Hematopoietic stem cell (HSC) transplantation offers a cure for hematologic malignancies, but it requires the use of toxic conditioning regimens (chemotherapy, radiation therapy) to prepare the host to receive donor transplants, and this treatment can be a killer.

If findings from a mouse study can be replicated in humans, in over 5 years patients may be able to forgo the toxic conditioning regimen and instead opt for using an immunotherapy approach that may be associated with significantly lower toxicities, including lower graft-vs-host disease.

The new study was published online on August 10 in Science Translational Medicine.

In the study, Stanford University researchers showed that mice can be conditioned for HSC transplantation with two biologic agents: a monoclonal antibody (ACK2) targeting c-kit and another antagonist targeting CD47.

ACK2 impairs and depletes the host stem cell compartment. CV1mb targets CD47, a transmembrane protein expressed on hematopoietic stem cells and other cells, acting as a marker of self — a “don’t eat me” signal — and is an innate immune checkpoint.

When used together, this combination led to a more than 99% depletion of host HSCs, and then a donor transplant led to a robust reconstitution of multilineage blood cells.

“This proof-of-concept study shows that these agents will be just as powerful as radiation or chemotherapy currently used in conditioning regimens,” corresponding and senior author Judith A. Shizuru, MD, PhD, a member of the Stanford Blood and Marrow Transplantation adult and pediatric faculty, the Stanford Immunology Program and the Institute of Stem Cell Biology and Regenerative Medicine, California, told Medscape Medical News.

Dr Shizuru is optimistic that the combination approach will be accelerated into clinical trials and that results from patients will be seen in 3 to 5 years.

“While blood stem cell or bone marrow transplantation is a powerful cellular therapy, the toxicity associated with the procedure outweighs the benefits in many cases. Our work is the first step to eliminate these toxic side effects, such as those caused by DNA-damaging agents used to prepare patients for transplant,” Akanksha Chhabra, PhD, a Stanford University postdoctoral fellow and one of the primary authors on the study, told Medscape Medical News.

Approached for comment on the new findings, Marcel R.M. van den Brink, MD, PhD, head of the Division of Hematologic Oncology, Memorial Sloan Kettering Cancer Center, New York, New York, told Medscape Medical News: “As proof of principle, this is elegant and worthwhile, but more needs to be done in larger animal systems before it can be taken to human subjects.”

What the Study Showed

In the first part of the study, the researchers injected a single dose (500 μg) of ACK2 into genetically engineered mice that were immunodeficient (Rag2^-/-cγ^-/- strain). Within 6 days, HSCs were depleted by more than 4 orders of magnitude. Such a robust response was not seen in wild-type animals, which have intact immune systems and showed less than a 10-fold decrease.

In another approach, the team showed that animals that were genetically altered to be deficient in the γ-chain subunit of FcRIII, FcRI, and FcRIV and that lacked the ability for antibody-dependent cell-mediated cytotoxicity (ADCC) and phagocytosis were unable to be conditioned with ACK2.

“Together, these results establish that the antibody Fc effector functions elicited by ACK2 are necessary for its in vivo HSC-depletive activity,” Dr Chhabra noted.

But this approach — using ACK2 alone — could not be used effectively in immunocompetent mice. The researchers had established from their earlier work that in mouse tumor models, blocking CD47 activity — the “don’t eat me signal” that attenuates ADCC and antibody-dependent cell-mediated phagocytosis (ADCP) — markedly increased ADCP or phagocytic activity. They figured that a combination of ACK2 and blocking CD47 may be their ideal conditioning regimen.

In experiments that used the combination of ACK2 and CV1mb (a CD47 antagonist) the researchers showed that, in immunocompetent mice, the combination depleted HSCs to undetectable levels — something not possible with ACK2 or CV1mb alone.

In fact, this approach depleted common myeloid progenitors, granulocyte macrophage progenitors, and megakaryocyte-erythroid precursors. This was shown both in reduced cell numbers and by immunohistochemistry.

So what happened when these animals received donor HSCs?

In immunocompetent animals preconditioned with ACK2 alone or CV1mb alone, very low levels of HSC engraftment were observed.

The researchers also showed that the synergy seen with ACK2 and CV1mb could be extended to the combination of ACK2 and other CD47 antagonists.

As expected from this approach, the principal toxicity from this approach was cytopenias, especially those associated with red blood cells. Dr Shizuru and colleagues suggest that in the clinical setting these toxicities can be minimized through careful monitoring and supportive care.

Profound Implications for the Future

Dr Shizuru explained to Medscape Medical News that the approach was novel not only in its conditioning regimen but also in its choice of donor cells.

In HSC transplantation, patients typically receive bone marrow transplant, which contains blood stem cells as well as immune cells from the donor such as T cells, which can attack the tissue of the recipient, leading to graft-vs-host disease, she pointed out. “In our approach, we used only blood-forming stem cells for engraftment,” she said.

Dr Shizuru has hopes that this approach will work in humans. “If it works in humans like it did in mice, we would expect that the risk of death from blood stem cell transplant would drop from 20 percent to effectively zero,” she said in a Stanford University press release.

“The extension of this approach to humans could obviate the need for nonspecific toxic therapies that are currently the standard of care and could subsequently allow for HSC transplantation to be extended to a broader set of patients,” the Stanford researchers write in their discussion.

Dr Shizuru indicated that phase I studies are already enrolling patients to determine a safe dose of a CD47 antibody — Hu5F9G4 — in patients across several malignancies.

However, the researchers indicate that although these approaches are being independently tested in the clinic, additional trials will be necessary to test the safety and feasibility of the combination regimen in patients.

Even more exciting is the prospect that this approach is not restricted to patients with cancer requiring HSC transplantation but may also be appropriate in the clinical management of a spectrum of diseases, including solid organ transplantation where patients have to take immunosuppressive drugs for life.

Dr Chhabra said, “We hope that in humans our biologic approach will not only transform the way we prepare patients who currently need a transplant, but also provide therapies for a much broader spectrum of disease such as autoimmune diseases.”

“Furthermore this approach could facilitate safer organ transplants where immune-suppressants will not be necessary,” she added.

“This approach lends itself to patients with lupus, diabetes, multiple sclerosis, and organ transplantation, and patients who may be candidates for gene therapy, such as those with sickle cell anemia and thalassemia,” Dr Shizuru told Medscape Medical News.

In fact, the anti-c-kit approach is being tested in a phase 1 study that recently opened at Stanford’s Lucile Packard Children’s Hospital using a humanized monoclonal antibody in patients with severe combined immunodeficiency.

Dr van den Brink, who was not involved in the study, agrees that the approach had great potential. “The major impact of this work will also be seen in the management of diseases beyond cancer,” he said.

He explained that in HSC transplantation in cancer, the initial conditioning regimen serves two purposes. “It creates a space in the stem cell niche and creates tolerance, and it helps in debulking the tumor,” he said.

But this conditioning is associated with serious toxicity. “Cancer itself is a lethal disease and we are willing to take a greater risk,” he said. But the toxicities with current conditioning regimens pose too great a risk for diseases that are not life-threatening, he suggested.

“For noncancer indications, such as lupus and rheumatoid arthritis, if one can reset the immune system and get rid of the bad clones of B and T cells with a conditioning regimen that is safer to tolerate, then the risk would be worth taking,” Dr van den Brink said.