The Germans at the Munich facility are in the less-is-more camp. The pigs they work with have three key genetic modifications originally made more than a decade ago—all designed to keep baboons and humans from rejecting their organs. Knocking out a gene that produces a sugar called galactosyltransferase prevented the recipient’s immune system from immediately rejecting an organ from a different species. The second change added a gene expressing human CD46, a protein that helps the immune system attack foreign invaders without overreacting and causing autoimmune disease; the third introduced a gene for a protein called thrombomodulin, which prevents the blood clots that would otherwise destroy the transplanted organ.
In 2018, the hearts of pigs from the Munich center were transplanted into 14 baboons. Two of the monkeys survived for six months, the longest any animal has lived with a heart from another species. In a report in Nature last December, the German researchers described their achievement as “a milestone on the way to clinical cardiac xenotransplantation.”