Development and maintenance of donor-specific chimerism in semi-allogenic and fully major histocompatibility complex mismatched facial allograft transplants

Background. The clinical application of composite tissue allograft transplants opened the discussion on the restoration of facial deformities by allotransplantation. We introduce a hemifacial allograft transplant model to investigate the rationale for the development of operational tolerance across a major histocompatibility complex (MHC) barrier. Material and Methods. Thirty rats were studied in five groups of six animals each. The composite hemiface isograft transplantations were performed in group 1. Allograft rejection controls included semi-allogenic transplantations from LBN (RT1(1+n)) donors (group 2) and fully allogenic transplantations from ACI (RTIa) donors (group 3) to LEW (RT1(l)) recipients. In the allograft treatment groups, recipients of LBN (group 4) and ACI donors (group 5) were treated with cyclosporine A monotherapy (16 mg/kg/day, tapered to 2 mg/kg/day). Face allografts were evaluated clinically and histologically. Donor-specific chimerism for MHC class I RT1(n) and RTV antigens was assessed by flow cytometry. Mixed lymphocyte reaction for donor-specific tolerance in vitro was tested at day 160 posttransplant. Results. Isograft controls survived indefinitely. All nontreated allografts rejected within 5 to 8 days posttransplant. Long-term survival was achieved in 100% of LBN (up to 400 days) and ACI (up to 330 days) recipients. At day 160, posttransplant donor-specific chimerism was present in recipients of LBN (10.14% CD4/RT1(n), 6.38% CD8/RT1(n), 10.02% CD45RA/RT1(n)) and ACI (17.54% CD4/RT1(a), 9.28% CD8/RT1(a)) transplants, and mixed lymphocyte reaction confirmed tolerance in recipients of LBN transplants and moderate reactivity in recipients of ACI allografts. Conclusion. Operational tolerance was induced in hemiface allograft transplants across an MHC barrier under cyclosporine A monotherapy protocol. It was associated directly with the presence of multilineage donor-specific chimerism.