POSITIVE SELECTION IS AN EARLY EVENT IN THYMOCYTE DIFFERENTIATION - HIGH TCR EXPRESSION BY CYCLING IMMATURE THYMOCYTES PRECEDES FINAL MATURATION BY SEVERAL DAYS

T cell antigen receptor expression by cycling and post-cycling thymocytes has been analysed by flow cytometry. Normal mice were pulsed with 5-bromo-2'-deoxyuridine (BrdUrd), a thymidlne analogue detectable with a monoclonal antibody. Thymocytes were surface-stained with antibodies against several Vβ gene products and against whole αβ receptors and detection of BrdUrd in the nuclei was performed after enzymatic generation of single-stranded DNA. A significant (10%) percentage of thymocytes expressing high levels of αβTCR were found in the cycle: these cells were immature, as shown by the CD4+8+ phenotype and by high HSA expression. After division, most αβhigh BrdUrd+ cells entered a resting state and their number remained constant for 3 days, decreasing in two steps thereafter. This post-mitotic evolution was not modified by injection of an anti-mitotic drug. After day 4, a majority of the studied subset acquired a single positive phenotype. Location of BrdUrd +ββ8.2high cells studied on frozen sections was found cortical at early times and medullary after day 3. Vβ6 expression by cycling and post-cycling thymocytes was analysed in various mouse strains, and early high expression by cycling thymocytes was found to be restricted to Mls1bstrains. These results suggest that high αβTCR expression by cycling immature thymocytes corresponds to positive selection, which must therefore be considered as an early event in Intrathymic differentiation. © 1990 Oxford University Press.