Tiny immune cell particles help blood vessels grow in lab-made human heart tissue

Researchers at the University of Toronto have discovered how tiny particles released by a special type of immune cell can help blood vessels form in engineered human heart tissue. The findings shed new light on how the heart may naturally support repair and could help scientists design better heart tissues for research and future therapies.

The study, led by PhD student Karl Wagner and Professor Milica Radisic at the Institute of Biomedical Engineering, was published in an recent issue of APL Bioengineering.

The human heart contains resident macrophages that play roles beyond immune defense, including maintaining tissue health. However, much of what is known about macrophage signaling comes from animal models or circulating immune cells, leaving a gap in understanding how human heart-resident macrophages communicate with other cardiac cells. In particular, the molecular content of extracellular vesicles released by these cells, and how they influence heart tissue development, has remained largely unexplored.

“Until recently, there was no easy way to access resident macrophages inside the body to study their roles in the human heart,” says Wagner. “Now that we know how to produce these cells in the lab from human stem cells, our group was curious study the extracellular vesicles released by macrophages. These vesicles are tiny particles used by cells to communicate with one another, and we thought that looking inside them might allow us to decode the ‘language of cells’ to help us better understand functions of resident macrophages in the human heart.”

To address this, the researchers first generated primitive macrophages from human pluripotent stem cells and collected the extracellular vesicles they released. These vesicles were analyzed using a combination of imaging and biochemical techniques to confirm their size, structure, and protein markers. The team then performed microRNA sequencing to map the molecular cargo within these vesicles and compared the results with vesicles from other major cardiac cell types.

Next, the team tested how these vesicles affect tissue development using a three-dimensional model of engineered human heart tissue. They embedded heart muscle cells, endothelial cells, and supporting cells within a fibrin gel to mimic the structure of cardiac tissue, then added macrophage-derived vesicles to the system. Using fluorescence imaging, they observed how blood vessel networks formed over time. Tissues treated with these vesicles showed more organized and connected vascular structures, including increased branching and junction formation, indicating improved early-stage vessel development.

“Growing stable networks of blood vessels within engineered heart tissues has been very difficult to achieve in the lab,” says Wagner. “Discovering beneficial effects of macrophages and their vesicles on vessel formation in heart tissue is exciting because it means we could use these cells and vesicles as tools to ‘instruct’ better vessel growth in lab-grown tissues. Engineered heart tissues containing complex blood vessel networks can act as more realistic platforms for studying heart diseases in the lab, testing new drugs, or even possible future implantation into the human body to replace damaged tissue after a heart attack while integrating with our body’s own blood supply.”