KGF enhances TEC numbers via induction of cell division in adult mice. (A-D) Adult TECs express the receptor for KGF. Thymic sections from 6-week-old, naive C57BL/6 mice were analyzed by confocal immunohistofluorescence for the expression of FgfR2IIIb on K18⁺ epithelial cells (A), K5⁺ cells (B), MTS24⁺ cells (C), and on TECs binding UEA1 (D). The colocalization denotes distinct TEC subpopulations expressing the KGF receptor (arrows). A total of 3 experiments were performed, providing comparable results. (E-H) TECs expand in response to KGF in vivo. Adult C57BL/6 mice were treated with KGF (5 mg/kg, ⊡) or HBSS (▪) on 3 consecutive days (days 0, 1, and 2). Frequencies of the 4 TEC subpopulations were assessed on day 3 by flow cytometry. Relative frequencies are given (x-fold changes KGF vs HBSS treatment, whereby frequencies [in %] for HBSS-treated mice were set as 1.0 and shown as dashed lines). Mean ± SD. *P < .01 versus HBSS. (I-N) KGF administration to adult mice does not alter the normal architecture and epithelial composition of the thymus. Confocal microscopy analysis was performed 15 days after treatment of adult C57BL/6 mice with HBSS (I-K) or KGF (L-N) for 4 TEC subpopulations, as indicated. mTEC indicates medullary TEC; cTEC, cortical TEC. (O) Adult C57BL/6 mice were treated as described in panels E-H and then injected with BrdU 48 and 24 hours before killing. BrdU⁺ cells among FSC^highSSC^highCD45⁻I-A^b+ cells present in freshly isolated thymic stromal cells (days 3 and 6) were analyzed (x-fold change KGF vs HBSS, whereby frequencies [in %] in HBSS-treated mice were set as 1.0 [dashed lines]). Mean ± SD; *P < .05 versus HBSS controls, with 6 mice per group and time point. A total of 3 experiments were performed, providing comparable results. (P) Absolute TEC numbers on days 6 and 15 following treatment of mice with either KGF (⊡) or HBSS (▪). Total CD45⁻I-A^{b int+high} stromal cells were counted by flow cytometry. Relative frequencies of total TECs are given, whereby the values for HBSS-treated mice were set as 1.0. The fractions of MTS24⁺ and MTS24⁻ cells (separated by horizontal lines) among total TECs and their absolute cell numbers are also indicated. Mean ± SD; *P < .01, total TEC numbers and #P < .02, total MTS24⁺ cells, respectively, in KGF-treated mice versus those in HBSS controls. A total of 12 individual mice were analyzed.