It is common to report T cell values as the number of T cells/ml and use this

value to set defined ranges and differences between ages [24]. In order to normalize the values with respect to T cells numbers in the blood we calculated the values of the number of sjTREC+ cells per ml of blood and then compared this with the values we obtained for leucocyte numbers and T cell numbers in the 7th–10th decades (Table 1). The results confirm the maintenance of both leucocyte and T cell numbers in the blood over the entire age range and also provide further confirmation of the degree of variation in the numbers of sjTREC+ cells. Interestingly, there is an observable convergence of the overall spread of the sjTREC levels with advancing age in the decades analysed. The standard deviations of the values progress from almost three times the average value in the 7th decade to twice Cisplatin clinical trial the ACP-196 value in the next decade to almost equal to the value in the 9th decade. During these decades the average sjTREC/ml

remains fairly steady between the ages of 60 and 89 at about 0·0006% of the T cells. The greatest change occurs in the 10th decade, and the degree of change is understood more easily if we calculate changes within the T cell pool as a whole. To do this we need to assume that each sjTREC is present as a single entity in a T cell (i.e. no doubles), that the blood volume in these individuals is close to average at 5 litres [25] and that approximately 2% of the total T cell pool resides within the blood [26]. From this we calculate that the average individual in their 9th decade has 2·35 × 106 sjTRECs in their T cell pool and that this value drops to 1·5 × 105 in the 10th decade. This is a dilution factor of almost fivefold without a comparative change in the overall T cell numbers. Using a linear regression model, further analysis was performed of the observed decline in sjTREC level as a function of age. Table 2 highlights the relationship between sjTREC levels and increasing age. Across the entire age range no significant correlation was observed; however, as the transition is made from the 8th to 9th decades a significant correlation coefficient is seen of r = −0·285

(P = 0·05), progressing to −0·463 (P = 0·02) by the 10th decade. We have analysed thymic output through following the change in the sjTREC values in the C1GALT1 peripheral blood CD3+ T cells of more than 200 individuals from five different European countries who were within the age range 60–100 years. Our results provide information about the potential end-point for thymic output and also provide a suggestion that sjTREC analysis may prove to be a biomarker of ageing. The observed convergence of the sample heterogeneity in the sjTREC levels with increasing age raises a number of interesting possibilities. First, are low sjTREC measurements reflective of an individuals immunosenescence status; if so, are the individuals in the lower left quadrant of Fig.