Researchers at Duke University and the University of California, Los Angeles (UCLA), have developed a hydrogel-based biomaterial that, when applied to wounds, triggers a regenerative immune response that could potentially help to heal skin injuries like burns, cuts, diabetic ulcers, and other wounds that would normally heal with scarring that is more susceptible to reinjury.
Tests in mice showed that the new hydrogel—which builds on the scientists’ previous research to develop scaffolds that support wound healing—activates a type of immune response that promotes more effective skin healing, reduced scarring, and complex skin structures, resulting in stronger, healthier healed skin.
“This study shows us that activating the immune system can be used to tilt the balance of wound healing from tissue destruction and scar formation to tissue repair and skin regeneration,” said Duke’s Tatiana Segura, PhD, senior author of the team’s paper in Nature Materials. “I am excited about the possibility of designing materials that can directly interact with the immune system to support tissue regeneration. This is a new approach for us.”
Segura and colleagues report on their developments in a paper titled, “Activating an adaptive immune response from a hydrogel scaffold imparts regenerative wound healing.”
In most mammals, the natural process of scar formation and tissue fibrosis is highly evolved and is designed to help restore critical barrier functions for survival, at the tissue level, the authors explained. “This process, however, is ultimately a biological ‘triage’ that favors the rapid deposition of a fibrotic matrix to restore the barrier at the expense of a loss of function of complex tissue.” The goal of regenerative medicine is thus to help to restore tissue function back to normal physiological activity, they continued. “For biomaterial scaffolds, the optimal strategy to achieve this requires balancing material degradation with tissue regrowth … a major goal when engineering skin regeneration is to allow for the rapid restoration of barrier function while providing an increased tissue tensile strength and higher tissue function.”
Current wound-healing hydrogels that are available for clinical use effectively sit on the surface of the wound, where they act as a dressing that helps to prevent the wound from drying out. That in turn helps to speed wound healing, but generally via scar formation. “The body forms scar tissue as fast as possible to reduce the chance of infection, to reduce pain, and, in larger wounds, to avoid water loss through evaporation,” said Maani Archang, a first author on the paper and an MD/PhD student in the labs of Scumpia and co-research lead Dino Di Carlo, PhD, at UCLA. “It’s a natural process of wound healing.”
In 2015, the research team, head by Segura and Di Carlo, developed microporous annealed particle (MAP) hydrogels, which are a microparticle-based biomaterial that can integrate into the wound rather than sit on the skin’s surface. The beads within the MAP gel link together but leave open spaces, creating a porous structure that provides a support for cells as they grow across the wound site. As the wound closes, the gel then slowly dissolves, leaving behind healed skin. “This unique design resulted in improved tissue closure and improved vascularization relative to a nanoporous (but chemically equivalent formulation) hydrogel in a cutaneous wound model,” the team explained.
Although the MAP hydrogels allowed for rapid cellular growth and faster repair, the researchers noticed that the formation of complex structures, such as hair follicles and sebaceous glands, was limited in the healed skin. They considered how they might alter their biomaterial to improve the quality of the healed skin. “Previously we’d seen that as the wound started to heal, the MAP gel started to lose porosity, which limited how the tissue could grow through the structure,” said Don Griffin, PhD, an assistant professor at the University of Virginia who is a first author on the paper and a former postdoctoral fellow in the Segura lab. “We hypothesized that slowing down the degradation rate of the MAP scaffold would prevent the pores from closing and provide additional support to the tissue as it grows, which would improve the tissue’s quality.”
To achieve this, and rather than create an entirely new gel with new materials, the team focused on the chemical linker that allowed the scaffold to be naturally broken down by the body. In their original MAP gels, this chemical linker is composed of an amino acid sequence taken from the body’s own structural proteins and arranged in a chemical orientation called L chirality. Because this peptide sequence and orientation is common throughout the body, this helps the gel avoid triggering a strong immune response, but it also enables ready degradation through naturally present enzymes.
“Previously, we used amino acid chirality to tune the proteolysis rate of peptide nanocapsules for the controlled release of encapsulated growth factors,” the investigators stated. “Therefore, we chose to use an analogous approach to slow the enzymatic degradation of our MAP scaffold by switching the chirality of the peptide crosslinker … To promote more extensive tissue ingrowth before scaffold degradation, we aimed to slow MAP degradation by switching the chirality of the crosslinking peptides from L- to D-amino acids.”
“Our body has evolved to recognize and degrade this amino acid structure, so we theorized that if we flipped the structure to its mirror image, which is D chirality, the body would have a harder time degrading the scaffold,” noted Segura, a professor of biomedical engineering at Duke.
When the investigators tested this new biomaterial in experiments in mice, they found that, contrary to their expectations, it didn’t take longer to degrade. ” … when we put the hydrogel into a mouse wound, the updated gel ended up doing the exact opposite,’ Segura said. In fact, while the updated material did integrate into the wound and support tissue as the wound closed, the team discovered that instead of lasting longer, the new gel had almost entirely disappeared from the wound site, leaving behind just a few particles. “Unexpectedly, despite showing the predicted slower enzymatic degradation in vitro, D-peptide crosslinked MAP hydrogel (D-MAP) hastened material degradation in vivo,” they wrote.
Despite this fast hydrogel degradation, the healed skin still turned out to be stronger, and included complex skin structures, such as hair follicles, which are typically absent in scars. After further investigation, the researchers discovered that the reason for the stronger healing—despite the lack of longevity—was a different immune response to the gel.
After a skin injury, the body’s innate immune response is immediately activated to ensure that any foreign substances that enter the body are quickly destroyed. If substances can escape this first immune response, the body’s adaptive immune response kicks in, which identifies and targets the invading material with more specificity.
Because the original MAP gel was made with the common L peptide structure, it generated a mild innate immune response. But when the team placed the reformulated gel into a wound, the foreign D chirality activated the adaptive immune system, which created antibodies and activated cells including macrophages that targeted and cleared out the gel more quickly after the wound closed. “Our findings suggest that an engineered type 2 immune response to sterile, degradable microparticle-based materials can trigger regeneration rather than fibrosis and further support a role of adaptive immune cells to restore tissue function.” the scientists wrote. They say their results “… demonstrate that the generation of an adaptive immune response from a biomaterial is sufficient to induce cutaneous regenerative healing despite faster scaffold degradation.”
“There are two types of immune responses that can occur after injury—a destructive response and a more mild regenerative response,” said Scumpia, an assistant professor in the division of dermatology at UCLA Health and the West Los Angeles VA Medical Center. “When most biomaterials are placed in the body, they are walled off by the immune system and eventually degraded or destroyed. But in this study, the immune response to the gel induced a regenerative response in the healed tissue.”
Working with Maksim Plikus, PhD, a regenerative tissue expert at the University of California, Irvine, the team also confirmed that key structures, such as hair follicles and sebaceous glands that they had observed, were correctly forming over the scaffold. Further investigation into the mechanism indicated that its cells of the adaptive immune system that are required for this regenerative response.
As the investigators continue to study the regenerative immune response to their MAP hydrogel, they are also exploring the possibility of its use as an immunomodulatory platform. “The team is now exploring the best way to release immune signals from the gel to either induce skin regeneration or develop the hydrogel as a vaccine platform,” said Scumpia.