Background. We reported that tolerance to skin grafts can be achieved by chimerism induction by way of nonlethal conditioning. In the present study, we evaluated the outcome of islet allografts implanted either simultaneously or after donor bone marrow cell (BMC) infusion when nonlethal conditioning was used. Methods and Results. B10 (H-2b) mice were conditioned with antilymphocyte serum (ALS), 100 cGy total body irradiation (TBI), and given 30x106 allogeneic (B10.BR, H-2k) BMC on day 0. On day 2, cyclophosphamide was given intraperitoneally (IP), followed by a second BMC infusion on day 3. After chimeras were typed for allogeneic BMC engraftment on day 28, animals were rendered diabetic chemically and transplanted under the kidney capsule with islet allografts genetically matched or disparate to the BM. Donor-specific islet grafts were accepted (median survival time [MST]> 180 days, n=6), whereas all major histocompatibility complex (MHC)-disparate third-party BALB/c (H-2d) islet grafts were rejected (MST=13.8 days, n=4). When B10.BR BMC and islets were given simultaneously, graft acceptance (MST>140 days, n=4) was observed. Surprisingly, when MHC-disparate third-party islets (BALB/c) were given together with B10.BR BMC, long-term survival was also observed (MST > 100 days, n = 3). These findings suggested that conditioning alone at the time of islet implant might induce long-term engraftment without further treatment. However, only chimeric animals accepted a second-set donor-specific graft, whereas all other groups rejected it. Conclusion. Our data indicates that stable allogeneic chimerism and islet indefinite survival can be achieved by the use of a nonmyeloablative protocol. The results of the conditioning-only experiments are consistent with the possibility of graft accommodation.
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