Novel strategy can make stem cell transplants safer

Stem cell transplantation (also called bone marrow transplantation) and gene therapy are among the most powerful curative approaches for blood diseases such as sickle cell disease, b-thalassemia, immune deficiencies, and some blood cancers. Replacing or correcting the blood-forming stem cells can offer the possibility of long-lasting benefit or a cure. However, before patients can receive these therapies, they usually need intensive and often toxic chemotherapy or radiation to clear space in the bone marrow for the new stem cells.

In a new study published today in Nature, a researchers describe a new strategy to make stem cell transplants safer by replacing chemotherapy-based treatment with a more targeted approach. Instead of using toxic agents that damage DNA throughout the body, they use antibodies that recognize specific markers on blood-forming stem cells. These antibodies can help clear the patient's existing stem cells from the bone marrow in a more selective, less toxic way.

An antibody normally cannot distinguish between the patient's original stem cells and the infused therapeutic stem cells from the treatment. If the antibody remains in the body, it may also attack the transplanted cells, preventing them from integrating. Researcher Pietro Genovese, PhD, of the Dana-Farber/Boston Children's Cancer and Blood Disorders Center, and his team solved this problem by giving the therapeutic stem cells a form of molecular protection.

Using precise genome-editing tools, they changed a tiny recognition site, or "epitope," on the surface of the donor stem cells. This small change prevented the antibody from binding to the therapeutic cells, while preserving the normal function of the protein. In simple terms, the edited stem cells were given a molecular camouflage: they could hide from the antibody, while the unedited cells remained vulnerable.

The results show that protected stem cells can survive antibody treatment, integrate in the bone marrow, and enrich gradually over time. This creates a new way to not only make room for transplanted cells, but also selectively favor the therapeutic cells after transplantation. The approach combined therapeutic editing of blood stem cells to increase fetal hemoglobin, a protective form of hemoglobin that can compensate for the defective adult hemoglobin found in sickle cell disease and b-thalassemia.

By avoiding chemotherapy, we can open up stem cell transplants for diseases that are less severe or for fragile patients normally too sick or too high risk for transplantation. Typically, bone marrow transplants are reserved for patients with life-threatening diseases but are simultaneously limited to those patients who can tolerate the chemotherapy."

Gabriele Casirati, MD, instructor in Genovese's lab and first author of the study

This work could have implications for the future of both stem cell and gene therapy. First, it may help enable chemotherapy-free or chemotherapy-sparing transplantation approaches, reducing the burden of treatment for patients who currently face the risks of DNA damage. Second, because the antibody can continue to select for protected cells after transplantation, the strategy could help therapeutic stem cells reach the levels needed for clinical benefit.

The broader significance extends beyond inherited blood disorders. In previous work published in Nature, the team used the same general principle of epitope editing  to protect healthy blood stem cells from powerful cancer immunotherapies, such as CAR-T cells or therapeutic antibodies, while allowing those therapies to attack leukemia cells. Together, these studies suggest that epitope editing could become a flexible platform: one application could make stem cell transplantation and gene therapy safer, while another could expand the use of cancer immunotherapy by protecting normal blood formation from unintended damage.

"Although this work is still preclinical, it points toward a future in which patients may receive curative stem cell therapies with less toxicity, less reliance on chemotherapy, and greater precision," said Genovese. "By combining targeted biological conditioning with molecularly protected therapeutic stem cells, this strategy offers a new framework for safer and more accessible treatments for a wide range of blood diseases."

The technology used in this study is jointly owned by Boston Children's and Dana-Farber Cancer Institute.