ITP progression increases BM CXCL12 expression and is associated with increased progenitor homing. (Ai) BM Cxcl12 expression (n = 4-6 mice; data are representative of 1-2 independent experiments) (control vs 2-week ITP P > .99, control vs 4-week ITP *P = .021, control vs 4-week ITP recovered P > .99, 2-week ITP vs 4-week ITP P = .14, 4-week ITP vs 4-week ITP recovered P = .43). P values were calculated by a Kruskal-Wallis test with Dunn’s multiple comparison test. (Aii) Spleen Cxcl12 expression (n = 6 mice; data are representative of 2 independent experiments; P = .82). P value was calculated by using a Mann-Whitney U test. (B) Representative confocal images of diaphysis BM from femurs of Cxcl12^DsRed/+ mice with sustained ITP vs controls. (C) CXCL12 enzyme-linked immunosorbent assay using (i) BM supernatant (n = 7 mice; **P = .0023), (ii) spleen homogenate (n = 7 mice; P = .38), and (iii) plasma (n = 3-4 mice; P = .23). Data are representative of 2 independent experiments. P values were calculated by using Mann-Whitney U tests. (D) Experimental outline for panel E. (E) Homing of naïve progenitors to the BM of mice with 4-week ITP vs controls (n = 8-9 mice; P = .004). Data are representative of 2 independent experiments. P value was calculated by using a Mann-Whitney U test. All data are mean ± SD.