EXPANSION OF MATURE THYMOCYTE SUBSETS BEFORE EMIGRATION TO THE PERIPHERY

A small population of DNA-synthesizing mature thymocytes could be defi ned by analyzing cell surface markers and 5-bromo-2'-deoxyuridine (Brd Urd) labeling by four-color cytofluorometry, These cells have a comple tely mature phenotype (CD4(-)CD8(+) or CD4(+)CD8(-)TCR(high), HSA(-), Qa-2(high)) and expand only weakly after BrdUrd incorporation, They re covered immediately in total number and in DNA synthesis rate after tr eatment with the antimitotic drug demecolcin, thus much faster than im mature CD4(+)CD8(+) thymocytes, These data demonstrate the existence o f a late intrathymic expansion phase, independent of that of developin g CD4(+)CD8(+) immature cells, and involving phenotypically mature cel ls renewed each day, In mixed chimeras prepared by transfer of bone ma rrow and lymph node cells into RAG-2(-/-) mice, all cycling mature thy mocytes were bone marrow derived, They are thus produced in situ and d o not correspond to peripheral T cells reentering the thymus, Double F ITC/BrdUrd detection showed that a high proportion (10-20%) of recent thymic emigrants were BrdUrd(+) just postcycling cells and that around 50% of cycling mature thymocytes are just ready to emigrate to the pe riphery in the few hours after DNA synthesis, The late intrathymic exp ansion phase demonstrated here increases the daily thymic cell export by at least 30%, It could play a role in the adjustment of the T cell repertoire before emigration and in the regulation of the thymic cell output into the peripheral T cell pool.