Sara van den Berg

154 Chapter 6 Differences between groups were assessed by Mann-Whitney U test, or Wilcoxon matched-pairs signed rank test for comparisons within the same individuals. Correlation was tested by Spearman’s correlation. P- values < 0.1 are presented in the figure, ns = not significant. Data from each individual are represented by unique symbols. Interestingly, the expression of CD57, KLRG-1, and CD28 showmoderate, but highly significant correlations with the estimated production rates ( r =  -0.6196, r =  -0.6075, r =  0.6243, respectively; for all markers p < 0.0001) ( Figure 5 ). Thus, late T-cell differentiation states are associated with reduced production rates in vivo . Expression of the proliferation marker Ki-67 correlated only weakly with the in vivo production rate ( r = 0.3304, p = 0.0285). This correlation was partly obscured due to the data points of one individual (E04), whose Ki-67 expression levels were relatively high given the corresponding production rate. Excluding this individual from the analysis yielded a moderate significant correlation between Ki-67 expression and the estimated production rate ( r = 0.5115, p = 0.0010). Figure 5. Association between T-cell production rates and expression of senescence, proliferation, and apoptosis markers. The estimated average production rates of different CD4 + and CD8 + T-cell subsets as esti- mated by heavy water labelling were plotted against their expression (% positive) of CD57, KLRG-1, CD28, Ki-67, and Bcl-2. Horizontal lines represent median values. Data points of donor E04 are circled in the Ki-67 graph. Correlation was tested by Spearman’s correlation, r -values are indicated in the graphs. Stars indicate significant differences as follows: * P -value <0.05, ** P -value <0.01, *** P -value <0.001, *** P -value <0.0001. Different symbols represent different memory subpopulations. DISCUSSION Immunological memory is indispensable for healthy ageing and therefore it is essential to understand the underlying biology of lymphocyte maintenance. CMV exerts a major effect on the composition of the T-cell pool. The abundancy of antigen-specific CD8 + T-cells