Image: University of Missouri; PHOTOGRAPHED BY JIM CURLEY

Given the limited supply of human organs available to patients who need replacements, scientists have long dreamed of being able to use other species as organ sources. Considering their body size, breeding characteristics and the fact that they are bred for food, pigs are generally viewed as the most appropriate prospective donors. Problematically, however, pig cells sport sugar molecules on their surfaces that can trigger a violent immune response in the transplant recipient¿an assault that can kill the donor organ in minutes. Efforts to outfit the organs with protective proteins have succeeded only in delaying rejection for a few months. But new research results suggest that a solution to the conundrum now lies within reach. According to a report released today by the journal Science, investigators have cloned a group of pigs genetically engineered to partially lack a key component of this immune response. The achievement could represent a major step toward making the dream of successful xenotransplantation a reality.

To make their pigs, Randall S. Prather of the University of Missouri and his colleagues started with fetal pig cells, "knocking out" a copy of the gene responsible for making the inflammatory sugar molecules. They then fused the modified pig cells with pig eggs and implanted the 3,000 cloned embryos in surrogate sows. The four resulting piglets that have survived each carry one inactive copy of the gene and one active one. The pigs can still manufacture the sugar molecules with the one active copy of the gene, though. To eliminate the sugar production altogether, researchers will have to produce swine in which both copies of the relevant gene are inactive¿a goal that Prather told Science he hopes to achieve within 18 months.