New method generates renewable supply of progenitor immune cells

In a paper published in Cell, a USC Stem Cell-led team reports a new way of generating a renewable and expandable supply of the progenitor cells that give rise to macrophages. These immune cells help drive the body's response against pathogens, and they hold strong promise as the basis for immunotherapies against cancer and other diseases.

The paper demonstrates that progenitor cells known as granulocyte-monocyte progenitors (GMPs), which give rise to macrophages and other immune cells, can be extensively expanded in the laboratory and engineered both to target specific cancer markers and help stimulate broader immune responses.

The study establishes a scalable and engineerable GMP platform for cellular immunotherapy and introduces concepts that we believe could have broad implications for both cancer immunotherapy and stem cell biology."

Qi-Long Ying, MD, PhD, paper's corresponding author, professor of stem cell biology and regenerative medicine, Keck School of Medicine of USC

One of these broader implications is that self-renewal, a defining property of stem cells but not of progenitor cells, can be maintained in a GMP, which is already committed to generating macrophages and other closely related immune cells.

"The prevailing view has been that long-term self-renewal in the blood system is primarily a property of the hematopoietic stem cells that can generate any type of blood or immune cell," said Ying. "We found that, under the right conditions, GMPs can also self-renew, dividing extensively while keeping their identity and ability to produce functional immune cells. That gives us a scalable starting point for engineering cell therapies for cancer, infectious disease and potentially many other conditions."

Straight to the source

Macrophages are attractive for cancer immunotherapy because they are naturally adapted to infiltrate tumors, engulf cancer cells and help coordinate immune responses. Unlike T-cell therapies, which have shown the greatest success against blood cancers, macrophage-based approaches could be particularly useful for solid tumors.

Unfortunately, mature macrophages are challenging to manufacture as immunotherapies, because they are difficult to expand to large numbers outside the body, hard to genetically engineer, and vulnerable to damage during freezing and storage. In addition, they tend to accumulate in organs such as the lungs and liver rather than distributing widely throughout the body.

So instead of attempting to work with mature macrophages, first author Shi Yue, MD, from the Ying Lab and his collaborators focused on their upstream progenitors, GMPs.

The scientists succeeded in growing and expanding GMPs long-term in the laboratory by using a defined chemical cocktail that prevented them from differentiating into more mature immune cell types.

Even after prolonged growth in the laboratory, the GMPs retained their cellular and molecular identity, as well as the ability to generate functional macrophages and other immune cell types.

Collaborators in the laboratory of Ravi Majeti, MD, PhD, at Stanford University also independently reproduced the long-term maintenance and genetic engineering of GMPs, helping validate the robustness of the platform for future cell-therapy applications.

Majeti, Director of the Institute for Stem Cell Biology and Regenerative Medicine at Stanford University, noted: "This method for the expansion and engineering of GMPs opens the door to numerous translational applications, much like T cell expansion and engineering. We have already demonstrated engineering of these cells to drive multiple potent functions, and there is a lot more to be explored."

Engineering a GMP immunotherapy

In addition to being maintained in the lab long-term, GMPs can be genetically engineered to perform as immunotherapies.

In the study, the team engineered GMPs to contain a chimeric antigen receptor, or CAR, which allows immune cells to recognize a specific marker on cancer cells. They further engineered the progenitor cells to carry an additional signal to help engage other nearby immune cells, which activate tumor-fighting T cells and amplify the body's natural defenses. This added signal works even when the donor cells and the recipient are immunologically mismatched, so the therapy could be made off the shelf, manufactured in advance from donor cells and given to many patients, rather than built individually from each patient's own cells.

After culturing and engineering mouse and human GMPs, the team tested their potential as an immunotherapy in mice. When injected into mice, the GMPs engrafted into the bone marrow and other blood-forming niches, where they generated a supply of engineered macrophages and other immune cells. Because the GMPs keep replenishing that supply from the bone marrow, they avoid the rapid clearance that has limited mature macrophage therapies, including in recent clinical trials.

In mice with blood cancer and solid tumors, the GMPs engineered with CARs delayed disease progression, while the GMPs engineered with both CARs and the immune-activating signal provided an even greater benefit.

The researchers also demonstrated potential applications beyond cancer. In mice with an inherited immune deficiency, known as chronic granulomatous disease, the GMPs restored the ability to fight bacterial infection.

"Our study suggests that the future of immunotherapy may depend not only on designing better CAR receptors, but also on choosing the right developmental stage of the cell," said Ying.