International Business Department           Liu Bojia           September 8, 2023

  For many patients, organ transplants are the last hope of saving a life. However, organ transplants face a serious shortfall worldwide. The World Health Organization estimates that fewer than 10 percent of those who need an organ transplant actually receive one. Many more people cannot wait for a suitable organ until the end of their lives.

  Among the potential solutions to the organ transplantation problem, xenotransplantation seems to be receiving more attention.In 2022, the first pig heart transplant into a human was performed. After receiving a genetically edited pig heart transplant, the patient died after surviving for 60 days. Meanwhile, pig kidney transplant trials have been conducted in brain-dead humans. These breakthroughs show the potential of xenotransplantation, but the risks of long-term immune rejection, viral infection, and causing dysfunction of porcine organs in humans still cannot be ruled out.

  Another promising solution, which could go in the opposite direction, is xenogeneration - the "growing" of transplantable human organs in animals. To do this, scientists would need to inject human pluripotent stem cells into the embryo of another mammal, and through such embryonic compensation techniques, human organ tissue would be grown in the mammal.

  Previously, scientists have achieved xenogamous chimera culture of organs such as pancreas and kidney between mice and rats. However, there are huge challenges to grow human organs using this technique.

  Pigs have become a popular target for "growing" human organs due to their similarities in physiology, organ size, and embryonic development, and a 2017 study produced the first human-pig chimeric embryos that developed in pigs for 3-4 weeks, but their chimeric efficiency was low. A major difficulty is that to obtain highly chimeric embryos, human-derived cells have to compete over pig cells in blastocysts; however, in real-world scenarios, porcine embryos can utilize cellular competition mechanisms to eliminate foreign human stem cells, making it difficult to grow human organs in a distantly related species.

  On September 7, a recent study published in Cell Stem Cell made an important breakthrough! The research team solved the above obstacles through a series of innovative means, and successfully cultured a human mesonephric kidney (an intermediate stage of kidney development) from a human-pig chimera embryo transplanted into a sow. 28 days later, the mesonephric kidney possessed the normal kidney structure, and the tubules and other functional structures were formed successfully. This is the first time for scientists to realize the xenogeneic in vivo regeneration of functional parenchymal organs of human origin, which opens up a brand-new path for the development of regenerative medicine and the study of kidney development. Dr. Lai Liangxue, Dr. Dai Zhen, Dr. Miguel A. Esteban, and Dr. Pan Guangjin of Guangzhou Institute of Biomedicine and Health (GIBH), Chinese Academy of Sciences (CAS), are the co-corresponding authors of the paper.

  The kidney is one of the earliest organs to develop in the human body and one of the organs in greatest demand for transplantation in the medical field. During development, there are three successive stages: the anterior, middle and posterior kidneys. With the degeneration of the mesonephros, the hind kidney originates from the ureteric bud and the hind renal mesenchyme.

  Previous studies on kidney development in different mammals have identified two genes that play key roles in the above kidney development process: SIX1 regulates the formation of mesonephric tubules and the branching of the ureteric buds during postnephrogenesis; and SALL1 maintains renal unit progenitors and neonatal renal units in the postnephric mesenchyme.

  Therefore, if these two genes are knocked out by genetic engineering, SIX1/SALL1-deficient porcine embryos will produce ecological niche vacancies in the kidney, manifesting as defective mesonephric development and absence of the hind kidney. In such an environment, foreign human pluripotent stem cells avoid competition with pig cells.

  It was not enough to modify pig embryos; the team also modified human pluripotent stem cells. By overexpressing the pro-survival genes MYCN and BCL2, the apoptotic program could be temporarily shut down, making them less likely to self-destruct, thus greatly enhancing the competitive ability of human pluripotent stem cells. In addition, the team developed a novel culture medium that allows human pluripotent stem cells to return to earlier stages of development, significantly enhancing the efficiency of cross-species chimerism. Before transplanting the chimeric embryos into sows, the researchers cultured the chimeras under optimal conditions to provide the human and pig cells in them with the nutrients and signaling molecules they each need.

  Based on this series of modifications, the research team transplanted chimeric embryos into 13 sows, six of which were successfully fertilized. After 25 or 28 days, the researchers terminated the pregnancies and removed five normally developing chimeric embryos (two of which were collected at 25 days post-transplantation and the other three at 28 days) to assess whether the chimeras had successfully produced human kidneys.

  Analysis showed that these mesonephric kidneys had a normal structure with up to 70% human-derived cells. By this time the mesonephric tubules and ureteric buds had formed, the latter eventually developing into a ureter that connects to the bladder.

  The team also found that the human-derived cells in the embryo were mainly localized in the kidney ecological niche they had constructed, while the rest of the embryo was still composed of pig cells. The researchers found only a very small number of human-derived nerve cells in the brain and spinal cord; no human cells were found in the germinal crest, suggesting that human pluripotent stem cells did not differentiate into germ cells. The results also support the ethical feasibility of the strategy. By knocking out more genes in human pluripotent stem cells, there is hope that the risk of human-derived cells appearing in the nervous and reproductive systems can be further avoided, the researchers said.

  The study thus confirms the ability to grow human-derived mesonephric organs in kidney-deficient pigs by modifying human pluripotent stem cells and optimizing embryonic compensation techniques. Although the pregnancy was terminated prematurely, the function of the removed mesonephric kidney suggests that this strategy has the potential to "grow" physiologically functional human kidneys in newborn pigs, providing an attractive alternative to overcome the shortage of organ transplant donors.

  The study also points out that a great deal of work remains to be done to realize the ultimate goal of human organ transplantation. In addition to further avoiding the ethical issues mentioned above, future research needs to address the high failure rate of porcine embryo development. In addition, because organs are made up of many different types of cells and tissues, growing fully functional humanized organs in pigs requires a number of additional steps. In this study, the researchers created favorable ecological niches for only one subpopulation of cells, which means that the vascular cells of the kidneys are still of porcine origin, which could lead to immune rejection of the organ if used for organ transplantation. And before eventual use in organ transplantation, the technique also offers new opportunities to study human organ development, tracking disease during development, and cell lineage formation.