Effects of sublethal irradiation on patterns of engraftment after murine bone marrow transplantation

Attempts to reduce the toxicity of hematopoietic stem cell transplantation have led to the use of various immunosuppressive, yet nonmyeloablative preparative regimens that often include low-dose irradiation. To determine the effects of low-dose irradiation on the dynamics of donor cell engraftment after bone marrow transplantation (BMT), we coupled standard endpoint flow cytometric analysis with in vivo longitudinal bioluminescence imaging performed throughout the early (<10 days) and late (days 10-90) post-BMT periods. To exclude the contribution of irradiation on reducing immunologic rejection, severely immune-deficient mice were chosen as recipients of allogeneic bone marrow. Flow cytometric analysis showed that sublethal doses of total body irradiation (TBI) significantly increased long-term (14 weeks) donor chimerism in the bone marrow compared with nonirradiated recipients (P < .05). Bioluminescence imaging demonstrated that the effect of TBI (P < .001) on chimerism occurred only after the first 7 days post-BMT. Flow cytometric analysis on day 3 showed no increase in the number of donor cells in irradiated bone marrow, confirming that sublethal irradiation does not enhance marrow chimerism early after transplantation. Local irradiation also significantly increased late (but not early) donor chimerism in the irradiated limb. Intrafemoral injection of donor cells provided efficient early chimerism in the injected limb, but long-term systemic donor chimerism was highest with i.v. administration (P < .05). Overall, the combination of TBI and i.v. administration of donor cells provided the highest levels of long-term donor chimerism in the marrow space. These findings suggest that the major effect of sublethal irradiation is to enhance long-term donor chimerism by inducing proliferative signals after the initial phase of homing.