Emergency hematopoiesis proceeds without contribution of hematopoietic stem cells

Summary Hematopoietic stem cells (HSCs) are the ultimate source of blood and immune cells. Under homeostatic conditions, these cells are considered a quiescent reserve population. However, it is not clear to what extent HSCs participate in emergency responses. Herein, we use fate mapping and proliferation tracking mouse models, which cumulatively record HSC activity in situ . We observed no direct contribution of HSCs to mature blood cell regeneration in response to common hematopoietic emergencies, including inflammation or blood loss. Innate immune training, in which HSCs were proposed to store and integrate information on previous infections, did not alter HSC activity upon secondary exposure. Only severe myeloablation resulted in a robust increase of HSC contribution. Our data demonstrates that HSCs do not directly participate in the regeneration of mature blood cells and therefore do not represent a reserve population to compensate for physiological hematopoietic perturbations.