AGING-RELATED DEFICIENCY OF CD28 EXPRESSION IN CD4-CELLS IS ASSOCIATED WITH THE LOSS OF GENE-SPECIFIC NUCLEAR FACTOR-BINDING ACTIVITY( T)

Changes in T cell populations and concomitant perturbation of T cell e ffector functions have been postulated to account for many aging-relat ed immune dysfunctions, Here, we report that high frequencies of CD28( null) CD4+ T cells were found in elderly individuals. Because deviatio ns in the function of these unusual CD4+ T cells might be directly rel ated to CD28 deficiency, we examined the molecular basis for the loss of CD28 expression in CD4+ T cells. In reporter gene bioassays, the mi nimal promoter of the CD28 gene was mapped to the proximal 400 base pa irs (bp) of the 5' untranslated region, CD28 deficiency was associated with the loss of two noncompeting binding activities within a 67-bp s egment of the minimal promoter, These binding activities were not comp eted by consensus Ets, Elk, or AP3 motifs that were found within the s equence stretch. The DNA-protein complexes were also not recognized by antibodies to Ets-related transcription factors, Furthermore, introdu ction of mutations into the 67-bp segment at positions corresponding t o the two DNA-protein interaction sites, i.e. nucleotides spanning -20 6 to -179 and -171 to -148, resulted in the loss of specific nuclear f actor binding activities and the abrogation of promoter activity. Thes e observations implicate at least two regulatory motifs in the constit utive expression of CD28. The loss of binding activity of transacting factors specific for these sequences may contribute to the accumulatio n CD4+CD28(null) T cells during aging.