Telomerase maintains telomere length by adding TTAGGG repeats to the 3′-ends of chromosomes. In human T-cells telomerase is transiently expressed upon activation and stimulation and, as shown previously, telomerase levels are able to control their lifespan. To understand the effects of culture parameters on telomerase activity and cell lifespan is of critical importance to improve T-cell expansion. Here, we investigated the influence of culture conditions and of stimulation on lifespan, clonogenicity (number of positive wells), cell cycle and telomerase activity in T-cells in vitro. Isolated peripheral blood mononuclear cells (PBMCs) from healthy individuals were cultured in RPMI 1640 medium containing IL-2, different sera (10%) (fetal calf serum (FCS), AB serum and autologous serum) and stimulated with phytohaemagglutinin (PHA) and irradiated allogeneic mononuclear feeder cells or anti-CD3/CD28 beads. T-cells were counted and passaged weekly until senescence. Expanded T-cells were analyzed for cell cycle by DNA staining with propidium iodide (PI) at various time points after stimulation and telomerase activity was measured using the telomerase PCR ELISA. Clonogenicity was determined by limiting dilution using different culture conditions. The proliferative lifespan of T-cells expanded with PHA/feeder cells and autologous serum from two different donors was significantly increased compared to T-cells stimulated with PHA/feeder cells and AB serum (44.7 vs. 14.2 PDs; 42.4 vs. 27.4 PDs). In contrast, no or only little difference was found for T-cells expanded with anti-CD3/CD28 beads and autologous or AB serum (8.7 vs. 8.1 PDs; 15.5 vs. 9.6 PDs). Most likely the latter findings reflect the loss of CD28 expression on T-cells. Furthermore, the use of autologous serum also increased the clonogenicity to about 4-fold compared to the use of AB serum or FCS without any signs for differences in the fractions of cycling cells. Interestingly, T-cells cultured with autologous serum exhibited a higher telomerase activity at day 3 after stimulation and a reduced decline of telomerase activity at day 6 and 9 compared to cultures with AB serum. Our results demonstrate that the use of autologous serum combined with PHA stimulation and feeder cells remarkably extends the proliferative lifespan and clonogenicity of human T-cells in vitro. In addition, T-cells cultured in medium containing autologous serum exhibit higher and prolonged telomerase activity compared to cultures with AB serum. Further studies are ongoing to elucidate the underlying factors involved in those observations. Nevertheless, we suggest that autologous serum together with PHA stimulation and feeder cells is superior to expand human T-cells especially for clinical applications where large numbers of specific T-cells are required for adoptive transfer strategies in cancer patients with for example severe viral infections.

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