Fish immunoglobulins and the genes that encode them

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

A spleen cDNA library was constructed from the Antarctic teleost Trematomus bernacchii and immunoscreened with rabbit IgG specific for T. bernacchii Ig heavy chain. Eleven cDNA clones, varying in size and encoding the entire heavy chain or parts of it, were isolated. Here the complete nucleotide and deduced amino acid sequences of clone 2C2 encoding the secretory IgH chain form are reported. Comparison of the amino acid sequence of the entire constant region of the T. bernacchii Ig heavy chain with those from other teleosts and two holostean fish showed percent identity ranging 53.6-60.6%, with the highest values found for Salmoniformes. The multiple sequence alignment revealed the presence of two remarkable insertions: one at the VH-CH1 boundary and a second one, not found in any other IgM heavy chain, localised at the CH2-CH3 boundary. The latter occurred in the region proposed to act as a 'hinge', and resulted in a CH2-CH3 hinge peptide longer than any other IgM hinge. Differences were also found in the number and position of putative N-glycosylation sites of the compared sequences. It is suggested that the unusual features found in the T. bernacchii Ig heavy chain might contribute to the flexibility of the Ig molecule and help understand more about the adaptation of Ig molecules to the polar sea environment.

Molecular cloning and expression analysis of carp (Cyprinus carpio) interleukin-1 beta, high affinity immunoglobulin E Fc receptor gamma subunit and serum amyloid A

Suppression subtractive hybridisation (SSH) is a powerful means to identify genes of cytokines and other genes that express small amount of mRNA. In this study, cDNA of normal fish (carp) head kidney cells (HKC) was subtracted from pooled cDNA of HKC and peritoneal cell (PC) obtained from fish which had been injected with sodium alginate (SA) and scleroglucan (SG) 3-48 h earlier. This subtraction produced 248 clones of cDNA fragments. After sequencing some of the fragments of interest were used as probes, and yielded full-length cDNAs homologous to mammalian interleukin-1 beta (IL-1 beta), the gamma subunit of high affinity Fc receptor for IgE (Fc epsilon RI gamma) and serum amyloid A (SAA); these were cloned and sequenced. Carp IL-1 beta shows 21.8-24.7% amino acid identities to mammalian mature IL-1 beta, and lacks a signal sequence, which is consistent with mammalian IL-1 beta. Carp Fc epsilon RI gamma, which was the first cloned non-mammalian Fc receptor subunit, shows 39.3-40.4% amino acid identities to mammalian Fc epsilon RI gamma, and contains the immunoreceptor tyrosin-based activation motif characteristic of the signal transduction subunit of antigen- and Fc-receptors. Carp SAA is most similar to mammalian acute phase responsive type SAA with 53.0-55.3% amino acid identities. Both SA-elicited and SG-elicited PC expressed higher amounts of IL-1 beta and SAA mRNA compared to saline-injected fish HKC and PC, indicating that these proteins are associated with inflammatory responses, similar to mammalian homologues. Fc epsilon RI gamma was constitutively expressed in leucocytes and not immunopotentiator-responsive, but this indicates that Fc receptor including Fc epsilon RI gamma subunit is likely functional in the carp immune system.

Cloning and structural analysis of IgM (mu chain) and the heavy chain V region repertoire in the marsupial Monodelphis domestica

To address the question of the Ig isotype repertoire of non placental mammals, we have examined the Ig expression in the marsupial Monodelphis domestica (grey short tailed opossum). Screening of an opossum spleen cDNA library has previously led to the isolation of full length clones for opossum IgG (gamma chain), IgE (epsilon chain) and IgA (alpha chain). We now present the isolation of several cDNA clones encoding the entire constant regions of the opossum IgM (mu chain). A comparative analysis of the amino acid sequences for IgM from various animal species showed that opossum IgM, within the various animals studied, is the most divergent member of its Ig class. However, it still conforms to the general structure of IgM in other vertebrates. Four Ig classes have now been identified in opossum and only one isotype is apparently present within each Ig class, IgM, IgG, IgA and IgE. Opossum has previously been shown to have a limited VH region diversity, with only two V gene families. Both of these belong to the group III of mammalian VH sequences. This limitation in variability is to some extent compensated for by a large variation in D, P and N regions, both in size and in sequence. However, evidence for the expression of only two functional J segments has so far been detected, which indicates a rather limited diversity also of the J segments in the opossum.

Influence of the mu-chain C-terminal sequence on polymerization of immunoglobulin M

Immunoglobulin (IgM) is found in various states of covalent polymerization (microL)n, where n is typically 8, 10, or 12. The usual form of IgM of bony fish is tetrameric (8 microL units) as compared to the pentameric form (10 microL units) observed in cartilaginous fish and mammals. Two hypotheses were tested in this study. First, that the length of the mu-chain C terminus following Cys575 determines whether an IgM polymerizes as a tetramer or as a pentamer. This was tested by examining the covalent polymerization state of mouse IgM mutated to contain a series of mu-chain C-termini from bony and cartilaginous fish. The results proved this hypothesis wrong: mouse IgM bearing the C-terminal sequence of shark, salmon and cod mu-chain behaved identically to native mouse IgM, forming predominantly (microL)10 and (microL)12 forms. The second hypothesis was that an additional Cys residue near the C terminus of the mu-chain is responsible for the multiple covalent structures seen in IgM of the channel catfish. The addition of a catfish C terminus to the mouse mu-chain resulted, as predicted, in the production of a series of covalently bonded forms, with the major species being (microL)4. When a Ser-Cys unit was removed from the catfish C terminus added to the mouse mu-chain, this resulted in production of IgM indistinguishable in structure from that of wild-type mouse IgM.

Catfish Oct2 binding affinity and functional preference for octamer motifs, and interaction with OBF-1

The DNA-binding (POU) domain of the catfish Oct2 transcription factor was shown, by electromobility shift assays and surface plasmon resonance techniques, to have an affinity for the consensus octamer motif (ATGCAAAT) that was slightly higher than its affinity for a variant motif (ATGtAAAT). This observation is consistent with the transcriptional activation potentials of catfish Oct2 alpha and Oct2 beta, which were shown to activate transcription in catfish B and T cell lines to an equivalent extent from both the consensus and variant octamer motifs. When tested in a mouse plasmacytoma cell line, catfish Oct2 alpha and Oct2 beta, as well as mouse Oct2, showed higher transcriptional activation with the variant, as compared to the consensus, octamer motif. Catfish Oct2 was shown to function synergistically with the mammalian co-activator, OBF-1, activating octamer-dependent transcription in catfish T cells. The strong transcriptional activity of OBF-1 in catfish cells was dependent on the presence of octamer motif(s) at the proximal (promoter) rather than the distal (enhancer) position.

Evolutionary variation of immunoglobulin mu heavy chain RNA processing pathways: origins, effects, and implications

Immunoglobulins (Ig) can occur in two physical forms, soluble (secreted) and membrane bound. The soluble form is secreted from B cells, and is present in the blood and other fluids where it plays a role as an immune effector molecule. The membrane-bound form of the Ig molecule is inserted into the B-cell membrane, where it serves as a receptor for antigen. The function of the membrane-bound Ig as a receptor for antigen requires additional accessory molecules, the membrane Ig plus accessory molecules are referred to, collectively, as the B-cell receptor (BCR) complex. The secreted and membrane-bound forms of an Ig result from alternative patterns of RNA processing of the primary transcript from the heavy chain gene. IgM is the only class of Ig known to be conserved in all vertebrate species (perhaps exclusive of the agnathan fish). While the structure of the IgM heavy (mu) chain gene has been highly conserved in vertebrate evolution, the patterns of alternative RNA processing of the mu transcript show surprising diversity. In particular, the bony fish (teleosts) produce membrane mu-chain message by a splicing pathway that is quite different from that seen in other vertebrates; it results in the production of membrane IgM that lacks the C mu 4 domain. How this unusual RNA splicing pattern could have evolved and its implications for the function of the BCR in the bony fishes are considered here.

The immunoglobulin light chain in poikilothermic vertebrates

The immunoglobulin light chains are classified as kappa or lambda in mammals and birds (homeothermic vertebrates), but the traditional criteria for this classification are not applicable to the light chains found in poikilothermic vertebrates. Still it is possible to find some relationships between Ig light chain sequences in these animals and in those of the homeothermic animals. It is generally accepted that the Ig light chains contribute to the antigen binding capacity of antibodies and the variability is approximately similar in all studied vertebrate species except the elasmobranchs. This might be explained by the organisation of the Ig light chain locus in these animals and the fact that the variable and joining DNA segments are joined in the genome. These conclusions are limited by the small number of species studied in this respect.

Ichthyophthirius multifiliis: a model of cutaneous infection and immunity in fishes

The parasitic ciliate Ichthyophthirius multifiliis offers a useful system for the study of cutaneous immunity against an infectious microorganism. Naive fish usually die following infection, but animals surviving sublethal parasite exposure become resistant to subsequent challenge. This resistance correlates with the presence of humoral antibodies in the sera and cutaneous mucus of immune fish. A mechanism of immunity has recently been elucidated that involves antibody binding to surface proteins (referred to as immobilization antigens or i-antigens) located on the parasite cell and ciliary membranes. Antibody-mediated cross-linking of i-antigens triggers a response by the parasite resulting in its exit from the host. These effects can be observed directly on the surface of live fish. In addition to allowing the observation of effector responses in vivo, Ichthyophthirius also provides a means to study the ontogeny of the mucosal immune response. The sites of antigen capture and presentation, and the sites of antibody production, are unknown with regard to cutaneous immunity. Because the external epithelial surfaces of fish are often the points of pathogen entry, a basic understanding of the inductive immune mechanisms and immune cell interactions in the skin and gills is extremely important with regard to vaccine development. The development of Ichthyophthirius as an experimental system and how it might be used to address these issues are discussed in this review.

Varied redox forms of teleost IgM: an alternative to isotypic diversity?

Teleosts (bony fish) are thought to primarily or exclusively possess a single structural form of immunoglobulin (Ig), a tetrameric IgM. However, in species wherein intact Ig has been electrophoretically analyzed under denaturing, non-reducing conditions, a significant degree of structural diversity has been revealed. This IgM molecule appears to be assembled with great latitude in the degree of disulfide crosslinking between monomeric or halfmer subunits composing the complete IgM molecule. This heterogeneity in the basic structure (herein referred to as redox forms) is not due to isotypic differences as each B cell produces this heterogeneity within its immunoglobulin product. Additionally, in the case of the catfish, a single fish/mouse chimeric Ig H gene is capable of producing IgM with a comparable amount of structural heterogeneity within the mouse cell. Thus, the piscine B lymphocyte routinely assembles a variety of redox forms from one IgM chain. This has both profound biosynthetic implications for macromolecular assembly processes as well as intriguing possibilities for the generation of teleost Ig functional diversity.

Complement diversity: a mechanism for generating immune diversity?

Unlike mammalian species, several cold-blooded species have been shown to possess multiple forms of complement components. The multiple forms of C3 characterized in several fish species can bind with different specificities to various complement-activating surfaces. Here, Oriol Sunyer, Ioannis Zarkadis and John Lambris explore the possible advantages conferred by having multiple forms of individual complement proteins in a single organism.

T-cell receptors in channel catfish: structure and expression of TCR alpha and beta genes

Herein are reported full length cDNA sequences for TCR alpha- and beta-chains of the channel catfish. Included are sequences belonging to four Valpha and six Vbeta families which share hallmarks in common with the Valpha and Vbeta genes of other species. Similar to the situation in other vertebrates, the catfish Calpha and Cbeta sequences exhibit distinct immunoglobulin, connecting peptide, transmembrane and cytoplasmic domains. However, the catfish TCR Calpha and Cbeta regions are shorter than those of mammals and the catfish Cbeta chain lacks a cysteine in its connecting peptide region. Two different catfish Cbeta cDNA sequences were identified, suggesting the existence of either two Cbeta loci or allotypes. Based on Southern blot analyses, each of the catfish TCR gene loci appear to be arranged in a translocon (as opposed to multicluster) organization with multiple V elements and a single or few copies of C region DNA. At the deduced amino acid level, the catfish Cbeta sequence exhibits 42% identity with the Cbeta of Atlantic salmon, 41% identity with the Cbeta of rainbow trout and 26% identity with Cbeta of the horned shark. The catfish Calpha amino acid sequence exhibits 44 and 29% identity with Calpha of the rainbow trout and southern pufferfish, respectively. TCRalpha and beta messages are selectively expressed and rearranged in a catfish clonal cell line that appears to be of the T lineage. This TCR alpha/beta expressing clonal lymphocyte line, designated 28S.1, has T-cell like function in that it constitutively produces a supernatant factor(s) with growth promoting activity. These findings should facilitate functional studies of fish TCRs and T cells in ways not previously possible with other 'lower' vertebrate models.

Mutation, selection, and memory in B lymphocytes of exothermic vertebrates

Unlike mammals, cold-blooded vertebrates produce antibodies of low heterogeneity that show little increase in binding affinity with time after immunization. In secondary responses, antibody titers and affinities are often little, if any, higher than in primary responses. That is, specificity, diversity, and memory--the hallmarks of the immune system--are rather meager in the humoral immune responses of exothermic vertebrates. As the genetic components of the immunoglobulin (Ig) gene systems in fishes, amphibians or reptiles are not deficient in number or diversity, their responses probably do not stem from restrictions in the primary antibody repertoire. Somatic hypermutation at the Ig locus, which generates diversity and higher affinity antibodies in mammals, is not lacking in the South African frog Xenopus or in the shark. However, the Ig mutants recovered are strongly biased toward alterations at GC pairs, an indication that they have not undergone effective selection. While cells resembling follicular dendritic cells are present in cold-blooded vertebrates, germinal centers do not form. It is suggested that this absence of germinal centers, the site of selection for the mutants with higher affinity receptors and of differentiation into memory B cells in mammals, may explain the principal differences between cold and warm-blooded vertebrates.

Structure and organization of the immunoglobulin M heavy chain genes in Atlantic salmon, Salmo salar

To determine the structure and organization of the germline immunoglobulin M heavy chain (IgH) genes in Atlantic salmon, Salmo salar, relevant clones from a genomic library (of one individual fish) have been characterized. Two closely related IgH constant region genes, CHA and CHB, have been sequenced completely. In addition, an allotypic variant of CHA was identified and partially sequenced. Five joining (JH) elements were found in a distance of 0.5-1.6 kb upstream of the first constant exon (CH1), in both CHA and CHB, substantiating the hypothesis that the entire gene complex is duplicated; possibly a remnant of a tetraploid event in the salmonid ancestor. An octamer motif (ATGTATTT, and its reverse complementary sequence) was found to be dispersed in the JH-CH1 region, but not elsewhere, signifying a role in these loci. Four closely related variable (VH) genes which were subcloned from three distinct lambda clones showed the classical structure of a two exon unit split by a 100 bp intron. The split-intron and a few hundred base pairs of the flanking sequences of the genes were highly similar. Three of the four genes were interrupted by stop codons and/or frame shifts, indicating a high proportion of VH-pseudogenes in this species. Based on the present results, and comparison with sequences of rainbow trout, Oncorhynchus mykiss, it is likely that the IgH loci have remained tetrasomicly inherited throughout the radiation of the genus Salmo and Oncorhynchus, and that the duplicated loci have gone into a disomic inheritance pattern in the comparatively recent past.

Fish immunology: the utility of immortalized lymphoid cells--a mini review

Long term cell lines can be readily established at high frequency with PBLs from normal channel catfish. Depending upon the mode of stimulation, morphologically and functionally distinct catfish lymphoid cell lines resembling B cells, T cells and monocytes have been developed. These fish cell lines appear unique from their putative mammalian counterparts in that they are immortalized without the need for exogenous factors or overt attempts at transformation.

Humoral immunoglobulins of the white sturgeon, Acipenser transmontanus: partial characterization of and recognition with monoclonal antibodies

White sturgeon (Acipenser transmontanus) immunoglobulin (Ig) was purified from serum by two methods, ion-exchange chromatography and gel filtration and precipitation of the euglobulin fraction. The purity of these immunoglobulin preparations was confirmed by gel electrophoresis. Sequence analysis of the N-terminal amino acids confirmed that the purified protein was immunoglobulin. The major portion of the immunoglobulin preparation consisted of two proteins with estimated molecular weights (m.w.) of 870 and 170 kDa. The m.w. of the H- and L-chains of the purified Ig were 73 and 27-30 kDa, respectively, as determined by SDS-PAGE. Ion-exchange purified Ig was used to immunize mice for the production of monoclonal antibodies. This resulted in the production of six stable hybrids that recognized sturgeon Ig, two specific for heavy chain and four specific for light chain. The two anti-H-chain mabs were highly specific for white sturgeon Ig while all four anti-L-chain mabs cross reacted with Ig from green sturgeon (A. medirostris), Atlantic sturgeon (A. oxyrhynchus oxyrhynchus), shovelnose sturgeon (Scaphirhynchus platorynchus), and paddlefish (Polyodon spathula), (all Chondrosteans), but not with channel catfish (Ictalurus punctatus), rainbow trout (Oncorhynchus mykiss) or striped bass (Morone saxatilis). The mabs were used to enumerate the percentage of sIg+ lymphocytes in the peripheral blood of white sturgeon by flow cytometry. The percentage of cells positively stained with the mabs ranged from 12 to 28%. In a comparison of mabs with polyclonal rabbit anti-sturgeon Ig serum by ELISA the mabs produced a larger signal and less background than the polyclonal serum.

Immune-complex trapping in the splenic ellipsoids of rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss), immunised with horseradish peroxidase, were given horseradish peroxidase intravenously, and the trapping of antigen in the spleen was followed 1, 24, and 48 h after injection. After 1 h, the localisation of horseradish peroxidase indicated that the antigen had been extensively trapped in the walls of the splenic ellipsoids. The colocalization of horseradish peroxidase with rainbow trout immunoglobulin M and complement factor 3 was shown with a double immunofluorescence technique and suggested that horseradish peroxidase was trapped in the form of immune complexes. After 24 and 48 h, very little horseradish peroxidase was detected in the ellipsoids, and horseradish peroxidase was mainly found in association with large cells with prominent cytoplasmic extensions. In non-immunized fish given horseradish peroxidase intravenously, antigen was not detected in ellipsoids. Thus, the observed difference between immunised and non-immunized trout suggests a specific role for the splenic ellipsoids in rapid immune-complex trapping and invites speculation on its significance in a secondary immune response.

Alternate pre-mRNA processing pathways in the production of membrane IgM heavy chains in holostean fish

A single gene encodes two forms of the IgM heavy chain (mu) in vertebrates: one (microseconds) present in serum as secreted IgM and the other (microns) as the antigen receptor form of IgM present on the B-lymphocyte membrane. The mRNAs encoding microseconds and microns are derived from a single primary transcript by alternate pathways of RNA processing. In all vertebrates so far examined, with the exception of teleosts, microns mRNA is produced by splicing the transmembrane (TM) encoding exon 1 into a cryptic donor site near the 3' end of the C mu 4 exon. In contrast, teleost species splice the TM exon 1 into the regular splice donor site at the 3' boundary of the C mu 3 exon. We have examined micron mRNAs in two species of primitive bony fish, the holostean bowfin and the longnose gar. These fish utilize both the C mu 3 to TM1 (teleost) pathway and the typical cryptic C mu 4 to TM1 pathway. In addition the bowfin possesses a cryptic splice donor site near the middle of C mu 3. This is used in the production of a third species of microns-encoding mRNA, but does not participate in the production of an alternate form of the microseconds mRNA. The structure and patterns of expression of their mu genes suggest that the gar and bowfin may be more closely related than implied by the current view of fish evolution.

B cell and immunoglobulin heterogeneity in carp (Cyprinus carpio L.); an immuno(cyto)chemical study

B cell and immunoglobulin (Ig) heterogeneity was demonstrated in carp, Cyprinus carpio L., using two monoclonal antibodies (MAbs; WC14, WCI12) produced against carp serum Ig. Immunochemical results showed that both WCI4 and WCI12 react with a protein determinant on the heavy chain of Ig (relative molecular mass approximately 70,000). Immunofluorescence microscopic and flow cytometric analyses of lymphoid cells suggest three distinct subpopulations of B cells and plasma cells: WCI4+12- cells, WCI4-12+ cells, and WCI4+12+ cells. WCI4-12+ and WCI4+12+ anti-DNP antibody-secreting cells were also demonstrated with the ELISPOT assay in pronephros and spleen cell suspensions from primary immunised carp. Affinity chromatography of carp serum and sequential immunoprecipitation of 125I-labelled peripheral blood leucocyte (PBL) membrane proteins only indicated the presence of two antigenically different Ig molecules, i.e., WCI4-12+ and WCI4+12+ molecules. WCI4+12- molecules could not be detected by affinity chromatography or immunoprecipitation. During ontogeny, a shift in percentages of WCI4+12- and WCI4-12+ cells was found in the spleen and the pronephros. In these organs, WCI4+12- cells formed the majority of B cells at 2 weeks of age, but the percentages of this cell type decreased during ontogeny. On the other hand, the percentages of WCI4-12+ cells increased during development, and these cells became the major population of B cells from 13 weeks onward. The proportion of WCI4+12+ cells remained almost constant during ontogeny. The distribution of B cell subpopulations in blood was more or less stable at all ages. The functional significance of Ig heterogeneity in fish and in particular carp is discussed.

Applications of monoclonal antibodies in aquaculture

Monoclonal antibodies (MAbs) currently are being applied to the study of fish immunology and fish infectious diseases. MAbs to fish immunoglobulins (Igs) have helped isolate fish Igs, identify heavy and light chain variants in fish Ig, study the ontogeny of B lymphocytes, and improve techniques for the measurement of fish Ig and specific antibodies (Abs). MAbs have been obtained against several leucocyte surface antigens and are being used as markers for different subsets of fish leukocytes: neutrophils, non-specific cytotoxic cells and cells responsible for the mixed leucocyte reaction. The sensitivity and specificity of many immunoassays for identifying fish pathogens have been improved by the use of MAbs. Further improvement of these tests is currently being attempted by the use of MAbs together with the polymerase chain reaction (PCR). Epizootiological studies of fish diseases are beginning to emerge from the use of these reagents and techniques. In the near future these new methods should detect low levels of pathogens in adult carriers and perhaps in eggs, thus helping to prevent the dissemination of fish diseases. MAbs to fish pathogens are also being used for passive immunization studies as well as for conformational probes in the development of genetically engineered vaccines.

Development of an immune system

Minimally, an immune response is an induced cellular and/or humoral defense mechanism specific for the challenging agent. The system is a cognitive one inasmuch as a second stimulus with the same antigen can specifically induce either an enhanced response (memory) or diminished response (tolerance). The cells responsible for the initial antigen-specific recognition in higher vertebrates are clonally restricted T and B lymphocytes. Accessory cells are necessary for the processing and presentation of antigen, and physiologic mediators (cytokines) are essential for proliferation, interaction, and regulation of the system. Although it now appears that the recombination mechanisms essential for the anticipatory immune response occurred late in the deuterostome stream leading to vertebrates, molecules required for cell adhesion and regulation are widely spread in phylogeny. Their emergence must have preceded the divergence between ancestral protostomes and deuterostomes. Genetic mechanisms underlying the generation of diversity in the light and heavy chains of antibodies of mammals may be quite distinct in primitive vertebrates, particularly elasmobranchs, the ancestors of which diverged from those of mammals more than 400 million years ago. Despite this, clonal selection of antigen receptors of lymphocytes is most probably universal within the vertebrates. There is no need to force induced recognition in protostomes (e.g. insects) or lower deuterostomes (e.g. echinoderms) into mammalian models of immunity.

Immunoregulation in fish. I: Intramolecular-induced suppression of antibody responses to haptenated protein antigens studied in Atlantic salmon (Salmo salar L)

We report here evidence for intramolecular-induced suppression of the in vivo antibody response in fish, using a panel of T-dependent hapten-carrier antigens. Atlantic salmon were immunized intraperitoneally with protein antigens (Limulus polyphemus hemocyanin, chicken gamma globulin, and Aeromonas salmonicida A-layer protein) given in their native form or haptenated with either 4-hydroxy-3-iodo-5-nitrophenyl-acetic acid (NIP), 2,4,6-trinitrophenyl-acetic acid (TNP), or fluorescein-5-iso-thiocyanate (FITC). The salmon immune system responds to these hapten-carrier antigens by eliciting high anti-hapten titers whereas the antibody titers against protein determinants were suppressed 87-99%, determined by ELISA. NIP also induced suppression of the anti-FITC response when NIP and FITC were intramolecularly conjugated to Limulus polyphemus hemocyanin (LPH). The suppression was found to be independent of haptenation ratios and time after immunization. The possibility that haptenation interferes with or blocks the protein determinants is not likely because antisera raised against native LPH recognize LPH-specific epitopes even on heavily NIP-substituted LPH. Although the mechanism behind intramolecular-induced suppression is poorly understood, even in mammals, this study demonstrates that intramolecular-induced suppression may be one means by which antibody responses in fish are regulated. The possible impact of antigen-induced suppression on immune responses against vaccine antigens in fish is discussed.

Expression of a mouse-channel catfish chimeric IgM molecule in a mouse myeloma cell

Fusion genes encoding a murine VH domain and the constant region domains of the mu chain from the channel catfish, Ictalurus punctatus, were stably expressed in the lambda light chain producing mouse myeloma cell line J558L. Although the pathways of pre-mRNA processing for expression of membrane (micron and secreted (microsecond) forms of the mu chain differ between mammals and teleosts, mRNAs encoding both catfish micron and microsecond were correctly expressed in the mouse myeloma cells. The mouse-channel catfish chimeric mu chain polypeptide was able to associate covalently with the mouse lambda light chain and assemble, intracellularly, into polymers of covalent structure (microL)2-8 which resembled those seen with native catfish IgM. In contrast to native catfish IgM, the mouse-catfish chimeric IgM showed the property of binding strongly to protein A of Staphylococcus aureus. The mouse-channel catfish chimeric IgM was core-glycosylated, but did not contain terminal sialic acid. Secretion rates for the chimeric IgM were low, and the possibility could not be excluded that extracellular chimeric IgM was released from dead or dying cells. The reason(s) for the intracellular retention of the chimeric IgM (probably in the endoplasmic reticulum) are not known, but those mechanisms involving retention via cysteine residues were excluded.

Molecular and biotechnological approaches to fish vaccines

Global aquaculture is projected to grow from 10 million to 20 million metric tons by the year 2000. To meet this projection, the aquaculture industry must bring some of its infectious disease problems under control without relying on antibiotics and chemotherapeutics. Thus, vaccines for fish and shellfish are being sought by the industry. The very first commercial fish vaccine was a killed vibriosis bacterin which was very effective and relatively easy to produce. Vaccines for other bacterial, viral, and parasitic fish pathogens are proving more difficult to develop and researchers have begun to use molecular and biotechnological approaches to develop such vaccines. This review describes the vaccines that are now available as well as the vaccines that are being developed and includes a discussion of live, attenuated vaccines, immunoadjuvants, and subunit vaccines for fish.

Phylogeny of B-cell development

All vertebrates with jaws (Gnathostomata) have B cells. With the exception of some B cells in cartilaginous fish that express germ-line joined Ig genes, all B cells, irrespective of the organization of their Ig genes (which varies among vertebrates), rearrange the Ig-gene segments somatically. Somatic diversification occurs in all species during rearrangement (junctional diversity) and later by somatic mutation of gene conversion. Somatic mutants are poorly selected in species that lack germinal centers, which may explain the differences in antibody repertoire among vertebrates. The early (larval or neonate) B-cell repertoire is restricted in all species so far studied because of a lack of N-region diversity and in some cases because of a special usage of the D segments of the heavy chain genes.