Effects of cessation of immunosuppression on skeleton reconstructed by vascularized bone allograft in rats

In the present study, we investigated the effects of cessation of immunosuppression on skeleton reconstructed by vascularized allogenic bone transplantation in a rat tibio-fibula graft model. Twelve-week-old male 25 Dark Agouti rats with the major histocompatibility antigen (MHC) RT1a were used as donors and age-matched male 25 Lewis rats with MHC RT1l were used as recipients. Among them, 20 rats were randomly allocated to 8-week cyclosporine A (CsA) followed by 8-week CsA vehicle group or continuous 16-week CsA group. The remaining 5 rats received CsA for 8 weeks followed by no further treatment for next 40 weeks (long-term observation group). In the CsA followed by vehicle group as well as the continuous CsA group, the structure of the reconstructed bones was maintained, though the transplanted bones in former group were found to be partly non-vital. The CsA followed by vehicle group had higher bone mineral density of the transplanted bones and stronger strength of the reconstructed bones than the continuous CsA group. In the long-term observation group, the structure of the reconstructed bones was still maintained and the transplanted bones were almost vital. These results suggest that long-term strong immunosuppression may not be necessary for successful reconstruction of large bone defect by vascularized bone allograft.