Flow Cytometric Assessment of Human T-cell Differentiation in Thymus and

Using multidimensional flow cytometry we have defined and quantified the human T-cell differentiation pathway, focusing on those events occurring among the most immature thymo-cytes and putative bone marrow (BM) T-precursors. Early thymocytes were found to express the CD34 antigen and consisted of a mean 1.2 % of cells within human pediatric (n = 9) and 2.0 % in fetal thymi (n = 4). All CD34 ' thymocytes were atypical blast by morphology, expressed intracytoplas-matic, but not cell surface, CD3, and were cell surface CD2'. CD5'. CD7+, CD38'. CD45+, CD45RA'. CD49d'. and LECAM-1(Leu8)h'gh. CD34hwh thymocytes lacked surface expression of CD4 and CD8, but as CD34 expression diminished there was a coordinate increase in CD4 levels, followed by the appear-ance of CD8. The expression of CDI and CDlO also increased concomitant with the loss of CD34. whereas expression o