Xenotransplantation refers to the transplantation of organs from non-human animals into human patients. Such a procedure can increase the availability of organs for transplantation, but proteins and sugars on the surfaces of animal cells that are not found in human bodies can elicit an immune response against these xenotransplanted organs and tissues. For example, the human immune system recognizes a sugar molecule that coats the surface of pig blood vessels but is absent from human tissues called alpha-1,3-galactose (α-gal). In 2003, David Cooper, who runs the transplantation program at the University of Cape Town Medical School, engineered pigs without the α-1,3-galactosyltransferase gene that produces the α-gal residues. However, there were other problems with pig organs as well.
Tissue engineered organs are grown from a patient’s own cells. Such organs should help increase the availability of organs and avoid the problems of immune rejection that plague the field of xenotransplantation. “Cartilage, skin, and bone are already on the market. Blood vessels are in clinical trials. The progress has been really gratifying,” says Laura Niklason of Yale University.
Such engineered tissues consist of either flat planes or hollow tubes and are relatively simple to produce. Also, they consist of a small number of cell types. However, solid organs, such as the lungs, heart, liver, and kidneys, pose a greater challenge, since they are bigger and contain dozens of cell types. In addition, they have a complex architecture and an extensive network of the most essential component, which are the blood vessels. “Every cell needs to eat and breathe, and each one needs to be close to a source of nutrition and oxygen,” says Joseph Vacanti, who is in charge of the liver transplantation program at Boston Children’s Hospital in Massachusetts. Still, Vacanti is optimistic that it should be possible to produce even these complex organs through tissue engineering. “People differ about whether it’ll be achieved in 5 or 100 years, but most people in the field believe that it’s a realistic goal,” he says.
In 2008, Harald Ott of Massachusetts General Hospital and Doris Taylor of the University of Minnesota dramatically demonstrated the potential of organ engineering by growing a beating heart in the laboratory. These know first-hand, the need for organs for transplantation, since as physician-scientists, they often see patients who badly need transplants, but have no available organs for transplantation. To make engineered hearts, they began by using detergents to strip the cells from the hearts of dead rats. This left behind an extracellular matrix (a white, ghostly, heart-shaped frame of connective proteins such as collagen and laminin). Ott and Taylor used this matrix as a scaffold, and they seeded it with cells from newborn rats and incubated it in a bioreactor, which is a vat that provides cells with the right nutrients, and simulates blood flow. Four days later, the muscles of the newly formed heart began contracting, and after eight days, it started to beat.
This technique is extremely labor-intensive and is known as whole organ decellularization. Think of it as knocking down a house’s walls to reveal its frame, and then replastering it anew with different materials. Because the frame is of the same structure as the original organ and retains the complicated three-dimensional architecture of the organ which includes the branching network of blood vessels. Additionally, it also preserves the armamentarium of complex sugars and growth factors that covers the matrix and provides signaling signposts for growing cells. These signals will nudge the cells into the proper shapes and structures. “The matrix really is smart,” says Taylor. “If we put human cells on human heart matrix, they organize in remarkable ways. We can spend the next 20 years trying to understand what’s in a natural matrix and recreate that, or we can take advantage of the fact that nature’s put it together perfectly.”
Ott and Taylor’s groundbreaking feat of tissue engineering has since been duplicated for several other organs, including livers, lungs, and kidneys. Rodent versions of all have been grown in labs, and some have been successfully transplanted into animals. Recellularized organs have even found their way into human patients. Between 2008 and 2011, Paolo Macchiarini from the Karolinska Institute in Sweden fitted nine people with new tracheas. These tracheas were built from their own cells grown on decellularized scaffolds. Most of these operations were successful (although three of the scaffolds partially collapsed for unknown reasons after implantation). Decellularization has one big drawback: it still depends on having an existing organ, either from a donor or an animal. These disadvantages led Macchiarini to devise a different approach. In March 2011, he transplanted the first trachea built on an artificial, synthetic polymer scaffold. His patient was an Eritrean man named Andemariam Teklesenbet Beyene, who had advanced tracheal cancer and had been given 6 months to live. “He’s now doing well. He’s employed, and his family have [sic] come over from Eritrea. He has no need for immunosuppression and doesn’t take any drugs at all,” says Macchiarini. A few months later, he treated a second patient—an American named Christopher Lyles—in the same way, although Lyles later died for reasons unrelated to the transplantation.
Macchiarini now has gained approval from the US Food and Drug Administration to perform these transplants in the United States on a compassionate basis, for those patients who have no other options. “The final organ will never ever be as beautifully perfect as a natural organ,” says Macchiarini, “but the difference is that you don’t need a donation. It can be offered to a patient in need within days or weeks.” By contrast, even if a donor is found, a simple trachea can take a few months to regrow using a decellularized scaffold.
Other scientists have enjoyed similar success with other organs. In 1999, Anthony Atala of the Wake Forest Institute for Regenerative Medicine successfully grew bladders using artificial scaffolds. He subsequently transplanted them into seven children afflicted with spina bifida. By 2006, all the children had gained better urinary control. Atala has just completed Phase II trials of his artificial bladders.
Vacanti thinks that artificial scaffolds are the future of organ engineering, and the only way in which organs for transplantation could be mass-produced. “You should be able to make them on demand, with low-cost materials and manufacturing technologies,” he says. Such mass production is relatively simple for organs such as tracheas or bladders, since these are simply hollow tubes or sacs. Such tissue engineering is much more difficult for the lung or liver, which have much more complicated structures. However, Vacanti thinks it will be possible to simulate their architecture with computer models, and then fabricate them with modern printing technology, which uses inkjet technology to squirt stem cells unto three-dimension scaffolds that fit the size of the organ of interest. “They obey very ordered rules, so you can reduce it down to a series of algorithms, which can help you design them,” says Vacanti. However, Taylor says that even if the architecture is correct, the scaffold would still need to contain the right surface molecules to guide the growth of any added cells. “It seems a bit of an overkill when nature has already done the work for us,” she says.
Whether the scaffold used by tissue engineers are natural or artificial, clinicians need to seed it with patient’s cells. For bladders or tracheas, enough cells can be collected from the patient by means of a small biopsy. Unfortunately, this will not work if the organ is diseased, or if it is a complex structure composed of multiple tissue types, or, as in the heart, if its cells do not normally divide normally. In such cases, clinicians will need either stem cells, which can divide and differentiate into any cell type, or progenitor cells that are restricted to specific organs. Since 2006, one source of stem cells has been adult tissues, which scientists can now reprogram back into a stem-cell like state using just a handful of genes. Induced pluripotent stem cells or iPSCs, could then be coaxed to develop into a tissue of choice. “For me, the cells have always been the most difficult part,” says Vacanti, “and I’d say the iPSCs are the ideal solution.”