Daan Pieren

31 Novel features of ageing in murine T cells INTRODUCTION The immune system reflects consequences of ageing by many alterations in the T-cell population that compromise T-cell responsiveness at old age [1,2]. Ageing-related changes have been widely reported in helper T cells (Th), cytotoxic T cells (Tc), and regulatory T cells (Treg) that act in concert to provide T cell-mediated immunity. Changes due to ageing occur among a wide variety of different immune parameters, such as the induction of cell surface activation markers, secretion of cytokines, and proliferative capacity [3-6]. The complexity to which ageing alters T-cell responses poses a major challenge in research on T-cell ageing. Whereas many studies address ageing-related T-cell phenotypes, only limited insight is available on the impact of ageing on the response kinetics over time [7]. In this study, we assessed ageing-related T-cell response kinetics by studying the effect of the duration and strength of in vitro stimulation on activation, proliferation, and cytokine secretion by T cells of young and aged mice. T cells of humans and mice rapidly upregulate expression of classical activation markers CD69 and CD25 after stimulation [8,9]. Upregulation of these markers at older age in human and murine T cells is reduced [10-13]. Expression of Programmed cell death-1 (PD-1) and Cytotoxic T-lymphocyte- associated antigen 4 (CTLA-4) is upregulated after T-cell activation [14,15], yet a substantial proportion of T cells of aged mice show constitutive expression of these inhibitory markers [16-19]. Additionally, ageing diminishes the capacity of T cells to proliferate in humans and mice [7]. Reduced proliferation is a major characteristic of T-cell senescence [6]. As both proliferation and expression of activation and inhibition markers by T-cell subsets are highly dynamic during an immune response, elucidating the kinetics of these parameters may reveal ageing-related alterations of T-cell responsiveness. Cytokine secretion after in vitro stimulation of T cells is also known to alter with ageing in both humans and mice [20]. However, findings are highly ambiguous in part due to the lack of studies addressing time kinetics of cytokine secretion, while these kinetics are vital for understanding ageing-related alterations [20]. For example, many studies in both humans and mice have shown contradicting results on the impact of ageing on IFN- γ [12,16,21-29] and IL-2 [20-23,27,30-35] secretion by cells. In addition, a suggested shift from a Type-1 towards a Type-2 cytokine secretion profile due to ageing [36,37] has also been counteracted in other studies [20,21,23,24]. The lack of consensus on the impact of ageing on secreted cytokines may be caused by a lack of 2