Sneaking Organs Past the Immune System

How to prevent those lucky enough to receive a transplant from losing them

The diagnosis comes in: patient X has end-stage renal failure. His kidneys no longer work, and he has the choice of either staying hooked up to a machine for dialysis treatment a few times each week, or obtaining a transplant organ. Luckily, he is able to receive a donated kidney—hard to come by.

But a new diagnosis comes in three months after the surgery: his new kidney is failing. His body has rejected the new organ, and his immune system is slowly eating away at it. Once again, he is forced to begin dialysis treatment, depending on a blinking machine to carry out the same function that his kidneys used to do.

This story is not uncommon: around fifteen to twenty percent of kidney transplants fail within five years of transplantation. Currently, the main way physicians attempt to decrease the rejection rate is by giving patients drugs that suppress the immune system, thereby reducing the body’s ability to attack and reject the transplant.

But Yale researchers are seeking to develop a new way to reduce immune rejection. A long-standing study done by Yale professors Mark Saltzman and Jordan Pober in collaboration with researchers at Cambridge University seeks to use carefully designed, tiny nanoparticles to deliver drugs to transplant organs before they are placed in the body. They hope their process will improve long-term outcomes for transplant patients. 

America’s Transplant Problem

The organ transplant waiting list is a national list compiled by the United Network for Organ Sharing (UNOS), a non-profit established to manage the federal organ transplant system and to objectify the complex matching process between donors and recipients. Factors such as a patient’s medical urgency, the compatibility between donor and recipient, and the time on the waiting list guide how organs are distributed.

Despite this, there are around 116,000 people waiting for a vital organ transplant, while in 2017 there were only about 35,000 organ transplants. The average wait time for a liver transplant is around 11 months; for a kidney transplant that number increases to 5 years.

Even after a patient receives a transplant, there’s still no guarantee that their new organ will prove an effective treatment. In the case of a bad match between the recipient and donor, the recipient’s immune system will recognize the new organ as foreign and will attack it, sometimes damaging it irreversibly. Though transplant rejection can be minimized using drugs that suppress the immune system, these immunosuppressant drugs can make the patient more susceptible to other diseases.

Patients need a more targeted approach to prevent the immune system from destroying the transplant, but also allow normal immune function to occur—perhaps through a delivery system of some sorts. Enter Professor of Biomedical Engineering Mark Saltzman, who has long worked on using nanoparticles to create better drug delivery systems.

Small but Mighty

What is the smallest object visible to the human eye? Those with imperfect vision might squint to see the words on a page in front of them. Others might say a human hair, just 0.1 millimeters wide, or 10 percent of the width of your typical credit card. The typical human cell is far smaller than what the eye can see—around ten cells can fit in the thickness of a single average hair. But on the nano-scale, thousands to hundreds of thousands of tiny nanoparticles can fit across a hair.

Science has turned its focus to nanoparticles as a potential drug delivery mechanism because of their size. Because they are so tiny, they not only have a comparatively large surface area available for reactions, but also are able to cross cell and tissue barriers that current delivery systems cannot, making them a more efficient system for drug delivery.

The key is finding ways to engineer nanoparticles to target specific cells, such as delivering growth-suppressing drugs to tumors in cancer patients. Nanoparticles can be designed with specific properties to increase their effectiveness—for instance, by giving them a positive charge to interact better with the drug they are carrying. Antibodies that recognize and bind to characteristic targets on the cell types of interest can also be attached to the surface of the nanoparticles to make the delivery more specific.

In a typical transplant organ, the circulating blood from the host primarily interacts with cells that line the blood vessels of the transplant organ called endothelial cells.  White blood cells, the fighters of the immune system, are found in the blood and interact with major histocompatibility complex (MHC) proteins found on the surface of the endothelial cells. There are thousands of slightly different MHC molecules in the population but each person only inherits a handful of different types from their parents. If an MHC protein not typically produced by the body is found, then the white blood cells hone in on those cells and initiate inflammatory responses, which can then kill the cells of the transplant organ. 

Some approaches now aim to mask the transplant organ from the immune system by decreasing the amount of MHC protein recognizable as foreign. Jordan Pober, Professor of Immunobiology at Yale, has long been interested in the role of endothelial cells in the immune response. Together with Saltzman, he worked on a project in which MHC protein was deleted in a mouse with a transplanted human artery by delivering molecules called small interfering RNAs (siRNAs) through a nanoparticle delivery system. This effectively prevented the immune system from attacking the transplant and allowed the new organ to heal.

But another problem with current drug delivery systems is that injecting drugs into the bloodstream may not get to the target at sufficient levels to be effective. Thus, Pober and Saltzman began collaborating with researchers from the University of Cambridge to create a system able to treat organs to improve long-term outcomes, before they are even transplanted into the body.

Targeting From the Start

Ex vivo normothermic machine perfusion (NMP) is a mouthful to pronounce, but it may be the key to improving transplant outcomes and increasing the number of transplant organs available. The process involves pumping warm blood at body-temperature through a transplant organ outside of the body, keeping the organ alive for longer and helping to repair damage to the organ. This allows even organs that previously did not seem viable to become suitable for transplantation.

“We thought, if these organs were being perfused for one to two hours, we could treat them in other ways to make them less prone to rejection,” Saltzman said.

The researchers decided to target CD31, a protein found on all endothelial cells. Nanoparticles coated with antibodies able to recognize CD31 were injected into the perfusion device while blood passed through a donor kidney, along with a non-targeted set of nanoparticles without the antibodies. The results, a colorful set of images showing where each set of nanoparticles accumulated, indicated that targeted particles could accumulate to levels two to five times higher than in the control group, whereas some areas showed profound targeting with levels up to ten times higher than the control.

“That was one surprise. No one ever looked at where particles go in a human organ before at this level,” Saltzman said. “It allowed us to make some hypotheses about what would give you the best distribution through the kidney.”

But by effectively showing that nanoparticles can be targeted to the endothelial cells of an organ through machine perfusion, the researchers are one step closer to engineering a drug delivery system that can use machine perfusion to improve transplant outcomes.

Combining Past and Present

There is still far more to do, and the researchers have received a new grant from the National Institutes of Health to work on the project. “It’s a new area. We’re treating human organs outside the body, and [there are] studies we need to do to show this is safe before we can use them in humans,” Saltzman said. “But I love the mystery.”

Right now, the research on machine perfusion has shown that scientists can target endothelial cells, but getting to the right targets—the cells on the transplant organ— is another question. “Now the question is can we improve the delivery, but also can we choose the right targets,” Pober said. Saltzman and Pober hope to combine their research on knocking down MHC proteins in the transplant organ with their research on machine perfusion, in hopes of creating a new transplant treatment system to improve outcomes.

“There just aren’t enough organs. Now there are two things you can do about that: one, to have people who have [transplants] to keep from losing them, and second, using tissue engineering to keep [transplants] from the invading immune system,” Pober said. They hope to decrease the frequency of the first.

In the future, hopefully cases like Patient X will be far less common through the help of treatments being developed by Saltzman and Pober’s labs. And with an increase in viability of organs, perhaps the organ transplant waiting list will decrease and more people will receive life-saving treatments. 

About the Author

Sonia is a current senior in Jonathan Edwards College majoring in Biochemistry and Economics. She used to be managing editor and news editor for the Yale Scientific, and looks forward to writing more for them this semester. She currently works in the Joan Steitz lab on microRNA degradation.

Optional Reading

1. Tietjen, Gregory T., et al. “Nanoparticle targeting to the endothelium during normothermic machine perfusion of human kidneys.” Science translational medicine 9.418 (2017): eaam6764.

2. Cui, Jiajia, et al. “Ex vivo pretreatment of human vessels with siRNA nanoparticles provides protein silencing in endothelial cells.” Nature Communications 8.1 (2017): 191.