Human–Monkey Chimeric Embryos: A Novel Tool for Biomedical Research

Researchers have successfully integrated human cells into animal tissue, producing human–monkey chimeric embryos that were able to grow for up to 20 days. The research, published in Cell, is a key step towards developing chimera-based tools for studying human health and disease, organ transplantation, drug discovery and many more research applications.

“A chimera is an organism that contains the cells of two or more species. Chimeras are developed using cell biology and molecular biology methods, often by inserting the cells of one organism into an early developmental stage of another,” explained Prof. Juan Carlos Izpisua Belmonte, senior author of the study.

Chimera development

Prof. Beatrice Mintz was the first person to develop and characterize mouse chimeras in the 1960s, and by the 1970s, research in the field had advanced sufficiently to enable the development of interspecies chimeras in mammals. Due to their evolutionary similarities, much of the work in this area has been performed using rats and mice, which have ~ 21 million years of evolutionary distance. Larger animal hosts with physiological properties that more closely align to that of humans are required to develop chimeras using human pluripotent stem cells (hPSCs), but historically, as Izpisua Belmonte noted in a recent press release, “the generation of human–animal chimeras has suffered from low efficiency and integration of human cells into the host species.”

The novel research published in Cell was built on previous work conducted by Izpisua Belmonte’s team: “In our 2017 study, in which we incorporated human cells into early-stage pig tissue, the contribution of human cells was fairly low, which we thought could be due to the large evolutionary distance (~ 90 million years) between the two species,” he explained. In the most recent study, the evolutionary distance is reduced. According to Izpisua Belmonte, this could explain the greater relative efficiency of integration of HSCs.

Izpisua Belmonte and colleagues harnessed a novel approach based on a modified in vitro cell culture protocol, developed by Prof. Weizhi Ji from  Kunming University of Science and Technology, that facilitates the development and survival of long-tailed macaque (Macaca fascicularis) embryos for up to 20 days.

The team labeled human extended (hE)PSCs with a fluorescent protein and injected the cells into the macaque embryos, enabling them to observe how well they integrated. The hEPSCs were indeed incorporated into the developmental program of the embryo until 19 days postfertilization. The human cells were detected in 132 embryos one day post-injection, and after 10 days, 103 of the chimeric embryos were still developing. By day 19, just three chimeras remained.

Identifying key communication pathways

To investigate molecular communication in cells comprising the chimeras, the researchers analyzed the transcriptome of the human–monkey chimeric embryos using single-cell RNA sequencing and compared this to the transcriptomes of control human and monkey embryos. The comparison revealed transcriptomic differences that the authors suggest are likely caused by the human cells impacting embryonic developmental niches within the host embryos following injection.

Izpisua Belmonte elaborated on the importance of determining key pathways involved in chimeric cell communication: “Without understanding how different types of cells communicate, it will be difficult to learn what cues human cells, tissues and organs use to develop in a healthy way. The process of two types of cells integrating is akin to communicating with different languages: human cells in pig tissue is akin to communications between two distant languages, such as Chinese and French, whereas human cells in macaques is like communications between two closely related languages, like Spanish and French.”

The team then set out to explore whether it was possible to uncover the signaling events responsible for these differences. They postulated that by modulating key signaling pathways, interspecies chimerism could be improved in future.

Several pathways were identified. The authors noted that phosphatidylinositol 3-kinase (PI3K)-Akt and mitogen-activated protein kinase (MAPK) signaling was strengthened in the human–monkey chimeric embryos and new pathways (e.g., WNT signaling) were also enriched.

Potential applications

Armed with a better understanding of the molecular communication underpinning chimeric embryos, researchers may be able to harness them as tools to explore a wide range of biomedical applications.

Izpisua Belmonte elaborated on some of their potential uses: “This work has the potential to shed light on many areas relevant for human health, from the earliest stages, when a fertilized egg begins to divide, to advanced age, which is a risk factor for many human diseases, including Alzheimer’s, heart disease and cancer. Chimeric models may offer more accurate ways to understand how human diseases develop, as well as to test drugs for those diseases.”

While human–animal chimeric embryos clearly possess great potential in many areas, it is also important to consider the ethical and legal considerations related to their use. Izpisua Belmonte discussed this point further: “This work was thoroughly and extensively reviewed both at the institutional level as well as via outreach to non-affiliated bioethicists with experience in state and national policies in this area of research. This process helped guide our experiments, which were entirely focused on ex vivo chimeric embryos.”

He continued, “Existing guidelines at the state, national and international levels vary, and keeping scientists, bioethicists, policymakers and funding agencies engaged and working together is an important step in keeping these guidelines up to date with the relevant science as well as for the welfare of society. All of our work is always directed by existing ethical and legal guidelines and will undergo review and approval by all relevant committees.”

The researchers plan to further explore the molecular pathways they identified as having a potential role in communication between cells of different species. “Next, it will be important to investigate that information and confirm which of those pathways are important for communication,” concluded Izpisua Belmonte.

Reference: Tan T, Wu J, Si C, et al. Chimeric contribution of human extended pluripotent stem cells to monkey embryos ex vivo. Cell. 2021. doi: 10.1016/j.cell.2021.03.020

Juan Carlos Izpisua Belmonte was speaking with Laura Elizabeth Lansdowne, Managing Editor for Technology Networks.