Fugu rubripes possesses genes for the entire set of the ITAM-bearing transmembrane signal subunits

A survey of the adaptive immune genes of the polka-dot batfish Ogcocephalus cubifrons

BACKGROUND

The anglerfish, belonging to the teleost order Lophiiformes, are a diverse and species-rich group of fish that are known to exhibit a number of unique morphological, reproductive and immunological adaptations. Work to date has identified the loss of specific adaptive immune components in two of the five Lophiiformes sub-orders (Lophioidei and Ceratioidei), while no anomalies have been identified to date in two other sub-orders, Antennaroidei and Chaunacoidei. The immunogenome of the fifth sub-order, Ogcocephaloidei has not yet been investigated, and we have therefore used whole genome shotgun sequencing, combined with RNA-seq, to survey the adaptive immune capabilities of the polka-dot batfish, O. cubifrons, as a representative of this as yet unexplored sub-order.

RESULTS

We find that the O. cubifrons genome encodes the core genes needed to mount adaptive T and B cell responses. These genes include those necessary for rearranging and editing antigen receptors, the antigen receptors themselves; as well as the co-receptors, signalling molecules, and antigen presenting molecules (both class I and class II) needed for B cell and T cell development and activation.

CONCLUSIONS

From an immune perspective, the polka-dot batfish has a canonical complement of adaptive immune genes, and does not exhibit any of the adaptive immune changes previously identified in monkfish and oceanic anglerfish.

Histocompatibility and Reproduction: Lessons from the Anglerfish

Reproduction in certain deep-sea anglerfishes involves the permanent attachment of dwarf males to much larger females and fusion of their tissues leading to the establishment of a shared circulatory system. This unusual phenomenon of sexual parasitism enables anglerfishes to maximize reproductive success in the vast and deep oceans, where females and males otherwise rarely meet. An even more surprising phenomenon relates to the observation that joining of genetically disparate male and female anglerfishes does not evoke a strong anti-graft immune rejection response, which occurs in vertebrates following allogeneic parabiosis. Recent studies demonstrated that the evolutionary processes that led to the unique mating strategy of anglerfishes coevolved with genetic changes that resulted in loss of functional genes encoding critical components of the adaptive immune system. These genetic alterations enabled anglerfishes to tolerate the histoincompatible tissue antigens of their mate and prevent the occurrence of reciprocal graft rejection responses. While the exact mechanisms by which anglerfishes defend themselves against pathogens have not yet been deciphered, it is speculated that during evolution, anglerfishes adopted new immune strategies that compensate for the loss of B and T lymphocyte functions and enable them to resist infection by pathogens.

On the relationship between extant innate immune receptors and the evolutionary origins of jawed vertebrate adaptive immunity

For over half a century, deciphering the origins of the genomic loci that form the jawed vertebrate adaptive immune response has been a major topic in comparative immunogenetics. Vertebrate adaptive immunity relies on an extensive and highly diverse repertoire of tandem arrays of variable (V), diversity (D), and joining (J) gene segments that recombine to produce different immunoglobulin (Ig) and T cell receptor (TCR) genes. The current consensus is that a recombination-activating gene (RAG)-like transposon invaded an exon of an ancient innate immune VJ-bearing receptor, giving rise to the extant diversity of Ig and TCR loci across jawed vertebrates. However, a model for the evolutionary relationships between extant non-recombining innate immune receptors and the V(D)J receptors of the jawed vertebrate adaptive immune system has only recently begun to come into focus. In this review, we provide an overview of non-recombining VJ genes, including CD8β, CD79b, natural cytotoxicity receptor 3 (NCR3/NKp30), putative remnants of an antigen receptor precursor (PRARPs), and the multigene family of signal-regulatory proteins (SIRPs), that play a wide range of roles in immune function. We then focus in detail on the VJ-containing novel immune-type receptors (NITRs) from ray-finned fishes, as recent work has indicated that these genes are at least 50 million years older than originally thought. We conclude by providing a conceptual model of the evolutionary origins and phylogenetic distribution of known VJ-containing innate immune receptors, highlighting opportunities for future comparative research that are empowered by this emerging evolutionary perspective.

ZAP70 activation is an early event of T cell immunity that involved in the anti-bacterial adaptive immune response of Nile tilapia

ZAP70 is essential for initiating the early events of T-cell antigen receptor (TCR) signaling cascade to ensure proper T cell activation and function. However, whether this molecule takes part in the T cell immune response of early vertebrates remains unclear. In the present study, using a teleost model Nile tilapia (Oreochromis niloticus), we investigated the potential involvement of ZAP70 in the T cell activation and adaptive immunity of fish species. Both primary and tertiary structures of O. niloticus ZAP70 (On-ZAP70) are highly conserved with those from other vertebrates. On-ZAP70 protein was widely expressed in lymphoid tissues, and with the highest level in thymus. Once Nile tilapia was infected by Aeromonas hydrophila, mRNA of On-ZAP70 in spleen lymphocytes was induced on day 5 and 8 after infection; meanwhile, phosphorylation of On-ZAP70 was also enhanced, suggesting that On-ZAP70 potentially participated in primary adaptive immune response of Nile tilapia. Furthermore, the frequency of ZAP70 positive lymphocytes was increased during the anti-bacterial adaptive immune response. More importantly, when spleen lymphocytes were activated by T cell specific mitogen PHA, a dramatical augment of On-ZAP70 could be observed at transcription, phosphorylation and cellular level, indicating the involvement of this molecule in T cells activation of Nile tilapia. Altogether, our results demonstrated that ZAP70 activation is an early event of T cell immunity that involved in the anti-bacterial adaptive immune response of Nile tilapia, and thus provided a new evidence to understand the evolution of the lymphocyte-mediated adaptive immunity.

Molecular cloning and expression analysis of CD79a and CD79b in rainbow trout (Oncorhynchus mykiss) after bacterial, parasitic, and viral infection

CD79a and CD79b heterodimers are important components that consist of B cell receptor compound, which play a crucial role in transduction activation signal of the antigen binding BCR, and B cell development and antibody production. In order to investigate the characters and potential functions of CD79a and CD79b in rainbow trout (Oncorhynchus mykiss), we firstly cloned and analyzed the expression of CD79a and CD79b and found that the cDNA sequences of CD79a and CD79b both contained open reading frame of 711 and 645 bp in length for encoding the protein of 237 and 215 amino acid residues, respectively. The predicted amino acid sequences from trout were highly conserved with those of other teleost fishes in structure. Phylogenetic tree was constructed to analyze the evolutionary relationship between the trout and other known species, the result indicated that CD79a and CD79b of trout clustered at high bootstrap values with Salmo salar. Moreover, three trout infection models with F. columnare G₄, I. multifiliis and infectious hematopoietic necrosis virus (IHNV) were constructed, which resulted in morphological changes and serious lesions in skin and gills. Importantly, the high expression of CD79a and CD79b occurred in skin, gills, and followed by head kidney in response to bacterial, parasitic, and viral infection, as its expression was closely related to that of Igs. Our findings indicated that CD79a and CD79b play vital roles in both systemic and mucosal immune responses of rainbow trout during bacterial, parasitic, and viral infection, which will contribute to explore the roles of CD79 subunits in B cell signaling during ontogeny and disease.

The immunogenetics of sexual parasitism

Sexual parasitism has evolved as a distinctive mode of reproduction among deep-sea anglerfishes. The permanent attachment of males to host females observed in these species represents a form of anatomical joining, which is otherwise unknown in nature. Pronounced modifications to immune facilities are associated with this reproductive trait. The genomes of species with temporarily attaching males lack functional aicda genes that underpin affinity maturation of antibodies. Permanent attachment is associated with additional alterations, culminating in the loss of functional rag genes in some species, abolishing somatic diversification of antigen receptor genes, the hallmark of canonical adaptive immunity. In anglerfishes, coevolution of innate and adaptive immunity has been disentangled, implying that an alternative form of immunity supported the emergence of this evolutionarily successful group of vertebrates.

B cell receptor accessory molecule CD79 gets involved in response against Streptococcus agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

CD79, composed of two distinct chains called CD79a and CD79b, is a transmembrane protein that forms a B cell antigen receptor with membrane immunoglobulin, and generates a signal following antigen recognition by the B cell receptor. In this study, the CD79a (OnCD79a) and CD79b (OnCD79b) were cloned and identified from Nile tilapia (Oreochromis niloticus). The cDNA of ORF for OnCD79a and OnCD79b are 669 and 627 bp, coding 222 and 208 amino acids, respectively. The deduced protein analysis showed that both CD79a andCD79b contain an immunoreceptor tyrosine-based activation motif in their intracellular tails that used to propagate a signal in a B cell. Expression analysis revealed that both CD79a and CD79b expressed at high levels in immune tissues, such as anterior kidney and spleen, and in IgM⁺ B cells. Upon Streptococcus agalactiae (S. agalactiae) infection, the expressions of OnCD79a and OnCD79b were significantly up-regulated in anterior kidney and spleen. The significant up-regulations of OnCD79a and OnCD79b were also detected in leukocytes after in vitro challenge with S. agalactiae. Further, stimulations of LPS and anti-OnIgM monoclonal antibody induced significant up-regulations of OnCD79a and OnCD79b in leukocytes. Taken together, the results of this study indicated that CD79 molecule, playing roles in BCR signaling, was likely to get involved in host defense against bacterial infection in Nile tilapia.

Zebrafish B Cell Development without a Pre-B Cell Stage, Revealed by CD79 Fluorescence Reporter Transgenes

CD79a and CD79b proteins associate with Ig receptors as integral signaling components of the B cell Ag receptor complex. To study B cell development in zebrafish, we isolated orthologs of these genes and performed in situ hybridization, finding that their expression colocalized with IgH-μ in the kidney, which is the site of B cell development. CD79 transgenic lines were made by linking the promoter and upstream regulatory segments of CD79a and CD79b to enhanced GFP to identify B cells, as demonstrated by PCR analysis of IgH-μ expression in sorted cells. We crossed these CD79-GFP lines to a recombination activating gene (Rag)2:mCherry transgenic line to identify B cell development stages in kidney marrow. Initiation of CD79:GFP expression in Rag2:mCherry⁺ cells and the timing of Ig H and L chain expression revealed simultaneous expression of both IgH-μ- and IgL-κ-chains, without progressing through the stage of IgH-μ-chain alone. Rag2:mCherry⁺ cells without CD79:GFP showed the highest Rag1 and Rag2 mRNAs compared with CD79a and CD79b:GFP⁺ B cells, which showed strongly reduced Rag mRNAs. Thus, B cell development in zebrafish does not go through a Rag^hi CD79⁺IgH-μ⁺ pre-B cell stage, different from mammals. After the generation of CD79:GFP⁺ B cells, decreased CD79 expression occurred upon differentiation to Ig secretion, as detected by alteration from membrane to secreted IgH-μ exon usage, similar to in mammals. This confirmed a conserved role for CD79 in B cell development and differentiation, without the requirement of a pre-B cell stage in zebrafish.

Grouper (Epinephelus coioides) BCR signaling pathway was involved in response against Cryptocaryon irritans infection

B cell antigen receptor (BCR) plays a crucial role in B cell development and antibody production. It comprises membrane immunoglobulin non-covalently associated with CD79a/CD79b heterodimer. After B cell activation, initial extracellular signals are transduced by BCR complex and amplified by two protein tyrosine kinases, LYN and SYK, which then trigger various pathways. In the present study, we cloned grouper genes for BCR accessory molecules, EcCD79a (669 bp) and EcCD79b (639 bp), as well as two protein tyrosine kinases, EcLYN (1482 bp) and EcSYK (1854 bp). Homology analysis showed that all four molecules had a relatively high amino acid identity compared with those in other animals. Among them, they all shared the highest identity with Takifugu rubripes (EcCD79a 49%, EcCD79b 52%, EcLYN 82% and EcSYK 77%). The conserved features and important functional residues were analyzed. Together with IgM and IgT, tissue distribution analysis showed that all six molecules were mainly expressed in immune organs, particularly systematic immune organs. In groupers infected with Cryptocaryon irritans, up-regulation of EcCD79a and b, EcIgM and EcIgT were not seen in the early stage skin and gill until 14-21 days. Up-regulation of EcCD79a was seen in head kidney at most time points, while EcCD79a and b were only significantly up-regulated in day 14 spleen. Significant up-regulation of EcIgT were seen in day 21 head kidney and day 1, day14 spleen. Significant up-regulation of EcIgM were seen in day 1 head kidney and 12 h spleen. In addition, two protein kinase genes, EcLYN and EcSYK, were up-regulated in the skin at most time points, which suggested that B cells may be activated at the skin local infection site.

The confounding complexity of innate immune receptors within and between teleost species

Teleost genomes encode multiple multigene families of immunoglobulin domain-containing innate immune receptors (IIIRs) with unknown function and no clear mammalian orthologs. However, the genomic organization of IIIR gene clusters and the structure and signaling motifs of the proteins they encode are similar to those of mammalian innate immune receptor families such as the killer cell immunoglobulin-like receptors (KIRs), leukocyte immunoglobulin-like receptors (LILRs), Fc receptors, triggering receptors expressed on myeloid cells (TREMs) and CD300s. Teleost IIIRs include novel immune-type receptors (NITRs); diverse immunoglobulin domain containing proteins (DICPs); polymeric immunoglobulin receptor-like proteins (PIGRLs); novel immunoglobulin-like transcripts (NILTs) and leukocyte immune-type receptors (LITRs). The accumulation of genomic sequence data has revealed that IIIR gene clusters in zebrafish display haplotypic and gene content variation. This intraspecific genetic variation, as well as significant interspecific variation, frequently confounds the identification of definitive orthologous IIIR sequences between teleost species. Nevertheless, by defining which teleost lineages encode (and do not encode) different IIIR families, predictions can be made about the presence (or absence) of specific IIIR families in each teleost lineage. It is anticipated that further investigations into available genomic resources and the sequencing of a variety of multiple teleost genomes will identify additional IIIR families and permit the modeling of the evolutionary origins of IIIRs.

Retention of duplicated ITAM-containing transmembrane signaling subunits in the tetraploid amphibian species Xenopus laevis

The ITAM-bearing transmembrane signaling subunits (TSS) are indispensable components of activating leukocyte receptor complexes. The TSS-encoding genes map to paralogous chromosomal regions, which are thought to arise from ancient genome tetraploidization(s). To assess a possible role of tetraploidization in the TSS evolution, we studied TSS and other functionally linked genes in the amphibian species Xenopus laevis whose genome was duplicated about 40 MYR ago. We found that X. laevis has retained a duplicated set of sixteen TSS genes, all except one being transcribed. Furthermore, duplicated TCRα loci and genes encoding TSS-coupling protein kinases have also been retained. No clear evidence for functional divergence of the TSS paralogs was obtained from gene expression and sequence analyses. We suggest that the main factor of maintenance of duplicated TSS genes in X. laevis was a protein dosage effect and that this effect might have facilitated the TSS set expansion in early vertebrates.

Chicken immunoregulatory Ig-like receptor families: an overview and expression details on ggTREM-A1

Paired immunoregulatory receptors facilitate the coordination of the immune response at the cellular level. In recent years, our group characterized chicken homologues to mammalian immunoregulatory Ig-like receptor families. The first part of this review focuses on the current progress on chicken immunoregulatory Ig-like receptor families. One of these receptors is gallus gallus TREM-A1, which was described as the only member of the chicken TREM family with activating potential. The second part of this review presents a study initiated to further characterize ggTREM-A1 expression. For this purpose we established real-time RT-PCR and generated a specific mab to analyze the expression profile of ggTREM-A1 on mRNA and protein level, respectively. GgTREM-A1 mRNA was predominantly expressed in macrophages, but was also detected in brain, bone marrow, bursa, thymus, spleen and PBMC. Analyzing ggTREM-A1 surface expression by mab staining validated the expression on macrophages. Additionally, we showed high expression on blood monocytes, heterophils and NK cells and on monocytes isolated from bone marrow. Moreover, we detected ggTREM-A1 protein also on thrombocytes, B and T cell subsets, but antigen expression seemed to be lower and more variable in these cells. Immunohistochemistry of chicken brain tissue, combining ggTREM-A1 mab and various markers specific for various brain cell subsets showed expression of ggTREM-A1 on microglial cells, but also on neurons, astrocytes and oligodendrocytes. In conclusion, ggTREM-A1 is expressed on a variety of cells, relevant for the immune system, possibly combining physiological function of different mammalian TREM.

B cell receptor accessory molecule CD79α: characterisation and expression analysis in a cartilaginous fish, the spiny dogfish (Squalus acanthias)

CD79α (also known as Igα) is a component of the B cell antigen receptor complex and plays an important role in B cell signalling. The CD79α protein is present on the surface of B cells throughout their life cycle, and is absent on all other healthy cells, making it a highly reliable marker for B cells in mammals. In this study the spiny dogfish (Squalus acanthias) CD79α (SaCD79α) is described and its expression studied under constitutive and stimulated conditions. The spiny dogfish CD79α cDNA contains an open reading frame of 618 bp, encoding a protein of 205 amino acids. Comparison of the SaCD79α gene with that of other species shows that the gross structure (number of exons, exon/intron boundaries, etc.) is highly conserved across phylogeny. Additionally, analysis of the 5' flanking region shows SaCD79α lacks a TATA box and possesses binding sites for multiple transcription factors implicated in its B cell-specific gene transcription in other species. Spiny dogfish CD79α is most highly expressed in immune tissues, such as spleen, epigonal and Leydig organ, and its transcript level significantly correlates with those of spiny dogfish immunoglobulin heavy chains. Additionally, CD79α transcription is up-regulated, to a small but significant degree, in peripheral blood cells following stimulation with pokeweed mitogen. These results strongly indicate that, as in mammals, spiny dogfish CD79α is expressed by shark B cells where it associates with surface-bound immunoglobulin to form a fully functional BCR, and thus may serve as a pan-B cell marker in future shark immunological studies.

Transcription of T cell-related genes in teleost fish, and the European sea bass (Dicentrarchus labrax) as a model

In recent years the cloning of genes coding for immuno-regulatory peptides, as well as the sequencing of genomes, provided fish immunologists with a growing amount of information on nucleotide sequences. Research is now also addressed in investigating the functional immunology counterpart of nucleotide sequence transcripts in various fish species. In this respect, studies on functional immunology of T cell activities are still at their beginning, and much work is needed to investigate T cell responses in teleost fish species. In this review we summarise the current knowledge on the group of genes coding for main T cell-related peptides in fish, and the expression levels of these genes in organs and tissues. Particular attention is paid to European sea bass (Dicentrarchus labrax), a marine species in which some information on functional immunology has been obtained, and we reassume here the expression of some T cell-related genes in basal conditions. In addition, we provide original data showing that T cells purified from the intestinal mucosa of sea bass with a specific mAb, express transcripts for TRβ, TRγ, CD8α, and RAG-1, thus showing similarities with intra-epithelial leucocytes of mammals.

T cell diversity and TcR repertoires in teleost fish

In vertebrates, the diverse and extended range of antigenic motifs is matched to large populations of lymphocytes. The concept of immune repertoire was proposed to describe this diversity of lymphocyte receptors--IG and TR--required for the recognition specificity. Immune repertoires have become useful tools to describe lymphocyte and receptor populations during the immune system development and in pathological situations. In teleosts, the presence of conventional T cells was first proposed to explain graft rejection and optimized specific antibody production. The discovery of TR genes definitely established the reality of conventional T cells in fish. The development of genomic and EST databases recently led to the description of several key T cell markers including CD4, CD8, CD3, CD28, CTLA4, as well as important cytokines, suggesting the existence of different T helper (Th) subtypes, similar to the mammalian Th1, Th2 and Th17. Over the last decade, repertoire studies have demonstrated that both public and private responses occur in fish as they do in mammals, and in vitro specific cytotoxicity assays have been established. While such typical features of T cells are similar in both fish and mammals, the structure of particular repertoires such as the one of gut intra-epithelial lymphocytes seems to be very different. Future studies will further reveal the particular characteristics of teleost T cell repertoires and adaptive responses.

Processing of fish Ig heavy chain transcripts: diverse splicing patterns and unusual nonsense mediated decay

While the diversification of the antigen-binding sites is realized by genomic VDJ rearrangements during B cell differentiation, different forms of immunoglobulin (Ig) heavy (H) chains can be produced through multiple splicing pathways. In most vertebrates, the secreted (S) and membrane (Mb) forms of IgM chain are created by alternative splicing through usage of a cryptic splice site in Cμ4 allowing the junction to the TM exon. The processing pattern for Igμ is different in teleosts, which generally use the Cμ3 donor site instead. In ancient fish lineages, multiple unusual splicing patterns were found for Ig H chain, involving donor sites that do not always follow the classical consensus. The production of IgD versus IgM H chains seems to be generally realized by alternative splicing in all vertebrates, but typical teleost IgD H chains are chimeric and contains a Cμ1 domain. Together, these observations raise questions on how different fish regulate RNA splicing and if their splicing machinery is especially complex. A preliminary scan of the zebrafish and stickleback genomes provides evidence that gene orthologs to the mammalian main splice factors are highly conserved as single copy genes, while the snRNPs U repertoire may be different and may explain other particular features of RNA processing in fish.

Immunoglobulin genes and their transcriptional control in teleosts

Immunoglobulin (Ig), which exists only in jawed vertebrates, is one of the most important molecules in adaptive immunity. In the last two decades, many teleost Ig genes have been identified by in silico data mining from the enormous gene and EST databases of many fish species. In this review, the organization of Ig gene segments, the expressed Ig isotypes and their transcriptional controls are discussed. The Ig heavy chain (IgH) locus in teleosts encodes the variable (V), the diversity (D), the joining (J) segments and three different isotypic constant (C) regions including Cμ, Cδ, and Cζ/τ genes, and is organized as a "translocon" type like the IgH loci of higher vertebrates. In contrast, the Ig light (L) chain locus is arranged in a "multicluster" or repeating set of VL, JL, and CL segments. The IgL chains have four isotypes; two κ L1/G and L3/F), σ (L2) and λ. The transcription of IgH genes in teleosts is regulated by a VH promoter and the Eμ3' enhancer, which both function in a B cell-specific manner. The location of the IgH locus, structure and transcriptional function of the Eμ3' enhancer are important to our understanding of the evolutional changes that have occurred in the IgH gene locus.

The chicken leukocyte receptor cluster

Receptors of the immunoglobulin-like superfamily are critically involved in virtually every aspect of immune responses. One large chromosomal area encoding such immunoregulatory receptors is the leukocyte receptor cluster. Here we review various aspects of the chicken Ig-like receptor (CHIR) family, located on microchromosome 31, an orthologous position to the mammalian leukocyte receptor cluster. The CHIR family has been massively expanded with over hundred CHIR genes that are further distinguished into activating, inhibitory and bifunctional receptors. Comparisons of various features such as amino acid motifs, genomic structure, expression and associated adaptor molecules reveal the homology of CHIR to both the killer Ig-like and the leukocyte Ig-like receptor families, with most pronounced correlation of certain CHIR to the NK cell receptor KIR2DL4. To date the CHIR ligands remain largely obscure with the exception of CHIR-AB1 that binds to chicken IgY. Detailed analyses of CHIR-AB1, its crystal structure, the interaction to IgY and functional capabilities allow us to draw conclusions regarding Fc receptor phylogeny and function.

A molecule in teleost fish, related with human MHC-encoded G6F, has a cytoplasmic tail with ITAM and marks the surface of thrombocytes and in some fishes also of erythrocytes

In teleost fish, a novel gene G6F-like was identified, encoding a type I transmembrane molecule with four extracellular Ig-like domains and a cytoplasmic tail with putative tyrosine phosphorylation motifs including YxN and an immunoreceptor tyrosine-based activation motif (ITAM). G6F-like maps to a teleost genomic region where stretches corresponding to human chromosomes 6p (with the MHC), 12p (with CD4 and LAG-3), and 19q are tightly linked. This genomic organization resembles the ancestral "Ur-MHC" proposed for the jawed vertebrate ancestor. The deduced G6F-like molecule shows sequence similarity with members of the CD4/LAG-3 family and with the human major histocompatibility complex-encoded thrombocyte marker G6F. Despite some differences in molecular organization, teleost G6F-like and tetrapod G6F seem orthologous as they map to similar genomic location, share typical motifs in transmembrane and cytoplasmic regions, and are both expressed by thrombocytes/platelets. In the crucian carps goldfish (Carassius auratus auratus) and ginbuna (Carassius auratus langsdorfii), G6F-like was found expressed not only by thrombocytes but also by erythrocytes, supporting that erythroid and thromboid cells in teleost fish form a hematopoietic lineage like they do in mammals. The ITAM-bearing of G6F-like suggests that the molecule plays an important role in cell activation, and G6F-like expression by erythrocytes suggests that these cells have functional overlap potential with thrombocytes.

Molecular characterisation of the CD79a and CD79b subunits of the B cell receptor complex in the gray short-tailed opossum (Monodelphis domestica) and tammar wallaby (Macropus eugenii): Delayed B cell immunocompetence in marsupial neonates

The B cell receptor (BCR) is a multiprotein complex that is pivotal to antigen recognition and signal transduction in B cells. It consists of an antigen binding component, membrane Ig (mIg), non-covalently associated with the signaling component, a disulphide-linked heterodimer of CD79a and CD79b. In this study, the gene and corresponding cDNA for CD79a and CD79b in the gray short-tailed opossum, as well as the cDNA sequences for CD79a and CD79b in the tammar wallaby, are described. Many of the structural and functional features of CD79a and CD79b were conserved in both marsupials, including the ITAM regulatory motif in the cytoplasmic tails of both subunits. The marsupial CD79 sequences shared a high degree of amino acid identities of 76% (CD79a) and 72% (CD79b) to each other, as well as 60-61% (CD79a) and 58-59% (CD79b) with their eutherian counterparts. RT-PCR analysis of CD79a and CD79b transcripts in the immune tissues of tammar pouch young revealed CD79a transcripts in the bone marrow, cervical thymus and spleen at day 10 postpartum. CD79b transcripts were detected in the bone marrow and cervical thymus at day 10 but were not detected in the spleen until day 21 postpartum. These results suggest that a functional BCR may not be assembled until day 21 postpartum and the tammar neonate may not be capable of mounting an effective adaptive immune response until this time. The molecular information presented here will allow further investigation of the role of the CD79 subunits in marsupial B cell signaling, especially during ontogeny and disease.

Rainbow trout (Oncorhynchus mykiss) possess multiple novel immunoglobulin-like transcripts containing either an ITAM or ITIMs

The recognition of pathogens by the innate immune system relies on a wide range of inhibitory and activating receptors. Some of these non-rearranging receptors belong to the immunoglobulin superfamily (IgSF) and in teleost fish the novel immune-type receptor (NITR) and the novel immunoglobulin-like transcript (NILT) have been reported. Here we describe the identification and characterisation of three new NILTs from rainbow trout (Oncorhynchus mykiss), with one NILT alternatively spliced into a long isoform containing two Ig domains and a short isoform containing one Ig domain. The cytoplasmic regions contain either immunoreceptor tyrosine-based inhibitory motifs (ITIMs) or an immunoreceptor tyrosine-based activating motif (ITAM) for downstream signalling. Alignment of the various NILT Ig domains revealed a high similarity, especially between Ig domains from NILTs found in this study. Furthermore, a phylogenetic tree showed that NILTs are more closely related to the triggering receptor expressed on myeloid (TREM) cells and NKp44 than to NITRs. The expression of NILTs was studied in six different tissues and two different cell lines, with expression apparent in immunologically important tissues. Expression of NILTs was also shown to be an early event in development, with both eyed eggs and embryos expressing all four genes. The results obtained in this study and future experiments will contribute to our knowledge of the immune system in fish and provide useful information for the control of inflammatory processes in rainbow trout.

Diverse splicing pathways of the membrane IgHM pre-mRNA in a Chondrostean, the Siberian sturgeon

Teleosts and tetrapods have evolved different splice patterns to generate their membrane-bound IgM. In the tetrapod lineage, the first transmembrane exon is spliced to an internal cryptic site located close to the end of the fourth constant exon. Because teleosts lack this site they use the regular 3'-splice site of the CH3 exon instead. We characterized the mum splicing patterns in a Chondrostean, the Siberian sturgeon. We observed a surprising diversity of splice patterns, the TM1 exon being spliced to a cryptic site at the end of CH4, to a cryptic site in CH3 or to the 3'-end of CH1. These different pathways lead to mIGHM transcripts encoding four, two or one complete C-domain(s), respectively. The short variant CH1-TM1 was found only in VH2 positive transcripts, while the two other variants were observed for IgHM transcripts expressing all VH families. These results shed light on the evolution of IgM splicing pathways.

DAP10- and DAP12-associated receptors in innate immunity

The DAP10 and DAP12 signaling subunits are highly conserved in evolution and associate with a large family of receptors in hematopoietic cells, including dendritic cells, plasmacytoid dendritic cells, neutrophils, basophils, eosinophils, mast cells, monocytes, macrophages, natural killer cells, and some B and T cells. Some receptors are able to associate with either DAP10 or DAP12, which contribute unique intracellular signaling functions. Studies of humans and mice deficient in these signaling subunits have provided surprising insights into the physiological functions of DAP10 and DAP12, demonstrating that they can either activate or inhibit immune responses. DAP10- and DAP12-associated receptors have been shown to recognize both host-encoded ligands and ligands encoded by microbial pathogens, indicating that they play an important role in innate immune responses.

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 the CD3zeta, CD3gammadelta and CD3epsilon subunits of the T cell receptor complex in Atlantic salmon

The CD3 subunits are essential components of the T cell receptor complex, transmitting signals to the inside of the cell. We report here cDNAs and corresponding genes encoding CD3zeta, CD3gammadelta and CD3epsilon in Atlantic salmon, and real-time RT-PCR analysis to reveal their tissue-specific expression. Salmon CD3zeta is the subunit that shows the highest sequence similarity to the mammalian counterparts, comprising of a short extracellular (EX) part, a transmembrane (TM) peptide and a long cytoplasmic (CY) tail with three immunoreceptor tyrosine-based activation motifs (ITAMs). The gene encoding CD3zeta in salmon has 7 exons. Salmon CD3gammadelta (a forerunner of CD3gamma and CD3delta in mammals) and CD3epsilon are related molecules each having an Ig-like EX domain, a TM peptide and a CY tail with one ITAM. Two distinct CD3gammadelta genes were found, each having 6 exons. The gene encoding CD3epsilon in salmon has 5 exons. RT-PCR also revealed a transcript from a degenerated CD3epsilon gene in salmon (Salmo salar) and brown trout (Salmo trutta). This pseudogene is located tail to tail to a CD3gammadelta gene in salmon and has a typical CD3epsilon gene structure with the exception of 1 extra exon. All the CD3 genes in salmon were most abundantly expressed in thymus but the expression of the CD3epsilon pseudogene was only a fraction of that from the intact CD3epsilon gene.

Genomic organization and expression of CD8alpha and CD8beta genes in fugu Takifugu rubripes

We have cloned cDNAs encoding the alpha and beta chains of CD8 from the tiger pufferfish (fugu), Takifugu rubripes. The cDNA sequences encode a putative leader peptide, extracellular immunoglobulin variable region-like domain, stalk region, transmembrane region, and cytoplasmic tail. A protein tyrosine kinase p56lck binding motif was not found in the putative fugu CD8alpha cytoplasmic tail. O-linked glycosylation sites were found in the stalk of both CD8 chains, suggesting possible stalk formation. Phylogenetic analysis showed that fugu CD8alpha and CD8beta chains cluster with other vertebrate CD8alpha and CD8beta chains, respectively. The fugu CD8 genes comprise six exons separated by five introns. The genes are tandemly aligned 3.6 kb apart and are in the same transcription orientation. Quantitative RT-PCR analysis demonstrated that fugu CD8 is expressed predominantly in lymphoid tissues. In situ hybridization showed that fugu CD8 genes are expressed in thymocytes and lymphocytes within lymphoid organs. Molecular characterization of CD8 in fish provides the basis for development of specific antibodies to identify T-cell subsets, as well as potentially important insights into the evolution of CD8 and the adaptive immunity.

Structural characteristics of zebrafish orthologs of adaptor molecules that associate with transmembrane immune receptors

Transmembrane bound receptors comprised of extracellular immunoglobulin (Ig) or lectin domains play integral roles in a large number of immune functions including inhibitory and activating responses. The function of many of the activating receptors requires a physical interaction with an adaptor protein possessing a cytoplasmic regulatory motif. The partnering of an activating receptor with an adaptor protein relies on complementary charged residues in the two transmembrane domains. The mammalian natural killer (NK) and Fc receptors (FcR) represent two of many receptor families, which possess activating receptors that partner with adaptor proteins for signaling. Zebrafish represent a powerful experimental model for understanding developmental regulation at early stages of embryogenesis and for efficiently generating transgenic animals. In an effort to understand developmental aspects of immune receptor function, we have accessed the partially annotated zebrafish genome to identify six different adaptor molecules: Dap10, Dap12, Cd3zeta, Cd3zeta-like, FcRgamma and FcRgamma-like that are homologous to those effecting immune function in mammals. Their genomic organizations have been characterized, cDNA transcripts have been recovered, phylogenetic relationships have been defined and their cell lineage-specific expression patterns have been established.

Siglec-15: an immune system Siglec conserved throughout vertebrate evolution

Siglecs are vertebrate cell-surface receptors that recognize sialylated glycans. Here we have identified and characterized a novel Siglec, named Siglec-15. Siglec-15 is a type-I transmembrane protein consisting of: (i) two immunoglobulin (Ig)-like domains, (ii) a transmembrane domain containing a lysine residue, and (iii) a short cytoplasmic tail. Siglec-15 is expressed on macrophages and/or dendritic cells of human spleen and lymph nodes. We show that the extracellular domain of Siglec-15 preferentially recognizes the Neu5Acalpha2-6GalNAcalpha- structure. Siglec-15 associates with the activating adaptor proteins DNAX activation protein (DAP)12 and DAP10 via its lysine residue in the transmembrane domain, implying that it functions as an activating signaling molecule. Siglec-15 is the second human Siglec identified to have an activating signaling potential; unlike Siglec-14, however, it does not have an inhibitory counterpart. Orthologs of Siglec-15 are present not only in mammals but also in other branches of vertebrates; in contrast, no other known Siglec expressed in the immune system has been conserved throughout vertebrate evolution. Thus, Siglec-15 probably plays a conserved, regulatory role in the immune system of vertebrates.

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.

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.

An evolutionarily mobile antigen receptor variable region gene: doubly rearranging NAR-TcR genes in sharks

Distinctive Ig and T cell receptor (TcR) chains define the two major lineages of vertebrate lymphocyte yet similarly recognize antigen with a single, membrane-distal variable (V) domain. Here we describe the first antigen receptor chain that employs two V domains, which are generated by separate VDJ gene rearrangement events. These molecules have specialized "supportive" TcRdeltaV domains membrane-proximal to domains with most similarity to IgNAR V. The ancestral NAR V gene encoding this domain is hypothesized to have recombined with the TRD locus in a cartilaginous fish ancestor >200 million years ago and encodes the first V domain shown to be used in both Igs and TcRs. Furthermore, these data support the view that gamma/delta TcRs have for long used structural conformations recognizing free antigen.

Costimulatory Receptors in a Teleost Fish: Typical CD28, Elusive CTLA4

T cell activation requires both specific recognition of the peptide-MHC complex by the TCR and additional signals delivered by costimulatory receptors. We have identified rainbow trout sequences similar to CD28 (rbtCD28) and CTLA4 (rbtCTLA4). rbtCD28 and rbtCTLA4 are composed of an extracellular Ig-superfamily V domain, a transmembrane region, and a cytoplasmic tail. The presence of a conserved ligand binding site within the V domain of both molecules suggests that these receptors likely recognize the fish homologues of the B7 family. The mRNA expression pattern of rbtCD28 and rbtCTLA4 in naive trout is reminiscent to that reported in humans and mice, because rbtCTLA4 expression within trout leukocytes was quickly up-regulated following PHA stimulation and virus infection. The cytoplasmic tail of rbtCD28 possesses a typical motif that is conserved in mammalian costimulatory receptors for signaling purposes. A chimeric receptor made of the extracellular domain of human CD28 fused to the cytoplasmic tail of rbtCD28 promoted TCR-induced IL-2 production in a human T cell line, indicating that rbtCD28 is indeed a positive costimulator. The cytoplasmic tail of rbtCTLA4 lacked obvious signaling motifs and accordingly failed to signal when fused to the huCD28 extracellular domain. Interestingly, rbtCTLA4 and rbtCD28 are not positioned on the same chromosome and thus do not belong to a unique costimulatory cluster as in mammals. Finally, our results raise questions about the origin and evolution of positive and negative costimulation in vertebrate immune systems.

Cytotoxic activities of fish leucocytes

Like mammalian leucocytes, white blood cells of fish are able to kill altered (e.g. virus-infected) and foreign (allogeneic or xenogeneic) cells. The existence of natural killer (NK)-like and specific cytotoxic cells in fish was first shown using allogeneic and xenogeneic effector/target cell systems. In addition to in vivo and ex vivo studies, very important contributions were made by in vitro analysis using a number of different long-term cytotoxic cell lines established from channel catfish. In mammals, specific cell-mediated cytotoxicity (CMC) as part of the adaptive immune response requires a number of key molecules expressed on effector leucocytes and target cells. CD8+ T lymphocytes kill infected cells only, if their antigen receptor (TCR) matches the MHC class I with bound peptide of the target cell. Expression patterns of the fish gene homologues for TCR, CD8 and MHC class I, as well as related genes, are in agreement with similar function. Convenient systems for the analysis of specific CMC have only recently become available for fish with the combination of clonal fish with syngeneic or allogeneic but MHC class I matching cell lines. It was demonstrated that both, NK- and cytotoxic T (Tc) cells are involved in the killing of virus infected MHC class I matching and mismatching target cells. Analysis of these lymphocyte subsets is only starting for fish. There is also evidence that the different viral proteins trigger different subsets of killer cells. This review further discusses findings on fish CMC with regard to temperature/seasons and ontogeny.

KARAP/DAP12/TYROBP: three names and a multiplicity of biological functions

The signaling adaptor protein KARAP/DAP12/TYROBP (killer cell activating receptor-associated protein / DNAX activating protein of 12 kDa / tyrosine kinase binding protein) belongs to the family of transmembrane polypeptides bearing an intracytoplasmic immunoreceptor tyrosine-based activation motif (ITAM). This adaptor, initially characterized in NK cells, is associated with multiple cell-surface activating receptors expressed in both lymphoid and myeloid lineages. We review here the main features of KARAP/DAP12, describing findings from its identification to recently published data, showing its involvement in a broad array of biological functions. KARAP/DAP12 is a wiring component for NK cell anti-viral function (e.g. mouse cytomegalovirus via its association with mouse Ly49H) and NK cell anti-tumoral function (e.g. via its association with mouse NKG2D or human NKp44). KARAP/DAP12 is also involved in inflammatory reactions via its coupling to myeloid receptors, such as the triggering receptors expressed by myeloid cells (TREM) displayed by neutrophils, monocytes/macrophages and dendritic cells. Finally, bone remodeling and brain function are also dependent upon the integrity of KARAP/DAP12 signals, as shown by the analysis of KARAP/DAP12-deficient mice and KARAP/DAP12-deficient Nasu-Hakola patients.

Identification and characterization of a second CD4-like gene in teleost fish

In fish, T cell subdivision is not well studied, although CD8 and CD4 homologues have been reported. This study describes a second teleost CD4-like gene, CD4-like 2 (CD4L-2). Two rainbow trout copies of this gene were found, -2a and -2b, encoding molecules sharing 81% aa identity. The 2a/2b duplication may be related to tetraploid ancestry of salmonid fishes. In the Fugu genome CD4L-2 lies head to tail with an earlier reported, very different CD4-like gene [Suetake, H., Araki, K., Suzuki, Y., 2004. Cloning, expression, and characterization of fugu CD4, the first ectothermic animal CD4. Immunogenetics 56, 368-374], which was designated CD4L-1 in the present article. The flanking genes of the Fugu CD4L-1 and CD4L-2 are reminiscent of the genes surrounding CD4 and LAG-3 in mammals. However, neither synteny nor phylogenetic analysis could decide between CD4 and LAG-3 identity for the fish CD4L genes. CD4L-1 and CD4L-2 share a tyrosine protein kinase p56(lck) binding motif in the cytoplasmic tail with CD4 but not with LAG-3. Trout CD4L-2 expression is highest in the thymus, similar to mammalian and chicken CD4, whereas Fugu CD4L-1 expression was highest in the spleen. However, CD4L-2 encodes only two IG-like domains, whereas CD4L-1, CD4 and LAG-3 encode four. The CD4-like genes 1 and 2 in fish apparently went through an evolution different from that of LAG-3 and CD4 in higher vertebrates.

Characterization and expression analysis of CD3? and CD3?/? in fugu, Takifugu rubripes

CD3 is an essential component of the CD3-TCR complex. In this report, we describe the cloning, characterization, and expression analysis of the CD3varepsilon and CD3gamma/delta chain genes from fugu, Takifugu rubripes. Two distinct CD3varepsilon homologue cDNAs, designated as CD3varepsilon-1 and CD3varepsilon-2, and a CD3gamma/delta homologue cDNA were isolated from the fugu thymus. The deduced amino acid sequences of these cDNAs exhibit conserved essential CD3 chain motifs and overall structures. RT-PCR analysis demonstrated that the CD3varepsilon and CD3gamma/delta genes were expressed in lymphoid organs (e.g. thymus, head kidney, trunk kidney and spleen), mucosal tissues (gill, skin, and intestine), and peripheral blood leucocytes (PBL). The CD3 and TCRalpha genes were expressed only in the surface IgM- population, which were separated from PBL using an anti-fugu IgM monoclonal antibody. In addition, in situ hybridization confirmed that CD3-expressing cells were distributed randomly in the head kidney, trunk kidney, and spleen, but in the thymus were restricted to the lymphoid outer zone and epithelioid inner zone only. Collectively, these results suggest that CD3 molecules are useful markers for the identification of T cells in teleost fish. The present study thus provides a critical step in identifying T cells in this model organism.