Development of lymphocyte subpopulations identified by monoclonal antibodies in human fetuses

We have used a panel of monoclonal antibodies to examine the development of lymphoid and myeloid subpopulations of cells in thymus, bone marrow, and liver of 16 fetuses from 12 to 16 weeks of gestational age. Pre-B and IgM+ B cells were present at a ratio of approximately 2:1 in all of the fetal bone marrow and liver samples; cells of both phenotypes were HLA-DR+ but did not express the mature B-cell antigen, HB-2. Cells expressing the myelomonocytic antigen, MMA or Leu-M1, were more frequent in bone marrow (40%) than in fetal liver (10%), and cells expressing the HNK-1 or Leu-7 antigen were rare (less than 1%) in all of the fetal tissues examined. Each of the T-cell antigens, T1, 4, 5, 6, and 8, was expressed by a majority of thymocytes irrespective of the age of the fetal donor. In contrast, cells with the T1, 4, 5, and 8 antigens were not seen in bone marrow and liver before the 13th week of gestation, and T6+ cells were never seen in these hemopoietic tissues. These results suggest that fetal liver and bone marrow precursors in humans do not express these T-cell antigens prior to thymic entry and the onset of thymocyte differentiation.