The first and major clinical obstacle in xenotransplantation is antibody-mediated hyperacute rejection. Although human natural antibodies against Galalpha1,3Gal (Gal) antigens, which are common on porcine cells and organs, have been identified to play a major role in hyperacute rejection, other natural antibodies against non-Gal epitopes may be also involved in the process. Here, we present evidence suggesting that the majority of human anti-non-Gal antibodies are specific for carbohydrate structures carrying terminally linked N-glycolylneuraminic acid (NeuGc), a xenoantigen existing in almost all animals except humans. Furthermore, this anti-NeuGc activity is detectable in 85% of healthy humans, implicating the involvement of NeuGc in hyperacute rejection and the importance of developing strategies for removing NeuGc for clinical xenotransplantation.

With an alpha-galactosidase, B erythrocytes can be converted to blood group O under conditions that neither impair their viability in vitro nor affect their ability to survive normally after transfusion to individuals of groups O, A, and B. Such an approach has the potential for producing enzymatically converted group O cells for use in transfusion therapy. It should also be possible to convert A cells to group O by using the appropriate alpha-N-acetylgalactosaminidase.

These results extend those observed in our earlier 1-unit transfusion studies and suggest that ECO RBCs pose little risk and will be useful in transfusion medicine.

ECO RBCs are safe and efficacious when transfused more than once or in multiple-unit volumes to group O or A subjects, and ECO RBCs prepared with recombinant or native enzyme are equivalent in vivo.

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Full-unit transfusions of RBC enzymatically converted from group B to group O by treatment with alpha-galactosidase (ECO RBC) to group O and A normal healthy individuals exhibit excellent in vivo survival times (24-hour survival 95.1% +/- 2.3%, T50 36.9 +/- 4.6 days). These results confirm our earlier findings describing ECO RBC in vitro viability and normal in vivo survival time after small-volume infusions. No significant increase in pretransfusion anti-B titer or score is observed in either group O or A subjects provided that sufficient enzyme is used to treat the cells: Cells transfused to group O recipients require higher levels of enzyme (185 to 200 U/mL RBC) than those infused to group A (90 U/mL RBC). Two separate single-unit transfusions of ECO RBC to one group O recipient (4.5 months apart) also survived normally (24-hour survival 96% and 92%, T50 40 and 36 days) and did not increase preexisting anti-B levels in this subject. ECO RBC were not agglutinated or lysed by recipient sera before or after transfusion. Similarly, no antibody development to the alpha- galactosidase used in cell treatment (and washed from the product before transfusion) could be detected in any subject. The sustained increase in hemoglobin levels after transfusion of ECO RBC suggests that this product will be useful in treatment of acute and chronic anemia.

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