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

Development of the Thymus and Kidney and Effects of Resveratrol on Their Aging in a Short-Lived Fish

Annual fishes of the genus Nothobranchius have been widely used in cognitive, behavioral, and genetic studies, and have become an excellent animal model for studying aging. However, the development and degeneration of immune organs in annual fishes and the antagonistic effects of resveratrol remain unclear. In the present study, the development of thymus and kidney was investigated systematically using Nothobranchius guentheri from larvae, juveniles, and young and old fish with hematoxylin and eosin staining. We found that thymus primordium was observed first in the larvae at 2 days after hatching (dah). After the lymphoid cells became evident at 5 dah, the thymus acquired an irregular shape at 7 dah. Then it formed a wedge shape at 15 dah. Thymus looked as homogeneous distribution of lymphocytes at 1 month old, and it differentiated into cortex and medulla approximately in 2-month-old fish. Combined with TUNEL and senescence-associated β-galactosidase (SA-β-gal) staining, it showed the degeneration of the thymus appeared in 4-month-old fish. Kidney primordium appeared on 1 dah, and the glomerulus was visible at 7 dah. The nephrogenic activity was most apparent in 1-month-old fish. A large hematopoietic tissue was arranged in the renal interstitium in 2- and 3-month-old fish. In 6-month-old fish, the kidney structure became less dense. By 12 months, the kidney exhibited the most pronounced histological characteristics of aging. Feeding resveratrol ameliorated renal fibrosis and SA-β-gal staining with age, increased SIRT1 and SIRT3 expression, and decreased the levels of NF-κB and inflammatory factors in thymus and kidney of the fish. We provided basic data for the development and degeneration of immune organs and resveratrol's anti-aging effects in short-lived fish.

Histology of some major immune system organs in Lake Van fish Alburnus tarichi (Güldenstädt, 1814) (Cyprinidae)

The teleostean immune system is variable between and within taxa in terms of morphology. Accordingly, in the current study, the histology of some immune organs (thymus, kidney and spleen) of Lake Van fish (Alburnus tarichi) was investigated. For this purpose, the tissues from eight mature fish were fixed and embedded in paraffin wax, and then the sections were stained with haematoxylin and eosin, Mallory's triple, periodic acid-Schiff and Alcian blue (pH of 2.5) for histologic examinations. In the thymus, no clear discrimination of the cortex and medulla was observed. Lymphoid cells within a reticulo-epithelial network were the predominant cell type in the thymus. The other components in the thymus were macrophages and myoid, mast-like, plasma-like, cystic cells, Hassall's corpuscles, and single or multiple epithelial cystic structures. In the kidney, the head kidney, a major haematopoietic site, consisted of lymphoid and non-lymphoid zones within a reticular network. Cells exhibiting mitotic figures were also detected in the haematopoietic tissue of the head kidney. Haematopoietic tissue was also found in the trunk kidney dispersed amongst the excretory components. The spleen was composed of red and white pulp. The red pulp comprised abundant erythrocytes, whilst the white pulp contained leucocytes with a reticular network. Ellipsoids were also determined in the white pulp. Melanomacrophage centres were found in all of the examined lymphoid tissues of the fish. These findings, which were reported, herein, for the first time will provide reference knowledge for future studies of this anadromous fish.

Innate and adaptive immune molecules of striped murrel Channa striatus

Channa striatus, also called snakehead murrel, is an important freshwater teleost fish which has been widely cultured for its tasty flesh along with nutritional and medicinal values. The growth of both cultured and wild murrels is affected by various physical, chemical and biological factors. As a teleost fish, C. striatus is an intermediate organism between invertebrates and vertebrates. They have a well‐developed innate immune system than invertebrates and a primitive adaptive immune system compared to that of higher vertebrates, thus an interesting unique immune structure to explore. Studies have identified that a few external stimulants do instigate the immune system to fight against the pathogens at the time of infection in C. striatus. This review discusses the physicochemical and biological stress factors, immune system and immune molecules of C. striatus which are potentially involved in combating the stress factors.

Oestrogen receptor distribution related to functional thymus anatomy of the European sea bass, Dicentrarchus labrax

In jawed vertebrates, the crosstalk between immune and endocrine system as well as many fundamental mechanisms of T cell development are evolutionary conserved. Oestrogens affect mammalian thymic function and plasticity, but the mechanisms of action and the oestrogen receptors involved remain unclear. To corroborate the oestrogenic regulation of thymic function in teleosts and to identify the implicated oestrogen receptor subtypes, we examined the distribution of nuclear and membrane oestrogen receptors within the thymus of the European Sea bass, Dicentrarchus labrax, in relation to its morpho-functional organisation. Immunohistological analysis specified thymus histology and organisation in teleosts and described, for the first time, Hassall's corpuscle like structures in the medulla of sea bass. All oestrogen receptors were expressed at the transcript and protein level, both in T cells and in stromal cells belonging to specific functional areas. These observations suggest complex regulatory actions of oestrogen on thymic function, notably through the stromal microenvironment, comprising both, genomic and non-genomic pathways that are likely to affect T cell maturation and trafficking processes. Comparison with birds, rodents and humans supports the thymic localization of oestrogen receptors and suggests that oestrogens modulate T cell maturation in all gnathostomes.

Histology and ultrastructure of the thymus during development in tilapia, Oreochromis niloticus

The thymus in teleost fishes plays an important role in producing functionally competent T-lymphocytes. However, the thymus in tilapia is not well known, which greatly hampers investigations into the immune responses of tilapia infected by aquatic pathogens. The histological structure and ultrastructure of the thymus in Oreochromis niloticus, including embryos and larvae at different developmental stages, juveniles, and adult fish, were systematically investigated using whole mount in situ hybridization (WISH), and light and transmission electron microscopy (TEM). The position of the thymus primordium was first labeled in the embryo at 2 days post-fertilization (dpf) with the thymus marker gene recombination activating gene 1 (Rag1), when the water temperature was 27 °C. Obvious structures of the thymus were easily observed in 4-dpf embryos. At this stage, the thymus was filled with stem cells. At 6 dpf, the thymus differentiated into the cortex and medulla. The shape of the thymus was 'broad bean'-like during the early stages from 4 to 10 dpf, and became wedge-shaped in fish larvae at 20 dpf. At 6 months post-fertilization (mpf), the thymus differentiated into the peripheral zone, central zone, and inner zone. During this stage, myoid cells and adipocytes appeared in the inner zone following thymus degeneration. Then, the thymus displayed more advanced degeneration by 1 year post-fertilization (ypf), and the separation of cortex and medulla was not observed at this stage. The thymic trabecula and lobule were absent during the entire course of development. However, the typical Hassall's corpuscle was present and underwent degeneration. Additionally, TEM showed that the thymic tissues contained a wide variety of cell types, namely lymphocytes, macrophages, epithelial cells, fibroblasts, and mastocytes.

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.

Transcriptional characterization of the T cell population within the salmonid interbranchial lymphoid tissue

Previously, our group has shown that the interbranchial lymphoid tissue (ILT) is a distinct structure largely consisting of T cells embedded in a meshwork of epithelial cells, with no direct resemblance to previously described lymphoid tissues. In this study, we aim to focus on the T cell population and the possibility of the ILT being a thymus analog. By characterizing structural responsiveness to Ag challenge, the presence of recombination activating genes, and different T cell-related transcripts, we attempt to further approach the immunological function of the ILT in salmonid gills. In addition to eight healthy individuals, a group of eight infectious salmon anemia virus-challenged fish were included to observe T cell responses related to infection. The results showed reduced size of ILT in the infected group, no expression of RAG-1 and -2, and a high degree of T cell diversity within the ILT. Taking into account that the ILT can be regarded as a strategically located T cell reservoir and possibly an evolutionary forerunner of mammalian MALTs right at the border to the external environment, the alteration in transcription observed may likely represent a shift in the T cell population to optimize local gill defense mechanisms.

Apoptosis in thymus of teleost fish

The presence and distribution of apoptotic cells during thymus development and in adult were studied by in situ end-labelling of fragmented DNA in three temperate species carp (Cyprinus carpio), sea bass (Dicentrarchus labrax) and dusky grouper (Epinephelus marginatus) and in the adult thymus of three Antarctic species belonging to the genus Trematomus spp. During thymus development some few isolated apoptotic cell (AC) firstly appeared in the central-external part of the organ (carp: 5 days ph; sea bass: 35 days ph grouper: 43 days ph). Initially the cells were isolated and then increased in number and aggregated in small groups in the outer-cortical region of the thymus larvae. The high density of apoptotic cells was observed in the junction between cortex and medulla from its appearance (border between cortex and medulla, BCM). ACs decreased in number in juveniles and adult as well as the ACs average diameter. In late juveniles and in adulthood, the apoptosis were restricted to the cortex. In Antarctic species the thymus is highly adapted to low temperature (high vascularisation to effort the circulation of glycoproteins enriched plasma and strongly compact parenchyma). The apoptosis process was more extended (4-7 fold) as compare with the thymus of temperate species, even if the distribution of ACs was similar in all examined species. Data suggested a common process of T lymphocyte negative-selection in BCM of thymus during the ontogeny. The selection process seems to be still active in adult polar fish, but restricted mainly in the cortex zone.

Heat shock protein 70 kDa (HSP70) increase in sea bass (Dicentrarchus labrax, L 1758) thymus after vaccination against Listonella anguillarum

Heat shock proteins 70 kDa (HSP70) and apoptosis were investigated in thymus of sea bass juveniles (Dicentrarchus labrax) subsequently to a vaccination against Listonella (syn. Vibrio) anguillarum. HSP70 expression was measured by immunohistochemistry and immunoenzymatic methods, resulting in increase in HSP70 after bath immunization and persistent in fish exposed to an intraperitoneal (i.p.) booster. The HSP70 increase in thymus was suggested as induction in lymphocytic cells, to be related to immune system stimulation after vaccination. However, a thymic recruitment of lymphocyte subpopulations, characterized by higher expression of HSP70, was also hypothesized after vaccination. No apparent relationships were found between HSP70 and apoptosis. In fact, the vaccination did not modulate the apoptosis response, as measured by TUNEL assay and by immunohistochemistry for active caspase-3 expression. The lack of apoptosis effects could be ascribed to the use of inactivated bacteria that appeared not able to interfere with programmed cell death mechanisms. This manuscript aims to contribute to the knowledge of some biochemical features underlying the immunization, with a particular emphasis on the modulation of HSP70. However, further parameters involved in innate/adaptative immunity and apoptosis pathways have to be taken into account to well establish the functional role of HSP70 in fish vaccination.

Antigen-dependent T lymphocytes (TcRβ+) are primarily differentiated in the thymus rather than in other lymphoid tissues in sea bass (Dicentrarchus labrax, L.)

All jawed vertebrates share lymphocyte receptors that allow the recognition of pathogens and the discrimination between self and non-self antigens. The T cell transmembrane receptor (TcR) has a central role in the maturation and function of T lymphocytes in vertebrates via an important role in positive selection of the variable region of TcR αβ/γδ chains. In this study, the TcRβ transcript expression and TcRβ(+) cell distribution during the ontogeny of the immune system of sea bass (Dicentrarchus labrax, L.) were analysed. RT-PCR analysis of larvae during early development demonstrated that the β chain transcript is expressed by 19 days post-fertilisation (p.f.). RNA probes specific for the β chain were synthesised and used for in situ hybridisation experiments on 30 day p.f. to 180 day old juvenile larvae. A parallel immunohistochemical study was performed using the anti-T cell monoclonal antibody DLT15 developed in our laboratory [Scapigliati et al., Fish Shellfish Immunol 1996; 6:383-401]. The first thymus anlage was detectable at 32-33 days p.f. (Corresponding to about 27 days post-hatch). DLT15(+) cells were detected at day 35 p.f. in the thymus whereas TcRβ(+) cells were recognisable at day 38 p.f. in the thymus and at day 41 p.f. in the gut. TcRβ(+) cells were observed in capillaries from 41 to 80 days p.f. At day 46 p.f., TcRβ(+) cells were identified in the head kidney and were detected in the spleen 4 days later. The present results demonstrate that TcRβ(+) cells can be differentiated first in the thymus and then in other organs/tissues, suggesting potential TcRβ(+) cell colonisation from the thymus to the middle gut. Once the epithelial architecture of the thymus is completed with the formation of the cortical-medullary border (around 70-75 days p.f.), DLT15(+) cells or TcRβ(+) cells are confined mainly to the cortex and cortical-medullary border. In particular, a large influx of TcRβ(+) cells was observed at the cortical-medullary border from 72 to 90 days p.f., suggesting a role in positive selection for this thymic region during the ontogeny of the fish immune system. This study provides novel information about the primary differentiation and distribution of TcRβ(+) cells in sea bass larvae and juveniles.

Expression of T-cell markers during Atlantic halibut (Hippoglossus hippoglossus L.) ontogenesis

The immune system of Atlantic halibut is relatively undeveloped at the time of hatching, and thus larvae are vulnerable to bacterial and viral diseases that can result in high mortalities. To enable establishment of effective prophylactic measures, it is important to know when the adaptive immune system is developed. This depends on both B- and T-cell functions. In the present study the expression of RAG1, TCRα, TCRβ, CD3γδ, CD3ɛ, CD3ζ, CD4, CD4-2, CD8α, CD8β, Lck, and ZAP-70 was analyzed in larval and juvenile stages during halibut development. Using real time RT-PCR, low basal mRNA levels of all 12 genes could be detected at early stages. An increase in mRNA transcripts for the genes was seen at different time points, from 38 days post hatching (dph) about the time when the first anlage of thymus is found, and onwards. The transcription patterns of the 12 mRNAs were found to be similar throughout the developmental stages tested. In situ hybridization on larval cross-sections showed that RAG1 and Lck could be detected in lymphocyte like cells within the thymus at 42 dph. CD4 expression could not be detected within the thymus before 66 dph, however, positive cells were restricted to the cortical region. At 87 dph, the zonation of the thymus in a cortical, cortico-medullary, and a medullary region seemed to be more evident with CD8α expressing cells found in all regions, indicating the presence of mature T-cells. This correlates with previous results describing thymus development and the appearance of IgM(+) cells during halibut ontogenesis.

Thymic nurse cells exhibit epithelial progenitor phenotype and create unique extra-cytoplasmic membrane space for thymocyte selection

Thymic nurse cells (TNCs) are epithelial cells in the thymic cortex that contain as many as 50 thymocytes within specialized cytoplasmic vacuoles. The function of this cell-in-cell interaction has created controversy since their discovery in 1980. Further, some skepticism exists about the idea that apoptotic thymocytes within the TNC complex result from negative selection, a process believed to occur exclusively within the medulla. In this report, we have microscopic evidence that defines a unique membranous environment wherein lipid raft aggregates around the alphabetaTCR expressed on captured thymocytes and class II MHC molecules expressed on TNCs. Further, immunohistological examination of thymic sections show TNCs located within the cortico-medullary junction to express cytokeratins five and eight (K5 and K8), and the transcription factor Trp-63, the phenotype defined elsewhere as the thymic epithelial progenitor subset. Our results suggest that the microenvironment provided by TNCs plays an important role in thymocyte selection as well as the potential for TNCs to be involved in the maintenance of thymic epithelia.

Effect of beta-glucan on immunity and survival of early stage of Anabas testudineus (Bloch)

The present experiment was carried out to study the effect of different dosages of beta-glucan suspension derived from barley on the innate immune response and disease resistance of Anabas testudineus spawns against infection caused by Aeromonas hydrophila. Four different dosages of beta-glucan suspension in phosphate buffered saline at the rate of 0, 5, 10, 15 mg l(-1) were taken and 8 days old spawn were exposed for 2 h and 3 h. The cell suspension of spawn was assayed for total protein, acid phosphatase activity, lysozyme activity, bactericidal and NBT. Further, the spawns were challenged with 3 x 10(5) cells ml(-1) of A. hydrophila and survivability percentage and immunological parameters were assayed upto day 7. On day 7, most of the immunological parameters such as lysozyme activity, bactericidal activity and NBT activity were significantly enhanced after exposing the fish to all the concentrations of beta-glucan. Challenge study indicated least mortality in the group of spawns immersed in 15 mg l(-1) beta-glucan suspension for 3 h. Thus, 3 h exposure to beta-glucan suspension could reduce the mortality and increase the immunity of A. testudineus spawns.

Rodlet cells in the thymus of the zebrafish Danio rerio (Hamilton, 1822)

The conspicuous rodlet cell (RC) of teleost fishes is a remarkable but still poorly understood phenomenon in the biology of this group. Since their discovery by Thelohan in 1892, a confusing discussion about their nature and physiological function has ensued with various interpretations of RCs as protozoan parasites, gland cells, or a specific type of immune cell. Here we present a study on RCs in the zebrafish Danio rerio (Hamilton, 1822) and the first observation of their occurrence in the thymus, based on light- and electron-microscopy. RCs were confirmed in thymus-surrounding connective tissue, the thymic epithelium and the parenchyma. Very few (late) immature stages of RCs could be identified, already displaying some rodlets and a developing fibrillar cell border but often also signs of a degenerating Golgi apparatus. Apoptotic RCs occurred mostly below the surface epithelium. The mature RCs ejected their rodlets not only at the surface of the epithelium but also into the extracellular environment of the thymic parenchyma. Coalescence of the rodlets into "multi-core" units seems to represent an organ-specific feature. No cellular immune reaction of reticulum cells or granulocytes against RCs and their cellular products was observed. RCs in tight contact with lymphocytes showed conspicuous formation of dense structures in the capsule-like cell membrane, but dense membrane appositions were also observed between RCs and erythrocytes in blood vessels. The possibility of immigration and defective development of RCs in the thymus is discussed.

Role of maternally derived immunity in fish

Maternal immunity is of paramount importance for protection of young ones at early stage of life since the immune factors of an immunocompetent female are transferred transplacentally or through colostrum, milk or yolk to an immunologically naive neonate. Both innate and adaptive type of immunity are transferred of from mother to offspring in fishes. These factors include immunoglobulin (Ig)/antibody, complement factors, lysozymes, protease inhibitors like alpha macroglobulin, different types of lectins and serine proteases like molecules. Among different types of Ig viz. IgM, IgD, IgT/IgZ and IgM-IgZ chimera types, IgM is present in most of the teleostean fishes. In teleosts, IgM either as a reduced/breakdown product or monomeric form is usually transferred to the offsprings. The maternally derived IgM usually persists for a limited duration, exhausts within the completion of yolk absorption process, and completely disappears thereafter during larval stages. Maternal transfer of immunity which provides defense to embryo and larvae depends upon the health as well as the immune status of brood fish. The overall health status of brood fish can affect breeding performances, quality seed production and protection of offsprings. However, factors such as age, maturation, reproductive behaviour and nutrition (micro and macro-nutrients) may affect the immunity in brood fishes. Besides these, seasonal changes such as photoperiods, temperature, adverse environmental conditions, and stress conditions like handling, crowding, and water pollution/contamination can also affect the immunity of brood fishes. The maintenance of the brood stock immunity at high level during vitellogenesis and oogenesis, is utmost important for reducing mortalities at larval/post larval stages through maximum/optimum transfer of maternal immunity. Brood stock immunization prior to breeding as well as selective breeding among the disease resistant families might be the ideal criteria for producing quality seed.

Lymphocyte differentiation in sea bass thymus: CD4 and CD8-alpha gene expression studies

Different developmental stages (from eggs to 1-year-old juveniles) of the teleost fish Dicentrarchus labrax (L.) were assayed for CD4 gene expression. RT-PCR revealed the appearance of CD4 transcripts in post-larvae from 51 days post-hatching (dph). This finding overlaps the first detection of CD8-alpha mRNA. Real-time PCR with specific primers quantified CD4, CD8-alpha and TCR-beta transcripts in larvae and post-larvae (25, 51, 75 and 92 dph) and 1-year-old thymus. At 92 dph, TcR-beta and CD8-alpha transcripts were significantly higher (P < 0.001) than in previous stages, as CD4 transcripts compared with 51 dph (P < 0.01). High levels of TCR-beta and CD8-alpha transcripts were found in the thymus, while CD4 transcripts were lower (P < 0.05 vs. TCR-beta). In situ hybridization identified CD4 mRNAs at 51 dph, localized in thymocytes of the outer and lateral zones of the thymic glands. From 75 dph on the signal was mainly detected in the outer region, drawing a cortex-medulla demarcation. Developmental expression of CD4 and CD8-alpha almost coincided. In each adult thymic lobe CD4(+) and CD8-alpha(+) thymocytes filled the cortex. The expression patterns of CD4 and CD8-alpha largely overlap, except in the medulla, where CD4(+) thymocytes were isolated, while CD8-alpha(+) ones mainly arranged in cords. These results provide new information about the thymic compartmentalization and lymphocyte differentiation pathways in a teleost, almost demonstrating that double negative thymocytes fill the cortex giving rise to further selection in the medulla.

Lymphoid tissue ontogeny in the mummichog, Fundulus heteroclitus

Most studies that examine the ontogeny of lymphoid organ development in teleostean fishes use species of interest to aquaculture or genetic research and, to date, have focused strictly on marine or freshwater species. The mummichog, Fundulus heteroclitus, also known as the estuarine killifish, is a unique model for studies on developmental immunobiology, because it is euryhaline, has a high degree of thermal tolerance, and has a unique reproductive strategy. Embryonic and larval mummichogs were examined for the ontogeny of lymphoid tissue development. The first lymphoid organ to appear was the head kidney at 1 dph, followed by the spleen at 1 wph, and then the thymus at 3 wph. Rag-1 was partially cloned and sequenced and shown to be highly conserved among other vertebrate Rag-1 genes. Using QT-PCR to monitor the temporal expression of Rag-1, it was shown to reach a maximum intensity at 3 and 4 wph and then to drop to pre-2-wph levels. Overall, this study suggests that juvenile mummichogs do not possess the ability to mount T- or B-cell responses until some time after 5 wph. Even though the estuarine killifish tolerates a wide range of salinities, the developmental patterns of lymphoid tissues are similar to what has been reported for strictly marine (stenohaline) teleosts. Thus, the mummichog should be a convenient model for understanding the developmental immunobiology of most marine teleosts.

Compartmentalisation of T cells expressing CD8alpha and TCRbeta in developing thymus of sea bass Dicentrarchus labrax (L.)

Eggs, larvae, post-larvae and sexually immature juveniles of the teleost Dicentrarchus labrax (L.) were assayed for the expression of genes encoding the T cell receptor beta and CD8alpha. RT-PCR of RNA extracted from larvae revealed TCRbeta transcripts from day 25 post-hatching (ph) and CD8alpha transcripts from 26 days later. At day 51 ph, CD8alpha and TCRbeta mRNAs were localised by in situ hybridisation in thymocytes of the outer and lateral zones of the thymic paired glands. From day 75 ph onwards the signal was mainly detected in the outer region, drawing a cortex-medulla demarcation. In 1-year-old fish, CD8alpha+ and TCRbeta+ thymocytes almost filled the cortex and extended in large cords in the medulla. A CD8alpha(-)TCRbeta+ subcapsular lymphoid zone was evident near the septa coming from the inner connective capsule that delimited the thymus. The localisation of CD8alpha and TCRbeta transcripts demonstrated a compartmentalisation of the juvenile thymus due to distinct localisation of thymocytes at different developmental stages.

Anatomy and cytology of the thymus in juvenile Australian lungfish, Neoceratodus forsteri

The anatomy, histology and ultrastructure of the thymus of a dipnoan, the Australian lungfish, Neoceratodus forsteri, was studied by light and transmission electron microscopy. The thymic tissue showed clear demarcation into a cortex and medulla with ample vascularization. Large cells including foamy and giant multinucleated cells with periodic acid Schiff/Alcian blue positive staining properties were localized mainly in the medulla. The major cellular components were epithelial cells and lymphoid cells. The epithelial cells were classified by location and ultrastructure into six sub-populations: capsular cells, cortical and medullary reticular cells, perivascular endothelial cells, intermediate cells, nurse-like cells and Hassall-like corpuscles. Myoid cells were found mainly in the cortico-medullary boundary and medulla. Macrophages and secretory-like cells were also present. These findings will provide a base of knowledge about the cellular immune system of lungfish.

Majority of TcRβ+ T-lymphocytes located in thymus and midgut of the bony fish, Dicentrarchus labrax (L.)

Real-time polymerase chain reaction (PCR) and in situ hybridization analyses were performed to investigate the occurrence and distribution of T-lymphocytes expressing TcRbeta in intestine and lymphoid tissues of the bony fish, Dicentrarchus labrax (sea bass). Immunohistochemistry with the monoclonal antibody DLT15 (pan-T-cell marker) was carried out to compare the cytology, distribution and number of T-cells and TcRbeta+ cells in the various sampled lymphoid organs. The highest TcRbeta expression was revealed by real-time PCR in the thymus, with high levels also being found in the gut. In the thymus, DLT15+ and TcRbeta+ cell populations were concentrated in the cortex and TcRbeta+ cells were notably reactive at the cortical-medullary border, suggesting a specialized role of this region in thymocyte selection. The density of DLT15+ T-cells increased from the anterior to posterior intestine, whereas TcRbeta+ lymphocytes were more numerous in the middle intestine compared with other segments. The existence, in fish thymus, of a medulla and a cortex comparable with those of mammals is revealed by this study. The concentration of TcRbeta+ cells in the sea bass midgut also strongly suggests a special role of this intestinal segment in antigen-specific cellular immunity. The large population of TcRbeta(-)/DLT15+ T-cells in the posterior gut can probably be ascribed to the TcRgammadelta phenotype fraction.

Distribution of the professional phagocytic granulocytes of the bony fish gilthead seabream (Sparus aurata L.) during the ontogeny of lymphomyeloid organs and pathogen entry sites

Although it is believed that fish fry depend fundamentally on their innate defence mechanisms, the ontogeny of fish innate immune cells is poorly understood. In the present study, we have used a specific monoclonal antibody against acidophilic granulocytes (AGs), the main professional phagocytic cell type of the bony fish gilthead seabream, to study their localization during the development of the main lymphomyeloid organs, namely the head kidney, spleen and thymus, and of the two major portals for pathogen entry, namely the gills and intestine. AGs were observed in the posterior intestine and in the blood earlier than in the haematopoietic kidney (21 vs. 27 days post-hatching, dph). AGs were observed scattered between other cells of the haematopoietic lineage in the head kidney of larvae, but were grouped around the blood vessels of this organ in juveniles and adults, where they were also much more numerous. In the spleen and in the thymus, AGs were observed much later (62 dph) and appeared scattered. AGs were also observed in the gill lamella and the posterior intestine near the anus throughout development.

Carp (Cyprinus carpio L.) innate immune factors are present before hatching

Expression of the innate immune factors, complement factor 3 (C3), alpha2-macroglobulin (alpha2M), serum amyloid A (SAA) and a complement factor 1 r/s--mannose binding lectin associated serine protease-like molecule (C1/MASP2), was determined with Real Time Quantitative-PCR in carp (Cyprinus carpio L.) ontogeny around hatching. Furthermore, the expression of C3 mRNA and the presence of C3 protein were studied in carp embryos and larvae using In Situ Hybridisation, Western Blotting and Immunohistochemistry. C3, alpha2M, SAA and C1/MASP2 mRNA were produced by embryos from 12 h post-fertilisation, which is relatively long before hatching (2 days post-fertilisation (dpf)), indicating either involvement of these factors in development itself or more probably a preparation of the immune system for the post-hatching period. In addition, maternal mRNA of the aforementioned innate immune factors and maternal C3- and immunoglobulin protein was present in unfertilised eggs. Furthermore, C3 mRNA production was situated in the yolk syncytial layer in embryos from 24 h post-fertilisation to 5 dpf, followed by the liver in larvae, providing a new sequence of C3 production in teleost development.

Ontogeny of the immune system of fish

Information on the ontogeny of the fish immune system is largely restricted to a few species of teleosts (e.g., rainbow trout, catfish, zebrafish, sea bass) and has previously focused on morphological features. However, basic questions including the identification of the first lympho-hematopoietic sites, the origin of T- and B-lymphocytes and the acquisition of full immunological capacities remain to be resolved. We review these three main topics with special emphasis on recent results obtained from the zebrafish, a new experimental model particularly suitable for study of the ontogeny of the immune system because of its rapid development and easy manipulation. This species also provides an easy way of creating mutations that can be detected by various types of screens. In some teleosts (i.e., angelfish) the first blood cells are formed in the yolk sac. In others, such as zebrafish, the first hematopoietic site is an intraembryonic locus, the intermediate cell mass (ICM), whereas in both killifish and rainbow trout the first blood cells appear for a short time in the yolk sac but later the ICM becomes the main hematopoietic area. Erythrocytes and macrophages are the first blood cells to be identified in zebrafish embryos. They occur in the ICM, the duct of Cuvier and the peripheral circulation. Between 24 and 30 hour post-fertilization (hpf) at a temperature of 28 degrees C a few myeloblasts and myelocytes appear between the yolk sac and the body walls, and the ventral region of the tail of 1-2 day-old zebrafish also contains developing blood cells. The thymus, kidney and spleen are the major lymphoid organs of teleosts. The thymus is the first organ to become lymphoid, although earlier the kidney can contain hematopoietic precursors but not lymphocytes. In freshwater, but not in marine, teleosts the spleen is the last organ to acquire that condition. We and other authors have demonstrated an early expression of Rag-1 in the zebrafish thymus that correlates well with the morphological identification of lymphoid cells. On the other hand, the origins and time of appearance of B lymphocytes in teleosts are a matter of discussion and recent results are summarized here. The functioning rather than the mere morphological evidence of lymphocytes determines when the full immunocompetence in fish is attained. Information on the histogenesis of fish lymphoid organs can also be obtained by analysing zebrafish mutants with defects in the development of immune progenitors and/or in the maturation of non-lymphoid stromal elements of the lymphoid organs. The main characteristics of some of these mutants will also be described.

The ontogeny of mucosal immune cells in common carp (Cyprinus carpio L.)

The ontogeny of carp (Cyprinus carpio L.) immune cells was studied in mucosal organs (intestine, gills and skin) using the monoclonal antibodies WCL38 (intraepithelial lymphocytes), WCL15 (monocytes/macrophages) and WCI12 (B cells). In addition, recombination activating gene 1 expression was examined in the intestine with real time quantitative PCR and in situ hybridization to investigate extrathymic generation of lymphocytes. WCL38(+) intraepithelial lymphocytes (putative T cells) appeared in the intestine at 3 days post-fertilization (dpf), which is shortly after hatching but before feeding, implying an important function at early age. These lymphoid cells appear in the intestine before the observation of the first thymocytes at 3-4 dpf, and together with the expression of recombination activating gene 1 in the intestine, suggests that similar to mammals at least part of these cells are generated in the intestine. WCL15(+)monocytes/macrophages appeared in the lamina propria of the intestine at 7 dpf, but considerably later in the epithelium, while WCI12(+) (B) cells appeared in intestine and gills at 6-7 weeks. From these results it can be concluded that putative T cells occur much earlier than B cells, and that B cells appear much later in the mucosae than in other internal lymphoid organs (2 wpf).

Effects of thyroid hormone on the development of immune system in zebrafish

Effects of thyroxine (T4) and methimazole (MMI) on the development of the zebrafish immune system were investigated using continuous immersion treatment experiments. The effects of the treatments on thymus development were determined using computer-aided thymus morphometric analyses on in situ hybridization serial sections of the thymus while the effects on immune-related gene expression levels were monitored using quantitative real-time PCR. The findings indicate that thymus development and thymopoiesis, as indicated by thymus size, thymus Rag-1-positive region, and TCRAC expression level, were affected by T4 and MMI-treatments. With the exception of Ikaros, MMI-treated fish has lower immune-related gene expression levels, although it is not certain whether the effect resulted indirectly from the concomitant growth-retardation and/or directly from an effect on lymphopoiesis itself. The findings were comparable with those in mammalian system, thus providing the first evidence that the thyroid relationship with thymus development and lymphopoiesis is likely to be conserved from fish to higher vertebrates. It suggests the possibility of using zebrafish as a model system to investigate the molecular mechanisms involved in thyroid hormone-dependent disorders in the immune system.

Three novel carp CXC chemokines are expressed early in ontogeny and at nonimmune sites

Three novel CXC chemokines were identified in common carp (Cyprinus carpio L.) through homology cloning. Phylogenetic analyses show that one of the three CXC chemokines is an unambiguous orthologue of CXCL14, whereas both others are orthologues of CXCL12, and were named CXCL12a and CXCL12b. Percentages of amino acid identity between each of these carp chemokines and their human and mouse orthologues are markedly higher than those reported previously for other carp CXC chemokines, suggestive of involvement in vital processes, which have allowed for relatively few structural changes. Furthermore, all three novel carp CXC chemokines are expressed during early development, in contrast to established immune CXC chemokines. In noninfected adult carp, CXCL12b and CXCL14 are predominantly expressed in the brain. CXCL12a is highly expressed in kidney and anterior kidney, but its expression is still more abundant in brain than any other carp CXC chemokine. Clearly, these chemokines must play key roles in the patterning and maintenance of the (developing) vertebrate central nervous system.

Morphologic transformation of the thymus in developing zebrafish

The morphologic transformation of the developing zebrafish thymus from 1 week postfertilization (wpf) to 15 wpf is described. The thymus overall morphology changed from a small pouch-like shape at 1 wpf to a conical shape between 2 and 3 wpf before acquiring a more complex shape from 4 wpf onward. Rapid growth rate along the lateral axis at the region near the pharyngeal epithelium occurred between 1 and 2 wpf, whereas rapid growth rate along the dorsal-ventral axis occurred between 3 and 6 wpf. Expansion of thymocyte population beginning from 1 wpf became more evident by 2 to 3 wpf, as indicated by the apparent increase of different sizes of lymphocytes, recombination activating gene-1 (rag-1), and T-cell antigen receptor alpha chain constant region (TCRAC) -positive cells. Tissue section in situ hybridization (ISH) analysis with rag-1 probe reveals that cortex-medullary regionalization has begun between 1 and 2 wpf as rag-1 expression clearly demarcated the cortex, whereas the medulla was rag-1 negative. The presence of TCRAC-positive cells in the medulla by 2-3 wpf, suggests that the thymic selection processes had begun. The zebrafish thymus is morphologically mature by 3 wpf. Early signs of thymic involution were observed in zebrafish aged 15 wpf.

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.

Ontogeny of channel catfish lymphoid organs

Previously, we showed that catfish could not mount a detectable antibody response after bacterial exposure until 21 days post-hatch (ph). In order to evaluate the changes associated with the development of a functional humoral response, we evaluated the temporal and spatial distribution of immune cell populations in developing catfish. Cells functioning in nonspecific immunity were present in the renal hematopoietic tissue (rht) and thymus at hatch and in the spleen by day 3 ph. Immunoglobulin (Ig) positive lymphocytes were first detected on day 7, 10, and 14 in the rht, thymus and spleen, respectively. Mature thymocytes were first detected on day 10 ph. Distinct thymic regionalization and splenic lymphoid tissue organization were not observed until day 21 ph. We suggest that the reason for a lack of antibody production until day 21 ph is the poor organization of secondary lymphoid tissue until that age.

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.