Mechano-mediated M2 macrophage polarization and immune suppression in stiffened tumor microenvironment

The tumor microenvironment (TME), which is composed of various cell types and the extracellular matrix (ECM), plays crucial roles in cancer progression and treatment outcomes. However, the impact of the mechanical properties of the ECM, specifically collagen fibril alignment and crosslinking, on macrophage behavior and polarization is less understood. To investigate this, we reconstituted 3D collagen matrices to mimic the physical characteristics of the TME. Our results demonstrated that stiffening the matrix through the alignment or crosslinking of collagen fibrils promotes macrophage polarization toward the anti-inflammatory M2 phenotype. This phenotype is characterized by increased expression of CD105 and CD206 and a distinct cytokine secretion profile. The increased stiffness and aligned fibrils activate mechanotransduction pathways, notably integrin β1 and PI3K signaling, leading to increased IL-4 secretion, which acts in an autocrine manner to further promote M2 polarization. Interestingly, these stiffened microenvironments also suppressed the proinflammatory response. In coculture experiments with breast cancer cell lines (MDA-MB-231 and MCF-7), macrophages within stiffened or aligned matrices significantly increased cancer cell proliferation and invasion. These findings suggest that the mechanical properties of the ECM, specifically its alignment and crosslinking, create a more favorable environment for tumor progression by modulating macrophage activity. Overall, our study underscores the critical role of ECM mechanics in shaping immune cell behavior within the TME, highlighting the potential for therapies that target ECM properties and macrophage polarization to inhibit cancer progression and enhance treatment efficacy.