Fig 1. ¹⁴C measurements of naive T-cell populations reveal age-dependent decrease in naive T-cell turnover.

(A) Schematic explaining how nuclear bomb test–derived ¹⁴C is incorporated into the DNA of newly produced naive T cells (either in the thymus or through cell division in the periphery). Depending on what year a new naive T cell is formed, the ¹⁴C content will mirror atmospheric levels, providing a cellular birth date. (B) Populations of naive T cells surveyed in this study include total CD8⁺ T cells and both CD4⁺CD31⁺ and CD31⁻ T-cell populations. CD8⁺ and CD4⁺CD31⁺ naive T cells are presumed to originate in the thymus and divide in the periphery with CD4⁺CD31⁺ cells, yielding CD4⁺CD31⁻ cells in the peripheral tissues. (C) Flow cytometry plots depicting gating strategies for purifying each population of naive T cells. Total CD3⁺ cells are shown for each gate, which were obtained after purifying all naive T cells from buffy coats subjected to red blood cell lysis. CD8⁺ naive T cells are >98% CD31⁺, so no distinction was made for sorting this population. (D) Average cell DNA age determined by ¹⁴C content analysis depicted for each population relative to donor age. Increased cell DNA age is observed for all populations relative to donor age (Student t test). CCR7, C-C chemokine receptor 7.