Molecular characterization and expression analysis of T cell receptor (TCR) γ and δ genes in dojo loach (Misgurnus anguillicaudatus) in response to bacterial, parasitic and fungal challenge

Identification and functional analyses of CD4-1+ cells in grass carp (Ctenopharyngodon idella)

In mammals, CD4 is found to be expressed on T cells and innate immune cells, however, teleost cells bearing CD4 have not been well identified and characterized. In this study, we identified two different CD4-1+ cell subsets in grass carp (Ctenopharyngodon idella): CD4-1+ lymphocytes (Lym) and CD4-1+ myeloid cells (Mye), both of which had the highest proportions in the head kidney. The mRNA expression analysis showed that CD4-1, CD4-2, TCRβ, CD3γ/δ, and LCK1 are highly expressed in CD4-1+ Lym and also expressed in CD4-1+ Mye. Furthermore, we found that CD4-1+ Lym have a Lym morphology and highly express T-cell cytokines, suggesting that they are CD4+ T cells equivalent to mammalian Th cells. On the other hand, CD4-1+ Mye were found to have a morphology of macrophage and highly express macrophage marker gene MCSFR, indicating that they are macrophages. In addition, functional analysis revealed that CD4-1+ Mye possess phagocytic ability and great antigen-processing ability. Taken together, our study sheds further light on the composition and function of CD4+ cells in teleost fish.

Identification, expression and molecular polymorphism of T-cell receptors α and β from the glacial relict Hucho bleekeri

The T-cell receptor (TCR) is a specific molecule on the surface of all T cells that mediates cellular adaptive immune responses to antigens. Hucho bleekeri is a critically endangered species and is regarded as a glacial relict that has the lowest-latitude distribution compared with any Eurasian salmonid. In the present study, two TCR genes, namely, TCR α and β, were identified and characterized in H. bleekeri. Both TCR α and TCR β have typical TCR structures, including the IgV domain, IgC domain, connecting peptide, transmembrane and cytoplasmic domains. The two TCR genes were constitutionally expressed in various tissues, with the highest expression found in the spleen for TCR α and in the trunk kidney for TCR β. Challenge of H. bleekeri with LPS or poly(I:C) resulted in significant upregulation of both TCR α and β expression in headkidney and spleen primary cells, indicating their potential roles in the immune response. Molecular polymorphism analysis of the whole ORF regions of TCR α and β in different individuals revealed high diversity of IgV domains of these two genes, especially in complementarity-determining region (CDR) 3. The ratio of nonsynonymous substitution occurred at a significantly higher frequency than synonymous substitution in the CDR of TCR α and β, demonstrating the existence of positive selection. The results obtained in the present study enhance our understanding of TCR roles in regulating immune mechanisms and provide new information for the study of TCR lineage diversity in fish.

Regulatory roles of cytokines in T and B lymphocytes-mediated immunity in teleost fish

T and B lymphocytes (T and B cells) are immune effector cells that play critical roles in adaptive immunity and defend against external pathogens in most vertebrates, including teleost fish. In mammals, the development and immune response of T and B cells is associated with cytokines including chemokines, interferons, interleukins, lymphokines, and tumor necrosis factors during pathogenic invasion or immunization. Given that teleost fish have evolved a similar adaptive immune system to mammals with T and B cells bearing unique receptors (B-cell receptors (BCRs) and T-cell receptors (TCRs)) and that cytokines in general have been identified, whether the regulatory roles of cytokines in T and B cell-mediated immunity are evolutionarily conserved between mammalians and teleost fish is a fascinating question. Thus, the purpose of this review is to summarize the current knowledge of teleost cytokines and T and B cells as well as the regulatory roles of cytokines on these two types of lymphocytes. This may provide important information on the parallelisms and dissimilarities of the functions of cytokines in bony fish versus higher vertebrates, which may aid in the evaluation and development of adaptive immunity-based vaccines or immunostimulants.

Endogenic upregulations of HIF/VEGF signaling pathway genes promote air breathing organ angiogenesis in bimodal respiration fish

Air-breathing has evolved independently serval times with a variety of air-breathing organs (ABOs) in fish. The physiology of the air-breathing in bimodal respiration fish has been well understood, while studies on molecular mechanisms of the character are very limited. In the present study, we first determined the gill indexes of 110 fish species including 25 and 85 kinds of bimodal respiration fishes and non-air-breathing fishes, respectively. Then combined with histological observations of gills and ABOs/non-ABOs in three bimodal respiration fishes and two non-air breathing fishes, we found that the bimodal respiration fish was always of a degeneration gill and a well-vascularized ABO. Meanwhile, a comparative transcriptome analysis of posterior intestines, namely a well vascularized ABO in Misgurnus anguillicaudatus and a non-ABO in Leptobotia elongata, was performed to expound molecular variations of the air-breathing character. A total of 5,003 orthologous genes were identified. Among them, 1,189 orthologous genes were differentially expressed, which were enriched in 14 KEGG pathways. More specially, the expressions of hemoglobin genes and various HIF/VEGF signaling pathway genes were obviously upregulated in the ABO of M. anguillicaudatus. Moreover, we found that HIF-1α, VEGFAa, and MAP2K1 were co-expressed dramatically higher in ABOs of bimodal respiration fishes than those of non-ABOs of non-air-breathing fishes. These results indicated that the HIF/VEGF pathway played an important role in ABO angiogenesis/formation to promote fish to do aerial respiration. This study will contribute to our understanding of molecular mechanisms of air-breathing in fish.

Molecular, Cellular and Functional Analysis of TRγ Chain along the European Sea Bass Dicentrarchus labrax Development

In jawed vertebrates, adaptive immune responses are enabled by T cells. Two lineages were characterized based on their T cell receptor (TcR) heterodimers, namely αβ or γδ peptide chains, which display an Ig domain-type sequence that is somatically rearranged. γδ T cells have been less extensively characterized than αβ and teleost fish, in particular, suffer from a severe scarcity of data. In this paper, we worked on the well-known model, the European sea bass Dicentrarchus labrax, to broaden the understanding of teleost γδ-T cells. The T cell receptor chain (TR) γ transcript was expressed at a later developmental stage than TRβ, suggesting a layered appearance of fish immune cells, and the thymus displayed statistically-significant higher mRNA levels than any other organ or lymphoid tissue investigated. The polyclonal antibody developed against the TRγ allowed the localization of TRγ-expressing cells in lymphoid organs along the ontogeny. Cell positivity was investigated through flow cytometry and the highest percentage was found in peripheral blood leukocytes, followed by thymus, gut, gills, spleen and head kidney. Numerous TRγ-expressing cells were localized in the gut mucosa, and the immunogold labelling revealed ultrastructural features that are typical of T cells. At last, microalgae-based diet formulations significantly modulated the abundance of TRγ⁺ cells in the posterior intestine, hinting at a putative involvement in nutritional immunity. From a comparative immunological perspective, our results contribute to the comprehension of the diversity and functionalities of γδ T cells during the development of a commercially relevant marine teleost model.

Expression analysis of taste receptor genes (T1R1, T1R3, and T2R4) in response to bacterial, viral and parasitic infection in rainbow trout, Oncorhynchus mykiss

Emerging evidence suggests that bitter and sweet Taste receptors (TRs) in the airway are important sentinels of innate immunity. TRs are G protein-coupled receptors that trigger downstream signaling cascades in response to activation of specific ligands. Among them, the T1R family consists of three genes: T1R1, T1R2, and T1R3, which function as heterodimers for sweet tastants and umami tastants. While the other TRs family components T2Rs function as bitter tastants. To understand the relationship between TRs and mucosal immunity in teleost, here, we firstly identified and analyzed the molecular characteristics of three TRs (T1R1, T1R3, and T2R4) in rainbow trout (Oncorhynchus mykiss). Secondly, by quantitative real-time PCR (qPCR), we detected the mRNA expression levels of T1R1, T1R3 and T2R4 and found that the three genes could be tested in all detected tissues (pharynx, buccal cavity, tongue, nose, gill, eye, gut, fin, skin) and the expression levels of T1R3 and T2R4 were higher in buccal mucosa (BM) and pharyngeal mucosa (PM) compare to other tissues. It may suggest that T1R3 and T2R4 play important roles in BM and PM. Then, to analyses the changes of expression levels of the three genes in rainbow trout infected with pathogens, we established three infection models Flavobacterium columnare (F. cloumnare), infectious hematopoietic necrosis virus (IHNV) and Ichthyophthirius multifiliis (Ich). Subsequently, by qPCR, we detected the expression profiles of TRs in the gustatory tissues (BM, PM and skin) of rainbow trout after infection with F. cloumnare, IHNV, and Ich, respectively. We found that under three different infection models, the expression of the T1R1, T1R3 and T2R4 showed their own changes in mRNA levels. And the expression levels of the T1R1, T1R3 and T2R4 changed significantly at different time points in response to three infection models, respectively, suggesting that TRs may be associated with mucosal immunity.

Purinergic signaling and gene expression of purinoceptors in the head kidney of the silver catfish Rhamdia quelen experimentally infected by Flavobacterium columnare

The head kidney is a lymphoid immune organ that plays a key role in the immune and inflammatory responses of teleost fish. It is associated with immunoglobulin G production and differentiation of B cells. The presence of a multi-enzymatic complex found anchored in the plasma membrane makes the head kidney an important purinergic-dependent tissue. Purinergic signaling has been associated with these responses under pathological conditions via regulation of extracellular adenosine triphosphate (ATP), the main damage molecular associated pattern agent released during bacterial infections. The aim of this study was to determine whether purinergic signaling is a pathway associated with impairment of immune responses in silver catfish (Rhamdia quelen) experimentally infected by Flavobacterium columnare, as well as to evaluate the role of P2 purine receptors in this response. Triphosphate diphosphohydrolase (NTPDase) activity in the head kidney was significantly lower in silver catfish experimentally-infected F. columnare 72 h post-infection (hpi) than in the control group, while no significant difference was observed with respect NTPDase activity on adenosine diphosphate, as well as on 5'-nucleotidase and adenosine deaminase activities. Extracellular ATP levels were significantly higher in the head kidney of experimentally-infected fish than in the control group at 72 hpi. Finally, p2ry11 and p2rx3 purine receptor levels were significantly higher in experimentally-infected fish than in the control group at 72 hpi. We conclude that purinergic signaling in the head kidney of silver catfish infected by F. columnare creates a pro-inflammatory profile that may contribute to impairment of immune and inflammatory responses via reduction of ATP hydrolysis and its accumulation in the extracellular milieu, accompanied by upregulation of p2ry11 and p2rx3 purine receptors, leading to pro-inflammatory status.

Branchial bioenergetics dysfunction as a relevant pathophysiological mechanism in freshwater silver catfish (Rhamdia quelen) experimentally infected with Flavobacterium columnare

Flavobacterium columnare, the causative agent of columnaris disease, is a serious bacterial disease responsible for causing devastating mortality rates in several species of freshwater fish, leading to severe economic losses in the aquaculture industry. Notwithstanding the enormous impacts this disease can have, very little is known regarding the interaction between the host and bacterium in terms of the mortality rate of silver catfish (Rhamdia quelen), as well its linkage to gill energetic homeostasis. Therefore, we conducted independent experiments to evaluate the mortality rates caused by F. columnare in silver catfish, as well as whether columnaris disease impairs the enzymes of the phosphoryl transfer network in gills of silver catfish and the pathways involved in this inhibition. Experiment I revealed that clinical signs started to appear 72 h post-infection (hpi), manifesting as lethargy, skin necrosis, fin erosion and gill discoloration. Silver catfish began to die at 96 hpi, and 100% mortality was observed at 120 hpi. Experiment II revealed that creatine kinase (CK, cytosolic and mitochondrial) and pyruvate kinase (PK) activities were inhibited in silver catfish experimentally infected with F. columnare, while no significant difference was observed between experimental and control groups with respect to adenylate kinase activity. Activity of the branchial sodium-potassium pump (Na⁺, K⁺-ATPase) was inhibited while reactive oxygen species (ROS) and lipid peroxidation levels were higher in silver catfish experimentally infected with F. columnare than in the control group at 72 hpi. Based on these data, the impairment of CK activity elicited by F. columnare caused a disruption in branchial energetic balance, possibly reducing ATP availability in the gills and provoking impairment of Na⁺, K ⁺ATPase activity. The inhibition of CK and PK activities appears to be mediated by ROS overproduction and lipid peroxidation, both of which contribute to disease pathogenesis associated with branchial tissue.

Nutrient sensing signaling functions as the sensor and regulator of immunometabolic changes in grass carp during Flavobacteriumcolumnare infection

In order to illustrate the immunometabolic changes of fish during bacterial infection, grass carp (Ctenopharyngodon idellus) was injected with Flavobacteriumcolumnare(F.columnare) and then the immune response, nutrient metabolism and related signaling pathways were assayed from 6 h post injection (hpi) to 7 days post injection (dpi). After F.columnare injection, gill lamellae showed obvious fusion and higher mRNA expression levels of pro-inflammatory cytokines. The mRNA expression levels of TNF-α, IL-1β and IL-8 in the head kidney were also significantly upregulated at 6 hpi and 3 dpi. Moreover, the expression of IgZ in the gill was significantly upregulated at 3 dpi and 7 dpi, while the expression of IgM in the head kidney was significantly upregulated at 1 dpi and 3 dpi after F.columnare injection. During bacterial infection, the systematic nutrient metabolism was also significantly affected. Hepatic glycolysis, indicated by GK mRNA expression and PK activity, was significantly upregulated at 1 dpi, while glucogenesis, indicated by PEPCK mRNA expression and enzyme activity, was significantly increased at later time, which resulted in the decreased hepatic glycogen content at 1dpi but increased glycogen content at 7 dpi in the experimental group. LPL, which catalyzed the lipid catabolism, showed decreased mRNA expression and enzyme activity at 6 hpi, while ACC, which was rate-limiting of FA synthesis, was significantly increased at 6 hpi, 3 dpi and 7 dpi. During this process, the nutrient sensing signaling was also significantly affected. TOR signaling in grass carp was significantly activated while ERK signaling was significantly inhibited after F.columnare infection, both of which might function as the sensor and regulator of fish immunometabolic changes.