Efficient thymic immigration of B220⁺ lymphoid-restricted bone marrow cells with T precursor potential

Using a human CD25 reporter transgene controlled by regulatory sequences from the gene encoding pre-T cell receptor α, we identified a common lymphocyte precursor (CLP-2) population that, in contrast to the previously identified CLP-1 population, was c-Kit^-B220⁺. In short-term culture, the CLP-2 could be derived from the CLP-1 subset, and contained cells that in clonogenic assays were assessed to be bipotent precursors of T and B cells. Intravenous injection of bone marrow cells yielded a selective accumulation of CLP-2 thymic immigrants that in thymic organ culture generated mature αβ T cells. Although the CLP-2 subset may represent the most differentiated population with T cell potential before commitment to the B cell lineage, other subsets of thymic immigrants capable of generating T cells may exist.