Ectopic human telomerase catalytic subunit expression maintains telomere length but is not sufficient for CD8(+) lymphocyte immortalization

Like most somatic human cells, T lymphocytes have a limited replicative lif e span. This phenomenon, called senescence, presents a serious barrier to c linical applications that require large numbers of Ag-specific T cells such as adoptive transfer therapy. Ectopic expression of hTERT, the human catal ytic subunit of the enzyme telomerase, permits fibroblasts and endothelial cells to avoid senescence and to become immortal. In an attempt to immortal ize normal human CD8(+) T lymphocytes, we infected bulk cultures or clones of these cells with a retrovirus transducing an hTERT cDNA clone. More than 90% of transduced cells expressed the transgene, and the cell populations contained high levels of telomerase activity, Measuring the content of tota l telomere repeats in individual cells (by flowFISH) we found that ectopic hTERT expression reversed the gradual loss of telomeric DNA observed in con trol populations during long term culture. Telomere length in transduced ce lls reached the levels observed in freshly isolated normal CD8(+) lymphocyt es. Nevertheless, all hTERT-transduced populations stopped to divide at the same time as nontransduced or vector-transduced control cells, When kept i n IL-2 the arrested cells remained alive. Our results indicate that hTERT m ay be required but is not sufficient to immortalize human T lymphocytes.