Treg cells require an IL-7Rα signal for thymic development but not for peripheral survival. (A) Treg cells require IL-7 or TSLP for thymic development. TSLPR^−/− or TSLPR^+/+ bone marrow was used to reconstitute irradiated IL-7^−/− or C57BL/6 recipients. Eleven weeks later, the thymus was analyzed for CD4⁺Foxp3⁺ cells. Representative data from 3 separate experiments is shown, and the total number of Treg cells per thymus is indicated in the upper right quadrant. (B) Mature Treg cells do not require IL-7 or TSLP for survival. Purified CD4⁺ spleen cells from TSLPR^−/− or TSLPR^+/+ mice were labeled with CFSE and transferred into irradiated recipients that expressed IL-7 or lacked it, as indicated. Six days later, spleen cells were analyzed for CD4 and Foxp3 expression. TSLPR^−/− Treg cells survived equally well in hosts with or without IL-7.