Humanized Kidneys Grown in Pig Embryos in World-First

Researchers have grown early humanized kidneys in pig–human embryos at 28 days of gestation – the first time a solid humanized organ has been grown in another species. The research, which has implications for our understanding of organ development as well as for transplant medicine, is published in Cell Stem Cell.

Shortages of transplantable kidneys

Kidneys are one of the first organs to form in the developing embryo, making them a tantalizing subject for the study of human organ development. They are also one of the most commonly transplanted organs with over 25,000 procedures performed across the US in 2022.

However, more than 90,000 people are on waiting lists for donor kidneys and almost 5,000 patients die on these lists each year due to shortages in suitable kidneys.

Alternatives to living and deceased donor transplants are now being explored by scientists in the form of human organs grown within other species of large mammals. For example, using gene editing techniques and human stem cells to produce embryos containing both human and animal cells, known as chimeras.

The first pig–human chimeras were developed in a milestone study in 2017 amid ethical concerns leading to restrictions preventing their development beyond the fetal stage. Although chimeras have been used to produce organs of other species, the current study – led by researchers from the Guangzhou Institutes of Biomedicine and Health – is the first to produce solid human organs inside another species.

“Rat organs have been produced in mice, and mouse organs have been produced in rats, but previous attempts to grow human organs in pigs have not succeeded,” said Dr. Liangxue Lai, senior author of the study and deputy director of Southern China Institute of Stem Cell Biology and Regenerative Medicine. “Our approach improves the integration of human cells into recipient tissues and allows us to grow human organs in pigs.”

Generating an interspecies chimera

The researchers had to take three key steps in developing the pig–human chimeric embryos owing to the different biological needs of pig and human cells.

They first removed two genes from pig embryos to prevent kidney development. Next, they grew human stem cells – which have the potential to grow into almost any human cell type – under special conditions to make them resemble embryonic cells. These were also genetically engineered to temporarily shut down a process called apoptosis, essentially the cells’ “self-destruct” button, to increase their chances of survival when added into the pig embryo. Lastly, the combined pig–human embryos were cultured with nutrients to cater for the needs of both species before their transfer into the surrogate sows.

A total of 1,820 embryos were transferred to 13 sows. At either 25 or 28 days of gestation, the embryos were terminated and extracted to see if they had begun developing humanized kidneys. However, only 5 embryos were obtained for analysis – 2 at 25 days and 3 at 28 days.

The few embryos obtained did, however, have structurally normal kidneys for their stage of development. They were composed of 50–60% human cells and had begun to form early structures that would eventually connect them to the bladder.

Additionally, research of this type also needed to investigate ethical concerns over the prospect of human cells being found in other tissues such as the nervous system or germ cells (sperm or egg cells). The researchers found that most human cells were localized to the kidneys and that the majority of the embryo was indeed composed of pig cells.

“We found that if you create a niche in the pig embryo, then the human cells naturally go into these spaces,” said the study’s senior author Dr. Zhen Dai. “We saw only very few human neural cells in the brain and spinal cord and no human cells in the genital ridge, indicating that the human pluripotent stem cells did not differentiate into germ cells.”

Therefore, the surviving embryos that were recovered showed little contribution of human cells outside the kidney, while “those embryos with excessive contribution to other tissues, including the placenta, [tended] to degenerate,” the authors write in the paper.

A window into human development

Now the initial growth of humanized kidneys in these chimeras has been achieved, the researchers are next aiming to study their growth over a longer duration while working on generating other organs such as the heart and pancreas.

The researchers also acknowledge that the use of this technology to produce human organs for transplant is a lofty goal that will take many more years to achieve, but in the meantime this technology could also provide insights into studying organ development and developmental diseases.

“Before we get to that late state of making organs that can be on the shelf for clinical practice, this method provides a window for studying human development,” said Dr. Miguel A. Esteban, co-senior author of the study and a professor at the Guangzhou Institutes of Biomedicine and Health. “You can trace the human cells you’re injecting and manipulate them so that you can study diseases and how cell lineages are formed.”

Additionally, kidneys are composed of several different cell types, and the researchers created a niche for only one subset of kidney cells, meaning the cells making up their blood vessels were pig-derived – a potential cause of rejection in a transplant scenario.

Reference: Wang J, Xie W, Li N, et al. Generation of a humanized mesonephros in pigs from induced pluripotent stem cells via embryo complementation. Cell Stem Cell. 2023. doi: 10.1016/j.stem.2023.08.003 

This article is a rework of a press release issued by Cell Press. Material has been edited for length and content.