The cellular protein expression of Foxp3 in lymphoid and non-lymphoid organs of Nile tilapia

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.

Expression analysis of grass carp Foxp3 and its biologic effects on CXCL-8 transcription in non-lymphoid cells

Teleost Forkhead box protein P3 (Foxp3) expression was discovered not only in regulatory T cells (Tregs) but also in other cells. Compared to the extensive study on its roles in lymphoid cells, the expression pattern and biological roles of Foxp3 in non-lymphoid cells have not been elucidated in both mammals and fish species. In the present study, grass carp Foxp3 (gcFoxp3) mRNA expression was detected in different cell types, showing that it has a moderate expression level in peripheral blood leukocytes (PBLs), head kidney leukocytes (HKLs) and grass carp fibroblast-like kidney cells (CIK cells). Interestingly, gcFoxp3 mRNA and protein expression could be significantly stimulated by polyinosinic-polycytidylic acid (poly I:C) in CIK cells, indicating its participation in poly I:C-induced immune response in non-lymphoid cells. To further investigate the function of gcFoxp3, its overexpression plasmid was constructed and transfected into CIK cells. After 24 h of transfection, grass carp C-X-C chemokine ligand (CXCL) 8 (gcCXCL-8) mRNA expression was elevated, implying the modulatory role of gcFoxp3 in gcCXCL-8 mRNA expression. This notion was further supported by the features of gcCXCL-8 promoter which contained a putative Foxp3 binding site at -2196 to -2190 region. Poly I:C or overexpression of gcFoxp3 obviously stimulated gcCXCL-8 promoter activity and deletion of gcFoxp3 binding region on the promoter abolished this stimulation, revealing that Foxp3 is pivotal for transcription of CXCL-8 induced by poly I:C. In conclusion, our results collectively demonstrate expression pattern of teleost Foxp3, and illuminate novel immune function of fish Foxp3 in regulating chemokine transcription in non-lymphoid cells.

Characterization of nanog in Nile tilapia (Oreochromis niloticus) and its spatiotemporal expression patterns during embryonic and gonadal development

Nanog is one of the well-characterized core transcription factors in pluripotency maintenance network. So far, studies on fishes indicate that the Nanog expression occurs from embryonic 1-cell stage to blastula stage, and is restricted to the gonadal germline cells in adult tissues, which is strikingly different from that in mammals. However, whether this expression profile is conservative in fishes remains to be investigated. Here Nile tilapia (Oreochromis niloticus) nanog (named as Ong) was identified and its spatiotemporal expression patterns during embryonic and gonadal development were investigated. The Ong cDNA contains an open reading frame of 678 bp, encoding 226 amino acids. The anti-Ong antibody was prepared through prokaryotic protein expression and its specificity was validated. The Ong expression in embryonic 1-cell stage did not appear until the early stage of blastocyst and continued to the late stage of blastocyst. In adult tissues, its expression was limited to gonads. Its expression patterns during gonadal development were further investigated by in situ hybridization and immunohistochemical staining. In testis, Ong was not expressed at 30 dah (days after hatching), but highly expressed in spermatogonia and spermatocytes at 150 dah; in ovaries at 30 and 150 dah, it was not expressed in germline cells but in all somatic cells. This expression profile is strikingly different from reports in fishes to date. Our study firstly indicates that the Nanog expression profile is not conservative in fishes. This study is valuable for further functional and evolutionary study of this gene.

Identification and Characterization of a Master Transcription Factor of Th1 Cells, T-bet, Within Flounder (Paralichthys olivaceus)

T-bet, a T-box family member, is a transcription factor essential for the differentiation of naive CD4⁺ T cells into Th1 cells that are involved in both innate and adaptive immune responses. In this study, the transcription factor T-bet of flounder (Paralichthys olivaceus) was cloned and characterized, and its expression profile after infection was analyzed. T-bet⁺ cells were identified in flounder, and the expression and localization of T-bet in T lymphocyte subsets and B lymphocytes were investigated. Finally, the proliferation of T-bet⁺ cells, T lymphocyte subsets, and B lymphocytes were studied after stimulation with IFN-γ, IL-2, and IL-6, respectively, and the variations of some transcription factors and cytokines in CD4⁺ T lymphocyte subsets were detected. The results showed that T-bet in flounder consists of 619 aa with a conserved T-box DNA binding domain. T-bet was abundantly expressed in the spleen, head kidney, and heart, and it was significantly upregulated after infection with Vibrio anguillarum, Edwardsiella tarda, and Hirame rhabdovirus, especially in the group of Edwardsiella tarda. A polyclonal antibody against recombinant protein of T-bet was prepared, which specifically recognized the natural T-bet molecule in flounder. T-bet⁺ cells were found to be distributed in the lymphocytes of peripheral blood, spleen, and head kidney, with the highest proportion in spleen, and the positive signals of T-bet occurred in the cell nucleus. T-bet was also detected in the sorted CD4-1⁺, CD4-2⁺, CD8⁺ T lymphocytes, and IgM⁺ B lymphocytes. In addition, T-bet⁺ cells, coordinated with CD4-1⁺ and CD4-2⁺ T lymphocytes, were proliferated after stimulation with IFN-γ, IL-2, and IL-6. Especially in sorted CD4-1⁺ and CD4-2⁺ T lymphocytes, IFN-γ and IL-2 were able to upregulate the expression of T-bet, forming a positive feedback loop in Th1-type cytokine secretion. These results suggest that T-bet may act as a master transcription factor regulating flounder CD4⁺ T lymphocytes involved in a Th1-type immune response.

Induction of foxp3 during the Crosstalk between Antigen Presenting Like-Cells MHCII+CD83+ and Splenocytes CD4+IgM- in Rainbow Trout

Simple Summary

In aquatic biological models, the communication between cells from the immune system remains poorly characterized. In this work, to determine the gene expression of master transcriptional factors that coordinate the polarization of T cells, co-cultures of rainbow trout splenocytes are analyzed after stimulation with Interferon-gamma and/or Piscirickettsia salmonis. The results showed an upregulation of foxp3 compared to the other transcriptional factors, suggesting a potential communication between cells in the spleen, which may induce a Treg phenotype.

Abstract

In fish, the spleen is one of the major immune organs in the animal, and the splenocytes could play a key role in the activation and modulation of the immune response, both innate and adaptive. However, the crosstalk between different types of immune cells in the spleen has been poorly understood. In this work, an in vitro strategy is carried out to obtain and characterize mononuclear splenocytes from rainbow trout, using biomarkers associated with lymphocytes (CD4 and IgM) and antigen-presenting cells (CD83 and MHC II). Using these splenocytes, co-cultures of 24 and 48 h are used to determine the gene expression of master transcriptional factors that coordinate the polarization of T cells (t-bet, gata3, and foxp3). The results show a proportional upregulation of foxp3 (compared to t-bet and gata3) in co-cultures (at 24 h) of IFNγ-induced splenocytes with and without stimulation of Piscirickettsia salmonis proteins. In addition, foxp3 upregulation was established in co-cultures with IFNγ-induced cells and in cells only stimulated previously with P. salmonis proteins at 48 h of co-culture. These results show a potential communication between antigen-presenting-like cells and lymphocyte in the spleen, which could be induced towards a Treg phenotype.

Who needs the hotspot? The effect of temperature on the fish host immune response to Tetracapsuloides bryosalmonae the causative agent of proliferative kidney disease

Proliferative kidney disease (PKD) of salmonids, caused by Tetracapsuloides bryosalmonae may lead to high mortalities at elevated water temperatures. However, it has not yet been investigated how temperature affects the fish host immune response to T. bryosalmonae. We exposed YOY (young of the year) rainbow trout (Oncorhynchus mykiss) to T. bryosalmonae at two temperatures (12 °C and 15 °C) that reflect a realistic environmental scenario and could occur in the natural habitat of salmonids. We followed the development of the parasite, host pathology and immune response over seven weeks. We evaluated the composition and kinetics of the leukocytes and their major subgroups in the anterior and posterior kidney. We measured immune gene expression profiles associated with cell lineages and functional pathways in the anterior and posterior kidney. At 12 °C, both infection prevalence and pathogen load were markedly lower. While the immune response was characterized by subtle changes, mainly an increased amount of lymphocytes present in the kidney, elevated expression of Th1-like signature cytokines and strong upregulation of the natural killer cell enhancement factor, NKEF at week 6 P.E. At 15 °C the infection prevalence and pathogen burden were ominously greater. While the immune response as the disease progressed was associated with a Th2-like switch at week 6 P.E and a prominent B cell response, evidenced at the tissue, cell and transcript level. Our results highlight how a subtle, environmentally relevant difference in temperature resulted in diverse outcomes in terms of the immune response strategy, altering the type of interaction between a host and a parasite.

Fish gut-liver immunity during homeostasis or inflammation revealed by integrative transcriptome and proteome studies

The gut-associated lymphoid tissue, connected with liver via bile and blood, constructs a local immune environment of both defense and tolerance. The gut-liver immunity has been well-studied in mammals, yet in fish remains largely unknown, even though enteritis as well as liver and gallbladder syndrome emerged as a limitation in aquaculture. In this study, we performed integrative bioinformatic analysis for both transcriptomic (gut and liver) and proteomic (intestinal mucus and bile) data, in both healthy and infected tilapias. We found more categories of immune transcripts in gut than liver, as well as more adaptive immune in gut meanwhile more innate in liver. Interestingly reduced differential immune transcripts between gut and liver upon inflammation were also revealed. In addition, more immune proteins in bile than intestinal mucus were identified. And bile probably providing immune effectors to intestinal mucus upon inflammation was deduced. Specifically, many key immune transcripts in gut or liver as well as key immune proteins in mucus or bile were demonstrated. Accordingly, we proposed a hypothesized profile of fish gut-liver immunity, during either homeostasis or inflammation. Current data suggested that fish gut and liver may collaborate immunologically while keep homeostasis using own strategies, including potential unique mechanisms.

Characterization of the POU5F1 Homologue in Nile Tilapia: From Expression Pattern to Biological Activity

POU5F1 (OCT4) is a crucial transcription factor for induction and maintenance of cellular pluripotency, as well as survival of germ cells in mammals. However, the homologues of POU5F1 in teleost fish, including zebrafish and medaka, now named Pou5f3, exhibit considerable differences in expression pattern and pluripotency-maintaining activity. To what extent the POU5F1 homologues are conserved in vertebrates has been unclear. In this study, we report that the POU5F1 homologue from the Nile tilapia (Oreochromis niloticus), OnPou5f3, displays an expression pattern and biological activity somewhat different from those in zebrafish or medaka. The expression of Onpou5f3 at both mRNA and protein levels was abundant in early development embryos until blastula stages, barely detectable as proceeding, and then displayed a transiently strong expression domain in the brain region during neurula stages similar to zebrafish but not medaka. Afterward, OnPou5f3 appeared as germline-restricted (including primordial germ cells and female and male gonad germ cells) expression just like medaka. Notably, OnPou5f3 depletion through morpholino oligos caused blastula blockage or lethality and failure of survival and proliferation of blastula cell-derived cells. These findings indicate that equivalent POU5F1-like expression and activity of Pou5f3 might be conserved accompanying with species-specific expression pattern during evolution. Our study provides insight into the evolutionary conservation of the POU5F1 homologues across vertebrates.