In vivo dynamics of CD4–8– thymocytes. Proliferation, renewal and differentiation of different cell subsets studied by DNA biosynthetic labeling and surface antigen detection

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Abstract
The proliferative status of CD4 −8− thymocyte subsets was determined by in vivo or in vitro labeling with bromodeoxyuridine (BrdUrd), a nonreutilized thymidine analogue detectable with a monoclonal antibody, simultaneously with relevant surface proteins. An actively cycling subset [J11d, interleukin 2 receptor-positive (IL2R)] was defined, besides a relatively resting one (J11d-, Pgpl, T cell receptor-positive). Continuous per os administration of BrdUrd showed that 85% only of CD4−8− thymocytes were labeled in 6 days confirming the existence of a relatively long-term resting subset. By contrast, CD48 thymocytes were all labeled in 3–4 days. Observation of labeled CD4−8− cells after pulse labeling showed an immediate decrease of their absolute number per thymus, confirming their low autorenewal capacity. However, a small number of labeled cells which were hydroxyurea or colchicine resistant remained CD4−8− for several days and progressively acquired surface expression of IL2R. IL2R expression by cycling CD4−8− cells during thymus regeneration after antimitogenic drug treatment was rapid, but very transient. According to these results most CD4−8− thymocytes appear as largely engaged in a proliferation-dependent differentiative process, and do not behave as true stem cells. Consequently, this subset is principally renewed by thymic immigration of exogenously produced resting cells. However, a tenfold expansion of CD4−8− cells was found in the fetal and regenerating thymus, suggesting two proliferative phases during intrathymic CD4−8− cell maturation, the first one yielding to cell expansion and the second to cell differentiation. A tentative evaluation of daily cell immigration is proposed starting with the determination of the number of cells beginning DNA synthesis each day. A global model is finally discussed by confronting our kinetic results with the known reconstitution capacities of CD4−8− thymocyte subsets.