To condition or not to condition-That is the question: The evolution of nonmyeloablative conditions for transplantation

New Insights on the Role of the Mesenchymal-Hematopoietic Stem Cell Axis in Autologous and Allogeneic Hematopoiesis

Cytoreductive protocols are integral both as conditioning regimens for bone marrow (BM) transplantation and as part of therapies for malignancies, but their associated comorbidities represent a long-standing clinical problem. In particular, they cause myeloablation that debilitates the physiological role of mesenchymal stem and precursor cells (MSPCs) in sustaining hematopoiesis. This review addresses the damaging impact of cytoreductive regimens on MSPCs. In addition, it discusses prospects for alleviating the resulting iatrogenic comorbidities. New insights into the structural and functional dynamics of hematopoietic stem cell (HSC) niches reveal the existence of "empty" niches and the ability of the donor-derived healthy HSCs to outcompete the defective HSCs in occupying these niches. These findings support the notion that conditioning regimens, conventionally used to ablate the recipient hematopoiesis to create space for engraftment of the donor-derived HSCs, may not be a necessity for allogeneic BM transplantation. In addition, the capacity of the MSPCs to cross-talk with HSCs, despite major histocompatibility complex disparity, and suppress graft versus host disease indicates the possibility for development of a conditioning-free, MSPCs-enhanced protocol for BM transplantation. The clinical advantage of supplementing cytoreductive protocols with MSPCs to improve autologous hematopoiesis reconstitution and alleviate cytopenia associated with chemo and radiation therapies for cancer is also discussed.