A recombinant soluble chimeric complement inhibitor composed of human CD46 and CD55 reduces acute cardiac tissue injury in models of pig-to-human heart transplantation

Inasmuch as complement plays a critical role in many pathological processes and in xenograft rejection, efficient complement inhibitors are of great interest. Because the membrane-associated complement inhibitors are very effective, recombinant soluble molecules have been generated. We tested the efficacy of complement activation blocker-2 (CAB-2), a recombinant soluble chimeric protein derived from human decay accelerating factor (DAF, CD55) and membrane cofactor protein (MCP, CD46), in two models of pig-to-human xenotransplantation in which tissue injury is complement mediated. The in vitro model consisted of porcine aortic endothelial cells and human serum, and the ex vivo model consisted of a porcine heart perfused with human blood. In vitro, addition of CAB-2 to serum inhibited cytotoxicity and the deposition of C4b and iC3b on the endothelial cells. Ex vivo, addition of CAB-2 to human blood prolonged organ survival from 17.3 +/- 6.4 min in controls to 108 +/- 55.6 min with 910 nM (100 microg/ml) CAB-2 and 219.8 +/- 62.7 min with 1820 nM (200 microg/ml) CAB-2. CAB-2 also retarded the onset of increased coronary vascular resistance. The complement activity of the perfusate was reduced by CAB-2, as was the generation of C3a and SC5b-9. The myocardial tissues had similar deposition of IgG, IgM, and Clq; however, CAB-2 reduced the deposition of C3, C4, and C9. Hearts surviving >240 min demonstrated trace to no deposition of C9 and normal histologic architecture. These results indicate that CAB-2 can function as an inhibitor of complement activation and markedly reduce tissue injury in models of pig-to-human xenotransplantation and thus may represent a useful therapeutic agent for xenotransplantation and other complement-mediated conditions.