Page 1134 - Veterinary Immunology, 10th Edition

from the species-specific activity of complement control proteins.
VetBooks.ir Thus the natural inhibitors of complement such as CD46, CD55,

and CD59 found on pig cells cannot control the activation of human
complement. Transgenic pigs have been produced that express

these human complement inhibitors on their cells and do not
trigger hyperacute rejection on grafting. Should the xenograft
survive attack by these natural antibodies and complement, it is still
susceptible to delayed attack from induced antibodies and from

antibody-dependent, cell-mediated cytotoxicity mediated through
NK cells and monocytes. Many barriers are yet to be overcome if
pig organs are ever to be routinely used as human organ
transplants.

One other point relevant to xenografting is that donor animals
may carry viruses that could cause disease in a severely
immunosuppressed recipient or, even worse, recombine with
human viruses to create new and potentially hazardous pathogens.

These xenograft-derived infections (xenozoonoses) are of special
concern if primates are used as organ donors. These animals are
known to carry viruses such as simian immunodeficiency virus and
herpes B virus that can infect humans. Pigs possess an endogenous

retrovirus that has the ability to infect some human cell lines in
tissue culture, although it is not known to cause human disease.

1134