Transcription and demethylation of TCR beta gene initiate prior to the gene rearrangement in c-kit+ thymocytes with CD3 expression: evidence of T cell commitment in the thymus.

In order to determine whether the cells immigrating into the thymus have been committed to the T cell lineage, we examined the gene expression of the TCR complex in fetal thymus (FT) and fetal liver (FL) precursors. We previously showed that c-kit bright-positive, Pgp-1 bright-positive and lineage markers negative fetal thymocytes (c-kit+ FT cells) are the most immature cells which do not undergo gene rearrangement of the TCR beta chain. In this study, we demonstrated that the gene rearrangement of TCR gamma as well as beta chains does not occur in c-kit+ FT cells, but that the germline transcript of their TCR beta was found in J-C regions. The TCR beta gene was demethylated in c-kit+ FT cells. CD3 gamma, delta and epsilon subunit genes were also expressed at the mRNA levels in c-kit+ FT cells, but cytoplasmic protein staining divided them into two populations: cytoplasmic CD3 epsilon positive and negative cells. These features were not observed in c-kit+ FL cells. Moreover, Ly-1 expression was found on c-kit+ FT cells but not on c-kit+ FL cells. These results indicate that DNA alteration on the TCR beta gene initiates with other phenotype expression determining the T cell lineage in the thymus prior to TCR gene rearrangement.