Most clinical allogeneic hemopoietic cell transplants (alloHCT) are now performed using reduced-intensity conditioning (RIC) instead of myeloablative conditioning (MAC); however, the biology underlying this treatment remains incompletely understood.Methods
We investigated a murine model of major histocompatibility complex–matched multiple minor histocompatibility antigen–mismatched alloHCT using bone marrow (BM) cells and splenocytes from B6 (H-2b) donor mice transplanted into BALB.B (H-2b) recipients after RIC with fludarabine of 100 mg/kg per day for 5 days, cyclophosphamide of 60 mg/kg per day for 2 days, and total body irradiation (TBI).Results
The lowest TBI dose capable of achieving complete donor chimerism in this mouse strain combination was 325 cGy given as a single fraction. Mice that underwent RIC had a reduced incidence and delayed onset of graft-versus-host disease (GVHD) and significantly prolonged survival compared with MAC-transplanted recipients (TBI of 850 cGy plus cyclophosphamide of 60 mg/kg per day for 2 days). Compared with syngeneic controls, RIC mice with GVHD showed evidence of BM suppression, have anemia, reduced BM cellularity, and showed profound reduction in BM B cell lymphopoiesis associated with damage to the endosteal BM niche. This was associated with an increase in BM CD8+ effector T cells in RIC mice and elevated blood and BM plasma levels of T helper1 cytokines. Increasing doses of splenocytes resulted in increased incidence of GVHD in RIC mice.Conclusions
We demonstrate that the BM is a major target organ of GVHD in an informative clinically relevant RIC mouse major histocompatibility complex–matched alloHCT model by a process that seems to be driven by CD8+ effector T cells.