A comparison of total and specific immunoglobulin levels in healthy Atlantic salmon (Salmo salar L.) and in salmon naturally infected with Aeromonas salmonicida subsp. achromogenes

Climate change can impair bacterial pathogen defences in sablefish via hypoxia-mediated effects on adaptive immunity

Low-oxygen levels (hypoxia) in aquatic habitats are becoming more common because of global warming and eutrophication. However, the effects on the health/disease status of fishes, the world's largest group of vertebrates, are unclear. Therefore, we assessed how long-term hypoxia affected the immune function of sablefish, an ecologically and economically important North Pacific species, including the response to a formalin-killed Aeromonas salmonicida bacterin. Sablefish were held at normoxia or hypoxia (100% or 40% air saturated seawater, respectively) for 6-16 weeks, while we measured a diverse array of immunological traits. Given that the sablefish is a non-model organism, this involved the development of a species-specific methodological toolbox comprised of qPCR primers for 16 key immune genes, assays for blood antibacterial defences, the assessment of blood immunoglobulin (IgM) levels with ELISA, and flow cytometry and confocal microscopy techniques. We show that innate immune parameters were typically elevated in response to the bacterial antigens, but were not substantially affected by hypoxia. In contrast, hypoxia completely prevented the ∼1.5-fold increase in blood IgM level that was observed under normoxic conditions following bacterin exposure, implying a serious impairment of adaptive immunity. Since the sablefish is naturally hypoxia tolerant, our results demonstrate that climate change-related deoxygenation may be a serious threat to the immune competency of fishes.

Anemia in salmon aquaculture: Scotland as a case study

Anemia in salmonid aquaculture is a recognized blood disorder resulting from the reduction of hemoglobin concentration and/or erythrocyte count. Because of sub-optimal oxygen supply to the tissues, as a negative impact of anemia fish will experience reduced growth and poor health. This health challenge may be linked with several factors including anthropogenic changes in the marine environment, infectious etiology (viral, bacterial, and parasitic), nutritional deficiencies, or hemorrhaging. From the mid-late summer of 2017 to 2019, Scottish salmon farming companies began to report the occurrence of anemic events in open-net marine sites. At that time, the industry had little understanding of the pathogenesis and possible mechanisms of anemia and limited the ability to formulate effective mitigation strategies. Clinical examination of fish raised suspicion of anemia and this was confirmed by generating a packed cell volume value by centrifugation of a microhematocrit tube of whole anticoagulated blood. Company health team members, including vets and biologists, reported discoloration of gills and local hemorrhages. This paper reviews various commercially significant cases and lesser-known cases of anemia in cultured salmonid species induced by various biological factors. The current methods available to assess hematology are addressed and some future methods that could be adopted in modern day fish farming are identified. An account of the most recent anemic event in Scottish farmed Atlantic salmon (Salmo salar) is presented and discussed as a case study from information provided by two major Scottish salmon producers. The percent of total marine sites (n = 80) included in this case study, that reported with suspected or clinical anemia covering the period mid-late summer 2017 to 2019, was between 1 and 13%. The findings from this case study suggest that anemia experienced in most cases was regenerative and most likely linked to blood loss from the gills.

Dietary arachidonic acid affects immune function and fatty acid composition in cultured rabbitfish Siganus rivulatus

The marbled spinefoot rabbitfish (Siganus rivulatus) is an economically valuable fish species that has potential for commercial production in aquaculture. To overcome challenges in its sustainable production, a formulated diet is required for imparting health and robustness. This study evaluates the effect of dietary supplementation with arachidonic acid (ARA; 20:4n-6) on growth, survival, immune function and fatty acid composition of red blood cells (RBCs) in rabbitfish. We conducted two feeding trials using juvenile fish (to evaluate growth and survival) and adults (to evaluate immune function and fatty acid incorporation). Fish were fed diets supplemented with three different levels of ARA (in % of total fatty acids): 0.6 (unsupplemented control), 2.6 (moderate) and 4.7 (high). The fish fed with moderate ARA levels exhibited improved (p < 0.05) growth over the control and the high ARA level groups. During an outbreak of Streptococcus iniae, fish fed with moderate ARA survived significantly (p < 0.05) better (89%) than the control and the high ARA groups (59% and 48%, respectively). Moderate ARA supplementation resulted in elevated lysozyme and complement levels in the plasma of rabbitfish. A significant increase in the total serum immunoglobulin levels was observed in both the medium and the high ARA level groups; however, a decrease in antiprotease activity was recorded in the supplemented groups as compared to the control. Fatty acid analysis in fish red blood cells revealed a significant (p < 0.05) increase in the proportion of ARA of total fatty acids in the groups fed with the medium and the high ARA level diets (9.5% and 11.2%, respectively, compared to 7.1% in the control). Concomitantly, there was a decrease in the proportion of eicosapentaenoic acid (EPA; 20:5n-3), dihomo-γ linolenic acid (DGLA; 20:3n-6) and several 18-carbon unsaturated fatty acids in these groups. In conclusion, ARA in rabbitfish feeds improved growth, survival as well as innate and acquired humoral immune functions. Thus ARA supplementation in the diet of this species could be a valuable step towards establishing the commercial culture of rabbitfish.

Development and application of monoclonal antibodies against IgM of black rockfish Sebastes schlegeli

Monoclonal antibodies (mAbs) against black rockfish Sebastes schlegeli serum immunoglobulin M (IgM) were developed, which showed a specific reaction with the heavy chain of S. schlegeli IgM in Western blotting and with surface IgM positive (sIgM⁺ ) lymphocytes in indirect immunofluorescence. mAb 2A6 was employed to investigate the antibody and sIgM⁺ lymphocyte responses of S. schlegeli injected with inactivated Edwardsiella tarda, by ELISA and flow cytometry. Compared with controls, the level of specific antibodies and the percentage of sIgM⁺ lymphocytes both increased in the immunized fish and simultaneously reached their peaks at day 35 after immunization.

The effects of increasing dietary levels of soy protein concentrate (SPC) on the immune responses and disease resistance (furunculosis) of vaccinated and non-vaccinated Atlantic salmon (Salmo salar L.) parr

Juvenile salmon, with an initial weight of 9 g, were fed three experimental diets, formulated to replace 35 (SPC35), 58 (SPC58) and 80 (SPC80) of high quality fishmeal (FM) with soy protein concentrate (SPC) in quadruplicate tanks. Higher dietary SPC inclusion was combined with increased supplementation of methionine, lysine, threonine and phosphorus. The experiment was carried out for 177 days. On day 92 salmon in each tank were bulk weighed. Post weighing eighty salmon from each tank were redistributed in two sets of 12 tanks. Salmon from the first set of tanks were vaccinated, while the second group was injected with phosphate buffer saline (PBS). Salmon were sampled on day 92 (pre-vaccination), day 94 (2 days post vaccination [dpv]/PBS injection [dpPBSinj]) and day 154 (62 dpv/dpPBSinj) of the trial for the assessment of their immune responses, prior to the performance of salmon bulk weights for each tank. On day 154, fish from each tank were again bulk weighed and then seventeen salmon per tank were redistributed in two sets of twelve tanks and intra-peritoneally infected with Aeromonas salmonicida. At Day 154, SPC80 demonstrated lower performance (weight gain, specific growth rate and thermal growth coefficient and feed conversion ratio) compared to SPC35 salmon. Reduced classical and total complement activities for salmon fed diets with over 58% of protein from SPC, were demonstrated prior to vaccination. Reduced alternative complement activity was detected for both SPC58 and SPC80 salmon at 2 dpv and for the SPC80 group at 62 dpv. Total and classical complement activities demonstrated no differences among the dietary groups after vaccination. Numerical increases in classical complement activity were apparent upon increased dietary SPC levels. Increased phagocytic activity (% phagocytosis and phagocytic index) was exhibited for the SPC58 group compared to SPC35 salmon at 62 dpPBSinj. No differences in serum lysozyme activity, total IgM, specific antibodies, protein, glucose and HKM respiratory burst were detected among the dietary groups at any timepoint or state. Mortalities as a result of the experimental infection only occurred in PBS-injected fish. No differences in mortality levels were demonstrated among the dietary groups. SPC58 diet supported both good growth and health in juvenile Atlantic salmon while SPC80 diet did not compromise salmon' immunity or resistance to intraperitoneally inflicted furunculosis.

Aeromonas salmonicida: updates on an old acquaintance

Aeromonas salmonicida is the oldest known infectious agent to be linked to fish disease and constitutes a major bacterial pathogen of fish, in particular of salmonids. This bacterium can be found almost worldwide in both marine and freshwater environments and has been divided into several sub-species. In this review, we present the most recent developments concerning our understanding of this pathogen, including how the characterization of new isolates from non-salmonid hosts suggests a more nuanced picture of the importance of the so‑called 'atypical isolates'. We also describe the clinical presentation regarding the infection across several fish species and discuss what is known about the virulence of A. salmonicida and, in particular, the role that the type 3 secretion system might play in suppressing the immune response of its hosts. Finally, isolates have displayed varied levels of antibiotic resistance. Hence, we review a number of solutions that have been developed both to prevent outbreaks and to treat them once they occur, including the application of pre- and probiotic supplements.

Optimisation and standardisation of functional immune assays for striped catfish (Pangasianodon hypophthalmus) to compare their immune response to live and heat killed Aeromonas hydrophila as models of infection and vaccination

Aquaculture production of Pangasianodon hypophthalmus is growing rapidly in South East Asia, especially in Vietnam. As it is a relatively new aquaculture species there are few reports evaluating its immune response to pathogens. Thus, functional assays for P. hypophthalmus were optimised to evaluate both innate and adaptive immune responses, and were then used to examine immune response following stimulation with live and heat-killed Aeromonas hydrophila. These were used as models of infection and vaccination, respectively. Four treatment groups were used, including a control group, a group injected intraperitonally (IP) with adjuvant only, a group injected with heat-killed A. hydrophila (1 × 10(9) cfu ml(-1) mixed with adjuvant), and a group injected with a subclinical dose of live A. hydrophila. Samples were collected at 0, 1, 3, 7, 14 and 21 days post-injection (d.p.i.) to assess their immune response. The results indicated that challenge with live or dead bacteria stimulated the immune response in P. hypophthalmus significantly above the levels observed in control groups with respect to specific antibody titre, plasma lysozyme and peroxidase activity, and phagocytosis by head kidney macrophages at 7 or/and 14 d.p.i. At 21 d.p.i., total and specific antibody (IgM) levels and plasma lysozyme activity in fish injected with either live or dead A. hydrophila were significantly different to the control groups. Differential immune responses were observed between fish injected with either live or dead bacteria, with live A. hydrophila significantly stimulating an increase in WBC counts and plasma peroxidase activity at 3 d.p.i., with the greatest increase in WBC counts noted at 21 d.p.i. and in phagocytosis at 14 d.p.i. By 21 d.p.i. only the macrophages from fish injected with dead A. hydrophila showed significantly stimulation in their respiratory burst activity. This study provides basic information on the immune response in pangasius catfish that can be useful in the health control of this species.

Toxoid construction of AsaP1, a lethal toxic aspzincin metalloendopeptidase of Aeromonas salmonicida subsp. achromogenes, and studies of its activity and processing

AsaP1 is a toxic aspzincin metalloendopeptidase secreted by the fish pathogen Aeromonas salmonicida subsp. achromogenes. The protease is highly immunogenic and antibodies against AsaP1 evoke a passive protection against infection with A. salmonicida subsp. achromogenes. The protease is expressed as 37 kDa pre-pro-protein and processed to an active enzyme of 19kDa in A. salmonicida subsp. achromogenes. Recombinant expression of AsaP1(rec) in E. coli results in a protease of 22 kDa that is not secreted. AsaP1(rec) induces comparable pathological changes in Atlantic salmon (Salmo salar L.) to native AsaP1(wt). The aim of the study was to construct AsaP1 toxoids by exchanging catalytically important amino acids in the active site region of the protease. Four different AsaP1 mutants (AsaP1(E294A), AsaP1(E294Q), AsaP1(Y309A), and AsaP1(Y309F)) were successfully constructed by one step site directed mutagenesis, expressed in E. coli BL21 C43 as pre-pro-proteins and purified by His-tag affinity chromatography and gel filtration. Three of the resulting mutants (AsaP1(E294A), AsaP1(E294Q), and AsaP1(Y309A)) were not caseinolytic active and are detected as unprocessed pre-pro-proteins of 37 kDa. Caseinolytic active AsaP1(rec) and a mutant with reduced activity, AsaP1(Y309F), were processed to a size of 22 kDa. Furthermore, AsaP1(rec) is able to process the inactive mutants to the mature size of 22 kDa, allowing the conclusion that AsaP1 is autocatalytically processed. All four mutants AsaP1(E294A), AsaP1(E294Q), AsaP1(Y309A) and AsaP1(Y309F) are non-toxic in fish but induce a specific anti-AsaP1 antibody response in Arctic charr (Salvelinus alpinus L.) and are therefore true toxoids and possible vaccine additives.

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.

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.

Cloning and Expression of Partial Japanese Flounder (Paralichthys olivaceus) IgD

The cDNA sequence of the Japanese flounder (Paralychthys olivaceus) IgD has been previously reported (GenBank accession no. AB052658) and this was followed by the detection of IgD mRNA expression in some flounder organ tissues. However, it has not been determined whether the flounder IgD gene is virtually expressed into IgD protein. To characterize the flounder immunoglobulins utilized in elucidating the mechanism, evolution and diversity of the flounder immune system, antibodies specific to IgD and IgM were necessary. In the present study, partial flounder recombinant IgD (rIgD), IgM (rIgM) and the conserved regions of IgD and IgM (rCIg) were produced by cloning the cDNA sequence using isotype specific primers which were designed to produce unique fragments of IgD and IgM specific amino acid sequences. The production of recombinant Igs was ascertained by SDS-gel electrophoresis and immunoblot analysis using anti-T7 d Taq antibody. The produced recombinant Igs were purified using affinity columns, and used as immunogens. Antibodies specific to the isotype of flounder Igs were generated by immunizing rabbits with rfIgs and the antibodies produced were identified by enzyme-linked immunosorbent assay (ELISA) and immunoblotting. Specificities of the generated antibodies were evaluated by testing cross-reactivity between recombinant IgM and IgD. By ELISA, rabbit antibodies against the rfIgD fragment (anti-rfIgD) failed to recognize any kind of flounder serum Igs, whereas respective antibodies against rfCIg (anti-rfCIg) and rfIgM fragments (anti-rfIgM) reacted with serum Igs. Likewise, in immunoblot assays, though anti-rfIgD did not, both anti-rfCIg and anti-rfIgM bound with the ~85 kd flounder IgM heavy chain. By flow cytometry analysis, anti-rfCIg, anti-rfIgD and anti-rfIgM reacted with 6%, 3% and 6.5% of cells, respectively, suggesting that flounder IgD is not secreted in serum but expressed on flounder B-like cell surfaces as in mammals. Antibodies produced against recombinant flounder Igs could be used to develop sandwich assay systems for detecting flounder Igs and for further investigating the flounder immune system.

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.

Experimental infection of turbot, Scophthalmus maximus (L.), by Moritella viscosa, vaccination effort and vaccine-induced side-effects

Moritella viscosa is the causative agent of winter ulcers in farmed salmonids and Atlantic cod in countries around the North Atlantic. The bacterium has also been isolated from various marine fish species. Bacterial diseases have been a limiting factor in farming of turbot, but M. viscosa has not so far been isolated. In this study, turbot was shown to be sensitive to M. viscosa infection in experimental challenges. Pathological changes in infected turbot were comparable with those previously described for winter ulcers in salmon. A multivalent commercial salmon vaccine, containing M. viscosa as one of five antigens and a mineral oil adjuvant, did not protect turbot against challenge 13 weeks post-vaccination. Weight gain of vaccinated turbot compared with controls was not reduced 7 weeks post-vaccination. Vaccination did not induce a specific anti-M. viscosa response, while elevated anti-M. viscosa antibody levels were detected both in vaccinated and unvaccinated fish 5 weeks post-challenge. The vaccine did, however, induce an antibody response against Aeromonas salmonicida, another vaccine component. Minor intra-abdominal adhesions were detected in vaccinated fish and fish injected with a mineral oil adjuvant. The measurement of various innate humoral immune parameters did not reveal significant differences between vaccinated and control groups.

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.

Development of an immunoassay to measure the humoral immune response of hybrid striped bass Morone chrysops x M. saxatilis

Hybrid striped bass (HSB) were immunized with bovine serum albumin (BSA) and the specific anti-BSA immunoglobulin (Ig) was affinity purified from the resulting serum by means of an agarose gel-BSA column. The native Ig had an apparent molecular size of 893 KDD, by size exclusion chromatography, and when examined by polyacrylamide gel electrophoresis (PAGE) under denaturing conditions, resolved to heavy (H) and light (L) chains of 76 and 27 KDD, respectively. Affinity purified native HSB Ig was used to immunize a goat which produced specific anti-HSB Ig antibody (Ab). Purified native HSB Ig was also used to produce two murine monoclonal antibodies (mAbs) with specific affinities for H and L chain moieties of the HSB Ig molecule. Both polyclonal and monoclonal antibodies could be used individually in an indirect enzyme-linked immunosorbent assay (ELISA) to measure specific anti-BSA Ig in HSB serum. These antibodies could also be used in combination to measure total Ig in a capture ELISA format. Using both assays, the kinetics of the humoral immune response of HSB was measured for 98 days following two injections of BSA.

Humoral immune parameters of cultured Atlantic halibut (Hippoglossus hippoglossus L.)

Several humoral immune factors were studied in a group of cultured halibut (Hippoglossus hippoglossus L.). The serum protein and IgM concentration was comparable to levels seen in other teleost species. A strong antibody activity against TNP-BSA was observed but not against other antigens tested. Lysozyme and anti-protease activity was detected and showed variable heat sensitivity. Unlike the anti-protease activity, the lysozyme activity of the sera was not sensitive to storage at -20 degrees C. No spontaneous haemolytic activity was observed and the sera had no bactericidal effect on any of the bacterial strains tested. Iron binding capacity of the sera was high. Individual variation was considerable in all the factors tested.

Immune parameters of immunised cod (Gadus morhua L.)

The immune response of cod (Gadus morhua L.) is unusual in that specific antibody response is limited or absent. In the present study cod was immunised with haptenated and non-haptenated protein antigen at two different temperatures and the antibody response monitored over a period of 18 months. Other humoral parameters of immunological importance were also analysed, namely total immunoglobulin concentration, anti-protease and spontaneous haemolytic activity. No specific antibody response was detected but increased activity of non-specific anti-TNP antibodies was observed 10-12 weeks after immunisation, irrespective of the antigen used. This antibody activity was attributed to the adjuvant used (FCA) and did not cross react with other antigens tested. Other parameters were probably not influenced by the immunisation but seasonal fluctuations were indicated. The immunoglobulin level appeared to peak in August-September and the anti-protease activity and the haemolytic activity in October-January.

Characterization of Japanese eel immunoglobulin M and its level in serum

Japanese eel immunoglobulin M (IgM) was purified from the sera of Anguilla japonica immunized with Edwardsiella tarda FPU 347 and characterized. Analysis of the purified IgM on sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE) under reducing and non-reducing conditions revealed that the eel IgM was a tetrameric protein with a molecular weight of 790,000; it contained an equimolar heavy chain and light chain with molecular weights of 72,000 and 25,000, respectively. While the N-terminal sequence of the heavy chain, VELTQPGSMVLKPGQSLTI, showed similarity to the variable regions of those of teleost fishes Igs, the N-terminal sequence of the light chain, DIVLTQSPAVQSVQLGDT, was similar to the variable regions of chondrostei and mammalian kappa chains. Lectin-binding assays showed that the binding of concanavalin A (Con A) to the Japanese eel IgM heavy chain was competitively inhibited by D-mannose and could be abolished by alpha-mannosidase treatment indicating the presence on the heavy chain of oligosaccharides, whose terminal were a bound mannoses. The average IgM concentration in the sera of the healthy eels was 3.4 mg ml(-1); it amounted to 10.3% of the total serum protein.

Single-dilution enzyme-linked immunosorbent assay for quantification of antigen-specific salmonid antibody

An enzyme-linked immunosorbent assay (ELISA) was developed on the basis of testing a single dilution of serum to quantify the level of antibody to the p57 protein of Renibacterium salmoninarum in sockeye salmon (Oncorhynchus nerka). The levels of antibody were interpolated from a standard curve constructed by relating the optical densities (OD) produced by several dilutions of a high-titer rainbow trout (O. mykiss) antiserum to the p57 protein. The ELISA OD values produced by as many as 36 test sera on each microplate were compared with the standard curve to calculate the antigen-specific antibody activity. Repeated measurements of 36 samples on 3 microplates on each of 6 assay dates indicated that the mean intraassay coefficient of variation (CV) was 6.68% (range, 0-23%) and the mean interassay CV was 8.29% (range, 4-16%). The antibody levels determined for the serum sample from 24 sockeye salmon vaccinated with a recombinant p57 protein generally were correlated with the levels determined by endpoint titration (r2 = 0.936) and with results from another ELISA that was based on extrapolation of antibody levels from a standard curve (r2 = 0.956). The single-dilution antibody ELISA described here increases the number of samples that can be tested on each microplate compared with immunoassays based on analysis of several dilutions of each test serum. It includes controls for interassay standardization and can be used to test fish weighing <3 g.

Association between immunisation, reduced weight gain and plasma cortisol concentrations in juvenile Baltic salmon (Salmo salar)

The changes in plasma cortisol levels, immune response parameters and growth of juvenile Atlantic salmon (Salmo salar) were monitored during a 50 days period following a DNP-HSA (di-nitrophenyl human serum albumin) immunisation program. Antibody titers rose significantly after a single immunisation. An increased plasma cortisol concentration was observed in association with injection of both antigen and saline. A single injection had a significant negative effect on growth of fish and fish subjected to 2 injections with a 25 days interval had an even larger growth reduction. The plasma cortisol concentration and the specific antibody response were compared at an individual level but no correlation was found. Total serum protein increased during the experimental period independently of immunisation. In contrast the total serum immunoglobulin 50 days after the first immunisation was clearly connected to antigen exposure. The observations are discussed in relation to immunophysiological changes during immunisation and stress induction.

Study of the humoral response of Atlantic salmon (Salmo salar L.), naturally infected with Aeromonas salmonicida ssp. achromogenes

The humoral antibody response of healthy Atlantic salmon and of two groups of salmon, naturally infected with Aeromonas salmonicida ssp. achromogenes, was examined in some detail. One diseased group was chronically infected and the other recently infected. It was found that the humoral response of these two infected groups was quite different. The chronically infected fish showed poor specific response to the causative agent whereas the recently infected salmon produced strong specific antibody response. The chronically infected fish showed evidence of increased unspecific response including an elevated level of natural antibodies. The specific humoral response of the recently infected fish was primarily directed against two cell-associated antigens of the A. salmonicida ssp. achromogenes bacterium, the A-layer protein and the o-polysaccharide component of LPS. In the chronically infected fish the humoral response was primarily directed against the A-layer protein.