Replication timing of CD4 and CD8 in single-positive peripheral blood lymphocytes

The regulatory events leading to the mutually exclusive expression of CD4 and CD8 on peripheral lymphocytes are not fully understood. In particular, the association between DNA replication timing and transcriptional activity of these genes has not been previously investigated. Here, the replication kinetics of the CD4 and CD8 loci in mature single-positive T-cell populations have been examined using a novel approach to the separation of CD4(+) or CD8(+) lymphocytes into discrete cell cycle fractions and a competitive PCR replication timing assay. While the timing of replication of each of these loci is independent of their expression in mature CD4 or CD8 single positive T-cells, the replication of CD8, but not of CD4, shifts to a later time in S phase in transcriptionally silent HS68 fibroblast cells. These findings suggest that changes in DNA replication timing are associated with the developmentally regulated but not with the tissue-specific expression of CD4 and CD8.