Effectiveness of intrathymic inoculation of soluble antigens in the induction of specific unresponsiveness to rat islet allografts without transient recipient immunosuppression.

Our finding that intrathymic inoculation of resting T cells but not dendritic cells induces donor-specific unresponsiveness to organ allografts led us to hypothesize that presentation of MHC class I alloantigens by thymic antigen-presenting cells to T cell precursors during their ontogeny may convey a tolerogenic signal to the recipient. In this study, we examined whether intrathymic inoculation of soluble antigen obtained from 3M KCl extracts of allogeneic T cells could induce donor-specific unresponsiveness to islet allografts in the Lewis-to-WF rat combination. Our results showed that while intrathymic injection of 0.5 mg soluble antigen on day -7 relative to islet transplantation caused acute graft rejection, intrathymic inoculation of 1.0 mg soluble antigen significantly prolonged the survival of Lewis islet allografts from 10.3 +/- 1.1 days in controls to 53.5 +/- 15.6 days (P < 0.001) in naive (nonimmunosuppressed) STZ (streptozotocin)-induced diabetic WF recipients. In contrast, intrathymic inoculation of 2.0 or 4.0 mg soluble Ag on day -7 led to indefinite Lewis islet survival (> 150 days) in all naive diabetic WF recipients; a finding that suggests that 2.0 mg soluble Ag is the optimal effective dose of intrathymic inoculum required to induce donor-specific unresponsiveness in naive recipients in this model. This finding could not be reproduced by intravenous injection of 2.0 mg soluble Ag, thus confirming the privileged position of the thymus in the induction of Ag-specific unresponsiveness. Third-party (BN) islet allografts were rejected in an acute fashion in similarly prepared recipients. Our results suggest that (1) intrathymic inoculation of soluble Ag, unlike cellular Ag, induces donor-specific unresponsiveness to islet allografts without the use of transient recipient immunosuppression; (2) induction of specific unresponsiveness appears to be dose dependent; and (3) the tolerogenic effect of soluble Ag is dependent on the indirect pathway of Ag-presentation in the thymus in the absence of donor antigen-presenting cells in the inoculum. This novel approach of thymic reeducation of adult animals by the deliberate intrathymic inoculation of soluble major histocompatibility complex Ag without the use of recipient immunosuppression may have therapeutic potential in the induction of transplantation tolerance.