The generation of transgenic pigs as potential organ donors for humans

Success of transplantation as a therapeutic option in terminal organ failure has resulted in the widespread diffusion of this life-saving treatment and a consequent worldwide organ shortage. Several approaches have been explored in an attempt to solve this donor crisis and xenotransplantation-the transplantation into humans of organs from other species-is currently viewed as a possible solution to this problem. Because of its anatomical and physiological similarities with humans and the relative simplicity with which it can be bred in large numbers, the pig is currently considered the most likely source of organs for human xenotransplantation. However, the pig-to-human combination is known to be discordant, thus when transplanted into humans, pig organs are rapidly attacked by the recipient's immune system in a very aggressive reaction termed hyperacute rejection (HAR). The HAR leads to destruction and loss of graft function within minutes or hours. Massive complement activation through both the classical and alternative pathways produces endothelial cell activation, loss of vascular integrity, oedema, haemorrhage and thrombosis-all of which characterize the pathology of hyperacutely rejected organs. The key role played by complement activation in HAR has previously been demonstrated by both in vitro and in vivo studies. In the latter experiments, both in spontaneous complement-deficient animal models or in animals deprived through in vivo administration of cobra venom factor or soluble complement receptor type 1 (sCR1), HAR was either delayed or did not occur. The complement 'cascade' is regulated at specific points by proteins such as decay-accelerating factor (DAF), membrane cofactor protein (MCP) and CD59 (collectively known as regulators of complement activation, RCAs) that are species-specific. It was hypothesized that expression of human RCAs by pig organs would downregulate the activation of human complement and could thus prevent its participation in HAR of the graft. In vitro experiments performed both by us and by others involving the expression of human RCAs on non-human cells clearly demonstrated the effectiveness of the de novo expression of these molecules in the prevention of lysis of these cells subsequent to human complement activation.