UNDETECTABLE LEVELS OF GAL-ALPHA-1,3-GAL EXPRESSION ARE REQUIRED TO AVOID HYPERACUTE OR ACCELERATED ACUTE REJECTION IN PIG-TO-BABOON RENAL TRANSPLANTATION:

O51* Aims: The major barrier of pig-to-primate xenotransplantation is Gal-alpha-1,3-Gal (Gal) expression on donor endothelium, which induces hyperacute rejection. Several approaches have been attempted to reduce or eliminate Gal expression on pig organs. However, thus far the only successful strategy in eliminating Gal epitopes in vivo has been to produce homozygous alpha-1,3-galactosyltransferase knockout (GalT-KO) pigs. As presented elsewhere at this meeting, we have recently found that neither complement inhibition nor depletion of natural anti-Gal antibodies is required to avoid hyperacute and/or accelerated acute rejection in baboons using these GalT-KO pigs as donors. During the production of GalT-KO pigs by nuclear transfer technology, some pigs were also produced which expressed extremely low levels (<5% of normal) of the Gal epitope (so called “GalT-Low”). These animals allowed us to ask whether this low level of Gal antigen was sufficient to cause humoral rejection in xenogeneic renal transplantation. Methods: The immunoperoxidase technique, using the biotinylated lectin Griffonia simplicifolia (IB4), was used for the assessment of Gal expression on kidneys and ureters in compound heterozygote pigs. Three kidneys from two GalT-Low, which expressed Gal antigen at almost undetectable levels in kidney parenchyma but expressed Gal weakly in the ureters by IB4 staining, were used as donor organs in three baboons. Immunosuppression consisted of T-cell depletion, MMF, steroids, anti-CD154 monoclonal Ab and cobra venom factor. The remaining kidney expressed Gal at very low levels in both kidney parenchyma and ureter, was used to assess early histologic changes seen in compound kidneys and ureters at 0 min, 1 hr, 3 hrs, 6 hrs and 9 hrs following transplantation to the fourth baboon, without immunosuppression. Results: Three baboon recipients of GalT-Low kidneys with immunosuppression rejected their grafts on days 15, 16 and 19, showing evidence of humoral rejection. Graft survival was slightly prolonged as compared to historic controls using Gal+ miniature swine kidneys treated similarly (mean 7.6 days). In correlation with Gal expression in the kidney/ureter, ureters were rejected earlier than kidneys. Sequential biopsies of the fourth GalT-Low kidney graft showed ureteral rejection by 3 hours after transplantation. The ureter was completely rejected 6 hours after revascularization and the kidney subsequently showed humoral rejection at 9 hours. Conclusions: These results indicate complete elimination of Gal antigen, such as that demonstrated in homozygous GalT-KO organs, is required to avoid hyperacute and/or accelerated acute rejection in pig-to-primate xenogeneic renal transplantation.