T cell progenitors in the murine fetal liver: differences from those in the adult bone marrow.

We compared the T cell generation from progenitors (T-progenitors) in the fetal liver (FL) with those in the fetal thymus (FT) and adult bone marrow (BM) by culturing these T-progenitors with hematopoietic cell-depleted FT lobes. As has previously been noted, 15-20 days were required for BM T-progenitors to give rise to CD4+ and/or CD8+ T cells. In marked contrast, only 7-10 days were enough for FL T-progenitors to produce those T cells. The kinetics of T cell differentiation from sorted c-kit+Lin- FL cells were quicker than those of BM cells and slower than those of FT cells. These results have indicated that prethymic FL T-progenitors are distinct from both prethymic BM T-progenitors and thymic FT T-progenitors in reference to their differentiational stage. Moreover, these FL type T-progenitors exist in the liver during fetal age from Day 12 of gestation but not in the bone marrow of 1-week-old neonatal mice, suggesting that the fetal liver as a hematopoietic organ could induce the early T cell differentiation of progenitor cells.