Fetal liver generates low CD4 hematopoietic cells in murine stromal cultures

We have demonstrated that 0.2% to 11% of cells from the fetal liver (FL) reacted specifically with high concentrations of anti-CD4 monoclonal antibody (MoAb). CD4+ cells from FL were similar in surface phenotype and fluorescence characteristics to the CD4+ population found previously in adult bone marrow (BM). FL and BM cells were seeded in cultures that allow differentiation to primitive precursors. FL cells released many low CD4+ and low Thy+ cells in the supernatant, while BM cells seeded under the same conditions did not. We studied the nonadherent cells harvested from 10-day FL cultures (greater than 90% low CD4+). In methylcellulose, they were able to produce more colonies that appear to be characteristic of earlier stages in the hierarchy of hematopoietic precursors (especially erythroid bursts and colonies composed of both myeloid and erythroid elements) in comparison with CD4- cells from 10-day BM cultures. CD4+ cells harvested from FL cultures initiated secondary cultures containing both a stromal layer and large hematopoietic colonies when replated under conditions similar to those of primary cultures. Furthermore, a limited number of CD4+ cells from 10-day FL cultures were able to repopulate lethally irradiated mice. Although we cannot formally exclude the possibility that the low CD4 cells produced in FL cultures were derived exclusively from the proliferation of the few CD4 cells found in fresh FL, the dynamic analysis of the development of these cells in culture favors the generation of this important population from a CD4- subset of hematopoietic stem cells (HSCs). We speculate that FL contains a prevalent population of very primitive cells not expressing the CD4 antigen, tentatively called "pre-low CD4 precursors." These primitive cells can differentiate into low CD4+ cells that share many characteristics with pluripotent HSCs of the adult type. These data indicate the possibility of using hematopoietic progenitors obtained by the expansion/differentiation of fetal stem cells in culture for transplantation purposes.