Académie royale de Médecine de Belgique


Pierre Gianello

(Séance du 28 octobre 2000)


par P. GIANELLO (Laboratoire de Chirurgie expérimentale – UCL).

The impact of the « Major Histocompatibility Complex » (MHC) on indefinite and specific tolerance to primarily vascularized allograft, is difficult to assess in outbred large animal models.  In order to precisely study the mechanisms of tolerance induction and maintenance in large animals, the use of inbred models is recommended.  The MGH inbred miniature swine model (D.H. Sachs, Boston, USA) seems, therefore, particularity appropriate to allows such studies.  Central or peripheral tolerance might be induced in order to obtain indefinite and specific survival of primarily vascularized allograft.  This lecture will, however, mainly focuse on tolerance induction to primarly vascularized allograft, without modulation on the central immune system of the recipient (thymus and bone marrow) and, by using only moderate and clinical relevant immunosuppression i.e. transient course of immunosuppressive drugs.

In a renal allograft model, a twelve-day course of cyclosporine A (CsA) at 10mg/kg has been shown to uniformly induce tolerance across a two haplotype class I disparity; in 78 % across a two haplotype class II mismatch, and in 67 % across a one haplotype class I plus class II mismatch in the MGH inbred pig model.  On the other hand, the CSA regimen was unable to induce tolerance to two haplotype fully mismatched renal allografts.  In long terme acceptor animals, the tolerant state was stable and mismatched renal allografts could be usually maintained with a normal renal function up to three years.  These results demonstrated that a transient course of CSA is able to induce specific, indefinite and stable long term tolerance to primarily vascularized renal allografts in miniature swine across several MHC mismatches, but never across a two haplotype full mismatch.  In order to induce tolerance across a full mismatch barrier, immunosuppressive protocols including establishment of mixed chimerism seems necessary.

Since a twelve-day course of CSA induced specific and indefinite tolerance to primarily vascularized class I mismatched, class II matched renal allografts in 100 % of the animals, it seemed interesting to study the mechanisms to tolerance involved.  Overall, in the two haplotype class I mismatched, class II matched model, induction of tolerance was dependent on (1) a lack of cytokine production at the time of the first antigen exposure to the host, (2) the development of a TH2 environment in the graft itself, and finally, (3) on the presence of intact thymic tissue.  During the maintenance phase, however, additional mechanism such as supression might be involved.  During maintenance phase, tolerance was stable, systemic, did not require graft adaptation, was not abrogated by sources of T-cell help and did not require an intact thymus.  All of these result suggested that tolerance was induced and maintained through consecutive and different mechanisms which occurred during the follow-up.

Finally, in order to specifically study a clinical situation which seems primordial in pediatric transplantation, we developed a semi-identical orthotopic liver  transplantation (OLT) model in order to study parent into infant living related liver transplantation.  In order to prolong graft survival in this model, a transient course of FK506 (twelve days) has been used and SLAdd pigs were used a recipients and SLAcd pigs as donors.  Without treatment, all the liver recipients rejected OLT within the first postoperative month, whereas FK-treated animals developed a specific and indefinite unresponsiveness to donor antigens and maintained a normal liver allograft function.  After more than one year, long term acceptors were challenged (three times) by sub-cutaneous injection of donor antigens (109 PBL) and demonstrated the maintenance of OLT, a normal hepatic function and signs of specific immune unresponsiveness, thereby allowing to affirm that tolerance was induced.  Microchimerism was detectable in the peripheral blood of some survivors up to ten weeks after OLT, but no microchimerism was detected in lymphoid tissues such as thymus, lymph nodes or in skin.

In conclusion, across a semi-identical SLA barrier, spontaneous liver graft tolerance did not occur in miniature swine.  A transient course of FK506 (0,05 mg/kg im. twice a day), however, allowed tolerance to OLT along with a specific cellular and humoral anti-donor unresponsiveness.  Microchimerism did not appear essential for maintenance of the liver allograft.  In this semi-identical model, the characterization on the early immune response to donor antigens has been carried out in order to study early biological signs of tolerance.