GCV administration controls GvHD-induced by T_CM suicide gene–modified human lymphocytes. NOD/Scid mice received a transplant of human skin, and were conditioned with anti-NK antibodies and sublethal irradiation prior to the intravenous infusion of baCD3/CD28⁺ IL-7/IL-15 TK⁺ lymphocytes or PBLs. Three to 6 days after injection, mice received a transplant of a 5 mg GCV or PBS-filled mini-osmotic pump. (A) After 7 days of treatment with or without GCV, circulating cells were analyzed by flow cytometry after staining with antihuman CD3 (x-axis in top panel), antimurine CD45 (y-axis in top panel), antihuman CD25 (x-axis in bottom panel), and antihuman LNGFR (y-axis in bottom panel) antibodies. Results from 1 representative of 3 independent experiments are shown. (B) At day 7 of treatment, the percentage of circulating TK⁺ lymphocytes in mice infused with baCD3/CD28⁺ IL-7/IL-15 was measured by flow cytometry. Median results of 3 independent experiments with 8 independent T-cell donors are shown. baCD3/CD28⁺ IL-7/IL-15 TK⁺ lymphocytes from mice treated with GCV (□, n = 4); baCD3/CD28⁺ IL-7/IL-15 TK⁺ lymphocytes from mice treated without GCV (■, n = 9). **P < .01. (C) At day 7 of treatment, transplanted human skin was excised from killed mice and analyzed by histopathology after staining with hematoxylin and eosin (×10 top low, ×40 middle low). Samples were simultaneously analyzed by immunohistochemistry after counterstaining with monoclonal anti-hCD3 antibodies and peroxidase-conjugated second-step reagent (×40 bottom low). Sections from 1 representative of 3 independent experiments are shown. (D) Pathological grading of transplanted human skin was performed on the basis of criteria currently used for pathological diagnosis of skin GVHD in patients who underwent transplantation. Frequency of severe skin GVHD (more than grade 2) in animals infused with baCD3/CD28⁺ IL-7/IL-15 TK⁺ lymphocytes or PBLs, and treated with or without GCV of 3 independent experiments with 8 independent T-cell donors are shown; baCD3/CD28⁺ IL-7/IL-15 TK⁺ lymphocytes with GCV treatment (□, n = 9), baCD3/CD28⁺ IL-7/IL-15 TK⁺ lymphocytes without GCV treatment (■, n = 11), PBLs with GCV treatment (, n = 3), and PBLs without GCV treatment (, n = 11). **P < .01.