primers to the 16S rRNA gene did not detect bacteria in DS. Although the evolution of DS most likely implies local exposure to antigens, neither this nor previous studies have found a necessary association between DS and pathogens or self-antigens.

Molecular characterization and expression analysis of immunoglobulins (IgM and IgT) heavy chains in black rockfish (Sebastes schlegelii) that response to bacterial challenge

Sebastes schlegelii is a kind of fish with great economic values. Recently, with the continuous expansion of aquaculture scale and the continuous improvement of aquaculture density, outbreak of various diseases has caused huge economic losses to its aquaculture industry. Study of fish immune system can help to understand the mechanism of immune response to external pathogens and can promote the development of immune prevention and control methods. Immunoglobulins (Igs) are complex glycoproteins that appear to be unique to the vertebrates that can recognize a wide variety of pathogens and recruit immune cells and molecules to destroy pathogens, which are generated by a series of rearrangement and somatic mutations. We therefore studied the immunoglobulin genes of S. schlegelii in view of their important roles in resisting to external pathogen infections. In this study, the immunoglobulin heavy chain genes (sIgM, mIgM, sIgT, and mIgT) of S. schlegelii were successfully identified and cloned. Phylogenetic analysis showed that the IgM and IgT genes of S. schlegelii were clustered together with homologous genes of other species, indicating that they were highly conserved during the evolutionary process. Collinearity analysis showed that the immunoglobulin genes and their adjacent genes were aligned with zebrafish, Atlantic salmon and tilapia, which further confirmed the conserved immunoglobulin gene of teleost. Expression analysis of healthy tissues showed that the expression levels of sIgM, sIgT and mIgT were the highest in the skin, while mIgM was the highest in spleen. After different bacterial infection, IgM and IgT were significantly expressed in skin and gill, which may be because skin and gill are the first line of defense against the infection pathogens. Subcellular localization showed that the mIgT protein was expressed in both the cell membrane and cytoplasm. Meanwhile, recombinant protein of mIgT was obtained in vitro, which laid a foundation for subsequent protein function studies. These results provide a theoretical basis for understanding the immunity role of immunoglobulin in S. schlegelii.

Mucosal immunoglobulin response in Epinephelus coioides after Cryptocaryon irritans infection

The teleost mucosal immune system consists mainly of the skin, gills and gut, which play crucial roles in local immune responses against invading organisms. Immunoglobulins are essential molecules in adaptive immunity that perform crucial biological functions. In our study, a mucosal immunity model was constructed in Epinephelus coioides groupers after Cryptocaryon irritans infection, according to previous experience. Total IgM and IgT in the groupers increased in the serum and mucus in the immune group, whereas only pathogen-specific IgM were detected existence. More critically, pathogen-specific IgM was detected in the head kidney, gill and skin supernatants, thus suggesting that the systematic immune and mucosal immune system secreted immunoglobulins. Furthermore, an early response in the skin was observed, on the basis of the detection of pathogen-specific IgM in the skin supernatant. In conclusion, this research characterized the grouper IgM and IgT in mucosal immune responses to pathogens in the gills and skin, thus providing a theoretical basis for future studies on vaccines against C. irritans.

Effects of Schyzocotyle acheilognathi (Yamaguti, 1934) infection on the intestinal microbiota, growth and immune reactions of grass carp (Ctenopharyngodon idella)

Our understanding of interactions among intestinal helminths, gut microbiota and host is still in its infancy in fish. In this study, the effects of Schyzocotyle acheilognathi infection on the intestinal microbiota, growth and immune reactions of grass carp were explored under laboratory conditions. 16S rDNA amplification sequencing results showed that S. acheilognathi infection altered the composition of intestinal microbiota only at the genus level, with a significant increase in the relative abundance of Turicibacter and Ruminococcus (P < 0.05) and a significant decrease in the relative abundance of Gordonia, Mycobacterium and Pseudocanthomonas (P < 0.05). Schyzocotyle acheilognathi infection had no significant effect (P > 0.05) on the alpha diversity indices (including Chao1, ACE, Shannon, Simpson index) of intestinal microbiota in grass carp, but PERMANOVA analysis showed that microbial structure significantly (P < 0.01) differed between hindgut and foregut. PICRUST prediction showed that some metabolism-related pathways were significantly changed after S. acheilognathi infection. The relative abundance of Turicibacter was positively correlated with the fresh weight of tapeworm (foregut: r = 0.48, P = 0.044; hindgut: r = 0.63, P = 0.005). There was no significant difference in the body condition of grass carp between the S. acheilognathi infected group and the uninfected group (P > 0.05). Intestinal tissue section with HE staining showed that S. acheilognathi infection severely damaged the intestinal villi, causing serious degeneration, necrosis and shedding of intestinal epithelial cells. The real-time fluorescent quantitative PCR results showed that S. acheilognathi infection upregulated the mRNA expression of the immune-related genes: Gal1−L2, TGF−β1 and IgM.

Molecular cloning, expression analysis of the IgT gene and detection of IgT+ B cells in the half-smooth tongue sole (Cynoglossus semilaevis)

IgT is a specific Ig isotype in teleosts, which plays extremely important roles in the mucosal immunity of fish. In the present study, the membrane-bound and secretory IgT of the half-smooth tongue sole (Cynoglossus semilaevis) were identified for the first time. The V-D-J-C structure of two forms of csIgT are translated by the same Cτ gene, and the secretory tail and transmembrane domain were encoded through alternative splicing at the 3' end of the Cτ4. The CH regions of csIgT had high similarity with that of other flatfish (P. olivaceus and S. maximus). In healthy C. semilaevis, sIgT and mIgT were mainly expressed in mucus related tissues such as skin, intestine and gill. The transcript levels of sIgT and mIgT mRNA showed a significant induction in the immune-related tissues upon Vibrio Harveyi infection. A polyclonal rabbit anti-csIgT was successfully prepared using the csIgT heavy chain recombinant protein. Using this antibody, we detected the native IgT with the molecular mass at 220 kDa in skin total protein under non-reducing SDS-PAGE condition. Immunofluorescence analysis indicated that IgT⁺ B lymphocytes were intensively located in the skin, gill, intestine, and head kidney of C. semilaevis. These results suggest that IgT may participate in the immune response of C. semilaevis, which will facilitate the investigations of the immunoglobulins of marine fish.

Identification and Characterization of Immunoglobulin T Heavy Chain in Large Yellow Croaker (Larimichthys crocea)

Three immunoglobulin (Ig) isotypes have been identified in teleosts, IgM, IgD, and IgT or IgZ. IgT, a new teleost Ig isotype, plays a vital role in mucosal immunity. However, information on molecular and functional characteristics of fish IgT is still limited. In this study, an IgT heavy chain (LcIgT) gene was cloned and characterized in large yellow croaker (Larimichthys crocea). Complete cDNA of LcIgT was 1930 bp in length, encoding a protein of 554 amino acids. The deduced LcIgT contains a VH region and only three CH regions (CH1, CH2, CH4), but no transmembrane region was predicted. Phylogenetic analysis showed that IgT heavy chain sequences from all fish species are grouped together. Homology comparison showed that LcIgT shares the highest amino acid identity of 58.73% with IgT heavy chain in Scophthalmus maximus. The VH domain of LcIgT has the highest identity of 72.50% with that of Scophthalmus maximus IgT. Relatively, each constant domain of LcIgT exhibits the highest amino acid identity with that of IgT in Oreochromis niloticus (67.61% identity for CH1, 61.11% identity for CH2, and 63.74% identity for CH4). LcIgT was constitutively expressed in various tissues tested, with the highest levels in mucosa-associated tissues such as gills and skin. After Cryptocaryon irritans infection, the mRNA levels of LcIgT were significantly up-regulated in the spleen (3.27-fold) at 4 d, in the head kidney (3.98-fold) and skin (2.11-fold) at 7 d, and in gills (4.45-fold) at 14 d. The protein levels in these detected tissues were all significantly up-regulated; the peak of its up-regulation was 6.33-fold at 28d in gills, 3.44-fold at 7d in skin, and 3.72-fold at 14d in spleen. These results showed that IgT response could be simultaneously induced in both systemic and mucosal tissues after parasitic infection and that IgT may be involved in systemic immunity and mucosal immunity against parasitic infection.

Serum N-Glycome Diversity in Teleost and Chondrostrean Fishes

Recent advances in carbohydrate chemistry, chemical biology, and mass spectrometric techniques have opened the door to rapid progress in uncovering the function and diversity of glycan structures associated with human health and disease. These strategies can be equally well applied to advance non-human health care research. To date, the glycomes of only a handful of non-human, non-domesticated vertebrates have been analyzed in depth due to the logistic complications associated with obtaining or handling wild-caught or farm-raised specimens. In contrast, the last 2 decades have seen advances in proteomics, glycoproteomics, and glycomics that have significantly advanced efforts to identify human serum/plasma biomarkers for various diseases. In this study, we investigated N-glycan structural diversity in serum harvested from five cultured fish species. This biofluid is a useful starting point for glycomic analysis because it is rich in glycoproteins, can be acquired in a sustainable fashion, and its contents reflect dynamic physiologic changes in the organism. Sera acquired from two chondrostrean fish species, the Atlantic sturgeon and shortnose sturgeon, and three teleost fish species, the Atlantic salmon, Arctic char, and channel catfish, were delipidated by organic extraction and the resulting protein-rich preparations sequentially treated with trypsin and PNGaseF to generate released N-glycans for structural analysis. Released N-glycans were analyzed as their native or permethylated forms by nanospray ionization mass spectrometry in negative or positive mode. While the basic biosynthetic pathway that initiates the production of glycoprotein glycan core structures is well-conserved across the teleost fish species examined in this study, species-specific structural differences were detected across the five organisms in terms of their monosaccharide composition, sialylation pattern, fucosylation, and degree of O-acetylation. Our methods and results provide new contributions to a growing library of datasets describing fish N-glycomes that can eventually establish species-normative baselines for assessing N-glycosylation dynamics associated with pathogen invasion, environmental stress, and fish immunologic responses.

Study on the characterization of grouper (Epinephelus coioides) immunoglobulin T and its positive cells

Immunoglobulins (Igs) play a vital role in the adaptive immunity of gnathostomes. IgT, a particular Ig class in teleost fishes, receives much attention concerning the mucosal immunity. While, the characteristic and function of Epinephelus coioides IgT is still unknown. In our study, a polyclonal antibody was first prepared with grouper IgT heavy chain recombinant protein. IgT was revealed to be polymeric in serum and mucus. In normal groupers, IgT had high expression level in head kidney and spleen, while little amount in gills, thymus, gut and liver. The number of IgT-positive cells in different tissues was in line with their IgT expression. Furthermore, IgT could coat fractional bacteria in the mucus. In conclusion, this research revealed the protein characteristic, basal expression and bacterial coverage of grouper IgT. This is the first study to identify the characteristic of grouper IgT and demonstrate the capacity of coating microbes.

Full-Length Transcriptome: A Reliable Alternative for Single-Cell RNA-Seq Analysis in the Spleen of Teleost Without Reference Genome

Fish is considered as a supreme model for clarifying the evolution and regulatory mechanism of vertebrate immunity. However, the knowledge of distinct immune cell populations in fish is still limited, and further development of techniques advancing the identification of fish immune cell populations and their functions are required. Single cell RNA-seq (scRNA-seq) has provided a new approach for effective in-depth identification and characterization of cell subpopulations. Current approaches for scRNA-seq data analysis usually rely on comparison with a reference genome and hence are not suited for samples without any reference genome, which is currently very common in fish research. Here, we present an alternative, i.e. scRNA-seq data analysis with a full-length transcriptome as a reference, and evaluate this approach on samples from Epinephelus coioides-a teleost without any published genome. We show that it reconstructs well most of the present transcripts in the scRNA-seq data achieving a sensitivity equivalent to approaches relying on genome alignments of related species. Based on cell heterogeneity and known markers, we characterized four cell types: T cells, B cells, monocytes/macrophages (Mo/MΦ) and NCC (non-specific cytotoxic cells). Further analysis indicated the presence of two subsets of Mo/MΦ including M1 and M2 type, as well as four subsets in B cells, i.e. mature B cells, immature B cells, pre B cells and early-pre B cells. Our research will provide new clues for understanding biological characteristics, development and function of immune cell populations of teleost. Furthermore, our approach provides a reliable alternative for scRNA-seq data analysis in teleost for which no reference genome is currently available.

Development of monoclonal antibody against IgT of a perciform fish, large yellow croaker (Larimichthys crocea) and characterization of IgT+ B cells

Teleost immunoglobulin T (IgT) is considered to be a primitive immunoglobulin class specialized in mucosal immunity. In the present study, a recombinant protein containing the CH2 region of large yellow croaker (Larimichthys crocea) IgT heavy chain was expressed, purified, and used as an immunogen to produce a monoclonal antibody (mAb) against large yellow croaker IgT. Western blotting results indicated that the obtained mouse anti-IgT mAb could specifically recognize a 45 kDa protein in the skin mucus of large yellow croaker, which was identified as the IgT heavy chain by mass spectrometric analysis. Immunofluorescence assay (IFA) analysis further demonstrated that this mouse anti-IgT mAb could recognize membrane-bound IgT (mIgT) molecules on large yellow croaker IgT⁺ leukocytes. This mAb also could be used for sorting of large yellow croaker IgT⁺ B cells by flow cytometry sorting technology. Then, flow cytometric immunofluorescence analysis (FCIA) results showed that the percentages of IgT⁺ B cells in skin, gills, gut, spleen, head kidney and peripheral blood lymphocytes were 27.553% ± 3.312%, 12.588% ± 3.538%, 12.355% ± 3.352%, 13.075 ± 2.258%, 5.552 ± 3.275%, and 2.600 ± 0.521%, respectively, indicating that mucosal tissues (skin, gills, and gut) contained a high ratio of IgT⁺ B cells. Accordingly, the high protein levels of IgT were also detected in these mucosal tissues, suggesting that IgT may play a role in mucosal immunity in large yellow croaker. Taken together, our data demonstrated that the mouse anti-IgT mAb developed in this study could be used for characterizing IgT⁺ B cells and studying the functions of IgT in large yellow croaker.

Development of species-specific IgM antibodies and elevation of mucosal immune response in Labeo rohita using recombinant bicistronic nano DNA vaccine priming

Immunoglobulin (IgM) is the primary immunoglobulin essential for defense mechanisms in fish. It is difficult to reliably quantify IgM because a lack of standardization in methodology and limited availability of commercially reagents. In the present study, a polyclonal antibody was developed for the specific detection and quantification of IgM in Labeo rohita. Recombinant bicistronic NanoDNA plasmid (RBND Vac) encoding the glyceraldehyde-3-phosphate dehydrogenase gene of Edwarsiella tarda conjugated with poly (lactic-co-glycolic acid) - Chitosan (PLGA-Chit) was developed and its potential as a DNA vaccine, to prevent the infection of E. tarda in L. rohita was investigated. Two treatment groups [T1 - (PLGA-Chit-NPs-pDNA), T2 - (PLGA-NPs-pDNA) and one control group (T0 - 1 × PBS)] were utilized. Polyclonal antibody was developed to estimate IgM titers in the serum and mucosal associated tissues (MAT) using Enzyme-linked Immunosorbent Assay (ELISA) technique. Additionally, immune gene expression was studied using qRT-PCR. Vaccinated groups also exhibited a significant increase in the total serum protein, globulin concentration and relatively less mortality was observed in T1 group. IgM level in serum and mucosal tissues (skin, gill and gut) increased significantly days post vaccination compared to control group, also non-specific immune parameters (myeloperoxidase and lysozyme levels) showed significant improvement in vaccinated fish. Furthermore, histopathological examination confirmed minor damage in physiological structure of kidney and liver tissues in vaccinated fish. Knowledge of the immunoglobulin in L. rohita primed with RBND Vac complex provides the better protection against E. tarda. The normal physiology findings of this study will aid in monitoring changes in the health status of fish, when the animals undergo vaccination by immersion method.

Monoclonal antibody against Nile tilapia (Oreochromis niloticus) IgM heavy chain: A valuable tool for detection and quantification of IgM and IgM+ cells

Nile tilapia (Oreochromis niloticus) is a freshwater fish, which is extensively cultivated worldwide and constitutes one of the model species for the study of fish immunology. Monoclonal antibodies are very advantageous molecular tools for studying teleost immune system. Specifically, monoclonal antibodies that react with immunoglobulins are used successfully in the study of the humoral immune response of several fish species. In the present study, we produced and characterized a monoclonal antibody against tilapia IgM heavy chain using a peptide-based strategy. The peptide sequence was selected from the surface-exposed region between CH3-CH4 domains. The specificity of the polyclonal serum and the hybridoma culture supernatant obtained by immunization with the peptide conjugated to keyhole limpet hemocyanin were evaluated by western blotting, both showing reactivity against tilapia serum IgM. The purified mAb was able to recognize secreted IgM by western blotting and ELISA and membrane IgM by flow cytometry. We also demonstrated that the antibody doesn't cross-react with a recombinant IgT fragment. This tool allowed us to study for the first time the stimulation of mucosal immunity after Pituitary Adenylate Cyclase Activating Polypeptide administration. Overall, the results demonstrated the utility of this mAb to characterize humoral immune response in O. niloticus.

Protein-Based Vaccine Protect Against Piscirickettsia salmonis in Atlantic Salmon (Salmo salar)

An effective and economical vaccine against the Piscirickettsia salmonis pathogen is needed for sustainable salmon farming and to reduce disease-related economic losses. Consequently, the aquaculture industry urgently needs to investigate efficient prophylactic measures. Three protein-based vaccine prototypes against Piscirickettsia salmonis were prepared from a highly pathogenic Chilean isolate. Only one vaccine effectively protected Atlantic salmon (Salmo salar), in correlation with the induction of Piscirickettsia-specific IgM antibodies and a high induction of transcripts encoding pro-inflammatory cytokines (i.e., Il-1β and TNF-α). In addition, we studied the proteome fraction protein of P. salmonis strain Austral-005 using multidimensional protein identification technology. The analyzes identified 87 proteins of different subcellular origins, such as the cytoplasmic and membrane compartment, where many of them have virulence functions. The other two prototypes activated only the innate immune responses, but did not protect Salmo salar against P. salmonis. These results suggest that the knowledge of the formulation of vaccines based on P. salmonis proteins is useful as an effective therapy, this demonstrates the importance of the different research tools to improve the study of the different immune responses, resistance to diseases in the Atlantic salmon. We suggest that this vaccine can help prevent widespread infection by P. salmonis, in addition to being able to be used as a booster after a primary vaccine to maintain high levels of circulating protective antibodies, greatly helping to reduce the economic losses caused by the pathogen.

Immunoglobulins in teleosts

Immunoglobulins are glycoproteins which are produced as membrane-bound receptors on B-cells or in a secreted form, known as antibodies. In teleosts, three immunoglobulin isotypes, IgM, IgT, and IgD, are present, each comprising two identical heavy and two identical light polypeptide chains. The basic mechanisms for generation of immunoglobulin diversity are similar in teleosts and higher vertebrates. The B-cell pre-immune repertoire is diversified by VDJ recombination, junctional flexibility, addition of nucleotides, and combinatorial association of light and heavy chains, while the post-immune repertoire undergoes somatic hypermutation during clonal expansion. Typically, the teleost immunoglobulin heavy chain gene complex has a modified translocon arrangement where the Dτ-Jτ-Cτ cluster of IgT is generally located between the variable heavy chain (VH) region and the Dμ/δ-Jμ/δ-Cμ-Cδ gene segments, or within the set of VH gene segments. However, multiple genome duplication and deletion events and loss of some individual genes through evolution has complicated the IgH gene organization. The IgH gene arrangement allows the expression of either IgT or IgM/IgD. Alternative splicing is responsible for the regulation of IgM/IgD expression and the secreted versus transmembrane forms of IgT, IgD, and IgM. The overall structure of IgM and IgT is usually conserved across species, whereas IgD has a large variety of structures. IgM is the main effector molecule in both systemic and mucosal immunity and shows a broad range of concentrations in different teleost species. Although IgM is usually present in higher concentrations under normal conditions, IgT is considered the main mucosal Ig.

Differential immune responses of immunoglobulin Z subclass members in antibacterial immunity in a zebrafish model

Immunoglobulin Z (IgZ) or its equivalent immunoglobulin T (IgT) is a newly identified immunoglobulin (Ig) class from teleost fish. This Ig class is characterized by its involvement in mucosa-associated lymphoid tissues (MALTs) for mucosal defence against pathogen infection. Recently, several subclass members of IgZ/IgT, such as IgZ, IgZ2, Igτ1, Igτ2 and Igτ3, have been further identified from zebrafish, common carp and rainbow trout. However, the functional diversity and correlation among these subclasses remain uncertain. Here, we explored the differential immune reactions of the IgZ and IgZ2 subclasses in antibacterial immunity in a zebrafish model. IgZ was extensively distributed in the peripheral serum and skin/gill MALTs and showed a rapid induction upon bacterial infection. IgZ2 was specialized in skin/gill MALTs and showed a strong induction following IgZ production. Correspondingly, the IgZ+ B cells had a wider distribution in the systemic primary/secondary lymphoid tissues and MALTs than the IgZ2+ B cells, which were predominant in MALTs. IgZ and IgZ2 exhibited a complementary effect in antibacterial immunity by possessing differential abilities. That is, IgZ is preferentially involved in bactericidal reaction that is in part C1q-dependent, and IgZ2 participates in neutralization action through bacteria-coating activity. The production of IgZ largely depended on the αβ T/CD4+ T cells, whereas that of IgZ2 did not, suggesting the different dependencies of IgZ and IgZ2 on systemic immunity. Our findings demonstrate that the functional behaviour and mechanism of the IgZ/IgT family are more diverse than previously recognized and thus improve the current knowledge about this ancient Ig class.

IgM+ and IgT+ B Cell Traffic to the Heart during SAV Infection in Atlantic Salmon

B cells of teleost fish differentiate in the head kidney, and spleen, and either remain in the lymphatic organs or move to the blood and peripheral tissues. There is limited knowledge about piscine B cell traffic to sites of vaccination and infection and their functional roles at these sites. In this work, we examined the traffic of B cells in Atlantic salmon challenged with salmonid alphavirus (SAV). In situ hybridization (RNAScope) showed increased numbers of immunoglobin (Ig)M+ and IgT+ B cells in the heart in response to SAV challenge, with IgM+ B cells being most abundant. An increase in IgT+ B cells was also evident, indicating a role of IgT+ B cells in nonmucosal tissues and systemic viral infections. After infection, B cells were mainly found in the stratum spongiosum of the cardiac ventricle, colocalizing with virus-infected myocardial-like cells. From sequencing the variable region of IgM in the main target organ (heart) and comparing it with a major lymphatic organ (the spleen), co-occurrence in antibody repertoires indicated a transfer of B cells from the spleen to the heart, as well as earlier recruitment of B cells to the heart in vaccinated fish compared to those that were unvaccinated. Transcriptome analyses performed at 21 days post-challenge suggested higher expression of multiple mediators of inflammation and lymphocyte-specific genes in unvaccinated compared to vaccinated fish, in parallel with a massive suppression of genes involved in heart contraction, metabolism, and development of tissue. The adaptive responses to SAV in vaccinated salmon appeared to alleviate the disease. Altogether, these results suggest that migration of B cells from lymphatic organs to sites of infection is an important part of the adaptive immune response of Atlantic salmon to SAV.

Rainbow trout Oncorhynchus mykiss skin responses to salmon louse Lepeophtheirus salmonis: From copepodid to adult stage

The marine crustacean Lepeophtheirus salmonis (salmon louse) is a common ectoparasite of wild and farmed salmonids. The parasite has a complex ontogeny comprising eight instars. The planktonic copepodid stage settles on host skin and pass through five instars to reach the adult stage. The present study comprises an experimental infestation of Oncorhynchus mykiss (rainbow trout) with salmon lice and describes histopathology and host immune responses in skin beneath the louse at multiple time points encompassing all louse developmental stages. Each fish was exposed to 80 infective copepodids, a mean no. of 32 parasites reached the preadult I stage whereas a mean no. of 11 parasites reached the adult stage. A progression in the severity of cutaneous lesions was observed, and levels of immune gene transcripts at the attachment site revealed a dynamic response, initially related to innate immunity. Later, immune cells accumulated in the dermis concomitant with a moderate decrease in levels of transcripts characteristic of both innate and adaptive immune responses. The present study also demonstrates that the cutaneous immune response was mainly induced at lice affected sites, while non-affected skin resembled the skin of untreated control. This indicates that the skin cannot be regarded as a uniform organ and requires careful sampling at all salmon louse stages.

Immune response to Ichthyophthirius multifiliis and role of IgT

The parasitic ciliate Ichthyophthirius multifiliis causes white spot disease in freshwater fish worldwide. The theront penetrates external surfaces of the naïve fish where it develops into the feeding trophont stage and elicits a protective immune response both at the affected site as well as at the systemic level. The present work compiles data and presents an overall model of the protective reactions induced. A wide spectrum of inflammatory reactions are established upon invasion but the specific protection is provided by adaptive factors. Immunoglobulin IgT is involved in protection of surfaces in several fish species and is thereby one of the first adaptive immune molecules reacting with the penetrating theront. IgT producing lymphocytes occur in epithelia, dispersed or associated with lymphoid cell aggregations (skin epidermis, fins, gills, nostrils and buccal cavities) but they are also present in central immune organs such as the head kidney, spleen and liver. When theronts invade immunized fish skin, they are encountered by host factors which opsonize the parasite and may result in complement activation, phagocytosis or cell-mediated killing. However, antibody (IgT, IgM and IgD) binding to parasite cilia has been suggested to alter parasite behaviour and induce an escape reaction, whereby specific IgT (or other classes of immunoglobulin in fish surfaces) takes a central role in protection against the parasite.

Effects of temperature on amoebic gill disease development: Does it play a role?

A relationship between increasing water temperature and amoebic gill disease (AGD) prevalence in Atlantic salmon (Salmo salar) has been noted at fish farms in numerous countries. In Scotland (UK), temperatures above 12°C are considered to be an important risk factor for AGD outbreaks. Thus, the purpose of this study was to test for the presence of an association between temperature and variation in the severity of AGD in Atlantic salmon at 10 and 15°C. The results showed an association between temperature and variation in AGD severity in salmon from analysis of histopathology and Paramoeba perurans load, reflecting an earlier and stronger infection post-amoebae exposure at the higher temperature. While no significant difference between the two temperature treatment groups was found in plasma cortisol levels, both glucose and lactate levels increased when gill pathology was evident at both temperatures. Expression analysis of immune- and stress-related genes showed more modulation in gills than in head kidney, revealing an organ-specific response and an interplay between temperature and infection. In conclusion, temperature may not only affect the host response, but perhaps also favour higher attachment/growth capacity of the amoebae as seen with the earlier and stronger P. perurans infection at 15°C.

Analysis of immunoglobulin and T cell receptor gene expression in ballan wrasse (Labrus bergylta) revealed an extraordinarily high IgM expression in the gut

The serum IgM concentration of ballan wrasse is relatively high, estimated to approximately 13 mg/ml in adult wild fish of 800 g. The present study revealed an unusual high abundance of IgM mRNA in the gut of ballan wrasse. Initially, transcripts encoding IgM, IgT, IgD, TCRα, TCRδ and CD3ε were quantified by RT-qPCR in several tissues of wild caught fish (approx. 800 g), indicating an elevated immune activity in hindgut and an extraordinarily high expression of IgM. Subsequently, a new RT-qPCR analysis was performed on the entire intestine, cut into four different segments, of reared fish (32-100 g). The analysis indicated immune activity along the entire intestine, but not as strong as in the hindgut. Furthermore, similar to the larger fish, the relative abundance of IgM transcripts was higher in the hindgut than in kidney and spleen, although the absolute level of IgM was in general higher in the larger fish. The secreted form of IgM was completely dominant in comparison to the membrane bound form of IgM and the other analysed genes. IgM was purified from gut mucus and external mucosal surfaces by magnetic beads coated with protein A. Mucus IgM reacted with rabbit antisera raised against serum IgM and contained subunits of the same size. Regarding the elevated immune activity in the intestine it is tempting to speculate on a possible compensatory strategy in this lineage of stomach-less fish, and that natural antibodies have an important role in the first line defence.

Profiling the Atlantic Salmon IgM+ B Cell Surface Proteome: Novel Information on Teleost Fish B Cell Protein Repertoire and Identification of Potential B Cell Markers

Fish immunology research is at a pivotal point with the increasing availability of functional immunoassays and major advances in omics approaches. However, studies on fish B cells and their distinct subsets remain a challenge due to the limited availability of differentially expressed surface markers. To address this constraint, cell surface proteome of Atlantic salmon IgM⁺ B cells were analyzed by mass spectrometry and compared to surface proteins detected from two adherent salmon head kidney cell lines, ASK and SSP-9. Out of 21 cluster of differentiation (CD) molecules identified on salmon IgM⁺ B cells, CD22 and CD79A were shortlisted as potential markers based on the reported B cell-specific surface expression of their mammalian homologs. Subsequent RT-qPCR analyses of flow cytometry-sorted subpopulations from head kidney leukocytes confirmed that both cd22 and cd79a genes were highly expressed in IgM⁺ lymphoid cells but were observed in barely detectable levels in IgM^- non-lymphoid suspension and adherent cells. Similarly, significantly high cd22 and cd79a mRNA levels were observed in IgM⁺ or IgT⁺ lymphoid cells from the spleen and peritoneal cavity, but not in their corresponding IgM^- IgT^- non-lymphoid fractions. This suggests that the B cell restrictive expression of CD22 and CD79A extend down to the transcription level, which was consistent across different lymphoid compartments and immunoglobulin isotypes, thus strongly supporting the potential of CD22 and CD79A as pan-B cell markers for salmon. In addition, this study provides novel information on the salmon B cell surface protein repertoire, as well as insights on B cell evolution. Further investigation of the identified salmon CD molecules, including development of immunological tools for detection, will help advance our understanding of the dynamics of salmon B cell responses such as during infection, vaccination, or immunostimulation.

IgM and IgD heavy chains of yellow catfish (Pelteobagrus fulvidraco): Molecular cloning, characterization and expression analysis in response to bacterial infection

Three different immunoglobulin (Ig) isotypes, namely IgM, IgD, and IgT/IgZ have been described in most teleost, among which IgM and IgT are considered crucial in systematic and mucosal immunity, respectively. However, some teleost have no IgT/IgZ and it is unclear how other Ig isotypes interact to perform immune-protective roles in both systematic and mucosal sites. In this study, the complete cDNA sequences of IgM and IgD heavy chains were cloned and analyzed from yellow catfish (Pelteobagrus fulvidraco). The full-length cDNA of Pf-IgM and Pf-IgD heavy chains contained an open reading frame (ORF) of 1710 and 2991 bp encoding a predicted protein of 570 and 997 amino acids, respectively. Tissue-specific expression analysis indicated that both IgM and IgD were highly expressed in kidney and spleen, and higher expression levels were found at zygote and 13th day post hatching during early development. Multiple sequence alignment and phylogenetic analysis showed IgM and IgD of yellow catfish are closely related to other fish of Siluriformes. Moreover, we also constructed the infection model of yellow catfish with bacteria (Flavobacterium columnare G₄) for the first time to study the function of Pf-IgM and Pf-IgD heavy chain genes in immune response. Quantitative real-time PCR (qRT-PCR) showed that significantly up-regulated expression of Pf-IgM was not only detected in liver and spleen, but also in mucosal tissues including skin and intestine, while Pf-IgD was just significantly increased in liver and spleen, which might suggest the main immune-protecting roles of IgM in mucosal tissues of yellow catfish.

Discovery of immunoglobulin T in Nile tilapia (Oreochromis niloticus): A potential molecular marker to understand mucosal immunity in this species

Immunoglobulin molecules play an important role in the immune defense system in all jawed vertebrates, by protecting the organism from a wide variety of pathogens. Nile tilapia (Oreochromis niloticus) is extensively cultivated worldwide, with a strong established market demand. It constitutes one of the model species for the study of fish immunology and its genome is currently fully sequenced. The presence of the immunoglobulin M gene in this species is well documented, as well as its major role in systemic immunity. To date, the IgT gene from O. niloticus has not been identified and, therefore, no information is available on the role of this immunoglobulin isotype in the immune response in tilapia. In the present work, novel secreted and membrane immunoglobulin T isotypes and a fragment of IgM were isolated from tilapia head kidney lymphocytes. Their transcriptional profiles were analyzed by quantitative PCR in larval development and in different tissues of healthy or lipopolysaccharide/Edwardsiella tarda-challenged tilapia adults. The presence of IgT and IgM were detected in early stages of larval development. Additionally, these genes exhibited differential expression profiles in basal conditions and after E. tarda infection in adult tilapia, in accord with the proposed effector functions of these immunoglobulins in the systemic and mucosal compartments. Our results suggest the potential involvement of this new Ig in mucosal immunity in tilapia.

Effect of yeast (Xanthophyllomyces dendrorhous) and plant (Saint John's wort, lemon balm, and rosemary) extract based functional diets on antioxidant and immune status of Atlantic salmon (Salmo salar) subjected to crowding stress

Salmon farming may face stress due to the intensive culture conditions with negative impacts on overall performance. In this aspect, functional feed improves not only the basic nutritional requirements but also the health status and fish growth. However, to date no studies have been carried out to evaluate the effect of functional diets in salmon subjected to crowding stress. Thus, the aim of this study was to evaluate the effect of yeast extract (Xanthophyllomyces dendrorhous; diet A) and the combination of plant extracts (common Saint John's wort, lemon balm, and rosemary; diet B) on the antioxidant and immune status of Atlantic salmon grown under normal cultured conditions and then subjected to crowding stress. Fish were fed with functional diets during 30 days (12 kg/m³) and then subjected to crowding stress (20 kg/m³) for 10 days. The lipid peroxidation in gut showed that both diets induced a marked decrease on oxidative damage when fish were subjected to crowding stress. The protein carbonylation in muscle displayed at day 30 a marked decrease in both functional diets that was more marked on the stress condition. The expression of immune markers (IFNγ, CD4, IL-10, TGF-β, IgM_mb, IgM_sec, T-Bet, and GATA-3) indicated the upregulation of those associated to humoral-like response (CD4, IL-10, GATA-3) when fish were subjected to crowding stress. These results were confirmed with the expression of secreted IgM. Altogether, these functional diets improved the antioxidant status and increased the expression of genes related to Th2-like response suggesting a protective role on fish subjected to crowding stress.

Profiling Atlantic salmon B cell populations: CpG-mediated TLR-ligation enhances IgM secretion and modulates immune gene expression

While TLR-activated pathways are key regulators of B cell responses in mammals, their impact on teleost B cells are scarcely addressed. Here, the potential of Atlantic salmon B cells to respond to TLR ligands was shown by demonstrating a constitutive expression of nucleic-acid sensing TLRs in magnetic sorted IgM+ cells. Of the two receptors recognizing CpG in teleosts, tlr9 was the dominating receptor with over ten-fold higher expression than tlr21. Upon CpG-stimulation, IgM secretion increased for head kidney (HK) and splenic IgM+ cells, while blood B cells were marginally affected. The results suggest that CpG directly affects salmon B cells to differentiate into antibody secreting cells (ASCs). IgM secretion was also detected in the non-treated controls, again with the highest levels in the HK derived population, signifying that persisting ASCs are present in this tissue. In all tissues, the IgM+ cells expressed high MHCII levels, suggesting antigen-presenting functions. Upon CpG-treatment the co-stimulatory molecules cd83 and cd40 were upregulated, while cd86 was down-regulated under the same conditions. Finally, ifna1 was upregulated upon CpG-stimulation in all tissues, while a restricted upregulation was evident for ifnb, proposing that salmon IgM+ B cells exhibit a type I IFN-response.

Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ+ CD83+ Antigen-Presenting Cells

The histozoic myxozoan parasite Kudoa thyrsites causes postmortem myoliquefaction and is responsible for economic losses to salmon aquaculture in the Pacific Northwest. Despite its importance, little is known about the host-parasite relationship, including the host response to infection. The present work sought to characterize the immune response in Atlantic salmon during infection, recovery, and reexposure to K. thyrsites After exposure to infective seawater, infected and uninfected smolts were sampled three times over 4,275 degree-days. Histological analysis revealed infection severity decreased over time in exposed fish, while in controls there was no evidence of infection. Following a secondary exposure of all fish, severity of infection in the controls was similar to that measured in exposed fish at the first sampling time but was significantly reduced in reexposed fish, suggesting the acquisition of protective immunity. Using immunohistochemistry, we detected a population of MHIIβ+ cells in infected muscle that followed a pattern of abundance concordant with parasite prevalence. Infiltration of these cells into infected myocytes preceded destruction of the plasmodium and dissemination of myxospores. Dual labeling indicated a majority of these cells were CD83+/MHIIβ+ Using reverse transcription-quantitative PCR, we detected significant induction of cellular effectors, including macrophage/dendritic cells (mhii/cd83/mcsf), B cells (igm/igt), and cytotoxic T cells (cd8/nkl), in the musculature of infected fish. These data support a role for cellular effectors such as antigen-presenting cells (monocyte/macrophage and dendritic cells) along with B and T cells in the acquired protective immune response of Atlantic salmon against K. thyrsites.

Molecular characterization and expression analysis of three subclasses of IgT in rainbow trout (Oncorhynchus mykiss)

As the teleost specific immunoglobulin, IgT plays important roles in systemic and mucosal immunity. In the current study, in rainbow trout, we have cloned the heavy chain (Igτ) genes of a secretory form of IgT2 as well as the membrane and secretory forms of a third IgT subclass, termed IgT3. Conserved cysteine and tryptophan residues that are crucial for the folding of the immunoglobulin domain as well as hydrophobic and hydrophilic residues within CART motif were identified in all IgT subclasses. Through analysis of the rainbow trout genome assembly, Igτ3 gene was found localized upstream of Igτ1 gene, while Igτ2 gene situated on another scaffold. At the transcriptional level, Igτ1 was mainly expressed in both systemic and mucosal lymphoid tissues, while Igτ2 was largely expressed in systemic lymphoid organs. After LPS and poly (I:C) treatment, Igτ1 and Igτ2 genes exhibited different expression profiles. Interestingly the transcriptional level of Igτ3 was negligible, although its protein product could be identified in trout serum. Importantly, a previously reported monoclonal antibody directed against trout IgT1 was able to recognize IgT2 and IgT3. These data demonstrate that there exist three subclasses of IgT in rainbow trout, and that their heavy chain genes display different expression patterns during stimulation. Overall, our data reflect the diversity and complexity of immunoglobulin in trout, thus provide a better understanding of the IgT system in the immune response of teleost fish.

Immunoglobulin T from sea bass (Dicentrarchus labrax L.): molecular characterization, tissue localization and expression after nodavirus infection

Background

Immunoglobulins (Igs) are fundamental components of the adaptive immune system of vertebrates, with the IgT/IgZ isotype specific of Teleosts. In this paper we describe the identification of an IgT heavy chain from the European sea bass (Dicentrarchus labrax L.), its molecular characterization and tissue mRNA localization by in situ hybridization.

Results

Sea bass IgT consists of 552 aa (Accession Number KM410929) and it contains a putative 19 amino acids long signal peptide and one potential N-glycosylation site. The C-region consists of four C_H domains; each contains the cysteine and tryptophan residues required for their correct folding. Based on the recent sequencing of sea bass genome, we have identified five different genomic contigs bearing exons unequivocally pertaining to IgT (C_H2, C_H3 and C_H4), but none corresponded to a complete IgH locus as IgT sequences were found in the highly fragmented assembled genomic regions which could not be assigned to any major scaffold. The 3D structure of sea bass IgT has been modelled using the crystal structure of a mouse Ig gamma as a template, thus showing that the amino acid sequence is suitable for the expected topology referred to an immunoglobulin-like architecture. The basal expression of sea bass IgT and IgM in different organs has been analysed: gut and gills, important mucosal organs, showed high IgT transcripts levels and this was the first indication of the possible involvement of sea bass IgT in mucosal immune responses. Moreover, sea bass IgT expression increased in gills and spleen after infection with nodavirus, highlighting the importance of IgT in sea bass immune responses. In situ hybridization confirmed the presence of IgT transcripts in the gut and it revealed a differential expression along the intestinal tract, with a major expression in the posterior intestine, suggesting the hindgut as a site for the recruitment of IgT⁺ cells in this species. IgT transcripts were also found in gill filaments and parallel lamellae and, for the first time, we identified scattered IgT positive cells in the liver, with a strong signal in the hepatic parenchyma.

Conclusions

In conclusion, we performed a full molecular characterization of IgT in sea bass that points out its possible involvement in mucosal immune responses of this species.

Differential Modulation of IgT and IgM upon Parasitic, Bacterial, Viral, and Dietary Challenges in a Perciform Fish

Three different immunoglobulin (Ig) isotypes can be found in teleost fish, IgM, IgD, and the teleost-specific IgT. IgM is considered to have a systemic activity, and IgT is attributed a mucosal role, similar to mammalian IgA. In this study, the complete sequence of gilthead sea bream IgM and IgT in their membrane (m) and soluble (s) forms are described for the first time in a perciform fish. Their constitutive gene expression is analyzed in different tissues, and their regulation upon viral, bacterial, parasitic, mucosal vaccination and dietary challenges are studied. GCB IgM and IgT have the prototypical structure when compared to other fish Igs. The constitutive expression of sIgM was the highest overall in all tissues, whereas mIgT expression was highest in mucosal tissues, such as gills and intestine. IgM and IgT were differentially regulated upon infection. IgT was highly upregulated locally upon infection with the intestinal parasite Enteromyxum leei or systemically after Nodavirus infection. Long-term intestinal parasitic infections increased the serum titer of both isotypes. Mucosal vaccination against Photobacterium damselae subsp. piscicida finely regulated the Ig response inducing a systemic increase of IgM titers in serum and a local IgT response in skin mucus when animals were exposed to the pathogen by bath challenge. Interestingly, plant-based diets inhibit IgT upregulation upon intestinal parasitic challenge, which was related to a worse disease outcome. All these results corroborate the mucosal role of IgT and emphasize the importance of a finely tuned regulation of Ig isotypes upon infection, which could be of special interest in vaccination studies.

Immunoglobulin Tau Heavy Chain (IgT) in Flounder, Paralichthys olivaceus: Molecular Cloning, Characterization, and Expression Analyses

Immunoglobulin tau (IgT) is a new teleost immunoglobulin isotype, and its potential function in adaptive immunity is not very clear. In the present study, the membrane-bound and secreted IgT (mIgT and sIgT) heavy chain genes were cloned for the first time and characterized in flounder (Paralichthys olivaceus), and found the nucleic acid sequence were exactly same in the Cτ1–Cτ4 constant domains of mIgT and sIgT, but different in variable regions and the C-terminus. The amino acid sequence of mIgT shared higher similarity with Bovichtus diacanthus (51.2%) and Dicentrarchus labrax (45.0%). Amino acid of flounder IgT, IgM, and IgD heavy chain was compared and the highest similarity was found between IgT Cτ1 and IgM Cμ1 (38%). In healthy flounder, the transcript levels of IgT mRNA were the highest in gill, spleen, and liver, and higher in peripheral blood leucocytes, skin, and hindgut. After infection and vaccination with Edwardsiella tarda via intraperitoneal injection and immersion, the qRT-PCR analysis demonstrated that the IgT mRNA level was significantly upregulated in all tested tissues, with similar dynamic tendency that increased firstly and then decreased, and higher in gill, skin, hindgut, liver, and stomach in immersion than in the injection group, but no significant difference existed in spleen and head kidney between immersion and injection groups. These results revealed that IgT responses could be simultaneously induced in both mucosal and systemic tissues after infection/vaccination via injection and immersion route, but IgT might play a more important role in mucosal immunity than in systemic immunity.

Recognition of purified beta 1,3/1,6 glucan and molecular signalling in the intestine of Atlantic salmon

Atlantic salmon was orally intubated with a highly purified β-glucan product (MacroGard(®)) to study the recognition of the molecule by the receptor genes, the regulation of the downstream signalling genes and global proteins, and the micromorphological changes in the intestine. The β-glucan receptor genes of Atlantic salmon, sclra, sclrb, sclrc and cr3, seem to recognize the molecule, and initiate the downstream ITAM-motif signalling, as evident from the significantly high mRNA levels of ksyk, mapkin2, il1b and mip2a levels. Among the altered proteins, the Apoa4 (involved in carbohydrate and lipid metabolism); Tagln, Actb (uptake of β-glucan); Psma2 (associated with substrate recognition); and Ckt (energy metabolism-related) were the overexpressed ones. The underexpressed proteins included the Uk114, Rpl9, Ctsb and Lgal that are connected to proliferation, LPS-stimulation, Il1b and lactose recognition, respectively. Furthermore, the mRNA levels of igt and the number of immune cells in the distal intestine were found to increase upon β-glucan uptake by the fish. This study provides some clues on the mechanisms by which the β-glucan evokes response in Atlantic salmon, particularly at the intestinal level.

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.

Functional characterization of IgM+ B cells and adaptive immunity in lumpfish (Cyclopterus lumpus L.)

The innate immune responses in lumpfish (Cyclopterus lumpus L.) have been shown to be functional, but little is currently known about the B cells, immunoglobulins or adaptive immune responses in this species. We have used anti-IgM antiserum to isolate B cells and compared them morphologically and functionally with other cell types. The fraction of IgM(+) cells among isolated peripheral blood leukocytes (PBL), head kidney leukocytes (HKL) and spleen leukocytes (SL) was in the range of 40%, 12% and 34%, respectively. The IgM(+) B cells had high phagocytic ability and were the predominant phagocytes in blood with higher capacity than IgM(+) B cells in HKL. Interestingly, among PBL, the most potent phagocytes were, in addition to monocytes, some small agranular uncharacterized IgM(-) cells. The IgM(+) B cells were positive for acid phosphatases (AcP), but negative for myeloperoxidase (MPO). Neutrophils were positive for MPO, while monocytes/macrophages and dendritic-like cells stained negatively. Monocytes/macrophages and the small, agranular IgM(-) cells stained most strongly positive for AcP corresponding to their high phagocytic capacity. Further, the ability to produce specific antibodies upon immunization verified adaptive immunity in the species. The high proportion of phagocytic IgM(+) B cells and their phagocytic ability indicate a significant role of phagocytic B cells in lumpfish innate immunity. The present analyses also give strong indications that vaccination and immunostimulation of farmed lumpfish can be used to prevent disease and mortality caused by pathogenic organisms.

Ontogeny, tissue distribution and expression analysis of IgZ in rohu, Labeo rohita in response to various stimuli

This study reports presence of IgZ transcripts, its ontogeny, tissue distribution and expression following various stimulations/infections in rohu. The derived rohu IgZ sequence clustered together with IgZ of Ctenopharyngodon idella and Cyprinus carpio in phylogenetic analysis. IgZ expression was detected at early developmental stages with higher expression at 1 day post-fertilization, and higher in anterior kidney and lower in skin tissues of juveniles. An inductive expression of IgZ was observed during both Edwardsiella tarda and Dactylogyrus catlaius infections in skin and/or gill tissues. In ConA treated HKLs, the response was prominent at 72 h post-stimulation where as in ConA-PMA treatment, it was higher during early time points. Increased expression of IgZ was found between 24 and 96 hps with formalin-killed Aeromonas hydrophila where as in poly I:C treated HKLs, the level increased at 96 hps. It seems to be the first report describing the functional existence of IgZ in rohu carp.

Systemic and mucosal immune response of rainbow trout to immunization with an attenuated Flavobacterium psychrophilum vaccine strain by different routes

Teleosts possess three immunoglobulin (Ig) heavy chain isotypes viz., IgM, IgT and IgD and all three isotypes are reported in rainbow trout. The expression of these Ig isotypes in response to different immunization routes was investigated and results provide a better understanding of the role these Igs in different tissues. Rainbow trout (Oncorhynchus mykiss) were immunized with an attenuated Flavobacterium psychrophilum strain, 259-93-B.17 grown under iron limiting conditions, by intraperitoneal, anal intubation and immersion routes. Serum, gill mucus, skin mucus and intestinal mucus samples were collected at 0, 3, 7, 14, 28, 42 and 56 days post immunization by sacrificing four fish from each treatment group and the unimmunized control group, and the IgM levels were estimated by an enzyme linked immunosorbent assay (ELISA). In addition, blood, gill, skin and intestinal tissue samples were collected for Ig gene expression studies. The secretory IgM, IgD and IgT gene expression levels in these tissues were estimated by reverse transcription quantitative real time PCR (RT-qPCR). Levels of IgM in serum, gill and skin mucus increased significantly by 28 days after immunization in the intraperitoneally immunized group, while no significant increase in IgM level was observed in fish groups immunized by other routes. Secretory IgD and IgT expression levels were significantly upregulated in gills of fish immunized by the immersion route. Similarly, secretory IgT and IgD were upregulated in intestines of fish immunized by anal intubation route. The results confirm mucosal association of IgT and suggest that IgD may also be specialized in mucosal immunity and contribute to immediate protection to the fish at mucosal surfaces.

Immunoglobulin isotypes in Atlantic salmon, Salmo salar

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

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

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

Adaptive immune responses at mucosal surfaces of teleost fish

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

Molecular cloning and characterization of secretory and membrane-bound IgM of turbot

In recent years, increasing diseases especially bacterial diseases have brought a host of losses with the expansive cultivation of turbot (Scophthalmus maximus). In order to do more research about the immune system of turbot for better understanding the mechanism of resisting diseases, the immunoglobulin genes related to secretory and membrane-bound IgM (s-IgM and m-IgM) of turbot were cloned using homology sequences cloning and SMART RACE PCR method. The heavy chain of s-IgM cDNA is 1900 bp in length including a leader region, a variable region, four constant regions (CH1, CH2, CH3 and CH4) and a C-terminal while the cDNA of m-IgM is 1795 bp with the same leader region, variable region, three constant regions (CH1, CH2 and CH3) and two transmembrane regions (TM1 and TM2). The sequence of IgM gene was also obtained and the structure consisted of V-CH1-CH2-CH3-CH4-TM1-TM2 is similar to other fishes. The highest level of s-IgM expression was observed in spleen, followed by kidney, gills, eyes, skin of the healthy turbot whereas the same profile of m-IgM expression is found with low level. And s-IgM takes up dominant proportion of total IgM expression. Also the relative expressions of s-IgM and m-IgM were analyzed in turbot vaccinated with the live attenuated vaccine Vibrio anguillarum. Not only the transcriptions of both s-IgM and m-IgM in liver, spleen and kidney of turbot injected with V. anguillarum MVAV6203 were up-regulated but also the expressions of s-IgM and m-IgM in spleen, kidney, gut, skin and gills of bath-vaccinated turbot were increased. Comparing the ratio changes of relative expression of m-IgM and s-IgM in vaccinated turbot, we found that the proportion of m-IgM were increasing in both administration routes, which probably indicated that the increasing expression of m-IgM strengthen the phagocytic ability of B cells.

Coevolution of Mucosal Immunoglobulins and the Polymeric Immunoglobulin Receptor: Evidence That the Commensal Microbiota Provided the Driving Force

Immunoglobulins (Igs) in mucosal secretions contribute to immune homeostasis by limiting access of microbial and environmental antigens to the body proper, maintaining the integrity of the epithelial barrier and shaping the composition of the commensal microbiota. The emergence of IgM in cartilaginous fish represented the primordial mucosal Ig, which is expressed in all higher vertebrates. Expansion and diversification of the mucosal Ig repertoire led to the emergence of IgT in bony fishes, IgX in amphibians, and IgA in reptiles, birds, and mammals. Parallel evolution of cellular receptors for the constant (Fc) regions of Igs provided mechanisms for their transport and immune effector functions. The most ancient of these Fc receptors is the polymeric Ig receptor (pIgR), which first appeared in an ancestor of bony fishes. The pIgR transports polymeric IgM, IgT, IgX, and IgA across epithelial cells into external secretions. Diversification and refinement of the structure of mucosal Igs during tetrapod evolution were paralleled by structural changes in pIgR, culminating in the multifunctional secretory IgA complex in mammals. In this paper, evidence is presented that the mutualistic relationship between the commensal microbiota and the vertebrate host provided the driving force for coevolution of mucosal Igs and pIgR.

Comparative analysis of IgM sub-variants in salmonid fish and identification of a residue in μ3 which is essential for MAb4C10 reactivity

In rainbow trout (Oncorhynchus mykiss) it has been shown that high affinity IgM antibodies have a higher degree of disulfide polymerization and a longer half life time. In the present study, distinct IgM sub-variants related to ancestral tetraploidy in salmonid fish were analyzed to reveal possible characteristic differences between these. A monoclonal antibody (MAb4C10) which distinguishes between IgM-A and IgM-B in Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) was further characterized. It was shown that substitution of a proline located in the loop between the B and C beta strands of the third constant domain (μ3) of salmon μA eliminated MAb4C10 reactivity. Accordingly, the reverse substitution in salmon μB restored MAb4C10 reactivity. Molecular cloning of μ cDNA from arctic char (Salvelinus alpinus) revealed two sub-variants (μA-1 and μA-2), i.e. a similar situation as in Atlantic salmon and brown trout. However, arctic char IgM eluted in one peak by anion exchange chromatography, in contrast to salmon and brown trout IgM that are eluted in two peaks. The only characteristic residue of salmon and brown trout μB is an additional cysteine in the C-terminal part of μ4. Most likely, this cysteine is involved in inter-chain disulfide bonding and influences the elution profiles of IgM-A and IgM-B on anion exchange chromatography. Neither of the μ sub-variants in arctic char have the additional cysteine, and char IgM, as well as salmon and brown trout IgM-A, showed a lower degree of inter-chain disulfide bonding than IgM-B when subjected to denaturation and gel electrophoresis under non-reducing conditions. Hybrids of char/salmon expressed μA-1, μA-2, μA and μB, indicating that there are two paralogous Ig heavy chain gene complexes in the haploid genome of char, like in Atlantic salmon. A comparison of salmonid μ sequences is presented, including representatives of Salmoninae (trout, salmon and char), Thymallinae (grayling) and Coregoninae (whitefish).