Mechanisms of mobilization of hematopoietic stem cells

Haematopoietic stem and progenitor cells circulate in the blood both under physiological and pathological circumstances. The evidence that several pharmacological agents can mobilize stem and progenitor cells in the blood suggests that mobilization represents a transient physiological reaction to haematopoietic stress. The circulation of haematopoietic stem and progenitor cells might be related either to their differentiative fate or the regulation of the self-renewal versus differentiation balance. Haematopoietic cell mobilization can be obtained with several techniques, including: (i) chemotherapy agents, (ii) growth factors, (iii) chemotherapy agents + growth factors. Different mobilization regimens allow the collection of different quantities of CD34+ cells, committed (CFU-Mix, BFU-E, CFU-GM) and primitive progenitors (LTC-IC). The biomolecular mechanisms of progenitor cell adhesion and mobilization are still unclear. Adhesion molecules (eg., VLA4/VCAM-1) and/or receptor-ligand interactions (eg., kit-receptor/kit-ligand) might play a role in the mobilization of haematopoietic cells. Although several aspects remain to be investigated, the availability of large amounts of haematopoietic cells for stem cell transplantation has dramatically changed both autologous and allogeneic transplantation procedures and allows the investigation of new cell therapy approaches.