Background.

We have previously reported that delaying histoincompatible transplantation after total body irradiation (TBI) conditioning markedly decreased the mortality of acute graft-versus-host disease (GVHD) in severe combined immunodeficiency mice. However, it was not clear whether the delayed transplantation would affect the final engraftment and acute GVHD mortality in normal hosts.

Methods.

BALB/c mice (H2 d) were lethally irradiated with 8.5 Gy TBI and transplanted with C57BL/6 (H2 d) bone marrow plus spleen cells on the same day (TBI+ day 0) or 4 days after TBI conditioning (TBI+ day 4).

Results.

We again demonstrated that delaying transplantation by 4 days after TBI conditioning markedly reduced acute GVHD mortality in normal hosts after major histoincompatible transplantation. The survival rates were 66% in TBI+ day 4 vs. 0% in TBI+ day 0 allogeneic transplanted animals by day+ 60 (P< 0.001). Further analysis demonstrated that the 4-day rest between the TBI and allogeneic transplantation broke the interaction of cell/inflammatory tumor necrosis factor-α, interleukin (IL)-1β, and IL-6 cytokine reactions stimulated by TBI and incompatible transplantation. Flow cytometry revealed 97% donor cells in host marrow by 2 weeks in TBI+ day 0 transplantation versus 57% in TBI+ day 4 transplantation. There was no difference in percentage of donor CD3+ T-cell engraftment between the TBI+ day 0 and TBI+ day 4 allogeneic transplanted animals. In TBI+ day 4 transplantation, the percentage of donor cells in host marrow steadily increased to 74% by day+ 60 and 93% by day+ 100.

Conclusions.

This 2-to 3-month early mixed chimerism in TBI+ day 4 transplanted animals might be related to lower levels of tumor necrosis factor-α and IL-6 both of which have been shown to stimulate lymphohematopoiesis and was associated with lower acute GVHD mortality. The data again demonstrated in immunologically normal BALB/c mice that delaying allogeneic transplantation after TBI is a simple and effective way to reduce acute GVHD mortality, achieve satisfactory engraftment and significantly increase overall survival.