Peripheral blood stem cell grafts in allogeneic hematopoietic cell transplantation: It is not all about the CD34+cell dose

Allogeneic Hematopoietic Cell Transplantation is a curative approach in various malignant and non-malignant disorders. The majority of adult transplants in the current era are performed using mobilized stem cells, harvested from the peripheral blood by leukapheresis. Peripheral blood stem cell (PBSC) collections are designed to target a dose of stem cells that will result in safe engraftment and hematopoietic recovery; however, 99 % of the cells contained in a PBSC graft are not stem cells and a growing number of studies attempt to characterize the associations between graft composition and transplant outcomes. A better understanding of the impact of the quantity and quality of various cell types in PBSC grafts may lead to development of novel collection strategies or improved donor selection algorithms. Here we review relevant findings from recent studies in this area.