Expression profiles of TCRβ and CD8α mRNA correlate with virus-specific cell-mediated cytotoxic activity in ginbuna crucian carp

Alloantigen-specific killing is mediated by CD8-positive T cells in fish

CD8-positive (CD8(+)) cytotoxic T lymphocytes (CTL) have antigen-specific cytotoxic activity. In fish, however, CTL expressing CD8 on their cell surface have not been identified. In order to characterize the cells involved in specific cell-mediated cytotoxicity in teleosts, we separated and sorted ginbuna kidney leucocytes into CD8alpha(+), CD4(+) and surface IgM (sIgM)(+) cells by magnetic activated cell sorting using monoclonal antibodies and examined their cytotoxic activities. Effector donor ginbuna (OB1 clone) were sensitized by allografting scales from S3N clone fish followed by injection of an allogeneic cell line (CFS) derived from S3N fish. In cytotoxic assays, target cells were labeled with CFSE and cytotoxicity was calculated based on the number of viable target cells using flow cytometry. CD8alpha(+) cells from sensitized OB1 fish showed relatively high cytotoxicity against CFS cells (immunogen) but not against allogeneic CFK cells (third party) nor isogeneic CFO cells. Pre-sensitized sIgM(+) cells exhibited cytotoxicity against not only CFS cells but also CFK cells. However, CD4(+) or CD8alpha(-) CD4(-)sIgM(-) cells as well as cells from non-sensitized fish did not show any significant cytotoxic activity. These results suggest that CD8alpha(+) cells in fish have characteristics similar to those of CTL in mammals, and that the sIgM(+) cells include NK-like cells which non-specifically killed the target cells.

Long-term hematopoietic reconstitution by transplantation of kidney hematopoietic stem cells in lethally irradiated clonal ginbuna crucian carp (Carassius auratus langsdorfii)

The study of hematopoietic stem cells (HSCs) has been facilitated by the transplantation of bone marrow cell populations into lethally irradiated mice. It is widely known that HSCs have the capacity for long-term and multilineage hematopoietic reconstitution in lethally irradiated hosts. Here, we developed a transplantation model system using clonal ginbuna crucian carp (Carassius auratus langsdorfii) that were exposed to a lethal dose of X-rays. The minimum lethal dose (MLD) of ginbuna was approximately 25Gy, which is lethal due to hematopoietic failure. The transplantation of kidney hematopoietic cells into lethally irradiated ginbuna resulted in the rescue of recipient fish for more than 180 days. We examined the reconstitution activity of head kidney (HK), body kidney (BK), and spleen cells. Transplantation experiments showed that only HK and BK cells had the long-term and multilineage reconstitution activity. These results indicate that teleost HSCs have the ability to fully reconstitute the hematopoietic system in lethally irradiated hosts, and that they are present in HK and BK, but not in spleen. This transplantation model system using clonal ginbuna is useful for studies of in vivo kinetics and functions of HSCs in teleosts.

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.

Cell-mediated immune responses in rainbow trout after DNA immunization against the viral hemorrhagic septicemia virus

To identify viral proteins that induce cell-mediated cytotoxicity (CMC) against viral hemorrhagic septicemia virus (VHSV)-infected cells, rainbow trout were immunized with DNA vectors encoding the glycoprotein G or the nucleocapsid protein N of VHSV. The G protein was a more potent trigger of cytotoxic cells than the N protein. Peripheral blood leukocytes (PBL) isolated from trout immunized against the G protein killed both VHSV-infected MHC class I matched (RTG-2) and VHSV-infected xenogeneic (EPC) target cells, suggesting the involvement of both cytotoxic T lymphocytes (CTL) and NK cells, respectively. In contrast, PBL from trout that were immunized against the N protein only killed VHSV-infected RTG-2 cells, indicating that this protein only elicits a CTL response. Further, a significant killing capacity of these PBL was only observed during summer months. PBL from fish that were immunized against the VHSV G protein significantly killed VHSV-infected but not infectious hematopoietic necrosis virus (IHNV)-infected targets indicating antigen specificity. Thus, this is the first report on cytotoxic immune responses after DNA vaccination in fish. Furthermore, cells isolated from the inflamed site of DNA injection were stained and transferred to isogeneic DNA-vaccinated recipients. Most of the stained donor leukocytes accumulated at the recipients' DNA injection site showing, for the first time, leukocyte homing in fish. Transferred donor leukocytes mainly migrated to the homologous vaccine injection site rather than to injection sites of heterologous vaccines, suggesting the antigen specificity of homing. By demonstrating CMC responses to distinct viral proteins and homing in rainbow trout, these results substantially contribute to the understanding of the teleost immune system.

Molecular cloning and expression analysis of T-bet in ginbuna crucian carp (Carassius auratus langsdorfii)

In the adaptive immune system of mammals, naive helper T (Th) cells differentiate into Th1 or Th2 cells. The T-box expressed in T cells (T-bet) is a member of a family of T-box transcription factors that regulates the expression of IFN-gamma and plays a crucial role in Th1 cell differentiation and cell-mediated immunity. We cloned and sequenced T-bet cDNA for the first time from non-mammalian species, ginbuna crucian carp. Ginbuna T-bet was composed of 608 predicted amino acids and showed 41.5% identity with human T-bet (Tbx21), and human and ginbuna T-bet share 77.3% identity in their T-box regions. Comparative genomic analysis showed conserved synteny in these regions between zebrafish, fugu, medaka and human T-bet. Phylogenetic analysis indicated that ginbuna T-bet is closely related to that of mouse and human. In unstimulated fish, ginbuna T-bet mRNA was strongly expressed in peripheral blood leukocytes (PBL), head kidney (HK) and spleen. RT-PCR analysis in kidney cells sorted by FACS revealed that T-bet was strongly expressed in surface-IgM-negative lymphocytes in comparison to IgM-positive lymphocytes. These results suggest that ginbuna T-bet is involved in the immune system, especially in T-cell function, and is an important tool to analyze teleost cell-mediated immunity.

Transcription of signal-3 cytokines, IL-12 and IFN alpha beta, coincides with the timing of CD8 alpha beta up-regulation during viral infection of common carp (Cyprinus carpio L)

Mammalian naïve CD8+ T cells are activated by antigen (signal 1) and CD28 costimulation (signal 2) to undergo several rounds of cell division, but programming for survival, effector function and memory requires a third signal that can be provided by IL-12 and/or type I interferons. Functional studies indicate that the route of antigen presentation and costimulation are conserved from fish to mammals. However, the potential of IL-12 and IFN alpha beta to act as signal-3 cytokines in infections inducing a CTL response has not been examined in fish. We report the cloning of CD8 alpha and CD8 beta homologues, each present in duplicate copies and of two TCR-C alpha isoforms in European common carp. The identification of (cytotoxic) T cell marker sequences and the availability of sequences coding for the signal-3 cytokines in the same fish species, allowed us to investigate by RT-qPCR their kinetics of gene expression during viral and parasitic infection. Our results show that transcription of signal-3 cytokines occurred concomitantly with CD8 alpha beta up-regulation exclusively at 4 days post-primary viral infection. No regulation of IL-12 and IFN alpha beta was observed after parasitic infection. Our data provide evidences for an evolutionary conservation of function for IL-12 and IFN alpha beta to act as third signal during CTL activation. In addition, we suggest that a CD8 alpha 2/beta1 and a p35p40b association could be the preferred combinations for the formation of a functional CD8 co-receptor and an IL-12p70 heterodimer during viral infection. The relevance of our findings to future vaccination strategies in fish is discussed.

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.

Expression analysis of two Eomesodermin homologues in zebrafish lymphoid tissues and cells

Eomesodermin (Eomes) is a T-box transcription factor that is involved in mesoderm formation in most vertebrates. Eomes is also expressed in CD8+ T cells and NK cells. No information is available on the role of Eomes in the immune system of lower vertebrates to date, although developmental studies on Eomes (Eomes1) have been performed in zebrafish. Here we report the identification of a second Eomes (Eomes2) in zebrafish and compare expression of the two Eomes genes in the immune system. Zebrafish Eomes1 and Eomes2, composed of 661 and 534 amino acids, respectively, share 49.3% amino acid identity in their coding regions and 88.7% amino acid identity in their T-box regions. Conserved synteny between regions of the human and zebrafish genomes, gene organization and phylogenetic analysis all indicate that the zebrafish Eomes2 gene is a homologue of mammalian Eomes, as previously found for zebrafish Eomes1. Eomes1 mRNA was found to be expressed in the gonad, body kidney, spleen and gill, while Eomes2 mRNA was not detected in any of these tissues. However, strong expression of both Eomes mRNAs was detected in the leukocytes from the spleen, followed by those from body kidney and peripheral blood, with expression of Eomes1 always stronger than that of Eomes2. RT-PCR analysis of body kidney cells sorted by FACS revealed that Eomes1 was expressed strongly in lymphocytes, weakly in blast cells, and was not expressed in granulocytes, while Eomes2 was expressed weakly in lymphocytes. These results suggest that both Eomes genes are involved in the zebrafish immune response, particularly in lymphocyte function as has been found in mammals.