Double fluorochrome analysis of human bone marrow lymphoid cells: Studies with terminal transferase and anti-T-cell monoclonal antibodies

Mature thymus-derived (T) lymphocytes are generally believed to be derived from a bone marrow progenitor cell. Data from studies with animals suggest that the enzyme terminal deoxynucleotidyl transferase (Tdt) is expressed in many T-cell progenitors in bone marrow. In this study we attempted to identify Tdt⁺ bone marrow cells in man that may be committed to T lineage based on coexpression of Tdt and antigens that have previously been useful in characterization of thymocytes or peripheral-blood T cells. We used a panel of ten monoclonal antibodies against such antigens to analyze Tdt⁺ bone marrow cells using two-color immunofluorescence. We found that T-cell-associated antigens were not expressed on Tdt⁺ bone marrow cells and that T cells in bone marrow have a phenotype similar if not identical to peripheral-blood T cells. These results support the hypothesis that many postthymic immunocompetent T cells are found in human marrow. Our results also suggest that if Tdt⁺ bone marrow cells are committed to T lineage, then the acquisition of mature T-cell-associated antigens is an intrathymic event.