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 thy- mus 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.