Engineered Macrophages Cross the Blood-Brain Barrier and Slow Lung Cancer Brain Tumors in Mice

Brain metastases from lung cancer are among the hardest problems in oncology. Nearly one in three patients with lung cancer will develop tumors in the brain, and the treatment options are severely limited by a fundamental anatomical fact: most drugs cannot cross the blood-brain barrier, the brain's tightly regulated cellular shield that keeps large molecules out. Surgery and radiation reach only what can be physically targeted. Systemic therapies mostly cannot get in.

The research published in Nature Biomedical Engineering by Wake Forest University School of Medicine exploits something unusual about macrophages: they can cross the blood-brain barrier naturally. That biological capability, evolved as part of the immune system's surveillance function, is the starting point for an engineered cell therapy designed to combine natural brain access with targeted tumor-killing ability.

The Engineering Strategy

The research team engineered macrophages - white blood cells that normally engulf and destroy pathogens and cellular debris - to express a chimeric antigen receptor, or CAR, that targets mesothelin. Mesothelin is a protein found at high levels on lung cancer cells that have spread to the brain, making it a useful target for directing an immune attack.

To strengthen the macrophages' killing capacity, the researchers added a signaling component called MyD88, which activates the cell's innate immune response and puts it in a more aggressive "attack" mode. This modification, known as the MyD88-CAR macrophage or CARMA, was compared against other CAR configurations in the laboratory and mouse models.

"Brain metastases are incredibly difficult to treat because most therapies simply can't get inside the brain," said Shih-Ying Wu, PhD, assistant professor of radiation oncology at Wake Forest and corresponding author. "Macrophages, however, naturally know how to cross into the brain. So, we asked: 'What if we could give them the ability to recognize and destroy cancer cells once they get there?'"

What the Preclinical Data Showed

Testing in laboratory models and in mouse models designed to replicate lung cancer brain metastasis yielded several notable results. The MyD88-enhanced CARMA cells successfully accumulated at brain tumor sites after crossing the blood-brain barrier. They showed stronger anti-tumor activity than non-MyD88 versions. In addition to directly attacking tumor cells expressing mesothelin, the macrophages released cytokines including TNF-alpha that harmed nearby cancer cells even when those cells did not express the target antigen - a bystander killing effect that could address the tumor heterogeneity that limits more targeted approaches.

Mouse models receiving CARMA treatment showed significant reductions in brain tumor progression and increased survival compared to controls. The macrophages also appeared to reshape the broader immune environment within the tumor - activating other immune cells and sustaining what the researchers describe as a longer-term anti-tumor response.

"These macrophages didn't just find the tumors; they reshaped the entire immune environment in the brain," said Kounosuke Watabe, PhD, professor of cancer biology at Wake Forest and corresponding co-author. "We were excited to see that they activated other immune cells and helped sustain a long-term anti-tumor response."

The Critical Limitations of Preclinical Data

All results to date come from laboratory cell culture experiments and mouse models. Mouse models of brain metastasis differ from human disease in important ways - the immune system, tumor microenvironment, and pharmacokinetics of cell-based therapies behave differently across species. Many approaches that show strong results in mouse cancer models have failed in human trials.

CARMA has not entered human clinical trials. The path from these preclinical results to clinical application requires manufacturing scale-up of the engineered cells, safety evaluation in larger animal models, regulatory approval of an investigational new drug application, and multiple phases of human trials. Each step is uncertain and time-consuming.

The finding that CARMA showed fewer toxicity signals than CAR-T approaches tested in the same study is promising but requires confirmation in more rigorous safety studies before drawing clinical conclusions.