Presence of an unique IgT on the IGH locus in three-spined stickleback fish (Gasterosteus aculeatus) and the very recent generation of a repertoire of VH genes

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.

Molecular structure and functional responses of IgM, IgT and IgD to Flavobacterium covae and Streptococcus iniae infection in Asian seabass (Lates calcarifer Bloch, 1790)

The Asian seabass (Lates calcarifer) faces significant disease threats, which are exacerbated by intensive farming practices and environmental changes. Therefore, understanding its immune system is crucial. The current study presents a comprehensive analysis of immune-related genes in Asian seabass peripheral blood leukocytes (PBLs) using Iso-seq technology, identifying 16 key pathways associated with 7857 immune-related genes, comprising 634 unique immune-related genes. The research marks the first comprehensive report on the entire immunoglobulin repertoire in Asian seabass, revealing specific characteristics of immunoglobulin heavy chain constant region transcripts, including IgM (Cμ, ighm), IgT (Cτ, ight), and IgD (Cδ, ighd). The study confirms the presence of membrane-bound form, ighmmb, ightmb, ighdmb of IgM, IgT and IgD and secreted form, ighmsc and ightsc of IgM and IgT, respectively, with similar structural patterns and conserved features in amino acids across immunoglobulin molecules, including cysteine residues crucial for structural integrity observed in other teleost species. In response to bacterial infections by Flavobacterium covae (formerly F. columnare genomovar II) and Streptococcus iniae, both secreted and membrane-bound forms of IgM (ighmmb and ighmsc) and IgT (ightmb and ightsc) show significant expression, indicating their roles in systemic and mucosal immunity. The expression of membrane-bound form IgD gene, ighdmb, predominantly exhibits targeted upregulation in PBLs, suggesting a regulatory role in B cell-mediated immunity. The findings underscore the dynamic and tissue-specific expression of immunoglobulin repertoires, ighmmb, ighmsc, ightmb, ightsc and ighdmb in Asian seabass, indicating a sophisticated immune response to bacterial pathogens. These findings have practical implications for fish aquaculture, and disease control strategies, serving as a valuable resource for advancing research in Asian seabass immunology.

Lunar-linked biological rhythms in the immune system of freshwater three-spined stickleback

Immune responses are widely accepted to be under circadian regulation via a molecular clock, with many practical consequences, but much less is known of how other biological rhythms could affect the immune system. In this study, we search for lunar rhythms (circalunar, circasemilunar, and circatidal cycles) in the immune expression of the recently marine-derived freshwater fish, the low-plate morph of the three-spined stickleback. We employed time series of immune expression (mRNA) measurements for 14 immune-associated genes, representing a variety of immunological pathways. Times series measurements were taken on fish populations in the wild, in seminatural outdoor mesocosms, and in the laboratory, according to sampling regimens originally designed to study circannual variation but with the additional potential to provide information about lunar variation. Our evidence best supported the existence of a very small endogenous tidal rhythm. This is consistent with previous suggestions of the existence of a primordial tidal endogenous clock, some elements of which may be conserved in animals evolving outside the marine environment.

Comparative analysis of the immune responses of CcIgZ3 in mucosal tissues and the co-expression of CcIgZ3 and PCNA in the gills of common carp (Cyprinus carpio L.) in response to TNP-LPS

Fish live in an aquatic environment rich in various microorganisms and pathogens. Fish mucosal-associated lymphoid tissue (MALT) plays a very important role in immune defence. This study was conducted to characterize the immune response mediated by CcIgZ3 in common carp (Cyprinus carpio.) and investigate the proliferating CcIgZ3+ B lymphocytes in gill. We determined the expression of CcIgZ3 in many different tissues of common carp following stimulation by intraperitoneal injection of TNP-LPS (2,4,6-Trinitrophenyl hapten conjugated to lipopolysaccharide) or TNP-KLH (2,4,6-Trinitrophenyl hapten conjugated to Keyhole Limpet Hemocyanin). Compared with TNP-KLH, TNP-LPS can induce greater CcIgZ3 expression in the head kidney, gill and hindgut, especially in the gill. The results indicate that the gill is one of the main sites involved in the immune response mediated by CcIgZ3. To examine the distribution of CcIgZ3+ B lymphocytes, immunohistochemistry (IHC) experiments were performed using a polyclonal antibody against CcIgZ3. The results indicated that CcIgZ3 was detected in the head kidney, hindgut and gill. To further examine whether CcIgZ3+ B lymphocytes proliferate in the gills, proliferating CcIgZ3+ B cells were analysed by immunofluorescence staining using an anti-CcIgZ3 polyclonal antibody and an anti-PCNA monoclonal antibody. CcIgZ3 and PCNA (Proliferating Cell Nuclear Antigen) double-labelled cells in the gills were located within the epithelial cells of the gill filaments of common carp stimulated with TNP-LPS at 3 dps and 7 dps, and relatively more proliferating CcIgZ3+ B cells appeared in the gills of common carp at 7 dps. These data imply that CcIgZ3+ B cells in the gills might be produced by local proliferation following TNP-LPS stimulation. In summary, compared with those in TNP-KLH, CcIgZ3 preferentially affects the gills of common carp following challenge with TNP-LPS. CcIgZ3+ B cells proliferate in the gills to quickly produce the CcIgZ3 antibody. In addition, CcIgZ3+ B cells can be activated to induce a strong immune response very early locally in the gill and produce the antibody CcIgZ3, which helps exert an immune-protective effect. These results suggest that an effective vaccine can be designed to promote production of the mucosal antibody CcIgZ3.

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.

Characterization of T-cell receptors and immunoglobulin heavy chains loci and identification of T/B cell clusters in teleost

Bacterial disease is one of the important factors leading to economic losses in the turbot (Scophthalmus maximus) cultivation industry. T lymphocytes are major components of cellular immunity, whereas B lymphocytes produce immunoglobulins (Ig) that are key elements of humoral immune responses against infection. However, the genomic organization of genes encoding T-cell receptors (TCR) and immunoglobulin heavy chains (IgHs) in turbot remains largely unknown. In this study, abundant full-length transcripts of TCRs and IgHs were sequenced by Isoform-sequencing (Iso-seq), and we investigated and annotated the V, D, J and C gene loci of TCRα, TCRβ, IgT, IgM and IgD in turbot. Furthermore, through single-cell RNA sequencing (scRNA-seq) of blood leukocytes, we confirmed that these identified TCRs and IgHs were highly expressed in T/B cell clusters, respectively. Meanwhile, we also identified the IgM⁺IgD⁺ B and IgT⁺ B cells with differential gene expression profiles and potential functions. Taken together, our results provide a comprehensive understanding of TCRs and IgHs loci in turbot, which will contribute to evolutionary and functional characterization of T and B lymphocytes in teleost.

Molecular and functional characterization of two IgM subclasses in large yellow croaker (Larimichthys crocea)

As the predominant immunoglobulin (Ig) isotype, IgM plays a crucial role in the acquired immunity of vertebrates. There is only one Igμ gene in mammals, except cattle, while the number of Igμ gene varies among teleost fish. In the current study, we found two functional Igμ genes (Igμ1 and Igμ2) and a pseudo Cμ gene (ψIgμ) in large yellow croaker (Larimichthys crocea). Both Igμ1 and Igμ2 genes possessed two transcript variants, which encoded the heavy chains of secreted (sIgM1 and sIgM2) and membrane-bound IgM1 and IgM2 (mIgM1 and mIgM2), respectively. Both the heavy chains of sIgM1 and sIgM2 consisted of a variable Ig domain, four constant Ig domains (CH1, CH2, CH3 and CH4) and a secretory tail, while those of mIgM1 and mIgM2 consisted of a variable Ig domain, three constant Ig domains (CH1, CH2 and CH3), a transmembrane domain and a short cytoplasmic tail. Cysteine residues that are necessary for the formation of intrachain and interchain disulfide bonds and tryptophan residues that are important for the folding of the Ig superfamily domain were well conserved in large yellow croaker IgM1 and IgM2. Interestingly, large yellow croaker IgM2 had an extra cysteine (C94) in the CH1 domain compared with IgM1, which may cause the structural difference between IgM1 and IgM2. A liquid chromatography-tandem mass spectrometry analysis revealed that both IgM1 and IgM2 were present at the protein level in large yellow croaker serum. Both the Igμ1 and Igμ2 genes were mainly expressed in systemic immune tissues, such as head kidney and spleen, but the expression level of Igμ2 was much lower than that of Igμ1. After Pseudomonas plecoglossicida infection, the expression levels of Igμ1 and Igμ2 in both the spleen and head kidney were significantly upregulated, with a higher upregulation of Igμ2 than that of Igμ1. These results suggested that Igμ1 and Igμ2 may play a differential role in the immune response of large yellow croaker against bacterial infection.

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.

Identification and characterization of immunoglobulin tau (IgT) in Asian Seabass (Lates calcarifer) and mucosal immune response to nervous necrosis virus

Mucosal immunity plays a critical role in the protection of teleost fish against infection, but mucosal immunoglobulin of important aquaculture species unique to Southeast Asia remained greatly understudied. In this study, the sequence of immunoglobulin T (IgT) from Asian sea bass (ASB) is described for the first time. IgT of ASB possesses the characteristic structure of immunoglobulin with a variable heavy chain and four CH4 domains. The CH2-CH4 domains and full-length IgT were expressed and CH2-CH4 specific antibody was validated against full-length IgT expressed in Sf9 III cells. Subsequent use of the anti-CH2-CH4 antibody in immunofluorescence staining confirmed the presence of IgT-positive cells in the ASB gill and intestine. The constitutive expression of ASB IgT was characterized in different tissues and in response to red-spotted grouper nervous necrosis virus (RGNNV) infection. The highest basal expression of secretory IgT (sIgT) was observed in the mucosal and lymphoid tissues such as the gills, intestine and head kidney. Following NNV infection, IgT expression was upregulated in the head kidney and mucosal tissues. Moreover, a significant increase in localized IgT was found in gills and intestines of infected fish on day 14 post-infection. Interestingly, a significant increase in NNV-specific IgT secretion was only observed in the gills of the infected group. Our results suggest that ASB IgT may play an important role in the adaptive mucosal immune responses against viral infection and could potentially be adapted as a tool for the evaluation of prospective mucosal vaccines and adjuvants for the species.

Tracing the origin of fish immunoglobulins

We have studied the origin of immunoglobulin genes in fish. There are two evolutionary lines of bony fish, Actinopterygii and Sarcopterygii. The former gave rise to most of the current fish and the latter to the animals that went to land. Non-teleost actinopterygians are significant evolutionary, sharing a common ancestor with sarcopterygians. There are three different immunoglob- ulin isotypes in ray-finned fish: IgM, IgD and IgT. We deduce that translocon formation in im- munoglobulins genes occurred already in non-teleost Actinopterygii. We establish a relationship between no teleosts and teleostean fish at the domain level of different immunoglobulins. We found two evolutionary lines of immunoglobulin. A line that starts from Immunoglobulin M and another from an ancestral Immunoglobulin W. The M line is stable, and the W line gives rise to the IgD of the fish. Immunoglobulin T emerges by recombination between both lines.

Immunoglobulin heavy-chain loci in ancient allotetraploid goldfish

As an ancient allotetraploid species, goldfish (Carassius auratus) have two sets of subgenomes. In this study, immunoglobulin heavy-chain (IGH) genes were cloned from the red crucian carp (Carassius auratus red var.), and the corresponding loci were identified in the gynogenetic diploid red crucian carp (GRCC) genome as well as the genomes of three other goldfish strains (Wakin, G-12, and CaTCV-1). Examination showed that each goldfish strain possessed two sets of parallel IGH loci: a complete IGHA locus and a degenerated IGHB locus that was nearly 40 × smaller. In the IGHA locus, multiple τ chain loci were arranged in tandem between the μ&δ chain locus and the variable genes, but no τ-like genes were found in the IGHB locus.

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.

The evolutionary puzzle solution for the origins of the partial loss of the Cτ2 exon in notothenioid fishes

Cryonotothenioidea is the main group of fishes that thrive in the extremely cold Antarctic environment, thanks to the acquisition of peculiar morphological, physiological and molecular adaptations. We have previously disclosed that IgM, the main immunoglobulin isotype in teleosts, display typical cold-adapted features. Recently, we have analyzed the gene encoding the heavy chain constant region (CH) of the IgT isotype from the Antarctic teleost Trematomus bernacchii (family Nototheniidae), characterized by the near-complete deletion of the CH2 domain. Here, we aimed to track the loss of the CH2 domain along notothenioid phylogeny and to identify its ancestral origins. To this end, we obtained the IgT gene sequences from several species belonging to the Antarctic families Nototheniidae, Bathydraconidae and Artedidraconidae. All species display a CH2 remnant of variable size, encoded by a short Cτ2 exon, which retains functional splicing sites and therefore is included in the mature transcript. We also considered representative species from the three non-Antarctic families: Eleginopsioidea (Eleginops maclovinus), Pseudaphritioidea (Pseudaphritis urvillii) and Bovichtidae (Bovichtus diacanthus and Cottoperca gobio). Even though only E. maclovinus, the sister taxa of Cryonotothenioidea, shared the partial loss of Cτ2, the other non-Antarctic notothenioid species displayed early molecular signatures of this event. These results shed light on the evolutionary path that underlies the origins of this remarkable gene structural modification.

Mucosal immunoglobulins of teleost fish: A decade of advances

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

A long reads-based de-novo assembly of the genome of the Arlee homozygous line reveals chromosomal rearrangements in rainbow trout

Currently, there is still a need to improve the contiguity of the rainbow trout reference genome and to use multiple genetic backgrounds that will represent the genetic diversity of this species. The Arlee doubled haploid line was originated from a domesticated hatchery strain that was originally collected from the northern California coast. The Canu pipeline was used to generate the Arlee line genome de-novo assembly from high coverage PacBio long-reads sequence data. The assembly was further improved with Bionano optical maps and Hi-C proximity ligation sequence data to generate 32 major scaffolds corresponding to the karyotype of the Arlee line (2 N = 64). It is composed of 938 scaffolds with N50 of 39.16 Mb and a total length of 2.33 Gb, of which ∼95% was in 32 chromosome sequences with only 438 gaps between contigs and scaffolds. In rainbow trout the haploid chromosome number can vary from 29 to 32. In the Arlee karyotype the haploid chromosome number is 32 because chromosomes Omy04, 14 and 25 are divided into six acrocentric chromosomes. Additional structural variations that were identified in the Arlee genome included the major inversions on chromosomes Omy05 and Omy20 and additional 15 smaller inversions that will require further validation. This is also the first rainbow trout genome assembly that includes a scaffold with the sex-determination gene (sdY) in the chromosome Y sequence. The utility of this genome assembly is shown through the improved annotation of the duplicated genome loci that harbor the IGH genes on chromosomes Omy12 and Omy13.

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.

Molecular characterization of a new IgZ3 subclass in common carp (Cyprinus carpio) and comparative expression analysis of IgH transcripts during larvae development

Background

Immunoglobulins (Igs) distributed among systemic immune tissues and mucosal immune tissues play important roles in protecting teleosts from infections in the pathogen-rich aquatic environment. Teleost IgZ/IgT subclasses with different tissue expression patterns may have different immune functions.

Results

In the present study, a novel secreted IgZ heavy chain gene was cloned and characterized in common carp (Cyprinus carpio). This gene exhibited a different tissue-specific expression profile than the reported genes IgZ1 and IgZ2. The obtained IgZ-like subclass gene designated CcIgZ3, had a complete open reading frame contained 1650 bp encoding a protein of 549 amino acid residues. Phylogenetic analysis revealed that CcIgZ3 was grouped with carp IgZ2 and was in the same branch as IgZ/IgT genes of other teleosts. Basal expression detection of the immunoglobulin heavy chain (IgH) in healthy adult common carp showed that CcIgZ3 transcripts were widely expressed in systemic immune tissues and mucosal-associated lymphoid tissues. CcIgZ3 was expressed at the highest levels in the head kidneys, gills, and gonads, followed by the spleen, hindgut, oral epithelium, liver, brain, muscle, foregut, and blood; it was expressed at a very low level in the skin. The transcript expression of CcIgZ3 in leukocytes isolated from peripheral blood cells was significantly higher than that in leukocytes isolated from the spleen. Different groups of common carp were infected with Aeromonas hydrophila via intraperitoneal injection or immersion. RT-qPCR analysis demonstrated that significant differences in CcIgZ3 mRNA levels existed between the immersion and injection groups in all the examined tissues, including the head kidney, spleen, liver, and hindgut; in particular, the CcIgZ3 mRNA level in the hindgut was higher in the immersion group than in the injection group. The different routes of A. hydrophila exposure in common carp had milder effects on the IgM response than on the CcIgZ3 response. Further study of the relative expression of the IgH gene during the development of common carp showed that the tissue-specific expression profile of CcIgZ3 was very different from those of other genes. RT-qPCR analysis demonstrated that the CcIgZ3 mRNA level increased gradually in common carp during the early larval development stage from 1 day post fertilization (dpf) to 31 dpf with a dynamic tendency similar to those of IgZ1 and IgZ2, and IgM was the dominant Ig with obviously elevated abundance. Analyses of the tissue-specific expression of IgHs in common carp at 65 dpf showed that CcIgZ3 was expressed at mucosal sites, including both the hindgut and gill; in contrast, IgZ1 was preferentially expressed in the hindgut, and IgZ2 was preferentially expressed in the gill. In addition to RT-qPCR analysis, in situ hybridization was performed to detect CcIgZ3-expressing cells and IgM-expressing cells. The results showed that CcIgZ3 and IgM transcripts were detectable in the spleens, gills, and hindguts of common carp at 65 dpf.

Conclusions

These results reveal that CcIgZ3 gene transcripts are expressed in common carp during developmental stage not only in systemic tissues but also in mucosal tissues. CcIgZ3 expression can be induced in immune tissues by A. hydrophila challenge via immersion and intraperitoneal injection with significantly different expression profiles, which indicates that CcIgZ3 is involved in the antimicrobial immune response and might play an important role in gut mucosal immunity.

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.

Gouania willdenowi is a teleost fish without immunoglobulin genes

Immunoglobulin (Ig) genes encode antibodies in jawed vertebrates. They are essential elements of the adaptive immune response. Ig exists in soluble form or as part of the B cell membrane antigen receptor (BCR). Studies of Ig genes in fish genomes reveal the absence of Ig genes in Gouania willdenowi by deletion of the entire Ig locus from the canonical chromosomal region. The genes coding for integral BCR proteins, CD79a and CD79b, are also absent. Genes exist for T α/β lymphocyte receptors but not for the T γ/δ receptors. The results of the genomic analysis are independently corroborated with RNA-Seq transcriptomes from other Gobiesocidae species. From the transcriptome studies, Ig is also absent from these other Gobiesocidae species, Acyrtus sp. and Tomicodon sp. Present evidence suggests that Ig is missing from all species of the Gobiesocidae family.

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.

Immunoglobulin T genes in Actinopterygii

In teleost fishes, there are three immunoglobulin isotypes named immunoglobulin M (IgM), D (IgD), and T (IgT). IgT was the last to be described in teleost fishes, and it is specific to them. From recent fish genomes, we identified and studied the immunoglobulin heavy chain genes in Actinopterygii. For this analysis, a custom bioinformatics and machine learning pipeline, we call CHfinder, was developed that identifies the exons coding for the CH domains of fish immunoglobulins. Some IgT in teleost and holostean fish found from this systematic study have not been previously described. Phylogenetic analysis of the deduced amino acid sequences of the IgT CH1 exons reveals they are similar to the CH1 of IgM. This analysis also shows that the other three domains (CH2, CH3, and CH4) were not the result of recent IgM duplication processes in Actinopterygii, demonstrating that it is an immunoglobulin of earlier origin. The bioinformatics program, CHfinder, is publicly available at https://github.com/compimmuno/CHfinder.

Insights into the evolution of IG genes in Amphibians and reptiles

Immunoglobulins are essential proteins of the immune system to neutralize pathogens. Gene encoding B cell receptors and antibodies (Ig genes) first appeared with the emergence of early vertebrates having a jaw, and are now present in all extant jawed vertebrates, or Gnathostomata. The genes have undergone evolutionary changes. In particular, genomic structural changes corresponding to genes of the adaptive immune system were coincident or in parallel with the adaptation of vertebrates from the sea to land. In cartilaginous fish exist IgM, IgD/W, and IgNAR and in bony fish IgM, IgT, IgD. Amphibians and reptiles witnessed significant modifications both in the structure and orientation of IG genes. In particular, for these amphibians and Amniota that adapted to land, IgM and IgD genes were retained, but other isotypes arose, including genes for IgA(X)1, IgA(X)2, and IgY. Recent progress in high throughput genome sequencing is helping to uncover the IG gene structure of all jawed vertebrates. In this work, we review the work and present knowledge of immunoglobulin genes in genomes of amphibians and reptiles.

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.

Immunoglobulins, Mucosal Immunity and Vaccination in Teleost Fish

Due to direct contact with aquatic environment, mucosal surfaces of teleost fish are continuously exposed to a vast number of pathogens and also inhabited by high densities of commensal microbiota. The B cells and immunoglobulins within the teleost mucosa-associated lymphoid tissues (MALTs) play key roles in local mucosal adaptive immune responses. So far, three Ig isotypes (i.e., IgM, IgD, and IgT/Z) have been identified from the genomic sequences of different teleost fish species. Moreover, teleost Igs have been reported to elicit mammalian-like mucosal immune response in six MALTs: gut-associated lymphoid tissue (GALT), skin-associated lymphoid tissue (SALT), gill-associated lymphoid tissue (GIALT), nasal-associated lymphoid tissue (NALT), and the recently discovered buccal and pharyngeal MALTs. Critically, analogous to mammalian IgA, teleost IgT represents the most ancient Ab class specialized in mucosal immunity and plays indispensable roles in the clearance of mucosal pathogens and the maintenance of microbiota homeostasis. Given these, this review summarizes the current findings on teleost Igs, MALTs, and their immune responses to pathogenic infection, vaccination and commensal microbiota, with the purpose of facilitating future evaluation and rational design of fish vaccines.

Identification of an IgD/IgT chimera in the European sea bass (Dicentrarchus labrax L.)

Three classes of immunoglobulins have been identified in Teleosts: IgM, IgT/Z and IgD. They are fundamental for fish immune responses and, therefore, their functional activities are heavily investigated. In this paper, we describe the identification of a new IgD/IgT chimera in sea bass (Dicentrarchus labrax) from a gills transcriptome. This transcript joined the first six constant domains of the IgD chain with the two terminal constant domains of IgT, generating a long in-frame coding sequence with a junction between the canonical δ6 exon splicing donor site and the τ3 exon splicing acceptor site. Studies performed on genomic DNA confirmed the presence of the sequence and identifies and intronic region of 656 bp within this joining region. The basal expression of the IgD/IgT chimera was investigated both in silico and in vivo: high level of expression was found in gills, gut and head kidney. Moreover, IgD/IgT transcripts were up-regulated after in vitro stimulation of sea bass HK leukocytes with LPS. The IgD/IgT chimera was found also in two congener species, Morone saxatilis and Morone chrysops. It is not possible to have a precise idea on the evolutionary scenario that lead to the appearance of this sequence due to the lack of genomic information, but we could speculate that an ancestral duplication of the entire IgH locus was followed by the chimerization of Cδ/Cτ in one of the two loci. Finally, the IgD/IgT high basal expression in tissues and organs fundamental for sea bass immune response and its modulation after LPS stimulation provide a very preliminary indication that this unusual Ig variant could have a functional activity.

Extreme genomic volatility characterizes the evolution of the immunoglobulin heavy chain locus in cyprinodontiform fishes

The evolution of the adaptive immune system has provided vertebrates with a uniquely sophisticated immune toolkit, enabling them to mount precise immune responses against a staggeringly diverse range of antigens. Like other vertebrates, teleost fishes possess a complex and functional adaptive immune system; however, our knowledge of the complex antigen-receptor genes underlying its functionality has been restricted to a small number of experimental and agricultural species, preventing systematic investigation into how these crucial gene loci evolve. Here, we analyse the genomic structure of the immunoglobulin heavy chain (IGH) gene loci in the cyprinodontiforms, a diverse and important group of teleosts present in many different habitats across the world. We reconstruct the complete IGH loci of the turquoise killifish (Nothobranchius furzeri) and the southern platyfish (Xiphophorus maculatus) and analyse their in vivo gene expression, revealing the presence of species-specific splice isoforms of transmembrane IGHM. We further characterize the IGH constant regions of 10 additional cyprinodontiform species, including guppy, Amazon molly, mummichog and mangrove killifish. Phylogenetic analysis of these constant regions suggests multiple independent rounds of duplication and deletion of the teleost-specific antibody class IGHZ in the cyprinodontiform lineage, demonstrating the extreme volatility of IGH evolution. Focusing on the cyprinodontiforms as a model taxon for comparative evolutionary immunology, this work provides novel genomic resources for studying adaptive immunity and sheds light on the evolutionary history of the adaptive immune system.

A Comparison of the Innate and Adaptive Immune Systems in Cartilaginous Fish, Ray-Finned Fish, and Lobe-Finned Fish

The immune system is composed of two subsystems-the innate immune system and the adaptive immune system. The innate immune system is the first to respond to pathogens and does not retain memory of previous responses. Innate immune responses are evolutionarily older than adaptive responses and elements of innate immunity can be found in all multicellular organisms. If a pathogen persists, the adaptive immune system will engage the pathogen with specificity and memory. Several components of the adaptive system including immunoglobulins (Igs), T cell receptors (TCR), and major histocompatibility complex (MHC), are assumed to have arisen in the first jawed vertebrates-the Gnathostomata. This review will discuss and compare components of both the innate and adaptive immune systems in Gnathostomes, particularly in Chondrichthyes (cartilaginous fish) and in Osteichthyes [bony fish: the Actinopterygii (ray-finned fish) and the Sarcopterygii (lobe-finned fish)]. While many elements of both the innate and adaptive immune systems are conserved within these species and with higher level vertebrates, some elements have marked differences. Components of the innate immune system covered here include physical barriers, such as the skin and gastrointestinal tract, cellular components, such as pattern recognition receptors and immune cells including macrophages and neutrophils, and humoral components, such as the complement system. Components of the adaptive system covered include the fundamental cells and molecules of adaptive immunity: B lymphocytes (B cells), T lymphocytes (T cells), immunoglobulins (Igs), and major histocompatibility complex (MHC). Comparative studies in fish such as those discussed here are essential for developing a comprehensive understanding of the evolution of the immune system.

Cloning and expressional analysis of secretory and membrane-bound IgM in rock bream (Oplegnathus fasciatus) under megalocytivirus infection and vaccination

In this study, for better understanding the humoral immunity of rock bream (Oplegnathus fasciatus), 2 transcripts of immunoglobulin M (IgM) heavy chain gene including membrane bound (m-IgM) and secretory (s-IgM) forms were sequenced and analyzed their tissue distribution and differential expression in rock bream under rock bream iridovirus (RBIV) infection and vaccination since RBIV has caused mass mortality in rock bream aquaculture in Korea. Consequently, s-IgM cDNA was 1902 bp in length encoding a leader region, a variable region, four constant regions (CH1, CH2, CH3, CH4) and a C-terminal region while m-IgM cDNA was 1689 bp in length encoding shorter three constant regions (CH1, CH2, CH3) and two transmembrane regions. The predicted s-IgM and m-IgM represent a high structural similarity to other species including human. In tissue distribution analysis in healthy fish, the highest expression of s-IgM was observed in head kidney followed by body kidney, spleen, and mid gut whereas m-IgM expression was the highest in blood followed by head kidney and spleen. In vitro, s-IgM expression was up-regulated by LPS in head kidney and spleen cells at 24 h with no change of m-IgM expression. In vivo upon vaccination, s-IgM expression was up-regulated in liver and blood but not in head kidney while m-IgM expression was only up-regulated in head kidney. After challenge with RBIV, s-IgM expression level was higher in vaccinated fish than in unvaccinated fish and m-IgM expression was up-regulated in head kidney of vaccinated group. In conclusion, differential expression of m-IgM and s-IgM may indicate their differential functions to produce the most effective IgM during adaptive immune response. Although it is not able to assess specific IgM at protein level due to a lack of antibody against rock bream IgM, the present study on s-IgM and m-IgM gene expressions upon infection and vaccination will be useful in developing efficient vaccines in the future.

Immunoglobulin (Ig) heavy chain gene locus and immune responses upon parasitic, bacterial and fungal infection in loach, Misgurnus anguillicaudatus

Teleost fish are the most primitive bony vertebrates that contain immunoglobulin (Ig). Although teleost Ig is known to be important during tetrapod evolution and comparative immunology, little is known about the genomic organization of the immunoglobulin heavy-chain (IgH) locus. Here, three Ig isotype classes, IgM, IgD and IgT, were firstly identified in dojo loach (Misgurnus anguillicaudatus), and the IgH locus covering τ, μ and δ genes was also illustrated. Variable (V) gene segments lie upstream of two tandem diversity (D), joining (J) and constant (C) clusters and the genomic organization of the IgH locus presented as V_n-D_n-J_n-C_τ-D_n-J_n-C_μ-C_δ, similar to some other teleost fish. However, unlike some other teleost fish, ten V_H, ten D and nine J genes were observed in this locus, which suggest teleost Igs might be conserved and diverse. Thus, it would be interesting to determine how Igs divide among themselves in immune response to different antigens. To address this hypothesis, we have developed three models by bath infection with parasitic, bacterial and fungal pathogens, respectively. We found that IgM, IgD and IgT were highly upregulated in the head kidney and spleen after infection with Ichthyophthirius multifiliis (Ich), suggesting that the three Igs might participate in the systemic immune responses to Ich. Moreover, the high expression of IgT in mucosal tissue, such as skin or gills, appeared after being infected with three different pathogens infection, respectively, in which the expression of IgT increased more rapidly in response to Ich infection. Interestingly, the expression of IgD showed a higher increase in spleen and head kidney being challenged with fungi, suggesting that IgD might play an important role in antifungal infection.

Comparative analysis of spleen transcriptome detects differences in evolutionary adaptation of immune defense functions in bighead carp and silver carp

The evolutionary divergence of the immune defense functions in bighead carp (A. nobilis) and silver carp (H. molitrix) is still not understood at the molecular level. Here, we obtained 48,821,754 and 55,054,480 clean reads from spleen tissue libraries prepared for bighead carp and silver carp using Illumina paired-end sequencing technology, respectively, and identified 4976 orthologous genes from the transcriptome data sets by comparative analysis. Adaptive evolutionary analysis showed that 212 orthologous genes and 255 Gene Ontology (GO) terms were subjected to positive selection(Ka/Ks values > 1) only in bighead carp, and 195 orthologous genes and 309 GO terms only in silver carp. Among immune defense functions with significant evolutionary divergence, the positively selected biological processes in bighead carp mainly included B cell-mediated immunity, chemokine-mediated signaling pathway, and immunoglobulin mediated immune response, whereas those in silver carp mainly included the antigen processing and presentation, defense response to fungus, and detection of bacteria. Moreover, we found 2974 genes expressed only in spleen of bighead carp and 3494 genes expressed only in spleen of silver carp, where these genes were mostly enriched in the same biological processes or pathways. These results provide a better understanding of the differences in resistance to some diseases by bighead carp and silver carp, and also facilitate the identification of candidate genes related to disease resistance.

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.

Novel Insights on the Regulation of B Cell Functionality by Members of the Tumor Necrosis Factor Superfamily in Jawed Fish

Most ligands and receptors from the tumor necrosis factor (TNF) superfamily play very important roles in the immune system. In particular, many of these molecules are essential in the regulation of B cell biology and B cell-mediated immune responses. Hence, in mammals, it is known that many TNF family members play a key role on B cell development, maturation, homeostasis, activation, and differentiation, also influencing the ability of B cells to present antigens or act as regulators of immune responses. Evolutionarily, jawed fish (including cartilaginous and bony fish) constitute the first animal group in which an adaptive immune response based on B cells and immunoglobulins is present. However, until recently, not much was known about the expression of TNF ligands and receptors in these species. The sequences of many members of the TNF superfamily have been recently identified in different species of jawed fish, thus allowing posterior analysis on the role that these ligands and receptors have on B cell functionality. In this review, we summarize the current knowledge on the impact that the TNF family members have in different aspects of B cell functionality in fish, also providing an in depth comparison with functional aspects of TNF members in mammals, that will permit a further understanding of how B cell functionality is regulated in these distant animal groups.

Innate and adaptive immune molecules of striped murrel Channa striatus

Channa striatus, also called snakehead murrel, is an important freshwater teleost fish which has been widely cultured for its tasty flesh along with nutritional and medicinal values. The growth of both cultured and wild murrels is affected by various physical, chemical and biological factors. As a teleost fish, C. striatus is an intermediate organism between invertebrates and vertebrates. They have a well‐developed innate immune system than invertebrates and a primitive adaptive immune system compared to that of higher vertebrates, thus an interesting unique immune structure to explore. Studies have identified that a few external stimulants do instigate the immune system to fight against the pathogens at the time of infection in C. striatus. This review discusses the physicochemical and biological stress factors, immune system and immune molecules of C. striatus which are potentially involved in combating the stress factors.

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.

Molecular cloning of IgZ heavy chain isotype in Catla catla and comparative expression profile of IgZ and IgM following pathogenic infection

Immunoglobulins serve as a crucial arm of the adaptive immune system against detrimental pathogenic threats in teleosts. However, whether the novel Ig isotype IgZ is present in the Indian major carp, Catla catla, has not yet been elucidated. The present study reports the presence of IgZ ortholog in C. catla (CcIgZ) and further demonstrates its comparative tissue specific expression with IgM (CcIgM) in response to bacterial and parasitic stimulation. The putative 139 amino acid sequence of IgZ heavy chain cDNA of C. catla showed homology with IgZ constant domains of other teleosts. Phylogenetic analysis of the predicted IgZ transcript sequence clustered with previously identified IgZ heavy chain sequences of Cyprinidae family members. The inductive expression profiles of IgZ and IgM genes were evaluated in immunologically relevant tissues at 24, 48 and 72 hr post infection with Aeromonas hydrophila, Streptococcus uberis and Argulus sp. Both CcIgZ and CcIgM were expressed most strongly in the kidneys of healthy fish. Basal expression of CcIgM transcript was higher than that of CcIgZ in all the examined tissues. Stimulation with bacteria triggered significant increase of IgZ in the intestine (P < 0.001) and spleen (P < 0.01), whereas IgM was relatively up-regulated in blood (P < 0.001) after stimulation with each of the three pathogens assessed. The study is the first to report identification of IgZ in C. catla. Further, it provides insights into the differential expression profiles of IgZ and IgM isotypes against various pathogenic infection in C. catla, which may facilitate better prophylaxis again such infections.

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.

Fish Immunoglobulins

The B cell receptor and secreted antibody are at the nexus of humoral adaptive immunity. In this review, we summarize what is known of the immunoglobulin genes of jawed cartilaginous and bony fishes. We focus on what has been learned from genomic or cDNA sequence data, but where appropriate draw upon protein, immunization, affinity and structural studies. Work from major aquatic model organisms and less studied comparative species are both included to define what is the rule for an immunoglobulin isotype or taxonomic group and what exemplifies an exception.

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.

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.

Changing expression of vertebrate immunity genes in an anthropogenic environment: a controlled experiment

Background

The effect of anthropogenic environments on the function of the vertebrate immune system is a problem of general importance. For example, it relates to the increasing rates of immunologically-based disease in modern human populations and to the desirability of identifying optimal immune function in domesticated animals. Despite this importance, our present understanding is compromised by a deficit of experimental studies that make adequately matched comparisons between wild and captive vertebrates.

Results

We transferred post-larval fishes (three-spined sticklebacks), collected in the wild, to an anthropogenic (captive) environment. We then monitored, over 11 months, how the systemic expression of immunity genes changed in comparison to cohort-matched wild individuals in the originator population (total n = 299). We found that a range of innate (lyz, defbl2, il1r-like, tbk1) and adaptive (cd8a, igmh) immunity genes were up-regulated in captivity, accompanied by an increase in expression of the antioxidant enzyme, gpx4a. For some genes previously known to show seasonality in the wild, this appeared to be reduced in captive fishes. Captive fishes tended to express immunity genes, including igzh, foxp3b, lyz, defbl2, and il1r-like, more variably. Furthermore, although gene co-expression patterns (analyzed through gene-by-gene correlations and mutual information theory based networks) shared common structure in wild and captive fishes, there was also significant divergence. For one gene in particular, defbl2, high expression was associated with adverse health outcomes in captive fishes.

Conclusion

Taken together, these results demonstrate widespread regulatory changes in the immune system in captive populations, and that the expression of immunity genes is more constrained in the wild. An increase in constitutive systemic immune activity, such as we observed here, may alter the risk of immunopathology and contribute to variance in health in vertebrate populations exposed to anthropogenic environments.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0751-8) contains supplementary material, which is available to authorized users.

sIgZ exhibited maternal transmission in embryonic development and played a prominent role in mucosal immune response of Megalabrama amblycephala

IgZ is considered to be the last immunoglobulin discovered in vertebrate species. In this study, the structure of secreted form of blunt snout bream (Megalabrama amblycephala) IgZ (sIgZ) heavy chain gene is VH-Cζ1-Cζ2-Cζ3-Cζ4, in which Cζ4 provides the specificity to the IgZ isotype. The deduced amino acid sequence of sIgZ shows highest similarity with that of Ctenopharyngodon idella (79%). The ontogeny of sIgZ gene expression showed a V-shape change pattern: decreased initially from unfertilized egg stage to 16-cell embryos and increased significantly from blastula stage, which exhibited maternal transmission effects. Compared with the juvenile fish, sIgZ mRNA level was higher in head kidney, spleen, trunk kidney, liver, intestine and gill of adult fish. In both juvenile and adult fish, sIgZ mRNA was detected in intestine and gill. Aeromonas hydrophila challenge experiment showed that sIgZ transcription significantly increased in skin, gill and intestine, indicating a prominent mucosal immune response. The results of Western blot also verified the protein alterations of sIgZ in mucosal organs in M. amblycephal. Our studies indicate a prominent role of IgZ in mucosa-associated lymphoid tissue immunity and further support the specialized role of IgZ in teleost mucosal immune responses.

Spatio-temporal expression of blunt snout bream (Megalobrama amblycephala) mIgD and its immune response to Aeromonas hydrophila

The function of IgD in fish and mammals has not been fully understood since its discovery. In this study, we have isolated and characterized the cDNA that encodes membrane-bound form of the immunoglobulin D heavy chain gene (mIgD) of blunt snout bream (Megalobrama amblycephala) using RT-PCR and rapid amplification of cDNA ends (RACE). The full-length cDNA of mIgD consisted of 3313 bp, encoding a putative protein of 943 amino acids. The structure of blunt snout bream mIgD is VDJ-μ1-δ1-δ2-δ3-δ4-δ5-δ6-δ7-TM. Multiple alignment and phylogenetic analyses indicated that blunt snout bream mIgD clusters with the homologues of cyprinid fish and that its highest identity is with that of C. idella (82%). The mIgD expression in early different developmental stages showed that the level of mIgD mRNA decreased dramatically from the unfertilized egg stage to the 32-cell stage, suggesting that mIgD mRNA was maternally transferred. As cell differentiation initially took place in the blastula stage, the mIgD expression increased significantly from the blastula stage to prelarva, which might be attributed to embryonic stem cell differentiation processes. Compared with juvenile fish, the expression and tissue distribution patterns of mIgD in adult individuals exhibited considerable variation. After the injection of Aeromonas hydrophila, mIgD expression was up-regulated in various tissues, reaching the peak expression at 5 d, 14 d or 21 d (depending on the tissue type). The present study provides a theoretical basis for further research of the teleost immune system.

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.