There were approximately 36,528 organ transplants in 2018, according the U.S. Department of Health and Human Services. Transplant rejection happens when a recipient’s immune system attacks the transplanted organ or tissue. Traditionally, the only way to determine if there has been serious damage to a transplanted organ because of rejection is through a biopsy.
The new method’s particles are able to send a fluorescent signal to the urine of a transplant recipient. The researchers have designed the sensor using highly biocompatible components and tested the method in mouse models.
A nanoparticle sensor in the detection method is able to detect a T cell weapon called granzyme B, which turns the transplanted organ’s cells against itself in a process known as apoptosis.
“Before any organ damage can happen, T cells have to produce granzyme B, which is why this is an early detection method,” Gabe Kwong, a researcher on the study, said in a press release.
The nanoparticles are crafted together in the middle like a ball using iron oxide. The are double-coated with dextran and polyethylene glycol to help keep the body from getting rid of it too quickly.
“This is sensitive enough to possibly detect budding rejection before you see significant injury to the transplanted organ and that could help clinicians treat early to prevent damage,” Dr. Andrew Adams, a researcher on the study and an associate professor of surgery at Emory University School of Medicine. “Right now, most tests are aimed at organ dysfunction, and sometimes they don’t signal there is a problem until organ function is below 50%.”
Amino acid bristles stick out from the iron ball with fluorescent reporter molecules attached at the tip.
The particles are then injected intravenously. They are small enough to accumulate in tissues of struggling transplanted organs to watch for rejection, according to the researchers. The particles are too big to accumulate in native tissue or to pass through kidneys and out of the body.
Once the T cells start making granzyme B, amino acid strands in the organ’s cells are cut off and the cells can begin to die.
“The nanoparticles’ bristles mimic granzyme’s amino acid targets in the cells, so the enzyme cuts the bristles on the nanoparticle at the same time,” Kwong said. “That releases the reporter molecules, which are so small that they easily make it through the kidney’s filtration and go into the urine.”
In animal tests, the urine glows and was able to be seen in bladders in near-infrared images.
The researchers now plan to use the sensor to detect other causes of transplant rejection like attacks by antibodies.
“Antibodies kill their target cells through similar types of enzymes. In the future, we envision a single sensor to detect both types of rejection,” said Kwong.
Kwong and the researchers tested the urine test mouthed in small skin grafts on mice and were able to get a very clear timely signal from the nanoparticle sensor.
“This method could be adapted to tease out multiple problems like rejection, infection or injury to the transplanted organ,” Adams said. “The treatments for all of those are different, so we could select the proper treatment or combination of treatments and also use the test to measure how effective treatment is.”
Biopsies are the current method for detecting if an organ transplant is working. However, biopsies can go wrong and the needle can damage tissue.
“This biggest risk of a biopsy is bleeding and injury to the transplanted organ,” Adams said. “Then there’s the possibility of infection. You’re also just taking a tiny fraction of the transplanted organ to determine what’s going on with the whole organ, and you may miss rejection or misdiagnose it because the needle didn’t hit the right spot.”
The researchers suggest that a urine test could have a more global reading on an entire organ while offering other advantages over biopsies.
“The biopsy is not predictive. It’s a static snapshot. It’s like looking at a photo of people in mid-jump. You don’t know if they’re on their way up or on their way down. With a biopsy, you don’t know whether the rejection is progressing or regressing,” Kwong said. “Our method measure biological activity rates, and tells us where things are going.”
Measuring biological activity could also allow clinicians to dose powerful immunosuppressant medications that most transplant patients receive.
“Adjusting the dose is very difficult but very important because heavy immunosuppression increases occurrence of infections and patients who receive it also get cancer more often,” Kwong said.
The research was published in the journal Nature Biomedical Engineering and was funded by the National Institutes of Health, the National Science Foundation and the Burroughs Wellcome Fund.