Identification of intrathymic T progenitor cells by expression of Thy‐1, IL 2 receptor and CD3

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Abstract
Immature (L3T4−/Lyt‐2− “double‐negative”) thymocytes were separated into several functionally distinct fractions based on their expression of IL 2 receptors, Thy‐1 and CD3. The majority (60–70%) of double‐negative thymocytes in young adult mice lack detectable IL 2 receptor expression, have high levels of Thy‐1 and rapidly “progress” to a L3T4+ or L3T4+/Lyt‐2+ stage when cultured for 20 h in simple medium. In contrast, the IL 2 receptor‐positive fraction retains the double‐negative phenotype for as long as it survives in culture and addition of IL 2 has little or no effect on such cells. IL 2 does generate strong proliferation from a fraction of cells expressing low levels of Thy‐1, but not detectable IL 2 receptors. Such culture generates an unusual population of double‐negative cells that expresses the pan‐B cell molecule B220 and which kill both the NK target cell line YAC‐1 and the NK‐resistant line EL4. This Thy‐1‐low fraction includes all of the double‐negative thymocytes capable of T cell reconstitution. Thy‐1‐low fraction could be further separated into two populations with regards to CD3 expression. CD3− but not CD3+ population could reconstitute mature T cells, indicating that Thy‐1‐low, IL 2R− and CD3− cells are the most enriched population of intrathymic T cell progenitors.