Indirect allorecognition in acquired thymic tolerance - Induction of donor-specific permanent acceptance of rat islets by adoptive transfer of allopeptide-pulsed host myeloid and thymic dendritic cells

Pancreatic islet transplantation remains a promising approach to the treatm ent of type 1 diabetes, Unfortunately,,graft failure continues to occur bec ause of immunologic rejection, despite the use of potent immunosuppressive agents. It is therefore reasoned that induction of peripheral tolerance by the use of self-dendritic cells (DCs) as a vehicle to deliver specific targ et antigens to self-T-cells without ex vivo manipulation of the recipient i s an attractive strategy in the treatment of type 1 diabetes, The finding t hat intrathymic inoculation of an immunodominant WF major histocompatibilit y complex (MHC) Class I (RT1.A(u)) peptide five (P5) or P5-pulsed host myel oid DCs induces acquired thymic tolerance raises the possibility that adopt ive transfer of allopeptide-primed host myeloid or lymphoid DCs might induc e transplant tolerance. To address this hypothesis, we studied the effects of intravenous transfer of in vitro P5-pulsed syngeneic myeloid DCs or in v ivo PB-primed syngeneic lymphoid (thymic) DCs on islet survival in the WF-t o-ACI rat combination, In vivo primed thymic DCs isolated from ACI rats giv en intrathymic inoculation of P5 for 2 days were capable of in vitro restim ulation of in vivo P5-primed T-cells (memory cells). In the first series of studies, we showed that intravenous-like intrathymic-inoculation of in vit ro P5-pulsed host myeloid DCs induced donor-specific permanent acceptance o f islets in recipients transiently immunosuppressed with antilymphocyte ser um (ALS), We next examined whether thymic DCs isolated from animals that ha d been previously intrathymically inoculated with P5 could induce T-cell to lerance. The results showed: that intravenous adoptive transfer of in vivo PB-primed thymic DCs led to donor-specific permanent acceptance of islets i n recipients transiently immunosuppressed with ALS. This finding suggested that the thymic DCs take up and present P5 to developing T-cells to induce T-cell tolerance, thus providing evidence of a direct link between indirect allorecognition and acquired thymic tolerance. The second series of studie s examined the mechanisms involved in this model by exploring whether in vi vo generation of peptide-specific alloreactive peripheral T-cells by intrav enous inoculation of P5-pulsed self-DCs was responsible for the induction o f T-cell tolerance, Intrathymic inoculation of splenic T-cells obtained fro m syngeneic ACI rats primed with intravenous injection of P5-pulsed DCs wit h a high in vitro proliferative response to P5 in the context of self-MHC i nduced donor-specific permanent acceptance of islets from WF donors. In add ition, the clinically relevant model of intravenous injection of P5-activat ed T-cells combined with transient ALS immunosuppression similarly induced transplant tolerance, which was then abrogated by thymectomy of the recipie nt before intravenous injection of the activated T-cells, These? data raise the possibility that circulation of peptide-activated T-cells to the host thymus plays a role in the induction and possibly the maintenance of T-cell tolerance in this model, Our findings suggest that intravenous administrat ion of genetically engineered host DCs expressing alloMHC peptides might ha ve therapeutic potential in clinical islet transplantation for the treatmen t of autoimmune diabetes.