The Loss of Telomerase Activity in Highly Differentiated CD8 super(+)CD28 super(-)CD27 super(-) T Cells Is Associated with Decreased Akt (Ser super(473)) Phosphorylation

The enzyme telomerase is essential for maintaining the replicative capacity of memory T cells. Although CD28 costimulatory signals can up-regulate telomerase activity, human CD8 super(+) T cells lose CD28 expression after repeated activation. Nevertheless, telomerase is still inducible in CD8 super(+)CD28 super(-) T cells. To identify alternative costimulatory pathways that may be involved, we introduced chimeric receptors containing the signaling domains of CD28, CD27, CD137, CD134, and ICOS in series with the CD3 zeta ( zeta ) chain into primary human CD8 super(+) T cells. Although CD3 zeta -chain signals alone were ineffective, triggering of all the other constructs induced proliferation and telomerase activity. However, not all CD8 super(+)CD28 super(-) T cells could up-regulate this enzyme. The further fractionation of CD8 super(+)CD28 super(-) T cells into CD8 super(+)CD28 super(-) CD27 super(+) and CD8 super(+)CD28 super(-)CD27 super(-) subsets showed that the latter had significantly shorter telomeres and extremely poor telomerase activity. The restoration of CD28 signaling in CD8 super(+)CD28 super(-)CD27 super(-) T cells could not reverse the low telomerase activity that was not due to decreased expression of human telomerase reverse transcriptase, the enzyme catalytic subunit. Instead, the defect was associated with decreased phosphorylation of the kinase Akt, that phosphorylates human telomerase reverse transcriptase to induce telomerase activity. Furthermore, the defective Akt phosphorylation in these cells was specific for the Ser super(473) but not the Thr super(308) phosphorylation site of this molecule. Telomerase down-regulation in highly differentiated CD8 super(+)CD28 super(-)CD27 super(-) T cells marks their inexorable progress toward a replicative end stage after activation. This limits the ability of memory CD8 super(+) T cells to be maintained by continuous proliferation in vivo.