High levels of anti-pig hemolytic antibodies are associated with hyperacute rejection of hDAF-transgenic pig cardiac and renal xenografts in cynomolgus monkeys

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Abstract 425
Introduction Hyperacute rejection (HAR) has been a rare phenomenon in non-human primate recipients of pig organs transgenic for human Decay Accelerating Factor (hDAF). We show, however, that hDAF cannot completely protect against injury in recipients with very high levels of natural anti-pig antibodies.
Materials and methods Cynomolgus monkeys received heterotopic cardiac xenografts (n = 6) or life-supporting renal xenografts (n = 4) from hDAF-transgenic pig donors. Immunosuppression was 4-dose cyclophosphamide induction, cyclosporine, steroids, and sodium mycophenolate. Pre-transplant anti-pig antibodies were measured by an anti-pig hemolytic antibody assay (APA) and expressed as titers normalized to a standard of sera pooled from 8 healthy humans (human standard = 1).
Results HAR occurred in 5/6 cardiac xenografts and 2/4 renal xenografts (heart: cessation of cardiac contractions; kidney: anuria within 4 days), and technical failures did not occur. All pigs were shown to express the hDAF transgene. The average pretransplant normalized APA titer of these 7 monkeys yielding HAR was 7.6 ± 2.5 (range 3.9-11.8). One cardiac xenograft (APA titer 5.7) did not undergo HAR but contractions were weak immediately post-transplantation. This graft continued to contract for 11 weeks and then succumbed to humoral xenograft rejection. In the 2 recipients of renal xenografts that did not undergo HAR, APA titers were 0.5 and 1 respectively.
Conclusions Our incidence of HAR is the highest ever reported for organs from hDAF-transgenic pig donors (7 out of 10). These HAR cases occurred in both cardiac and renal xenografts. One explanation for this phenomenon is the remarkably high preformed natural antibody levels. When these antibody levels are substantially higher than the normal human standard (in the present HAR cases, on average 7.6-fold higher), complement inhibition conferred by hDAF may not always be sufficient to prevent HAR. Our findings may have important implications for the use of hDAF-transgenic pig organs in man: either patients with high levels of natural anti-pig antibodies may need to be excluded, or means to reduce these levels should be used before transplantation.
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