mKITL expressed by ECs is required for sustaining systemic sKITL levels but not BM HSCs in adolescent mice. (A) qPCR analysis of Kitl messenger RNA (mRNA) levels using Kitl exon 2-3–spanning (measuring total Kitl mRNA) and exon 7-8–spanning (measuring mRNA generating mKITL) amplicons on FACS-purified BM ECs (CD45^–TER119^–CD31⁺) from 4- to 6-week-old Tie2ΔEx7 (n = 6) and wild-type littermate control mice (n = 13). Mean (± SEM) expression relative to Hprt is shown. (B) sKITL levels measured by ELISA in the blood serum from 4-week-old wild-type (control; n = 17) and Tie2ΔEx7 (n = 11) mice. The parametric Student t test was used for statistical analysis. (C) sKITL levels measured by ELISA in the BM extracellular fluid from 4-week-old wild-type (control; n = 10) and Tie2ΔEx7 (n = 4) mice. (D-F) Mean (± SEM) BM cellularity (D), number of committed myelo-erythroid progenitor cells (E) and LSK CD150⁺CD48^– HSCs (F) from 2 femurs and 2 tibias from 4- to 5-week-old wild-type (control; n = 11) and Tie2ΔEx7 (n = 9) mice. No significant (P < .05) differences detected by either a parametric or nonparametric t test. (G-H) Percent CD45.2 contribution to total WBCs, MAC1⁺GR1⁺ myeloid cells, CD19⁺ B cells, and CD4/CD8a⁺ T cells in the blood at 4 to 6 and 16 to 19 weeks (G), and BM LSK CD150⁺CD48^– HSCs at 16 to 19 weeks (H) after transplantation of BM from 5-week-old wild-type (control; n = 4) and Tie2ΔEx7 (n = 5) CD45.2 mice, in competition with wild-type CD45.1 BM cells. Unless otherwise specified, all data represent mean ± SEM, the nonparametric Mann-Whitney test was used to assess statistical significance. ∗P < .05; ∗∗P < .01; ∗∗∗P < .001.