Purpose: Clinical application of composite tissue allograft transplants opened discussion on the restoration of facial deformities by allotransplantation. Here we introduce hemi-facial allograft transplant model to investigate rationale for development of functional tolerance across MHC barrier. Material and Methods: Thirty rats were studied in five groups of six animals each. The composite hemi-face isograft transplantations were performed in group 1. Allograft rejection controls included semi-allogenic transplantations from LBN (RT1l+n) donors to LEW (RT1l) recipients in group 2 and fully-allogenic transplantations from ACI (RT1a) donors in group 3. In allograft treatment groups, recipients of LBN (RT1l+n) donors in group 4 and of ACI (RT1a) donors in group 5 were treated with standard immunosuppressive protocol of CsA monotherapy at dose of 16 mg/kg/day, tapered to 2 mg/kg/day and maintained at this level thereafter. Signs of graft rejecting were checked on daily basis. Flow cytometry was used to evaluate donor specific chimerism for MHC class I - RT1n and RT1a specific antigens. The effect of immunosuppression of CD4+ and CD8+ T-cell repertoire behavior was evaluated at days 21, 63 and 160. Mixed lymphocyte reaction (MLR) for donor specific tolerance in vitro was tested at day 160 post-transplant. Following H+E staining histological grading of graft rejection was evaluated. Results: Isograft controls survived indefinitely. All non-treated allografts rejected within 5 to 8 days post-transplant. Long-term survival was achieved in 100% of LBN recipients and in 67% of ACI recipients. Five out of six face transplants (83%) from LBN donors and 2 out of 6 flaps (67%) from ACI donors did not show any signs of rejection at long-term follow-up 270 days and 200 days respectively. The rejection signs were reversed by CsA dose adjustments in remaining allografts. At day 160 donor specific chimerism was achieved in both LBN recipients (10.14% CD4/RT1n, 6.38% CD8/RT1n T-lymphocyte subpopulation and 10.02% CD45RA/RT1n B-lymphocyte, and in ACI recipients (17.54% CD4/RT1a and 9.28% CD8/RT1a). MLR assay at day 160 post-transplant revealed suppressed response against donor (LBN) antigens in semi-allogenic transplant recipients (group 4) but moderate reactivity to donor (ACI) antigens in fully mismatched transplant recipients (group 5). Skin biopsies taken from animals showing sign of rejection demonstrated Grade II histopathological lesions in LBN and ACI recipients at days 140 and 180 post-transplant. Conclusion: Functional tolerance was induced in hemifacial allograft transplants across MHC barriers under low maintenance dose of CsA monotherapy. Tolerance was associated with presence and maintenance of donor specific multilinege chimerism.
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