Once immune reconstitution is achieved, selective elimination of TK⁺ cells by activation of the suicide gene does not impair immunity against pathogens. (A) Histogram bars represent IFN-γ release in response to CMV-infected (top section of each bar) or uninfected (bottom section of each bar) fibroblasts by T cells harvested from HSC donors (in black), from TK007 patients who achieved T-cell immune reconstitution and did not experience GVHD (in white), and from TK007 patients who suffered GVHD assessed ex vivo before and after the administration of GCV and the resolution of GVHD (in gray). All studied patients were transplanted from CMV-seropositive donors. Shown is average with SEM. (B) Frequency of viral reactivations over time after HSCT in patients who received TK⁺ cells and achieved (solid line, black diamonds) or did not achieve (dashed line, white diamonds) T-cell recovery. Gray bars represent the frequency of viral reactivations in the 2 months preceding and after GVHD treatment in those TK007 patients in whom this complication occurred. Frequencies are expressed as number of viral reactivations per patient per month.