INDUCTION OF PERIPHERAL TOLERANCE BY INTRATHYMIC INOCULATION OF SOLUBLE ALLOANTIGENS - EVIDENCE FOR THE ROLE OF HOST ANTIGEN-PRESENTING CELLS AND SUPPRESSOR-CELL MECHANISM

Intrathymic (IT) inoculation of soluble alloantigens (Ag) obtained fro m 3 M KCl extracts of resting T-cells induces donor-specific tolerance to cardiac allografts and islet allografts. This study examined the c ellular basis of induction of transplantation tolerance by IT injectio n of soluble Ag. Our results show that while IT inoculation of 2 mg so luble donor Ag on Day -7 relative to Lewis islet transplantation induc ed specific unresponsiveness to islet allografts (>200 days) in naive diabetic recipients, IT inoculation of 2 mg soluble Ag on the same day as islet transplantation did not prolong islet allograft survival in the same Lewis-to-WF rat combination. To define the role of donor APCs in intrathymic tolerance, we showed that IT injection of an admixture of 1 x 10(4) donor DC and 2 mg soluble Ag caused acute islet graft re jection. In contrast, addition of 1 x 10(4) recipient-type DC to the I T inoculum did not prevent long-term graft survival. This finding sugg ests that while the presence of donor APCs in the inoculum does not ap pear neccessary for IT-alloantigen to induce peripheral tolerance, pre sentation of the soluble Ag in the thymus is dependent on host APCs. T his conclusion is supported by our in vitro MLR experiments which show ed that in vivo WF-Ag-primed Lewis T-cells proliferated specifically t o WF-soluble Ag and that the response was enhanced 14-fold by the addi tion of responder-type DC. Addition of anti-lewis MHC class II mAb spe cifically blocked the alloresponse, thus suggesting that in vivo Ag-pr imed T-cells are capable of recognizing and proliferating in response to allopeptides presented by responder APCs. We also showed that adopt ive transfer of syngeneic naive T-cells into unresponsive recipients f ailed to break tolerance to long-term surviving islet allografts. This finding suggests that tolerance in this model is not due to a lack of T help. On the other hand, the adoptive transfer of spleen cells, but not sera, from the unresponsive WF recipients bearing long-term (>120 days) functioning Lewis islets resulted in prolonged survival of dono r-type but not third-party islet allografts in secondary syngeneic hos ts. Our data suggest that the tolerogenic effect of IT inoculation of soluble Ag is dependent on the indirect pathway of Ag presentation and clonal deletion of alloreactive T-cells in the thymus, while suppress or/regulatory mechanism may be involved in the maintenance of peripher al tolerance. (C) 1995 Academic Press, Inc.