Non–cell-autonomous effect of KGF on early thymic progenitors. ETPs (Lin⁻CD44⁺CD25⁻CD117^hi cells) were purified on day 6 after initiation of treatment of B6.CD45.1 donors (CD45.1⁺) that had received KGF (5 mg/kg per day, ⊡) or HBSS (▪, ▨) on days 0, 1, and 2. Afterward, ETPs (1 × 10³) from either group were transferred separately to a single thymic lobe of adult C57BL/6 recipients (CD45.2⁺) that had received 15 days earlier either HBSS (▪, ⊡) or KGF (5 mg/kg per day, ▨) on 3 consecutive days. The panels depict recipient thymic total cellularity (A) and CD45.1⁺ congenic ETP progeny (B) that were determined 7 days after intrathymic (i.t.) injection. Mean ± SD; *P < .001 versus HBSS controls (black bars), with 5 mice per group and time point.