Cellular-Scale Matrix Stiffness Gradient at Soft-Hard Tissue Interfaces Regulates Immunophenotype of Mesenchymal Stem Cells

Soft-hard tissue interfaces are widespread in the human body, where matrix stiffness gradient at the cellular scale is a typical biophysical cue that cells experience. However, most studies have focused on the large-scale matrix stiffness gradient that affected cell fate. It is still necessary to elucidate the impact of cellular-scale matrix stiffness gradient on cell behavior and function, such as mesenchymal stem cell (MSC) immunoregulation. In this study, it is observed that stiffness gradient strength (SGS) at a typical soft-hard tissue interface (e.g., periodontal ligament (PDL)-alveolar bone (AB) enthesis) in periodontitis is lower than that in health, due to low mineral content gradient there. Meanwhile, MSCs residing at PDL-AB enthesis in periodontitis show an anti-inflammatory phenotype (i.e., MSC2). To investigate how the stiffness gradient affects the MSC immunoregulation, both high and low SGS in vitro is recapitulated. Combined with mathematical modeling, low SGS minimizes the ability of cell polarization in guiding the MSC2, which is probably driven by the reduced polarization of integrin β1 clusters and myosin IIB, leading to H4K16ac localization-dependent chromatin opening that facilitates TLR3 gene expression. This study establishes the molecular mechanisms by which MSCs respond to cellular-scale stiffness gradient at PDL-AB enthesis for regulating the immunophenotype development.