Fish lymphocytes differ in the expression of surface immunoglobulin

Molecular structure and functional responses of IgM, IgT and IgD to Flavobacterium covae and Streptococcus iniae infection in Asian seabass (Lates calcarifer Bloch, 1790)

The Asian seabass (Lates calcarifer) faces significant disease threats, which are exacerbated by intensive farming practices and environmental changes. Therefore, understanding its immune system is crucial. The current study presents a comprehensive analysis of immune-related genes in Asian seabass peripheral blood leukocytes (PBLs) using Iso-seq technology, identifying 16 key pathways associated with 7857 immune-related genes, comprising 634 unique immune-related genes. The research marks the first comprehensive report on the entire immunoglobulin repertoire in Asian seabass, revealing specific characteristics of immunoglobulin heavy chain constant region transcripts, including IgM (Cμ, ighm), IgT (Cτ, ight), and IgD (Cδ, ighd). The study confirms the presence of membrane-bound form, ighmmb, ightmb, ighdmb of IgM, IgT and IgD and secreted form, ighmsc and ightsc of IgM and IgT, respectively, with similar structural patterns and conserved features in amino acids across immunoglobulin molecules, including cysteine residues crucial for structural integrity observed in other teleost species. In response to bacterial infections by Flavobacterium covae (formerly F. columnare genomovar II) and Streptococcus iniae, both secreted and membrane-bound forms of IgM (ighmmb and ighmsc) and IgT (ightmb and ightsc) show significant expression, indicating their roles in systemic and mucosal immunity. The expression of membrane-bound form IgD gene, ighdmb, predominantly exhibits targeted upregulation in PBLs, suggesting a regulatory role in B cell-mediated immunity. The findings underscore the dynamic and tissue-specific expression of immunoglobulin repertoires, ighmmb, ighmsc, ightmb, ightsc and ighdmb in Asian seabass, indicating a sophisticated immune response to bacterial pathogens. These findings have practical implications for fish aquaculture, and disease control strategies, serving as a valuable resource for advancing research in Asian seabass immunology.

Catfish lymphocytes expressing CC41-reactive leukocyte immune-type receptors (LITRs) proliferate in response to Edwardsiella ictaluri infection in vitro

Monoclonal antibodies (mAbs) CC34 and CC41 recognize overlapping subsets of leukocyte immune-type receptors (LITRs). The mAb CC34 was raised against the clonal TS32.15 cytotoxic T cell line and the mAb CC41 was raised against the clonal NK cell line TS10.1. In this study, an in vitro model was developed to monitor CC34- and CC41-reactive cells in response to Edwardsiella ictaluri infection. Briefly, head kidney leukocytes and peripheral blood lymphocytes (PBL) were isolated from individual catfish and labeled with CellTrace Violet and CellTrace FarRed dye, respectively. Head kidney-derived macrophages were infected with E. ictaluri and then cocultured with autologous PBL. The combined cell cultures were then analyzed using flow cytometry. A significant increase in CC41 staining was observed in the PBL population at 2, 5 and 7 days after culture, which suggest that LITRs are involved in cell-mediated immunity to E. ictaluri.

Recombinant ATPase of Virulent Aeromonas hydrophila Protects Channel Catfish Against Motile Aeromonas Septicemia

Channel catfish farming dominates the aquaculture industry in the United States. However, epidemic outbreaks of motile Aeromonas septicemia (MAS), caused by virulent Aeromonas hydrophila (vAh), have become a prominent problem in the catfish industry. Although vaccination is an effective preventive method, there is no vaccine available against MAS. Recombinant proteins could induce protective immunity. Thus, in this work, vAh ATPase protein was expressed, and its protective capability was evaluated in catfish. The purified recombinant ATPase protein was injected into catfish, followed by experimental infection with A. hydrophila strain ML09-119 after 21 days. Results showed catfish immunized with ATPase exhibited 89.16% relative percent survival after challenge with A. hydrophila strain ML09-119. Bacterial concentrations in liver, spleen, and anterior kidney were significantly lower in vaccinated fish compared with the non-vaccinated sham group at 48 h post-infection (p < 0.05). Catfish immunized with ATPase showed a significant (p < 0.05) higher antibody response compared to the non-vaccinated groups. Overall, ATPase recombinant protein has demonstrated potential to stimulate protective immunity in catfish against virulent A. hydrophila infection.

Evaluation of three recombinant outer membrane proteins, OmpA1, Tdr, and TbpA, as potential vaccine antigens against virulent Aeromonas hydrophila infection in channel catfish (Ictalurus punctatus)

A virulent clonal population of Aeromonas hydrophila (VAh) is recognized as the etiological agent in outbreaks of motile aeromonas septicemia (MAS) in catfish aquaculture in the southeastern United States since 2009. Genomic subtraction revealed three outer membrane proteins present in VAh strain ML09-119 but not in low virulence reference A. hydrophila strains: major outer membrane protein OmpA1, TonB-dependent receptor (Tdr), and transferrin-binding protein A (TbpA). Here, the genes encoding ompA1, tdr, and tbpA were cloned from A. hydrophila ML09-119 and expressed in Escherichia coli. The purified recombinant OmpA1, Tdr, and TbpA proteins had estimated molecular weights of 37.26, 78.55, and 41.67 kDa, respectively. Catfish fingerlings vaccinated with OmpA1, Tdr, and TbpA emulsified with non-mineral oil adjuvant were protected against subsequent VAh strain ML09-119 infection with 98.59%, 95.59%, and 47.89% relative percent survival (RPS), respectively. Furthermore, the mean liver, spleen, and anterior kidney bacterial concentrations were significantly lower in catfish vaccinated with the OmpA1 and Tdr than the sham-vaccinated control group. ELISA demonstrated that catfish immunized with OmpA1, Tdr, and TbpA produce significant antibody response by 21 days post-immunization. Therefore, OmpA1 and Tdr proteins could be used as potential candidates for vaccine development against virulent A. hydrophila infection. However, TbpA protein failed to provide strong protection.

A Leukocyte Immune-Type Receptor Subset Is a Marker of Antiviral Cytotoxic Cells in Channel Catfish, Ictalurus punctatus

Channel catfish, Ictalurus punctatus, leukocyte immune type receptors (LITRs) represent a multigene family that encodes Ig superfamily proteins that mediate activating or inhibitory signaling. In this study, we demonstrate the use of mAb CC41 to monitor viral cytotoxic responses in catfish and determine that CC41 binds to a subset of LITRs on the surface of catfish clonal CTLs. Homozygous gynogenetic catfish were immunized with channel catfish virus (CCV)-infected MHC-matched clonal T cells (G14D-CCV), and PBL were collected at various times after immunization for flow cytometric analyses. The percentage of CC41(+) cells was significantly increased 5 d after primary immunization with G14D-CCV and at 3 d after a booster immunization as compared with control fish only injected with G14D. Moreover, CC41(+) cells magnetically isolated from the PBL specifically killed CCV-infected targets as measured by (51)Cr release assays and expressed messages for CD3γδ, perforin, and at least one of the CD4-like receptors as analyzed by RNA flow cytometry. When MLC effector cells derived from a G14D-CCV-immunized fish were preincubated with CC41 mAb, killing of G14D-CCV targets was reduced by ∼40%, suggesting that at least some LITRs have a role in target cell recognition and/or cytotoxicity. The availability of a LITR-specific mAb has allowed, to our knowledge for the first time, functional characterization of LITRs in an autologous system. In addition, the identification of an LITR subset as a cytotoxic cell marker will allow for more effective monitoring of catfish immune responses to pathogens.

Physiology and immunology of mucosal barriers in catfish (Ictalurus spp.)

The mucosal barriers of catfish (Ictalurus spp) constitute the first line of defense against pathogen invasion while simultaneously carrying out a diverse array of other critical physiological processes, including nutrient adsorption, osmoregulation, waste excretion, and environmental sensing. Catfish depend more heavily on mucosal barriers than their terrestrial counterparts as they are continuously interacting with the aquatic microbiota. Our understanding of these barriers, while growing, is still limited relative to that of mammalian model systems. Nevertheless, a combination of molecular and cellular studies in catfish over the last few decades, and particularly within the last few years, has helped to elucidate many of the primary actors and pathways critical to their mucosal health. Here we describe aspects of innate and adaptive immune responses in the primary mucosal tissues (skin, gill, and intestine) of catfish, focusing on mucus-driven responses, pathogen recognition, soluble mediators, and immunoglobulin and T-cell derived immunity. Modulation of mucosal barriers will be critical moving forward for crafting better diets, improving vaccine delivery, enhancing water quality, and ensuring sustainable production practices in catfish.

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.

Antibody response of channel catfish after channel catfish virus infection and following dexamethasone treatment

Channel catfish virus (CCV, Ictalurid herpesvirus 1) and CCV disease have been extensively studied. Yet, little is known about CCV-host interaction after resolution of the primary infection. In order to determine potential recrudescence of CCV from latency, we established latency by exposing channel catfish juveniles with CCV or a thymidine kinase-negative recombinant (CCVlacZ) at a dose that caused less than 20% mortality. Then, we evaluated antibody response by serially sampling the same fish at 0 (pre-infection), 30, 60 and 90 d post challenge (DPC). We then attempted to induce viral recrudescence by intramuscular administration of dexamethasone and sampled the fish at 2, 4, 7, or 10 d post treatment. Recrudescence was evaluated by leukocyte co-cultivation and cell culture of tissue homogenates but no virus was detected. Western blot data demonstrated the highest number of seropositive fish by 30 DPC and a secondary antibody induction after dexamethasone treatment. The antigen specificity of the secondary response corresponded to viral proteins with molecular masses similar to those recognized by the same fish by 30 DPC. The recognized proteins were predominantly large, ranging from approximately 90 to >200 kDa. Expression analysis of selected virus genes at 90 DPC and following dexamethasone treatment demonstrated occasional immediate-early virus gene expression in peripheral blood leukocytes. Early and late gene expression was rarely detected. The combined data suggest restricted re-activation of CCV in our experimental system. Primary and secondary responses and virus gene expression were demonstrated in CCVlacZ-exposed fish but were less frequent than in CCV-exposed fish.

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

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

Characterization of anti-channel catfish IgL sigma monoclonal antibodies

This study characterizes three monoclonal antibodies (mAbs) developed against the constant (C) region of the immunoglobulin light (IgL) sigma chain isotype of the channel catfish, Ictalurus punctatus. Microsphere bead assays and Western blot analyses utilizing different recombinant (r) proteins show that these anti-catfish IgL sigma chain mAbs each specifically recognize the denatured form of IgL sigma. Importantly, Western blotting of catfish sera using the anti-IgL sigma mAbs also identified an IgL chain-sized immunoreactive band(s) of approximately 27kDa. It is anticipated that these mAbs in combination with the already existing anti-catfish Ig heavy (H) and IgL chain mAbs will be useful in future studies examining the functional roles of the different catfish IgL isotypes.

Production, characterisation and applicability of monoclonal antibodies to immunoglobulin of Japanese flounder (Paralichthys olivaceus)

Immunoglobulin (Ig) of Japanese flounder (Paralichthys olivaceus) was purified by a combination of salting-out and DEAE Sepharose Column chromatography. The purified immunoglobulin had an apparent molecular weight of 74 kDa (heavy chain) and 24 kDa (light chain) in SDS-PAGE. Eighteen hybridomas secreting monoclonal antibodies (MAbs) against Japanese flounder Ig were obtained by immunisation of Balb/C mice with purified Ig preparations, which were selected on the basis of the double indirect enzyme-linked immunosorbent assay (D-ELISA). Two of them designated as 2D8 and 2H1 were cloned by limiting dilution and characterised with western blotting, indirect immunofluorescence assay test (IIFAT) and fluorescence-activated cell sorter (FACS) analysis. Under reducing conditions in western blotting, both MAb 2D8 and MAb 2H1 were specific for the heavy chain of Japanese flounder Ig. MAb 2D8 was used to identify surface Ig-positive lymphocytes in the peripheral blood, spleen and pronephros of healthy Japanese flounder by flow cytometry. FACS analysis revealed that 40.48% of lymphocytes in the peripheral blood, 17.32% in the spleen and 9.67% in the pronephros were reactive to 2D8.

Channel catfish immunoglobulins: repertoire and expression

The channel catfish, Ictalurus punctatus, is widely recognized as an important model for studying immune responses in ectothermic vertebrates. It is one of the few fish species for which defined viable in vitro culture systems have been established and is currently the only fish species from which a variety of functionally distinct clonal leukocyte lines are available. Moreover, there is a large basis of biochemical and molecular information on the structure and function of catfish immunoglobulins (Igs). Catfish, as other teleosts, have a tetrameric homolog of IgM as their predominant serum Ig plus a homolog of IgD. They also have genetic elements basically similar to those of mammals, which encode and regulate their expression. The catfish Ig heavy (H) chain locus is a translocon-type locus with three Igdelta genes linked to an Igmu gene or pseudogene. The catfish IgH locus is estimated to contain approximately 200 variable (V) region genes representing 13 families as well as at least three diversity (D) and 11 joining (JH) genes. The catfish has two light (L) chain isotypes, F and G, both encoded by loci organized in multiple cassettes of VL-JL-CL with the VL in the opposite transcriptional orientation. Hence, all requisite components for encoding antibodies are present in the catfish, albeit with certain variations. In the future, whether or not additional unique features of Ig function and expression will be found remains to be determined.

The zebrafish: a new model of T-cell and thymic development

T-cell and thymic development are processes that have been highly conserved throughout vertebrate evolution. Mammals, birds, reptiles and fish share common molecular signalling pathways that regulate the development of the adaptive immune system. This Review article focuses on defining the similarities and differences between zebrafish and mammalian T-cell immunobiology, and it highlights the advantages of using the zebrafish as a genetic model to uncover mutations that affect T-cell and thymic development. Finally, we summarize the use of the zebrafish as a new model for assessing stem-cell function and for drug discovery.

Channel catfish NK-like cells are armed with IgM via a putative FcmicroR

Two-color flow cytometry demonstrated that 4-8% of channel catfish PBL are positive for both F and G IgL chain isotypes, suggesting that they passively acquire serum IgM via a putative FcmicroR. These cells show spontaneous killing toward allogeneic targets, and in vitro stimulation of PBL with allogeneic cells results in an increase of double IgL chain positive cells with a concomitant increase in nonspecific cytotoxicity. Long-term cultures of alloantigen-stimulated PBL contain both sIgM(+) and sIgM(-) cytotoxic cells that transcribe message for the catfish homolog of the FcepsilonR gamma chain, but not for Igmicro and TCR-alpha,-beta, or -gamma chains. Immunoprecipitation of lysates from sIgM(+) NK-like cells with anti-IgM co-immunoprecipitated a putative FcmicroR of approximately 64 kDa. Finally, removal of IgM from sIgM(+) NK-like cells and replacement with anti-hapten antibody enabled antibody-armed effectors to kill haptenated targets that were refractory to killing by effectors armed with normal IgM. This is the first report suggesting that teleost NK-like cells express a putative FcmicroR which participates in antibody-dependent cell-mediated cytotoxicity.

Survivability and immune responses after challenge with Edwardsiella ictaluri in susceptible and resistant families of channel catfish, Ictalurus punctatus

Diseases in catfish farming are prevalent and costly, particularly the bacterial disease Enteric Septicemia of Catfish. Considerable research has focused on different aspects of this disease, including the biology of the causative agent, Edwardsiella ictaluri. However, no satisfactory treatment or preventive has resulted from these efforts. One solution is to increase the natural disease resistance of the fish through genetic selection. Recent research has demonstrated that genetic factors influence resistance to infection in mammals as well as fish. Selective breeding for disease resistance in channel catfish is ongoing, however differences in defence mechanisms among E. ictaluri challenged strains and families are only now being investigated. Antigen-specific as well as non-specific immune responses of full-sib families of channel catfish to laboratory challenge with E. ictaluri have been investigated. Both resistant and sensitive families produce a humoral response as specific antibody, but there were no differences found in the level of specific antibody produced. The sensitive family produced a slightly higher percentage of B lymphocytes in mononuclear cell preparations from peripheral blood, while the resistant family had a higher percentage of T lymphocytes in those preparations. The most significant observation was that the resistant family produced more macrophage aggregations in the spleen and posterior kidney throughout the infection than the sensitive family. Neither family produced stress-associated amounts of cortisol.

Immunocytochemical detection of Ig-positive cells in blood, lymphoid organs and the gut associated lymphoid tissue of the turbot (Scophthalmus maximus)

The present study was designed to search for the sites of the B-cell lineage in the different lymphoid organs of turbot (Scophthalmus maximus) by immunoperoxidase staining with a rabbit polyclonal antiserum against deglycosylated turbot IgM (TUDG-6). A turbot immunoglobulin (Ig) fraction, isolated by protein A, was checked for purity by gel filtration and SDS-PAGE under reducing conditions. The turbot IgM was deglycosylated and used to raise an antiserum. The antiserum titre was evaluated in ELISA. It was then used to analyse turbot peripheral blood leucocytes for membrane and cytoplasmic Ig and for immunohistochemistry with turbot lymphoid tissues. Very low numbers of Ig+ cells were found in thymus sections. In sections of spleen, Ig+ cells were observed in white pulp, around ellipsoids but were mostly concentrated and associated with melanomacrophage centers (MMCs). The lymphoid Ig+ cells in the kidney tended to be dispersed among haematopoietic and granulopoietic cell populations and were in intimate association with the MMCs and blood vessels. This association between MMCs and Ig+ cells in the spleen and the kidney, is discussed with respect to the role played by these organs in the immune system of fish. Last, the lymphoid population in the gut associated lymphoid tissue (GALT) of turbot was characterised with respect to staining for Ig. Immunoreactive cells were rarely detected in the epithelial layer although many lymphocytes were present, but they were frequently observed in the lamina propria, presumably as part of the GALT and involved in mucosal immune responses.

Allelic exclusion in anamniotic vertebrates

Clonal selection, a central principle in immunology, is predicated on one lymphocyte making one kind of antibody or T cell receptor. At loci encoding antigen receptors only one allele is used, and this has been shown in normal lymphocytes from frogs to humans. Fish antibody chains, however, are encoded by multiple loci, and at some of these loci the gene segments are already rearranged in the germline. The differences in germline organization and the uncoupling of rearrangement and expression raise questions as to whether some of the early vertebrates might be an exception to the clonal selection theory.

Functional and molecular characterization of teleost leukocytes

The coupling of immunologically relevant in vitro assay systems, cell separation techniques, and the development of distinct clonal leukocyte lines has established the existence of T, B, natural killer, and accessory cell equivalents in teleosts. B cells are directly defined by monoclonal antibodies to teleost immunoglobulin (Ig) and identification of Ig H and L chain genes. As in mammals, fish B cells show Ig H-chain gene rearrangements, allelic exclusion, produce both membrane-bound and secreted forms of Ig, and transduce intracellular proliferative signals upon anti-Ig cross-linking. It has also been found that some fish B cells express a unique chimeric Ig chain with sequence homology to mammalian Ig delta. Teleost T cells are still indirectly defined as sIg- lymphocytes due to a lack of definitive surface markers. These mIg- lymphocytes are the responding cells in mixed leukocyte cultures, proliferate specifically to autologously processed and presented antigen, provide helper function for in vitro antibody responses, and produce interleukin-like factors upon activation. Recent identification of teleost T-cell receptor alpha and beta genes has now permitted the unequivocal genetic demonstration that some of these mIg- cells are bona fide T cells. It is anticipated that such long-term clonal cell lines will be indispensable tools for dissecting the physiology, biochemistry and molecular biology of teleost immune responses.

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.

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.

Immunocytochemical analysis of a monoclonal antibody specific for rainbow trout (Oncorhynchus mykiss) granulocytes and thrombocytes

A monoclonal antibody against rainbow trout peripheral blood leucocytes was selected for its lack of reactivity with rainbow trout immunoglobulin. Its reactivity with leucocytes from peripheral blood, head kidney and spleen was analysed by flow cytometry and electron microscopy, and compared with that of monoclonal antibodies directed against rainbow trout immunoglobulin, which reacted with B cells, B lymphoblasts and plasma cells. The antibody reacted with 5-20% of the peripheral blood leucocytes, 8-9% of head kidney leucocytes and 5-7% of spleen leucocytes. Electron microscopical immunocytochemistry revealed that the antibody reacted strongly with granulocytes and weakly with thrombocytes, and not with erythrocytes, lymphocytes, monocytes or macrophages. The antibody has possible applications in the identification and isolation of rainbow trout leucocytes, either alone or in combination with other monoclonal antibodies.

Production, characterisation and applicability of monoclonal antibodies to European eel (Anguilla anguilla L., 1758) immunoglobulin

Monoclonal antibodies (mAbs) to European eel (Anguilla anguilla L., 1758) immunoglobulin (Ig) were produced, characterised and tested for applicability in a number of immuno(cyto)chemical assays. The selected mAbs, WEI 1 and WEI 2, were specifically reactive with Ig heavy and light chain, respectively. WEI 1 appeared to react with all or nearly all Ig molecules, B cells and plasma cells. WEI 2 was reactive with a subpopulation of those cells, indicating that European eel possesses at least two antigenically different light chain types. Both mAbs could be used for detection of antigen-specific antibodies in plasma by means of an enzyme-linked immunosorbent assay (ELISA).

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.

Phylogeny of lymphocyte heterogeneity: cytotoxic activity of channel catfish peripheral blood leukocytes directed against allogeneic targets

Channel catfish peripheral blood contains leukocytes that function as cytotoxic effectors directed against a variety of long-term cultured allogeneic, but not xenogeneic, targets. These effector cells are probably distinct from macrophages, B cells, and nonspecific cytotoxic cells. The cytotoxic activity of these effector cells was inhibited with monoclonal antibody 1H5. Although this reagent appears to react with a catfish cell surface molecule akin to the integrin LFA-1 present on the surface of nearly all leukocytes, it does not clarify the question as to whether or not these effectors are related to T cells.

Monoclonal antibodies to turbot (Scophthalmus maximus) immunoglobulins: characterization and applicability in immunoassays

Five monoclonal antibodies (mAbs) to immunoglobulins (Igs) of the turbot Scophthalmus maximus were produced and characterized. All the mAbs (denominated UR1, UR3, UR4, UR6 and UR7) are of isotype IgG1/kappa and show good anti-turbot Ig reactivity in enzyme-linked immunosorbent assay (ELISA) and immunoblotting. Results of competitive ELISA and immunoblotting analysis indicate that these five mAbs react with at least three different epitopes on the turbot Ig H chain. Except in the case of UR1, reactivity with periodate-treated purified turbot Ig was much lower than with the untreated Ig, suggesting that carbohydrate residues are involved in epitope recognition. All the mAbs showed reactivity with sera from the closely related species Scophthalmus rhombus but not with sera from species of other flatfish genera. One of these mAbs (UR3) has been successfully applied for the detection of antibodies against Vibrio anguillarum in ELISA.

Ontogeny of IgM and IgM-bearing cells in rainbow trout

We have studied the ontogenic development of immunoglobulin M (IgM) and of IgM-bearing cells in the rainbow trout, Oncorhynchus mykiss. Lymphocytes showing cytoplasmic IgM were first observed in embryos at 12 days before hatch (14 degrees C). At this stage, no cells positive for surface IgM were present. Lymphocytes bearing surface IgM were observed at 8 days before hatch (14 degrees C). Unfertilized trout eggs contained detectable amounts of IgM (11.2 +/- 2.6 micrograms/g of egg weight), indicating that transfer of IgM from mother to embryo can occur in salmonids. The levels of IgM from whole fish increase slowly after the appearance of intraembryonic cells that express surface IgM. The amount of IgM/g of tissue peaks around hatch, but this parameter shows lower values up to 2 months after hatch.

Plasmid and serological differences between Edwardsiella ictaluri strains

Several studies have shown that isolates of Edwardsiella ictaluri obtained from infected channel catfish in the southeastern United States harbor two cryptic plasmids, designated pCL1 (5.7 kb) and pCL2 (4.9 kb). These isolates appear to be serologically homogeneous. To extend these studies, we focused our analyses on two isolates of nonictalurid origin. Plasmid analyses of a danio isolate showed that it harbored plasmids which were similar if not identical to pCL1 and pCL2. This strain was also serologically indistinguishable from those isolated from channel catfish. In contrast, a green knife fish (GNF) isolate harbored four plasmids with relative mobilities of 6.0, 5.7, 4.1, and 3.1 kb. Southern blot analyses indicated that only the 5.7- and 4.1-kb plasmids strongly hybridized under high-stringency conditions to probes specific for pCL1 and pCL2, respectively. The GNF isolate showed minimal reactivity when reacted with polyclonal antiserum prepared against a channel catfish isolate. However, polyclonal antiserum to the GNF isolate strongly reacted with the GNF isolate in both surface fluorescence and agglutination reactions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses of cell lysates showed that the protein banding patterns of the strains compared were similar. However, Western blots of proteinase K-digested cell extracts showed that O antigen of the GNF isolate was antigenically distinct from the O antigen of the other isolates. These studies indicate that there are different serotypes of E. ictaluri and suggest that plasmid and serological analyses of future isolates of E. ictaluri can be used to determine whether structurally distinct strains are emerging in major channel catfish aquaculture areas.

Immunoglobulin heavy chain cDNA from the teleost Atlantic cod (Gadus morhua L.): nucleotide sequences of secretory and membrane form show an unusual splicing pattern

Rabbit antibodies to Atlantic cod (Gadus morhua L.) immunoglobulin were affinity purified and used to screen cDNA libraries from spleen and head kidney mRNA. cDNA clones for both the secretory and membrane-bound heavy (H) chain were isolated, the nucleotide and deduced amino acid sequences of which are reported here. Comparisons of the cod secretory H chain amino acid sequence show 24%, 27%, 30% identity to the mu chain of Mus, Xenopus and Ictalurus, respectively. The highest degree of identity was observed in the CH4 domain. The cDNA encoding the transmembrane form shows a novel splicing pattern where the TM1 exon is spliced directly onto the CH3 domain and not to the CH4 domain as in other animal groups. Southern blot analyses with VH and C probes on genomic DNA from cod erythrocytes indicate that there is a unique C gene but several V genes in the cod immunoglobulin H chain locus.

Mitogen induced proliferative responses of lymphocytes from spot (Leiostomus xanthurus) exposed to polycyclic aromatic hydrocarbon contaminated environments

The marine fish spot, Leiostomus xanthurus, was collected from five sites in the lower Chesapeake Bay system representing a gradient of sediment polycyclic aromatic hydrocarbon (PAH) concentrations. The proliferative responses to mitogens by anterior kidney lymphocytes were assessed using [3H]-thymidine uptake by replicating DNA. The data shows two different mitogen-dependent lymphocytic responses as the sediment PAH levels increase at the sampling sites; a suppression of the response to the T cell mitogens, concanavalin A (Con A) and phytohemagglutinin, and a sharp augmentation of the response to B cell mitogen, lipopolysaccharide (LPS), as well as to poke weed mitogen and peanut agglutinin. The magnitude of the lymphoproliferative responses correlated strongly with the total sediment PAH concentrations (r2 greater than 0.8). A similar correlation was also observed with 15 selected individual PAH compounds regardless of their molecular weights. By maintaining the fish in clean York River water for up to 24 weeks, it was possible to reverse the augmented proliferative responses to LPS of fish from all sampling sites and to increase the reduced responses to Con A, in fish from three sites, and partially in two sites where sediments were highly contaminated with PAH. These results suggest that the proliferative responses of fish lymphocytes to mitogens may be a potentially sensitive biomarker of exposure to, and effects of xenobiotics.

Characterization of monoclonal antibodies to channel catfish, Ictalurus punctatus, leucocytes

Four monoclonal antibodies (MAbs) were evaluated for specific reactions to various catfish peripheral blood leucocytes and anterior kidney cells. The MAb 9E1 served as a standard and positive control for all test reactions because of its defined reactivity with channel catfish immunoglobulin and immunoglobulin bearing cells. Of the four MAbs, two have been characterized as being specific for a non-immunoglobulin marker on lymphocytes, thus marking T lymphocytes and two were specific for catfish neutrophils. Morphological, flow cytometric and functional analysis of the reactive cells verified these findings.

Autonomic innervation of the spleen of the coho salmon, Oncorhynchus kisutch. a histochemical demonstration and preliminary assessment of its immunoregulatory role

Regulation of immunity by the nervous system, now a well-established phenomenon in mammals, is effected in part through the autonomic innervation of lymphoid tissues. Noradrenergic fibers specifically target lymphocyte-rich areas in mammalian lymphoid tissues, and their ablation, or the administration of adrenergic agents, can significantly alter immune responses. This study demonstrates that the spleen of the coho salmon is also richly innervated by adrenergic neurons. While this innervation enters the spleen and remains largely associated with the splenic vasculature, fibers can also be observed entering the parenchyma. Although the coho spleen does not possess a well-developed white pulp, aggregations of leukocytes are found adjacent to the major blood vessels in close proximity to the vascular nervous tissue and parenchymal fibers. Chemical sympathectomy with 6-hydroxydopamine results in a significant enhancement of the splenic antibody-secreting cell response to trinitrophenylated sheep red blood cells. These results suggest that sympathectomy is removing a constraint, in the form of inhibitory catecholamines, on the immune response. The potential benefits from a teleost model of neural-immune interactions are discussed.

Immunological classification of a pike lymphoma

Monoclonal and polyclonal antibodies against mammalian intermediate filament (IF) proteins were used to demonstrate IF expression in tissues and lymphoma tumors of northern pikes, Esox lucius L., from the Aland Islands of Finland by indirect immunofluorescence microscopy. Frozen sections of pike tissues demonstrated IFs in a manner confirming their evolutionary conservation and subclass specificity. Tumor cells showed morphological resemblance to head kidney cells and were positive for vimentin while negative for cytokeratin, desmin, neurofilament proteins, and glial fibrillary acidic protein. The results show that the neoplasm is a mesenchymal as opposed to an epithelial, muscle, neural, or glial tumor, and is probably hemic cell derived. A rabbit anti-pike IgM antiserum showed that up to 90% of mononuclear (MN) cells isolated on Ficoll-Isopaque gradients from peripheral blood, spleen and head kidney were surface- and cytoplasmic-immunoglobulin positive by indirect immunofluorescence, while a maximum of 5% of tumor cells were positive. A maximum of 5% of MN cells from hemic tissues exhibited rosettes when incubated with AET-treated sheep red blood cells; however, only 1% of cells in the tumor formed rosettes. Lymphocyte proliferation assays were performed on MN cells from hemic tissues and tumor using phytohemagglutinin P, concanavalin A, tuberculin purified protein derivative and lipopolysaccharide W in medium supplemented with fetal calf serum or autologous pike plasma. Proliferation indices in hemic tissues were similar in the groups. However, all proliferation indices in tissues were significantly higher than corresponding values in the tumor. These assays show that pike MN cells respond when stimulated by T and B cell mitogens, but that this reactivity is lacking in the tumor.

Structural diversity of channel catfish immunoglobulins

This paper describes a number of observations which show that the covalent structure of catfish Ig is heterogeneous and secondly that different Ig L chain classes are present.

Preferential expression of catfish light chain immunoglobulin isotypes in anti-dinitrophenyl antibodies

Previous studies indicated that immunoglobulins of the channel catfish, Ictalurus punctatus, contained two distinct isotypes of light chains. These different light chains, designated F and G, are reminiscent of kappa and lambda light chains of higher animals. This study was undertaken to establish whether or not there was preferential expression of one of the light chains in antibodies induced to the dinitrophenyl moiety during the course of the catfish humoral antibody response. The results indicated that, although antibody produced very early (1-2 weeks) after primary immunization contained significant amounts (approximately 20%) of G light chains, the vast majority (approximately 90%) of antibody produced later (3 weeks through 1 year) was of the F isotype. Thus fish, as well as higher vertebrates, can express different isotypes of antibody during the immune response.

Evolution of antibody structure and effector functions: comparative hemolytic activities of monomeric and tetrameric IgM from rainbow trout, Salmo gairdnerii

Monomeric and tetrameric IgM anti-haptin antibodies isolated from the sera of rainbow trout (S. gairdnerii) by immunoaffinity chromatography were compared both immunochemically and with regard to their functional abilities to lyse haptenated trout erythrocytes in the presence of trout complement. The two populations had similar binding affinities for hapten and apparently identical L chains, but differed in H chain peptide maps and immunoreactivity with rabbit anti-trout H chain serum. These differences could not be attributed to J-chain. The abilities of the two antibody subpopulations to activate C to lyse haptenated trout erythrocytes also differed dramatically. Such functional differences are not simply explained by the greater avidity of the tetrameric form since preliminary studies show that the monomeric form of trout IgM activates C via an alternative pathway mechanism while the tetrameric form activates both classical and alternative pathway mechanisms. Results suggest divergent evolution of antibody structures involved in the familiar effector functions (C activation, transport, etc.).

Microsystem for in vitro primary and secondary immunization of channel catfish (Ictalurus punctatus) leukocytes with hapten-carrier conjugates

Methods are described for the in vitro generation and detection of antibody-secreting cells (PFC) from channel catfish. Hapten-specific PFC can readily be enumerated by an indirect plaque assay employing rabbit antiserum to catfish Ig and guinea pig complement. A modified Mishell-Dutton-type culture system was developed for effectively generating significant in vitro anti-hapten PFC responses with catfish leukocytes at 27 degrees C. The classical hapten-carrier effect and primary responses to both TI and TD antigens were demonstrable with catfish cells. Variables found to be important with catfish cells included the serum supplement, cell densities and, to a lesser extent, antigen form. Optimistically these methods will prove useful in attempts to delineate the functional roles of different lymphocyte subpopulations in fish.

Nonspecific cytotoxic cells in fish (Ictalurus punctatus). I. Optimum requirements for target cell lysis

Nonspecific cytotoxic cells (NCC) obtained from the head (anterior) kidney of fish (Ictalurus punctatus) lyse human transformed B-cell targets. Lysis depended on direct cell-cell contact. Fish size, age, environmental holding temperatures, and lytic reaction conditions such as osmolality and optimum effector:target cell ratios were optimized. Experiments to characterize optimum kinetics demonstrated highly efficient killing after two hours incubation. This rapid cytolysis was further studied by determining NCC activity against appropriately labeled target cells after 30, 60, 90 and 120 minutes of cocultivation. At 160:1 (E:T) greater than 40% of the 5 hour percent specific release value was produced after 30 minutes. After 90 minutes, more than 90% of total percent specific release was observed. At least one mechanism of regulation of NCC killing was described. In the presence of normal (homologous or heterologous) catfish serum (CFS), essentially no NCC activity was observed. This suppression was reversible by preincubation in 10% fetal bovine serum (FBS). NCC "activation" by preincubation in 10% FBS was time-dependent (at least four hours was required to generate significant lysis). NCC activation could be reversed by treating potentially lytic cells with supernatants containing dissociated CFS. In addition, reversible activation could be demonstrated by treating potentially lytic effector cells with CFS to produce suppression. Regulation occurred at the effector cell level because treated target cells did not suppress NCC activity. These data demonstrate a population of nonspecific effector cytolytic cells that potentially represent a phylogenetic precursor to mammalian natural killer cells.

Immunoglobulin-bearing blood leucocytes in the Pacific hagfish

The occurrence of immunoglobulin (Ig) bearing leucocytes in the blood of the Pacific hagfish, Eptatretus stoutii , was examined using a murine monoclonal antibody (45.3) and a rabbit antiserum specific for hagfish serum Ig. Binding of antibody 45.3 to hagfish leucocytes assessed by radioimmunoassay was inhibited by preincubation of antibody with purified serum Ig thus verifying the presence of cell surface Ig cross reactive with serum Ig. The monoclonal antibody identified approximately 65% of blood leucocytes as Ig+ve while the rabbit antiserum indicated 81% Ig+ve cells. Both antibody preparations failed to react specifically with cells from mouse, horned shark, tunicate or sea star; this indicates the distinctive nature of hagfish Ig. The high percentage of blood cells bearing surface Ig in the hagfish raises the possibility that lymphocyte divergence to separate B and T pathways may not have occurred in this most primitive vertebrate. Alternatively, an Ig-like specificity characteristic of both "T" and "B" lymphocytes may have been detected. In any event, a subset of Ig negative leucocytes is evident in hagfish.

Temperature-mediated processes in teleost immunity: differential effects of in vitro and in vivo temperatures on mitogenic responses of channel catfish lymphocytes

The in vitro mitogenic responses of channel catfish peripheral blood leucocytes to ConA and LPS were differentially affected by both in vitro and in vivo temperatures. The magnitude of the response to LPS was relatively independent of both in vitro culture temperature and in vivo acclimation temperature. The magnitude of the response to ConA was suppressed at lower in vitro temperatures although this suppression could be reduced by lower in vivo acclimation temperatures. In vitro temperature-shift experiments indicated that channel catfish PBL could respond to ConA at a lower in vitro temperature if first stimulated with ConA at a higher in vitro temperature. The converse, however was not true in that channel catfish PBL did not respond at a higher in vitro temperature after an initial stimulation with ConA at a lower in vitro temperature. This latter failure to respond could not be attributed to the induction of a suppressor cell (or factor) by exposure to ConA at a lower temperature. These studies, when coupled with other available data on channel catfish PBL subpopulations, are interpreted as supporting the hypothesis that low temperature immunosuppression in fish may result from preferential inhibitory effects on T cells rather than B cells.