Characterization of a T cell lineage marker in carp (Cyprinus carpio L.)

RNA-Seq of Single Fish Cells - Seeking Out the Leukocytes Mediating Immunity in Teleost Fishes

The immune system is a complex and sophisticated biological system, spanning multiple levels of complexity, from the molecular level to that of tissue. Our current understanding of its function and complexity, of the heterogeneity of leukocytes, is a result of decades of concentrated efforts to delineate cellular markers using conventional methods of antibody screening and antigen identification. In mammalian models, this led to in-depth understanding of individual leukocyte subsets, their phenotypes, and their roles in health and disease. The field was further propelled forward by the development of single-cell (sc) RNA-seq technologies, offering an even broader and more integrated view of how cells work together to generate a particular response. Consequently, the adoption of scRNA-seq revealed the unexpected plasticity and heterogeneity of leukocyte populations and shifted several long-standing paradigms of immunology. This review article highlights the unprecedented opportunities offered by scRNA-seq technology to unveil the individual contributions of leukocyte subsets and their crosstalk in generating the overall immune responses in bony fishes. Single-cell transcriptomics allow identifying unseen relationships, and formulating novel hypotheses tailored for teleost species, without the need to rely on the limited number of fish-specific antibodies and pre-selected markers. Several recent studies on single-cell transcriptomes of fish have already identified previously unnoticed expression signatures and provided astonishing insights into the diversity of teleost leukocytes and the evolution of vertebrate immunity. Without a doubt, scRNA-seq in tandem with bioinformatics tools and state-of-the-art methods, will facilitate studying the teleost immune system by not only defining key markers, but also teaching us about lymphoid tissue organization, development/differentiation, cell-cell interactions, antigen receptor repertoires, states of health and disease, all across time and space in fishes. These advances will invite more researchers to develop the tools necessary to explore the immunology of fishes, which remain non-conventional animal models from which we have much to learn.

Can a biopesticide based on Bacillus thuringiensis affect the physiology and histomorphology of Arapaima gigas?

The use of biopesticides based on Bacillus thuringiensis (Berliner 1915) in agriculture has been considered harmless for non-target organisms such as fish. The present study aimed to investigate the effects of a biopesticide based on B. thuringiensis on the physiology and histology of the liver, kidney and intestine of Arapaima gigas (Schinz, 1822), via exposure to water (0.13 g/L) and in the diet (0.13 g), after 24 and 48 h. Fish subjected to B. thuringiensis in the water of their breeding and feeding tanks exhibited mortality due to changes in erythrogram (hematocrit, hemoglobin, erythrocytes), thrombogram and leucogram blood parameters, and plasma (sodium, chloride, potassium, cholesterol, glucose, triglycerides, cortisol and total proteins) and enzymatic (Aspartate Amino Transferase and Alanine Amino Transferase) biochemistry. Histopathological alterations in the liver and kidney ranged from mild to severe and were characterized by the presence of cytoplasmic vacuolization, nuclear hypertrophy and atrophy, melanomacrophage centers and necrosis, and in the intestine by changes to the number of villi and goblet cells. Therefore, these physiological and histopathological alterations indicate that care should be taken with the dispersion of biopesticides based on agricultural B. thuringiensis in fish farming.

Development and characterization of a monoclonal antibody against the putative T cells of Labeo rohita

In this study, we have described the development and characterization of monoclonal antibodies (MAbs) directed against thymocytes of rohu, Labeo rohita. MAbs were obtained by immunizing BALB/c mice with freshly isolated and nylon wool column enriched mononuclear cells of thymus. Positive clones against thymocytes were screened by cellular ELISA. The hybridoma showing strong reactivity with nylon wool enriched mononuclear cells, and non-reactivity with a rohu thymus macrophage cell line and rohu serum was selected and subjected to single cell cloning by limiting dilution. The MAbs secreted by a positive clone were designated as E6 MAb. Western blotting of reduced protein from enriched thymocytes showed that E6 reacted with a 166.2 kDa polypeptide and belongs to the IgG1 subclass. Flow cytometric analysis of gated lymphocytes, revealed that the percentage of E6 positive (E6+) cells in thymus (n = 5, 720.4 ± 79.70 g) was 89.7 %. Similarly, the percentage of E6+ cells in kidney, spleen and blood (n = 5) was 6.71, 1.71 and 1.88 %, respectively. In indirect immunoperoxidase test, E6+ cells appeared to be lymphoid cells with a high nucleus to cytoplasmic ratio and were densely packed in the central region of thymus whereas, a few cells were found to be positive in kidney and spleen sections. E6 MAb also reacted with a small population of lymphocytes in blood smear. This MAb appears to be a suitable marker for T lymphocytes and can be a valuable tool in studying immune response and ontogeny of L. rohita immune system.

Characterization and expression analysis of an interferon-γ2 induced chemokine receptor CXCR3 in common carp (Cyprinus carpio L.)

Chemokine and chemokine receptor signalling pairs play a crucial role in regulation of cell migration, morphogenesis, and cell activation. Expressed in mammals on activated T and NK cells, chemokine receptor CXCR3 binds interferon-γ inducible chemokines CXCL9-11 and CCL21. Here we sequenced the carp CXCR3 chemokine receptor and showed its relationship to CXCR3a receptors found in other teleosts. We found high expression of the CXCR3 gene in most of the organs and tissues of the immune system and in immune-related tissues such as gills and gut, corroborating a predominantly immune-related function. The very high expression in gill and gut moreover indicates a role for CXCR3 in cell recruitment during infection. High in vivo expression of CXCR3 at later stages of inflammation, as well as its in vitro sensitivity to IFN-γ2 stimulation indicate that in carp, CXCR3 is involved in macrophage-mediated responses. Moreover, as expression of the CXCR3 and CXCb genes coincides in the focus of inflammation and as both the CXCb chemokines and the CXCR3 receptor are significantly up-regulated upon IFN-γ stimulation it is hypothesized that CXCb chemokines may be putative ligands for CXCR3.

Production and characterization of a monoclonal antibody against putative T lymphocytes of Catla catla

Catla catla is the fastest growing Indian major carp and one of the major aquaculture species in South Asia. A monoclonal antibody (MAb) designated B8 MAb was produced against nylon wool-enriched thymus mononuclear cells of C. catla. This MAb did not show reactivity with macrophage and epithelial cell lines derived from catla thymus in cellular ELISA. In flow cytometric analysis of gated lymphocytes, the percentage of B8 positive (B8+) cells in thymus (n = 10, 500-600 g) was determined to be 77.7 %. Similarly, the percentage of B8+ cells in kidney, spleen and blood (n = 5) was 15.08, 1.1 and 32.17 %, respectively. Western blotting of reduced membrane proteins showed that B8 MAb reacted with a polypeptide having a molecular weight of 168.2 kDa. In indirect immunoperoxidase test, B8+ cells appeared to be lymphoid cells with a high nucleus to cytoplasmic ratio. B8 reactive cells were densely packed in central region of thymus whereas, a few cells were found to be positive in kidney and spleen sections. B8 MAb also reacted with a significant population of lymphocytes in blood smears. Considering the economic importance of C. catla, this MAb should be a useful tool for studying immune response of this fish species.

Differential expression of two interferon-gamma genes in common carp (Cyprinus carpio L.)

Two interferon gamma (IFN-gamma) genes are expressed in immune cells of teleost fish and are potentially implicated in B- and T-lymphocyte responses. IFN-gamma-2 shows structural and functional characteristics to other vertebrate IFN-gamma genes and is associated with T-lymphocyte function. Expression profiling shows IFN-gamma-2 upregulation in T-lymphocytes after phytohemagglutinin (PHA) stimulation in vitro. Unexpectedly, we found IFN-gamma-1, which is structurally different from IFN-gamma-2, to be expressed in lipopolysacharide (LPS)-stimulated IgM+ (B- lymphocyte enriched) fractions. Expression of T-box transcription factor T-bet, but not of GATA-binding protein 3 (GATA3), correlated with expression of both IFN-gamma genes. In-vivo parasite infection, but as predicted not zymosan-induced inflammation, resulted in concomitant upregulation of T-bet and IFN-gamma-2. This corroborates a genuine T-lymphocyte associated role for IFN-gamma-2.

Cross-reactivity of human leukocyte differentiation antigen monoclonal antibodies on carp and rainbow trout cells

Three hundred and seventy-seven monoclonal antibodies (mabs) directed against human CD antigens and non-classified human leukocyte surface antigens were assayed for their reactivity with common carp (Cyprinus carpio L.) and rainbow trout (Oncorhynchus mykiss) peripheral blood leukocytes (PBL) and thymocytes within the animal homologue section of the 8th International Workshop on Human Leukocyte Differentiation Antigens (HLDA8). Four of the mabs clearly reacted with rainbow trout PBL and two with carp PBL. Positive mabs were investigated further by two-colour flow cytometry with established mabs directed against carp and rainbow trout leukocyte subpopulations. None of these mabs were suitable for Western blotting and immunoprecipitation. Three mabs were found to stain cells in fixed cryostate sections of the lymphatic organs thymus, pronephros and spleen. In this study, for the first time an anti-CD14 mab was found to cross-react with fish cells. This mab could be a valuable tool complementing the limited toolbox of population-specific mabs in fish. The low number of cross-reactive mabs analyzed in this workshop is another indication for the great phylogenetic difference between mammals and osteichthyes.

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.

Monoclonal antibodies to lymphocytes of rainbow trout (Oncorhynchus mykiss)

Monoclonal antibodies (Mabs) to lymphocytes of rainbow trout have been developed by immunisation with synthetic peptides, prepared from selected parts of the alpha- and beta-gene sequences of the T-cell receptor (TCR). Mab 1C2 (TCR beta immunisation) identified lymphocytes in blood (11%), spleen (18%) and in thymus (9%) in flow cytometry analysis (FCM). Immune complexes of lymphocytes coupled to Mab 1C2 was used for further immunisations resulting in numerous supernatants reactive with lymphocytes in FCM, of which Mabs 7A5 and 8H4 were selected for further characterisation. Mab 7A5 identified 31% of lymphocytes in blood and 9% in the spleen. Mab 8H4 labelled 61% and 85% of lymphocytes in the same organs. Mab 8H4 reacted with the majority of the lymphocytes in the thymus (98%). Mabs 1C2, 7A5 and 8H4 recognised surface markers on both Ig(-) and Ig(+) lymphocytes in peripheral blood and in spleen in double staining experiments. An increased proportion of Ig(-) lymphocytes were identified when Ig(+) lymphocytes were eliminated by immunomagnetic separation. No cross-reactivity of Mabs 1C2, 7A5 or 8H4 to anti-thrombocyte Mabs was detected. Mab 1C2 captured molecules of about 40 and also of 55-60kDa, in an immunoprecipitation assay. Mab 7A5 recognised an antigen of approximately 75-80kDa and Mab 8H4 identified proteins of about 70, 100 and 150kDa. Immunohistochemical staining by Mab 8H4 of fixed thymus, revealed a strong labelling of lymphoid cells in the outer zones of thymus. The 8H4 positive lymphoid cells surrounds circular structures, which were not labelled by Mab 8H4. These distinctly appearing structures have a similar shape as nurse cells described in mammals.

Cytology of lymphomyeloid head kidney of Antarctic fishes Trematomus bernacchii (Nototheniidae) and Chionodraco hamatus (Channicthyidae)

Species that live in extreme conditions have specially adapted physiology and tissue/organ organisation. The adaptation of lymphoid organs to low temperatures in polar species could be an original field of study, indicating how the immune system works under extreme conditions. In fishes, the head kidney is a key organ for immunity and here the cytology of this organ is studied in two common Antarctic species: Trematomus bernacchii and Chionodraco hamatus. Ultrastructural analysis revealed heterogeneity of epithelial cells, with reticular cells, subcapsular- and perivascular-limiting cells. Differences in the size and morphology of epithelial cells were observed between the polar species and warm water species of fish. Intermingled with epithelial cell leucocytes, such as lymphocytes, thrombocytes and macrophages, had comparable morphology in both species, contrary to sharp differences observed in the morphology of erythrocytes and granulocytes. The functional adaptation of the head kidney to the low temperatures of polar water is discussed.

A screen of mammalian antibodies on snapper (Pagrus auratus, Sparidae) peripheral blood leukocytes reveals cross reactivity of an anti-human CD3 antibody with a population of mIg(-) cells

Detailed immunological studies of the teleosts have been hampered by a lack of antibodies against cell-specific markers. Furthermore, where antibodies have been raised, in many instances they have been found to be species-specific. In comparison, many monoclonal and polyclonal antibodies exist with specificities for mammalian proteins and glycoproteins that effectively differentiate leukocyte sub-populations. In this study, we have tested a panel of 54 commercial antibodies against human and murine cell surface receptors for their ability to bind leukocytes isolated from the peripheral blood of snapper (Pagrus auratus). From this panel, one antibody, A452, which is specific for the intracytoplasmic tail of the epsilon (epsilon) chain of the T cell receptor-associated CD3 complex (CD3epsilon) bound to a subpopulation of peripheral blood leukocytes. Mutually exclusive counterstaining was observed when this antibody was used in conjunction with a monoclonal anti-snapper immunoglobulin antibody. This suggests that A452 may be binding to putative snapper T cells.

Monoclonal antibodies to European flat oyster Ostrea edulis hemocytes: characterization and tissue distribution of granulocytes in adult and developing animals

Monoclonal antibodies specific for hemocyte sub-populations of the European flat oyster, Ostrea edulis, were prepared using separated granulocytes and hyalinocytes as antigen sources. Six monoclonal antibodies specific for hemocytes were selected. Five of them showed a specificity for more than one hemocyte type and one hybridoma produced a monoclonal antibody reacting specifically with granulocytes. At the ultrastructural level, this monoclonal antibody demonstrated epitopes principally in dense cytoplasmic granules of granulocytes. Western blotting analysis indicated that a peptide of 50kDa was recognized by this antibody. It was therefore used to investigate granulocyte distribution and ontogenesis in European flat oysters using immunohistochemistry. Granulocytes were mostly observed in connective tissues in different organs. Their distribution pattern in digestive gland, mantle, gills and gonad may indicate different functional status. Moreover, the absence of granulocytes in early larval stages can partly correspond to an immature immune system in oyster larvae.

Ontogeny of B and T cells in sea bass (Dicentrarchus labrax, L.)

Monoclonal antibodies specific to sea bass Ig heavy (WDI 1) and light (WDI 3) chains and T cells (DLT15) were used in an ontogenetic study of sea bass by flow cytometry and immunocytochemistry. The influence of weight and age, as well as season, on B cell development was studied in the fastest and slowest growing offspring from the same spawn (5-305 days post hatch: dph). Additionally, B and T cell development was followed in samples of different offspring (5-137 dph). The results suggest that DLT15 recognises very early (pre-?) T cells as well as mature T cells and that these very early T cells might have their origin in a different compartment and subsequently mature in the thymus. They also appeared much earlier in ontogeny (between 5-12 dph onwards) than pre-B cells having cytoplasmic Ig (from 52 dph onwards). With the monoclonal antibodies used, adult levels of T and B cells were both reached between 137-145 dph, suggesting that sea bass is immunologically mature from at least that age onwards. As in other teleosts, the thymus appears to be the primary organ for T lymphocytes and head kidney the primary organ for B lymphocytes. For sea bass, age seems to be more important in determining B cell maturation than body weight.

Immunopurification of T-cells from sea bass Dicentrarchus labrax (L.)

The monoclonal antibody DLT15, specific for thymocytes and peripheral T-cells of the teleost fish Dicentrarchus labrax (sea bass), was used to purify immunoreactive cells from blood and gut-associated lymphoid tissue. The purification was performed by immuno-magnetic sorting of leucocyte fractions enriched by Percoll density gradient centrifugation, and the purity of the isolated cells was estimated by cytofluorimetric analysis. Following a single step, the percentage of DLT15-purified cells was 88 +/- 10% for gut-associated lymphoid tissue and 79 +/- 18% for blood leucocytes. DLT15-purified cells from gut-associated lymphoid tissue were employed for RNA extraction and cDNA synthesis. In RT-PCR experiments using as primers degenerate oligonucleotides corresponding to the peptide sequence MYWY and VYFCA of the trout TcR beta chain, a 203 bp product was amplified. When sequenced, the cDNA was found to show 60% nucleotide identity to the trout TcRV beta 3. By 3'-RACE the cDNA was elongated to obtain the TcR constant region, with high similarity to other fish TcR sequences. These results strongly suggest that cells recognised by DLT15 are putative T lymphocytes.

Viral-antigen dependence and T-cell receptor expression in leucocytes from rhabdovirus immunized trout

This work describes the characterization of trout haematopoietic in vitro long-term cell cultures showing specific viral antigen-dependent cell (ADC) proliferation. The ADC cultures were developed from outbreed trout after surviving viral hemorrhagic septicaemia virus (VHSV) infections or after immunization with purified VHSV. For in vitro long-term proliferation of the ADC cultures, adherent (Ad) cells obtained from autologous trout were pulsed with VHSV recombinant glycoprotein G4 (G4-pulsed Ad cells) and added periodically to the cultures. ADC did not proliferate in cultures obtained from non-infected control trout treated in parallel with G4 or from VHSV survivor/VHSV immunized trout kidney donors treated with non-viral proteins. After months in culture, the ADC acquired an increasingly homogeneous morphology compatible with that of mature trout lymphocytes, secreted supernatant 'factors', and were stained with rabbit antibodies to the ectodomain of recombinant trout T-cell receptor (TcR) beta-chain. Together with all the above mentioned properties, the presence of TcR sequences in the ADC cultures confirmed by the expression of alpha- and beta-chain TcR by nested PCR amplification and sequencing of the amplified bands, suggests that these ADC cultures contain trout T-like cells engaged in a VHSV response. These trout ADC cultures offer a first opportunity to further analyze fish anti-viral immunological responses.

Ontogeny of the thymus in a teleost fish, Cyprinus carpio L.: developing thymocytes in the epithelial microenvironment

A monoclonal antibody, WCL9, specific for membrane molecules of a thymocyte subpopulation was used to detect these cells in situ during the ontogeny of thymus. Cryo-sections revealed WCL9+ cells in the rudiment of the thymus (day 4 post fertilization); thereafter, the positive cells were observed exclusively in the cortex from the first appearance of thymic regionalization (week 4 post fertilization) until adult age. Whole-mount immunostaining of the thymus with WCL9 revealed the three-dimensional structure of the cortex by specific staining. The presence and distribution of apoptotic cells during thymus development was studied by in situ end-labelling of fragmented DNA. From week 4 post fertilization onwards, apoptotic cells were more frequently detected in the cortex than medulla, suggesting a continuous selection of thymocytes in the cortex. Ultrastructural studies confirmed the presence of numerous cortical apoptotic cells inside macrophages. Electron microscopy provided evidence for the existence of epithelial heterogeneity in the thymus. During the ontogeny, the differentiation of epithelial cells was followed from the first weeks until the juvenile age. Cell types were classified on the basis of their localization and cytological characteristics as: i) limiting epithelial cells located in subcapsular, perivascular and peritrabecular zones; ii) reticular epithelial cells situated in medullary and cortical zones; iii) nurse-like cells at the border between the cortex and medulla, iiii) Hassall's body-like structures localized in the medulla. This study could suggest the occurrence of a wide range of lympho-epithelial interactions throughout thymocytes differentiation.

Immunodetection of lymphocyte subpopulations involved in allograft rejection in a teleost, Dicentrarchus labrax (L.)

Monoclonal antibodies which recognize antigenic determinants expressed by T-cells and Ig-bearing cells, respectively, allowed lymphocyte subpopulations involved in allograft rejection of muscle transplants to be identified in the teleost fish Dicentrarchus labrax (L.). The monoclonal antibody DLT15 first allowed recognizing T-cells involved in an in vivo antigen-driven cellular response in teleosts. Immunohistochemical studies showed a high density of lymphocytes in allografts and provided evidence of predominance of T-cells. The heterogeneity of the cell populations recognized by the antibodies was evidenced by the different size, cytology, and staining patterns of T-cells and Ig-bearing cells.

Expression of MhcCyca class I and class II molecules in the early life history of the common carp (Cyprinus carpio L.)

In this study transcription of class I alpha chain (Cyca-UA), beta2-microglobulin (Cyca-B2m) and class II alpha (Cyca-DXA) and beta (Cyca-DAB) during the early stages of embryo development was investigated by semiquantitative PCR. No transcripts of the genes under investigation were detected in the unfertilized egg. The expression of the genes encoding for the class II molecules revealed to be synchronized starting at day 1, unlike those for the class I molecules. Transcription of Cyca-B2m was first detected at day 7, whereas Cyca-UA was already present on day 1. This discrepancy would suggest absence of class I molecules during early development. The transcription of the Mhc genes in lymphoid organs was well established on day 21, with the exception of the spleen. In later stages of ontogeny cell surface expression of class I molecules was studied using polyclonal antibodies to Cyca-UA and Cyca-B2m in conjunction with detection of surface Ig. In week 3-10 Cyca-B2m was found on a higher percentage of cells from pronephros, spleen and thymus compared to Cyca-UA, suggesting the use of an alternative class I alpha chain. In the thymus, unlike the other organs, this difference remained present in the adult stage. The most likely candidates are alpha chains encoded by non-classical class I genes.

Indications for a distinct putative T cell population in mucosal tissue of carp (Cyprinus carpio L.)

A monoclonal antibody against carp intestinal T cells (WCL38; of IgM class) was produced by immunization of mice with isolated membrane molecules of carp intestinal intraepithelial lymphoid cells. Flow cytometric analysis showed that WCL38 reacted with 50-70% of the lymphoid cells isolated from intestine, gills or skin, with less than 6% of lymphoid cells isolated from thymus, head kidney or spleen and with a negligible number of PBL. WCL38+ cells were abundant in the intestinal epithelium and less numerous in the lamina propria. Immunogold labelling confirmed that WCL38 reacted with lymphoid cells; in gills and skin some of them have the morphology of large granular lymphoid cells. Immunochemical analysis showed that WCL38 reacted with dimeric membrane molecule on mucosal lymphoid cells with an Mr of 76 kDa, consisting of two 38 kDa subunits. WCL 38+ lymphoid cells are postulated to T cells, since WCL38 does not react with B cells, macrophages or non-specific cytotoxic cells. In conclusion, like higher vertebrates, carp seem to have a distinct (Putative) T cell population in their mucosal tissues.