Tracing lymphopoiesis with the aid of a pTα-controlled reporter gene

A transgenic reporter mouse strain, which expressed the human CD25 (hCD25) surface marker as a reporter under the control of the pre-T cell receptor α (pTα) promoter, was used to identify lymphoid precursors that expressed pTα intracellularly. The hCD25 reporter marked intra- and extrathymic precursors of lymphocytes but not myeloid cells. The earliest intrathymic precursors were the extrathymic precursors were common lymphoid progenitors (CLPs) that included CD19⁻, B220⁺, Thyl⁺ and CD4⁺ cells. Thus, the pTα reporter can be used to trace lymphopoiesis between CLPs and αβ T cells. The slower extinction of the hCD25 reporter compared to pTα enabled us to define points at which pTα⁻ lineages branched off.