Early T cell development can be traced in rat fetal liver

The degree of T cell commitment reached by cell precursors present in the fetal liver is a controversial issue. In the present work, the occurrence of fully T cell‐committed progenitors among CD45+Thy‐1+CD44+ 13‐day‐old rat fetal liver cells was demonstrated when limiting numbers of these cells in vitro reconstituted SCID mouse fetal thymic lobes providing single lineage‐containing lobes for T, natural killer or dendritic cells. In addition, expression of rat pre‐TCRα chain mRNA was detected in the CD45+ but not in the CD45– fetal liver cells and fully rearranged TCR Vβ8‐Cβ mRNA transcripts were specifically detected in the former population, demonstrating early transcription of some rearranged TCRVβ genes in the rat fetal liver of 13 days of gestation. Finally, fetal liver organ cultures provided low numbers of TCRγ δ T cells and CD2+CD8+NKR‐P1A– intracytoplasmic CD3+ immature T cells, which intracellularly reacted with a mAb specific to the TCRα β molecule. These results prove T, NK and DC cell lineage determination at a prethymic stage in the fetal liver.