Characterization of the subset of immature thymocytes which can undergo rapid in vitro differentiation

Recently, we reported that thymocytes expressing the CD8 molecule on their surface can give rise to CD4⁺CD8⁺ double‐positive and CD4⁺ single‐positive progeny following intrathymic transfer into an irradiated host mouse. Thymcoytes expressing a high density of CD8, referred to as CD8^hi, and those expressing a low density of the molecule, CD8^lo, were both able to differentiate in vivo. In this study we examined the ability of these CD8⁺ thymocyte populations and of CD4⁻CD8⁻ double‐negative thymocytes to change their phenotype during brief in vitro culture. CD8⁻ thymocytes were prepared by anti‐CD4 plus complement lysis followed by positive selection of the survivors on anti‐CD8‐coated plates. After 16 h of culture, > 60% of CD8⁺ thymocytes became double‐positive. Both CD8^hi and CD8^lo cells were able to show this in vitro change: about 30% of the former and about 80% of the latter became double‐positive. In contrast to this, double‐negative thymocytes which had been depleted of cells expressing low densities of CD8 did not show such a phenotypic conversion in vitro. Further panning experiments suggested that all of the CD8⁺ thymocytes actually express a low surface density of the CD4 molecule which is undetectable in our cytofluorometric assays.