Infusion of purified TK⁺ cells after T cell–depleted HSCT prompts the immune recovery of TK⁻-naive T cells. (A) Composition of 43 genetically modified T-cell products infused into 25 patients enrolled in the TK007 clinical trial. After magnetic selection, the large majority of infused cells expressed the surface marker ΔLNGFR (TK⁺ cells, shown in black), whereas only a minor fraction was negative for the transgene (TK⁻ cells, shown in white). (B) Absolute counts of circulating T (circles), B (squares), and natural killer (NK; triangles) lymphocytes in TK007 patients. Shown is the average with SD. (C) Absolute counts of circulating T cells in TK007 patients before (baseline) and at different time points after the infusion of TK⁺-cell add-backs (at immune reconstitution, defined as the attainment of a CD3 cell count above 100 cells/μL; at 6 months after HSCT; and at 12 months after HSCT). Histogram bars are subdivided to represent the relative frequency of circulating TK⁺ (in black) and TK⁻ (in white) T cells at each time point. Shown is the average with SD. (D) Correlation between the frequency of PBMCs expressing on their surface the ΔLNGFR marker (on the x axis) and that of PBMCs having integrated in their genome the HSV-TK transgene as assessed by specific qPCR (on the y axis) in 25 patients enrolled in the TK007 trial. (E) Absolute counts of circulating TK⁺ (left panels) and TK⁻ (right panels) T cells at different time points after HSCT and TK⁺ cell add-backs subdivided according to the CD4 or CD8 subtype (in black and white, respectively; top panels) and to their naive, central memory or effector phenotype (in black, gray, and white, respectively; bottom panels). Shown is the average with SD of the results obtained from 10 TK007 patients.