Unlocking the potential to grow human organs in animals has long been a dream in regenerative medicine, but it's faced significant scientific obstacles—until now. But here's where it gets controversial: recent research reveals that the immune response in nonhuman cells may be a key reason why human pluripotent stem cells (PSCs) struggle to survive when introduced into other species. This groundbreaking discovery not only deepens our understanding of interspecies cell compatibility but also paves the way for overcoming one of the biggest hurdles in generating transplantable organs in animals.
Imagine a future where the shortage of human organs is drastically reduced because we can grow organs inside animals like pigs or sheep. That’s the ultimate goal driving scientists at UTSouthwestern Medical Center and beyond. Their latest study, published in the journal Cell, suggests that a previously overlooked immune mechanism—called RNA innate immunity—might be responsible for the failure of human PSCs to thrive in these cross-species environments.
Jun Wu, Ph.D., an Associate Professor of Molecular Biology at UTSouthwestern and a New York Stem Cell Foundation (NYSCF)-Robertson Investigator, explains, “Our primary aim is to harness human PSCs to cultivate tissues and organs within animals, which could revolutionize transplant medicine. This research uncovers a previously unrecognized role for RNA-based immune responses in cell competition and interspecies chimerism that are barriers we need to address.” His team’s work is crucial because it reveals why human cells tend to be at a disadvantage when co-cultured with cells from distantly related species such as mice or rats. In prior experiments, human PSCs gradually deteriorated when grown with nonhuman PSCs, whereas the nonhuman cells prospered. While scientists initially thought this might be solved with genetic modifications that protect human cells from programmed death, such solutions risk causing issues in the tissues destined for transplantation.
In this latest research, Dr. Wu and his colleagues aimed to identify whether the immunity of the nonhuman cells themselves was a contributing factor. They grew mouse and human PSCs together in lab dishes and examined the gene activity of the mouse cells compared to those grown alone. The team discovered that a specific immune response pathway—known as the retinoic acid-inducible gene I-like receptor (RLR) pathway—was significantly more active in the co-cultured mouse cells. This pathway serves an important function in viruses-infected cells, detecting foreign RNA molecules and activating immune defenses.
To test if this pathway was responsible for the death of human cells, researchers shut down a key gene in the pathway called MAVS, which encodes a protein critical to the immune response. By disabling MAVS in mouse cells, they observed a remarkable increase in human cell survival. This evidence strongly indicated that the immune activity driven by RNA sensing in mouse cells was actually harming the human cells.
Further investigations showed that small quantities of RNA were exchanged between human and mouse cells, likely through tiny membrane bridges called tunneling nanotubes (TNTs). When these TNTs were blocked, more human cells survived. Additionally, injecting human cells into mouse embryos lacking MAVS resulted in significantly higher survival rates compared to those in normal mice. This suggests that the innate immune response to foreign RNA is a crucial barrier to establishing viable human-mouse chimeras.
Dr. Wu emphasizes that these findings open multiple avenues for scientific intervention. By targeting the immune pathways or interfering with RNA exchanges between cells, future strategies could significantly improve the survival of human PSCs within nonhuman hosts. This step brings us closer to the realistic possibility of growing functional human organs in animals—potentially revolutionizing transplant medicine.
As with all groundbreaking research, these findings also spark important discussions and debates. Could tweaking immune responses in host animals carry unforeseen risks? Are there ethical considerations in creating human-animal chimeras? What do you think about using immune pathway suppression as a solution? Do you agree that overcoming these biological barriers is the key to saving millions of lives or do you see potential risks we might overlook? Share your thoughts and join the conversation.
