Immune cells help older muscles heal like new

The results were posted online on 1 October in Nature Biomedical Engineering.

In 2014, the group led by Nenad Bursac, professor of biomedical engineering at Duke, launched the first self-healing skeletal muscle developed in the laboratory. It has contracted powerfully, quickly integrated with mice and has demonstrated its ability to heal itself both inside the lab and inside an animal.

This step was taken by taking muscle samples from rats as young as two days old, removing the cells and "planting" them in a laboratory environment that is perfectly adapted to their growth. In addition to a three-dimensional scaffold and many nutrients, this environment promotes the formation of niches for muscle stem cells, called satellite cells, which activate in case of injury and facilitate the regeneration process.

However, for potential applications with human cells, muscle samples would be predominantly taken from adult donors rather than neonates. Many degenerative muscle diseases do not appear before adulthood, and use of the patient's adult cells would help develop muscle in the laboratory to test the drug response of these patients.

There is just one problem: adult muscle tissue manufactured in the laboratory does not have the same regenerative potential as newborn tissue.

"I spent a year exploring methods of engineering muscle tissue from samples of adult rats able to heal themselves after an injury," said Mark Juhas, a former Duke PhD student in the Bursac laboratory, which directed the original and new research.

"The addition of various drugs and growth factors known to help with muscle repair had little effect, so I started to consider adding a cell population." support able to respond to injuries and stimulate muscle regeneration, "said Juhas. "That's how I came up with macrophages, immune cells needed for the muscles' ability to self-repair in our body."

Macrophages are a type of white blood cell in the immune system. Translated literally from Greek by "big eater", macrophages engulf and digest cellular debris, pathogens and anything that they think should not be dragging while secreting factors that promote tissue survival and repair.

After a muscle injury, a class of macrophages appears on the scene to clean the debris left behind, increase inflammation and stimulate other parts of the immune system. One of the cells that they recruit is a second type of macrophage, called M2, which decreases inflammation and promotes tissue repair. Although these anti-inflammatory macrophages were used in muscle healing therapies, they had never been integrated into a platform for the growth of complex muscle tissue outside the body.

It took several more months of work for Juhas to understand how to incorporate macrophages into the system. But once he's done it, the results have changed dramatically. New muscle tissue not only performed better in the lab, but also performed better when transplanted to live mice.

"When we damaged the artificial muscle derived from adults with a toxin, we found no functional recovery and the muscle fibers could not be rebuilt," said Bursac, co-director of the team. Duke's Regeneration Next initiative. "But after adding the macrophages into the muscle, we had a shock moment, the muscle pushed back in 15 days and contracted almost as before the injury." It was really remarkable.

The success seems to come mainly from macrophages that act to protect damaged muscle cells from apoptosis – programmed cell death. While the muscle cells of newborns naturally resist the urge to throw away the sponge, adult muscle cells need macrophages to help them get through the initial damage without getting into cell death. These surviving muscle fibers then provide a "scaffold" allowing the muscle stem cells to hook to perform their regenerative functions.

Bursac believes this discovery could lead to a new line of research on potential regenerative therapies. According to a widespread theory, fetal and neonatal tissues heal much better than adult tissues, at least in part because of an initial supply of macrophage-resident tissue, similar to M2 macrophages. As individuals age, this original macrophage intake is replaced by less regenerating and more inflammatory macrophages from bone marrow and blood.

"We believe that the macrophages of our advanced muscular system can behave more like resident muscle macrophages, with which people are born," Bursac said. "We are currently trying to understand if this is indeed the case, we could then consider" training "macrophages to be better healers in a system like ours or increase them by genetic modifications and then implant them in damaged sites in patients. "

This work is, of course, still in the years to come. Although this study has also shown that human macrophages promote the healing of lab-developed rat muscle and that separate work in the Bursac group has developed complex human muscles containing macrophages, there is as yet no system for laboratory or animal adequate to test the regenerative powers that this approach can have in humans.

"Building a platform to test these results in modified human tissue is a clear next step," Bursac said. "In the same vein, we want to better understand the potential roles that macrophages within the transformed muscle play in its vascularization and innervation after implantation.We hope our approach of supplementing the muscles developed in the laboratory with Immune system cells will prove to be a general strategy to increase the survival and function of other tissues developed in the laboratory during future regeneration therapies. "

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More information:
Mark Juhas et al., The incorporation of macrophages into a modified skeletal muscle allows for increased muscle regeneration, Nature Biomedical Engineering (2018). DOI: 10.1038 / s41551-018-0290-2