Naive T cells circulating after TK⁺-cell add-backs are CD31⁺ recent thymic emigrants. (A) Frequency of CD4-naive T cells among total CD3⁺ lymphocytes in healthy subjects (white bar) and in TK007 patients before treatment and at different time points during follow-up (black bars). Shown is average with SD. (B) Correlation between age of the subject (x-axis) and the frequency of CD31⁺ cells among the CD4-naive subset (y-axis) in 26 healthy subjects. (C) CD31 expression analysis in a representative patient from the TK007 clinical trial (UPN#4, age 57). In the top row are the dot plots of CD3⁺CD4⁺ΔLNGFR⁻ T cells according to their expression of CD45RA (horizontal axis) and CD62L (vertical axis) at different time points during treatment. Gated in black are naive cells identified as CD45RA⁺ and CD62L⁺. Histograms below the dot plots represent CD31 positivity among naive CD4 cells (in black) and, as a negative control, among effectors (in white). (D) Frequency of CD31⁺ cells among naive CD4⁺ T lymphocytes in healthy subjects (white diamonds), in TK007 patients at different time points during follow-up (black diamonds), or in a control group of patients receiving haploidentical HSCT without T-cell depletion of the graft, followed by pharmacologic GVHD prophylaxis (gray diamonds). Shown is average with SD. *P < .05; **P < .001 in a paired-sample Student t test.