Genetic Engineering Hides Donor Organs from Host Immune System

Worldwide, more than three million people lose their lives each year to lung-damaging conditions like chronic obstructive pulmonary disease or cystic fibrosis.¹ In contrast, fewer than 5,000 individuals per year are fortunate enough to receive life-saving lung transplants.² Transplant recipients still face many challenges, however, including a lifetime of immunosuppressant medications with potentially severe side effects and a more than 40 percent transplant failure rate in the first five years.³

In a recent study in Science Translational Medicine, transplantation immunologist Rainer Blasczyk and his team at the Hannover Medical School presented a potential solution to these problems in a minipig model.⁴ Instead of administering immunosuppressants, which Blasczyk likened to blinding the patient’s immune system, leaving the individual vulnerable to infections and even certain types of cancer, the researchers suppressed key immune proteins in the donor lung, rendering it immunologically invisible.⁵

Modifying the organ instead of the recipient isn’t a new idea, but it is an important one, said Jeffrey Platt, a transplantation biologist at the University of Michigan Medical School who was not involved in this work. “If you can introduce something that will affect the donor organ, then you can preserve the immune system of the recipient. And in lung transplantation, that’s really important because the lung is one of the first targets of infectious organisms.”

Except in transplants between identical twins, however, some kind of immune manipulation is necessary. After a transplant, host immune cells can easily spot the donor organ thanks to cell surface molecules called major histocompatibility complex (MHC) proteins. These are also known as human leukocyte antigens (HLA) or swine leukocyte antigens (SLA), depending on the species, and are encoded by genes that are highly variable between individuals. Differences between the donor and host MHC molecules allow the immune system to recognize the donor organ as non-self; greater MHC mismatches provoke a stronger immune response and increase the risk of rejection. Therefore, the researchers reasoned that reducing the organ’s MHC expression might help it evade immune attack.

Completely eliminating MHC expression, however, would create its own set of problems. “[The antigens] are not made for making transplantation more complicated,” said Blasczyk. Instead, they provide the immune system with crucial information about what’s happening within the cell.

“When these antigens are not present on a cell anymore, the immune system thinks that something has gone wrong with the cell, and that the cell doesn’t want to be surveyed by the immune system anymore,” he said. These MHC-less cells are then targeted for destruction by natural killer cells.

Blasczyk and his team needed a strategy that would allow them to reduce, but not eliminate, SLA expression in the donor minipig lungs. They also needed a treatment that they could administer quickly since an organ remains viable for a limited time after being removed from the donor organism.

During the lung’s brief time ex vivo, the researchers used lentiviral vectors to introduce short hairpin RNAs (shRNAs) that targeted mRNA transcripts crucial for the expression of SLA proteins and triggered their destruction. By intentionally designing imperfect shRNA sequences, the researchers ensured that some transcripts could sneak through, resulting in residual levels of SLA expression.

Blasczyk and his team previously demonstrated the effectiveness of this strategy in ex vivo minipig lungs.⁶ After this proof-of-concept study, it was time to test how the organs fared when transplanted into SLA-mismatched minipigs. One group of minipigs received shRNA-treated lung transplants, while a control group received an unmodified transplant. Following surgery, both groups received immunosuppressants for 28 days, after which the drugs were withdrawn.

Five of the seven pigs in the treatment group survived for the duration of the two-year monitoring period. In contrast, all seven pigs that received unmodified organs died within three months. In keeping with these findings, pigs in the treatment group had improved immunological profiles: Seventy days after the transplant, they had fewer donor-specific antibodies, their T cells were less reactive to donor cells, and they had lower levels of granzyme B, a protein that promotes cell death.

This research may represent a step towards improving the process of lung transplantation by reducing rejection risk as well as reliance on potentially harmful immune-supressing drugs. However, Platt expressed some reservations about how this work might translate to human patients. “The fact that you don't get rejection here with this manipulation—yes, that's significant and it may be the first report of something that could ameliorate or prevent [transplant rejection]. But the barrier is lower than it would be for human-to-human transplants and therefore it may not really predict the kind of outcome that you would see in a human.”

Indeed, a recent genetic analysis of this minipig breed concluded that the data pointed towards “a restricted SLA diversity in this pig breed, which could be a limiting factor in later mismatch donor allotransplant studies.”⁷ And while the genetically modified lungs displayed long-term survival, Platt noted that, “Control lungs also survived far longer than one might expect after cessation of immunosuppression.”

“Still, there are several aspects of the paper I consider important,” he noted. “One is the enduring effect achieved with the lentiviral transfer...That could, for example, make it possible to genetically repair organs ex vivo or to introduce or suppress expression of other genes.”

Overall, said Platt, this study sought to address a pressing issue in medicine. “The problem of lung failure is a very big one…and transplantation could solve that problem. That would have an enormous impact on public health.”

Disclosure of conflicts of interest: Rainer Blasczyk holds a patent entitled “Method for genetically modifying a vascularized tissue" and is the founder and CEO of the start-up Allogenetics.

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