CD4 and CD8 expression and T cell antigen receptor gene rearrangement in early intrathymic precursor cells

The earliest T precursor population in the adult mouse thymus, considered to have the surface phenotype CD4(lo)8(-)3(-)44(-)Thy-1(lo) c-kit+ (termed the low CD4 precursor), has been shown to have the capacity to produce B cells and dendritic cells, as well as T cells, and to hove the T cell antigen receptor (TCR) C beta, gene region in germ-line configuration. Because of evidence that this precursor population may have low levels of CDS as well as CD4 on the cell surface, it nas isolated, stained for surface CD4 and CD8 and assayed For the expression of messenger RNA (mRNA) for CD4 and CDS by the reverse transcriptase polymerase chain reaction (RT-PCR). The low CD4 precursors gave definite, moderate levels of staining for both CD8 and CD4. in contrast to downstream precursors which showed only marginal staining and so could be considered as genuine CD4(-)8(-)3(-) triple negatives. The low CD4 precursor expressed a significant level of mRNA for CD4, indicating that the surface C4 was produced by these cells. However. the low CD4 precursor did nor express a detectable level of mRNA for CDS. suggesting that the surface CDS was acquired from other cells. Since the lon CD4 precursor population was found already to express mRNA fur enzymes involved in TCR gene rearrangement, including in this study terminal deoxynucleotidyl transferase (TdT). a PCR procedure was used to assay early precursors for D-J rearrangements at the TCR beta gene locus. However. the low CD4 precursor had the TCR beta D-J genes in germ-line configuration, D-J gene rearrangements being first detected several stages downstream in the CD3(-)4(-)8(-)44(-)25(-) precursor population. We conclude that a transient synthesis of CD4, but not of CD8. characterizes these early thymus precursors. Although they have initiated synthesis of some recombination-associated enzymes. full commitment to the T lineage and TCR gene rearrangement is a later event.