CD4 AND CD8 EXPRESSION AND T-CELL ANTIGEN RECEPTOR GENE REARRANGEMENTIN EARLY INTRATHYMIC PRECURSOR CELLS

The earliest T precursor population in the adult mouse thymus, conside red 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 t o 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 co nfiguration. Because of evidence that this precursor population may ha ve low levels of CDS as well as CD4 on the cell surface, it nas isolat ed, stained for surface CD4 and CD8 and assayed For the expression of messenger RNA (mRNA) for CD4 and CDS by the reverse transcriptase poly merase chain reaction (RT-PCR). The low CD4 precursors gave definite, moderate levels of staining for both CD8 and CD4. in contrast to downs tream precursors which showed only marginal staining and so could be c onsidered as genuine CD4(-)8(-)3(-) triple negatives. The low CD4 prec ursor expressed a significant level of mRNA for CD4, indicating that t he surface C4 was produced by these cells. However. the low CD4 precur sor did nor express a detectable level of mRNA for CDS. suggesting tha t the surface CDS was acquired from other cells. Since the lon CD4 pre cursor population was found already to express mRNA fur enzymes involv ed in TCR gene rearrangement, including in this study terminal deoxynu cleotidyl transferase (TdT). a PCR procedure was used to assay early p recursors 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 configur ation, D-J gene rearrangements being first detected several stages dow nstream in the CD3(-)4(-)8(-)44(-)25(-) precursor population. We concl ude that a transient synthesis of CD4, but not of CD8. characterizes t hese early thymus precursors. Although they have initiated synthesis o f some recombination-associated enzymes. full commitment to the T line age and TCR gene rearrangement is a later event.