Organ xenografts in discordant combinations such as pig-to-man undergo hyperacute rejection due to the presence of naturally occurring human anti-pig xenoantibodies. The galactose alpha(1,3)-galactose epitope on glycolipids and glycoproteins is the major porcine xenoantigen recognized by these xenoantibodies. This epitope is formed by alpha(1,3)-galactosyltransferase, which is present in all mammals except man, apes, and Old World monkeys. We have generated mice lacking this major xenoantigen by inactivating the alpha(1,3)-galactosyltransferase gene. These mice are viable and have normal organs but develop cataracts. Substantially less xenoantibody from human serum binds to cells and tissues of these mice compared with normal mice. Similarly, there is less activation of human complement on cells from mice lacking the galactose alpha(1,3)-galactose epitope. These mice confirm the importance of the galactose alpha(1,3)-galactose epitope in human xenoreactivity and the logic of continuing efforts to generate pigs that lack this epitope as a source of donor organs.

There is now considerable evidence implicating anti‐Gal xenoantibodies as a key instigator in the hyperacute rejection of discordant xenografts. As a consequence it is generally held that elimination or reduction of the Gal/anti‐Gal component is critical to overcoming hyperacute rejection. We have recently shown that in mice inactivation of the GalT gene by homologous recombination completely eliminates the expression of the Gal‐epitope and that hearts from these mice demonstrate prolonged survival when perfused ex vivo with human plasma. Unfortunately this strategy is currently not feasible in pigs because the technology to isolate porcine embryonic stem cells, which are critical for homologous recombination, is not yet available. This study investigates an alternative competition‐based transgenic strategy to suppress the level of the Gal epitope by expression of H‐transferase (α1,2‐fucosyltransferase) an enzyme which has the same substrate specificity (lactobiose) as α1,3‐galactosyltransferase. In vitro transfection of murine cells with H‐transferase reduced Gal‐epitope expression by 80–90%. A similar reduction in Gal expression was observed on PBL and thymocytes from H‐transferase transgenic mice. This reduction in Gal epitope expression resulted in a marked reduction in the reactivity of these cells with human serum. In tissues from these mice the reduction in Gal expression was inversely proportional to the endogenous level of Gal. The results of this study support pursuing this strategy as a means to reduce the xenoantigenicity of porcine tissues.

These findings demonstrate that expression of human DAF in association with elimination of the Gal epitope provides added protection from complement-mediated injury in these models of HAR.

High-level endothelial-specific expression of CD59 was effective in prolonging the function of mouse hearts perfused with 20% human plasma, whereas low-level, broader expression did not provide protection from 6% plasma.

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