Kinetics of lymphocyte subpopulations in allogeneic grafted scales of ginbuna crucian carp

Immune Rejection Mediated by prf1 and gzmb Affects the Colonization of Fat Greenling (Hexagrammos otakii) Spermatogonia in Heterotransplantation

Fish germ cell transplantation holds great potential for conserving endangered species, improving cultured fish breeds, and exploring reproductive techniques. However, low transplantation efficiency is a common issue in heterotransplantation. This study transplanted fat greenling (Hexagrammos otakii) spermatogonia into the testes of spotted sea bass (Lateolabrax maculatus) to investigate factors that might affect the colonization and fixation of heterologous transplanted germ cells. Results indicated that transplanted fat greenling spermatogonia cells were successfully detected in the early transplantation phase in spotted sea bass. Their numbers gradually decreased over time, and after 10 days post-transplantation, more than 90% of the transplanted cells underwent apoptosis. Transcriptome sequencing analysis of the testes of spotted sea bass and fat greenling spermatogonia on days 1 and 10 post-transplantation revealed that this apoptosis process involved many immune-related genes and their associated signaling pathways. Acute immune rejection marker genes prf1 and gzmb were detected in the spotted sea bass testes, while immune tolerance genes lck and zap-70 were expressed in the fat greenling spermatogonia. Additionally, differential expression of prf1 and gzmb genes was screened from spotted sea bass, with experimental evidence indicating that PRF1 and GZMB protein from spotted sea bass primarily induce apoptosis in transplanted fat greenling spermatogonia via the mitochondrial apoptosis pathway, at the protein level. This suggests that the difficulties in heterotransplantation are primarily related to acute immune rejection, with PRF1 and GZMB playing significant roles.

CKLF instigates a "cold" microenvironment to promote MYCN-mediated tumor aggressiveness

Solid tumors, especially those with aberrant MYCN activation, often harbor an immunosuppressive microenvironment to fuel malignant growth and trigger treatment resistance. Despite this knowledge, there are no effective strategies to tackle this problem. We found that chemokine-like factor (CKLF) is highly expressed by various solid tumor cells and transcriptionally up-regulated by MYCN. Using the MYCN-driven high-risk neuroblastoma as a model system, we demonstrated that as early as the premalignant stage, tumor cells secrete CKLF to attract CCR4-expressing CD4+ cells, inducing immunosuppression and tumor aggression. Genetic depletion of CD4+ T regulatory cells abolishes the immunorestrictive and protumorigenic effects of CKLF. Our work supports that disrupting CKLF-mediated cross-talk between tumor and CD4+ suppressor cells represents a promising immunotherapeutic approach to battling MYCN-driven tumors.

Adjuvant effect of allogeneic blood in vaccines against edwardsiellosis in ginbuna crucian carp Carassius auratus langsdorfii

Edwardsiella tarda (E. tarda), an intracellular pathogen, has caused severe economic losses in aquaculture. Effective vaccine development for E. tarda prevention is urgently needed. A previous study indicates that cell-mediated immunity (CMI) might play an important role in E. tarda infection. We believe that the involvement of allograft rejection and CMI has now been well documented in mammals and some fishes. However, there is still little research on the application of blood allograft rejection in vaccine development. In the current study, we investigate the immune response and vaccine effect in fish vaccinated with allogeneic blood + formalin-killed cells vaccine (FKC), allogeneic blood + phosphate-buffered saline (PBS), PBS + FKC and PBS + PBS. In the challenge test, the relative percentage survival (RPS) of the allogeneic + FKC, the allogeneic blood + PBS and the PBS + FKC group was 61.46, 35.41, and 30.63 % respectively. The up-regulated expression of Th1-related genes IFN-γ 1, IFN-γ 1rel2, IL-12p35 and T-bet suggests the protection is via CMI induction. Only in the allogeneic + FKC group, gene expression of IFN-γ 1, IL-12p35 and T-bet is significantly higher, indicating synergy between the two substances. Furthermore, among the fish injected with the allogeneic blood cells, syngeneic blood cells and PBS group, only in the fish of the allogenic blood cells injection group, did expression of IFN-γ 1, IFN-γ 2 and IFN-γ rel2 gene expression significantly increased. The results indicate that the rejection was induced by allogeneic components. Thus, our findings might provide essential information and insights into vaccine development in aquaculture.

CD4+ T lymphocyte responses to viruses and virus-relevant stimuli in teleost fish

Fish diseases caused by viruses are a major threat to aquaculture. Development of disease protection strategies for sustainable fish aquaculture requires a better understanding of the immune mechanisms involved in antiviral defence. The innate and adaptive arms of the vertebrate immune system collaborate to mount an effective defence against viral pathogens. The T lymphocyte components of the adaptive immune system, comprising two major classes (helper T, Th or CD4+ and cytotoxic T lymphocytes, CTLs or CD8+ T cells), are responsible for cell-mediated immune responses. In particular, CD4+ T cells and their different subsets orchestrate the actions of various other immune cells during immune responses, making CD4+ T cells central drivers of responses to pathogens and vaccines. CD4+ T cells are also present in teleost fish. Here we review the literature that reported the use of antibodies against CD4 in a few teleost fish species and transcription profiling of Th cell-relevant genes in the context of viral infections and virus-relevant immunomodulation. Studies reveal massive CD4+ T cell proliferation and expression of key cytokines, transcription factors, and effector molecules that evoke mammalian Th cell responses. We also discuss gaps in the current understanding and evaluation of teleost CD4+ T cell responses and how development and application of novel tools and approaches to interrogate such responses could bridge these gaps. A greater understanding of fish Th cell responses will further illuminate the evolution of vertebrate adaptive immunity, inform strategies to address viral infections in aquaculture, and could further foster fish as model organisms.

Cytokine networks provide sufficient evidence for the differentiation of CD4+ T cells in teleost fish

Cytokines, a class of small molecular proteins with a wide range of biological activities, are secreted mainly by immune cells and function by binding to the corresponding receptors to regulate cell growth, differentiation and effects. CD4⁺ T cells can be defined into different lineages based on the unique set of signature cytokines and transcription factors, including helper T cells (Th1, Th2, Th17) and regulatory T cells (Treg). In teleost, CD4⁺ T cells have been identified in a variety of fish species, thought to play roles as Th cells, and shown to be involved in the immune response following specific antigen stimulation. With the update of sequencing technologies, a variety of cytokines and transcription factors capable of characterizing CD4⁺ T cell subsets also have been described in fish, including hallmark cytokines such as IFN-γ, TNF-α, IL-4, IL-17, IL-10, TGF-β and unique transcription factors such as T-bet, GATA3, RORγt, and Foxp3. Hence, there is increasing evidence that the subpopulation of Th and Treg cells present in mammals may also exist in teleost fish. However, the differentiation, plasticity and precise roles of Th cell subsets in mammals remain controversial. Research on the identification and differentiation of fish Th cells is still in its infancy and requires more significant effort. Here we will review recent research advances in characterizing the differentiation of fish CD4⁺ T cells by cytokines and transcription factors, mainly including the identification of Th and Treg cell hallmark cytokines and transcription factors, the regulatory role of cytokines on Th cell differentiation, and the function of Th and Treg cells in the immune response. The primary purpose of this review is to deepen our understanding of cytokine networks in characterizing the differentiation of CD4⁺ T cells in teleost.

Induction of both local and systemic immunity by in vivo injection of PHA into ginbuna carp fin

Phytohemagglutinin (PHA) is a well-known mitogen inducing activation and proliferation of lymphocytes, particularly T lymphocytes in vitro. PHA has also been used in vivo for assessing cell-mediated immunity in non-mammalian vertebrates, particularly in birds. However, it has been suggested that local inflammation as a direct result of tissue damage could be responsible for skin swelling after PHA injection, in addition to induction of T lymphocyte mitogenesis. In order to understand the complex nature of this response in fish we investigated the accumulation of cell types chronologically in dorsal fin of ginbuna crucian carp Carassius auratus langsdorfii after PHA injection. Neutrophils appeared first and showed a peak response on day 1, decreasing gradually and followed by macrophages and blast cells while lymphocytes increased later with a peak response on day 5. The number of accumulated cells was significantly higher in PHA-injected fish than controls in most cases. Lymphocytes identified as CD4-1⁺and CD8α⁺ were significantly more abundant in PHA-injected fish than in control fish throughout the 7-day experimental period except on day 1, while the number of IgM⁺ lymphocytes was higher in PHA-injected fish only on day 1. In the blast cell fraction, the number of CD4-1⁺ lymphocytes was significantly higher in PHA-injected fish than in control fish throughout experimental period, except on day 1. We also document the migration of neutrophils from the kidney to the fin through blood, followed by granulopoiesis in the kidney. These results suggest that adaptive immunity as well as innate immunity was induced by in vivo stimulation with PHA.

Teleost CD4+ helper T cells: Molecular characteristics and functions and comparison with mammalian counterparts

CD4⁺ helper T cells play key and diverse roles in inducing adaptive immune responses in vertebrates. The CD4 molecule, which is found on the surfaces of CD4⁺ helper T cells, can be used to distinguish subsets of helper T cells. Teleosts are the oldest living species with bona-fide CD4 coreceptors. Although some components of immune systems of teleosts and mammals appear to be similar, many physiological differences are represented between them. Previous studies have shown that two CD4 paralogs are present in teleosts, whereas only one is present in mammals. Therefore, in this review, the CD4 molecular structure, expression profiles, subpopulations, and biological functions of teleost CD4⁺ helper T cells were summarized and compared with those of their mammalian counterparts to understand the differences in CD4 molecules between teleosts and mammals. This review provides suggestions for further studies on the CD4 molecular function and regulatory mechanism of CD4⁺ helper T cells in teleost fish and will help establish therapeutic strategies to control fish diseases in the future.

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.

Cross-reactivity of monoclonal antibodies against CD4-1 and CD8α of ginbuna crucian carp with lymphocytes of zebrafish and other cyprinid species

We have monoclonal antibodies (mAbs) against CD4-1 (6D1) and CD8α (2C3) in ginbuna crucian carp Carassius auratus langsdorfii. In our previous studies we showed that 2C3 mAb positive cells are the primary cell type showing specific cytotoxicity against allogeneic targets, suggesting that CD8α⁺ lymphocytes in ginbuna are equivalent to cytotoxic T lymphocytes (CTLs) in mammals. We further demonstrated the helper T cell function of 6D1 mAb positive cells by studying mixed leukocyte culture (MLC) and hapten/carrier effects. Here, we report that our mAbs cross-react with zebrafish lymphocytes. First, mAbs 6D1 and 2C3 recognized 7-11% of zebrafish lymphocytes that were ZAP-70 positive and had the typical morphology of lymphocytes. Second, to verify the cell types reacting with the 6D1 and 2C3 mAbs we examined the expression profiles of zebrafish lymphocyte surface markers in FACS-sorted lymphocytes from kidney. cd4-1 (cd8a) and tcrac but not iglc transcripts were detected in 6D1(2C3)⁺ lymphocytes, whereas cd4-1 (cd8a) transcripts were not found in 6D1 (2C3)^- lymphocytes. Third, we further confirmed that 6D1 reacted with zebrafish CD4-1 but not CD4-2, and 2C3 recognized zebrafish CD8α expressed on HEK293T cells. Collectively, these findings suggest that 6D1⁺ and 2C3⁺ lymphocytes in zebrafish are equivalent to CD4⁺ and CD8α⁺ T lymphocytes in mammals, respectively. Furthermore, we found the cross-reactivity of our 6D1 and 2C3 mAbs with other cyprinid species including goldfish, common carp and grass carp.

A simple and non-invasive method for analyzing local immune responses in vivo using fish fin

Immunocompetence is an important parameter that reflects disease resistance in fish. Very few methods to examine immunocompetence in vivo have been developed, even for mammals. In the present study, we present a unique method for analyzing local immune responses using fish fin. We first demonstrated the migration of granulocytes to the site of zymosan injection in fin in a dose-dependent manner and that this could be easily observed macroscopically due to the fin membrane transparency. We also demonstrated phagocytic activity of accumulated leukocytes after zymosan administration and that almost all phagocytes were granulocytes. In addition, we succeeded to detect respiratory burst activity of granulocytes in vivo without any in vitro treatment of cells, indicating that our present method is suitable for the analysis of granulocyte phagocytic function in vivo. The method provides a unique tool applicable for fishes that possess transparent fins and may lead to better understanding of the mechanisms of local immune responses in fish.

Stimulatory effects of heat-killed Enterococcus faecalis on cell-mediated immunity in fish

Intracellular bacterial and viral diseases are widespread in the aquaculture industry and cause serious economic losses. Development of effective vaccines and adjuvants that can induce cell-mediated immunity is urgently needed for prevention of these diseases. Here we report the immunostimulatory effects of probiotic bacteria ''E. faecalis'' in ginbuna crucian carp Carassius auratus langsdorfii. Intraperitoneal injection of heat-killed E. faecalis induced an increase in CD4-1⁺ lymphocytes, CD8α⁺ lymphocytes and macrophages in vivo. Expression of Th1 cytokine genes was enhanced by exposure to the bacteria in vitro. We identified the leukocyte subsets that expressed specific Th1 cytokine genes: granulocytes and macrophages produced IL12 and IFNγrel2, respectively, while lymphocytes produced IFNγs including IFNγ1 and IFNγ2. Finally, expression of Th1 cytokines was also enhanced by intraperitoneal injection of heat-killed E. faecalis in vivo, while expression of Th2 cytokine was unchanged. Together, these findings suggest that heat-killed E. faecalis can induce cell-mediated immunity in fish.

Comprehensive validation of T- and B-cell deficiency in rag1-null zebrafish: Implication for the robust innate defense mechanisms of teleosts

rag1^−/− zebrafish have been employed in immunological research as a useful immunodeficient vertebrate model, but with only fragmentary evidence for the lack of functional adaptive immunity. rag1-null zebrafish exhibit differences from their human and murine counterparts in that they can be maintained without any specific pathogen-free conditions. To define the immunodeficient status of rag1^−/− zebrafish, we obtained further functional evidence on T- and B-cell deficiency in the fish at the protein, cellular, and organism levels. Our developed microscale assays provided evidence that rag1^−/− fish do not possess serum IgM protein, that they do not achieve specific protection even after vaccination, and that they cannot induce antigen-specific CTL activity. The mortality rate in non-vaccinated fish suggests that rag1^−/− fish possess innate protection equivalent to that of rag1^+/− fish. Furthermore, poly(I:C)-induced immune responses revealed that the organ that controls anti-viral immunity is shifted from the spleen to the hepatopancreas due to the absence of T- and B-cell function, implying that immune homeostasis may change to an underside mode in rag-null fish. These findings suggest that the teleost relies heavily on innate immunity. Thus, this model could better highlight innate immunity in animals that lack adaptive immunity than mouse models.

Effects of IFNγ administration on allograft rejection in ginbuna crucian carp

In vertebrates, the rejection of allografts is primarily accomplished by cell-mediated immunity. We recently identified four IFNγ isoforms with antiviral activity in ginbuna crucian carp, Carassius auratus langsdorfii. However, involvement of the IFNγ isoforms in cell-mediated immunity, especially in T cell function remains unknown. Here we investigate expression of the IFNγ isoforms and effects of administration of recombinant IFNγ (rgIFNγ) isoforms in ginbuna scale allograft rejection. All four IFNγ isoforms showed significantly higher expression with the progression of graft rejection. Administration of rgIFNγrel 1 but not rgIFNγrel 2, rgIFNγ1 nor rgIFNγ2 enhanced allograft rejection. The number of CD4(+) and CD8α(+) cells increased in early stages of rejection, while sIgM(+) cells were higher than controls at day 0 and 5 in the rgIFNγrel 1 administrated group. Expression of IFNγ1 and IFNγ2 mRNA was significantly up-regulated by rgIFNγrel 1 administration, while that of IFNγrel 1 and IFNγrel 2 was not. These results suggest different contributions of the four IFNγ isoforms toward the immune responses comprising allograft rejection.

Selection for brain size impairs innate, but not adaptive immune responses

Both the brain and the immune system are energetically demanding organs, and when natural selection favours increased investment into one, then the size or performance of the other should be reduced. While comparative analyses have attempted to test this potential evolutionary trade-off, the results remain inconclusive. To test this hypothesis, we compared the tissue graft rejection (an assay for measuring innate and acquired immune responses) in guppies (Poecilia reticulata) artificially selected for large and small relative brain size. Individual scales were transplanted between pairs of fish, creating reciprocal allografts, and the rejection reaction was scored over 8 days (before acquired immunity develops). Acquired immune responses were tested two weeks later, when the same pairs of fish received a second set of allografts and were scored again. Compared with large-brained animals, small-brained animals of both sexes mounted a significantly stronger rejection response to the first allograft. The rejection response to the second set of allografts did not differ between large- and small-brained fish. Our results show that selection for large brain size reduced innate immune responses to an allograft, which supports the hypothesis that there is a selective trade-off between investing into brain size and innate immunity.

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.