MHC II-β chain gene expression studies define the regional organization of the thymus in the developing bony fish Dicentrarchus labrax (L.)

• MHC II-β real-time PCR and ISH detailed thymic development and regionalization.
• Thymocyte selection depends on evolving patterns of MHC II-β expression.
• MHC II-β+ cTECs and medullary dendritic cells are likely involved in thymocyte selection.

MHC II-β chain gene transcripts were quantified by real-time PCR and localised by in situ hybridization in the developing thymus of the teleost Dicentrarchus labrax, regarding the specialization of the thymic compartments.MHC II-β expression significantly rose when the first lymphoid colonization of the thymus occurred, thereafter increased further when the organ progressively developed cortex and medulla regions. The evolving patterns of MHC II-β expression provided anatomical insights into some mechanisms of thymocyte selection. Among the stromal cells transcribing MHC II-β, scattered cortical epithelial cells appeared likely involved in the positive selection, while those abundant in the cortico-medullary border and medulla in the negative selection. These latter most represent dendritic cells, based on typical localization and phenotype.These findings provide further proofs that efficient mechanisms leading to maturation of naïve T cells are operative in teleosts, strongly reminiscent of the models conserved in more evolved gnathostomes.