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Animal transplants may one day be used to treat people with type 1 diabetes, according to a new study.

In a groundbreaking study, scientists were able to successfully transplant insulin-producing cells from one species to another, without the use of immunosuppressive drugs.

Rejection of foreign cells is a huge problem in xenotransplantation, transplants between species. However, researchers at Northwestern University have developed a new method that prevented the rejection of islets, the cells that produce insulin, in transplants between mice and rats.

"This is the first time that an interspecies transplant of islet cells has been achieved for an indefinite period of time without the use of immunosuppressive drugs," study co-senior author Stephen Miller said in a news release. "It's a big step forward."

Researchers said their ultimate goal is to find a way to successfully transplant pig islets into humans.  Previous studies have shown that pig islets produce insulin that controls blood sugar in humans.

Currently, people with hard-to-control type 1 diabetes can get transplants of insulin-producing islets from deceased donors.  However, many patients on waiting lists don't receive the transplant or suffer significant damage to their organs while they wait.

Researchers said using islets from other species could help shorten waiting lists and provide wider access to transplants for humans. Until now, concerns about rejection of transplants from a different species have made xenotransplantation unfeasible.

In the latest study, researchers were able to persuade the immune systems of mice to recognize rat islets as their own and not reject them. What's more, the new method did not require the long-term use of drugs to suppress the immune system. Researchers said this is very important because the use of immunosuppressive drugs often come with serious side effects.

The study revealed that the rat islets live and produced insulin in mice for at least 300 days, which is as long as scientists studied the mice.

Co-senior researcher Dr. Xunrong Luo noted that the barrier from rats to mice is probably lower than from pigs to humans, but the most important thing was that the latest findings showed that interspecies islet transplants are possible and without immunosuppressive drugs.

For the study, researchers had removed and treated rat splenocytes, a type of white blood cell located in the spleen, with a chemicals.  Afterwards, researchers injected the dead splenocytes into laboratory mice.  The cells then entered the spleen and liver and were taken up by scavenger cells, which processed the splenocytes and presented fragments of them on their cell surface.  Researchers said this triggered a reaction that instructed the T cells to accept the subsequently transplanted rat islets and not attack them.

However, researchers noted that rejection was still a threat.  Controlling antibody-producing B cells is crucial in preventing rejection in interspecies transplants.  When scientists initially transplanted rat islets into mice, the mouse immune system started producing antibodies against the rat cells causing rejection.

To stop the B cells from killing off the islets, researchers injected mice with B-cell depleting antibodies. Researchers said that after the transplant, the B-cells naturally returned and no longer attacked the rat islets.

"With this method, 100 percent of the islets survived indefinitely," Luo concluded. "Now we're trying to figure out why the B-cells are different when they come back."

The latest findings are published in the journal Diabetes.