Biomaterial Directed Immune Modulation and Tissue Regeneration in the Context of Skeletal Muscle Injury

Abstract

Chronic wounds and inflammatory diseases pose a significant health challenge worldwide. In fact, chronic inflammation contributes to the pathogenesis of 7 of the 10 leading causes of death in the United States (CDC, 2016) and the prevalence of diseases associated with chronic inflammation is anticipated to rise. Dysregulation of the inflammatory response can inhibit healing and cause progressive tissue damage. The successful regeneration of functional tissues requires both appropriate modulation of the inflammatory response, and activation of tissue resident cells. Biomaterials offer unique opportunities to spatiotemporally control cytokine delivery, and may provided significant advantages in the modulation of immune cells and tissue resident stem cells alike to promote regeneration. The aim of this thesis is to develop and explore biomaterial systems capable of both modulating the inflammatory response and directly promoting tissue regeneration, independently, in the context of skeletal muscle. In order to modulate the inflammatory response, gold nanoparticles (AuNPs) were designed to deliver immunomodulatory cytokines that could direct macrophage polarization. Partial PEGylation of the AuNP surface, followed by cytokine conjugation resulted in the generation of stable nanoparticles that retained the bioactivity of the conjugated cytokines. Conjugated IL-4, IL-13, and IL-10, all potent promoters of anti-inflammatory and pro-regenerative macrophage phenotypes (M2), were able to direct the polarization of human macrophages in vitro towards M2 phenotypes. Further, PEGylated, IL-4 conjugated AuNPs were able to shift the balance of macrophages in vivo, in ischemic muscle tissue, away from the inflammatory and towards the pro-regenerative M2a phenotype. Importantly, this shift in macrophage polarization resulted in improved muscle histology, and ultimately improved muscle function as evidenced by significant increases in muscle contraction force and velocity. This thesis also explored the use of an established biomaterial system, an injectable alginate hydrogel, to deliver growth factors that could promote angiogenesis, innervation, and muscle regeneration in the contexts of muscle nerve injury and microvascular muscle transplantation. First, in a model of sciatic nerve injury in aged mice, the combined delivery of vascular endothelial growth factor (VEGF) and insulin-like growth factor-1 (IGF-1) promoted the regeneration on functional muscle innervation, ultimately leading to improved toe spreading, significant increases in muscle fiber area, and increased muscle contraction force and velocity. Subsequently, this strategy was tested in a more clinically relevant model of autologous muscle transplantation in rabbits. Here, clinical electromyography demonstrated a significant increase in compound muscle action potential, indicative of improved engraftment and muscle function, in response to alginate delivery of VEGF + IGF-1. In summary, this thesis demonstrates the ability of biomaterials to both modulate the innate inflammatory response and directly promote tissue regeneration, independently, in skeletal muscle tissue. The ability to promote resolution of inflammatory processes in vivo, shift macrophage polarization towards regenerative phenotypes, and directly stimulate tissue regeneration opens new opportunities for biomaterial based regenerative therapies.