Functional Disruption of the CD28 Gene Transcriptional Initiator in Senescent T Cells

We recently reported that aging is accompanied by the emergence of CD4 ⁺CD28^null T cells, a functionally aberrant lymphocyte subset rarely seen in individuals younger than 40 years. Here, we directly examined whether the lack of CD28 expression is due to a defect at the level of transcriptional initiation. Molecular studies reveal that CD28 gene transcription is controlled by two sequence motifs, sites α and β. In vitro transcription assays using initiator-dependent DNA templates revealed that reversed polarity or the deletion of either motif inhibited transcription, indicating that α/β sequences constitute a composite initiator. Moreover, nuclear extracts from CD28^null cells failed to activate transcription of αβ-initiator DNA templates. Transcription of such templates was, however, restored with the addition of extracts from CD28 ⁺ cells. Although previously described initiator elements have been defined by a consensus sequence, the αβ-initiator has no homology to such sequence. These studies demonstrate that initiators have functions other than positioning elements for the basal transcription complex. Rather, initiators can have a direct role in regulating the expression of specific genes. The gain or loss of initiator activity can be an important determinant of cell phenotypes.