Regulation and kinetics of premigrant thymocyte expansion

Normal mature thymocytes proliferate before emigrating to the periphery, an d continuous bromodeoxyuridine labeling showed that more than 30% of fully mature thymic emigrants have replicated DNA in the 24 h before exit. The pe rcentage of DNA-synthetizing single-positive (SP) thymocytes is transiently augmented during the postnatal period, with peaks on days 2 and 4 for CD4 and CD8 cells, respectively. Similar kinetics were observed in mouse chimer as made by transfer of normal bone marrow cells into RAG-2-deficient mice. These data show that proliferation of mature thymocytes is developmentally regulated. The proliferation peaks (on days 16 and 18 post transfer) observ ed in simple bone marrow chimeras were abolished when lymph node T cells we re mixed with the bone marrow cell inoculum, suggesting that the peripheral pool controls the late thymic expansion. The phenotype of cycling SP thymo cytes is atypical. they do not regulate activation and adhesion surface mol ecules like peripheral activated T cells.