Today, we here at CIRM wanted to provide an update on the fascinating world of hematopoietic (blood) stem cell-based therapies.  What is the current status of this promising field and what are some of the challenges that need to be overcome? Dr. Kelly Shepard, Associate Director of Discovery and Translation here at CIRM, answers these questions and many more in the blog entry below.

There have been a number of exciting advances in regenerative medicine over the past few years, especially in the use of gene therapy and hematopoietic (blood) stem cell transplantation to treat and even cure various diseases of the blood and immune system. These studies built off groundbreaking research by Till and McCulloch in the 1950-60’s, who identified a rare and special stem cell in the bone marrow of mice that gives rise to all cells of the blood and immune system for the lifetime of the animal, the “hematopoietic stem cell”, or HSC. It wasn’t long before scientists and doctors realized the therapeutic implications of this discovery, and the journey to identify the human counterpart began. Fast forward to the present, and HSC transplantation (HSCT) has become a standard medical procedure for treating various cancers and genetic disorders of the blood. The basic premise is this: a patient with a diseased or defective blood/immune system receives an infusion of healthy HSCs, which are typically procured from donated bone marrow or umbilical cords, but in certain situations, might come from the patient him/herself. Once established in the recipient, these healthy cells will divide and regenerate a new blood and immune system over the course of the patient’s lifetime.

For HSCT to be successful, the donor cells must “engraft”, or take up permanent residence in their new environment. This usually necessitates “conditioning” the recipient with some form of chemotherapy or radiation, which eliminates some of the patient’s own cells to create room for the new arrivals. Unfortunately, conditioning creates a situation where the patient is extremely vulnerable to infections and other complications during the period of recovery, as it will take weeks for his/her blood and immune systems to be reestablished. These inherent risks mean HSC transplants can only be offered to patients with life threatening diseases such as leukemia, or to those with significant blood/immune disorders who are sufficiently healthy to tolerate the toxic conditioning regimen and to weather the extended period of recovery.

A second major issue preventing a more widespread use of HSCT is the shortage of healthy donor HSCs that are available for transplant, which must be immune matched to the recipient to prevent rejection. Immune matching is also critical to avoid a dangerous complication called graft vs. host disease, where the transplanted cells or their progeny launch an immune attack against the recipient’s organs, often leading to chronic disease and sometimes, death. Unfortunately, there are many people who have no compatible donors and for whom the risk of even a partially matched transplant is unacceptable.

Scientists and clinicians have long sought means to overcome the technical challenges of HSCT in order to “unleash” its true potential to cure and treat a wider variety of diseases, and to  make it feasible (and affordable) for a much larger number of patients. CIRM has endeavored to support novel approaches that could hopefully produce game changing advances for the field. Some of these approaches were recently highlighted in a Perspective article, published in Stem Cells Translational Medicine in early 2020, along with a discussion of other important advances in related areas, listed below. More information can be found in that article or referring to our website to learn more about the individual projects.

Developing New Sources of Healthy and Immune Compatible HSCs for transplant

• Exploring ways to produce HSCs from pluripotent stem cells in the lab
• Expanding populations of HSCs that are already present in donated tissues such as cord blood
• Using genetic engineering to “repair” defects in the DNA of HSCs from patients with inherited blood and/or immune disorders
• Using genetic engineering to create “immune invisible” or “universal donor” HSCs that will not be rejected after transplantation

Developing Safer and More Tolerable Conditioning Regimens

• Exploring reduced intensity forms of conditioning with drugs or radiation
• Using antibodies rather than chemicals to free up space in the bone marrow for incoming, donor HSCs
• Using dietary methods to free up space in the bone marrow for incoming, donor HSCs

Accelerating Reovery of Immune Function Lost Through Conditioning

• Adding back key populations of immune cells to protect the host during regeneration of their immune system
• Discovering new drugs and treatments to accelerate the pace of regeneration after transplant, or to prevent the death of HSCs that survived conditioning

Overcoming these scientific and technical challenges could create a paradigm shift in the way HSCT is applied and used and consequently, reduce the costs and risks associated with the procedure. In this way, the true potential of HSCT could be unleashed for the greatest good.