Early T-cell and thymus development in Japanese eel Anguilla japonica

Artificial cultivation of the Japanese eel has not been feasible due to difficulties in seed production and larval rearing, leading to poor seed and larval survival. We have focused on the importance of the thymus and T-cell function in eel larval stages because only the thymus, of all the immune system organs, develops well in eel leptocephalus. In this study, we identified lymphocyte-specific protein tyrosine kinase () as the T-cell marker gene and analyzed expression in eel by reverse transcription (RT)-PCR and in situ hybridization. Eel gene encodes a predicted protein made up of 506 amino acids, including an essential motif that binds to the cytoplasmic domain of both CD4 and CD8 in T cells. RT-PCR analysis revealed that eel was expressed in major lymphoid organs (thymus, kidney, and spleen), mucosal organs (intestine and skin), and T-cell populations. In situ hybridization demonstrated the presence of -positive cells in the thymus. Eel was also expressed in the leptocephalus and even preleptocephalus at 3 and 7 days post-fertilization (artificial fertilization). Our results show that eel is an appropriate molecular marker for T-cell identification and for the study of thymus development, and that it offers a better understanding of the eel immune system.