[The roles of orphan nuclear receptors in T-lymphocyte development in the thymus]

T-lymphocyte development in the thymus proceeds through successive steps of differentiation,selection and expansion of thymocytes. Multiple signaling proteins and transcription factors play important roles in mediating the response to signals delivered through the pre-T-cell receptor (pre-TCR) and T-cell receptor (TCR) at two checkpoints during T-cell development. The orphan nuclear receptors RORg and Nur77 are involved in the differentiation and selection processes that are induced following activation of the pre-TCR and TCR, respectively, and they exert opposite effects on the survival of thymocytes. RORg activates the gene encoding the antiapoptotic protein Bcl-X(L) and is required for the survival of CD4+CD8+ thymocytes, while Nur77 is involved in the apoptosis of CD4+CD8+ thymocytes bearing TCRs with a high affinity for self antigens presented in the thymus.