Compartmentalisation of T cells expressing CD8α and TCRβ 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 β and CD8α. RT-PCR of RNA extracted from larvae revealed TCRβ transcripts from day 25 post-hatching (ph) and CD8α transcripts from 26 days later.At day 51 ph, CD8α and TCRβ 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, CD8α+ and TCRβ+ thymocytes almost filled the cortex and extended in large cords in the medulla. A CD8α−TCRβ+ subcapsular lymphoid zone was evident near the septa coming from the inner connective capsule that delimited the thymus.The localisation of CD8α and TCRβ transcripts demonstrated a compartmentalisation of the juvenile thymus due to distinct localisation of thymocytes at different developmental stages.