Xenotransplantation

A pig organ transplanted into a human without genome editing dies within minutes. The cause is not cellular immunity but antibodies a human already carries before any contact with the donor.

Three sugars

Pigs express alpha-1,3-galactosyltransferase (the GGTA1 gene), which caps endothelial glycoproteins with galactose in an α-1,3 linkage — the α-Gal antigen. Humans and higher primates have that gene inactivated, but gut microbiota present the same epitope continuously, so every human carries high titres of pre-formed antibody against it. On perfusion those antibodies bind the graft endothelium at once, activate complement by the classical pathway, and produce hyperacute rejection.

Knocking out GGTA1 alone is not enough, because two further carbohydrate antigens remain. CMAH converts N-acetylneuraminic acid into N-glycolylneuraminic acid (Neu5Gc), which humans do not synthesise and against which they also carry antibody; B4GALNT2 makes the Sda blood-group antigen. Hence today’s standard construct — the triple knockout of GGTA1, CMAH and B4GALNT2, which removes the bulk of human pre-formed reactivity.

Coagulation is the second, less obvious incompatibility

Even with the sugars gone, a species mismatch in proteins remains. Porcine thrombomodulin activates human protein C poorly; porcine tissue factor pathway inhibitor acts weakly on human factor Xa; porcine von Willebrand factor spontaneously binds human platelet glycoprotein Ib. The result is thrombotic microangiopathy and platelet consumption inside the graft while the humoral response looks controlled. Donors are therefore given human transgenes: the complement regulators CD46 and CD55, thrombomodulin and endothelial protein C receptor, and CD47 to damp phagocytosis. The growth hormone receptor is knocked out separately, or the organ keeps growing to the donor’s programme inside the recipient.

What actually decided the outcome

The first pig-to-human heart transplant, at the University of Maryland in 2022, ended after two months, and post-mortem analysis found porcine cytomegalovirus — more precisely porcine roseolovirus — DNA in the graft. The donor had tested negative on standard blood screening, but latent infection is invisible to that test. It reset the field’s priorities: donor screening now runs PCR across many tissues alongside serology and early weaning, because donor virological status turned out to be a determinant of graft survival on a par with genotype.

A separate and much more discussed risk is porcine endogenous retrovirus, integrated in the genome in dozens of copies and therefore not removable by screening; inactivation by editing has been demonstrated, but the clinical significance of transmission remains unproven in either direction. In the United States the work proceeds through FDA expanded-access protocols and the first cleared clinical studies; the regulatory frame requires lifelong recipient surveillance and specimen archiving, which is unprecedented in transplantation and reflects infectious rather than immunological uncertainty.