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

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.