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Summary
Heterologous organ transplantation is an effective way to replace organ function, but is limited by a severe organ shortage. Although generating human organs in other large mammals through embryo complementation would be an innovative solution, it faces many challenges, especially the poor integration of human cells into recipient tissues. To produce human cells with superior competitiveness within the niche, we combined optimized pluripotent stem cell culture conditions with inducible overexpression of two pro-survival genes (MYCN and BCL2). The resulting cells had substantially improved viability in the xeno environment of cross-species chimeric blastocysts and successfully formed organized human-pig mid-stage kidney (mesonephros) chimeric structures up to embryonic day 28 within pig embryos with nephritic defects that they lack SIX1 and SALL1. Our findings demonstrate proof of principle for the possibility of generating a humanized primordial organ in organogenically disabled pigs, opening an exciting avenue for regenerative medicine and an artificial window to study human kidney development.
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For the first time, a solid humanized organ has been grown from scratch in an animal, a first step in a process that could potentially solve the organ shortage and save countless lives. Chinese researchers have grown early-stage partially human kidneys inside pig embryos, using a variety of genetic engineering techniques, a new report reveals.
"This study demonstrates proof-of-principle for the possibility of generating a humanized organ in pigs, which opens up exciting possibilities before us," said lead researcher Liangxue Lai. He is deputy director of the South China Institute of Stem Cell Biology and Regenerative Medicine at the Guangzhou Institutes of Biomedicine and Health.
The researchers generated a series of five humanoid kidneys in pigs that had reached the mesonephros stage , the second stage of kidney development, after 25 to 28 days of development. About 50% to 70% of the cells in these early-stage kidneys were human, Lai said.
"If the gestation time were extended, the proportion of human cells may increase, but there may also be other technical barriers," Lai said. "We are working on that."
Massimo Mangiola, director of the Immunogenetics Laboratory at New York University Langone Transplant Institute, referred to these as "primordial organs." "They clearly showed that they can make human cells grow into primordial organs," said Mangiola, who was not involved in the research. "That hasn’t been done before. It’s a little early to say whether or not this will become a complete human kidney, but this was the necessary initial step toward that goal."
Previous research has used similar methods to generate human tissues such as blood or muscle in pigs, but this is the first to grow a partially human organ.
The researchers focused on kidneys because they are one of the first organs to develop and are also the most commonly transplanted organ in humans, according to background notes.
In this effort, the researchers first created genetically modified pig embryos that were missing two genes that are key to kidney development. This created an empty space (a kidney-shaped "niche") within the embryos that human stem cells could fill with less competition from pig cells.
One of the challenges of integrating human stem cells into pig embryos has been that pig cells outcompete human cells during development, the study authors noted.
Next, the researchers engineered human stem cells to make them more likely to integrate into pig embryos and less likely to self-destruct. These cells were then grown into "naïve" cells that resemble early human embryonic cells.
Pig embryos containing human stem cells were then grown in the laboratory, using unique nutrients and hormones to promote the growth of human and porcine cells, before the researchers implanted the developing embryos into surrogate pigs. In total, researchers transferred 1,820 embryos to 13 surrogate mothers, according to the study.
After 25 to 28 days, the researchers terminated the gestation and removed the embryos to see if they had successfully produced humanized kidneys. The researchers collected five pig embryos that had structurally normal kidneys for their stage of development.
The researchers discovered that genetic engineering had managed to localize the development of human cells mainly in the kidneys, while the rest of the embryo was made up of pig cells.
The study findings were published in the journal Cell Stem Cell .
"We found that if a niche is created in the pig embryo, then human cells naturally enter these spaces," lead author Zhen Dai of the Guangzhou Institutes of Biomedicine and Health said in a journal news release. . "We saw only very few human neural cells in the brain and spinal cord and no human cells in the genital ridge, indicating that human pluripotent stem cells did not differentiate into germ cells." This slight proliferation could be further prevented by knocking out more genes in human stem cells in future studies, the researchers said.
The next step is to allow the kidneys to develop longer, to see if the human cells continue to outcompete the pig cells during development. Researchers are also working to generate other human organs in pigs, including the heart and pancreas.
The long-term goal is to create human organs raised in pigs ready for transplant, but the researchers acknowledged that the work will be complex and could take many years.
"Because organs are not made up of a single cell lineage, to have an organ where everything comes from the human, we would probably need to engineer the pigs in a much more complex way and that also brings some additional challenges," said the lead author. . Miguel Esteban of the Guangzhou Institutes of Biomedicine and Health said in the press release.
Scientists will need to find a way to completely displace competing pig cells if they want to grow a fully human kidney inside a pig, Mangiola said. "They need to overcome two barriers," she said. "One is: Can you create a fully functional organ? And the second barrier would be: Can you create an organ that is made entirely of human cells?"
Any pig cells left in the kidney increase the likelihood of rejection of the organ after transplant, he said. The ultimate goal would be to develop a custom-made kidney composed of the patient’s own cells in a pig, Mangiola said. "My immune system will see it as its own and won’t reject it," she explained.
But there is another potential drawback: the possibility of porcine viruses growing in human cells, Mangiola added. "Some of these viruses don’t create much of a problem in the pig, but of course when they’re in our body, they could jump species and actually create some new pandemic, which is one of the concerns of the [Food and Drug Administration. US Drugs] in terms of xenotransplants," he said.
What does this mean
If successful, this process could help kidney patients waiting for organ donation by allowing human kidneys to be grown in pigs.
