Normal human bone marrow precursors that express terminal deoxynucleotidyl transferase include T-cell precursors and possible lymphoid stem cells.

To compare the differentiation of early B- and T-lymphoid precursors, we have used immune adherence combined with analytical flow cytometric techniques to enrich and characterize subsets of the small population of bone marrow mononuclear cells that express the enzyme terminal deoxynucleotidyl transferase (TdT) but lack the CD19 B-lymphoid marker. Two percent to five percent of bone marrow TdT + mononuclear cells belong to the T-lymphoid lineage by virtue of expression of CD7 or CD5. Three-color immunofluorescence studies showed that, like early B-lymphoid precursors, most bone marrow TdT + T cells express HLA-DR and the progenitor cell antigen CD34, and about half express CD10. All CD5 + TdT + cells express surface CD3 and T-cell receptor alpha, beta, while a subset of CD7 + TdT + cells lack these "mature" T cell features. CD2 is low or absent on CD5 + TdT + cells. Examination of isolated CD34 + cells showed that approximately 70% of CD34 + TdT + cells expressed neither CD19, CD22, CD7, nor CD5, and 15% to 50% also lacked CD10. Thus, a major subset of CD34 + TdT + cells lack lineage-specific surface antigens. TdT expression may be the earliest available marker of lymphoid differentiation, and CD34 + TdT + cells are likely to include progenitor cells for both the B and T lineages.