Lymphoid reconstitution after neonatal treatment. (A) Number of mature T and B cells in the peripheral circulation of GT (^–/– GT) and transplant-recipient ADA^–/– mice (^–/– BMT) treated at neonatal age. As control, untreated age-matched ADA^+/+ mice (^+/+; n = 6) are reported. FACS analysis was used to determine the absolute number, represented as mean ± SD, of white blood cells, CD3⁺CD4⁺ and CD3⁺CD8⁺ T cells, B220⁺IgM⁺ B cells, and NK cells evaluated 5 months after GT (n = 5) or BMT (n = 8). Cell counts from untreated 18-day-old ADA^–/– (^–/–; n = 14) and ADA^+/+ (^+/+; n = 8) mice are shown on the right. (B) H&E stainings of thymus and spleen sections (× 5 magnification) in ADA^–/– mice (untreated, treated with GT or BMT) and ADA^+/+ mice. Tissues from ADA^–/– untreated controls were collected postnatally at day 18, while all other mice were analyzed 6 months after treatment.