MONOCLONAL-ANTIBODIES DIRECTED AGAINST HUMAN C5 AND C8 BLOCK COMPLEMENT-MEDIATED DAMAGE OF XENOGENEIC CELLS AND ORGANS

The hyperacute rejection (HAR) of xenotransplanted organs is initiated by the deposition of natural antibodies on donor endothelium followed by the activation of the recipient complement system, which rapidly d estroys the graft. Studies of the role of activated complement in HAR have suggested that natural antibody as well as early (C3a, C3b) and l ate (C5a, C5b-9) activated complement components may contribute to cel l activation and damage. Attenuation of HAR has been achieved by block ade of C3 activation with soluble CR1 or consumptive depletion of comp lement with cobra venom factor; however, similar studies using specifi c inhibitors of terminal complement components have not been described . To address the contribution of C5a and the membrane attack complex ( C5b-9, MAC) to complement-mediated xenogeneic cell and organ damage, w e utilized functionally blocking monoclonal antibodies directed agains t the human terminal complement components C5 and C8. Our data show th at both anti-C5 and anti-C8 mAbs protect porcine aortic endothelial ce lls from membrane damage mediated by human C5b-9. Additionally, both t he anti-C5 and anti-C8 mAbs blocked complement-mediated generation of membrane prothrombinase activity on porcine aortic endothelial cells c hallenged with human serum. To test the ability of these antibodies to attenuate antibody and complement-mediated damage of xenogeneic organ s, an ex vivo model was developed wherein isolated rat hearts were per fused with human serum in the presence or absence of the anti-C5 and a nti-C8 mAbs. Our data demonstrate that mAbs directed against human C5 and C8 prevented organ damage by human serum complement and suggest th at these molecules may serve as potent inhibitors of HAR.