Down‐regulation of CD28 via Fas (CD95): influence of CD28 on T‐cell apoptosis

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Abstract
Following antigen engagement of the T-cell receptor (TCR), T-cell survival is largely dictated by the provision of additional signals, such as those from costimulatory receptors and cytokine receptors. Whilst CD28-mediated signalling is increasingly associated with survival, ligation of alternative T-cell antigens, such as Fas (CD95), can trigger apoptosis. The T-cell response following antigen engagement may therefore be influenced by the relative expression levels of these coreceptors as well as by the availability of their ligands (CD80/86 and Fas-L). In this study we demonstrate functional interplay between the death receptor Fas and the costimulatory receptor CD28 in human T cells. In Jurkat T cells, we show that Fas signalling leads to rapid and selective CD28 down-regulation, and that this is associated with a specific decrease in mRNA for CD28, indicating that mechanisms exist which target CD28 at a transcriptional level. Moreover, cells that down-regulate CD28 also undergo apoptosis. Studies on activated human peripheral blood T cells demonstrate that cells expressing high levels of CD28 are resistant to Fas-mediated apoptosis whereas cells expressing low levels are more susceptible, implicating CD28 in the provision of anti-apoptotic signals. Consistent with this hypothesis, direct ligation of CD28 using B7 transfectants concomitant with anti-Fas challenge protects from apoptosis. Since antigen-presenting cells may express Fas-L under certain circumstances, the maintenance of T-cell CD28 expression may be crucial for the prevention of Fas-mediated apoptosis during the course of antigen engagement.