Identification of a committed T cell precursor population in adult human peripheral blood

Here, we report data concerning the discovery in adult human peripheral blood of a precursor cell population able to differentiate into CD4(+)CD3(+)alpha beta(+) mature T cells. These cells, which represent 0.1-0.5% of total peripheral blood mononuclear cells (PBMC), express substantial levels of CD4, but lack CD3 surface expression. At a molecular level, they express the pre-T cell receptor alpha (pT alpha) gene, CD3-gamma, CD-delta and CD-epsilon, and RAG-1 recombination enzyme and have initiated rearrangements in the T cell receptor (TCR)-beta locus (D-J). Moreover, low levels of CD3 epsilon protein, but not of TCR-beta chain, can be detected in their cytoplasm. Our results suggest that CD4(+)CD3(-) cells identified in peripheral blood are different from CD3(-)CD4(+)CD8(-) thymocytes and may contain precursors of an extrathymic T cell differentiation pathway.