Can a Stem Cell Transplant Truly Reset the Immune System?
Miller School researchers led by Dr. Robert Levy propose adding post-transplant cyclophosphamide during immune recovery to target autoreactive cells that may survive autologous stem cell transplantation.
Key Takeaways
- Autologous hematopoietic stem cell transplantation can reduce autoreactive immune cells, but some cells may survive and contribute to relapse.
- Miller School of Medicine researchers propose administering post-transplant cyclophosphamide while surviving immune cells are rapidly expanding.
- The strategy remains a hypothesis and has not been tested in clinical trials for autoimmune disease. Future studies must examine safety, timing, dosing, patient selection and the restoration of healthy immune regulation.
For people living with severe autoimmune diseases, one of the greatest challenges is convincing the immune system to stop attacking the body it is designed to protect. Even some of the most powerful immune-modifying therapies do not always provide lasting disease control.
Researchers at the University of Miami Miller School of Medicine have proposed a new strategy that could potentially strengthen autologous hematopoietic stem cell transplantation (auto-HSCT), one of the most effective treatment options available for selected patients with certain autoimmune diseases. The approach centers on adding post-transplant cyclophosphamide (PTCy), a drug already used in donor stem cell transplantation, during the critical period when the immune system is rebuilding.
Robert Levy, Ph.D., professor of microbiology and immunology and ophthalmology at the Miller School, and Dietlinde Wolf, Ph.D., a Miller School researcher, suggest that PTCy may help reduce autoreactive immune cells that survive transplantation and could contribute to disease relapse. Multiple sclerosis serves as the model disease for the hypothesis, although the concept could eventually be explored in other autoimmune disorders.
Why do Autoimmune Diseases Return?
Autoimmune diseases occur when the immune system mistakenly attacks healthy tissues. More than 80 autoimmune diseases have been identified, affecting an estimated 5% to 10% of the global population. Conditions such as multiple sclerosis, lupus, rheumatoid arthritis and systemic sclerosis can cause lifelong disability and often require continuous treatment.
Auto-HSCT has emerged as an effective treatment option for selected patients with certain severe autoimmune diseases, particularly relapsing-remitting multiple sclerosis. Physicians collect a patient’s own blood-forming stem cells, use chemo/radiation conditioning therapy to dramatically reduce existing immune-cell populations, including disease causing auto-reactive T lymphocytes, and then return the stem cells to rebuild blood cells and immune function.
The objective of the treatment is to reset the immune system. That includes reducing autoreactive immune cells and accomplishing immune reconstitution. Yet relapse can still occur.
It’s possible that not all autoreactive immune cells are eliminated during conditioning and transplantation. Some disease-causing cells may survive and later contribute to renewed disease activity as the immune system recovers.
“Autologous stem cell transplantation can be remarkably effective at interrupting autoimmune disease. However, relapse tells us that the immune reset may not always be complete,” Dr. Levy said. “The question we’re asking is whether there is an opportunity during the early period of immune reconstitution to further reduce the autoreactive cells that survive the transplant process.”
A “Therapeutic Window” During Immune Rebuilding
Following transplantation, the immune system enters a state of lymphopenia. Immune-cell numbers are temporarily low. During recovery, the surviving T cells begin proliferating in response to signals that help rebuild the immune system. Dr. Levy and his team believe autoreactive T cells are also expanding during this phase.
Cyclophosphamide preferentially affects rapidly dividing cells. The investigators hypothesize that administering the drug several days after transplantation during the “therapeutic window” could reduce autoreactive cells that escaped the initial conditioning regimen and are actively proliferating during immune reconstitution. In theory, that could produce a more complete immune reset than transplantation alone.
“The period immediately after transplantation may represent a unique window,” Dr. Levy said. “As the immune system rebuilds, some surviving T cells begin proliferating. Administering post-transplant cyclophosphamide during this precise time could make the immune reset more complete by targeting cells that escaped earlier treatment.”
The concept builds on decades of experience using PTCy after allogeneic stem cell transplantation. In that setting, the drug helps control harmful immune responses that can cause graft-versus-host disease while still allowing hematopoietic recovery and immune reconstitution.
Frequently Asked Questions
What is autologous hematopoietic stem cell transplantation?
It is a procedure in which clinicians collect a patient’s own blood-forming stem cells, reduce existing immune-cell populations and return the stem cells so blood cells and immune function can rebuild.
Why can autoimmune disease return after transplantation?
One possible explanation is that some autoreactive immune cells survive the initial conditioning and later expand as the immune system recovers.
What role could post-transplant cyclophosphamide play?
Researchers hypothesize that administering the drug during immune recovery could target rapidly dividing autoreactive cells that survived the initial treatment.
Has this strategy been tested in autoimmune disease?
No. The proposed strategy remains a hypothesis and has not been tested in clinical trials for patients with autoimmune disease.
More Than Cell Elimination
Removing autoreactive cells is only part of the challenge, however.
Long-term disease control depends on the immune system’s ability to maintain self-tolerance (recognizing the body’s own tissues without attacking them while identifying foreign pathogens to protect against infections). The research team posits that, even if PTCy successfully reduces additional autoreactive T cells, some potentially harmful cells likely persist. As a result, restoring immune regulation may be just as important as eliminating disease-causing cells.
The researchers highlight the role of regulatory T cells, or Tregs, which help suppress inappropriate immune responses and maintain self-tolerance. They envision auto-HSCT plus PTCy as a potential platform for future immune-regulatory strategies, including approaches using novel engineered proteins that simultaneously expand Treg activity and enhance immune regulation of other components contributing to autoimmune disease.
What Comes Next
The proposed strategy remains a hypothesis and has not been tested in clinical trials for autoimmune disease patients. Important questions remain regarding safety, timing, dosing and patient selection. Cyclophosphamide can cause significant side effects, including bladder, cardiac and reproductive toxicities. Researchers will need to determine whether potential benefits outweigh those risks.
“The most important question is whether we can turn a temporary immune reset into a more durable one,” Dr. Levy said. “If future pre-clinical and clinical studies support this approach, it could open new avenues for improving long-term outcomes in autoimmune disease. We are working together with clinicians who treat patients with MS and perform hematopoietic stem cell transplants here at UM to translate our proposed strategies into the clinic. ”
For now, the question is, after a stem cell transplant attempts to reset the immune system, can researchers use the rebuilding phase to make that reset more durable? Answering that question could help inform future strategies for treating multiple sclerosis and other autoimmune diseases.
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Tags: autoimmune disease, Department of Microbiology and Immunology, Dr. Robert Levy, hematopoietic stem cells, immune system, microbiology and immunology, multiple sclerosis, ophthalmology, stem cells, transplant