Bendamustine with Total Body Irradiation Limits Murine Graft-versus-Host Disease in Part Through Effects on Myeloid-Derived Suppressor Cells

Graft-versus-host disease (GVHD) remains a significant challenge in allogeneic hematopoietic cell transplantation (HCT). An underinvestigated strategy to reduce GVHD is the modification of the preparative conditioning regimen. In the present study, we aimed to evaluate GVHD associated with bendamustine (BEN) conditioning in conjunction with total body irradiation (TBI) as an alternative to the standard myeloablative regimen of cyclophosphamide (CY) and TBI. We demonstrate that BEN-TBI conditioning, although facilitating complete donor chimerism, results in significantly less GVHD compared with CY-TBI. In BEN-TBI-conditioned mice, suppressive CD11b⁺Gr-1^high myeloid cells are increased in the blood, bone marrow, spleen, and intestines. When Gr-1^high cells are depleted before transplantation, the beneficial effects of BEN-TBI are partially lost. Alternatively, administration of granulocyte colony-stimulating factor, which promotes CD11b⁺Gr-1⁺ myeloid cell expansion, is associated with a trend toward increased survival in BEN-TBI-conditioned mice. These findings indicate a potential role of myeloid-derived suppressor cells in the mechanism by which BEN allows engraftment with reduced GVHD. BEN-TBI conditioning may present a safer alternative to CY-TBI conditioning for allogeneic HCT.