Germ-line transcription of the T cell receptor delta gene in mouse hematopoietic cell lines.

T cell receptor (TCR) genes are initially activated in hematopoietic stem cells that have been committed to the T cell lineage during ontogeny. We have established cell lines derived from hematopoietic organs which express truncated TCR delta mRNA (1.3 kbp). In this study, cDNA clones of this mRNA were analyzed to characterize TCR delta gene activation in early T cell development. Transcription started from an intron between D delta 2 and J delta 1, 72 bp upstream from J delta 1, and continued to J delta 1 and C delta exons, which were directly spliced to J delta 1. Thus, the truncated mRNA expressed by the cell lines was determined to be a germ-line transcript. The promoter activity of the DNA fragment between D delta 2 and J delta was assessed by its ability to drive the transcription of a reporter gene linked to it in the cell line expressing the TCR delta germ-line transcript. TCR delta germ-line transcription was found not only in these cell lines but also in fetal liver and thymus cells. These findings indicate that the TCR delta germ-line transcription is a naturally occurring event in developing T cells. The significance of germ-line transcription of TCR delta genes is unknown, but it may be an event that follows T-lineage commitment or, at least, may be closely associated with this commitment.