DELETION OF ALLOANTIGEN-REACTIVE THYMOCYTES AS A MECHANISM OF ADULT TOLERANCE INDUCTION FOLLOWING INTRATHYMIC ANTIGEN ADMINISTRATION

Direct injection of foreign antigen into the adult thymus is a potent route of antigen delivery for the induction of tolerance in vivo. In t his report, we demonstrate that tolerance to C57BL/10 (H2(b)/BL10) all oantigens can be induced in CBA/Ca (H2(k)/CBA) mice by intrathymic (IT ) administration of BL10 spleen leukocytes coincident with transient p eripheral immunomodulation of CD4(+) T cells using a depleting anti-CD 4 monoclonal antibody. T cell receptor (TCR) transgenic mice (BM3.6; H 2(k)) expressing a CD8-independent TCR specific for H2K(b) were used a s recipients to facilitate investigation of the mechanisms responsible for tolerance induction by allowing visualization of events in the th ymus following IT injection. IT administration of 5 x 10(7) BL10 splee n leukocytes and concomitant transient peripheral T cell. depletion in BM3.6 mice resulted in a substantial H2K(b)-specific deletion of tran sgenic-TCR+ (tg-TCR) thymocytes which was dependent on the level of tg -TCR expression. IT deletion and the failure to export CD8(+) T cells to the peripheral lymphoid organs correlated with the induction of tol erance to H2K(b); TCR transgenic mice that had received IT injection o f BL10 splenocytes and peripheral T cell depletion accepted a H2K(b+) cardiac allograft indefinitely. Analysis of tolerant BM3.6 mice reveal ed that there were low numbers of CD8(+) T cells in the periphery givi ng rise to a substantially reduced reactivity in vitro despite the fac t that no donor cells or IT deletion were observed in the thymi of the majority of tolerant mice. These results demonstrate for the first ti me that IT injection of foreign alloantigen into an adult thymus resul ts in the deletion of thymocytes expressing a TCR specific for the inj ected alloantigen and suggest that this is an important mechanism of t olerance induction following IT injection of alloantigen in vivo. Furt hermore, analysis of tolerant TCR-transgenic mice suggests that IT del etion is not required for the maintenance of tolerance, and that perip heral mechanisms enforce continued hyporesponsiveness to H2K(b) follow ing transplantation.