Phenotypic and functional characterization of committed and primitive myeloid and lymphoid hematopoietic precursors in human fetal liver

We studied the phenotypic and functional properties of colony-forming cells (CFCs), primitive long-term culture initiating cells (LTC-ICs) and lymphoid precursors present in human fetal liver (FL) and compared these with their adult bone marrow (BM) counterparts. FL (7-14-week) cells were selected by fluorescence-activated cell sorting based on increasing CD34 antigen expression (34⁺, 34⁺⁺, or 34⁺⁺⁺), CD38 antigen expression (CD34(++/+++)CD38⁺, or CD38^-), and HLA-DR antigen expression (CD34(++/+++)HLA-DR⁺ or HLA-DR^-). 13 ± 0.6% of FL CD34-positive cells were 34⁺⁺⁺. Significantly more FL CD34(++/+++) cells were CD38^- (49 ± 2.4%) and HLA-DR^- (72 ± 6.7%) than BM CD34⁺⁺ cells (6.8 ± 0.7% CD38^- and 13.3 ± 3.2% HLA-DR^-). FL and BM CFCs were CD34(+/++), CD38⁺, and HLA-DR⁺. However, significantly more FL CFCs were erythroid (40%) than adult BM CFCs (15%), and FL colonies were larger (8111 ± 738 cells/CFC) than BM colonies (3466 ± 272 cells/CFC, p < 0.001). As is seen in adult BM, FL LTC-ICs were CD34(++/+++) CD38^-. In contrast to BM LTC-ICs, FL LTC-ICs were almost exclusively CD34(++/+++) HLA-DR⁺. In addition, a single FL LTC-IC gave rise to >30 CFCs at 5 weeks compared with only 5 ± 0.9 CFCs per LTC-IC from BM. Finally, we demonstrate that the FL CD34(++/+++)/CD38^-/HLA-DR⁺ population, which contains 3.7% LTC-ICs, also contains primitive lymphoid progenitors capable of differentiating into natural killer (NK) cells. In conclusion, the phenotype of primitive human FL progenitors such as LTC-IC and primitive NK progenitors is CD34(++/+++)/CD38^-/HLA-DR⁺, suggesting that this population may contain FL hematopoietic stem cells. The phenotypic characterization of FL primitive LTC-ICs and NK progenitors will facilitate further studies of the functional properties of these progenitors.