By David N. Leff
In our guts, we all offer hospitality to populations of bacteria. Some of these flaunt antigenic carbohydrates on their cell surfaces. These epitopes, alpha-galactosyl (alpha-Gal) by name, excite the attention of immune-system antibodies specifically programmed to bind those molecules.
That coupling of antibody and antigen runs a flag up the flagpole, summoning the body's complement system to wipe out these alien molecules. Complement proteins duly jump on the antibody-antigen complex and lyse its cells full of lethal holes.
This process of hyperacute rejection is what complicates the lives of transplant surgeons. The donor organs — hearts, lungs, liver, kidneys, whatever — that they stitch into the bodies of their recipient patients are naturally paved with the antigens that the host's antibodies will see as foreign, and move to destroy.
Even with massive, debilitating doses of immunosuppressive drugs, the host's immune system turns the living donor organ in minutes from a healthy flesh-color to the blackened hue of necrosis — death.
"For instance, suppose you put a pig heart into a baboon," observed molecular immunologist John Iacomini. "The blood vessels of that heart would be destroyed very quickly, if you don't remove the antibodies. Basically starved of nutrients, the donor organ becomes necrotic very quickly, because the baboon's blood starts flowing through the arteries and veins in that porcine organ, and the first thing that's exposed is the alpha-Gal antigen on vascular endothelium."
Even as transplant technology continues to improve, more and more eligible candidate patients continue to die while waiting for donor organs that remain in critically short supply.
Enter Sus scrofa, the common domestic pig. This animal's vital organs resemble human ones in size and function. And because pigs reach full size rapidly, they qualify as an endless source of ever-scarcer replacement spare parts. (See BioWorld Today, Aug. 31, 1998, p. 1.)
But there's a cruel catch: Sus scrofa's cells bristle with the same antigenic alpha-Gal epitopes targeted by antibodies on human bacterial flora. Today's issue of Science, dated Sept. 18, 1998, reports a preclinical solution to this impasse. Its title is, "Inhibition of xenoreactive natural antibody production by retroviral gene therapy." The article's senior author is Iacomini, of Harvard Medical School and the Transplantation Research Center at Massachusetts General Hospital, both in Boston.
Antigens On Stem Cells Fooled Antibodies
"What we've done," Iacomini told BioWorld Today, "is use gene therapy and retroviruses to deliver genes encoding alpha-Gal to the host's bone-marrow cells. There, they reprogram the blood-forming stem cells. These give rise to multiple lineages in our blood — red cells, T-cells, antibody-forming B cells, macrophages, etc. B cells producing antibodies to alpha-Gal are thus eliminated."
Iacomini and his co-authors enlisted a genetically engineered mouse model that recapitulates part of the human immune system and contains antibodies to the carbohydrate alpha-Gal epitope. They expressed that antigen in the mice, via retroviruses carrying porcine alpha-galactosyl genes. Then they infused that modified bone marrow back into genetically identical animals.
"The animals' reconstituted immune systems," Iacomini explained, "became repopulated with bone marrow, which now considers the gene product for the alpha-Gal to be part of itself, so its antibodies no longer elicit the response to the antigen."
As the endpoint of the experiment, his paper reported, "The level of alpha-Gal-reactive natural antibodies remained undetectable in the mice up to 51 weeks after transplantation."
"This is pretty long in the life of a mouse," Iacomini commented, "particularly one that's undergone such manipulations. That's one of the limitations with these experiments.
"We've analyzed mice past 51 weeks," he went on, "but usually sacrificed them about that time, to see if the gene product is still being expressed in their tissues." His lab has a couple of the transgenic animals still alive at almost 70 weeks. "They're pretty old-looking mice, but pretty healthy as far as we can tell," he observed.
"What we're pursuing now," Iacomini recounted, "is trying to do a similar experiment in a baboon model, because their immune system is very similar to what humans have. So we are testing whether or not we can effectively infect bone marrow cells from baboons and look for gene expression."
As for clinical trials, he went on, "I really wouldn't even suggest that there are going to be studies in humans any time soon. We have a lot of basic work to do in a large primate model to really make this feasible."
Method Tuned To Autoimmune 'Rejection' Too
Transplant rejection and autoimmune disease have much in common.
"Autoimmune disease," Iacomini pointed out, "can be mediated by B-cell (antibody) immunity or T-cell immunity, sometimes both. We hypothesize that our approach, as we've done it, is a potent way to reprogram the immune response.
"So, in the case of autoimmunity, we may be able, by gene therapy, to re-establish discrimination between self- and non-self antigens. In other words, imagine an autoimmune disease where you know what the gene product is that antibodies are attacking, and causing the pathology. If we could somehow express the antigenic product on or in bone-marrow-derived cells, maybe in the future we could eventually treat patients to re-program their immune systems, much as we've done in these alpha-Gal-reactive antibodies."
By way of example, he cited multiple sclerosis (MS), an autoimmune disease that is T-cell-mediated. We think our method could also work in T-cells," Iacomini suggested. "It's well known that MS myelin basic protein in the central nervous system is the antigen to which autoreactive cells are expressed. So it's a good candidate, because we know what antigen we're hunting down. In fact we're trying to find a very good mouse model and embark on those studies." *