Multi-omic profiling of paired pig liver and kidney xenografts in a human decedent

The dream of using organs from other animals to save human lives has moved from science fiction to a fragile reality. For decades, scientists have tried to bridge the gap between species, primarily by editing the DNA of pigs to make their tissues less recognizable to the human immune system. The goal is to create a supply of organs that the body will not immediately attack. However, a major hurdle remains: we do not fully understand how a pig organ behaves once it is inside a human body. Does the immune system treat a pig liver the same way it treats a pig kidney? Do the two organs react to the same internal signals in the same way, or do they have their own unique stories to tell? Answering these questions is critical because if we cannot predict how different organs will respond, we cannot safely transplant them into living patients.

To find these answers, researchers recently conducted a unique experiment using a human donor who had passed away. They took a liver and a kidney from the same genetically modified pig and transplanted them into the human body. This setup allowed them to watch two different organs from the same animal react to the same human environment at the same time. By taking frequent blood samples and carefully analyzing tissue from both organs over eleven days, the team created a detailed map of how the body and the new organs interacted. They looked at the genetic activity of individual cells, the proteins floating in the blood, and the chemical signals in the fluids the organs produced. The study revealed that while the two organs started with a similar reaction, they quickly diverged, developing distinct patterns of injury and repair that were specific to their type.

The experiment began with a pig that had been engineered with six specific genetic changes. Three of these changes removed pig proteins that typically trigger a violent immune rejection, while the other three added human proteins to help the pig cells blend in better. The researchers transplanted a pig kidney into the side of the human body and placed a pig liver next to the remaining human liver. Crucially, they connected separate tubes to drain the bile from the pig liver and the urine from the pig kidney, allowing them to measure exactly what each organ was doing without mixing the results. Over the next eleven days, the team monitored the patient's blood and collected tissue samples from the organs at specific intervals. They used advanced technology to read the genetic instructions inside millions of individual cells, distinguishing between cells that came from the pig and those that came from the human.

In the first few days, the human body reacted with a predictable surge of immune cells. The blood was filled with neutrophils, a type of white blood cell that acts as a first responder to injury, releasing chemical signals that cause inflammation. This was followed by a second wave dominated by monocytes, another type of immune cell that arrives later to clean up debris and coordinate repair. This pattern of early inflammation followed by later repair is common in many types of tissue injury. However, when the researchers looked closely at the organs themselves, they found that the two grafts did not follow the same script. The pig liver and the pig kidney, despite sharing the same genetic makeup and the same blood supply, developed very different internal environments.

In the pig liver, the immune response evolved into a specific state dominated by a type of macrophage, a large immune cell that acts as a scavenger. By the eleventh day, nearly all the immune cells inside the pig liver belonged to this specific group, which is known for helping to remodel damaged tissue. These cells were found throughout the liver, interacting with the blood vessels and the supporting structure of the organ. The liver also showed signs of a specific injury: a blood clot had formed in a major vein, leading to a lack of oxygen in that area. The cells in this damaged region showed signs of stress and hypoxia, but the liver continued to produce bile almost immediately after the surgery. The chemical makeup of this bile quickly shifted to resemble the bile of the human host, suggesting the liver was rapidly adapting to its new environment.

The pig kidney told a different story. While it also faced immune pressure, the cells inside did not settle into the same uniform state as the liver. Instead, the kidney developed distinct zones of injury. The inner part of the kidney, called the medulla, showed the most severe damage, with many of the tiny tubes that filter urine becoming injured and losing their normal function. In these damaged areas, a different type of immune cell appeared, one that is often associated with chronic inflammation and scarring. Unlike the liver, which started working almost immediately, the kidney took longer to produce urine. When it finally did, it was producing nearly two liters a day, yet the tissue inside still showed strong signs of injury and stress. This mismatch suggested that the kidney could produce fluid even while its internal machinery was still struggling to recover.

The most striking finding was that the two organs, though they came from the same animal and lived in the same body, were essentially having two different conversations with the immune system. The liver developed a response focused on remodeling and adapting to a blood clot, while the kidney developed a response centered on repairing damaged tubes and managing inflammation in its inner regions. The immune cells inside the liver were different from those inside the kidney, even though they were both reacting to the same human host. This means that a single genetic fix for a pig organ might not be enough to protect all organs. An organ that works well in the liver might fail in the kidney, and vice versa.

The researchers also looked at the proteins and chemicals in the blood to see if they could predict what was happening inside the organs. They found that the blood contained signals from both the pig and the human, but these signals did not tell the whole story. For example, the blood showed signs of inflammation, but it could not reveal that the liver was adapting quickly while the kidney was still struggling with deep tissue damage. This highlights the importance of looking directly at the organs themselves rather than relying solely on blood tests. The study suggests that future efforts to make pig organs safe for humans will need to be tailored to each specific organ. Just as a key fits one lock but not another, the genetic modifications needed to protect a liver might be different from those needed to protect a kidney.

This experiment, conducted on a human donor, provided a rare and detailed look at the early days of a pig-to-human transplant. It showed that the journey of a transplanted organ is not a single, uniform event but a complex, organ-specific process. The liver and kidney responded to the same environment in their own unique ways, developing different patterns of injury and repair. While the pig organs were able to function for a time, the differences in their responses suggest that there is still much to learn about how to make them last. The findings offer a roadmap for the future, pointing scientists toward the need for organ-specific solutions that address the unique challenges of each type of tissue. By understanding these differences, researchers can move closer to a future where pig organs can reliably save human lives.