Macrophages retain splenic HSCs via VCAM-1. (A) Flow cytometric plots showing VCAM-1 expression on splenic endothelial cells and macrophages after VCAM-1 knockdown. Flow cytometric plots show percentage of LSKs, HSCs, and GMPs in the spleen (B) and blood (C) of mice treated with siRNA against VCAM-1 (siVCAM-1) or control siRNA (siCON). Levels of LSKs, HSCs, and GMPs in the spleen (B), blood (C), and bone marrow (D; n = 8–10). Myeloid cells and monocytes in the blood (E) and spleen (F) after siVCAM-1 treatment in LPS-challenged mice (n = 4). (G) Reduced HSPC retention in the spleen after VLA-4 neutralization (n = 4–5). Two independent experiments were performed. Data are mean ± SEM. Significance was determined by Mann-Whitney test. *, P < 0.05; **, P < 0.01.