Since the discovery of adult human mesenchymal stem cells in the late 1900’s, the potential of utilizing these cells in the clinic for cell-therapy applications has been an everpresent goal. Unfortunately, clinical trials using these cells have garnered lackluster results with a high degree of variability in patient outcome and in many cases no difference between patients who received these adult stem cell or placebo. Various factors account for such results including the inability to properly control cell presence via the routine method of intravenous administration, the inability to control cell phenotype once the cells are injected into the patient and the harsh microenvironment cells are injected into. Biomaterials can provide solutions for these factors through engineering scaffolds to present needed signals to both encapsulated stem cells and the surrounding microenvironment. The objective of this project is to engineer bioartificial hydrogels presenting specific signals in the form of integrin-specific ligands and covalently-bound proteins to enhance vascularization and mesenchymal stem cell activity and efficacy in wound and disease models.

We investigated the application of these bioarticifial hydrogels towards two different goals:​ 1) to enhance vascularization and bone regeneration in a critical size bone defect and 2) to enhance immunomodulation of encapsulated stem cells. For the first aim, cell-mediated degradable poly(ethylene) glycol-based (PEG) hydrogels were functionalized with the vasculogenic protein, vascular endothelial growth factor (VEGF), along with one of two types of adhesive peptides, either the α₂β₁ integrin-targeting ‘GFOGER’ peptide or the mainly αᵥβ₃ integrin-targeting ‘RGD’ peptide. These hydrogels were implanted within a murine segmental bone defect and bone repair monitored via microcomputed tomography and vascularization assessed via vessel perfusion. We hypothesized that incorporation of VEGF and different peptides would result in a differential effect on vascularization which would enhance both stem cell therapy and bone repair. For our second goal, we synthesized PEG-based hydrogels functionalized with a tethered form of the protein interferon-gamma (IFN-γ), a protein known to enhance the immunomodulatory properties of hMSCs. The capacity for these hMSC-laden hydrogels to modulate immune responses was tested in vitro with monocytes and T-cells before assessing the platform in a colonic wound model. We hypothesized that covalent tethering of this protein onto hydrogels would result in enhanced immunomodulation by encapsulated stem cells which would translate into more rapid wound resolution.

For our first goal, we found that hydrogels presenting the α₂β₁ ligand ‘GFOGER’ resulted in enhanced vascularization of bone defects compared to hydrogels presenting the αᵥβ₃ ligand ‘RGD’ in the absence of vasculogenic protein. For our second goal, we found that hydrogels functionalized tethered IFN-γ enhanced the immunomodulatory properties of encapsulated hMSCs which led to enhanced tissue resolution in a colonic wound model. Together, our findings elucidate novel ways to enhance adult stem cell efficacy and further the applicability of these cells in clinical settings.