Bone marrow derived mesenchymal stem/stromal cells (MSCs) are widely used in clinical trials. In addition to their multipotency, MSC therapeutic efficacy relies on their secretory abilities; specifically, their secretion of growth factors, chemokines, and cytokines can influence endogenous cell behaviors and direct wound healing. Because MSCs only constitute 0.001-0.01% of all mononuclear cells in the bone marrow, their in vitro expansion is required to achieve clinically relevant cell numbers. Unfortunately, expansion of MSCs on tissue culture polystyrene (TCPS) can significantly reduce their in vivo regenerative capacities. Additionally, when injected intravenously or delivered carrier-free to a defect site, MSCs have low survival and engraftment. To begin to address these challenges, this thesis focused on developing hydrogel platforms to direct and maintain MSC secretory properties during in vitro expansion and in vivo delivery. We used PEG-based synthetic macromers reacted via bio-click reactions to create hydrogel environments to direct matrix mechanics, MSC-matrix interactions, and MSC-MSC contacts.
First, by controlling MSC exposure to matrix mechanics, we designed a 2D soft hydrogel intervention to increase MSC cytokine secretion during in vitro TCPS expansion. Next, we designed a granular hydrogel system, comprised of micron-scale PEG-based hydrogels, to promote MSC secretion in 3D. By modulating scaffold porosity and including integrin and cadherin binding epitopes, we controlled MSC cell-cell and cell-matrix interactions. Pore-directed cell clustering increased MSC trophic factor secretion, specifically through N-cadherin mediated interactions. Exploiting our understanding of the influence of cell clustering and the inclusion of N-cadherin peptide mimetics on MSC secretion, we next evaluated the capacity of MSC-laden porous scaffolds to direct bone regeneration in critical-sized rat calvarial defects. Finally, we used these scaffolds to better understand how MSC secretion is changed in osteoporotic environments. Large clusters of osteoporotic MSCs had a pro-resorptive secretory profile, which was reduced by blocking N-cadherin interactions. Overall, we designed multiple hydrogel platforms to exert precise control over the in vitro MSC microenvironment and increased our understanding of how biophysical factors can be tuned to influence MSC secretory properties.</p
