A good amount of evidence has been assembled by the scientific community to demonstrate that the innate immune cells called macrophages play a central role in tissue regeneration. Regeneration is an intricate dance of signaling between numerous cell types and cell states: stem cells, somatic cells, immune cells, and others. Macrophages supply necessary signals that help to guide regenerative processes. They are also responsible for destroying the temporary population of senescent cells that arises in wounds, cells that also deliver signals that promote regenerative activity. Senescent cells are useful in the short term, but if they linger they become disruptive and harmful.
One of the lines of evidence for the importance of macrophages in healing involves comparisons with species capable of highly proficient regeneration. In salamanders, regeneration of organs is dependent on the presence of macrophage signaling. Similarly, African spiny mice exhibit an unusually comprehensive regenerative capacity for mammals, and here again that is due to their macrophages.
Much of the investigative work on macrophages and regeneration has focused on muscle tissue, and the materials noted here today continue that theme. Researchers have been able to engineer small sections of functional muscle tissue for a number of years, with the inability to reliably produce capillary networks being the primary roadblock to the creation of large muscle sections for transplantation. Blood and nutrients can only perfuse through a few millimeters of solid tissue. These small organoids may be functional when it comes to the core capabilities of muscle tissue, but they are lacking when it comes to regenerative capacity. One logical approach to fixing this problem is to incorporate macrophages into the mix of cells, and judging from the results here, this works fairly well once the initial hurdles are overcome.
In 2014, researchers debuted the world's first self-healing, lab-grown skeletal muscle. The milestone was achieved by taking samples of muscle from rats just two days old, removing the cells, and "planting" them into a lab-made environment perfectly tailored to help them grow. For potential applications with human cells, muscle samples would be mostly taken from adult donors rather than newborns. There's just one problem - lab-made adult muscle tissues do not have the same regenerative potential as newborn tissue. "I spent a year exploring methods to engineer muscle tissues from adult rat samples that would self-heal after injury. Adding various drugs and growth factors known to help muscle repair had little effect, so I started to consider adding a supporting cell population that could react to injury and stimulate muscle regeneration. That's how I came up with macrophages, immune cells required for muscle's ability to self-repair in our bodies."
After a muscle injury, one class of macrophages shows up on the scene to clear the wreckage left behind, increase inflammation and stimulate other parts of the immune system. One of the cells they recruit is a second kind of macrophage, dubbed M2, that decreases inflammation and encourages tissue repair. While these anti-inflammatory macrophages had been used in muscle-healing therapies before, they had never been integrated into a platform aimed at growing complex muscle tissues outside of the body. "When we damaged the adult-derived engineered muscle with a toxin, we saw no functional recovery and muscle fibers would not build back. But after we added the macrophages in the muscle, we had a wow moment. The muscle grew back over 15 days and contracted almost like it did before injury. It was really remarkable."
The discovery may lead to a new line of research for potential regenerative therapies. According to a popular theory, fetal and newborn tissues are much better at healing than adult tissues at least in part because of an initial supply of tissue-resident macrophages that are similar to M2 macrophages. As individuals age, this original macrophage supply is replaced by less regenerative and more inflammatory macrophages coming from bone marrow and blood. "We believe that the macrophages in our engineered muscle system may behave more like the muscle-resident macrophages people are born with. We are currently working to understand if this is indeed the case. One could then envision 'training' macrophages to be better healers in a system like ours or augmenting them by genetic modifications and then implanting them into damaged sites in patients."
Adult skeletal muscle has a robust capacity for self-repair, owing to synergies between muscle satellite cells and the immune system. In vitro models of muscle self-repair would facilitate the basic understanding of muscle regeneration and the screening of therapies for muscle disease. Here, we show that the incorporation of macrophages into muscle tissues engineered from adult-rat myogenic cells enables near-complete structural and functional repair after cardiotoxic injury in vitro.
First, we show that-in contrast with injured neonatal-derived engineered muscle-adult-derived engineered muscle fails to properly self-repair after injury, even when treated with pro-regenerative cytokines. We then show that rat bone-marrow-derived macrophages or human blood-derived macrophages resident within the in vitro engineered tissues stimulate muscle satellite cell-mediated myogenesis while significantly limiting myofibre apoptosis and degeneration. Moreover, bone-marrow-derived macrophages within engineered tissues implanted in a mouse model augmented blood vessel ingrowth, cell survival, muscle regeneration, and contractile function.