Isolation and partial characterisation of immunoglobulin from southern bluefin tuna Thunnus maccoyii Castelnau

Development of a monoclonal antibody specific to burbot (Lota lota) IgM and optimization of an ELISA to measure anti-Aeromonas sp. antibody titers following pathogen challenge

Burbot (Lota lota) are an ideal candidate for cool or cold-water aquaculture and are gaining interest because of their high economic value, low temperature requirements, and fast growth rate. Limited information exists on the innate and adaptive immune systems of this species. This is partly due to the lack of species-specific tools to determine antibody responses following disease or vaccination or to characterize the immune response in general. An anti-IgM monoclonal antibody (mAb 27C) was developed and characterized via enzyme-linked immunosorbent assay (ELISA) and Western blot for species specificity, affinity to the heavy chain of burbot IgM, and cross-reactivity to other reagents used in the analysis. The 27C monoclonal antibody was further utilized to develop an ELISA protocol to measure the specific antibody response of burbot following exposure to two pathogenic strains of Aeromonas sp. (A141 and IR004). This ELISA confirmed that vaccinated burbot that survived the challenge with either strain developed statistically higher titers of anti-Aeromonas antibodies specific for the relative strain when compared to fish that were not vaccinated or challenged. Western blot analysis further demonstrated that burbot surviving challenge had serum IgM that recognized distinct antigens specific to the strain they were challenged with, A141 bound to antigens in the 50-250Kda range and IR004 bound to a distinct 150Kda antigen. Western blots further indicated that each strain shared antigenic regions regardless of experimental Aeromonas strain exposure. Finally, immunofluorescent staining confirmed that mAb 27C binds to membrane-bound IgM (presumably B cells) on burbot head kidney cells. Taken together, results from this study demonstrate that mAb 27C specifically recognized burbot IgM and will be an important tool to further characterize the adaptive and cellular immune responses of this fish species.

Acquirement of HRP conjunct IgG anti-IgMs from most widely cultured freshwater fishes in China and its immunoreactivity

Until now, custom-made or commercial polyclonal antibody against only one kind of fish IgM limited application of the antibody. During our research on development of vaccine against infection of Clonorchis sinensis (C. sinensis) in several kinds of fish, we were conscious of the urgency of secondary antibody to evaluate immune effect and screen C. sinensis infection with immunological technology instead of labor-intensive and time-consuming squash or artificial digestion of fish flesh. So that, we purified IgM of grass carp, bighead carp, crucian carp, common carp and tilapia which were widely cultured freshwater fishes in most areas of China. On this basis, we generated HRP-conjunct rabbit IgG anti-fish IgMs with high titers. IgM of other freshwater fishes including oshima, yellow catfish, bream, silver carp and so on could be recognized by the IgG sensitively. Additionally, The ELISA detection displayed that the IgG could be more specific and sensitive than custom-made rabbit IgG anti-grass carp IgM. The acquirement of HRP-conjunct rabbit IgG anti-fish IgMs was the cornerstone for studying the immune system of teleost fish, developing immunoassay methods and evaluation of fish vaccine with more convenience.

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.

Development of an indirect ELISA assay to evaluation of the adaptive immune response of pacu (Piaractus mesopotamicus)

The pacu is one of the most important species for Brazilian fish farming and is considered emerging in the global aquaculture. Despite its importance, no effective tool for evaluation of the adaptive immune response of this species has been developed. Therefore, this study aimed the development and standardization of indirect ELISA for the measurement of pacu antigen-specific antibodies using polyclonal rabbit anti-pacu IgM used as detector antibody. For this purpose was isolated and purificated of pacu IgM using mannose-binding protein affinity chromatography and produced specific polyclonal antibodies against heavy and light chains pacu IgM, that showed a molecular weight of 72 kDa and 26 kDa, respectively. Polyclonal antibodies obtained demonstrated specificity with heavy and light Ig chains of pacu serum in western blotting. These polyclonal antibodies allowed the development of an indirect ELISA assay of high sensitivity and specificity for the detection and quantification of pacu IgM antibodies immunized with bovine IgG. In conclusion, this approach has great potential to improve the monitoring of vaccine-induced immune responses and help develop immunodiagnostic and epidemiological studies of infectious diseases in pacu systems.

Characterization of an IgM-like immunoglobulin from silver catfish (Rhamdia quelen) serum and its use for the production of polyclonal antibodies and development of immunoassays

Abstract: Knowledge on fish immunoglobulin (Ig) characteristics and the availability of monoclonal or polyclonal antibodies to fish Igs are essential to evaluate the humoral immune response and the Ig distribution on leukocyte cells. We demonstrated that silver catfish serum Ig is composed of one immunodominant H chain with approximately 75k Da and one L chain with approximately 28 kDa, similar to human IgM. Rabbit polyclonal antibodies to the catfish IgM-like Ig recognized both the H and L chain and were useful in developing an indirect ELISA to measure the production of antibodies in fish immunized with bovine serum albumin. Dot blot and western blot cross-reactivity studies indicated a wide degree of epitope sharing amongst Ig from several Siluriformes and Characiformes fish indigenous to Brazilian rivers. In these fish species, polyclonal antibodies reacted mostly with the H chain. The results presented here are central to the development of tools and strategies to investigate the antibody production to inoculated antigens and tissue distribution of Ig molecules in native fish species. Furthermore, because of the wide range of cross-reactivity, polyclonal antibodies to silver catfish IgM-like Ig might be used to develop immunoassays to measure the humoral immune response in other fish species.

Evaluation of guppy (Poecilia reticulata Peters) immunization against Tetrahymena sp. by enzyme-linked immunosorbent assay (ELISA)

Analysis of the effectiveness of guppy (Poecilia reticulata Peters) immunization based on measurements of antibody (Ab) titers suffers from a shortage of reagents that can detect guppy antibodies (Abs). To overcome this problem, we immunized mice with different preparations of guppy immunoglobulins (Igs) and used the mouse antisera to develop a quantitative enzyme-linked immunosorbent assay (ELISA). The most efficient immunogen for mouse immunization was guppy Igs adsorbed on protein A/G beads. Antisera from mice boosted with this immunoglobulin (Ig) preparation were highly specific and contained high Ab titers. They immunoreacted in a Western blot with Ig heavy and light chains from guppy serum, and Ig heavy chain from guppy whole-body homogenate. The mouse anti-guppy Ig was applied in an ELISA aimed at comparing the efficiency of different routes of guppy immunization against Tetrahymena: (i) anal intubation with sonicated Tetrahymena (40,000 Tetrahymena/fish in a total volume of 10 μL) mixed with domperidon, deoxycholic acid and free amino acids (valine, leucine, isoleucine, phenylalanine and tryptophan), or (ii) intraperitoneal (i.p.) injection of sonicated Tetrahymena in complete Freund's adjuvant (15,000 Tetrahymena/fish in total a volume of 20 μL). Negative control fish were anally intubated with the intubation mixture without Tetrahymena, or untreated. ELISA measurement of anti-Tetrahymena Ab titer revealed a significantly higher level of Abs in i.p.-immunized guppies, compared to the anally intubated and control fish. In addition, the efficiency of immunization was tested by monitoring guppy mortality following (i) i.p. challenge with Tetrahymena (900 Tetrahymena/fish) or (ii) cold stress followed by immersion in water containing 10,000 Tetrahymena/mL. Fish mortality on day 14 post-Tetrahymena infection by i.p. injection exceeded 50% in the control and anally intubated fish, compared to 31% in i.p.-immunized fish. Immunization did not protect from pathogen challenge by immersion. The results suggest a direct correlation between the anti-Tetrahymena Ab response and fish resistance to i.p.-injected Tetrahymena, but not to infection by immersion preceded by cold stress.

Expressed IgH μ and τ transcripts share diversity segment in ranched Thunnus orientalis

It is now appreciated that in addition to the immunoglobulin (Ig)M and D isotypes fish also make the mucosal IgT. In this study we sequenced the full length of Ig τ as well as μ in the commercially important Thunnus orientalis (Pacific bluefin tuna), the first molecular analysis of these two Ig isotypes in a member of the order Perciformes. Tuna IgM and IgT are each composed of four constant (CH) domains. We cloned and sequenced 48 different variable (VH) domain gene rearrangements of tuna immunoglobulins and grouped the VH gene sequences to four VH gene segment families based on 70% nucleotide identity. Three VH gene families were used by both IgM and IgT but one group was only found to be used by IgM. Most interestingly, both μ and τ clones appear to use the same diversity (DH) segment, unlike what has been described in other species, although they have dedicated IgT and IgM joining (JH) gene segments. We complemented this repertoire study with phylogenetic and tissue expression analysis. In addition to supporting the development of humoral vaccines in this important aquaculture species, these data suggest that the DH-JH recombination rather than the VH-DH recombination may be instructive for IgT versus IgM/D bearing lymphocyte lineages in some fish.

Serum carbohydrate-binding IgM are present in Vietnamese striped catfish (Pangasianodon hypophthalmus) but not in North African catfish (Clarias gariepinus)

Pangasianodon hypophthalmus serum was fractionated by affinity chromatography on 12 different Sepharose-carbohydrate columns and proteins eluted by the corresponding sugar. Binding to the affinity matrices is dependent on Ca(2+) ions. Upon gel filtration using Superose-12, essentially one fraction was obtained, eluting as a protein with a molecular mass of about 900 kDa. SDS-PAGE in reducing conditions revealed the presence of large (72 kDa) subunits (H-chains) and one up to three small (24, 26 and/or 28-29 kDa) subunits (L-chains). The isolated proteins were shown to be IgM since they bind monoclonal anti-P. hypophthalmus IgM antibodies. Rabbit polyclonal anti-galactose-binding IgM only cross-react with some sugar-binding IgM. The H-chains of the anti-carbohydrate IgM are glycosylated. Circular dichroism studies revealed that the IgMs have an "all-β" type of structure, and that Ca(2+) ions, though essential for carbohydrate-binding activity, are not required for the structural integrity of the molecules. In non-reducing SDS-PAGE, only monomers and halfmers were obtained, showing that there are no disulfide bonds linking the monomers, and that a disulfide bond connecting both H-chains within one monomer is only present in 45% of the molecules. Both the monomers and the halfmers display molecular mass heterogeneity which is indicative for redox forms at the level of the intradomain disulfide bonds. The native carbohydrate-binding IgMs agglutinate erythrocytes from different animals, as well as fish pathogenic bacteria. Similar proteins could not be isolated from another catfish, Clarias gariepinus.

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.

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.

Development of monoclonal antibodies to rohu [Labeo rohita] immunoglobulins for use in immunoassays

Serum immunoglobulins [Ig] of rohu [Labeo rohita] were purified by affinity chromatography using bovine serum albumin as capture ligand. The purified rohu Ig [r-Ig] had a molecular weight [MW] of 880 kDa as determined with gel filtration chromatography. The heavy chain of r-Ig had an MW of 77.8 kDa and that of light chain was 26.4 kDa in SDS-PAGE. Purified r-Ig was used for the production of two anti-rohu Ig monoclonal antibodies [D7 and H4] that belonged to subclass IgG2b and IgG1, respectively. Both the MAbs were specific to heavy chain of r-Ig as seen in Western blotting. Anti-rohu Ig MAb was used as a diagnostic reagent in ELISA and immunocytochemical assays to demonstrate its application for sero-surveillance and for immunological studies in rohu. A competitive ELISA was used to demonstrate the antigenic relatedness of r-Ig with whole serum Ig of other fish species. Cross reactivity of anti-rohu Ig MAb was observed with serum Ig of Catla catla and Cirrihinus mrigala. No reactivity to serum Ig of Ophiocephalus striatus and Clarias gariepinus was seen. Anti-rohu Ig MAb was found to be suitable for the detection of pathogen specific [Edwardsiella tarda] antibodies in serum of immunized rohu by an indirect ELISA. In flow cytometry using D7 MAb, the mean percentage [+/-SE] of Ig positive cells in spleen and blood of rohu were found to be 64.85% [+/-2.34] and 51.84% [+/-2.55] of gated lymphocytes, respectively. Similarly, D7 MAb also stained 52.84% [+/-1.30] and 10.5% of gated lymphocytes in kidney and thymus, respectively. The anti-rohu Ig MAbs also showed specific staining of Ig bearing cells in spleen sections by the indirect immunoperoxidase test.

Effects of fish protein hydrolysate on growth performance and humoral immune response in large yellow croaker (Pseudosciaena crocea R.)

We investigated the effects of fish protein hydrolysate (FPH) on growth performance and humoral immune response of the large yellow croaker (Pseudosciaena crocea R.). One thousand and two hundred large yellow croakers [initial average weight: (162.75+/-23.85) g] were divided into four groups and reared in floating sea cages (3 m x 3 m x 3 m). The animals were fed with 4 diets: basal diet only (control) or diets supplemented with 5%, 10% and 15% (w/w) FPH. The results show that dietary FPH levels significantly influenced the growth and immunity of the large yellow croaker. Compared with the control group, total weight gain (TWG) in all treatment groups, relative weight gain (RWG) and specific growth rate (SGR) in fish fed with diets supplemented with 10% and 15% FPH were significantly increased (P<0.05). Similar results were observed in immune parameters [lysozyme activity, serum complements, immunoglobulin M (IgM)]. Lysozyme activity, complement C4 and IgM were also significantly increased (P<0.05) in fish fed with diets supplemented with 10% and 15% FPH, while complement C3 level was significantly increased (P<0.05) in all treatment groups. In general, with the supplementation of FPH, particularly at dose of 10%, the growth performance and immunity of the large yellow croaker can be improved effectively.

Immune responses and enhanced disease resistance in Chinese drum, Miichthys miiuy (Basilewsky), after oral administration of live or dead cells of Clostridium butyrium CB2

Clostridium butyrium CB2 isolated from chickens was tested as a potential fish probiotic in the Chinese drum, Miichthys miiuy. Fish were fed live (CB), dead CB2 (D-CB) cells (10(8) cells g(-1)) or spent culture supernatant (SCS), for 30 days and challenged with Vibrio anguillarum or Aeromonas hydrophila. Survival was higher in both the CB and the CB-D fed groups, but the SCS group was not significantly different from the control. After feeding live or dead CB2 cells, there was increase in phagocytic activity of the head kidney macrophages, the lysozyme activity of serum and gut mucosa and immunoglobulin (Ig) level. The SCS group showed no obvious change in immune parameters. The results suggest that live or dead Clostridium butyrium CB2 has an immunomodulatory effect on fish.

Histopathology of five species of Didymocystis spp. (Digenea: Didymozoidae) in cage-reared Atlantic bluefin tuna (Thunnus thynnus thynnus)

The first parasitological examinations of Atlantic bluefin tuna Thunnus thynnus thynnus reared in the Adriatic Sea revealed high numbers of different digenean trematodes, belonging to family Didymozoidae. Five species were the most frequently isolated: Coeliodidymocystis, abdominalis, Didymocystis wedli, Koellikerioides internogastricus, K. intestinalis and K. apicalis. Parasites were isolated from gills, gill rakers, intestine, serosa of pyloric caeca, and inner layers of the fundic region of stomach, encysted in pairs in connective tissue capsules originating from host tissue. Gross pathology revealed no pathological changes. Histological alterations comprised tissue reaction resulting in lymphocyte infiltration and formation of a collagenous envelope around the parasite cysts; the intensity of observed reaction depended on the size of the cyst and on the tissue infected. The most notable changes were observed in the intestine under provocation by K. intestinalis, while the most potentially pathogenic didymozoid under fish rearing conditions is D. wedli, which if present in high abundance can induce secondary bacterial infections.

Effects of dietary supplementation with clostridium butyricum on the growth performance and humoral immune response in Miichthys miiuy

The effects of dietary supplementation with Clostridium butyricum on growth performance and humoral immune response in Miichthys miiuy were evaluated. One hundred and fifty Miichthys miiuy weighing approximately 200-260 g were divided into five groups and reared in 15 tanks with closed circuiting culture system. The animals were fed 5 diets: basal diet only (control) or supplemented of the basal diet with C. butyricum at doses of 10(3) (CB1), 10(5) (CB2), 10(7) (CB3) or 10(9) (CB4) CFU/g. Compared with the control, the serum phenoloxidase activity was significantly increased by the supplementation (P<0.05), acid phosphatases activity was increased significantly (P<0.05) at the doses of 10(9) CFU/g. Serum lysozyme activity peaked at dose of 10(7) CFU/g and in the skin mucus at dose of 10(9) CFU/g. Immunoglobulin M level in the serum and skin mucus was increased except at dose of 10(3) CFU/g (P<0.05). The growth at the dose of 10(9) CFU/g was higher than that of the control (P<0.05). It is concluded that supplementation of C. butyricum can mediate the humoral immune responses and improve the growth performance in Miichthys miiuy.

Efficacy of protein A-HRP in an immunological study of black rockfish (Sebastes schlegeli Higendorf) humoral immune responses

The efficacy of protein A-horse radish peroxidase (HRP), as compared to that of mouse polyclonal antibody raised against purified Ig, in detection of black rockfish (Sebastes schlegeli Higendorf) immunoglobulin (Ig) was examined. Protein A affinity chromatography successfully purified Ig from black rockfish serum; the purified-Ig could be visualised as two protein bands (MW 70 and 25kDa) following resolution with sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. In SDS-PAGE immunoblot profiles of the purified-Ig, the mouse polyclonal antibody recognised both the light chain and heavy chains of rockfish Ig, whereas protein A-HRP immunostained only the heavy chain of rockfish Ig. These results suggest that protein A-HRP may be used to detect rockfish antibody-antigen complexes in immunoassays. In a 2-DE immunoblot assay for exploring antigenic profiles of Lactococcus garvieae KG9408, protein A-HRP successfully detected specific antibodies to antigenic proteins of L. garvieae in the rockfish Ig. In addition, enzyme linked immunosorbent assay (ELISA) showed a high correlation between the results obtained for positivity of L. garvieae when protein A-HRP and the mouse polyclonal antibody-was used to analyse samples from 25 diseased rockfish. These results collectively indicate that protein A-HRP has a high affinity for Ig, and may be useful for new investigations into the humoral immune responses of rockfish.

Monoclonal antibodies recognising serum immunoglobulins and surface immunoglobulin-positive cells of puffer fish, torafugu (Takifugu rubripes)

Immunoglobulin of the torafugu, Takifugu rubripes, was purified by a combination of precipitation by low ionic strength dialysis and gel filtration. The Ig was used to immunise mice for the production of monoclonal antibody (MAb). Supernatants of hybridoma cultures were screened by enzyme-linked immunosorbent assay using purified-torafugu Ig-coated plates, and two stable hybridomas producing MAbs against torafugu Ig were obtained. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis under reducing conditions and Western blotting indicated that one MAb (16F3) was specific for the deglycosylated heavy chain of torafugu, and the other MAb (4H5) did not bind to the reduced Ig, suggesting that 4H5 recognised the higher-order structure of Ig. Under non-reduced conditions, both MAbs recognised mainly a 750 kDa band and also minor bands of 672, 410 and 205 kDa. MAb 16F3- and 4H5-primed magnetic beads (Dynabeads) adsorbed 84.9+/-3.3% and 63.6+/-4.4% of the torafugu Ig, respectively. The Ig adsorbed by MAb 16F3-primed Dynabeads was reactive to 4H5 on immunoblotting, and vice versa, indicating that the epitopes for both MAbs are held on the same Ig molecule. Both of these MAbs cross-reacted extensively with the Ig of other Takifugu species, but not with other genus. The MAbs were used to identify surface Ig-positive lymphocytes in the spleen, pronephros, peripheral blood and thymocytes of torafugu by flow cytometry. Flow cytometric analysis of the cells in the lymphocyte-enriched fraction revealed that 50.2+/-6.9% in the PBL, 11.8+/-1.7% in the mesonephros, 13.3+/-2.1% in the pronephros, 42.5+/-4.3% in the spleen and 3.2+/-0.6% in thymus were reactive to 4H5 or 16F3.

Development and maturation of the immune system in zebrafish, Danio rerio: a gene expression profiling, in situ hybridization and immunological study

The development and maturation of the immune system in zebrafish was investigated using immune-related gene expression profiling by quantitative real-time polymerase chain reaction, in situ hybridization (ISH), immunoglobulin (Ig) detection by immuno-affinity purification and Western blotting as well as immersion immunization experiments. Ikaros expression was first detected at 1 day post-fertilization (dpf) and thereafter increased gradually to more than two-fold between 28 and 42dpf before decreasing to less than the initial 1dpf expression level in adult fish (aged 105dpf). Recombination activating gene-1 (Rag-1) expression levels increased rapidly (by 10-fold) between 3 and 17dpf, reaching a maximum between 21 and 28dpf before decreasing gradually. However, in adult fish aged 105dpf, the expression level of Rag-1 had dropped markedly, and was equivalent to the expression level at 3dpf. T-cell receptor alpha constant region and immunoglobulin light chain constant region (IgLC) isotype-1, 2 and 3 mRNAs were detected at low levels by 3dpf and their expression levels increased steadily to the adult range between 4 and 6 weeks post-fertilization (wpf). Using tissue-section ISH, Rag-1 expression was detected in head kidney by 2wpf while IgLC-1, 2 and 3 were detected in the head kidney and the thymus by 3wpf onwards. Secreted Ig was only detectable using immuno-affinity purification and Western blotting by 4wpf. Humoral response to T-independent antigen (formalin-killed Aeromonas hydrophila) and T-dependent antigen (human gamma globulin) was observed in zebrafish immunized at 4 and 6wpf, respectively, indicating that immunocompetence was achieved. The findings reveal that the zebrafish immune system is morphologically and functionally mature by 4-6wpf.

Vitellogenin isolation, purification and antigenic cross-reactivity in three teleost species

Vitellogenin (Vtg) was isolated from male greenback flounder (Rhombosolea tapirina), rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar) plasma, following induction by estradiol (E(2)) inoculation. The molecular weight of each native molecule, as determined by gel filtration, was 540, 383 and 557 kDa, respectively. With sodium dodecyl sulphate polyacrylamide gel electrophoresis under reducing conditions, Atlantic salmon and greenback flounder Vtg appeared as three major bands (approximately 159, 117, 86 kDa and 155, 104, 79 kDa, respectively), whereas rainbow trout Vtg appeared as one major band (approximately 154 kDa). Several minor bands were also present in each Vtg isolate. Polyclonal antisera, produced against only the highest molecular weight band from each species following excision from reducing gels, were reactive with all major bands in Western blots. In competition ELISA, parallel binding slopes were demonstrated between purified Vtg and plasma from vitellogenic females of the same species, but there was no reaction with plasma from untreated males. These antisera were highly species-specific and little cross-reactivity was noted, even between the two salmonid species. These data suggest that excision of bands from gels is a simple procedure for the preparation of species-specific antisera, and confirm that cross-species assays give highly variable results.