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

Abstract

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.