EFFECTIVENESS OF INTRATHYMIC INOCULATION OF SOLUBLE-ANTIGENS IN THE INDUCTION OF SPECIFIC UNRESPONSIVENESS TO RAT ISLET ALLOGRAFTS WITHOUT TRANSIENT ALLOGRAFTS WITHOUT TRANSIENT RECIPIENT IMMUNOSUPPRESSION

Our finding that intrathymic inoculation of resting T cells but not de ndritic cells induces donor-specific unresponsiveness to organ allogra fts led us to hypothesize that presentation of MHC class I alloantigen s by thymic antigen-presenting cells to T cell precursors during their ontogeny may convey a tolerogenic signal to the recipient. In this st udy, we examined whether intrathymic inoculation of soluble antigen ob tained from 3M KCl extracts of allogeneic T cells could induce donor-s pecific unresponsiveness to islet allografts in the Lewis-to-WF rat co mbination. 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 antig en 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 (n onimmunosuppressed) STZ (streptozotocin)-induced diabetic WF recipient s. 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 naiv e diabetic WF recipients; a finding that suggests that 2.0 mg soluble Ag is the optimal effective dose of intrathymic inoculum required to i nduce donor-specific unresponsiveness in naive recipients in this mode l. This finding could not be reproduced by intravenous injection of 2. 0 mg soluble Aff, thus confirming the privileged position of the thymu s in the induction of Ag-specific unresponsiveness. Third-party (BN) i slet allografts were rejected in an acute fashion in similarly prepare d recipients. Our results suggest that (1) intrathymic inoculation of soluble Ag, unlike cellular Ag, induces donor-specific unresponsivenes s to islet allografts without the use of transient recipient immunosup pression; (2) induction of specific unresponsiveness appears to be dos e dependent; and (3) the tolerogenic effect of soluble Ag is dependent on the indirect pathway of Aff-presentation in the thymus in the abse nce of donor antigen-presenting cells in the inoculum. This novel appr oach of thymic reeducation of adult animals by the deliberate intrathy mic inoculation of soluble major histocompatibility complex Ag without the use of recipient immunosuppression may have therapeutic potential in the induction of transplantation tolerance.