Genes of the constant regions of functional immunoglobulin heavy chain of Japanese flounder, Paralichthys olivaceus

An Introduction to Relevant Immunology Principles with Respect to Oral Vaccines in Aquaculture

Vaccines continue to play an enormous role in the progression of aquaculture industries worldwide. Though preventable diseases cause massive economic losses, injection-based vaccine delivery is cost-prohibitive or otherwise impractical for many producers. Most oral vaccines, which are much cheaper to administer, do not provide adequate protection relative to traditional injection or even immersion formulas. Research has focused on determining why there appears to be a lack of protection afforded by oral vaccines. Here, we review the basic immunological principles associated with oral vaccination before discussing the recent progress and current status of oral vaccine research. This knowledge is critical for the development and advancement of efficacious oral vaccines for the aquaculture industry.

Mystifying Molecular Structure, Expression and Repertoire Diversity of IgM Heavy Chain Genes (Ighμ) in Clarias Catfish and Hybrids: Two Novel Transcripts in Vertebrates

Two novel immunoglobulin heavy chain (Ighμ) transcripts encoding membrane-bound forms of IgM (mIgM) were discovered in bighead catfish, Clarias macrocephalus. The first transcript contains four constant and two transmembrane domains [Cμ1-Cμ2-Cμ3-Cμ4-TM1-TM2] that have never been reported in teleosts, and the second transcript is an unusual mIgM that has never been identified in any vertebrate [Cμ1-(Cδ2-Cδ3-Cδ4-Cδ5)-Cμ2-Cμ3-TM1-TM2]. Fluorescence in situ hybridization (FISH) in bighead catfish, North African catfish (C. gariepinus) and hybrid catfish revealed a single copy of Ighμ in individual parent catfish, while two gene copies were found in diploid hybrid catfish. Intensive sequence analysis demonstrated multiple distinct structural variabilities in the VH domain in Clarias, and hybrid catfish were defined and used to generate diversity with various mechanisms. Expression analysis of Ighμ in Aeromonas hydrophila infection of the head kidney, peripheral blood leukocytes and spleen revealed significantly higher levels in North African catfish and hybrid catfish than in bighead catfish.

Transcriptome analysis and protein-protein interaction in resistant and susceptible families of Japanese flounder (Paralichthys olivaceus) to understand the mechanism against Edwardsiella tarda

Edwardsiella tarda is one of the most harmful bacterial pathogens for aquaculture flatfish. After artificial infection of 47 Japanese flounder (Paralichthys olivaceus) families, resistant and susceptible families were identified in this study. High-throughput sequencing was performed on the liver transcriptome of uninfected groups (PoRU and PoSU) and infected groups (PoRC and PoSC). Through assembly and annotation, a total of 3012 and 1386 differentially expressed genes (DEGs) were identified in PoRU vs. PoSU and PoRC vs. PoSC. The significant enrichment pathways between PoRU and PoSU were mainly in metabolic and biosynthesis pathways. A total of thirty dominant enrichment pathways between PoRC and PoSC mainly focused on some immune-related pathways, including the hematopoietic cell lineage, cytokine-cytokine receptor interaction, complement and coagulation cascades, antigen processing and presentation, the intestinal immune network for immunoglobulin A (IgA) production and T/B cell receptor signaling pathway. Under the protein-protein interaction (PPI) analysis, hub genes, including CD molecules, complement component factors and chemokines, were identified in the network, and 16 core genes were differentially expressed in resistant and sustainable families in quantitative polymerase chain reaction (qPCR) validation. This study represents the first transcriptome analysis based on resistant and susceptible families and provides resistant genes to understand the potential molecular mechanisms of antibacterial function in marine fish. The results obtained in this study provide crucial information on gene markers for resistant breeding of Japanese flounder.

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.

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.

Development of a Monoclonal Antibody Specific to the IgM Heavy Chain of Bighead Catfish (Clarias macrocephalus): A Biomolecular Tool for the Detection and Quantification of IgM Molecules and IgM+ Cells in Clarias Catfish

Catfish is a commonly-cultivated freshwater fish in Thailand and many Southeast Asian countries. The molecular data obtained for the IgM heavy chain (IgMH) of catfish have been useful for distinguishing monoclonal antibodies (mAbs). A mAb specific to Cμ₁ of the IgMH of catfish (IgMHCμ₁ mAb) was developed in a rabbit model using sequence information from bighead catfish (Clarias macrocephalus). The IgMHCμ₁ mAb strongly recognized the IgM heavy chain of the tested catfish, namely, bighead catfish, African catfish (Clarias gariepinus) and their hybrid (C. macrocephalus × C. gariepinus), in immunological Western blot analysis and competitive ELISAs. Additionally, the IgMHCμ₁ mAb successfully recognized IgM⁺ cells by detecting IgM molecules in both secreted and membrane-bound forms in peripheral blood leukocytes (PBLs). The IgMHCμ₁ mAb was further used to quantify the percentage of IgM⁺ cells among PBLs through flow cytophotometry. The IgM⁺ cell percentages of healthy bighead catfish, African catfish and their hybrid were 38.0–39.9%, 45.6–53.2%, and 58.7–60.0%, respectively. Furthermore, the IgMHCμ₁ mAb showed no cross-reactivity with the IgM of zebrafish. These findings suggest that this mAb can be used as an immunological tool for monitoring the health, immune status, and immune development of cultivated Clarias catfish.

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.

Molecular characterization and expression analysis of secretory immunoglobulin M (IgM) heavy chain gene in rohu, Labeo rohita

Immunoglobulin M (IgM) is the major isotype among teleost immunoglobulins. The present study was aimed to explore IgM heavy chain gene and its expression profile in rohu. Full-length IgM heavy chain cDNA of rohu consisted of 1994 bp encoding a polypeptide of 576 amino acid residues including a leader peptide, variable (VH) and constant (CH1-CH2-CH3-CH4) domains confirming the secretory form of IgM. The sequence carries conserved residues such as cysteine, tryptophan and amino acid motifs like 'YYCAR' and 'FDYWGKGT-VTV-S'. The predicted 3 D model confirmed various domains of rohu IgM heavy chain. Phylogenetic tree analysis revealed that IgM heavy chain gene of rohu shared the same cluster with that of other cyprinid fishes. Tissue distribution analysis showed the predominant level of IgM heavy chain gene expression in kidney, spleen and intestine. IgM heavy chain gene expression in rohu kidney was found to be up-regulated and reached a maximum at 7 days post-challenge with Aeromonas hydrophila. These findings demonstrate the first report of full-length secretory IgM heavy chain gene in rohu. Besides, IgM heavy chain gene was highly expressed in major lymphoid tissues and bacterial challenge influenced its expression which further confirmed its role in the adaptive humoral immune response.

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.

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.

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.

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.

Characterization of the Z lineage Major histocompatability complex class I genes in zebrafish

Zebrafish (Danio rerio) are a valuable model for studying immunity, infection, and hematopoietic disease and have recently been employed for transplantation assays. However, the lack of syngeneic zebrafish creates challenges with identifying immune-matched individuals. The MHC class I genes, which mediate allogeneic recognition in mammals, have been grouped into three broad lineages in zebrafish: the classical U genes on chromosome 19, the Z genes which have been reported to map to chromosome 1, and the L genes that map to multiple loci. Transplantations between individual zebrafish that are matched at the U locus fail to consistently engraft suggesting that additional loci contribute to allogeneic recognition. Although two full-length zebrafish Z transcripts have been described, the genomic organization and diversity of these genes have not been reported. Herein we define ten Z genes on chromosomes 1 and 3 and on an unplaced genomic scaffold. We report that neither of the Z transcripts previously described match the current genome assembly and classify these transcripts as additional gene loci. We characterize full-length transcripts for 9 of these 12 genes. We demonstrate a high level of expression variation of the Z genes between individual zebrafish suggestive of haplotypic variation. We report low level sequence variation for individual Z genes between individual zebrafish reflecting a possible nonclassical function, although these molecules may still contribute to allogeneic recognition. Finally, we present a gene nomenclature system for the Z genes consistent with MHC nomenclature in other species and with the zebrafish gene nomenclature guidelines.

The astonishing diversity of Ig classes and B cell repertoires in teleost fish

With lymphoid tissue anatomy different than mammals, and diverse adaptations to all aquatic environments, fish constitute a fascinating group of vertebrate to study the biology of B cell repertoires in a comparative perspective. Fish B lymphocytes express immunoglobulin (Ig) on their surface and secrete antigen-specific antibodies in response to immune challenges. Three antibody classes have been identified in fish, namely IgM, IgD, and IgT, while IgG, IgA, and IgE are absent. IgM and IgD have been found in all fish species analyzed, and thus seem to be primordial antibody classes. IgM and IgD are normally co-expressed from the same mRNA through alternative splicing, as in mammals. Tetrameric IgM is the main antibody class found in serum. Some species of fish also have IgT, which seems to exist only in fish and is specialized in mucosal immunity. IgM/IgD and IgT are expressed by two different sub-populations of B cells. The tools available to investigate B cell responses at the cellular level in fish are limited, but the progress of fish genomics has started to unravel a rich diversity of IgH and immunoglobulin light chain locus organization, which might be related to the succession of genome remodelings that occurred during fish evolution. Moreover, the development of deep sequencing techniques has allowed the investigation of the global features of the expressed fish B cell repertoires in zebrafish and rainbow trout, in steady state or after infection. This review provides a description of the organization of fish Ig loci, with a particular emphasis on their heterogeneity between species, and presents recent data on the structure of the expressed Ig repertoire in healthy and infected fish.

Mucosal immunoglobulins and B cells of teleost fish

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

Processing of fish Ig heavy chain transcripts: diverse splicing patterns and unusual nonsense mediated decay

While the diversification of the antigen-binding sites is realized by genomic VDJ rearrangements during B cell differentiation, different forms of immunoglobulin (Ig) heavy (H) chains can be produced through multiple splicing pathways. In most vertebrates, the secreted (S) and membrane (Mb) forms of IgM chain are created by alternative splicing through usage of a cryptic splice site in Cμ4 allowing the junction to the TM exon. The processing pattern for Igμ is different in teleosts, which generally use the Cμ3 donor site instead. In ancient fish lineages, multiple unusual splicing patterns were found for Ig H chain, involving donor sites that do not always follow the classical consensus. The production of IgD versus IgM H chains seems to be generally realized by alternative splicing in all vertebrates, but typical teleost IgD H chains are chimeric and contains a Cμ1 domain. Together, these observations raise questions on how different fish regulate RNA splicing and if their splicing machinery is especially complex. A preliminary scan of the zebrafish and stickleback genomes provides evidence that gene orthologs to the mammalian main splice factors are highly conserved as single copy genes, while the snRNPs U repertoire may be different and may explain other particular features of RNA processing in fish.

Immunoglobulin genes and their transcriptional control in teleosts

Immunoglobulin (Ig), which exists only in jawed vertebrates, is one of the most important molecules in adaptive immunity. In the last two decades, many teleost Ig genes have been identified by in silico data mining from the enormous gene and EST databases of many fish species. In this review, the organization of Ig gene segments, the expressed Ig isotypes and their transcriptional controls are discussed. The Ig heavy chain (IgH) locus in teleosts encodes the variable (V), the diversity (D), the joining (J) segments and three different isotypic constant (C) regions including Cμ, Cδ, and Cζ/τ genes, and is organized as a "translocon" type like the IgH loci of higher vertebrates. In contrast, the Ig light (L) chain locus is arranged in a "multicluster" or repeating set of VL, JL, and CL segments. The IgL chains have four isotypes; two κ L1/G and L3/F), σ (L2) and λ. The transcription of IgH genes in teleosts is regulated by a VH promoter and the Eμ3' enhancer, which both function in a B cell-specific manner. The location of the IgH locus, structure and transcriptional function of the Eμ3' enhancer are important to our understanding of the evolutional changes that have occurred in the IgH gene locus.

Immunoglobulin heavy chains in medaka (Oryzias latipes)

BACKGROUND

Bony fish present an immunological system, which evolved independently from those of animals that migrated to land 400 million years ago. The publication of whole genome sequences and the availability of several cDNA libraries for medaka (Oryzias latipes) permitted us to perform a thorough analysis of immunoglobulin heavy chains present in this teleost.

RESULTS

We identified IgM and IgD coding ESTs, mainly in spleen, kidney and gills using published cDNA libraries but we did not find any sequence that coded for IgT or other heavy chain isotypes described in fish. The IgM - ESTs corresponded with the secreted and membrane forms and surprisingly, the latter form only presented two constant heavy chain domains. This is the first time that this short form of membrane IgM is described in a teleost. It is different from that identified in Notothenioid teleost because it does not present the typical splicing pattern of membrane IgM. The identified IgD-ESTs only present membrane transcripts, with Cμ1 and five Cδ exons. Furthermore, there are ESTs with sequences that do not have any VH which disrupt open reading frames. A scan of the medaka genome using transcripts and genomic short reads resulted in five zones within a region on chromosome 8 with Cμ and Cδ exons. Some of these exons do not form part of antibodies and were at times interspersed, suggesting a recombination process between zones. An analysis of the ESTs confirmed that no antibodies are expressed from zone 3.

CONCLUSIONS

Our results suggest that the IGH locus duplication is very common among teleosts, wherein the existence of a recombination process explains the sequence homology between them.

Retained orthologous relationships of the MHC Class I genes during euteleost evolution

Major histocompatibility complex (MHC) class I molecules play a pivotal role in immune defense system, presenting the antigen peptides to cytotoxic CD8+ T lymphocytes. Most vertebrates possess multiple MHC class I loci, but the analysis of their evolutionary relationships between distantly related species has difficulties because genetic events such as gene duplication, deletion, recombination, and/or conversion have occurred frequently in these genes. Human MHC class I genes have been conserved only within the primates for up to 46-66 My. Here, we performed comprehensive analysis of the MHC class I genes of the medaka fish, Oryzias latipes, and found that they could be classified into four groups of ancient origin. In phylogenetic analysis using these genes and the classical and nonclassical class I genes of other teleost fishes, three extracellular domains of the class I genes showed quite different evolutionary histories. The α1 domains generated four deeply diverged lineages corresponding to four medaka class I groups with high bootstrap values. These lineages were shared with salmonid and/or other acanthopterygian class I genes, unveiling the orthologous relationships between the classical MHC class I genes of medaka and salmonids, which diverged approximately 260 Ma. This suggested that the lineages must have diverged in the early days of the euteleost evolution and have been maintained for a long time in their genome. In contrast, the α3 domains clustered by species or fish groups, regardless of classical or nonclassical gene types, suggesting that this domain was homogenized in each species during prolonged evolution, possibly retaining the potential for CD8 binding even in the nonclassical genes. On the other hand, the α2 domains formed no apparent clusters with the α1 lineages or with species, suggesting that they were diversified partly by interlocus gene conversion, and that the α1 and α2 domains evolved separately. Such evolutionary mode is characteristic to the teleost MHC class I genes and might have contributed to the long-term conservation of the α1 domain.

Common carp have two subclasses of bonyfish specific antibody IgZ showing differential expression in response to infection

Immunoglobulin heavy chains identified in bony fish are broadly classified into three classes namely IgM, IgD and IgZ. The most recently described isotype is IgZ, a teleosts-fish specific isotype that shows variations in gene structure across teleosts. In this study we have identified two IgZ subclasses in common carp. IgZ1 is a four constant heavy chain domains containing antibody isolated across teleosts and IgZ2 is a two constant domains containing heavy chain chimera with a μ1 and ζ4 domain. Sequence analyses suggest that these subtypes are expressed from two separate genomic loci. Expression analyses show that IgZ1 is more abundant in systemic organs and IgZ2 chimera is preferentially expressed at mucosal sites. The basal expression level of IgM in fish is much higher than of the other isotypes. We show that IgZ1 expression in systemic and mucosal organs is responsive to blood parasites, while mucosal parasite infection induces IgM and IgZ2 gene expression. This report is the first to show differential expression of the IgZ variants in response to pathogens and suggests that the IgZ subtypes in carps may have mutually exclusive humoral functions.

A second generation genetic linkage map of Japanese flounder (Paralichthys olivaceus)

BACKGROUND

Japanese flounder (Paralichthys olivaceus) is one of the most economically important marine species in Northeast Asia. Information on genetic markers associated with quantitative trait loci (QTL) can be used in breeding programs to identify and select individuals carrying desired traits. Commercial production of Japanese flounder could be increased by developing disease-resistant fish and improving commercially important traits. Previous maps have been constructed with AFLP markers and a limited number of microsatellite markers. In this study, improved genetic linkage maps are presented. In contrast with previous studies, these maps were built mainly with a large number of codominant markers so they can potentially be used to analyze different families and populations.

RESULTS

Sex-specific genetic linkage maps were constructed for the Japanese flounder including a total of 1,375 markers [1,268 microsatellites, 105 single nucleotide polymorphisms (SNPs) and two genes]; 1,167 markers are linked to the male map and 1,067 markers are linked to the female map. The lengths of the male and female maps are 1,147.7 cM and 833.8 cM, respectively. Based on estimations of map lengths, the female and male maps covered 79 and 82% of the genome, respectively. Recombination ratio in the new maps revealed F:M of 1:0.7. All linkage groups in the maps presented large differences in the location of sex-specific recombination hot-spots.

CONCLUSIONS

The improved genetic linkage maps are very useful for QTL analyses and marker-assisted selection (MAS) breeding programs for economically important traits in Japanese flounder. In addition, SNP flanking sequences were blasted against Tetraodon nigroviridis (puffer fish) and Danio rerio (zebrafish), and synteny analysis has been carried out. The ability to detect synteny among species or genera based on homology analysis of SNP flanking sequences may provide opportunities to complement initial QTL experiments with candidate gene approaches from homologous chromosomal locations identified in related model organisms.

Identification of two IgD+ B cell populations in channel catfish, Ictalurus punctatus

Channel catfish Ictalurus punctatus express two Ig isotypes: IgM and IgD. Although catfish IgM has been extensively studied at the functional and structural levels, much less is known about IgD. In this study, IgM(+)/IgD(+) and IgM(-)/IgD(+) catfish B cell populations were identified through the use of anti-IgM and anti-IgD mAbs. Catfish IgM(+)/IgD(+) B cells are small and agranular. In contrast, IgM(-)/IgD(+) B cells are larger and exhibit a plasmablast morphology. The use of cell sorting, flow cytometry, and RT-PCR demonstrated that IgD(+) B cell expression varies among individuals. For example, some catfish have <5% IgM(-)/IgD(+) B cells in their PBLs, whereas in others the IgM(-)/IgD(+) B cell population can represent as much as 72%. Furthermore, IgD expressed by IgM(-)/IgD(+) B cells preferentially associates with IgL σ. Comparatively, IgM(+)/IgD(+) B cells can express any of the four catfish IgL isotypes. Also, transfection studies show that IgD functions as a typical BCR, because Igδ-chains associate with CD79a and CD79b molecules, and all membrane IgD transcripts from sorted IgM(-)/IgD(+) B cells contain viable VDJ rearrangements, with no bias in family member usage. Interestingly, all secreted IgD transcripts from IgM(+)/IgD(+) and IgM(-)/IgD(+) B cells were V-less and began with a leader spliced to Cδ1. Importantly, transfection of catfish clonal B cells demonstrated that this leader mediated IgD secretion. Together, these findings imply that catfish IgM(-)/IgD(+) B cells likely expand in response to certain pathogens and that the catfish IgD Fc-region, as has been suggested for human IgD, may function as a pattern recognition molecule.

The up-regulation of large yellow croaker secretory IgM heavy chain at early phase of immune response

An immunoglobulin M (IgM) heavy-chain gene homologue was isolated from the spleen cDNA library of the large yellow croaker Pseudosciaena crocea (LycIgH). The complete cDNA of LycIgH is 1,987 nucleotides long, encoding a protein of 585 amino acids with a putative molecular weight of 64.5 kDa. The deduced LycIgH possesses a typical secretory IgM heavy chain organization with a variable region (V(H)) connected to four constant regions (C(H1-4)) by a diversity segment (D(H)) and a joining segment (J(H)). Tissue expression profile analysis showed that LycIgH was constitutively expressed in gills, intestine, liver, kidney, heart, spleen, muscle, and blood, while at a higher level in spleen, kidney and intestine. Upon stimulation with poly (I: C), the LycIgH transcripts were quickly increased in spleen and kidney at 12 h post induction (with 5.87- and 5.48-fold mRNA increases, respectively), followed by a recovery to normal level at 24 h. The LycIgH transcripts in spleen and kidney induced by inactivated bacterial vaccine reached their peak levels at 48 h (14.53-fold) and 12 h (3.70-fold), respectively. These results indicated the up-regulation of LycIgH expression in spleen and kidney by poly (I: C) or bacterial vaccine occurred at the early phase of induction and was differentially modulated in the two tissues by different stimulations.

Identification of immune related genes in Atlantic halibut (Hippoglossus hippoglossus L.) following in vivo antigenic and in vitro mitogenic stimulation

To identify and characterize genes and proteins of the Atlantic halibut (Hippoglossus hippoglossus) immune system, six cDNA libraries were constructed from liver, kidney, spleen, peripheral blood, and thymus. Halibut were injected with nodavirus, infectious pancreatic necrosis virus (IPNV), or vibriosis vaccine and tissue samples were collected at various time points. Leukocytes from peripheral blood and spleen from stimulated and mock-injected fish were isolated and further in vitro activated with the mitogens, concanavalin A (Con A) and phorbol myristate acetate (PMA) to facilitate activation and proliferation. A total of 5117 high quality expressed sequence tags (ESTs) were identified and assembled into 781 contigs and 2796 singletons. Amongst these ESTs, 147 different putative immune related genes were identified. Several genes involved in innate and adaptive immune responses such as complement proteins, immunoglobulins, cell surface receptors, and cytokines and chemokines were identified. Of the immune related genes identified in this study, 44% had no match against any of the publicly available sequence data for halibut and thus can be considered as novel identification in halibut species. The approach of combining in vivo antigenic with in vitro mitogen stimulation, in addition to preparation of cDNA libraries from thymus enabled identification of many of the interesting genes including those involved in T-cell receptor complex.

Role of maternally derived immunity in fish

Maternal immunity is of paramount importance for protection of young ones at early stage of life since the immune factors of an immunocompetent female are transferred transplacentally or through colostrum, milk or yolk to an immunologically naive neonate. Both innate and adaptive type of immunity are transferred of from mother to offspring in fishes. These factors include immunoglobulin (Ig)/antibody, complement factors, lysozymes, protease inhibitors like alpha macroglobulin, different types of lectins and serine proteases like molecules. Among different types of Ig viz. IgM, IgD, IgT/IgZ and IgM-IgZ chimera types, IgM is present in most of the teleostean fishes. In teleosts, IgM either as a reduced/breakdown product or monomeric form is usually transferred to the offsprings. The maternally derived IgM usually persists for a limited duration, exhausts within the completion of yolk absorption process, and completely disappears thereafter during larval stages. Maternal transfer of immunity which provides defense to embryo and larvae depends upon the health as well as the immune status of brood fish. The overall health status of brood fish can affect breeding performances, quality seed production and protection of offsprings. However, factors such as age, maturation, reproductive behaviour and nutrition (micro and macro-nutrients) may affect the immunity in brood fishes. Besides these, seasonal changes such as photoperiods, temperature, adverse environmental conditions, and stress conditions like handling, crowding, and water pollution/contamination can also affect the immunity of brood fishes. The maintenance of the brood stock immunity at high level during vitellogenesis and oogenesis, is utmost important for reducing mortalities at larval/post larval stages through maximum/optimum transfer of maternal immunity. Brood stock immunization prior to breeding as well as selective breeding among the disease resistant families might be the ideal criteria for producing quality seed.

Characterization of polyclonal antibodies against Japanese flounder IgM derived from recombinant IgM constant region proteins

Cell and determinant markers are important in fish immunology and have vast applications in aquaculture but the availability of such markers are quite limited. Hence, there is a need to identify and also further improve existing markers in fish. Here, we developed effective polyclonal antibodies (pAbs) targeting specific parts of the Japanese flounder (Paralichthys olivaceus) IgM constant (C) region. Recombinant proteins from the CHmu2 and CHmu3 termed IgM fragment 1 (rIgM1) and from CHmu4 termed IgM fragment 2 (rIgM2) were expressed and used to construct mouse pAb-IgM1 and pAb-IgM2, respectively. pAb-IgM1 detected both the approximately 77 kDa and the approximately 72 kDa heavy chains detected while pAb-IgM2 marked only the approximately 77 kDa heavy chain of Japanese flounder. Both pAbs detected IgM heavy chain in immune-related tissues, heart and serum. pAb-IgM2, but not pAb-IgM1, revealed cross reactions with other fish species detecting pronounced multiple IgM bands suggesting that the CHmu4 is an important functional region in the teleost IgM molecules. Finally, the pAb-IgMs detected surface IgM+ (sIgM+) and cytoplasmic IgM+ (cIgM+) B cells in Japanese flounder kidney in vivo.

Hepcidin gene expression induced in the developmental stages of fish upon exposure to Benzo[a]pyrene (BaP)

Hepcidin is known to be expressed in fish with bacterial challenge and iron overload. Here we first report the hepcidin expression induced in the developmental stages from embryo to fry of red sea bream (Pagarus major) and in juvenile black porgy (Acanthopagrus schlegelii B.) upon continuous waterborne exposure to BaP. The gene expression of CYP1A1 and IgL (immunoglobulin light chain) were both measured. Expression of the Pagarus major hepcidin gene (PM-hepc) was increased in post hatch fry at 24 h and 120 h exposure to BaP at concentrations of 0.1, 0.5 and 1.0 microg/l, respectively. The gene expression pattern was comparable to that of CYP1A1 but different from that of IgL. In addition, a high number of AS-hepc2 transcripts (Acanthopagrus schlegelii B. hepcidin gene) were detected in the liver upon exposure to 1.0 microg/l BaP. This study demonstrates that hepcidin gene expression is significantly induced in BaP-exposed red sea bream and black porgy.

Interactions between immune and biotransformation systems in fish: a review

In animals biotransformation and immune system are not totally independent, there are numerous functional interrelationships between these two systems. They are both implicated in the capacity of organisms to resist to a wide variety of environmental components such as viruses, bacteria and xenobiotics. It is known for a long time that the immune system functions as a physiologic system and interacts with all the other components of the organism including nervous or endocrine ones. In the same manner, the biotransformation system (especially the cytochrome P450 monooxygenases) is involved in the regulation of numerous hormone productions. In this way, many studies in mammals have revealed the possible interaction between immune and biotransformation systems. Among these interactions, the capacity of the activation of host defense mechanisms to down-regulate microsomal cytochrome P450 and the role of biotransformation system in the xenobiotic-mediated immunotoxicity have been underlined. Advances in the basic knowledge of fish immune and biotransformation systems should lead to a better understanding of the possible interactions between both systems and should improve fish health monitoring which is a crucial ecotoxicological goal.

A new isotype of immunoglobulin heavy chain in the urodele amphibian Pleurodeles waltl predominantly expressed in larvae

Up to now, it was thought that urodele amphibians possessed only two IgH isotypes, IgM (mu) and IgY (upsilon). By screening a Pleurodeles waltl Ig cDNA mini-library, we identified three isotypes: IgM, IgY and a previously unknown class. IgM are multimeric molecules and represent the most abundant isotype throughout the life of P. waltl. IgY are likely the counterpart of mammalian IgA. The new isotype has typical Ig H-chain characteristics and is expressed as both secretory and membrane forms. Our analyses indicate that this isotype is restricted to Pleurodeles. Consequently, we named it "IgP" (pi) for Pleurodeles. This isotype is mainly expressed after hatching. Its expression decreases after metamorphosis. Our data indicate that IgP-expressing B cells present some similarities with mammalian B1-cells.

Molecular characterization of immunoglobulin D in mammals: immunoglobulin heavy constant delta genes in dogs, chimpanzees and four old world monkey species

Antibodies are adaptor molecules that neutralize pathogens and link humoral and cellular defence mechanisms. Immunoglobulin D (IgD), one of the five antibody classes present in mammals, is expressed as an antigen receptor on naïve B cells. The functional role that IgD plays in the immune response is still poorly understood, but the recent characterization of immunoglobulin heavy constant delta genes (IGHD) in a variety of species challenges the view that IgD is of minor importance and is not present in many animals. On the basis of serological studies, IgD appears to be expressed in the majority of mammalian species examined. To confirm, at the molecular level, that IgD is present in different species, we cloned and sequenced IGHD cDNA from dogs and five non-human primate species (chimpanzee, rhesus macaque, cynomolgus macaque, baboon and sooty mangabey). Our results show that in all six species, IgD heavy chains possess three immunoglobulin domains and a long hinge region encoded by two exons. Only the hinge region of non-human primates is similar to the human hinge region, with conservation of O-glycosylation sites and multiple charged residues at opposing ends. The preservation of IgD in primates, dogs and previously characterized species suggests an important functional role for IgD, possibly involving binding to a receptor. The high degree of similarity existing between the structural features of human and non-human primate IgD suggests that non-human primates are suitable for in vivo studies designed to define the role that IgD plays in the immune response.

Conservation and divergence of the Eμ3′ enhancer in the IGH locus of teleosts

The core region of the Emicro3' transcriptional enhancer that drives the expression of the teleost IGH locus has been characterized functionally in two species, the catfish (Ictalurus punctatus) and the zebrafish (Danio rerio). These studies have suggested important differences: whereas the catfish enhancer acts through an E-box and two octamer motifs, the zebrafish enhancer exerts its major effects through two E-box motifs alone. In this study, the function of the catfish enhancer was reexamined in a broader comparative context within the teleosts. Electrophoretic mobility shift assays of motifs from catfish, zebrafish, and Fugu were conducted to determine their ability to bind catfish E-protein and Oct transcription factors. Transient expression assays were conducted using a region of the catfish core enhancer that includes a newly described hybrid octamer/E-box motif. Sequences homologous to the Emicro3' enhancer region from six teleosts were aligned to determine conserved regions ("phylogenetic footprinting"). These studies allowed the following conclusions to be drawn: (1) The important 3'E-box motif described in the zebrafish corresponds in the homologous region of the catfish enhancer to an Oct motif with a newly described negative regulatory function and (2) Comparison of the Emicro3' enhancer sequences of six teleosts indicates that while a variety of octamer and E-box motifs are found in this region, strict evolutionary conservation of the important functional elements of the teleost Emicro3' enhancer has not occurred.

Characterization of serum immunoglobulin M of grouper and cDNA cloning of its heavy chain

Immunoglobulin M (IgM) from the whole serum of grouper fish, Epinephelus coioides was purified by affinity chromatography using protein A-Sepharose column. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions revealed that the relative molecular masses (Mr) of the equimolar heavy and light chains of IgM were 78,000 and 27,000, respectively. The cDNAs encoding IgM heavy chain comprising its variable (VH) and constant (CH) regions have been cloned and sequenced from a grouper kidney cDNA library by antibody screening method. Five VH (130-142 amino acids) and four CH (450-454 amino acids) families were identified. The variable and constant regions were conserved with their putative domains. All the four constant region domains (CH1-CH2-CH3-CH4) contained each three conserved cysteine residues, which are considered to form the inter- and intra-chain disulfide bridges. There were three carbohydrate acceptor sites in the constant region. In general, the pattern of IgM gene organization seems to resemble that of other teleosts. Moreover, the CH genes in grouper IgM occur as multifamily as reported in Atlantic salmon and common carp.

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

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

Serum immunoglobulin M in Atlantic halibut (Hippoglossus hippoglossus): characterisation of the molecule and its immunoreactivity

Three preparations of purified immunoglobulin (IgM) were isolated from serum of Atlantic halibut (Hippoglossus hippoglossus) by means of three different methods, and each of the three IgM preparations was used to produce a polyclonal rabbit anti-halibut IgM antiserum. One of the IgM preparations was employed in the characterisation of halibut serum immunoglobulin. Halibut IgM was shown to consist of two subunits, compatible with heavy (mu) and light (L) chains. A single mu chain at approximately 76 kDa, and six possible molecular weight (MW) variants of L chain were found (range approximately 25 to approximately 28.5 kDa). IgM was glycosylated on the heavy chain and N-linked carbohydrate constituted approximately 10.3% (w/w) of the total MW of IgM. The dominant form of non-reduced IgM had a MW of approximately 780 kDa, suggesting a tetrameric structure. Non-reduced IgM also showed a number of minor protein bands. Based on estimated MW, the relative carbohydrate content and the reactivity with all three anti-halibut IgM antisera, mono-, di- and trimeric redox forms of IgM were identified. The three antisera were characterised as to specificity and reactivity by means of enzyme linked immuno-sorbent assay (ELISA), crossed immuno-electrophoresis (CIE), and immunoblotting methods. The antisera showed a considerable diversity in their specificity to the suggested MW variants of halibut Ig light chain. A method for immunohistochemical detection of IgM in tissue was established. Protein A or protein G affinity for the IgM was not detectable.

Evolution of transcriptional enhancers in the immunoglobulin heavy-chain gene: functional characteristics of the zebrafish Emu3' enhancer

Past studies of the channel catfish immunoglobulin heavy-chain (IgH) locus indicates that it lacks an Emu enhancer in the J(H)-Cmu1 intron but does have an enhancer, termed Emu3', in the mu-delta intergenic region. The positioning of the catfish enhancer downstream of the mu-chain exons is predicted to be unfavorable for antibody-affinity maturation in catfish, and would also have been an impediment to the evolution of class switch recombination, had it existed in early tetrapods. To determine if this downstream enhancer is a general feature of teleost fish, we have identified the location of the transcriptional enhancer in the zebrafish IgH locus. We find that zebrafish, like catfish, only have an Emu3'-like enhancer that has cross-species activity, but which is B-cell-specific in its activity. A 300-bp region of the zebrafish enhancer shares sequence homology with the core of the catfish Emu3', although there has been loss and gain of specific octamer enhancer motifs. Mutagenesis studies demonstrate that the zebrafish IgH enhancer depends on a pair of E-box motifs that are found in the enhancer core. Similarly spaced E-box motifs appear to exist in the Emu3' enhancer regions of other teleost fish, suggesting this is a common feature among fish IgH enhancers. We discuss how this distal positioning of the enhancer may influence affinity maturation in extant teleosts as well as the evolution of this process in the early tetrapods.

The immunoglobulin heavy-chain locus in zebrafish: identification and expression of a previously unknown isotype, immunoglobulin Z

The only immunoglobulin heavy-chain classes known so far in teleosts have been mu and delta. We identify here a previously unknown class, immunoglobulin zeta, expressed in zebrafish and other teleosts. In the zebrafish heavy-chain locus, variable (V) gene segments lie upstream of two tandem diversity, joining and constant (DJC) clusters, resembling the mouse T cell receptor alpha (Tcra) and delta (Tcrd) locus. V genes rearrange to (DJC)(zeta) or to (DJC)(mu) without evidence of switch rearrangement. The zebrafish immunoglobulin zeta gene (ighz) and mouse Tcrd, which are proximal to the V gene array, are expressed earlier in development. In adults, ighz was expressed only in kidney and thymus, which are primary lymphoid organs in teleosts. This additional class adds complexity to the immunoglobulin repertoire and raises questions concerning the evolution of immunoglobulins and the regulation of the differential expression of ighz and ighm.