Published in the journal Cell, the study focuses on granulocyte-monocyte progenitors (GMPs), a type of progenitor cell that produces macrophages and several other immune cells. Macrophages play a key role in defending the body against infections and have attracted growing interest as potential tools for treating cancer.

The researchers showed that GMPs can be expanded extensively in the laboratory and genetically modified to recognize cancer cells while also boosting 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 the study's most significant findings relates to self-renewal, a characteristic traditionally associated with stem cells. Self-renewal allows cells to repeatedly divide while maintaining their identity. Scientists generally have not considered progenitor cells to possess this long-term capability.

"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."

Why Macrophage Precursors Matter

Macrophages are appealing candidates for cancer immunotherapy because they naturally enter tumors, consume cancer cells, and help organize immune responses. While T-cell therapies have achieved major success against blood cancers, macrophage-based therapies may offer particular advantages against solid tumors.

However, mature macrophages have several drawbacks as therapeutic products. They are difficult to grow in large numbers outside the body, challenging to genetically engineer, and can be damaged during freezing and storage. They also tend to accumulate in organs such as the lungs and liver instead of spreading widely throughout the body.

To overcome these obstacles, first author Shi Yue, MD, and colleagues in the Ying Lab focused on GMPs, which sit earlier in the developmental pathway that produces macrophages.

Using a carefully defined chemical cocktail, the team prevented GMPs from maturing into other immune cell types and succeeded in maintaining and expanding them over long periods in the laboratory.

Even after extended growth, the cells preserved their molecular and cellular characteristics and continued to generate functional macrophages and other immune cells.

Researchers in the laboratory of Ravi Majeti, MD, PhD, at Stanford University independently reproduced the long-term maintenance and genetic engineering of GMPs, providing additional support for the platform's reliability and potential therapeutic value.

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 GMPs To Fight Cancer

Beyond their ability to grow long term in the laboratory, GMPs can also be genetically engineered for use as immunotherapies.

In this study, the researchers equipped GMPs with a chimeric antigen receptor, or CAR, enabling the cells to recognize a specific marker found on cancer cells. The team also added a second signal designed to activate nearby immune cells that help stimulate tumor-fighting T cells and strengthen the body's natural defenses.

Importantly, this additional signal remains effective even when donor and recipient cells are immunologically mismatched. That raises the possibility of creating off-the-shelf therapies produced in advance from donor cells and used in many patients, rather than generating a custom treatment for each individual.

After expanding and engineering both mouse and human GMPs, the researchers tested them in mice. The cells successfully settled into the bone marrow and other blood-forming tissues, where they continuously generated engineered macrophages and additional immune cells.

Because the GMPs maintained an ongoing supply of these cells from the bone marrow, they avoided the rapid loss that has limited mature macrophage therapies, including those evaluated in recent clinical trials.

In mice with blood cancers and solid tumors, CAR-engineered GMPs slowed disease progression. GMPs carrying both the CAR and the additional immune-activating signal produced even stronger benefits.

Potential Beyond Cancer

The platform may have applications that extend beyond oncology.

The researchers tested the approach in mice with chronic granulomatous disease, an inherited immune disorder. GMP treatment restored the animals' ability to fight bacterial infections, demonstrating the potential of the technology for immune deficiencies as well.

"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 the Study

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.