Hematopoietic stem cell mobilization

R ecruitment of hematopoietic stem/progenitor cells (HSC) from the bone marrow into peripheral blood following treatment with chemotherapy and/or cytokines is termed mobilization.The release of HSC from the bone marrow is a physiological phenomenon for the protection of HSC from toxic injury, as circulating cells can re-engraft bone marrow, or to maintain a fixed number of HSC in the bone marrow (homeostatic mechanism).In fact, trafficking to blood is an important death pathway to regulate the steady-state number of HSC. 1 Bone marrow cells also enter peripheral blood in response to stress signals during injury and inflammation of hematopoietic and nonhematopoietic tissues.[2][3][4] Several randomized trials have demonstrated the clinical advantages of the reinfusion of autologous mobilized peripheral blood stem cells over bone marrow HSC.5,6 These advantages include shorter duration of granulocytopenia, enhanced immune reconstitution, reduced morbidity and mortality, shorter hospital stay and saving of financial resources.However, several factors affect HSC mobilization: age, type and dose of cytokines used and, in the autologous setting, the patient's diagnosis, mobilizing chemotherapy regimen, number and type of previous chemotherapy cycles or radiation, and interval from last chemotherapy cycle.Indeed, depending on these characteristics, a significant proportion of patients receiving standard mobilization for the purpose of autologous transplantation or allogeneic donors still fail to mobilize bone marrow cells into the periphery.7,8 Although an extensive description of factors influencing HSC mobilization 9 and current clinical trials involving transplantation of peripheral blood stem cells is beyond the scope of this perspective article, we report here some of the bio-molecular mechanisms leading to the release of HSC from the bone marrow to help the understanding of novel strategies under development to make stem cell collection more effective.