Development of leukocyte cell lines from the channel catfish (Ictalurus punctatus)

TLR7 in channel catfish (Ictalurus punctatus) is expressed in the endolysosome and is stimulated by synthetic ssRNA analogs, imiquimod, and resiquimod

Toll-like receptors (TLRs) are pivotal pattern recognition receptors (PRRs) and key mediators of innate immunity. Despite the significance of channel catfish (Ictalurus punctatus) in comparative immunology and aquaculture, its 20 TLR genes remain largely functionally uncharacterized. In this study, our aim was to determine the catfish TLR7 agonists, signaling potential, and cellular localization. Using a mammalian reporter system, we identified imiquimod and resiquimod, typical ssRNA analogs, as potent catfish TLR7 agonists. Notably, unlike grass carp TLR7, catfish TLR7 lacks the ability to respond to poly (I:C). Confocal microscopy revealed predominant catfish TLR7 expression in lysosomes, co-localizing with the endosomal chaperone protein, UNC93B1. Furthermore, imiquimod stimulation elicited robust IFNb transcription in peripheral blood leukocytes isolated from adult catfish. These findings underscore the conservation of TLR7 signaling in catfish, reminiscent of mammalian TLR7 responses. Our study sheds light on the functional aspects of catfish TLR7 and contributes to a better understanding of its role in immune defense mechanisms.

A recombinant 9E1 monoclonal antibody binds membrane and soluble channel catfish immunoglobulin M

The development and validation of the recombinant 9E1 monoclonal antibody against channel catfish IgM is described. The variable heavy and light chain domains of the 9E1 hybridoma were cloned into murine IgG1 and IgK expression vectors. These expression plasmids were co-transfected into 293F cells and mature IgG was purified from culture supernatant. It is demonstrated that the recombinant 9E1 monoclonal antibody binds to soluble IgM in ELISA and ELISPOT assays and to membrane-bound IgM by immunofluorescence with different B-cell types. The recombinant 9E1 monoclonal antibody will be a valuable tool in the continued examination of the channel catfish adaptive immune system.

A subset of leukocyte immune-type receptors (LITRs) regulates phagocytosis in channel catfish (Ictalurus punctatus) leukocytes

Channel catfish, Ictalurus punctatus, leukocyte immune-type receptors (LITRs) constitute a large family of paired, immunoregulatory receptors unique to teleosts. A role for LITRs in phagocytosis has been proposed based on studies in mammalian cell lines; however, LITR-mediated phagocytosis has not been examined in the catfish model. In this study, we use two anti-LITR monoclonal antibodies, CC41 and 125.2, to contrast the effects of crosslinking subsets of inhibitory and activating LITRs. Briefly, LITRs expressed by catfish γδ T cells, αβ T cells, and macrophage cell lines were crosslinked using mAb-conjugated fluorescent microbeads, and bead uptake was evaluated by flow cytometry and confirmed by confocal microscopy. A clear difference in the uptake of 125.2- and CC41-conjugated beads was observed. Crosslinking LITRs with mAb 125.2 resulted in efficient bead internalization, while mAb CC41 crosslinking of inhibitory LITRs resulted predominantly in a capturing phenotype. Pretreating catfish macrophages with mAb CC41 resulted in a marked decrease in LITR-mediated phagocytosis of 125.2-conjugated beads. Overall, these findings provide insight into fish immunobiology and validate LITRs as regulators of phagocytosis in catfish macrophages and γδ T cells.

Interferons and interferon receptors in the channel catfish, Ictalurus punctatus

In this work, we describe the complete repertoire of channel catfish, Ictalurus punctatus, IFNs and IFN receptor genes. Based on multiple genomic and transcriptomic resources we identified 16 type I IFN genes, which represent the six type I IFN subgroups previously defined in salmonids (a-f.) No representatives of subgroup h previously only found in percomorphs were identified. An expansion in copy numbers of subgroup d IFN genes was of particular interest, as this has not been reported in other fish species to date. Furthermore, we confirmed the presence of two type II ifn genes encoding orthologs of IFNγ and the teleost-specific IFNγRel. Six homologs of IFN type I receptor genes were found in an array that shows conserved synteny with human chromosome 21. Three homologs of type II IFN receptor genes were also identified. These type I and type II receptor sequences are compatible with the dual type I IFN receptors, and the potentially more complex type II IFN receptors described in teleosts. Our data provide a comprehensive resource for future studies of channel catfish innate antiviral immunity.

A Comprehensive Annotation of the Channel Catfish (Ictalurus punctatus) T Cell Receptor Alpha/Delta, Beta, and Gamma Loci

The complete germline repertoires of the channel catfish, Ictalurus punctatus, T cell receptor (TR) loci, TRAD, TRB, and TRG were obtained by analyzing genomic data from PacBio sequencing. The catfish TRB locus spans 214 kb, and contains 112 TRBV genes, a single TRBD gene, 31 TRBJ genes and two TRBC genes. In contrast, the TRAD locus is very large, at 1,285 kb. It consists of four TRDD genes, one TRDJ gene followed by the exons for TRDC, 125 TRAJ genes and the exons encoding the TRAC. Downstream of the TRAC, are 140 TRADV genes, and all of them are in the opposite transcriptional orientation. The catfish TRGC locus spans 151 kb and consists of four diverse V-J-C cassettes. Altogether, this locus contains 15 TRGV genes and 10 TRGJ genes. To place our data into context, we also analyzed the zebrafish TR germline gene repertoires. Overall, our findings demonstrated that catfish possesses a more restricted repertoire compared to the zebrafish. For example, the 140 TRADV genes in catfish form eight subgroups based on members sharing 75% nucleotide identity. However, the 149 TRAD genes in zebrafish form 53 subgroups. This difference in subgroup numbers between catfish and zebrafish is best explained by expansions of catfish TRADV subgroups, which likely occurred through multiple, relatively recent gene duplications. Similarly, 112 catfish TRBV genes form 30 subgroups, while the 51 zebrafish TRBV genes are placed into 36 subgroups. Notably, several catfish and zebrafish TRB subgroups share ancestor nodes. In addition, the complete catfish TR gene annotation was used to compile a TR gene segment database, which was applied in clonotype analysis of an available gynogenetic channel catfish transcriptome. Combined, the TR annotation and clonotype analysis suggested that the expressed TRA, TRB, and TRD repertoires were generated by different mechanisms. The diversity of the TRB repertoire depends on the number of TRBV subgroups and TRBJ genes, while TRA diversity relies on the many different TRAJ genes, which appear to be only minimally trimmed. In contrast, TRD diversity relies on nucleotide additions and the utilization of up to four TRDD segments.

Hepatic antioxidant activity, immunomodulation, and pro-anti-inflammatory cytokines manipulation of κ-carrageenan (κ-CGN) in cobia, Rachycentron canadum against Lactococcus garvieae

The effects of dietary k-Carrageenan (k-CGN) at 10, 20, and 30 g kg^-1 on growth rate, hemato-biochemical indices, innate-adaptive parameters and modification of pro- and/or anti-inflammatory cytokines and chemokines pathway in cobia, Rachycentron canadum against Lactococcus garvieae is reported. The weight gain (WG) increased substantially (P < 0.05) in all k-CGN treated groups; the specific growth rate (SGR) was significant in healthy uninfected normal (HuN) and L. garvieae challenged (LaC) groups fed with 20 g kg^-1k-CGN diet on 45 and 60 days. The white blood cell (WBC) counts, total protein (TP) level, total anti-oxidant (T-AOC), catalase (CAT), and glutathione (GSH) activities increased significantly when fed with 20 g and 30 g kg^-1k-CG diets on 45th and 60th day. The immunological parameters such as phagocytic (PC) index and the activity of phagocytic (PC), respiratory burst (RB), superoxide dismutase (SOD), alternate complement pathway (ACH₅₀), and lysozyme (LZM) were significantly enhanced with all k-CG diets in 45 and 60 days of treatment. No cumulative mortality (CM) in HuN group fed by control or any k-CGN diets. CM was 5% in LaC group fed with 20 g kg^-1k-CGN diet whereas in LaC groups fed with 10 g and 30 g kg^-1k-CGN diets the CM was 10%. The interleukin 1 beta (IL-1β) and tumor necrosis factor alpha (TNFα) pro-inflammatory cytokines mRNA transcripts were one-fold high (P < 0.05) in both HuN and LaC group fed all k-CGN enriched diets on 45 and 60 days. Similarly, IL-18 and TLR2 mRNA was one-fold high expression in both groups fed the 20 g and 30 g kg^-1k-CGN enriched diets on 45 or 60 days. Interferon gamma (IFNγ) and interferon regulatory factor 3/7 (IRF3/IRF7) mRNA transcripts did not change with any diet. IL-6, IL-10, and IL-11 mRNA were one-fold high expressions in both groups fed the 20 g and 30 g kg^-1k-CGN enriched diets on 45 and 60 days. However, the expression of CC1, CC3, and CCR9 pro-inflammatory chemokines mRNA did not vary with any control or k-CGN enriched diets. The results indicate that diet enriched with k-CGN at 20 g kg^-1 significantly influences the growth, antioxidant and innate-adaptive immune performance, and pro-anti-inflammatory cytokines and chemokines regulation in cobia against L. garvieae.

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.

Cloning and characterization of antiviral cytotoxic T lymphocytes in channel catfish, Ictalurus punctatus

To determine the role of piscine anti-viral cytotoxic cells, we analyzed the response of channel catfish to Ictalurid herpesvirus 1, commonly designated channel catfish virus (CCV). Peripheral blood leukocytes (PBL) from catfish immunized with MHC-matched, CCV-infected G14D cells (G14D-CCV) showed marked lysis of G14D-CCV but little to no lysis of uninfected allogenic (3B11) or syngeneic (G14D) cells. Expansion of effectors by in vitro culture in the presence of irradiated G14D-CCV cells generated cultures with enhanced cytotoxicity and often broader target range. Cytotoxic effectors expressed rearranged TCR genes, perforin, granzyme, and IFN-γ. Four clonal cytotoxic lines were developed and unique TCR gene rearrangements including γδ were detected. Furthermore, catfish CTL clones were either CD4⁺/CD8^- or CD4^-/CD8^-. Two CTL lines showed markedly enhanced killing of G14D-CCV targets, while the other two lines displayed a broader target range. Collectively, catfish virus-specific CTL display unique features that illustrate the diversity of the ectothermic vertebrate immune response.

Insights into the dynamics of memory, effector and apoptotic cytotoxic T lymphocytes in channel catfish, Ictalurus punctatus

In this study, we used the channel catfish model clonal TS32.15 alloantigen-specific cytotoxic T cell (CTL) line to examine the dynamics of memory CTL expansion and senescence in teleosts. Although TS32.15 has been routinely cultured to study catfish CTL responses and killing mechanisms, little is known about the dynamics of the CTLs in these cultures. Here we show that this cell line consists of small non-cytotoxic T cells and larger granular effector T cells and that their ratios vary with time after stimulation. Small CTLs, when exposed to their irradiated targets, replicate and differentiate to morphologically distinct cytotoxic effectors, which do not replicate. After lysing target cells, or with prolonged absence of stimulation, the effector cells transition to a non-cytolytic senescent stage or become apoptotic. In addition, we demonstrate that natural IgM in catfish serum binds lipids, including PIP₂, on early apoptotic CTLs, and that these IgM⁺ CTL can be cleared by catfish head kidney-derived macrophages.

Orange-spotted grouper Epinephelus coioides that have encountered low salinity stress have decreased cellular and humoral immune reactions and increased susceptibility to Vibrio alginolyticus

Orange-spotted grouper Epinephelus coioides reared at 34‰ and 27 °C were abruptly transferred to 6‰, 20‰ and 34‰ (control) and examined for innate cellular and humoral parameters after 3-96 h. Total leucocyte count (TLC), respiratory burst (RB), phagocytic activity (PA), alternative complement pathway (ACP) and lysozyme activity were significantly decreased 3-6 h, 3-6 h, 3-96 h, 3-96 h and 3-96 h, respectively after transferal into 6‰ salinity. TLC, RB and PA significantly increased after 3-48 h, 3-96 h and 3-24 h, respectively, with recovery of TLC and PA after 96 h and 48-96 h, whereas ACP and lysozyme activity significantly decreased 3-96 h after being transferred to 20‰. In another experiment, grouper reared at 34‰ and 27 °C were injected with Vibrio alginolyticus grown in tryptic soy broth (TSB) at 2.3 × 10⁹ colony-forming units (cfu) fish^-1 and then transferred to 6‰, 20‰ and 34‰ (control). The cumulative mortalities of V. alginolyticus-injected fish held in 6‰ were significantly higher than in injected fish held at 20‰ and 34‰. It was concluded that grouper E. coioides encountering a 34‰-6‰ salinity drop stress exhibited a depression in immunity as evidenced by decreased cellular and humoral parameters and increased susceptibility to V. alginolyticus. Grouper encountering a salinity stress drop from 34‰ to 20‰, however, exhibited decreased humoral immune parameters but also increased TLC and cellular immune parameters, indicating immunomodulation.

Expression of Bcl-2 genes in channel catfish after bacterial infection and hypoxia stress

Bcl-2 proteins are of vital importance in regulation of apoptosis, and are involved in a number of biological processes such as carcinogenesis and immune responses. Bcl-2 genes have been well studied in mammals, while they are not well investigated in teleost fish including channel catfish, the major aquaculture species in the United States. In this study, we identified 34 bcl-2 genes from the channel catfish genome, and verified their identities by conducting phylogenetic and syntenic analyses. The expression profiles of the bcl-2 genes in response to bacterial infections (Edwardsiella ictaluri and Flavobacterium columnare) and hypoxia stress were determined by performing meta-analysis using the existing RNA-Seq datasets. Differential expressions of bcl-2 genes were observed after bacterial infections and hypoxia treatment, including 22 bcl-2 genes after E. ictaluri infection, 22 bcl-2 genes after F. columnare infection, and 19 bcl-2 genes after hypoxia stress. Overall, the expression of the pro-apoptotic bcl-2 genes were repressed after bacterial infection and hypoxia stress, indicating that bcl-2 genes are potentially involved in the stress response by reducing cell apoptosis. Some bcl-2 genes, such as bcl2b, mcl1a, bmf1, and bnip3, showed different expression pattern during the E. ictaluri and F. columnare infection, suggesting the difference in the pathogenicity of diseases. This work presented the first systematic identification and annotation of bcl-2 genes in catfish, providing essential genomic resources for further immune and physiological studies.

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.

Identification of SHIP-1 and SHIP-2 homologs in channel catfish, Ictalurus punctatus

Src homology domain 2 (SH2) domain-containing inositol 5'-phosphatases (SHIP) proteins have diverse roles in signal transduction. SHIP-1 and SHIP-2 homologs were identified in channel catfish, Ictalurus punctatus, based on sequence homology to murine and human SHIP sequences. Full-length cDNAs for catfish SHIP-1 and SHIP-2 (IpSHIP-1 and IpSHIP-2) were obtained using 5' and 3' RACE protocols. Catfish SHIP molecules share a high degree of sequence identity to their respective SHIP sequences from diverse taxa and both are encoded by single copy genes. IpSHIP-1 and IpSHIP-2 transcripts were expressed in all catfish tissues analyzed except for skin, and IpSHIP-1 message was more abundant than IpSHIP-2 message in lymphoid tissues. Catfish clonal B, cytotoxic T, and macrophage cell lines also expressed message for both molecules. IpSHIP-1 and IpSHIP-2 SH2 domains were expressed as recombinant proteins and were both found to be bound by cross-reacting rabbit anti-mouse SHIP-1 pAb. The anti-mouse SHIP-1 pAb also reacted with cell lysates from the cytotoxic T cell lines, macrophages and stimulated PBL. SHIP-1 is also phosphorylated at a conserved tyrosine residue, as shown by immunoprecipitation studies.

Identification and characterization of TCRγ and TCRδ chains in channel catfish, Ictalurus punctatus

Channel catfish, Ictalurus punctatus, T cell receptors (TCR) γ and δ were identified by mining of expressed sequence tag databases, and full-length sequences were obtained by 5'-RACE and RT-PCR protocols. cDNAs for each of these TCR chains encode typical variable (V), diversity (D), joining (J), and constant (C) regions. Three TCRγ V families, seven TCRγ J sequences, and three TCRγ C sequences were identified from sequencing of cDNA. Primer walking on bacterial artificial chromosomes (BACs) confirmed that the TRG locus contained seven TRGJ segments and indicated that the locus consists of (Vγ3-Jγ6-Cγ2)-(Vγ1n-Jγ7-Cγ3)-(Vγ2-Jγ5-Jγ4-Jγ3-Jγ2-Jγ1-Cγ1). In comparison for TCRδ, two V families, four TCRδ D sequences, one TCRδ J sequence, and one TCRδ C sequence were identified by cDNA sequencing. Importantly, the finding that some catfish TCRδ cDNAs contain TCR Vα-D-Jδ rearrangements and some TCRα cDNAs contain Vδ-Jα rearrangements strongly implies that the catfish TRA and TRD loci are linked. Finally, primer walking on BACs and Southern blotting suggest that catfish have four TRDD gene segments and a single TRDJ and TRDC gene. As in most vertebrates, all three reading frames of each of the catfish TRDD segments can be used in functional rearrangements, and more than one TRDD segment can be used in a single rearrangement. As expected, catfish TCRδ CDR3 regions are longer and more diverse than TCRγ CDR3 regions, and as a group they utilize more nucleotide additions and contain more nucleotide deletions than catfish TCRγ rearrangements.

A short history of research on immunity to infectious diseases in fish

This review describes the history of research on immunity to infectious diseases of fish in the period between 1965 and today. Special attention is paid to those studies, which are dealing with the interaction between immune system and invading pathogens in bony fish. Moreover, additional biographic information will be provided of people involved. In the 1960s and 1970s the focus of most studies was on humoral (Ig, B-cell) responses. Thorough studies on specific cellular (T-cell) responses and innate immunity (lectins, lysozyme, interferon, phagocytic cells) became available later. In the period between 1980 and today an overwhelming amount of data on regulation (e.g. cell cooperation, cytokines) and cell surface receptors (e.g. T-cell receptor; MHC) was published. It became also clear, that innate responses were often interacting with the acquired immune responses. Fish turned out to be vertebrates like all others with a sophisticated immune system showing specificity and memory. These basic data on the immune system could be applied in vaccination or in selection of disease resistant fish. Successful vaccines against bacterial diseases became available in the 1970s and 1980s. Effective anti-viral vaccines appeared from the 1980s onwards. There is no doubt, that Fish Immunology has become a flourishing science by the end of the 20th century and has contributed to our understanding of fish diseases as well as the success of aquaculture.

Transcriptional regulation of teleost Aicda genes. Part 1 - suppressors of promiscuous promoters

In order to better understand antibody affinity maturation in fishes we sought to identify gene regulatory elements that could drive expression of activated B-cell specific fluorescent reporter transgenes in zebrafish. Specifically the promoter and several non-coding regions of the channel catfish (Ictalurus punctatus) and zebrafish (Danio rerio) were tested for transcriptional activity using a dual luciferase reporter system in transfected fish leukocytes and two mammalian cell lines that constitutively express Aicda (activation-induced cytidine deaminase). The promoters of both fish Aicda genes were as transcriptionally active as an SV40 promoter control in all cell lines tested, regardless of the cells ability to express Aicda. Coupling of a putative intron 1 enhancer or a region 10 kb upstream of the zebrafish promoter effectively silenced transcription from the fish Aicda promoter. Paradoxically these suppressor elements enhanced transcription when they were coupled to the mouse Aicda intron 1 enhancer. The results are considered in context of similar observations for Aicda transcriptional regulation in mice and in light of recent evidence that Aicda is utilized for epigenetic reprogramming of several non-lymphoid cell types.

Malachite green toxicity assessed on Asian catfish primary cultures of peripheral blood mononuclear cells by a proteomic analysis

The potential genotoxic and carcinogenic properties reported for malachite green (MG) and the frequent detection of MG residues in fish and fish products, despite the ban of MG, have recently generated great concern. Additional toxicological data are required for a better understanding of the mechanism of action and a more comprehensive risk assessment for the exposure of fish to this fungicide. To date, the use of fish peripheral blood mononuclear cells (PBMCs) has not been exploited as a tool in the assessment of the toxicity of chemicals. However, PBMCs are exposed to toxicants and can be easily collected by blood sampling. The present study aims at better understanding the effects of MG by a proteomic analysis of primary cultured PBMC from the Asian catfish, Pangasianodon hypophthalmus, exposed to MG. The two lowest concentrations of 1 and 10 ppb were selected based on the MTS (water soluble tetrazolium salts) cytotoxicity test. Using a proteomic analysis (2D-DIGE), we showed that 109 proteins displayed significant changes in abundance in PBMC exposed during 48 h to MG. Most of these proteins were successfully identified by nano LC-MS/MS and validated through the Peptide and Protein Prophet of Scaffold™ software, but only 19 different proteins were considered corresponding to a single identification per spot. Our data suggest that low concentrations of MG could affect the mitochondrial metabolic functions, impair some signal transduction cascades and normal cell division, stimulate DNA repair and disorganize the cytoskeleton. Altogether, these results confirm that the mitochondrion is a target of MG toxicity. Further studies on the identified proteins are needed to better understand the mechanisms of MG toxicity in fish produced for human consumption.

The zebrafish IgH locus contains multiple transcriptional regulatory regions

Many fish have, in addition to IgM and IgD, a third isotype called IgZ or IgT. The ζ-chain locus is embedded among the Ig heavy chain V-, D- and J-elements in a manner reminiscent of the TcR δ/α locus. Isotype selection thus occurs during VDJ recombination, a process that is facilitated by intralocus transcription. Using in silico analyses and enhancer reporter vectors we identified 3 new regions within the zebrafish IgH locus through which transcription can be activated in catfish B-cell lines. Two of these, termed Eζi (Jζ to Cζ1 intronic) and Eζ3' regions flank the ζ-chain constant domain exons. A third region, Eδ3', resides downstream of the δ-chain exons. All regions contain predicted binding sites for transcription factors that contribute to B-cell specific transcription in fish and mammals. Each region also has proximal matrix attachment regions, which may further contribute to transcriptional activation and chromatin remodeling. We discuss possible roles for these regions during VDJ recombination.

Development, characterization, conservation and storage of fish cell lines: a review

Cell lines provide an important biological tool for carrying out investigations into physiology, virology, toxicology, carcinogenesis and transgenics. Teleost fish cell lines have been developed from a broad range of tissues such as ovary, fin, swim bladder, heart, spleen, liver, eye muscle, vertebrae, brain, skin. One hundred and twenty-four new fish cell lines from different fish species ranging from grouper to eel have been reported since the last review by Fryer and Lannan (J Tissue Culture Methods 16: 87-94, 1994). Among the cell lines listed, more than 60% were established from species from Asia, which contributes more than 80% of total fish production. This includes 59 cell lines from 19 freshwater, 54 from 22 marine and 11 from 3 brackish water fishes. Presently, about 283 cell lines have been established from finfish around the world. In addition to the listing and a scientific update on new cell lines, the importance of authentication, applications, cross-contamination and implications of overpassaged cell lines has also been discussed in this comprehensive review. The authors feel that the review will serve an updated database for beginners and established researchers in the field of fish cell line research and development.

Expression of alternatively spliced CD45 isoforms by channel catfish clonal T and B cells is dependent on activation state of the cell and regulated by protein synthesis and degradation

In mammals, expression of the three alternatively spliced exons of the tyrosine phosphatase CD45 is regulated by the developmental and activation state of the cell. In comparison, the channel catfish, Ictalurus punctatus, CD45 homolog contains 18 functional alternatively spliced exons. Since very little is known about CD45 regulation in ectothermic vertebrates, this study examines the regulation of catfish CD45 mRNA isoform expression in clonal T and B cells in response to stimulation. Results show that mitogenic stimulation using catfish serum or concanavalin A induced expression of mRNAs for small CD45 isoforms, and isoform message expression was growth curve dependent, i.e. cells in logarithmic phase express message for smaller CD45 isoforms, whereas stationary phase cells express message for longer CD45 isoforms. In addition, cells treated with the protein synthesis inhibitor cycloheximide expressed message for longer CD45 isoforms, and treatment with lactacystin, which blocks protein degradation, rescued smaller isoform message expression. Collectively these data suggested that expression of CD45 isoforms, in catfish, at least at the mRNA level, is "constitutively dynamic" and highly dependent on extracellular stimuli.

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.

Effects of temperature change on the innate cellular and humoral immune responses of orange-spotted grouper Epinephelus coioides and its susceptibility to Vibrio alginolyticus

Orange-spotted grouper Epinephelus coioides held at 27 degrees C were then further cultured at 19, 27 (control), and 35 degrees C, and were examined for innate cellular and humoral responses after 3-96 h. The total leucocyte count, respiratory burst, and phagocytic activity significantly decreased 3, 48, and 96 h after fish were transferred to 19 and 35 degrees C. Both the alternative complement pathway (ACH(50)) and the lysozyme activity significantly decreased at 3-96 h after fish were transferred to 19 and 35 degrees C. In another experiment, groupers reared at 27 degrees C at 34 per thousand salinity were injected with Vibrio alginolyticus grown in tryptic soy broth (TSB) at a dose of 2.3 x 10(9) colony-forming units (cfu) fish(-1), and then further reared in water temperatures of 19, 27 (control), and 35 degrees C. The cumulative mortalities of V. alginolyticus-injected fish held in 19 and 35 degrees C were significantly higher than that of injected fish held in 27 degrees C. Resistance had decreased after 12 h for the challenged grouper held at 35 degrees C. All injected fish held in 19 degrees C had died after 72 h. It was concluded that at 12 h after transfer of grouper from 27 to 19 and 35 degrees C, immunity was suppressed and resistance against V. alginolyticus had decreased.

Identification and expression analyses of poly [I:C]-stimulated genes in channel catfish (Ictalurus punctatus)

Channel catfish (Ictalurus punctatus) have proven to be an excellent model with which to study immune responses of lower vertebrates. Identification of anti-viral antibodies and cytotoxic cells, as well as both type I and II interferon (IFN), demonstrates that catfish likely mount a vigorous anti-viral immune response. In this report, we focus on other elements of the anti-viral response, and identify more than two dozen genes that are induced following treatment of catfish cells with poly [I:C]. We showed that poly [I:C] induced type I interferon within 2 h of treatment, and that characteristic interferon stimulated genes (ISGs) appeared 6-12 h after exposure. Among the ISGs detected by RT-PCR assay were homologs of ISG15, Mx1, IFN regulatory factor 1 (IRF-1), inhibitor of apoptosis protein-1 (IAP-1) and the chemokine CXCL10. Microarray analyses showed that 13 and 24 cellular genes, respectively, were upregulated in poly [I:C]-treated B cell and fibroblast cultures. Although many of these genes were novel and did not fit the profile of mammalian ISGs, there were several (ISG-15, ubiquitin-conjugating enzyme E2G1, integrin-linked kinase, and clathrin-associated protein 47) that were identified as ISGs in mammalian systems. Taken together, these results suggest that dsRNA, either directly or through the prior induction of IFN, upregulates catfish gene products that function individually and/or collectively to inhibit virus replication.

Expression profiles of cloned channel catfish (Ictalurus punctatus) lymphoid cell lines and mixed lymphocyte cultures

Clonal channel catfish lymphoid cell lines and mixed lymphocyte cultures (MLCs) have proven extremely useful in examining immune responses at the cellular and molecular levels. To date clonal catfish cell lines and MLCs have been biologically and phenotypically characterized using a variety of techniques including reverse transcription polymerase chain reaction (RT-PCR), as well as Northern and Southern blotting. To expand the molecular characterization of these cultures, microarray analysis was employed. Clonal B (3B11), macrophage (42TA), and cytotoxic T cell (TS32.15 and TS32.17) lines and MLCs were examined using a cDNA array containing approximately 2500 probes derived from EST libraries prepared from the 42TA macrophage cell line, a MLC, and 5-14-day-old catfish fry. Analysis showed that each cell line displayed a unique RNA expression profile that included a variety of immune-related genes. Pearson correlation analysis indicated that one cytotoxic T cell line (TS32.15) clustered with the MLC, whereas a second cytotoxic T cell line (TS32.17) was more closely associated with a second cluster containing B cells and macrophages. This study illustrates the utility of microarray analyses in profiling RNA expression patterns in catfish lymphoid cell lines and will serve as a platform for examining catfish immune responses following virus infection or poly [I:C] treatment.

B cell receptor accessory molecules in the channel catfish, Ictalurus punctatus

B cell receptor (BCR) accessory molecules CD79a and CD79b homologs were identified in the channel catfish, Ictalurus punctatus. Both are found as single copy genes that encode proteins containing a signal peptide, an extracellular immunoglobulin domain, a transmembrane region and a cytoplasmic tail containing an immune-receptor tyrosine-dased activation motif (ITAM). IpCD79a and IpCD79b transcripts correlate well with IgM message expression. They are highly expressed in peripheral blood leukocytes (PBL) enriched in membrane (m) IgM+ cells and catfish clonal B cell lines, but not in catfish clonal T cells, indicating that IpCD79a and IpCD79b expression is B cell restricted. Studies using catfish clonal B cells (3B11) transfected with constructs encoding epitope-tagged IpCD79a and IpCD79b revealed that IpCD79a was expressed as a 45 kDa protein and IpCD79b was expressed as a 32 kDa protein. Furthermore, co-immunoprecipitations of epitope-tagged CD79 proteins demonstrate that these molecules are non-covalently associated with mIgM. These data correlate with some of the previous immunoprecipitation data demonstrating that catfish mIgM associates with proteins of 45 and 32 kDa.

Characterization of additional novel immune type receptors in channel catfish, Ictalurus punctatus

Mining of channel catfish (Ictalurus punctatus) expressed sequence tag databases identified seven new novel immune type receptors (IpNITRs). These differed in sequence, but not structure, from previously described IpNITR1-11. IpNITR12a, 12b, 13, and 14 encode proteins containing a single variable (V)-like immunoglobulin (Ig) domain. IpNITR12a and 13 encode a transmembrane (TM) region and cytoplasmic tail (CYT) containing immunoreceptor tyrosine inhibition motifs (ITIMs). IpNITR14 contains a TM and short CYT devoid of signaling motifs and is similar in structure to IpNITR7. IpNITR12b lacks a TM and may represent an IpNITR12a splice variant. In contrast, IpNITR15a, 15b, and 16 encode two Ig domains (V-like domain 1 and V/C2-like domain 2). IpNITR15a and 15b contain TM and CYT with ITIMs. IpNITR16 appears to be a secreted form. The first V-like domains of IpNITR12-16 (except a/b pairs) share 17-32% amino acid identity with each other and with V domains of IpNITR1-11. They therefore represent five additional IpNITR V families (defined as possessing 70% or more amino acid identity). The V/C2 domains of IpNITR15a, 15b and 16 have 94-98% amino acid identity, but share 37-50% amino acid identity with corresponding V/C2 domains found in IpNITR1-4. Phylogenetic analyses indicate IpNITR12-16 are more closely related to other teleost NITRs than to IpNITR1-11. Gene mapping indicates that IpNITRs are linked, and members of the ten known IpNITR families are interspersed. IpNITR12-16 are differentially expressed in various catfish immune-type cells and preferentially up regulated in peripheral blood leukocytes by allogeneic stimulation.

Channel catfish, Ictalurus punctatus, CD4-like molecules

Two CD4-like (CD4L) molecules have been identified in channel catfish, Ictalurus punctatus. Although phylogenetically related to other vertebrate CD4 molecules, they exhibit only 19% amino acid identity to each other. IpCD4L-1 encodes a predicted protein containing four immunoglobulin domains, a transmembrane region and a cytoplasmic tail containing a p56(lck) binding site. In contrast, IpCD4L-2 encodes for a similar protein with three immunoglobulin domains. The gene organization of IpCD4L-1 is very similar to that of other vertebrate CD4 genes, while the genomic organization of IpCD4L-2 is different. Southern blots indicate both catfish molecules are likely single copy genes and mapping studies show that both are found on a single Bacterial Artificial Chromosome suggesting close linkage. At the message level, IpCD4L-1 and -2 are expressed in various catfish lymphoid tissues and in non-B-cell peripheral blood leukocytes (PBL). Both messages are upregulated in concanavalin A (ConA) and alloantigen stimulated PBL, but not in lipopolysaccharide (LPS)-stimulated cultures. In contrast, they are differentially expressed among the catfish clonal T cell lines. While both molecules appear to be T cell specific, their functional significance in catfish is unknown.

Channel catfish leukocyte immune-type receptors contain a putative MHC class I binding site

The recent identification of a large and diverse family of leukocyte immune-type receptors (IpLITRs) in channel catfish (Ictalurus punctatus) indicates that immunoglobulin superfamily (IgSF) members related to both mammalian Fc receptors (FcRs) and leukocyte receptor complex (LRC)-encoded proteins exist in fish. In the present study, it was found that IpLITR messages were preferentially up regulated in catfish peripheral blood leukocytes (PBL) and clonal cytotoxic T cells (CTL) after alloantigen stimulation. Detailed sequence analyses of the expressed IpLITR cDNAs from two clonal CTL lines indicated an unexpectedly large array of putative activatory and inhibitory IpLITR-types containing variable numbers of extracellular immunoglobulin (Ig)-like domains. Importantly, all expressed IpLITRs shared similar membrane distal Ig domains (i.e., D1 and D2), suggesting that they may bind a common type of ligand. Sequence alignments and comparative homology modeling revealed that IpLITR domains, D1 and D2, have similar predicted 3-D structural properties with the corresponding domains of the human LRC-encoded leukocyte Ig-like receptor (LILR) family. Furthermore, conservation of key major histocompatibility class I (MHC I)-binding residues were located at similar positions within the membrane distal tip of D1 between representative IpLITRs and group 1 LILRs. Taken together, these results suggest that fish LITRs have an orthologous relationship to LRC-encoded receptors such as the human LILRs and could potentially function as a diverse family of MHC class I-binding receptors.

Structure of the catfish IGH locus: analysis of the region including the single functional IGHM gene

The catfish IGH locus is large ( approximately 1 Mb) and complex, having undergone multiple internal duplications and transpositions. To define the structure of the locus that contains the single expressed IGHM gene, two overlapping bacterial-artificial-chromosome (BAC) clones spanning the most 3' end of the channel catfish immunoglobulin heavy (IGH) chain locus have been completely sequenced. The analyses created a contig of 257,153 bp containing 55 VH, 6 D, 12 JH genes and the IGH constant region genes encoding the functional secreted and membrane forms of IgM and the membrane form of IgD. This analysis revealed three major features. First, no C-region genes were found aside from the previously described IGHM1 and IGHD1, with the latter gene being the most 3' C-region gene of the catfish IGH locus. There was no evidence in the region sequenced for genes that could encode an Ig class similar to the IgZ/IgT described in zebrafish, trout and pufferfish. Second, there are a high number of VH pseudogenes, 28 out of 55 (51%). In contrast, the entire zebrafish IGH locus has 40 functional VH genes and eight pseudogenes (17%). Third, an internal duplication of a 52.4-kb block of VH genes has occurred. These observations suggest that the IGH locus of teleost fish varies significantly from species to species in the diversity of C-region genes as well as the numbers of genes encoding V regions.

Identification and Characterization of a FcR Homolog in an Ectothermic Vertebrate, the Channel Catfish (Ictalurus punctatus)

An FcR homolog (IpFcRI), representing the first such receptor from an ectothermic vertebrate, has been identified in the channel catfish (Ictalurus punctatus). Mining of the catfish expressed sequence tag databases using mammalian FcR sequences for CD16, CD32, and CD64 resulted in the identification of a teleost Ig-binding receptor. IpFcRI is encoded by a single-copy gene containing three Ig C2-like domains, but lacking a transmembrane segment and cytoplasmic tail. The encoded Ig domains of IpFcRI are phylogenetically and structurally related to mammalian FcR and the presence of a putative Fc-binding region appears to be conserved. IpFcRI-related genomic sequences are also present in both pufferfish and rainbow trout, indicating the likely presence of a soluble FcR in other fish species. Northern blot and qualitative PCR analyses demonstrated that IpFcRI is primarily expressed in IgM-negative leukocytes derived from the lymphoid kidney tissues and PBL. Significantly lower levels of IpFcRI expression were detected in catfish clonal leukocyte cell lines. Using the native leader, IpFcRI was secreted when transfected into insect cells and importantly the native IpFcRI glycoprotein was detected in catfish plasma using a polyclonal Ab. Recombinant IpFcRI binds catfish IgM as assessed by both coimmunoprecipation and cell transfection studies and it is presumed that it functions as a secreted FcR akin to the soluble FcR found in mammals. The identification of an FcR homolog in an ectothermic vertebrate is an important first step toward understanding the evolutionary history and functional importance of vertebrate Ig-binding receptors.

A novel family of diversified immunoregulatory receptors in teleosts is homologous to both mammalian Fc receptors and molecules encoded within the leukocyte receptor complex

Three novel and closely related leukocyte immune-type receptors (IpLITR) have been identified in channel catfish (Ictalurus punctatus). These receptors belong to a large polymorphic and polygenic subset of the Ig superfamily with members located on at least three independently segregating loci. Like mammalian and avian innate immune regulatory receptors, IpLITRs have both putative inhibitory and stimulatory forms, with multiple types coexpressed in various lymphoid tissues and clonal leukocyte cell lines. IpLITRs have an unusual and novel relationship to mammalian and avian innate immune receptors: the membrane distal Ig domains of an individual IpLITR are related to fragment crystallizable receptors (FcRs) and FcR-like proteins, whereas the membrane proximal Ig domains are related to several leukocyte receptor complex encoded receptors. This unique composition of Ig domains within individual receptors supports the hypothesis that functionally and genomically distinct immune receptor families found in tetrapods may have evolved from such ancestral genes by duplication and recombination events. Furthermore, the discovery of a large heterogeneous family of immunoregulatory receptors in teleosts, reminiscent of amphibian, avian, and mammalian Ig-like receptors, suggests that complex innate immune receptor networks have been conserved during vertebrate evolution.

Identification and expression analysis of cDNAs encoding channel catfish type I interferons

Previously a cDNA encoding a putative interferon gene, designated CF IFN-1, was identified from a catfish EST library. However, its constitutive expression, absence of a signal peptide, and apparently low level of biological activity suggested that this cDNA likely encoded an expressed pseudogene. Since Southern blot analysis suggested the presence of two to three IFN genes, additional cDNAs were generated from catfish fibroblast and lymphoid cell lines using primers designed to conserved regions of zebrafish and catfish interferon. Using this approach, three novel CF IFN genes, two of which likely encode functional interferon molecules, were identified. At the amino acid level, similarity among CF IFNs ranged from 71% to 82%, whereas similarity to other fish IFNs ranged from 15% to 35%. Although CF IFN-3, like CF IFN-1, lacks a signal peptide, CF IFN-2 and -4 appear to encode full-length, signal sequence-bearing genes. Consistent with their putative identification as functional genes, CF IFN-2 and -4 were not expressed in unstimulated cell lines, and CF IFN-2 was rapidly upregulated in CCO cells in response to virus infection or treatment with dsRNA. Moreover, as with salmon, fugu, and zebrafish interferon genes, CF IFN-1 contained four introns whose locations were conserved not only with respect to other fish IFNs, but also with respect to mammalian IFN-lambda. While it is likely that CF IFNs represent Type I IFNs, several characteristics preclude assigning these cytokines to any particular subfamily.

NK-lysin of channel catfish: Gene triplication, sequence variation, and expression analysis

Antimicrobial peptides (AMPs) are important components of the host innate immune response against microbial invasion. In addition to the previously known four classes of antimicrobial peptides, a fifth class of antimicrobial peptides has been recently identified to include NK-lysins that have a globular three-dimensional structure and are larger with 74-78 amino acid residues. NK-lysin has been shown to harbor antimicrobial activities against a wide spectrum of microorganisms including bacteria, fungi, protozoa, and parasites. To date, NK-lysin genes have been reported from only a limited number of organisms. We previously identified a NK-lysin cDNA in channel catfish. Here we report the identification of two novel types of NK-lysin transcripts in channel catfish. Altogether, three distinct NK-lysin transcripts exist in channel catfish. In this work, their encoding genes were identified, sequenced, and characterized. We provide strong evidence that the catfish NK-lysin gene is tripled in the same genomic neighborhood. All three catfish NK-lysin genes are present in the same genomic region and are tightly linked on the same chromosome, as the same BAC clones harbor all three copies of the NK-lysin genes. All three NK-lysin genes are expressed, but exhibit distinct expression profiles in various tissues. In spite of the existence of a single copy of NK-lysin gene in the human genome, and only a single hit from the pufferfish genome, there are two tripled clusters of NK-lysin genes on chromosome 17 of zebrafish in addition to one more copy on its chromosome 5. The similarity in the genomic arrangement of the tripled NK-lysin genes in channel catfish and zebrafish suggest similar evolution of NK-lysin genes.

Identification and expression analysis of interferon gamma genes in channel catfish

Multiple species of type I interferon (IFN) were recently identified in catfish (CF) (Ictalurus punctatus). Herein we extend these studies and report the existence of two distinct type II IFN genes in channel CF. As with zebrafish and the green spotted pufferfish, the two CF IFN-gamma genes are dissimilar in sequence but closely linked on the same chromosome. One of the genes (IFN-gamma2) encodes two distinct messages that likely arose via alternative splicing at two closely spaced splice donor sites within the first intron. Sequence analysis indicates that CF IFN-gamma genes contain the hallmarks of authentic IFN-gamma including: (1) a conserved nuclear localization site at the C terminus (CF IFN-gamma2 only), (2) an IFN-gamma signature sequence, (3) six putative helical regions within the mature protein, (4) one or more potential glycosylation sites, and (5) multiple mRNA instability motifs within the 3' untranslated region. Moreover, well-characterized CF T and NK cell clones were shown to synthesize IFN-gamma transcripts. This is the first unequivocal demonstration in any lower vertebrate species that NK and T cells synthesize IFN-gamma and is consistent with results in mammalian systems where T cells and NK cells are the major sources of type II IFN production. Collectively, these studies indicate that Siluriformes possess two evolutionarily conserved IFN-gamma genes and demonstrate that CF possess three key elements of the innate immune response: NK cells and types I and II IFN.

Characterization of glycans on major histocompatibility complex class II molecules in channel catfish, Ictalurus punctatus

The glycans associated with mammalian major histocompatibility complex (MHC) class II molecules have been studied extensively. Co-translational and post-translational addition of sugar molecules to proteins confers many structural and modulatory functions. In the present study we characterized the glycans associated with MHC class II molecules in the channel catfish to compare glycosylation patterns in a teleost to those known to occur in mammals. This study made use of enzymatic methods and two-dimensional (2D) gel electrophoresis to characterize the N-linked sugars. Unlike mammalian T cells which expressed complex N-linked sugars, channel catfish derived 28S T cells were found to express high-mannose/hybrid N-glycans on class II molecules. However studies with Endoglycosidase H in conjunction with cell surface labeling on peripheral blood leukocytes revealed that catfish possess the machinery to modify the intermediate high-mannose sugars to complex type sugars. Nonetheless, the majority of the class II cell surface glycoproteins were of the high-mannose type. Resolution of catfish MHC class II molecules by 2D gel analyses revealed multiple bands for class II beta chains whereas class II alpha chains focused as a single spot. Glycosylation in the channel catfish, a premier model system for studying the immune system of teleosts, has significant differences from the glycosylation patterns characterized in mammalian systems, likely with functional implications.

Identification and characterization of a FasL-like protein and cDNAs encoding the channel catfish death-inducing signaling complex

To elucidate cytolytic mechanisms in the channel catfish, lysates from catfish lymphoid and fibroblast cell lines were screened by Western blot analysis using a panel of antibodies reactive with components of the mammalian apoptotic pathway. Strong reactivity with three proteins (approximate Mr 70,000, 37,000, and 15,000) was seen using an antibody targeted to mammalian Fas ligand (FasL). The sizes of the two smaller proteins are consistent with their tentative designation as membrane-bound (37,000 Mr) and soluble (15,000 Mr) FasL. Treatments known to induce FasL in mammalian systems (e.g., PMA/calcium ionophore, UV-irradiation) induced expression of the 37,000- Mr protein in catfish T-cell lines. Moreover, expression of the 37,000- Mr protein in clonal T cells was up-regulated by increasing cell density. At the nucleotide level, homologues of Fas receptor (FasR), FADD, and caspase 8 were identified and characterized. These gene products likely constitute the teleost equivalent of the death-inducing signaling complex (DISC). FADD was constitutively expressed in all (T, B, macrophage, and fibroblast) cell lines examined as well as in peripheral blood lymphocytes (PBL), whereas FasR and caspase 8 were expressed in all cell lines except CCO, a FasL-positive fibroblast line. In contrast to FasL, expression of FasR and caspase 8 was inversely proportional to cell density. Collectively these studies identified four membrane-proximal proteins involved in the initiation of apoptosis in channel catfish and suggest that mechanisms of cell-mediated cytotoxicity in teleosts are similar to those used by mammals.

Molecular identification and expression analysis of tumor necrosis factor in channel catfish (Ictalurus punctatus)

A tumor necrosis factor (TNF) alpha-like gene, encoding a propeptide of 230 amino acids and a mature (soluble) peptide of 162 amino acids, was identified in channel catfish (Ictalurus punctatus). While the catfish protein shared features in common with both mammalian TNFalpha and TNFbeta homologs, overall sequence identity/similarity was slightly higher vs. TNFalpha genes when mature TNF sequences were compared. Phylogenetic analysis placed catfish and other fish TNF sequences within their own cluster apart from mammalian TNFalpha and beta genes, and supported the suggestion that TNFalpha and beta genes separated after the divergence of mammals and teleosts. In contrast to trout and carp, but similar to flounder, catfish TNF was present as a single copy gene. Expression studies demonstrated that catfish TNFalpha mRNA was present in all tested tissues (i.e. liver, spleen, head kidney, mesonephros, gill, thymus, and PBLs) from an unstimulated fish. Moreover, catfish TNF was constitutively expressed in actively proliferating, but otherwise unstimulated, macrophage (42TA) and T cell (G14D; TS32.17) lines, but not in B cell (1G8 or 3B11) or fibroblast lines. TNF expression was upregulated in PBLs, and in G14D and 42TA cells, but not in 3B11 cells, by PMA/calcium ionophore treatment. These results demonstrate that a catfish homolog of TNFalpha has been identified, and indicate that catfish TNFalpha is expressed in catfish in a manner similar to that seen in mammals.

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.

Evaluation of dorsal aorta cannulation for immunological studies of grouper (Epinephelus malabaricus)

Blood is often withdrawn to study the immune responses of fish. However, netting, handling and anaesthetising the experimental fish, and drawing blood samples cause severe stress that may alter the effects of immune study protocols and treatments. We evaluated the effect of aorta cannulation, for use in immune studies, on grouper (Epinephelus malabaricus) plasma cortisol, total red and white blood cell counts and phagocytosis. Plasma cortisol increased from 30 to 88 ng/ml 1 day after insertion of the cannula, to a maximum of 951 ng/ml 3 to 5 days after surgery, indicating the groupers were stressed by cannulation and post-cannulation inflammation. Total RBC count decreased, and total WBC count increased after surgery. Following cannulation, the phagocytic index of peripheral blood leukocytes decreased from 100% to 46%. The adverse effects of cannulation were mitigated by continuously immersing groupers in oxytetracycline (OTC), which decreased the recovery period for treated fish. In contrast, OTC-treatment did not markedly improve the recovery of groupers subjected to caudal vessel puncture. Cortisol levels in OTC-treated grouper with caudal vessel puncture were significantly higher than in OTC-treated, cannulated grouper, and remained at a high level until day 13 of the experiment. From day 7 to 13, total RBC and WBC counts in OTC-treated, cannulated groupers were significantly different from those in OTC-treated groupers with caudal vessel puncture. OTC treatment improved the phagocytic index of groupers subjected to caudal vessel puncture, but the phagocytic index was lower than that of groupers subjected to cannulation. Cannulation minimises visual and handling disturbances, and facilitates standardisation of experimental conditions and quick and easy sampling via the dorsal aorta cannula. Therefore, dorsal aorta cannulation minimises the stress of blood sampling and should prove useful for immune studies in fish.

Activation of channel catfish (Ictalurus punctatus) T cells involves NFAT-like transcription factors

Cyclosporin A (CsA) specifically inhibits mammalian T cells by preventing activation of transcription factors (termed nuclear factor of activated T cells (NFAT)) involved in cytokine gene expression. In this study, catfish peripheral blood lymphocytes (PBL) and antigen specific T cells were treated with CsA to gain insights into the intracellular processes involved in fish T cell activation. To this end, CsA was observed to inhibit the in vitro proliferation of Con A stimulated catfish PBL, and specific alloantigen stimulated T cells. However, the inhibitory effect of CsA on catfish T cells was obviated by treatment with Con A, antigen activation or culture supernatant from activated catfish T cells prior to the addition of CsA. The use of a phosphatase assay coupled with Western blot analysis employing a polyclonal antibody to mammalian NFAT indicated that CsA prevents the dephosphorylation and subsequent nuclear translocation of an NFAT-like molecule in catfish T cells. Finally, a nuclear protein selection protocol demonstrated that a catfish NFAT-like protein binds to a known murine IL-2 promoter sequence. These results suggest that cytokines are involved in the activation of teleost T cells, and argue that T cell activation processes are conserved over a wide phylogenetic distance.

The IgH locus of the channel catfish, Ictalurus punctatus, contains multiple constant region gene sequences: different genes encode heavy chains of membrane and secreted IgD

The delta-chain of catfish IgD was initially characterized as a unique chimeric molecule containing a rearranged VDJ spliced to C micro 1, seven C domain-encoding exons (delta1-delta7), and a transmembrane tail. The presence of cDNA forms showing splicing of delta7 to an exon encoding a secretory tail was interpreted to indicate that membrane (deltam) and secreted (deltas) forms were likely expressed from a single gene by alternative RNA processing. Subsequent cloning and sequence analyses have unexpectedly revealed the presence of three delta C region genes, each linked to a micro gene or pseudogene. The first (IGHD1) is located 1.6 kb 3' of the functional C micro (IGHM1). The second (IGHD3) is positioned immediately downstream of a pseudo C micro (IGHM3P), approximately 725 kb 5' of IGHM1. These two delta genes are highly similar in sequence and each contains a tandem duplication of delta2-delta3-delta4. However, IGHD1 has a terminal exon encoding the transmembrane region, whereas IGHD3 has a single terminal exon encoding a secreted tail. The occurrence of IGHD3 immediately downstream of a micro pseudogene indicates that the putative deltas product may not be expressed as a chimeric micro delta molecule. Western blots and protein sequencing data indicate that an IGHD3-encoded protein is expressed in catfish serum. Thus, catfish deltam transcripts appear to originate from IGHD1, whereas deltas transcripts originate from IGHD3 rather than, as previously inferred, from a single expressed delta gene. The third delta (IGHD2) is associated with a pseudo C micro (IGHM2P); its presence is inferred by Southern blot analyses.

Immortal and mortal clonal lymphocyte lines from channel catfish: comparison of telomere length, telomerase activity, tumor suppressor and heat shock protein expression

Channel catfish autonomous (immortal) and nonautonomous (mortal) leukocyte lines were phenotyped with respect to telomere length and the expression of telomerase, Hsp70 and p53, potentially important factors in cellular immortalization. The autonomous cells constitutively expressed telomerase whereas the nonautonomous cells expressed this activity only transiently. This observation, coupled with the low telomerase activity level seen in freshly isolated leukocytes, suggests that telomerase expression in catfish leukocytes is activation induced. In contrast both types of cell lines exhibited quite similar patterns of significantly shortened telomeres, suggesting that telomerase does not stabilize catfish telomeres until a critical short length is reached. Northern analyses indicated that, like telomerase, Hsp70 gene expression was constitutive in autonomous cells and transient in nonautonomous cells. In contrast, p53 mRNA levels appeared similarly low and noncycling in both long-term cultured types of catfish cells, regardless of the culture situation. Furthermore it was noted, by Western analyses, that both types of cells display multiple sized forms of p53 proteins. This latter observation implies that truncation of p53 protein is probably not directly involved in the in vitro immortalization process of channel catfish leukocytes.

Genomic organization and differential expression of channel catfish MHC class I genes

Two clones, designated Icpu-UA/3 and Icpu-UA/26, were isolated from a genomic library prepared from a single homozygous gynogenetic channel catfish. Sequence analysis showed that each clone encoded a gene product containing features conserved among MHC class I molecules. The genomic organization of both clones indicated that each domain, with the exception of the cytoplasmic, was encoded by a separate exon. Moreover, like mammals, catfish cytoplasmic regions were encoded by three exons rather than two as previously described for other teleost MHC class I genes. Analysis of nucleotide sequences upstream of catfish class I genes revealed the presence of several regulatory motifs similar to those seen in mammalian class I genes. These included a TATA box, Enhancer B, Site alpha, ISRE, and GAS elements. To determine the functional significance of these elements, EMSAs and tissue expression assays were performed. EMSAs demonstrated that an Enhancer B element within Icpu-UA/26, and an imperfect Enhancer B element and/or a GC-rich region within Icpu-UA/3 were responsible for formation of specific DNA/protein complexes. Expression studies detected Icpu-UA/26 transcripts in all tissues tested, whereas Icpu-UA/3 encoded messages were seen in a limited number of tissues. These results define the intron/exon organization of catfish MHC class I genes, suggest that Icpu-UA/3 encodes a nonclassical gene, and provide the first functional evidence that upstream sequences, similar to those seen in mammalian class I genes, play important roles in regulating teleost MHC gene expression.

Heterogeneity of channel catfish CTL with respect to target recognition and cytotoxic mechanisms employed

Two types of catfish alloantigen-dependent cytotoxic T cells were cloned from PBL from a fish immunized in vivo and stimulated in vitro with the allogeneic B cell line 3B11. Because these are the first clonal cytotoxic T cell lines derived from an ectothermic vertebrate, studies were undertaken to characterize their recognition and cytotoxic mechanisms. The first type of CTL (group I) shows strict alloantigen specificity, i.e., they specifically kill and proliferate only in response to 3B11 cells. The second type (group II) shows broad allogeneic specificity, i.e., they kill and proliferate in response to several different allogeneic cells in addition to 3B11. "Cold" target-inhibition studies suggest that group II CTL recognize their targets via a single receptor, because the killing of one allotarget can be inhibited by a different allotarget. Both types of catfish CTL form conjugates with and kill targets by apoptosis. Killing by Ag-specific cytotoxic T cells (group I) was completely inhibited by treatment with EGTA or concanamycin A, and this killing is sensitive to PMSF inhibition, suggesting that killing was mediated exclusively by the secretory perforin/granzyme mechanism. In contrast, killing by the broadly specific T cytotoxic cells (group II) was only partially inhibited by either EGTA or concanamycin A, suggesting that these cells use a cytotoxic mechanism in addition to that involving perforin/granzyme. Consistent with the presumed use of a secretory pathway, both groups of CTL possess putative lytic granules. These results suggest that catfish CTL show heterogeneity with respect to target recognition and cytotoxic mechanisms.

Thioredoxin acts as a B cell growth factor in channel catfish

To identify differentially expressed genes from channel catfish macrophages, a cDNA library from LPS-stimulated catfish macrophages was screened by subtractive hybridization. This screening yielded a 552-bp cDNA coding for catfish thioredoxin (CF-TRX). The deduced amino acid sequence revealed that CF-TRX contains 107 amino acids and is 59% homologous to human adult T cell leukemia-derived factor/TRX, originally described as an IL-2R alpha-inducing factor. Northern blot analyses showed that CF-TRX is expressed in catfish T and macrophage cell lines, but weakly in B cell lines. Similar results were also observed in Western blot analyses using a mAb specific for recombinant CF-TRX (rTRX). The use of rTRX in functional studies demonstrated that rTRX induces in vitro proliferative responses of catfish PBL that were synergistically enhanced by the addition of culture supernatants from catfish T cell lines. In addition, cell separation studies and flow cytometric analyses revealed that the cells proliferating in rTRX-stimulated cultures were mostly B cells. These results suggest that CF-TRX may have an important role in the activation and proliferation of channel catfish B cells.

Differential in vitro suppressive effects of steroids on leukocyte phagocytosis in two teleosts, tilapia and common carp

The objectives of this study were to investigate the potential roles of cortisol and gonadal steroids in the phagocytic activity of peripheral blood leukocytes in two teleosts, tilapia (Oreochromis niloticus x O. aureus) and common carp (Cyprinus carpio). An in vitro microtiter plate assay, measuring incorporation of FITC-latex beads into peripheral blood leukocytes, was developed for the first time in teleosts. Peripheral blood leukocytes were cultured in AL medium with tested compounds in a microfluor black plate at 25 degrees C. FITC-latex beads were further incubated for phagocytosis and engulfed fluorescent intensity in phagocytes was detected fluorometrically. Cortisol suppressed leukocyte phagocytosis in a dose (10(-14) to 10(-4) M)- and time (0.5 to 8 h)-dependent manner in tilapia. The glucocorticoid agonist dexamethasone had a suppressive effect similar to that of cortisol, while cortisone and the mineralocorticoid aldosterone had only a weak effect in tilapia. High doses of estradiol and ethynylestradiol, but not of estrone, suppressed phagocytosis in tilapia. No suppressive effect on phagocytosis was observed with various concentrations of progesterone, testosterone, and 11-ketotestosterone. Triiodothyronine was also inactive on phagocytosis. A combination of estradiol and cortisol potentiatingly suppressed phagocytosis. Actinomycin D and cycloheximide blocked the suppressive effects of cortisol and estradiol. Cortisol had weaker suppressive effects on the phagocytosis of leukocytes in common carp than tilapia. Other steroids had no suppressive action on phagocytosis in common carp. It is concluded that the suppressive effects of cortisol and estradiol on phagocytosis in tilapia are mediated by specific glucocorticoid receptors and estrogen receptors, respectively. Cortisol would play a main and important role on the down-regulation of phagocytic activity. Sexual steroids, such as estradiol, also could interact with cortisol to further suppress immunity in tilapia. Differential responsiveness of the immune system to suppressive effects of steroids, among teleosts species, has been demonstrated.