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

IL-4/13 expressing CD3γ/δ+ T cells regulate mucosal immunity in response to Flavobacterium columnare infection in grass carp

Interleukin (IL) 4 and 13 are signature cytokines orchestrating Th2 immune response. Teleost fish have two homologs, termed IL-4/13A and IL-4/13B, and have been functionally characterized. However, what cells express IL-4/13A and IL-4/13B has not been investigated in fish. In this work, the recombinant IL-4/13A and IL-4/13B proteins of grass carp (Ctenopharyngodon idella) were produced in the Escherichia coli (E. coli) cells and purified. Monoclonal antibodies (mAbs) against the recombinant CiIL-4/13A and CiIL-4/13B proteins were prepared and characterized. Western blotting analysis showed that the CiIL-4/13A and CiIL-4/13B mAbs could specifically recognize the recombinant proteins expressed in the E. coli cells and HEK293T cells and did not cross-react with each other. Confocal microscopy revealed that the CiIL-4/13A⁺ and CiIL-4/13B⁺ cells were present in the gills, intestine and spleen and could be upregulated in fish infected with Flavobacterium columnare (F. columnare). Interestingly, the cells expressing CiIL-4/13A and CiIL-4/13B were mostly CD3γ/δ⁺ cells. The CD3γ/δ⁺/IL-4/13A⁺ and CD3γ/δ⁺/IL-4/13B⁺ cells were significantly upregulated in the gill filaments and the intestinal mucosa after F. columnare infection. Our results imply that the CD3γ/δ⁺/IL-4/13A⁺ and CD3γ/δ⁺/IL-4/13B⁺ cells are important for homeostasis and the regulation of mucosal immunity.

Characterization of CD3γ/δ gene and its immune response in Qihe crucian carp Carassius auratus after challenged by Aeromonas veronii and Poly(I:C)

CD3γ/δ found in non-mammalian vertebrates is a CD3 homolog with structural characteristics similar to both mammalian CD3γ and CD3δ, and plays important roles in T cell recognization and immune response in fish. In this study, the full-length of CD3γ/δ from Qihe crucian carp (named CaCD3γ/δ) was cloned and characterized, then the expression response profiles and potential immune functions was explored after Aeromonas veronii and Poly(I:C) challenge. The results showed that the full-length of CaCD3γ/δ was 819 bp including a 5'-UTR of 141 bp, a 3'-UTR of 168 bp, and an ORF of 510 bp encoding a putative 169-aa protein with an estimated MW of 18.71 kD and a theoretical pI of 8.77. The protein sequence of CaCD3γ/δ contained a Leu-Leu and a CXXXC motif in the extracellular domain, and an ITAM and a Leu-Ile motif in the cytoplasm, and a residue of Asn in the transmembrane. CaCD3γ/δ was constitutively expressed in the spleen, liver, gill, and blood of Qihe crucian carp. After the carp were challenged with Poly(I:C) and Aeromonas veronii, the mRNA expression levels of CaCD3γ/δ were significantly changed in the spleen, head kidney, intestine and gill, according to the results of qPCR. However, compared with A. veronii, Poly(I:C) challenge can rapidly induce the CaCD3γ/δ expression levels in head kidney, intestine and spleen, which suggested CaCD3γ/δ may be differentially modulated by different pathogens. Moreover, the results of immunohistochemical analysis showed that the CaCD3γ/δ⁺ secreted cells in the spleen and gills of Qihe crucian were increased after challenged with Poly(I:C), as well as the spleen challenged with A. veronii, but at different levels. Combined with the fact that vascular congestion, necrosis of parenchymal cells, and inflammatory cells including lymphocytes infiltration were also observed in the gill and spleen of Qihe crucian carp treated with A. veronii and Poly(I:C) revealed by pathological analysis, it was predicted that CaCD3γ/δ⁺ T lymphocytes may participated in the immune response against pathogens. This study will contribute to understand the important role of CaCD3γ/δ⁺ T lymphocytes in the immune response of Qihe crucian carp, and provide new insights for the prevention and treatment of the diseases of Qihe crucian carp.

Cluster of differentiation antigens: essential roles in the identification of teleost fish T lymphocytes

Cluster of differentiation (CD) antigens are cell surface molecules expressed on leukocytes and other cells associated with the immune system. Antibodies that react with CD antigens are known to be one of the most essential tools for identifying leukocyte subpopulations. T lymphocytes, as an important population of leukocytes, play essential roles in the adaptive immune system. Many of the CD antigens expressed on T lymphocytes are used as surface markers for T lymphocyte classification, including CD3, CD4 and CD8 molecules. In this review, we summarize the recent advances in the identification of CD molecules on T lymphocytes in teleosts, with emphasis on the functions of CD markers in the classification of T lymphocyte subsets. We notice that genes encoding CD3, co-receptors CD4 and CD8 have been cloned in several fish species and antibodies have been developed to study protein expression in morphological and functional contexts. T lymphocytes can be divided into CD4⁺ and CD8⁺ cells discriminated by the expression of CD4 and CD8 molecules in teleost, which are functionally similar to mammalian helper T cells (Th) and cytotoxic T cells (Tc), respectively. Further studies are still needed on the particular characteristics of teleost T cell repertoires and adaptive responses, and results will facilitate the health management and development of vaccines for fish.

Molecular and cellular characterization of European sea bass CD3ε+ T lymphocytes and their modulation by microalgal feed supplementation

The CD3 coreceptor is a master T cell surface marker, and genes encoding CD3ζ, γδ, and ε chains have been reported in several teleost fish. Here, a complete cDNA sequence of CD3ɛ chain was identified from a sea bass (Dicentrarchus labrax L.) gill transcriptome. Its basal expression was quantified in both lymphoid and non-lymphoid organs of sea bass juveniles with real-time qPCR analysis. After either in vitro stimulation of head kidney leukocytes with the T-cell mitogen phytohaemagglutinin or in vivo stimulation with an orally administered Vibrio anguillarum vaccine, CD3ε expression levels increased in head kidney leukocytes, confirming that CD3ε T cells may play important roles in fish systemic protection against pathogens. Further, three peptides were designed on the CD3ɛ cytoplasmic tail region and employed as immunogens for antibody production in rabbit. One antiserum so obtained, named RACD3/1, immunostained a band of the expected size in a western blot of a sea bass thymocyte lysate. The distribution of CD3ε⁺ lymphocyte population in the lymphoid organs and mucosal tissues was addressed in healthy fish by IHC. In decreasing percentage order, CD3ε⁺ lymphocytes were detected by flow cytometry in thymus, peripheral blood leukocytes, gills, head kidney, gut, and spleen. Finally, a significant in vivo enhancement of CD3ε⁺ T intestinal lymphocytes was found in fish fed on diets in which 100% fish meal was replaced by the microalgae Nannochloropsis sp. biomass. These results indicate that CD3ε⁺ T cells are involved in nutritional immune responses.

Characterization of CD3γ/δ+ cells in grass carp (Ctenopharyngodon idella)

CD3 is an essential component of the TCR-CD3 complex which plays a key role in adaptive immunity. Non-mammalian CD3 complex consists of CD3γ/δ, CD3ε and CD3ζ subunits. In this study, homologues of CD3γ/δ and CD3ε (termed CiCD3γ/δ and CiCD3ε) have been identified in grass carp (Ctenopharyngodon idella). Like their counterparts from other vertebrates, the CiCD3γ/δ and CiCD3ε are clustered in the same locus in the genome and encode proteins which are structurally conserved, comprising a signal peptide, an extracellular domain, a transmembrane domain and a cytoplasmic tail containing two ITAM motifs. Sequence analyses identified two novel conserved motifs in the cytoplasmic tail of CiCD3γ/δ and CiCD3ε, one is composed of an arginine and lysine motif (RK or RR) at the C terminus of CiCD3γ/δ and a proline rich domain (PxxPxP/Q) located at the N terminus of ITAM motifs of CiCD3ε. Both genes were highly expressed at the mRNA level in the spleen and gills of healthy fish and could be modulated by infection of Flavobacterium columnare and grass carp reovirus. A monoclonal antibody against the CiCD3γ/δ (GC38T) was produced and showed good reactivity with the native molecule in Western blotting analysis and flow cytometry. The CiCD3γ/δ⁺ cells were analysed in the primary leucocytes, accounting for 5.5% of lymphocytes isolated from spleen, 4.5% from head kidney and 2.8% from peripheral blood. The CiCD3γ/δ⁺ cells were localized in the gills and head kidney by fluorescent confocal microscopy.

Revisiting the Teleost Thymus: Current Knowledge and Future Perspectives

Simple Summary

The thymus is the immune organ producing T lymphocytes that are essential to create immunity after encountering pathogens or vaccination. This review summarizes the thymus localization and histological studies, cell composition, and function in teleost fishes. We also describe how seasonal changes, photoperiod, water temperature fluctuations, and hormones can affect thymus development in fish species. Overall, the information helps identify future studies needed to understand thymus function in fish species and the immune system’s evolutionary origins. Since fish are exposed to pathogens, especially under aquaculture conditions, knowledge about the fish thymus and T lymphocyte can also help improve fish farming protocols, considering intrinsic and environmental conditions that can contribute to achieving the best vaccine responsiveness for disease resistance.

Abstract

The thymus in vertebrates plays a critical role in producing functionally competent T-lymphocytes. Phylogenetically, the thymus emerges early during evolution in jawed cartilaginous fish, and it is usually a bilateral organ placed subcutaneously at the dorsal commissure of the operculum. In this review, we summarize the current understanding of the thymus localization, histology studies, cell composition, and function in teleost fishes. Furthermore, we consider environmental factors that affect thymus development, such as seasonal changes, photoperiod, water temperature fluctuations and hormones. Further analysis of the thymus cell distribution and function will help us understand how key stages for developing functional T cells occur in fish, and how thymus dynamics can be modulated by external factors like photoperiod. Overall, the information presented here helps identify the knowledge gaps and future steps needed for a better understanding of the immunobiology of fish thymus.

Teleost cytotoxic T cells

Cell-mediated cytotoxicity is one of the major mechanisms by which vertebrates control intracellular pathogens. Two cell types are the main players in this immune response, natural killer (NK) cells and cytotoxic T lymphocytes (CTL). While NK cells recognize altered target cells in a relatively unspecific manner CTLs use their T cell receptor to identify pathogen-specific peptides that are presented by major histocompatibility (MHC) class I molecules on the surface of infected cells. However, several other signals are needed to regulate cell-mediated cytotoxicity involving a complex network of cytokine- and ligand-receptor interactions. Since the first description of MHC class I molecules in teleosts during the early 90s of the last century a remarkable amount of information on teleost immune responses has been published. The corresponding studies describe teleost cells and molecules that are involved in CTL responses of higher vertebrates. These studies are backed by functional investigations on the killing activity of CTLs in a few teleost species. The present knowledge on teleost CTLs still leaves considerable room for further investigations on the mechanisms by which CTLs act. Nevertheless the information on teleost CTLs and their regulation might already be useful for the control of fish diseases by designing efficient vaccines against such diseases where CTL responses are known to be decisive for the elimination of the corresponding pathogen. This review summarizes the present knowledge on CTL regulation and functions in teleosts. In a special chapter, the role of CTLs in vaccination is discussed.

Peculiar Expression of CD3-Epsilon in Kidney of Ginbuna Crucian Carp

TCR/CD3 complex is composed of the disulfide-linked TCR-αβ heterodimer that recognizes the antigen as a peptide presented by the MHC, and non-covalently paired CD3γε- and δε-chains together with disulfide-linked ζ-chain homodimers. The CD3 chains play key roles in T cell development and T cell activation. In the present study, we found nor or extremely lower expression of CD3ε in head- and trunk-kidney lymphocytes by flow cytometric analysis, while CD3ε was expressed at the normal level in lymphocytes from thymus, spleen, intestine, gill, and peripheral blood. Furthermore, CD4-1⁺ and CD8α⁺ T cells from kidney express Zap-70, but not CD3ε, while the T cells from other tissues express both Zap-70 and CD3ε, although expression of CD3ε was low. Quantitative analysis of mRNA expression revealed that the expression level of T cell-related genes including tcrb, cd3ε, zap-70, and lck in CD4-1⁺ and CD8α⁺ T cells was not different between kidney and spleen. Western blot analysis showed that CD3ε band was detected in the cell lysates of spleen but not kidney. To be interested, CD3ε-positive cells greatly increased after 24 h in in vitro culture of kidney leukocytes. Furthermore, expression of CD3ε in both transferred kidney and spleen leukocytes was not detected or very low in kidney, while both leukocytes expressed CD3ε at normal level in spleen when kidney and spleen leukocytes were injected into the isogeneic recipient. Lower expression of CD3ε was also found in kidney T lymphocytes of goldfish and carp. These results indicate that kidney lymphocytes express no or lower level of CD3ε protein in the kidney, although the mRNA of the gene was expressed. Here, we discuss this phenomenon from the point of function of kidney as reservoir for T lymphocytes in teleost, which lacks lymph node and bone marrow.

Omics in fish mucosal immunity

The mucosal immune system of fish is a complex network of immune cells and molecules that are constantly surveilling the environment and protecting the host from infection. A number of "omics" tools are now available and utilized to understand the complexity of mucosal immune systems in non-traditional animal models. This review summarizes recent advances in the implementation of "omics" tools pertaining to the four mucosa-associated lymphoid tissues in teleosts. Genomics, transcriptomics, proteomics, and "omics" in microbiome research require interdisciplinary collaboration and careful experimental design. The data-rich datasets generated are proving really useful at discovering new innate immune players in fish mucosal secretions, identifying novel markers of specific mucosal immune responses, unraveling the diversity of the B and T cell repertoires and characterizing the diversity of the microbial communities present in teleost mucosal surfaces. Bioinformatics, data analysis and storage platforms should be developed to facilitate rapid processing of large datasets, especially when mammalian tools such as bioinformatics analysis software are not available in fishes.

Development of a monoclonal antibody against the CD3ε of olive flounder (Paralichthys olivaceus) and its application in evaluating immune response related to CD3ε

The T cell receptor (TCR) is the binding site of antigen and is responsible for specifically activating the adaptive immune response. CD3, an essential component of the CD3-TCR complex, is known to be composed of γδ and ε chains in teleost. However, there are few monoclonal antibodies (mAb) available to identify these molecules on T cells, so we aimed to produce a mAb against CD3ε to improve our understanding of T cell immune response in olive flounder (Paralichthys olivaceus). CD3ε recombinant protein was expressed in yeast, the expression of which was confirmed by SDS-PAGE, MALDI-TOF/TOF MS and Western blot analysis. A CD3ε-specific mAb 4B2 was selected, the specificity of which was examined by confocal microscopy, flow cytometry and RT-PCR, and the mAb was subsequently used to examine the CD3ε lymphocyte population in several different immune organs, with relatively high percentages of these cells seen in trunk-kidney and spleen, while lower percentages were seen in the liver and peripheral blood of olive flounder. During a viral hemorrhagic septicemia virus (VHSV) infection in olive flounder, the number of CD3ε lymphocytes was seen to gradually increase in the liver, spleen and trunk-kidney of infected fish until 7 days post infection (dpi). In peripheral blood, on the other hand, the increase in CD3ε lymphocyte numbers peaked by 3 dpi. These results suggest that CD3ε lymphocytes might be involved in the immune response against VHSV.

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

T cell activation is a complicated process accompanying with the activation of T cell receptor (TCR) signaling pathway, which is not well described in teleost fish. The initiation of this pathway depends on the interaction of membrane TCR co-receptors (e.g. CD4/8, CD3 and CD45) and a series of cytoplasmic protein tyrosine kinases (e.g. Lck, Fyn and ZAP70). Cyptocaryon irritans is a ciliate pathogen of marine fish white spot disease causing huge economic lost in marine aquaculture. This parasite can infect fish gill and skin and is considered to be a good pathogen model for fish gill and skin mucosal immunity. Our previous studies showed the locally mucosal antibody response was important for fish defense against this parasite. While how TCR signaling pathway involved in T cell activation to help B cell activation in C. irritans infected fish is still not known. In the present study, we cloned a grouper TCR co-receptor gene EcCD3ε (537 bp) and its three kinase genes, including EcLck (1512 bp), EcFyn (1605 bp) and EcZAP70 (1893 bp). Homology analysis showed that they all shared the highest identity with corresponding genes from Takifugu rubripes (EcCD3ε 41%, EcLck 88%, EcFyn 98% and EcZAP70 93%), and their conserved motifs involved in the signaling transduction were analyzed. The tissue distribution analysis showed these four genes were high expressed in thymus, and it is interesting to find their comparative high expression in skin, gill and midgut mucosal immune tissues. In C. irritans infected grouper, the expression of three TCR co-receptors (EcCD4-1, EcCD3ε and EcCD45) and three kinases (EcLck, EcFyn and EcZAP70) was tested in skin, gill, head kidney and spleen at 0, 12 h, 24 h, 2 d, 3 d, 5 d and 7 d. All six genes were significantly up-regulated in skin at most tested time points, which indicate the possibility of skin local T cell activation to support the local antibody response. Compared to three TCR co-receptors, significantly up-regulation of three kinases were seen in the spleen, and the spleen fold changes of these three kinases were much higher than head kidney, which indicates spleen maybe the major systematic immune organs for T cell activation in C. irritans infected fish.

Characterization of CD3+ T lymphocytes of Japanese flounder (Paralichthys olivaceus) and its response after immunization with formalin-inactivated Edwardsiella tarda

The CD3 complex is an important cell surface marker of T lymphocytes and essential for T lymphocytes activation in higher vertebrates. In the present work, the CD3ε of Japanese flounder (Paralichthys olivaceus) was recombinantly expressed in E. coli BL21 (DE3) and used as an immunogen to produce mouse anti-rCD3ε polyclonal antibodies, which could specifically recognize a 20 kDa protein in the membrane proteins of peripheral blood lymphocytes (PBL) of Japanese flounder by co-immunoprecipitation assay. Mass spectrometric analysis showed the 20 kDa protein was the native CD3ε of Japanese flounder. Both the flow cytometric analysis and double immunofluorescence assay (DIFA) showed that the CD3⁺ T lymphocytes could be identified specifically by the mouse anti-rCD3ε polyclonal antibodies, which didn't cross-react with the sIgM⁺ lymphocytes. Immunohistochemistry showed that CD3⁺ T lymphocytes could be detected in gill, skin, stomach, intestine, spleen, liver, head-kidney and mid-kidney. Flow cytometric analysis showed the percentages of CD3⁺ T lymphocytes in the PBL, spleen lymphocytes (SL) and head-kidney lymphocytes (HKL) of Japanese flounder increased rapidly after immunization with formalin-inactivated Edwardsiella tarda, and reached their peak levels at 5th day with 12.6%, 9.7% and 8.7%, respectively, and then decreased gradually. These results suggested that CD3⁺ T lymphocytes play important roles in mucosal and cell-mediated immunity, and the results would deepen our understanding on the roles of teleost T lymphocytes in the immune response.

Characterizations of CD4-1, CD4-2 and CD8β T cell subpopulations in peripheral blood leucocytes, spleen and head kidney of Japanese flounder (Paralichthys olivaceus)

In the previous study, antibodies against CD3 molecule have been produced and were used in labeling T cells in Japanese flounder (Paralichthys olivaceus). In this paper, CD4⁺ and CD8⁺ lymphocytes subpopulations in peripheral blood leucocytes (PBL), spleen and head kidney of flounder were investigated. The flounder CD4-1, CD4-2 and CD8β recombinant proteins and their antibodies (Abs) were produced, then the cross-reactivity of the Abs to CD4-1, CD4-2 and CD8β was detected by Western blotting, respectively, and the reactions of Abs to PBL were analyzed by immunofluorescence staining (IFS) and flow cytometry (FCM). CD4-1⁺/CD3⁺, CD4-2⁺/CD3⁺, and CD8β⁺/CD3⁺ lymphocytes in PBL, spleen and head kidney were observed by double IFS, then their proportions were analyzed using two-color FCM, respectively. Further, CD4-1/CD8β, CD4-2/CD8β, or CD4-1/CD4-2 lymphocytes were analyzed using double-IFS and two-color FCM. Finally, CD4-1⁺, CD4-2⁺, and CD8β⁺ lymphocytes in spleen and head kidney were observed by immunohistochemistry. The results showed that the Abs were specific for CD4-1, CD4-2 and CD8β molecules, respectively. The proportions of CD4-1⁺/CD3⁺, CD4-2⁺/CD3⁺, and CD8β⁺/CD3⁺ lymphocytes were 6.7±2.0%, 8.6±2.8%, 2.1±1.3% in PBL; 13.6±3.6%, 15.6±5.2%, 2.8±1.4% in spleen; 20.0±4.6%, 20.5±4.6%, 3.2±1.5% in head kidney, respectively. Most CD4⁺ and CD8⁺ cell subpopulations belonged to CD3⁺ cells; there were no cross-reactivity between CD4⁺ and CD8⁺ cells. CD4-1⁺/CD4-2^-, CD4-1^-/CD4-2⁺, and CD4-1⁺/CD4-2⁺ cells presented different proportions in PBL, spleen and head kidney, among them, CD4-1⁺/CD4-2⁺ cell is the majority of CD4T cell subpopulation.

T Cells in Fish

Cartilaginous and bony fish are the most primitive vertebrates with a thymus, and possess T cells equivalent to those in mammals. There are a number of studies in fish demonstrating that the thymus is the essential organ for development of T lymphocytes from early thymocyte progenitors to functionally competent T cells. A high number of T cells in the intestine and gills has been reported in several fish species. Involvement of CD4⁺ and CD8α⁺ T cells in allograft rejection and graft-versus-host reaction (GVHR) has been demonstrated using monoclonal antibodies. Conservation of CD4⁺ helper T cell functions among teleost fishes has been suggested in a number studies employing mixed leukocyte culture (MLC) and hapten/carrier effect. Alloantigen- and virus-specific cytotoxicity has also been demonstrated in ginbuna and rainbow trout. Furthermore, the important role of cell-mediated immunity rather than humoral immunity has been reported in the protection against intracellular bacterial infection. Recently, the direct antibacterial activity of CD8α⁺, CD4⁺ T-cells and sIgM⁺ cells in fish has been reported. In this review, we summarize the recent progress in T cell research focusing on the tissue distribution and function of fish T cells.

The Mucosal Immune System of Teleost Fish

Teleost fish possess an adaptive immune system associated with each of their mucosal body surfaces. Evidence obtained from mucosal vaccination and mucosal infection studies reveal that adaptive immune responses take place at the different mucosal surfaces of teleost. The main mucosa-associated lymphoid tissues (MALT) of teleosts are the gut-associated lymphoid tissue (GALT), skin-associated lymphoid tissue (SALT), the gill-associated lymphoid tissue (GIALT) and the recently discovered nasopharynx-associated lymphoid tissue (NALT). Teleost MALT includes diffuse B cells and T cells with specific phenotypes different from their systemic counterparts that have co-evolved to defend the microbe-rich mucosal environment. Both B and T cells respond to mucosal infection or vaccination. Specific antibody responses can be measured in the gills, gut and skin mucosal secretions of teleost fish following mucosal infection or vaccination. Rainbow trout studies have shown that IgT antibodies and IgT(+) B cells are the predominant B cell subset in all MALT and respond in a compartmentalized manner to mucosal infection. Our current knowledge on adaptive immunity in teleosts is limited compared to the mammalian literature. New research tools and in vivo models are currently being developed in order to help reveal the great intricacy of teleost mucosal adaptive immunity and help improve mucosal vaccination protocols for use in aquaculture.

Genome complexity in the coelacanth is reflected in its adaptive immune system

We have analyzed the available genome and transcriptome resources from the coelacanth in order to characterize genes involved in adaptive immunity. Two highly distinctive IgW-encoding loci have been identified that exhibit a unique genomic organization, including a multiplicity of tandemly repeated constant region exons. The overall organization of the IgW loci precludes typical heavy chain class switching. A locus encoding IgM could not be identified either computationally or by using several different experimental strategies. Four distinct sets of genes encoding Ig light chains were identified. This includes a variant sigma-type Ig light chain previously identified only in cartilaginous fishes and which is now provisionally denoted sigma-2. Genes encoding α/β and γ/δ T-cell receptors, and CD3, CD4, and CD8 co-receptors also were characterized. Ig heavy chain variable region genes and TCR components are interspersed within the TCR α/δ locus; this organization previously was reported only in tetrapods and raises questions regarding evolution and functional cooption of genes encoding variable regions. The composition, organization and syntenic conservation of the major histocompatibility complex locus have been characterized. We also identified large numbers of genes encoding cytokines and their receptors, and other genes associated with adaptive immunity. In terms of sequence identity and organization, the adaptive immune genes of the coelacanth more closely resemble orthologous genes in tetrapods than those in teleost fishes, consistent with current phylogenomic interpretations. Overall, the work reported described herein highlights the complexity inherent in the coelacanth genome and provides a rich catalog of immune genes for future investigations.

Identification of CD3ε, CD4, CD8β splice variants of Atlantic salmon

In vertebrates, CD3 complex and CD4 and CD8 co-receptors are essential for signal transduction during T cell activation. In the present study, we report the mRNA spliced variants of the Atlantic salmon CD3ε, CD4 and CD8β and the effect of pathogen encounter on the expression of these variants. CD3ε is alternatively spliced in thymus, head kidney, spleen and gills to give rise to the complete mRNA sequence and to an alternative product that lacks the transmembrane exon. CD4 is also alternatively spliced in the thymus, head kidney, spleen and gills to form two variants, although the alternative product is barely detectable. The alternative product lacks the exon 1B encoding the D1 domain, which is essential for binding to MHC class II proteins. Two amplicons were also found for the CD8β gene; sequencing analysis revealed that the main PCR product corresponds to the previously reported CD8β sequence, whereas the variant sequence encodes a potential protein that lacks the Ig-like domain. The expression of CD3, CD4, CD8β genes also analyzed in head kidney of LPS-treated and IPNV infected salmon and different patterns of expression were observed. The presence and balance of the different variants of T cell co-receptors could be related to the ability of fish to induce a particular type of immune response, as well as, the ability of the pathogen to modify the fish immune response.

Fish T cells: recent advances through genomics

This brief review is intended to provide a concise overview of the current literature concerning T cells, advances in identifying distinct T cell functional subsets, and in distinguishing effector cells from memory cells. We compare and contrast a wealth of recent progress made in T cell immunology of teleost, elasmobranch, and agnathan fish, to knowledge derived from mammalian T cell studies. From genome studies, fish clearly have most components associated with T cell function and we can speculate on the presence of putative T cell subsets, and the ability to detect their differentiation to form memory cells. Some recombinant proteins for T cell associated cytokines and antibodies for T cell surface receptors have been generated that will facilitate studying the functional roles of teleost T cells during immune responses. Although there is still a long way to go, major advances have occurred in recent years for investigating T cell responses, thus phenotypic and functional characterization is on the near horizon.

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.

Diversity of teleost leukocyte molecules: role of alternative splicing

Alternative splicing is an important mechanism of gene expression control that also produces a large proteome from a limited number of genes. In the immune system of mammals, numerous relevant genes have been found to undergo alternative splicing that contributes to the complexity of immune response. An increasing number of reports have recently indicated that alternative splicing also occurs in other vertebrates, such as fish. In this review we summarize the general features of such molecular events in cytokines and leukocyte co-receptors and their contribution to diversity and regulation of fish leukocytes.

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.

Molecular and biochemical characterization of the Mexican axolotl CD3 (CD3ε and CD3γ/δ)

In mammals, the T-cell receptor (TCR) complex expressed on mature T-cells consists of α/β or γ/δ clonotypic heterodimers non-covalently associated with four invariant chains forming the CD3 complex (CD3γ, CD3δ, CD3ε and CD3ζ). The TCR is the unit implicated in the antigenic peptide recognition whereas the CD3 subunits present as three different dimers (δ-ε, γ-ε and ζ-ζ) in the receptor complex participate to the signal transduction and are indispensable for the expression of the TCR at the cell surface. We report the cloning, characterization and expression analysis of CD3γ/δ and CD3ε genes in an amphibian urodele, the Mexican axolotl. Amino acid comparisons show that important motifs and residues were preserved between the axolotl CD3 chains and various vertebrate CD3ε, CD3γ, CD3δ and CD3γ/δ chains. During ontogeny, CD3ε transcripts are first detected in the dorsal region of tail-bud embryos before thymus organogenesis. CD3γ/δ transcripts are first detected in the head of 4-week-old larvae. A cross-reactive polyclonal anti-CD3ε antibody was used for the co-immunoprecipitation of the two CD3 proteins of 25 and 29 kDa, respectively, associated with the 90-kDa αβ TCR heterodimer.

The plasmablast-like leukocyte in the kidney of fugu (Takifugu rubripes)

In teleosts, the kidney is the major immune organ. From the kidney of fugu (Takifugu rubripes), we isolated a unique leukocyte population. This population shows properties similar to those of mammalian plasmablasts. First, adherent cells expressing IgM protein on their surface were obtained from the fugu kidney. Flow cytometry (FCM) showed that these cells were mainly composed of two cell populations: IgM+CD8α⁻ cells and IgM+CD8α+ cells. Further characterization of the IgM+CD8α⁻ population by RT-PCR demonstrated that the cells expressed secretory-type IgM as well as Bcl-6 and Blimp-1, developmental marker genes for the B cell lineage. Western blotting also showed that the cells secreted IgM protein. These results indicate that the IgM+CD8α⁻ cells are similar to cells at the plasmablast stage in mammals. This is the first report isolating plasmablast-like leukocytes in fish species. Our data also suggests that the teleosts kidney is a organ where B cells terminally differentiate into the plasma cells.

CD3γ/δ in sea bass (Dicentrarchus labrax): Molecular characterization and expression analysis

The CD3 complex is the common marker on the surface of both αβ and γδ T cells and is essential for formation of the T-cell receptor complex and for T-cell activation. In this paper, we report the gene cloning and molecular characterization of a CD3γ/δ homologue in sea bass (Dicentrarchus labrax), the analysis of transcription levels in lymphoid and non-lymphoid organs and the gene regulation after in vitro stimulation with LPS and PHA. Four cysteine residues in the extracellular domain, involved in the constitution of immunoglobulin-like domain, are present in sea bass CD3γ/δ sequence and they are conserved both in number and position from mammals to teleost sequences. Similar to other known CD3γ/δs, in sea bass CD3γ/δ there is also a conserved immunoreceptor tyrosine-based activation ITAM motif that could be responsible for its individual signal transduction capacity. The real time RT-PCR basal analysis shows the highest level of CD3γ/δ mRNA in thymus, followed by peripheral blood leucocytes, spleen, gills, gut, liver, head kidney, brain and muscle. The expression analysis under stimuli condition reveals a significant decrease of CD3γ/δ expression after LPS stimulation and a significant increase after PHA-L stimulation, in agreement with mammals results. In conclusion, these data allow us to affirm that sea bass CD3γ/δ can be used as a T cell marker and will help in adding new insight on the immune response mechanisms of sea bass.

Cloning and expression analysis of Atlantic halibut (Hippoglossus hippoglossus) CD3 genes

The CD3 complex is in higher vertebrates shown to be important for the activation of T-cells. The T-cell system in fish is believed to be similar to that in higher vertebrates, and the CD3 chains could therefore be an important marker for identification of T-cells in fish. Here, we report the cDNA and corresponding gene sequence of Atlantic halibut (Hippoglossus hippoglossus) CD3gammadelta, CD3varepsilon, and CD3zeta chains, and the tissue-specific expression pattern of CD3 and T- cell receptor (TCR) genes. Important structural characteristics defining the CD3 genes seemed to be conserved in the halibut CD3 chains, such as a signal peptide, an extracellular region, a transmembrane helix having a negatively charged residue, and an ITAM bearing cytoplasmic tail. The extracellular domain of halibut CD3gammadelta and CD3varepsilon included two cysteines presumably involved in Ig-fold stabilisation and the CxxCxE motif important for dimerization. A spliced variant of CD3varepsilon was identified, lacking the Ig-fold, but with the CxxCxE motif intact. The real time RT-PCR analysis revealed a highly similar expression pattern of the CD3 genes and the TCRalpha and TCRbeta genes, indicating that the functional relationship between the TCR and the CD3 genes are preserved in teleosts.

N-terminal negatively charged residues in CD3varepsilon chains as a phylogenetically conserved trait potentially yielding isoforms with different isoelectric points: analysis of human CD3varepsilon chains

CD3varepsilon chains are essential to the structure, expression and signaling of T cell receptors. Here, we extend to human CD3varepsilon our previous data in mouse CD3varepsilon showing that, in T cells, proteolytic processing of the acidic N-terminal sequence of CD3varepsilon chains generate distinct polypeptide species that can be identified by two-dimension (IEF-SDS PAGE) electrophoresis and immunoblot. This was shown first by showing the processing of a fusion protein of GFP and the extracellular domain of mouse CD3varepsilon (mCD3GFP) expressed in Jurkat cells. Secondly, pI heterogeneity was also found in human CD3varepsilon chains immunoprecipitated from the surface of Jurkat cells or PHA blasts of human blood T lymphocytes. Comparison of CD3varepsilon chains from 27 different species shows that their N-terminal sequences share a strong acidic nature, despite the large differences in terms of length and composition, even among closely related species. Our results suggest that generation of CD3varepsilon chain isoforms with different N-terminal sequence and pI is a general phenomenon. Thus, as previously observed in the mouse, the relative abundance of CD3varepsilon chain species might regulate TCR/CD3 structure and function, including the strength of the interactions between CD3 dimers and the TCR clonotypic receptors, as well as TCR/CD3 activation thresholds. Interestingly, CD3varepsilon chains from 7 out of 27 species studied have putative N-glycosylation (NxS or NxT) motifs in their Ig extracellular domain. Their location, plus the conservation of residues involved in domain organization, the interactions with other CD3 chains, or the TCR, and signal triggering add new data useful to establish a permissive topology for the interaction between CD3 dimers and the TCR chains.

Teleost B7 expressed on monocytes regulates T cell responses

In mammals, professional APCs induce adaptive immunity via the activation of T cells. During this process, B7 family molecules present upon APCs are known to play crucial roles in optimal T cell stimulation. In contrast, the confirmation of APCs in a nonmammalian vertebrate has yet to be achieved. To obtain further insights into the evolutionary origin of APCs, we have identified three members of the B7 family in the teleost Takifugu rubripes (fugu): B7-H1/DC, B7-H3, and B7-H4. The three fugu B7s were expressed on the surface of blood monocytes. The B7(+) monocytes, which are composed of at least two distinct populations, expressed the MHC class II component gene. The fugu B7 molecules bound to activated T cells, indicating that putative B7 receptors were expressed upon T cells. Fugu B7-H1/DC inhibited T cell proliferation concomitant with increasing levels of both IL-10 and IFN-gamma expression, whereas both B7-H3 and B7-H4 promoted T cell growth following IL-2 induction and the suppression of IL-10. These observations indicate that fugu B7s regulate T cell responses via receptors upon T cells. We suggest that fish B7(+) monocytes are APCs and that a costimulatory system has already developed in fish via the evolutionary process.

Molecular cloning and characterization of common carp (Cyprinus carpio L.) TCRgamma and CD3gamma/delta chains

Partial cDNA sequences of TCRgamma and CD3gamma/delta were isolated from the thymus of common carp (Cyprinus carpio L.) by the method of suppression subtractive hybridization (SSH). Subsequently the full length cDNAs of carp TCRgamma and CD3gamma/delta were obtained by means of 3' RACE and 5' RACE, respectively. The full length of carp TCRgamma chain is 1368bp and encodes 326 amino acids including a signal peptide region of 19 amino acids and a transmembrane region of 23 amino acids at the C-terminal region from aa 291 to 313. The V region of carp TCRgamma contains 109 amino acids, the core motif FGXG in J segment was also found in carp TCRgamma. The C region of carp TCRgamma contains the characteristic CX6PX6WX45C motif. The CP region of carp TCR Cgamma contains 37 amino acids. The full length of carp CD3gamma/delta is 790bp and encodes 175 amino acids including a signal peptide region of 17 amino acids and a transmembrane region of 23 amino acids from aa 93 to 115. Similar to other known CD3gamma/deltas, four cysteine residues in the extracellular domain and an immunoreceptor tyrosine-based activation motif ITAM (YxxL/Ix6-8YxxL/I) in the intracellular domain are also included in carp CD3gamma/delta. Differing from other known CD3gamma/deltas, carp CD3gamma/delta lacks the CXXCXE motif in the extracellular domain. RT-PCR analysis demonstrated that the expression of TCRgamma gene was mainly in the thymus and gill of 6-month carp, but in 18-month carp, TCRgamma gene was detected in all the examined tissues. The expression of CD3gamma/delta gene was detected in all examined tissues of 6 and 18-month carp; among them, the highest expression level was in the thymus of 6-month carp. In situ hybridization showed that CD3gamma/delta-expressing cells were widely distributed in the head kidney, spleen and kidney of carp, whereas in the thymus, they were densely distributed in the lymphoid outer zone and scattered in the epithelioid inner zone.

Characterization of CD8+ leukocytes in fugu (Takifugu rubripes) with antiserum against fugu CD8alpha

We have investigated the characteristics of CD8+ leukocytes by using an anti-CD8alpha antiserum raised in mouse by DNA-immunization. The magnetically sorted CD8alpha+ peripheral blood leukocyte (PBL) population comprised lymphocytes/thrombocytes and monocytes, whereas CD8alpha- PBLs consisted of lymphocytes/thrombocytes, monocytes, and neutrophils. Expression analysis demonstrated that both groups of cells expressed the CD3epsilon and TCRalpha genes. The CD8alpha and CD8beta genes were detected only in CD8alpha+ cells, whereas expression of CD4 and immunoglobulin light chain (IgL) was observed only in CD8alpha- cells. These results suggest that fugu CD8alpha+ leukocytes contain CD8+ T cells, but not CD4+ T cells or B cells. Furthermore, mitogenesis of the CD8+ lymphocyte/thrombocyte population was induced by phytohemaglutinin stimulation, suggesting that fish CD8+ lymphocytes/thrombocytes (probably CD8+ T cells) have characteristics similar to mammalian CD8+ T cells. Neutrophils and monocytes/macrophages infiltrating a subcutaneous inflammatory site expressed only CD8alpha, but not CD8beta, CD4, TCRalpha, or IgL. This result suggests that similar to mammalian dendritic cells, fugu monocytes/macrophages express CD8alpha.

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.

Evidence of positive selection on the Atlantic salmon CD3γδ gene

Atlantic salmon are typically anadromous, spending the majority of their lifetime in oceans and returning to fresh water to breed. This diversity of environments likely results in strong selective forces shaping their genome. In this paper, we present the first genomics approach to detect positive selection operating on the Salmo salar (salmon) lineage, an important aquaculture species. We identify a panel of candidate genes that may have been subject to adaptive evolution in this species. In particular, we identify a robust signature of positive selection operating on the salmon CD3gammadelta gene, which encodes one of the protein chains essential for formation of the T-cell receptor complex and for T-cell activation. Furthermore, we identified the particular codon sites that have been subject to positive selection in fish and highlight two sites flanking an important N-glycosylation site in this molecule.

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.