DONOR-SPECIFIC UNRESPONSIVENESS TO MURINE CARDIAC ALLOGRAFTS INDUCED BY INTRATHYMIC-SOLUBLE ALLOANTIGENS IS DEPENDENT ON ALTERNATE PATHWAY OF ANTIGEN PRESENTATION

This study extends the finding that intrathymic (IT) inoculation of uv -B irradiated donor spleen cells (SC) or soluble alloantigens (Ag) ind uces peripheral tolerance to organ allografts in the rat to the murine cardiac allograft model, In our initial experiment, we showed that IT inoculation of uv-B irradiated SC combined with transient immunosuppr ession of the recipient with either sublethal TBI or ALS on Day -7 led to donor-specific, long-term cardiac allograft survival (>300 days) i n the completely mismatched A/J-to-C57BL/6 mice combination. To test o ur hypothesis that peripheral tolerance induced by IT injection of don or Ag is dependent on presentation of the foreign MHC molecule by thym ic APC to T cell precursors, we examined the effect of IT injection of donor APC-free-soluble Ag inoculum obtained from 3 M KCl extracts of purified MHC class I resting T cells on cardiac allograft survival in the A/J-to-C3H mice combination. The results showed that IT injection of the optimal dose of 500 mu g soluble Ag combined with 0.5 ml ALS on Day -7 led to donor-specific permanent graft survival (>200 days). Th is finding could not be reproduced by intravenous administration of so luble Ag, thus confirming the privileged position of the thymus in tol erance induction. To further define the role of host APC in allorecogn ition, we studied the presentation of soluble Ag by responder APC in M LR. The results showed that primed T cells obtained from A/J skin graf t-sensitized C3H T cells specifically developed alloreactivity to A/J- soluble Ag in MLR. Addition of anti-Ia mAb blocked the proliferative r esponse, thus confirming that primed T cells can respond to soluble Ag presented by responder-type APC. This study suggests that induction o f peripheral tolerance by IT inoculation of soluble Ag is reproducible in the murine cardiac allograft model, where tolerance is dependent o n the Ag dose and the indirect pathway of Ag presentation. This approa ch of immunologic manipulation of the adult thymus may have clinical t herapeutic applications. (C) 1995 Academic Press, Inc.