Tolerance of vascularised composite allografts: the effect of major histocompatibility matching

Background: Vascularised composite allografts (VCAs), such as face, hand and abdominal wall transplants, have resulted in excellent functional and quality-of-life outcomes. The development of VCA tolerance protocols would obviate the need for immunosuppression and allow widespread use of this reconstructive modality. But tolerance of VCA skin has been particularly difficult to achieve. I hypothesised that sharing of MHC class I and/or class II between donor and recipient influences VCA tolerance. Methods: Two large animal experimental models were used. In model 1, MGH miniature swine recipients (n=8) underwent non-myeloablative conditioning and haematopoietic stem cell transplantation (HSCT), generating mixed chimeras across either a class I (n=4) or class II (n=4) MHC mismatch. VCAs were transplanted at the time of HSCT. In model 2, VCAs were transplanted onto animals already tolerant of heart and kidney transplants across haploidentical and full MHC barriers through regulatory pathways (n=4). Serial biopsies of VCAs were performed to assess rejection and cellular infiltrate. Flow cytometry was used to assess chimerism in the peripheral blood and tissues. Immune responsiveness was assessed using mixed lymphocyte reaction, cell-mediated lymphocytotoxicity and alloantibody assays. Results: In model 1, animals displayed stable mixed chimerism in lymphoid, myeloid and granulocyte lineages. MHC class II-mismatched chimeras remained tolerant of VCAs (>500 days). In contrast, MHC class I-mismatched chimeras experienced rejection of VCA skin, characterised by infiltration of recipient-type CD8+ T lymphocytes. All animals demonstrated systemic donor-specific non-responsiveness in vitro, including after rejection episodes. In model 2, animals rejected all components of VCAs placed across haploidentical and full MHC-mismatched barriers. VCA rejection correlated with re-establishment of in vitro donor responsiveness. Despite VCA rejection, tolerance of heart and kidney allografts persisted. Conclusions: MHC matching of donor and recipient has not been considered in clinical VCAs to date. This study shows that in a mixed chimerism model, MHC class I antigen mismatching mediates VCA skin rejection whereas robust VCA tolerance can be induced across MHC class II barriers. In addition, we found that local regulation of immune tolerance is critical in long-term acceptance of VCAs. Based on the tolerance of VCAs and non-vascularised skin grafts in MHC class II-mismatched chimeras, we hypothesised that establishing tolerance of allogeneic MHC class II antigens would facilitate more robust tolerance, via induction of regulatory pathways. However, tolerance of VCAs was not seen in a large animal model in which regulatory tolerance of solid organs was established across MHC class II barriers. Taken together, our results identify mechanisms underlying VCA skin tolerance, with important implications for future clinical strategies.Open Acces