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," said the paper's corresponding author Qi-Long Ying, MD, PhD , professor of stem cell biology and regenerative medicine at the 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.
About this research
The paper in Cell is titled "Expansion and CAR engineering of granulocyte-monocyte progenitors for cellular immunotherapy."
In addition to Ying, Yue and Majeti, additional authors are: Zheng Guo, Crystal Pan, Xueyuan A. Jing, Tai Nguyen, Jiaqi Tang, Yanpui Chan, Humberto Contreras-Trujillo, Du Jiang, Xue Yan, Hang Xiang, Xugeng Liu, Xiao Wang, Ziyuan Wang, Natalie Shu, Daniel B. McKim, Rong Lu and Chao Zhang from USC; and Litao Tao and Celia Bloom from Creighton University; Asiri Ediriwickrema and Sebastian Koschade from Stanford University School of Medicine; and Yingxiao Shi from Harvard Medical School and the Dana-Farber Cancer Institute.
This work was supported by the Chen Yong Foundation of the Zhongmei Group, a sponsored research project from Myelogene Inc., the L.K. Whittier Foundation, the Eli and Edythe Broad Innovation Award, the Ming Hsieh Institute for Research on Engineering Medicine for Cancer Award, the USC SBIR/STTR Planning Award, the Xia Research Fund, and the Wu & Jiang Research Fund. Majeti reports support from the Ludwig Institute for Cancer Research, and Guo was supported by the California Institute for Regenerative Medicine Predoctoral Training Fellowship.
Disclosures
Ying, Yue, Jing, Guo, Majeti, Zhang, Nguyen and Tang are co-inventors on patents related to this study, filed by USC and licensed to Myelogene Inc. Ying, Yue, Zhang and Majeti are co-founders of Myelogene Inc. Majeti is on the Advisory Boards of Kodikaz Therapeutic Solutions, Pheast Therapeutics, Prelude Therapeutics, Mubadala Capital, Aculeus Therapeutics, Sequentify, BMS and Bectas Therapeutics. Majeti is also a co-founder and equity holder of Pheast Therapeutics.
