Sustained ITP drives progenitor expansion and LT-HSC proliferation. (A) Total cell counts from flushed BM (n = 7; data are representative of 2 independent experiments; Mann-Whitney U test P = .46). (B) Flow cytometry analysis of hematopoietic progenitor populations (n = 13; data are representative of 3 independent experiments): (i) Lin^–c-Kit⁺ (LK) *P = .034, (ii) Lin^–Sca^–1⁺c-Kit⁺ (LSK) **P = .0055, (iii) LSK CD48⁺CD150^–P = .068, (iv) LSK CD48^–CD150^– *P = .019, (v) LSK CD48^–CD150⁺ ***P = .0005. P values were calculated by Mann-Whitney U tests. (C) Experimental outline for C-E. (D) Cumulative time taken for a single LT-HSC to complete first (i), second (ii), and third (iii) division (n = 4-5 mice; data are representative of 2 independent experiments). P values were calculated by a two-way ANOVA with Sidak’s multiple comparison test. (E) Colony size after 10 days in culture (n = 3-4 mice; data are representative of 2 independent experiments). *P = .020, calculated by a two-way ANOVA. Colonies were categorized as very small (VS; <50), small (S; 51-500), medium (M; 501-10 000), or large (L; >10000). (F) After 10 days in culture, colonies were analyzed by flow cytometry for the expression of lineage and LSK markers (n = 143 clones from controls and n = 208 clones from mice with sustained ITP; n = 3 mice; data are representative of 2 independent experiments). LSK numbers (i, **P = .006), LSK frequency (ii, P = .92), and Lin⁺ cell frequency (iii, P = .55). P values were calculated by Mann-Whitney U tests. All data are shown as mean ± SD except in panel E, which shows mean ± standard error of the mean (SEM). MPP, multipotent progenitors; ST-HSC, short-term LT-HSC.