Measuring the Effects of Cytokines on the Modification of Pericellular Rheology by Human Mesenchymal Stem Cells.

Implantable hydrogels are designed to treat wounds by providing structure and delivering additional cells to damaged tissue. These materials must consider how aspects of the native wound, including environmental chemical cues, affect and instruct delivered cells. One cell type researchers are interested in delivering are human mesenchymal stem cells (hMSCs) due to their importance in healing. Wound healing involves recruiting and coordinating a variety of cells to resolve a wound. hMSCs coordinate the cellular response and are signaled to the wound by cytokines, including transforming growth factor-β (TGF-β) and tumor necrosis factor-α (TNF-α), present in vivo. These cytokines change hMSC secretions, regulating material remodeling. TGF-β, present from inflammation through remodeling, directs hMSCs to reorganize collagen, increasing extracellular matrix (ECM) structure. TNF-α, present primarily during inflammation, cues hMSCs to clear debris and degrade ECM. Because cytokines change how hMSCs degrade their microenvironment and are naturally present in the wound, they also affect how hMSCs migrate out of the scaffold to conduct healing. Therefore, the effects of cytokines on hMSC remodeling are important when designing materials for cell delivery. In this work, we encapsulate hMSCs in a polymer-peptide hydrogel and incubate the scaffolds in media with TGF-β or TNF-α at concentrations similar to those in wounds. Multiple particle tracking microrheology (MPT) measures hMSC-mediated scaffold degradation in response to these cytokines, which mimics aspects of the in vivo microenvironment post-implantation. MPT uses video microscopy to measure Brownian motion of particles in a material, quantifying structure and rheology. Using MPT, we measure increased hMSC-mediated remodeling when cells are exposed to TNF-α and decreased remodeling after exposure to TGF-β when compared to untreated hMSCs. This agrees with previous studies that measure: (1) TNF-α encourages matrix reorganization and (2) TGF-β signals the formation of new matrix. These results enable material design that anticipates changes in remodeling after implantation, improving control over hMSC delivery and healing.