Translate 
A transplant of blood-forming stem cells, also known as a hematopoietic-cell or bone-marrow transplant, offers the prospect of cure for many blood, immune, autoimmune and metabolic disorders. But the treatment required before the transplant — chemotherapy and radiation — is toxic enough that most patients either cannot receive a transplant or suffer from bad side effects. Now, research conducted at Boston Children's Hospital and Dana-Farber Cancer Institute finds that a new pre-treatment, combining a specially-targeted antibody with a drug, has virtually no toxicity, at least in mice.
If the findings hold up in clinical trials, the new pre-treatment could enable patients to receive stem-cell transplants for a much wider range of disorders, perhaps even from unmatched donors, and could also greatly expand the use of gene-modified stem cells. Results were published in two back-to-back papers February 6 in Nature Communications.
"Our findings suggest a huge opportunity to do transplants in a way that's extremely safe and extremely effective," says Agnieszka Czechowicz, MD, PhD, who was co-first author on the first study with Rahul Palchaudhuri, PhD from Harvard University and co-first author on the second study with Zhanzhuo Li, MD, PhD of the National Institute of Allergy and Infectious Diseases (NIAID). "This also opens up the possibility of easily using blood-forming stem cells that have undergone gene therapy and gene editing, without additional toxicity."
The whole-body chemotherapy and radiation currently used prior to stem-cell transplantation are designed in part to deplete patients' own defective blood-forming stem cells. This provides space for the donor stem cells to engraft, but it wipes out the immune system, raising the risk for infections, and can cause other serious side effects such as anemia, infertility, other organ damage and secondary cancers. Moreover, if the stem-cell donor isn't an exact match, patients' immune systems must be suppressed for prolonged periods, to prevent the body from rejecting the donor cells.
Targeting pre-treatment to avoid side effects
The new studies build upon research Czechowicz conducted as a graduate student at Stanford University, where she is now on the faculty. That research found that pre-treatment with an antibody that blocks the CD117 receptor killed off blood-forming stem cells selectively, since few other cells carry this receptor. This enabled mice to receive new blood-forming stem cells safely, without the side effects of chemotherapy and radiation. Clinical trials with a human version of this antibody are now underway at Lucile Packard Children's Hospital Stanford in patients undergoing stem cell transplants to treat severe combined immunodeficiency.
"Although the 'naked' antibody was very helpful, we have long wanted to make the antibody more powerful," says Czechowicz. Czechowicz continued her work during her residency and fellowship at the Dana-Farber/Boston Children's Cancer and Blood Disorders Center in the lab of Derrick Rossi, PhD, at the Program in Cellular and Molecular Medicine at Boston Children's. To enhance the effects of the CD117 antibody, the researchers attached a drug called saporin, which has already been used in cancer patients and inhibits ribosomes, the protein-building structures in all cells. The team hoped that the combination would efficiently kill blood-forming stem cells, and only these cells, by stopping their ability to make important proteins.
"We theorized it would be effective, but were both reassured and excited when it worked so well," says Czechowicz.
The first study showed that a single dose of the antibody-drug combination eliminated more than 99 percent of blood-forming stem cells in mice. This allowed high levels of transplanted stem cells to take up residence in the host animals and effectively replace their blood and immune systems. Importantly, the antibody-drug conjugate specifically targeted the hosts' stem cells without harming other kinds of blood cells and without causing clinically significant side effects. The animals' immune cell function was preserved and responded effectively to pathogens.
Transplants from any type of donor?
While the first study used matched donors, the second study tried the same approach with fully mismatched donors (based on the major histocompatibility complex or MHC). The mice receiving the transplants also received immunosuppression, but only for a very short period. They showed up to 50 percent engraftment of the donor cells. This was enough for the mice to also tolerate skin grafts from the same donors, even many months after their first transplants.
"Transplants of blood-forming stem cells not only create new blood and immune systems, they also lead to tolerance of other donor tissues and organs without the need for chronic immune suppression," explains Czechowicz. "But this approach isn't used often, since it has, until now, required toxic radiation or chemotherapy pre-treatment. Our modified approach could be transformative for the transplant field, and could potentially enable both stem cell transplantation and subsequent solid organ transplantation from any type of donor."
"If the principles demonstrated in these studies translate to the clinic, they open the possibility of expanding stem cell transplantation to many more patients who we know would benefit if only it were made more tolerable," says Rossi, who was a co-senior author on the first study with David Scadden, MD of Harvard University and a co-senior author on the second with Philip Murphy, MD of NIAID.
The researchers plan further studies to confirm the safety and efficacy of the combination using a human CD117 antibody. They also plan to investigate other antibody-drug combinations. Magenta Therapeutics (Cambridge, Mass.) has licensed the technology and is working towards developing and testing this approach in patients. The company presented pre-clinical data on anti-CD117 antibody-drug conjugates at the American Society of Hematology (ASH) meeting in December 2018, using another ribosome inhibitor, amanitin.
Supporters of the studies include: The National Institutes of Health (R01 A I132638, RO1HL107630, R00AG029760, UO1DK072473-01), the NIH National Institute of Allergy and Infectious Diseases (AI000615-25), the NIH National Heart, Lung and Blood Institute (K99/R00 HL119559, HL44851, HL129903), a Potter Fellowship to the Boston Children's Hospital Trust, the Gerald and Darlene Jordan Chair of Medicine of Harvard University, Harvard Blavatnik Biomedical Accelerator Fund, The Leona M. and Harry B. Helmsley Charitable Trust, The New York Stem Cell Foundation, The Harvard Stem Cell Institute and the American Federation for Aging Research.
Czechowicz, Rossi and certain other study co-authors are founders and equity holders in Magenta Therapeutics and are co-listed as inventors on related U.S. patent applications. Boston Children's Hospital is also an equity holder in Magenta Therapeutics. See the papers for a complete list of authors, funders and disclosures.