In an advance that could eventually offer patients with blood disorders curative therapies derived from their own cells, researchers at the Ansary Stem Cell Institute, WCMC, have been able to directly reprogram vascular cells to long-lasting blood cells that resemble bone marrow stem cells capable of forming the various components of blood necessary for life. Dr. Shahin Rafii is a Professor of Medicine and the Director of the Ansary Stem Cell Institute.
The findings, reported as an article in the July 2 issue of Nature, show that blood cells engineered from cells lining blood vessels, or endothelial cells, could potentially provide an abundant and safe source of new blood stem cells to treat a variety of diseases. The potential application of this novel approach includes treatment for genetic disorders, such as thalassemia and sickle cell disease, blood cancers and even certain infections such as HIV.
The technique uses endothelial cells from a biopsied bit of skin and adds four genetic transcription factors – proteins that bind to and activate genes – that provide an initial push to direct the cells to become blood-forming cells. Scientists then expose these morphing cells to a bed of supporting specialized vascular cells that "educates" them to complete the "reprogramming" process generating cells that produce red cells to carry oxygen, white cells to fight infections, and platelets to prevent bleeding.
When the scientists transplanted these engineered human blood cells into immune-deficient mice, they lodged in the rodents' bone marrow manufacturing all of those blood parts.
"We have conceived a reproducible approach to manufacture engraftable durable blood cells" says the study's senior investigator, Dr. Shahin Rafii, director of the Ansary Stem Cell Institute and professor of medicine and genetic medicine at Weill Cornell. "The key to the success of this reprogramming process was development of supporting vascular cells, that form a nurturing niche for the survival and growth of the reprogrammed blood cells, similar to what happens developmentally during blood production. We expect that our method of directly reprogramming a person's own readily accessible endothelial cells to abundant and stable blood cells will offer the first safe technology to cure a wide spectrum of benign and malignant disorders."
Currently, the only source of blood cells is a patient's own blood, umbilical cord blood, or a matched bone marrow donor. Many patients cannot find a suitable donor, and those who do may develop complications including graft-versus-host disease, in which the donor cells attack the recipient. The study, led by Dr. Vladislav Sandler, who is a instructor in medicine in Dr. Rafii's lab, overcomes these complications by allowing patients' own cells to be reprogrammed into blood cells free of the defects that caused their original diseases.
"The potential applications and importance of this work are just enormous. Imagine, this new approach gives us a chance to use a patient's own endothelial cells to produce an entirely new blood forming system for them to fix their disease be it leukemia or sickle cell anemia or HIV. It is very exciting," says Dr. Joseph Scandura, a senior co-author of the study and an assistant professor in the Department of Hematology-Oncology at Weill Cornell Medicine.
In the study, the scientists found four transcription factors that were in blood cells but not in endothelial cells. Because endothelial cells make the first blood forming cells in fetuses, the scientists believed these factors would kick-start the process of turning endothelial cells into blood cells.
The investigators gathered endothelial cells from human skin and human umbilical cords, and added the transcription factors. "What we got were cells that weren't yet blood cells. They wouldn't expand and wouldn't engraft when transplanted," says Dr. Raphael Lis, a post-doctoral fellow in Dr. Rafii's lab and a co-author on the study. However, once the scientists bathed these partially reprogrammed blood cells in a plate of supportive vascular niche cells developed by Angiocrine Biosciences, of which Dr. Rafii is a co-founder, they morphed into working blood cells that continued to increase and generate multipotent blood cells. However, one drawback in this study is that while the reprogrammed blood cells can generate immune B cells they are apparently less efficient in forming T cells. "We are developing ways to fix this issue," Dr. Sandler says.
"Nonetheless, the genetic makeup of these reprogrammed multipotent blood cells were very similar to those of normal blood cells, suggesting that our reprogramming strategy has converted an endothelial cell fate to a true blood cell fate", commented co-author Dr. Olivier Elemento, associate professor in computational medicine.
Previous attempts to engineer multipotent blood cells from embryonic stem cells or laboratory-induced pluripotent stem cells have yielded blood cells that do not home in on and engraft efficiently in the bone marrow of mice, notes co-author Dr. Daylon James, an assistant professor in reproductive medicine. "Indeed, many laboratories are focusing on exploiting the potential use of pluripotent stem cells as a source of new blood cells," adds co-author Dr. Jason Butler, an assistant professor of genetic medicine. "However, there are concerns because unless these pluripotent cells fully complete the transformation to blood cells, they could yield tumors. This is a major obstacle to bringing these cells to the clinic because we don't currently have a way to ensure that cells derived from pluripotent cells are safe. Our approach could potentially bypass the use of pluripotent stem cells."
"More importantly the stepwise reprogramming model devised here will allow scientists to decipher the as yet unknown pathways that participate in generating long-lasting blood cells. This discovery that is the culmination of 10 years of investigation at the Ansary Stem Cell Institute has brought us closer to capitalize on the potential of vascular cells to rebuild injured or genetically defective bone marrow," concludes Dr. Rafii.