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.

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.

Conservation and divergence of the Eμ3′ enhancer in the IGH locus of teleosts

The core region of the Emicro3' transcriptional enhancer that drives the expression of the teleost IGH locus has been characterized functionally in two species, the catfish (Ictalurus punctatus) and the zebrafish (Danio rerio). These studies have suggested important differences: whereas the catfish enhancer acts through an E-box and two octamer motifs, the zebrafish enhancer exerts its major effects through two E-box motifs alone. In this study, the function of the catfish enhancer was reexamined in a broader comparative context within the teleosts. Electrophoretic mobility shift assays of motifs from catfish, zebrafish, and Fugu were conducted to determine their ability to bind catfish E-protein and Oct transcription factors. Transient expression assays were conducted using a region of the catfish core enhancer that includes a newly described hybrid octamer/E-box motif. Sequences homologous to the Emicro3' enhancer region from six teleosts were aligned to determine conserved regions ("phylogenetic footprinting"). These studies allowed the following conclusions to be drawn: (1) The important 3'E-box motif described in the zebrafish corresponds in the homologous region of the catfish enhancer to an Oct motif with a newly described negative regulatory function and (2) Comparison of the Emicro3' enhancer sequences of six teleosts indicates that while a variety of octamer and E-box motifs are found in this region, strict evolutionary conservation of the important functional elements of the teleost Emicro3' enhancer has not occurred.

Oct2 transcription factors in fish--a comparative genomic analysis

The Oct2 transcription factor is important in driving expression of the IgH locus of the channel catfish, Ictalurus punctatus. Two isoforms, catfish Oct2alpha and Oct2beta, have been characterized at the level of expression and function, but little is known of the structure of the Oct2 gene in catfish. To gain insight into the diversity of Oct2 gene structure and expression in the teleost fish, a comparative genomic analysis of Oct2 was undertaken in the pufferfish (Fugu rubripes) and the zebrafish (Danio rerio). The orthologues of zebrafish and Fugu Oct2 were identified, and share with catfish Oct2 the expression of a limited number (two in zebrafish, three in Fugu) of isotypes produced by alternative pathways of RNA processing. The alternatively spliced variants of catfish Oct2 showed a different pattern of exon use from those of Fugu and zebrafish. The analysis also identified a novel homologue of Oct2 in both zebrafish and Fugu. This homologue, termed Oct2x, shares similarities to both Oct1 and Oct2. A phylogenetic analysis of the relationships of Oct2x gave an unexpected result, with Oct2x occupying a position basal to the Oct gene families of both vertebrates and Drosophila.

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.

Evolution of vertebrate E protein transcription factors: comparative analysis of the E protein gene family in Takifugu rubripes and humans

E proteins are essential for B lymphocyte development and function, including immunoglobulin (Ig) gene rearrangement and expression. Previous studies of B cells in the channel catfish ( Ictalurus punctatus) identified E protein homologs that are capable of binding the μE5 motif and driving a strong transcriptional response. There are three E protein genes in mammals, HEB (TCF12), E2A (TCF3), and E2-2 (TCF4). The major expressed E proteins found in catfish B cells are homologs of HEB and of E2A. Here we sought to define the complete family of E protein genes in a teleost fish, Takifugu rubripes, taking advantage of the completed genome sequence. The catfish CFEB (HEB homolog) sequence identified homologous E-protein-encoding sequences in five scaffolds in the Takifugu genome database. Detailed comparative analysis with the human genome revealed the presence of five E protein homologs in Takifugu. Single genes orthologous to HEB and to E2-2 were identified. In contrast, two members of the E2A gene family were identified in Takifugu; one of these shows the alternative processing of transcripts that identifies it as the ortholog of the E12/E47-encoding mammalian E2A gene, whereas the second Takifugu E2A gene has no predicted alternative splice products. A novel fifth E protein gene (EX) was identified in Takifugu. Phylogenetic analysis revealed four E protein branches among vertebrates: EX, E2A, HEB, and E2-2.

Regulation of immunoglobulin gene transcription in a teleost fish: identification, expression and functional properties of E2A in the channel catfish

The function of the transcriptional enhancer, Emu3', of the IgH locus of the channel catfish, Ictalurus punctatus, involves the interaction of E-protein and Oct family transcription factors. The E-proteins [class I basic helix-loop-helix (bHLH) family] are encoded in mammals by three genes: E2A (of which E12/E47 are alternatively spliced products), HEB, and E2-2. An E2A homologue has been identified in a catfish B-cell cDNA library and contains regions homologous to the bHLH and activation domains of mammalian and other vertebrate E2A proteins. E2A message is widely expressed, being readily detected in catfish B cells, T cells, kidney, spleen, brain, and muscle. Its expression is lower than that previously observed for TF12/CFEB, the catfish homologue of HEB. E2A strongly activated transcription of a muE5 motif-dependent construct in catfish B cells, and also activated transcription from the core region of the catfish IgH enhancer (Emu3') in a manner dependent on the presence of the muE5 site. Catfish E2A, expressed in vitro, bound the muE5 motif present in the core region of Emu3'. These results document the conservation of structure and function in vertebrate E2A and suggest a potential role of E2A in driving expression of the IgH locus at the phylogenetic level of a teleost fish.

MHC RFLP analyses in channel catfish full-sibling families: identification of the role of MHC molecules in spontaneous allogeneic cytotoxic responses

Genes encoding MHC class I and II molecules have been identified in a number of fish species, including the channel catfish, but there is still a dearth of knowledge concerning their functional roles in teleost immune responses. This has in part been due to a lack of appropriate MHC class I and II matched and mismatched animals. To identify such animals, MHC segregation and linkage studies in the channel catfish were undertaken. The results of restriction fragment length polymorphism and fluorescent in situ hybridization studies showed that all the MHC class II genes are linked and most if not all MHC class I genes are linked. These studies also demonstrated that in catfish, as in other teleosts, MHC class I and II genes are not linked. Consequently, catfish matched and mismatched for MHC class I and II genes were identified and preliminary functional studies indicate that spontaneous non-specific allogeneic cytotoxic responses are likely mediated by differences in MHC class I, but not class II, region molecules.

Evolution of Transcriptional Control of theIgHLocus: Characterization, Expression, and Function of TF12/HEB Homologs of the Catfish

The transcriptional enhancer (Emu3') of the IgH locus of the channel catfish, Ictalurus punctatus, differs from enhancers of the mammalian IgH locus in terms of its position, structure, and function. Transcription factors binding to multiple octamer motifs and a single muE5 motif (an E-box site, consensus CANNTG) interact for its function. E-box binding transcription factors of the class I basic helix-loop-helix family were cloned from a catfish B cell cDNA library in this study, and homologs of TF12/HEB were identified as the most highly represented E-proteins. Two alternatively spliced forms of catfish TF12 (termed CFEB1 and -2) were identified and contained regions homologous to the basic helix-loop-helix and activation domains of other vertebrate E-proteins. CFEB message is widely expressed, with CFEB1 message predominating over that of CFEB2. Both CFEB1 and -2 strongly activated transcription from a muE5-dependent artificial promoter. In catfish B cells, CFEB1 and -2 also activated transcription from the core region of the catfish IgH enhancer (Emu3') in a manner dependent on the presence of the muE5 site. Both CFEB1 and -2 bound the muE5 motif, and formed both homo- and heterodimers. CFEB1 and -2 were weakly active or inactive (in a promoter-dependent fashion) in mammalian B-lineage cells. Although E-proteins have been highly conserved in vertebrate evolution, the present results indicate that, at the phylogenetic level of a teleost fish, the TF12/HEB homolog differs from that of mammals in terms of 1) its high level of expression and 2) the presence of isoforms generated by alternative RNA processing.

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.

Identification of a cDNA encoding channel catfish interferon

Despite considerable advances in our understanding of teleost immunity, relatively few cytokine genes, including those for interferon (IFN), have been identified at the molecular level. In contrast, numerous studies have shown that following virus infection or exposure to double-stranded RNA, fish or fish cells produce a soluble factor that is functionally similar to mammalian IFN. A putative catfish (CF) IFN cDNA was identified by BLASTX screening of a catfish EST library generated from a mixed lymphocyte culture enriched for NK-like cells. Consistent with its designation as a putative cytokine cDNA, the 3' non-translated region contained multiple copies of an RNA instability motif. Analysis of the deduced amino acid sequence of CF IFN showed low levels of identity/similarity to a panel of mammalian and avian IFN proteins, and markedly higher similarity to a recently identified zebrafish IFN. To determine if the identified cDNA encoded CF IFN, expression was monitored following infection of channel catfish ovary (CCO) cells with UV-inactivated catfish reovirus or exposure to double-stranded RNA, treatments which induce IFN or IFN-like activity in catfish and other species. In both cases, upregulation of putative CF IFN mRNA was detected. Moreover, upregulation of CF IFN mRNA was accompanied by the appearance of an antiviral factor in the culture medium. To confirm these results, recombinant CF IFN was synthesized in COS-7 cells and shown to have antiviral activity in CCO cells. Collectively, these results argue strongly that the identified catfish cDNA is an IFN homolog.

Identification and characterization of clonal NK-like cells from channel catfish (Ictalurus punctatus)

TcR alpha, beta, and gamma chain negative cytotoxic NK-like cells were cloned from alloantigen-stimulated PBL obtained from nai;ve channel catfish. Stimulation with allogeneic cells and growth promoting factors are required for their continued in vitro proliferation and cytotoxic activity. These granular cells kill not only the stimulating allogeneic cells, but also unrelated allogeneic targets by a perforin/granzyme-mediated apoptosis pathway. In addition, they are negative for markers that define neutrophils, monocytes/macrophages, and non-specific cytotoxic cells. Although these NK-like clones kill a number of different allogeneic targets, they display interclonal variation in cytotoxicity toward a panel of allogeneic targets, i.e. some clones have no apparent target specificity, while others display a target preference. In addition, flow cytometric analyses revealed that expression of a putative FcmuR, an LFA-1-like molecule, and a putative thymocyte/T cell antigen varies among the different clones, with no clear correlation between surface antigen expression and cytotoxic activity. Although not all clones express a putative FcmuR, it was noted that they all expressed an ITAM containing FcepsilonR gamma chain homolog. This finding suggests that the catfish FcepsilonR gamma chain may potentially be used as an accessory molecule for not only FcmuRs, but also for other unknown activation receptors. These results support the hypothesis that catfish NK-like cells are heterogeneous in terms of target specificities and cell surface phenotype.

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.

The T cell receptor beta locus of the channel catfish, Ictalurus punctatus, reveals unique features

Previously, a series of clonal alloantigen-dependent T cell lines established from the channel catfish revealed distinctly different TCR beta rearrangements. Here, a follow-up study of the junctional diversity of these TCR gene rearrangements focuses on characterization of the genomic organization of the TCRB locus. Surprisingly, a total of 29 JB genes and two substantially different CB genes were identified downstream of a single DB gene. This is in contrast to the situation in mammals, where two clusters of a DB gene, six or seven JB genes, and a CB gene are found in tandem. The catfish CB genes are approximately 36% identical at the amino acid level. All 29 catfish JB gene segments appear functional. Thirteen were used in the 19 cDNAs analyzed, of these eight were used by the 11 catfish clonal alloantigen-dependent T cell lines. As might be expected, CDR3 diversity is enhanced by N-nucleotide additions as well as nucleotide deletions at the V-D and D-J junctions. Taken together, compared with that in mammals, genomic sequencing of the catfish TCR DB-JB-CB region reveals a unique locus containing a greater number of JB genes and two distinct CB genes.

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.

Oct2 transcription factor of a teleost fish: activation domains and function from an enhancer

Oct2 transcription factors of the catfish (Ictalurus punctatus) are expressed as alternatively spliced alpha and beta isoforms. Functional analysis revealed an N-terminal glutamine (Q)-rich transactivation domain common to both isoforms of catfish Oct2. A C-terminal proline, serine, threonine (PST)-rich activation domain was identified exclusively in the beta isoform. Activation domains of fish and mammalian Oct2 showed cell type- and species-specific activity correlated with their biochemical composition (Q-rich vs PST-rich). In contrast the activation domains of the aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator of fish and mammals showed no correlation of activity with biochemical composition or species of origin. Although isolated catfish Oct2 activation domains were unable to drive transcription from a site 1.9kb distal to the promoter, Oct2beta activated transcription from both an IgH enhancer and an array of octamer motifs at this distal position. The properties of catfish Oct2 activation domains differ depending on whether they are studied in isolation or as components of the intact transcription factor.

Channel catfish cytotoxic cells: a mini-review

The use of allogeneic and autologous lymphoid cell lines has facilitated studies of cytotoxic T lymphocytes (CTL) and natural killer (NK)-like cells in channel catfish. Naïve catfish leukocytes were shown to spontaneously kill allogeneic cells and virally-infected autologous cells without the need for prior sensitization, and allogeneic cytotoxic responses were greatly enhanced by in vitro alloantigen stimulation. Both catfish CTL and NK-like cells have been successfully cloned from these alloantigen-stimulated cultures, and represent the first cytotoxic cell lines derived from any ectothermic vertebrate. These cloned cytotoxic cells contain granules and likely induce apoptosis in sensitive targets via a putative perforin/granzyme mechanism. In addition, some catfish CTL clones may also kill targets by an additional mechanism, possibly by Fas/FasL-like interactions. Importantly, these cytotoxic cells do not express the marker for catfish nonspecific cytotoxic cells (NCCs), and thus represent cell types distinct from NCCs. The use of monoclonal antibodies against the catfish F and G immunoglobulin light chain isotypes revealed the presence of a putative Fc receptor for IgM (Fc mu R) on some catfish NK-like cells that appears to 'arm' these cells with surface IgM. In addition, a potentially important monoclonal antibody (CC41) developed against catfish NK-like cells was found to recognize an approximately 150kDa molecule on the surface of catfish cytotoxic cells. These studies clearly demonstrate that catfish possess an array of different cytotoxic cells. The availability of various cloned cytotoxic cell lines should enable unambiguous functional studies to be performed in ways not currently possible with any other fish species.

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.

An IgH Enhancer That Drives Transcription through Basic Helix-Loop-Helix and Oct Transcription Factor Binding Motifs

The transcriptional enhancer (E(mu)3') of the IgH locus of the channel catfish, Ictalurus punctatus, shows strong B cell-specific activity and differs from the mammalian E(mu) enhancer in both location and structure. It occurs between the mu and delta genes and contains numerous transcription factor binding sites, predominantly octamer and muE5 motifs of consensus and variant sequences. It lacks the classical muA-muE3(CBF)-muB core array of binding motifs seen within mammalian IgH E(mu) enhancers. To determine the functionally important motifs, a series of mutant enhancers was created using sequence-targeted polymerase chain reaction. Whereas the mutation of consensus and variant octamer motifs (individually or in multiples) decreased enhancer function, mutation of a single consensus muE5 motif destroyed the function of this enhancer in mammalian plasmacytomas. Mutation of this consensus muE5 site, combined with mutations of certain octamer sites, destroyed function in catfish B cells. Experiments using artificial enhancers containing multimers of motifs or short regions of the native enhancer suggested that the minimal E(mu)3' enhancer (a) contains a consensus muE5 site and two octamer sites, (b) is B cell-specific, and (c) is active across species. The dependence of an Ig enhancer on sites that bind basic helix-loop-helix and Oct transcription factors has not previously been observed and confirms large differences in structure and function between fish and mammalian IgH enhancers.

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.

Telomerase expression and telomere length in immortal leukocyte lines from channel catfish

Normal channel catfish leukocytes readily undergo spontaneous in vitro immortalization yielding functionally active diploid cell lines. Since telomerase activation appears to be a critical step in the establishment of immortal mammalian cells, studies were undertaken to determine if and when telomerase expression occurs during the in vitro immortalization process of channel catfish leukocytes. To this end, freshly isolated peripheral blood leukocytes (PBL) from normal fish were shown to exhibit low to undetectable levels of telomerase activity and within four days after culture initiation showed dramatic increases in telomerase activity which typically remained high for at least four weeks. This activity then declined, concomitant with decreases in cellular proliferation and increases in cell death. Cells which escaped this culture "crisis" re-expressed high levels of telomerase activity indefinitely. Although telomerase activity was expressed early in the immortalization process, clonal cell lines derived from these cultures had relatively short telomeres. These results suggest that telomerase expression in catfish leukocytes is activation-induced, and its expression does not necessarily stabilize telomere length until a critically, albeit ill-defined, short length is reached.

MHC class II A genes in the channel catfish (Ictalurus punctatus)

In order to characterize the Major histocompatibility complex (MHC) class II A genes of the channel catfish (Ictalurus punctatus) a cDNA library was screened and PCR was performed. Four different full-length cDNA sequences for MHC class II A genes were obtained from a clonal B cell line derived from an outbred fish. Two different genomic sequences and corresponding cDNAs were obtained from a presumably homozygous gynogenetic catfish. The A genes have five exons and four phase one introns. The first exon encodes the 5' untranslated region (UTR) and leader peptide; the second and third exons encode the alpha1 and alpha2 domains, respectively. The connecting peptide, transmembrane and cytoplasmic domains, as well as part of the 3' UTR, are encoded by the fourth exon and the rest of the 3' UTR is encoded by the fifth exon. Southern blot analyses using an exon three probe revealed two to four hybridizing fragments with considerable restriction fragment length polymorphisms evident among randomly selected outbred channel catfish. These findings are consistent with the presence of at least two functional polymorphic MHC class II A gene loci. An unusual aspect of the channel catfish MHC class II alpha chain is its lack of N-linked glycosylation sites.

Development and analysis of various clonal alloantigen-dependent cytotoxic cell lines from channel catfish

To determine the phenotypes of cytotoxic cells in channel catfish, clonal alloantigen-dependent leukocyte lines were established from mixed leukocyte cultures. Each clone was analyzed for expression of TCR alpha and beta genes by RT-PCR and for target cell specificity by 51Cr-release assay. Based on the above criteria, the following five different cell types were identified among the 19 clones analyzed: 1) TCR alphabeta+ allospecific cytotoxic cells, 2) TCR alphabeta+ nonspecific cytotoxic cells, 3) allospecific TCR alphabeta+ noncytotoxic cells, 4) TCR alphabeta- nonspecific cytotoxic cells, and 5) TCR alphabeta- allospecific cytotoxic cells. The demonstration of cloned, TCR alphabeta+, allospecific cytotoxic effectors provides the strongest evidence to date for the existence of cytotoxic T cells in fish.

Induction of target cell apoptosis by channel catfish cytotoxic cells

This study examines cytotoxic mechanisms used by channel catfish peripheral blood-derived effector cells. Transmission electron microscopy (TEM), coupled with [(3)H]thymidine DNA fragmentation (JAM) and terminal deoxynucleotidyl nick-end labeling (TUNEL) assays, provided the first evidence that catfish peripheral blood cytotoxic effectors killed allogeneic targets via an apoptotic pathway. TEM demonstrated that the effector cell population present within peripheral blood leukocytes (PBLs) was composed of agranular lymphocytes that formed conjugates with, and induced apoptosis in, allogeneic target cells. Both JAM and TUNEL assays showed that PBLs induced target cell DNA fragmentation within 1 h of coculture. In addition, fixed effectors did not induce target cell necrosis or apoptosis, and target cell lysis was completely inhibited by chelation of free Ca(2+) by EGTA. These results suggest that catfish peripheral blood-derived effector cells utilize a secretory mechanism rather than a ligand-based mechanism to trigger apoptosis.

Molecular and Immunologic Characterization of Gynogenetic Channel Catfish (Ictalurus punctatus)

: Second-generation gynogenetic channel catfish were characterized by molecular and immunologic assays to determine if they were isogenic at major histocompatibility complex loci. Southern blot analyses, using channel catfish MHC class II B and class I A gene probes, revealed identical banding patterns among second-generation gynogenetic fish. In contrast, banding patterns from outbred fish differed not only from gynogenetic animals, but also among themselves. Nucleotide sequence analysis of the MHC class II beta(1) domain, which encompasses the peptide binding region, from four randomly selected gynogenetic fish showed a single DNA sequence. In contrast, analysis of the same region from three outbred fish showed sequences that differed not only among themselves, but also from those of gynogenetic animals. In cytotoxic assays, peripheral blood leukocytes from outbred fish lysed both gynogenetic and allogeneic targets, whereas those from gynogenetic fish lysed only allogeneic targets. Taken together, these results suggest that this group of second-generation gynogenetic channel catfish is isogenic at MHC loci and may provide an excellent system with which to study cell-mediated immunity in teleosts.

MHC class I genes of the channel catfish: sequence analysis and expression

Four cDNAs encoding the major histocompatibility complex (MHC) class I alpha chain were isolated from a channel catfish clonal B-cell cDNA library. Sequence analysis suggests these cDNAs represent three different MHC class I loci. All cDNAs encoded conserved residues characteristic of the MHC class I alpha chain: namely, those involved in peptide binding, salt bridges, disulfide bond formation, and glycosylation. Southern blot analyses of individual outbred and second-generation gynogenetic fish indicated the existence of both polygenic and polymorphic loci. Northern blot studies demonstrated that catfish B, T, and macrophage cell lines transcribed markedly higher levels of class I alpha and beta2-microglobulin (beta2m) mRNA than fibroblast cell lines. In addition, immunoprecipitation data showed that a 41 000 Mr glycoprotein (presumably class I alpha) was associated with beta2m on the surface of catfish B cells. This latter finding is the first direct evidence for the cell surface association of beta2m with the MHC class I alpha chain on teleost cells and supports the notion that functional MHC class I proteins exist in teleosts.

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

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

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

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

Identification and characterization of the tumor suppressor p53 in channel catfish (Ictalurus punctatus)

Herein is presented the sequence of a catfish full-length p53 cDNA obtained from a cloned B cell line cDNA library. Southern blot analyses determined that a restriction fragment linked polymorphism (RFLP) existed with PstI among outbred catfish. Western blot analyses demonstrated that, when compared to PBLs, the catfish leukocyte lines express higher levels of p53 protein. Additionally, the results of Western blot analyses and in vitro translation experiments suggest that the catfish leukocyte lines may produce truncated forms of p53 due to internal initiation.

Evidence for Jak-STAT interaction in channel catfish lymphoid cells

Herein is described the identification of a putative Jak molecule in the channel catfish. A commercially available polyclonal antibody against human Jak1 was shown to react in Western blot analyses with molecules of approximately 140, 80, and 60 kDa from cell lysates of catfish long-term lymphoid cell lines. It is postulated that the 140 kDa protein is a catfish Jak homolog and the 80 and 60 kDa proteins may represent breakdown products of the 140 kDa protein. Both the 140 and 60 kDa proteins appear to be constitutively tyrosine phosphorylated in channel catfish long-term leukocyte lines. Furthermore, immuno-precipitation studies indicate an association between the Jak molecule and a STAT molecule in cloned T cell lines, and to a much lesser extent in cloned B cell lines, indicative of a possible autocrine stimulatory pathway in catfish long-term T cell lines.

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

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

Characterization of Oct2 from the Channel Catfish: Functional Preference for a Variant Octamer Motif

The Ig heavy chain enhancer of the channel catfish (Ictalurus punctatus) has an unusual position and structure, being found in the 3′ region of the μ gene and containing eight functional octamer motifs of consensus (ATGCAAAT) and variant sequences. The presence of multiple octamer motifs suggests that an Oct2 homologue may play an important role in driving expression of the Ig heavy chain locus in a teleost fish. To test this hypothesis, two catfish Oct2 cDNAs (α and β) were cloned by screening a catfish B cell cDNA library. Catfish Oct2 α and β isoforms are derived by alternative RNA splicing; as determined by Southern analysis, Oct2 is a single copy gene. In comparisons with mammalian Oct2, the catfish Oct2 isoforms show high sequence conservation in their N-terminal regions and POU domains, but extensive divergence in their C-terminal regions. Catfish Oct2 α and β are tissue restricted, bind both consensus and variant octamer motifs, and activate transcription in both catfish and murine cells. In contrast, mouse Oct2 activated transcription in mouse but not catfish cells. Catfish Oct2 β is a more potent transcriptional activator than Oct2 α. In transient expression assays, catfish Oct2 β showed a marked preference for the octamer variant, ATGtAAAT, which occurs twice in the catfish enhancer. Mouse Oct2 also showed increased activity with the variant octamer when tested in mouse B cells. Gel-shift analysis competition assays indicated that catfish Oct2 binds the consensus octamer motif with an apparently higher affinity than it does the variant motif.

Characterization of Oct2 from the channel catfish: functional preference for a variant octamer motif

The Ig heavy chain enhancer of the channel catfish (Ictalurus punctatus) has an unusual position and structure, being found in the 3' region of the mu gene and containing eight functional octamer motifs of consensus (ATGCAAAT) and variant sequences. The presence of multiple octamer motifs suggests that an Oct2 homologue may play an important role in driving expression of the Ig heavy chain locus in a teleost fish. To test this hypothesis, two catfish Oct2 cDNAs (alpha and beta) were cloned by screening a catfish B cell cDNA library. Catfish Oct2 alpha and beta isoforms are derived by alternative RNA splicing; as determined by Southern analysis, Oct2 is a single copy gene. In comparisons with mammalian Oct2, the catfish Oct2 isoforms show high sequence conservation in their N-terminal regions and POU domains, but extensive divergence in their C-terminal regions. Catfish Oct2 a and beta are tissue restricted, bind both consensus and variant octamer motifs, and activate transcription in both catfish and murine cells. In contrast, mouse Oct2 activated transcription in mouse but not catfish cells. Catfish Oct2 beta is a more potent transcriptional activator than Oct2 alpha. In transient expression assays, catfish Oct2 beta showed a marked preference for the octamer variant, ATGtAAAT, which occurs twice in the catfish enhancer. Mouse Oct2 also showed increased activity with the variant octamer when tested in mouse B cells. Gel-shift analysis competition assays indicated that catfish Oct2 binds the consensus octamer motif with an apparently higher affinity than it does the variant motif.

Mitogen and growth factor-induced activation of a STAT-like molecule in channel catfish lymphoid cells

This article describes the identification of a putative STAT molecule in the channel catfish (Ictalurus punctatus), the first report of such a molecule in a 'lower' vertebrate. A monoclonal antibody against human STAT6 recognizes an approximately 100 kDa molecule that becomes activated and translocates to the nucleus upon both growth factor and mitogen stimulation of catfish leukocytes. This presumed catfish STAT binds the mammalian interferon-gamma activation site, a known motif of mammalian STAT binding, as shown by electromobility shift assays. Purification of the proteins present in these DNA complexes confirms that the catfish reactive molecule binds to the interferon-gamma activation site sequence. These results suggest that STAT molecules have been highly conserved in vertebrate evolution.

Effects of oleic acid on murine CD4+ T cell death and anti-CD3 or superantigen induced proliferation at low temperature

Studies were conducted to examine the effects of low, yet physiologically relevant, temperatures on murine T lymphocyte responses. Akin to ectothermic vertebrate responses, lectin-induced murine T cell proliferation was previously shown to be ablated at temperatures 10 degrees C below optimal (i.e. 27 degrees C); responsiveness at 27 degrees C was restored by the addition of oleic acid (18:1). The aim of the present study was to address the mechanism involved in such low temperature suppression. Murine splenic CD4+ T lymphocytes stimulated with either alpha CD3 or SEB exhibited cell death, as opposed to anergy, at 27 degrees C. However proliferation was observed in the presence of 18:1. Thus low temperature-suppression of murine CD4+ T cells is also mediated through TCR and/or CD3 pathways. Additional studies examining the temporal effects of adding 18:1, as well as temperature shifts, indicated that the cell death induced by stimulation at low temperature was preventable by 18:1.

Cytotoxic activity generated from channel catfish peripheral blood leukocytes in mixed leukocyte cultures

In previous work, lysis of allotargets was routinely observed with PBL from nonimmune channel catfish. In the work reported here, greatly increased (approximately 100-fold) cytotoxic responses were generated by stimulation of channel catfish PBL with irradiated cells of allogeneic cloned B cell lines in mixed leukocyte cultures (MLC). This increased cytotoxicity did not appear to be simply a consequence of cell proliferation since stimulation of catfish PBL proliferative responses with polyclonal mitogens did not result in increased lysis. Somewhat surprisingly, the MLC-generated cytotoxicity did not exhibit allospecificity; i.e., allogeneic targets from other fish were as susceptible to lysis as were the cells used as stimulators. This apparent lack of allospecificity in MLC-generated cytotoxicity was confirmed by "cold" target inhibition assays. However, autologous targets were not killed, clearly demonstrating that MLC-generated effectors could distinguish "self" from "nonself" at the level of lysis/recognition. Although their origin is unresolved, the MLC-generated effectors may be a source of highly enriched fish cytotoxic cells and thus facilitate directly addressing questions pertaining to the evolution of such cells.

A novel chimeric Ig heavy chain from a teleost fish shares similarities to IgD

IgD is considered to be a recently evolved Ig, being previously found only in primates and rodents. Here we describe, from a teleost fish (the channel catfish, Ictalurus punctatus), a novel complex chimeric Ig heavy chain, homologous, in part, to the heavy chain (delta) of IgD. In addition to alternative secretory or membrane-associated C termini, this chimeric molecule contains a rearranged variable domain, the first constant domain of mu, and seven constant domains encoded by a delta gene homolog. Identification of the catfish gene as delta is based on the following properties: sequence relatedness to mammalian delta; a location within the IgH locus that is immediately downstream of the mu gene; separate terminal exons for the secretory and membrane forms; coexpression with the complete mu chain in some but not all B cells. These results (i) suggest that IgD is an ancient immunoglobulin that was present in vertebrates ancestral to both the mammals and the ray-finned fishes, and (ii) raise the possibility that this Ig isotype may have served an as yet unidentified important function early in the evolution of the immune system.

Environmental effects on fish immune mechanisms

Environmental stress factors which influence fish immune (and likely many other physiological) functions can be divided into two broad, but not mutually exclusive, categories, namely those which occur naturally and those which are artificial. Natural environmental stress factors include season, temperature, salinity and photoperiod as well as social stress factors such as crowding and hierarchy. In general, artificial environmental stress factors are man made, and mainly involve pollutants such as acid rain, heavy metals and organic compounds. The available data indicate that regardless of which immune parameters are assessed, both natural and artificial environmental stress factors appear to suppress immune functions. Of the numerous environmental stress factors considered, pollutants, handling/confinement and low temperature are probably the best studied forms in fish. All three forms of stress factors have been shown to suppress components of both the innate (non-specific) and adaptive arms of the immune system. Since immune responses which protect against invading pathogens frequently involve interactions between both the innate and adaptive arms of the immune system, it seems reasonable to conclude that either acute or chronic exposure to stress factors may predispose fish to infectious diseases. Signalling mechanisms responsible for the effects of these various stress factors on immunity in fish are poorly understood, although elevated serum ACTH and cortisol levels appear to be involved in some cases. A better understanding of the mechanism(s) resulting in immunosuppression should facilitate future in vivo manipulations to reduce susceptibility to disease in aquaculture situations.

MHC class II B genes in the channel catfish (Ictalurus punctatus)

Two different cDNA sequences for major histocompatibility complex (MHC) class II beta chains from the channel catfish (Ictalurus punctatus) have been identified. Homology between these sequences and those previously identified as MHC class II B genes in other teleosts suggests they represent alleles of the DAB locus. The inferred amino acid sequences show strong evidence for a functional polypeptide chain with a peptide binding region. Southern blot analysis reveals polymorphism in the MHC class II B gene(s) of the channel catfish and suggests the presence of two to four genes.

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

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

Membrane immunoglobulin-associated molecules on channel catfish B lymphocytes

Membrane immunoglobulin (mIgM) on the surface of channel catfish B lymphocytes is non-covalently associated with 64 and 70 kDa molecules which are composed of covalent 32 kDa dimers and covalent 45/25 kDa subunits, respectively. Cross-linking of mIgM on catfish B cells leads to rapid phosphorylation of tyrosine residues in these presumed accessory as well as numerous other cytoplasmic molecules. These data indicate that fish likely use a signal transduction system containing elements similar to those of mammalian B cells.

Anti-viral cytotoxic cells in the channel catfish (Ictalurus punctatus)

Cytotoxic cells isolated from the head kidney and peripheral blood of the channel catfish appear to represent distinct subpopulations of effector cells. Previous studies showed that the former lyse xenogeneic natural killer (NK) cell targets, whereas the latter preferentially lyse allogeneic cells. Here we extend these studies and present data suggesting a third class of cytotoxic effectors responsible for killing virus-infected allogeneic and autologous cells. Peripheral blood leukocytes (PBLs) freshly isolated from unimmunized catfish lyse uninfected allogeneic target cells as well as virus-infected allogeneic and autologous cells. Cell depletion and unlabeled ("cold") target inhibition studies discriminated between putative effector classes and supported the view that at least two populations of cytotoxic cells are present within peripheral blood leukocytes. One population lyses allogeneic targets, whereas a second population kills channel catfish virus (CCV)-infected cells. In addition, inhibitor studies demonstrated that early virus gene products are sufficient to render infected cells susceptible to lysis. These results suggest that channel catfish possess distinct populations of NK-like, PBL-derived cytotoxic cells capable of lysing allogeneic and virus-infected target cells.