“Neutrophils are known to kill and clear out pathogens and damaged tissue, but in this study we identified their direct impacts on muscle progenitor cell behaviors,” said co-second author Stephanie McNamara, a former post-graduate fellow at the Wyss Institute who is now an M.D.-Ph.D. student at Harvard Medical School and the Graduate School of Arts and Sciences. “While the inflammatory response is important for regeneration in the initial stages of healing, it is equally important that inflammation is quickly resolved to enable the regenerative processes to run its full course.”
Seo and her colleagues then turned back to their in vivo model and analyzed the types of muscle fibers in the treated vs. untreated mice 14 days after injury. They found that type IIX fibers were prevalent in healthy muscle and treated muscle, but untreated injured muscle contained smaller numbers of type IIX fibers and increased numbers of type IIA fibers. This difference explained the enlarged fiber size and greater force production of treated muscles, as IIX fibers produce more force than IIA fibers.
Finally, the team homed in on the optimal amount of time for neutrophil presence in injured muscle by depleting neutrophils in the mice on the third day after injury. The treated mice’s muscles showed larger fiber size and greater strength recovery than those in untreated mice, confirming that while neutrophils are necessary in the earliest stages of injury recovery, getting them out of the injury site early leads to improved muscle regeneration.
“These findings are remarkable because they indicate that we can influence the function of the body’s immune system in a drug-free, non-invasive way,” said Walsh, who is also the Paul A. Maeder Professor of Engineering and Applied Science at SEAS and whose group is experienced in developing wearable technology for diagnosing and treating disease. “This provides great motivation for the development of external, mechanical interventions to help accelerate and improve muscle and tissue healing that have the potential to be rapidly translated to the clinic.”
The team is continuing to investigate this line of research with multiple projects in the lab. They plan to validate this mechanotherpeutic approach in larger animals, with the goal of being able to test its efficacy on humans. They also hope to test it on different types of injuries, age-related muscle loss, and muscle performance enhancement.
“The fields of mechanotherapy and immunotherapy rarely interact with each other, but this work is a testament to how crucial it is to consider both physical and biological elements when studying and working to improve human health,” said Mooney, who is the corresponding author of the paper and the Robert P. Pinkas Family Professor of Bioengineering at SEAS.
Additional authors of the paper include Benjamin Freedman, Brian Kwee, Sungmin Nam, Irene de Lázaro, Max Darnell, Jonathan Alvarez, and Maxence Dellacherie from the Wyss Institute and SEAS, and Herman H. Vandenburgh from Brown University.
This research was supported by the National Institute of Dental & Craniofacial Research under Award Number R01DE013349, the Eunice Kennedy Shriver National Institute of Child Health & Human Development under Award Number P2CHD086843, the Materials and Research Science and Engineering Centers grant award DMR-1420570 from the National Science Foundation, the National Institute of Arthritis and Musculoskeletal and Skin Diseases, the National Institute of Health (F32 AG057135), and the National Cancer Institute (U01CA214369).