A pT -Negative Subpopulation of CD25+ TN Thymocytes Revealed by a Transgenic Marker

We have recently generated 5′λ5-huTAC mice, which express the human CD25 (huTAC) gene under the control of the 5′-flanking region of the mouse λ5-gene. The huTAC-transgene was expressed in pre-B cells but neither in mature B cells nor in T cells of these mice. In this report we demonstrate that the transgene is also transiently expressed by adult CD25⁺ CD3^-CD4^-CD8^- (triple negative, TN) thymocytes and in fetal thymocytes. The huTAC⁺, in contrast to the huTAC^- subpopulation of the CD44⁺CD25⁺ TN cells, was unexpectedly found not to express the pTα-gene. Still the huTAC⁺CD44⁺CD25⁺ TN cells reconstituted the development of both αβ and γδ lineage cells equally efficiently as the pTα-expressing huTAC^- fraction, demonstrating that this pTα-negative subpopulation contained precursors for both T-cell lineages. Single cell reverse transcription-polymerase chain reaction (RT-PCR) experiments demonstrated that also in normal mice only a fraction of CD44⁺CD25⁺ and CD44^-CD25⁺ TN cells expressed this gene. Taken together, these data indicate that huTAC transgene expression revealed a truly pTα-negative fraction of the CD44⁺CD25⁺ TN cells. The observation that not all precursors in the CD25⁺ TN population express the pTα-gene has important implications for the understanding of early T-cell development and T-cell lineage commitment.