ABO-incompatible allotransplantation as a basis for clinical xenotransplantation

The 8th Congress of the International Xenotransplantation Association (IXA), held in Goteborg in September, 2005, immediately followed the 2nd International Symposium on ABO-Incompatibility in Transplantation, both congresses organized by Michael Breimer and Lennart Rydberg. The Proceedings of the Symposium on ABO-Incompatibility in Transplantation have been published (Xenotransplantation 2006; 13 (2)). The present paper provides an overview of a workshop held at the 8th Congress of the IXA, and highlights the immunological concepts emerging from ABO-incompatible allotransplants and discusses them in relation to xenotransplantation. Using specified immunomodulatory protocols, ABO-incompatible solid organ allotransplantation has become a clinical reality for a small number of patients over the last two decades. ABO-incompatible adult kidney and infant heart transplants have similar patient and graft survivals as their ABO-compatible counterparts. In contrast, ABO-incompatibility is present in up to 30% of all patients receiving hematopoietic stem cell transplants in the absence of specific immunomodulation, without affecting overall survival. Consequently, ABO-incompatible solid organ transplants and hematopoietic stem cell transplants may serve as in vivo models to elucidate the immunological mechanisms of accommodation and/or tolerance in the clinical setting. Because of similarities in the immunological hurdles that need to be overcome, knowledge obtained from ABO-incompatible allotransplantation might further promote advances in the field of xenotransplantation. The similarities and differences between ABO-incompatible allotransplantation and xenotransplantation are discussed.