Title: Chondrogenesis of Stem/Progenitor Cells by Chemotaxis Using Novel Cell Homing Systems
Description: The predominant approach for cartilage tissue engineering involves cell transplantation with or without cytokine delivery, biomaterial scaffolds, bioreactors, applied mechanical stimulation and altered oxygen tension. Despite its scientific merit, cell delivery faces drawbacks including scarce cell availability, donor site trauma, possible immune rejection and potential tumorigenesis. Tissue regeneration by cell homing is a novel concept and may offer the advantage of accelerated clinical translation. Promising results have been shown using a cell homing approach to engineer a number of tissue types including dental pulp, vascular tissue and bone. Various stem/progenitor cell populations are present in tissues adjacent to an articular cartilage defect including subsets of cells that have the ability to differentiate into cartilage-like tissue. Furthermore, several factors have been elucidated that stimulate stem/progenitor cell homing and selected cytokines have been discovered to be potent at inducing chondrogenic differentiation of stem/progenitor cells.

Cell homing is an exciting area of regenerative medicine but many critical questions remain such as cell origin, homing distance, and effective chemotactic cues. In addition to currently studied cell homing cues, other cytokines present during inflammation that are not typically known for their homing abilities might be helpful in recruiting additional cells to the scaffold and improving the quality of cartilage tissue formation. The effect of concurrently exposing a cell population to multiple cytokine signals, similar to conditions that cells experience in vivo, has not been fully investigated. Determining which cytokine or groups of cytokines that induce high levels of chemotaxis would be critical for designing effective bioactive scaffolds for cell recruitment and chondrogenesis. This thesis develops novel systems to characterize stem/progenitor cell migration and uses the knowledge gained from these systems to develop new methods for inducing chondrogenesis by cytotactic homing. First, the concept of stem/progenitor cell homing for cartilage tissue regeneration is reviewed (Chapter 1).

Next, a system was developed for the in vitro recruitment and chondrogenesis of Adipose Stem Cells (ASCs), Mesenchymal Stem Cells (MSCs) and Synovium Stem Cells (SSCs), all of which are natively located adjacent to a full-thickness articular cartilage defect (Chapter 2). Using microfluidic principles, novel assay systems were designed and built to characterize the process of stem cell migration in the presence of single and competing cytokine signals (Chapter 3). An in-depth study was conducted investigating the process of stem/progenitor cell migration in the presence of competing cytokine signals (Chapter 4). Lastly, the knowledge gained through extensive chemokine testing using these novel assay systems was used to develop a bioactive scaffold to induce cell homing and chondrogenesis for rhinoplasty augmentation in a rat model (Chapter 5). The novel migration devices developed herein offer a rare opportunity for screening of cell homing efficacy, potentially applicable to any stem cell population including embryonic, iPS, skeletal, muscular, neural, cardiac and adipose.

A number of basic biological concepts have been examined by studies using these devices such as cell motility behavior and optimal migratory distances. The competitive cytotactic assay system provided new insight into stem cell behavior in response to gradients of multiple cytotactic cues, thus mimicking native in vivo conditions. By determining combinations of cytokines effective at maximizing cell homing, novel approaches for cartilage tissue engineering without the need for cell delivery, were developed for rhinoplasty augmentation. These systems for inducing chondrogenesis by chemotactic homing were shown to be an effective alternative to cell transplantation for cartilage tissue regeneration therapies.