Molecular basis for the loss of CD28 expression in senescent T cells

CD28^null T cells are the most consistent biological indicator of the aging immune system in humans and are predictors of immunoincompetence in the elderly. The loss of CD28 is the result of an inoperative transcriptional initiator (INR), which consists of two nonoverlapping α and β motifs that have distinct protein binding profiles but function as a unit. In CD28^null T cells, there is a coordinate loss of α/β-bound complexes, hence the αβ-INR is inactive. In the present work therefore, studies were conducted to identify the components of such complexes that may account for the trans-activation of the αβ-INR. By affinity chromatography and tandem mass spectrometry, two proteins, namely, nucleolin and the A isoform of heterogeneous nuclear ribonucleoprotein-D0 (hnRNP-D0A), were identified to be among the key components of the site α complex. In DNA binding assays, specific antibodies indicated their antigenic presence in α-bound complexes. Transcription assays showed that they are both required in the trans-activation of αβ-INR-driven DNA templates. Because CD28 is T cell-restricted, and nucleolin and hnRNP-D0A are ubiquitous proteins, these results support the notion that cell-specific functions can be regulated by commonly expressed proteins. The present data also provide evidence for INR-regulated transcription that is independent of the known components of the basal transcription complex.