Daan Pieren

50 Chapter 2 memory cells [2,21]. Based on our study, it would be a contribution to the field of ageing for future studies to reveal how purified Th, Tc, and Treg cell subsets respond separately, as well as naive and memory cells within these cell subsets. Collectively, our data show that stimulatory strength and time kinetics of cytokine secretion, activation markers, and proliferation of Th, Tc, and Treg cells are crucial in understanding the impact of ageing on T cells. Despite low proliferative capacity, T cell subsets of aged mice do respond to stimulation by upregulation of activation markers and secretion of cytokines. These findings therefore indicate that replicative senescence of aged T cells is not a measure of unresponsiveness per se, but rather stress that ageing influences the kinetics of proliferation, upregulation of activation markers and cytokine secretion each to a different extent. Moreover, our multi-dimensional analyses revealed phenotypical T-cell subpopulations that further delineate the impact of ageing on T cells. ACKNOWLEDGEMENTS We thank the Biotechnicians of the Institute for Translational Vaccinology for their assistance, Jeroen Hoeboer for assisting with performing the experiments and Debbie van Baarle for critically reading the manuscript. This study was supported by grants (V/152106 and S/000301) provided by the Dutch Ministry of Health, Welfare, and Sport.