ADULT VASCULARIZED THYMUS TRANSPLANTATION RESTORES HOST IMMUNOCOMPETENCE

Abstract

504 Introduction: Intrathymic mechanisms are fundamental to the development of self-tolerance and may be responsible for the tolerance seen in mixed chimeras. Transplantation of a chimeric thymus would enable the study of intrathymic events by isolating the thymus from the effects of peripheral alloantigen in mixed radiation and neonatal chimeras. However traditional methods of nonvascularized thymus transplantation are limited as the grafts undergo ischemia and necrosis and exhibit destruction of the thymic architecture. To circumvent these drawbacks, we developed a method of adult vascularized thymus transplantation (VTTx). Prior to chimeric VTTx the ability to restore host immunocompetence with a vascularized syngeneic thymus needs to be established. Methods: Thymectomized Lewis rats underwent lethal irradiation and reconstitution with T-cell depleted syngeneic bone marrow prior to VTTx. One group received a syngeneic VTTx (N=5), while in a second group the syngeneic thymus donors received high dose hydrocortisone (N=5) to eliminate the transfer of intrathymic T-cells to the recipient. A third group did not receive a VTTx (N=9), but had a sham operation. Thirty days after VTTxin vitro immunocompetence was measured by mixed lymphocyte reaction(MLR), while in vivo immunocompetence was assessed with an ACI heterotopic allograft and delayed type hypersensitivity response to keyhole limpet hemocyanin (KLH). Reconstitution of α/β T-cells, CD4, and CD8 populations was documented by flow cytometry. Results: Animals receiving a nontreated or hydrocortisone pretreated VTTx rejected allogeneic hearts within a normal time course, had reconstitution of T-cells to normal levels, and demonstrated significant T-cell responses to alloantigen in MLR and KLH (p<0.006 vs athymic group) assays. Athymic animals had extended(4/9 > 140 days) survival of their grafts and diminished functional T-cell responses. Normal thymic architecture was observed in biopsies from the thymus transplants. TableConclusion: Adult VTTx restores recipient T-cell function and phenotype providing a novel method for studying thymic function. We believe the VTTx will be an important tool in understanding intrathymic events associated with the tolerant state in mixed chimeras.