Analysis of a cDNA sequence encoding the immunoglobulin heavy chain of the Antarctic teleost Trematomus bernacchii

Comparative Analysis of the pIgR Gene from the Antarctic Teleost Trematomus bernacchii Reveals Distinctive Features of Cold-Adapted Notothenioidei

The IgM and IgT classes were previously identified and characterized in the Antarctic teleost Trematomus bernacchii, a species belonging to the Perciform suborder Notothenoidei. Herein, we characterized the gene encoding the polymeric immunoglobulin receptor (pIgR) in the same species and compared it to the pIgR of multiple teleost species belonging to five perciform suborders, including 11 Antarctic and 1 non-Antarctic (Cottoperca gobio) notothenioid species, the latter living in the less-cold peri-Antarctic sea. Antarctic pIgR genes displayed particularly long introns marked by sites of transposable elements and transcription factors. Furthermore, analysis of T. bernacchii pIgR cDNA unveiled multiple amino acid substitutions unique to the Antarctic species, all introducing adaptive features, including N-glycosylation sequons. Interestingly, C. gobio shared most features with the other perciforms rather than with the cold-adapted relatives. T. bernacchii pIgR transcripts were predominantly expressed in mucosal tissues, as indicated by q-PCR and in situ hybridization analysis. These results suggest that in cold-adapted species, pIgR preserved its fundamental role in mucosal immune defense, although remarkable gene structure modifications occurred.

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.

On Origin and Evolution of the Antibody Molecule

Simple Summary

Like many other molecules playing vital functions in animals, the antibody molecule possesses a complex structure with distinctive features. The structure of the basic unit, i.e., the immunoglobulin domain of very ancient origin is substantially simple. However, high complexity resides in the types and numbers of the domains composing the whole molecule. The emergence of the antibody molecule during evolution overturned the effectiveness of the organisms’ defense system. The particular organization of the coding genes, the mechanisms generating antibody diversity, and the plasticity of the overall protein structure, attest to an extraordinary successful evolutionary history. Here, we attempt to trace, across the evolutionary scale, the very early origins of the most significant features characterizing the structure of the antibody molecule and of the molecular mechanisms underlying its major role in recognizing an almost unlimited number of pathogens.

Abstract

The vertebrate immune system provides a powerful defense because of the ability to potentially recognize an unlimited number of pathogens. The antibody molecule, also termed immunoglobulin (Ig) is one of the major mediators of the immune response. It is built up from two types of Ig domains: the variable domain, which provides the capability to recognize and bind a potentially infinite range of foreign substances, and the constant domains, which exert the effector functions. In the last 20 years, advances in our understanding of the molecular mechanisms and structural features of antibody in mammals and in a variety of other organisms have uncovered the underlying principles and complexity of this fundamental molecule. One notable evolutionary topic is the origin and evolution of antibody. Many aspects have been clearly stated, but some others remain limited or obscure. By considering a wide range of prokaryotic and eukaryotic organisms through a literature survey about the topic, we have provided an integrated view of the emergence of antibodies in evolution and underlined the very ancient origins.

Gene expression profiles of antigenic proteins of third stage larvae of the zoonotic nematode Anisakis pegreffii in response to temperature conditions

Anisakis pegreffii, a recognised etiological agent of human anisakiasis, is a parasite of homeothermic hosts at the adult stage and of ectothermic hosts at the third larval stage. Among distinct factors, temperature appears to be crucial in affecting parasite hatching, moulting and to modulate parasite-host interaction. In the present study, we investigated the gene transcripts of proteins having an antigenic role among excretory secretory products (ESPs) (i.e., a Kunitz-type trypsin inhibitor, A.peg-1; a glycoprotein, A.peg-7; and the myoglobin, A.peg-13) after 24 h, in A. pegreffii larvae maintained in vitro, under controlled temperature conditions. Temperatures were 37 °C and 20 °C, resembling respectively homeothermic and ectothermic hosts conditions, and 7 °C, the cold stress condition post mortem of the fish host. Primers of genes coding for these ESPs to be used in quantitative real-time PCR were newly designed, and qRT-PCR conditions developed. Expression profiles of the genes A.peg-1 and A.peg-13 were significantly up-regulated at 20 °C and 37 °C, with respect to the control (larvae kept at 2 °C for 24 h). Conversely, transcript profiles of A.peg-7 did not significantly change among the chosen temperature conditions. In accordance with the observed transcript profiles, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed the presence of the three target ESPs at 37 °C, while only A.peg-13 was observed at 7 °C. The results suggest that temperature conditions do regulate the gene expression profiles of A.peg-1 and A.peg-13 in A. pegreffii larvae. However, regulation of the glycoprotein A.peg-7 is likely to be related to other factors such as the host's immune response.

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.

Site-Specific N-Glycan Characterization of Grass Carp Serum IgM

Immunoglobulin M (IgM) is the major antibody in teleost fish and plays an important role in humoral adaptive immunity. The N-linked carbohydrates presenting on IgM have been well documented in higher vertebrates, but little is known regarding site-specific N-glycan characteristics in teleost IgM. In order to characterize these site-specific N-glycans, we conducted the first study of the N-glycans of each glycosylation site of the grass carp serum IgM. Among the four glycosylation sites, the Asn-262, Asn-303, and Asn-426 residues were efficiently glycosylated, while Asn-565 at the C-terminal tailpiece was incompletely occupied. A striking decrease in the level of occupancy at the Asn-565 glycosite was observed in dimeric IgM compared to that in monomeric IgM, and no glycan occupancy of Asn-565 was observed in tetrameric IgM. Glycopeptide analysis with liquid chromatography-electrospray ionization tandem mass spectrometry revealed mainly complex-type glycans with substantial heterogeneity, with neutral; monosialyl-, disialyl- and trisialylated; and fucosyl-and non-fucosyl-oligosaccharides conjugated to grass carp serum IgM. Glycan variation at a single site was greatest at the Asn-262 glycosite. Unlike IgMs in other species, only traces of complex-type and no high-mannose glycans were found at the Asn-565 glycosite. Matrix-assisted laser desorption ionization analysis of released glycans confirmed the overwhelming majority of carbohydrates were of the complex-type. These results indicate that grass carp serum IgM exhibits unique N-glycan features and highly processed oligosaccharides attached to individual glycosites.

Characterization of IgM in Norwegian cleaner fish (lumpfish and wrasses)

The use of cleaner fish in Norwegian aquaculture has to a large extent been based on wild catches, but breeding of lumpfish and ballan wrasse is currently increasing. Due to disease problems and required vaccine development, tools to study immune responses and a better understanding of the immune system in these species is demanded. The present study comprises lumpfish (Cyclopterus lumpus) and five species of wrasses: Ballan wrasse (Labrus bergylta), rock cook (Centrolabrus exoletus), cuckoo wrasse (Labrus mixtus), corkwing wrasse (Symphodus melops), and goldsinny wrasse (Ctenolabrus rupestris). We present a comparison of the IgM sequences, phylogenetic relationship to other teleosts and characteristic features of IgM in the species studied. The lumpfish IgM heavy chain sequence was assembled from high throughput cDNA sequencing whereas the wrasse sequences were determined by molecular cloning. The secreted form of the IgM heavy chain from all species consisted of four constant Ig domains. IgM was purified from lumpfish and ballan wrasse sera by gel filtration followed by anion exchange chromatography, and polyclonal sera were produced against these proteins. Antisera against ballan wrasse IgM showed cross-reactivity to all analyzed species of wrasses, some cross-reactivity to lumpfish, very low reaction to salmon, and no reaction to cod. Anti- IgM sera against lumpfish cross-reacted to the light chain of all species studied. Wrasses and lumpfish IgM showed high binding affinities for protein A. IgM concentration in adult ballan wrasse (700-800 g) was measured by single radial immunodiffusion assay and found to be 13.4 mg/ml which is about 36% of the total protein concentration. The IgM concentration in lumpfish (600-3600 g) was estimated to 1-2.6 mg/ml, which corresponds to approximately 3% of the total protein concentration.

Analysis and characterization of the head kidney transcriptome from the Antarctic fish Trematomus bernacchii (Teleostea, Notothenioidea): a source for immune relevant genes

In this study we describe the de novo assembled head kidney transcriptome of the Antarctic notothenioid fish Trematomus bernacchii, an important model species for biochemical, environmental and immunological studies. RNA-seq data was generated using Illumina paired-end sequencing, obtaining ~7 Gbp of sequence data, which were assembled into 96,641 contigs and annotated with the Trinotate pipeline. Since this sequence collection is expected to contain a relevant number of immunity-related transcripts, it will be used as a reference for future immunological studies in this species.

A piscidin-like antimicrobial peptide from the icefish Chionodraco hamatus (Perciformes: Channichthyidae): molecular characterization, localization and bactericidal activity

Antimicrobial peptides (AMPs) are considered one of the most ancient components of the innate immune system. They are able to exert their protection activity against a variety of microorganisms, and are widely distributed in both vertebrates and invertebrates. In this paper we focused on an AMP identified in the Antarctic teleost Chionodraco hamatus, an icefish species. The cDNA sequence of the AMP, named chionodracine, is comprised of 515 bp and translates for a putative protein precursor of 80 amino acids, with a signal peptide of 22 amino acids. The structural features evidenced in the primary sequence of chionodracine lead to the inclusion of the peptide in the antimicrobial family of piscidins. The analysis by real-time PCR of the basal gene transcripts of chionodracine in different icefish tissues showed that the highest expression was found in gills, followed by head kidney. The chionodracine expression levels in head kidney leukocytes were up-regulated in vitro both by LPS and poly I:C, and in vivo by LPS. A putative chionodracine mature peptide was synthesized and employed to obtain a polyclonal antiserum, which was used in immunohistochemistry of gills sections and revealed a significant positivity associated with mast cells. The bactericidal activity of the peptide was investigated and found significant against Antarctic psychrophilic bacteria strains (Psychrobacter sp. TAD1 and TA144), the Gram-positive Bacillus cereus, and at a lesser extent against the Gram-negative Escherichia coli. Interestingly, the haemolytic activity of chionodracine was tested in vitro on human erythrocytes and no significant lysis occurred until peptide concentration of 50 μM.

Antarctic teleost immunoglobulins: more extreme, more interesting

We have investigated the immunoglobulin molecule and the genes encoding it in teleosts living in the Antarctic seas at the constant temperature of -1.86 °C. The majority of Antarctic teleosts belong to the suborder Notothenioidei (Perciformes), which includes only a few non-Antarctic species. Twenty-one Antarctic and two non-Antarctic Notothenioid species were included in our studies. We sequenced immunoglobulin light chains in two species and μ heavy chains, partially or totally, in twenty species. In the case of heavy chain, genomic DNA and the cDNA encoding the secreted and the membrane form were analyzed. From one species, Trematomus bernacchii, a spleen cDNA library was constructed to evaluate the diversity of VH gene segments. T. bernacchii IgM, purified from the serum and bile, was characterized. Homology Modelling and Molecular Dynamics were used to determine the molecular structure of T. bernacchii and Chionodraco hamatus immunoglobulin domains. This paper sums up the previous results and broadens them with the addition of unpublished data.

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.

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.

Evolution of the Antarctic teleost immunoglobulin heavy chain gene

Notothenioid teleosts underwent major modifications of their genome to adapt to the cooling of the Antarctic environment. In order to identify specific features of the Antarctic teleost immunoglobulin, transcripts encoding the constant region of the IgM heavy chain from 13 Antarctic and non-Antarctic notothenioid species were sequenced. The primary mRNA splicing for the membrane form was found to be atypical in the majority of Antarctic species, because it led to exclusion of two entire constant exons, and to inclusion of 39-nucleotide exons encoding an unusually long Extracellular Membrane-Proximal Domain (EMPD). Genomic DNA analysis revealed that each 39-nucleotide exon fell within a long sequence that was the reverse complement of an upstream region. Deduced amino acid sequence analysis lead to the identification of cysteine encoding codons in the 39-nucleotide exons, but not in the respective sequence counterpart, suggesting that these residues might play an important role in the folding of the EMPD.

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.

Cytogenetic mapping of immunoglobulin heavy chain genes in Antarctic fish

The chromosomal location of the IgH locus has been analyzed in several bony fish of the Antarctic perciform group Notothenioidei. Two IgH probes were prepared from the species Trematomus bernacchii (family Nototheniidae, tribe Trematominae) and mapped onto the chromosomes of ten species belonging to the same genus (Trematomus) and in two outgroups, through one-color and two-color FISH. A single location of the IgH locus was found in the majority of the species examined, including the outgroups, whereas in four of them the IgH genes splited to two chromosomal loci. RT-PCR experiments revealed the presence of three allelic sequences in T. newnesi, a species in which the IgH genes were organized in two chromosomal loci. Possible pathways leading to IgH genes duplication during the diversification of trematomine fishes were inferred from the analysis of the FISH patterns in a phylogenetic context. The present work provides the first comprehensive picture of IgH genes organization at chromosomal level in a bony fish group.

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.

Total serum immunoglobulin M levels are affected by immunomodulators in seabream (Sparus aurata L.)

Immunoglobulin M (IgM) is a major component of the teleost humoral immune system. Despite the significance of IgM levels as an immune parameter, there are relatively few studies on changes induced in its total levels in serum. This study examines the effects of several immunomodulators (vitamin A, chitin, yeast cells or levamisole, which act as immunostimulants, and crowding, hypoxia or anaesthetics, which act as stressors) upon the total serum IgM levels of non-immunized gilthead seabream (Sparus aurata L.). Total serum IgM levels of fish fed with the assayed immunostimulant-supplemented diets were statistically higher than those in fish fed a non-supplemented diet, especially in the case of levamisole. On the other hand, serum IgM levels of fish subjected to different stressors were not affected by crowding, hypoxia or certain anaesthetics. However, benzocaine and a narcotic dose of 2-phenoxyethanol provoked a great reduction, while quinaldine sulphate increased IgM levels to a significant degree. These results show how the seric IgM levels can be differently affected by some immunomodulators and the important role they may play in the regulation of total circulating IgM levels in seabream. The possibility of using total serum IgM for assessing immunostimulation, disease diagnosis and stress symptoms during fish farming is discussed.

Immunoglobulin V(H)families and light chain isotypes in the spotted wolffish (Anarhichas minor Olafsen)

The spotted wolffish (Anarhichas minor Olafsen) is a species of the Perciformes, the most diverse and numerous order of all fish. A cDNA library from head kidney tissue was screened for immunoglobulin (Ig) heavy and light chain transcripts, and showed highest identity to Ig sequences from other perciform species. So far only one Ig class is described in spotted wolffish, but three V(H)families were identified among the heavy chain transcripts. Highest diversity was located at the CDR3 region and demonstrates the importance of this gene element to the antibody repertoire. Two V(L)families were identified among the light chain clones and three distinct isotypes were present. Use of polymerase chain reaction and in situ hybridisation techniques revealed individual variations in the relative expression of the three isotypes of light chains.

Positive Darwinian selection operating on the immunoglobulin heavy chain of Antarctic fishes

The cooling of the Southern Ocean to the freezing point of seawater (-1.9 degrees C) over the past 25 million years played a dominant selective role in the evolution of the Antarctic fish fauna. During this period, the perciform suborder Notothenioidei, which is largely endemic to the Antarctic, diversified and developed numerous cold-adapted characters. In this report, we provide compelling evidence that the immunoglobulin heavy chain (IgH) of the notothenioid fishes has undergone adaptive selection. Two and four IgH clones were isolated, respectively, from spleen cDNA libraries prepared from the Antarctic icefish Chaenocephalus aceratus and the yellowbelly rockcod Notothenia coriiceps. The transmembrane region of the membrane form of the rockcod IgM heavy chain was located at the end of the second constant (C(H)) domain, in contrast to other teleost IgMs in which the transmembrane region is located at the end of the third constant domain. Phylogenetic analyses of C(H) regions revealed that rates of nonsynonymous nucleotide substitution were higher than rates of synonymous nucleotide substitution. Many of the nonsynonymous substitutions introduced charge changes, consistent with positive Darwinian selection acting to adapt the structure of the notothenioid immunoglobulins. The rates of nonsynonymous nucleotide substitutions were higher than the rates of synonymous nucleotide substitutions in complementarity determining regions of variable regions, suggesting that diversity at antigen binding sites is enhanced by genomic and/or somatic selection. Results of Southern blot hybridization experiments were consistent with a translocon type of IgH gene organization reminiscent of bony fishes and tetrapods.

Limited diversity of the immunoglobulin heavy chain variable domain of the emerald rockcod Trematomus bernacchii

To investigate the diversity of the immunoglobulin heavy chain variable domain of the cold adapted teleost Trematomus bernacchii, 45 cDNA clones, containing complete or partial sequences of rearranged VH/D/JH segments, were analysed. Clones were isolated from a spleen library constructed by 5' RACE or from an expression library previously constructed and immunoscreened with rabbit anti- T. bernacchii Ig heavy chain antibodies. VH sequences shared, on average, 79.9% nucleotide identity and defined only two gene families referred to as Trbe VH I and Trbe VH II, the latter comprising 89% of the VH sequences analysed in this study. A Southern blot analysis, performed with family specific probes, revealed that there are at least 25 genomic VH genes. A phylogenetic tree showed that Trbe VH I clustered with VH genes belonging to group D and Trbe VH II with those of group C. Four putative distinct D segments were found to contribute to the diversity of CDR3, which showed a high glycine content. The Shannon analysis revealed that FRs are very highly conserved. Of CDRs, CDR2 exhibits a mean entropy value higher than CDR1, contributing to variability in a significant manner. Moreover, eight distinct JH segments were identified. These findings provide several clues suggesting a limited diversity of the VH genes in the Antarctic teleost T. bernacchii.

Specific features of immunoglobulin VH genes of the Antarctic teleost Trematomus bernacchii

The somatic recombination of different germline-encoded gene segments constitutes a principal source of antibody diversity. In order to investigate the diversity in recombined gene segments encoding the immunoglobulin heavy chain of the Antarctic teleost Trematomus bernacchii, a VH library was constructed by 5'-RACE (rapid amplification of cDNA ends) using RNA isolated from the spleen of an individual specimen. Analysis of cDNA sequences of 45 rearranged VH/D/JH segments revealed specific features, such as: high number of repeats, up to 8 bp long, and palindromic sequences, especially in CDRs (complementary determining regions); occurrence of the RGYW consensus, known as mutational hot spot, higher than in other species. Sixty-four percent of single base substitutions was found within this motif. In addition, the usage of serine codons showed a clear bias for AGY in CDRs, particularly in CDR2, and for TCN in FRs (framework regions). In CDRs, the frequency of non-synonymous changes was higher than that of synonymous changes. Diversity generated by insertions/deletions occurred more often than in other species; inserted bases were often repeats of adjacent bases. In particular the CDR2 showed the highest length variability as compared to other species. Alignment of VH sequences indicated that also the gene conversion mechanism may contribute to generating diversity. These data indicate a CDR mutability higher than in other species and provide some insights into the hypermutational events that may also occur in teleosts.

Cytology of lymphomyeloid head kidney of Antarctic fishes Trematomus bernacchii (Nototheniidae) and Chionodraco hamatus (Channicthyidae)

Species that live in extreme conditions have specially adapted physiology and tissue/organ organisation. The adaptation of lymphoid organs to low temperatures in polar species could be an original field of study, indicating how the immune system works under extreme conditions. In fishes, the head kidney is a key organ for immunity and here the cytology of this organ is studied in two common Antarctic species: Trematomus bernacchii and Chionodraco hamatus. Ultrastructural analysis revealed heterogeneity of epithelial cells, with reticular cells, subcapsular- and perivascular-limiting cells. Differences in the size and morphology of epithelial cells were observed between the polar species and warm water species of fish. Intermingled with epithelial cell leucocytes, such as lymphocytes, thrombocytes and macrophages, had comparable morphology in both species, contrary to sharp differences observed in the morphology of erythrocytes and granulocytes. The functional adaptation of the head kidney to the low temperatures of polar water is discussed.

Immunoglobulin genes and antibody responses in the spotted wolffish (Anarhichas minor Olafsen)

The spotted wolffish Anarhichas minor Olafsen is a promising new species in aquaculture in the cold waters of northern Norway. In this paper, some basic immunological studies of this marine species are reported. Of comparative interest are the cDNA sequences of the immunoglobulin transcript and the antibody responses to model antigens. Of more practical importance are the humoral immune responses and antibody specificities to potentially pathogenic bacteria. Full length cDNA clones encoding the immunoglobulin heavy and light chains in the spotted wolffish were sequenced demonstrating variable degrees of similarity to other teleost fish species. Also in the spotted wolffish the CH4 domain was deleted in the transmembrane form of the immunoglobulin heavy chain (IgH) as a receptor on B cells, with the transmembrane exon spliced directly to the CH3 domain. The antibody responses to various antigens like hapten-carrier molecules, protein antigens and bacterial pathogens were relatively high, but with some interesting exceptions. Anti-hapten responses to NIP and FITC were high while anti-DNS responses were low, but more surprisingly, there was hardly any B-cell response to the carrier molecule LPH. On the other hand, protein antigens like CGG and BSA were highly immunogenic in the spotted wolffish as were the bacterial antigens Vibrio anguillarum, V. salmonicida and Aeromonas salmonicida.