Xenotransplantation: is the risk of viral infection as great as we thought?

Two major hurdles remain before xenotransplantation can enter the clinic. T he first is the more technical issue of being able to overcome the human im mune response that leads to rejection of transplanted organs/cells from oth er species, The second, reviewed here, concerns the potential risk of inadv ertent transfer of animal viruses present in the xenotransplant that are ab le to infect the human recipient, The threat from viruses is a particularly contentious topic because it poses a risk not only to those individuals wh o receive xenotransplants, but also to healthy individuals who come into co ntact, either directly or indirectly with the xenotransplant recipient. In this review, we describe some of the virus types, in addition to the much d iscussed porcine endogenous retroviruses that might cross the species barri er, and assess the risk of such viruses causing disease in human hosts.