Isolation of the most immature population of murine fetal thymocytes that includes progenitors capable of generating T, B, and myeloid cells.

Thymus cells of murine fetuses at day 12 of gestation are exclusively of the CD3-CD4-CD8-CD44+CD25- phenotype, which is known as a hallmark of the most immature subset of thymus cells. In the present study, we show that day 12 fetal thymus (FT) cells express Fc gamma RII/ III (FcR) at a broad range of levels on their surface. The FcR+ FT cells seem to represent T lineage cells, because a large majority of them express the T lineage specific transcription factors TCF-1 and GATA-3 as well as CD3 epsilon in the cytoplasm. Also shown is that the FcR- population contains progenitors capable of developing into not only T cells but also B and myeloid cells, whereas FcR+ progenitors are mostly committed to the T lineage. These findings indicate that thymic T lineage cells express FcR on their surface at the earliest stage of differentiation, and thus FcR is a useful marker in isolating the most immature population of murine FT cells.