Overexpression of hTERT increases the lifespan of CD4⁺ T lymphocytes. Polyclonal populations as well as cells derived from a single sorted naive and memory CD4⁺ lymphocyte from 2 different donors (aged 34 years [A] and 24 years [B]) were transduced with hTERT (right panels) and a GFP control vector (left panels) on the indicated days and cultured until no further growth was obtained. Population doublings (PDs) were determined by counting viable cells once a week. The proliferative lifespan of the hTERT-transduced populations was increased to over 120 PDs in polyclonal as well as clonally derived naive CD4⁺ cells (A and B) as well as CD4⁺ memory T cells (C, results obtained with cells from donor B) transduced with hTERT compared with GFP controls that stopped proliferation after 25 to 60 PDs. Accumulated total PDs are shown in panel C, whereas the number of PDs relative to the days in culture are shown in panels A and B.