Xenograft rejection—all that glitters is not Gal

Xenotransplantation is being developed in the hope of resolving the critical shortage of donor organs for transplantation. The Eurotransplant waiting lists [1] for donor organs of various kinds number almost 16 000 patients and the US lists [2] more than 90 000 patients. Renal transplantation, for instance, cost-effectively confers a significant survival advantage [3] and improvement of quality of life [4]. But whereas currently, in Europe, nearly 12 000 end-stage renal disease patients await a suitable donor, only 3383 kidney transplants were performed in 2005, with an average waiting time of 1174 days [1]. Substantial research efforts are being made in the field of xenotransplantation, and the immunological barriers are gradually being elucidated. Pig-to-human xenogeneic organ transplantation is considered the combination of choice for anatomical, physiological and biochemical reasons, and the use of porcine tissues is thought to confer reduced risks for cross species transmission of (viral) agents such as porcine endogenous retroviruses (PERV). Pigs are also easy to breed and their use as organ donors for humans is ethically more accepted than that of non-human primates [5]. However, the pig is immunologically discordant to human and non-human primates and, as a consequence, hyperacute rejection (HAR) and acute humoral xenograft rejection (AHXR) (also called acute vascular rejection) pose major hurdles to xenograft survival in experimental pig-to-baboon models. The HAR occurs within minutes as a result of antibody-mediated complement activation and is histologically characterized by widespread interstitial haemorrhage and thrombosis [6]. The putative antibodies are predominantly directed against Gala1,3Galb1,4GlcNAc (aGal) carbohydrate residues, present on cell-surface glycoproteins and glycolipids of porcine tissue [7,8]. Primates lack the functional a1,3-galactosyltransferase (a1,3GalT) gene to synthesize aGal, and high titres of ‘natural anti-aGal antibodies’ arise early in life upon encounter of aGal-expressing micro-organisms in the gastrointestinal tract [9]. Strategies have been developed to prevent HAR and include the removal or inactivation of anti-aGal antibodies [10], the depletion or inhibition of complement, or alternatively, the production of donor pigs transgenic for either one or more complement regulatory proteins such as hDAF, MCP and CD59 [11]. However, these approaches have not been able to prevent the return of anti-aGal antibodies, but rather showed that when HAR was avoided, AHXR occurred [12]. This second form of humoral rejection develops within days and is mediated by elicited xenoreactive antibodies, directed to a large extent at aGal but also at non-aGal epitopes, and complement may also be involved [13]. With the successful development of a1,3GT / pigs, donor organs became available in which aGal epitope expression was completely eliminated [14,15], and it was hoped that the use of a1,3GT / donor organs would not only prevent HAR but also AHXR. In January 2005, encouraging results were reported by Kuwaki and coworkers [16] and by Yamada and coworkers [17], who achieved prolonged survival of a1,3GT / porcine heart, respectively kidney grafts in baboons. Yamada and coworkers [17] performed concomitant recipient thymectomy and splenectomy, with vascularized xenothymus transplantation, T-cell depletion, CD154–blockade and treatment with mycophenolate mofetil, and achieved xenokidney graft survival of up to 80 days. Neither aGal-specific nor non-aGal-specific xenoreactive antibodies were detected, nor did kidney grafts show histological signs of rejection [17]. In the recent 2005 December issue of Nature Medicine, Chen and coworkers [18] equally studied a1,3GT / pig xenokidney graft survival in baboons, who were given a preclinical immunosuppressive regimen consisting of a short course of antithymocyte globulin (ATG), tacrolimus, mycophenolate mofetil and steroids, or a single high dose of ATG with subsequent tacrolimus monotherapy. Although HAR was prevented, graft survival was Correspondence and offprint requests to: Mark Waer, Laboratory of Experimental Transplantation, University of Leuven, Campus Gasthuisberg, O&N 811, Herestraat 49, B-3000 Leuven, Belgium. Email: mark.waer@med.kuleuven.be