We used an allograft pretreatment regimen involving lethal total-body gamma irradiation of donor rats eight days prior to transplantation to compare directly the immunogenicty of immediately-vascularized heart allografts with isolated, neovascularized pancreatic islet allografts. TBI pretreatment of heart donors (IHT) caused a modest prolongation in cardiac allograft survival compared with controls in a low-responding fully -allogeneic strain combination, Lewis-to-ACI. The addition of 10s donor-type dendritic cells (DCs) at the time of transplantation reversed the prolongation of IHT in this strain combination. Donor pretreatment with TBI did not lead to prolonged cardiac allograft survival in either a high-responding combination, ACI-to-Lewis, or in an MHC class H-compatible strain combination, F344-to-Lewis. In contrast, islets from irradiated donors (IIT) showed markedly prolonged survival in both low-responding (Lewis-to-ACI) and high-responding (W/F-to-Lewis) strain combinations. The addition of 105 donor-type DCs to the islets at the time of transplantation caused the rejection of only two of six IIT in the Lewis-to-ACI combination. Combined pretreatment regimens involving TBI-pretreated cardiac allografts— such as recipient immunosuppression with peritrans-plant cyclosporine (10 mg/kg on days 0, 1, and 2) or the combination of TBI (1000 cGy on day —3) and cyclophosphamide (300 mg/kg on day —5) in a pretreatment regimen—did not lead to synergistic graft prolongation in the low-responding Lewis-to-ACI combination. Im-munohistologic studies showed that eight days after TBI rat hearts and islet were both >90% depleted of class II (0X6+) interstitial dendritic cells while class I (0X18+) expression was unchanged. IIT are able to increase class II expression when donors are treated with recombinant gamma interferon (4000 U/day on days —3, —2, and —1). MLR data showed that ACI recipients of Lewis IIT reacted normally to donor (Lewis) and third-party (W/F) stimulator cells >100 days after transplantation. The apparent greater immunogenicity of heart allografts as compared with islets in this model could be due to (1) a quantitative difference between the number of residual class II—positive cells in hearts and islets or a greater non-MHC antigenic load in the heart grafts given its larger size, or (2) a qualitative difference between hearts and islets due to the presence of a vascular endothelium in the immediately vascularized heart that can present alloantigen and provide a major stimulus to rejection.
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