Subpopulations of normal and leukemic human thymocytes: an analysis with the use of monoclonal antibodies

Combinations of antibodies to membrane antigens and to terminal deoxynucleotidyl transferase (TdT) were used to study human thymocyte and bone marrow subpopulations and leukemia cells. Cortical thymocytes were TdT+ and expressed T-cell antigens (HuTLA+), a thymocyte-specific antigen (HTA-1+), and a leukocyte antigen (HLe-l++) but lacked detectable HLA-A,B,C and la (HLA-D) antigens. In contrast, medullary thymocytes were TdT-, HuTLA+, HTA-1-, HLe-l++. A small subpopulation of larger, probably immature, thymocytes were strongly TdT+, HuTLA+, la-, HTA-1-, HLe-l +/-. Many blast cells from cases of thymic acute lymphoblastic leukemia (Thy-ALL) showed the phenotype of this small subset, and only a proportion of Thy-ALL blast cells exhibited HTA-1 and HLe-l antigens as strongly as was observed on normal cortical thymocytes. In contrast, TdT+ cells observed in normal juvenile bone marrow were HuTLA, HTA-1-, HLA+, la+. This phenotype corresponded to the phenotype of the common form of ALL (non-T, non-B) and indicated that further studies are necessary to analyze the differentiation of bone marrow precursors to thymic cells.