The cells in many privileged sites express FasL on their surface. When threatened by a cytotoxic T cell , they avoid being killed by forcing the T cell to commit suicide by apoptosis . The cytotoxic T cells express Fas on their surface. Link to more of the story of the role of Fas and FasL in apoptosis. So if the organs of transgenic pigs can be made to expresses human FasL, perhaps they will be resistant to T-cell mediated rejection.
Animals often contain latent viruses in their cells.
The danger seems greater for xenotransplants from other primates . However, retroviruses have also been found in some pigs. One of these is able to infect human cells grown in tissue culture. For these reasons, many biologists are urging that transplant surgeons proceed cautiously with xenotransplants.