Matrix Stiffness Induces Suppressor Cell Phenotype in CD14+ Bone Marrow mononuclear Cells

Myeloid-derived suppressor cells (MDSCs) suppress anti-tumor immune responses and contribute to tumor resistance to immunotherapy, but how their function is regulated by the tumor microenvironment is unclear. As tumors typically are stiffer than normal healthy tissue, we hypothesized that matrix mechanical properties play a significant role in the immune function of MDSCs. Tumor stiffness was modeled with a collagen-alginate interpenetrating network (IPN), and oscillatory shear rheology was performed to characterize the hydrogels’ resulting viscoelastic properties, i.e., storage and loss moduli and stress relaxation. We isolated primary human CD14+ mononuclear cells from bone marrow, which include a subset of myeloid progenitors that give rise to monocytes, macrophages, and dendritic cells in peripheral blood. The isolated cells were encapsulated in hydrogels with a range of composition and stiffness. qPCR analysis after three days of culture revealed that increased stiffness of the collagen-alginate IPN was associated with elevated expression of MDSC markers, including cyclooxygenase-2 (PTGS2), indoleamine 2,3-dioxygenase (IDO1), and IL6. Further, analysis of conditioned media revealed that stiffer hydrogels were associated with increased amounts of secreted prostaglandin E2 (PGE2), which is associated with tumorigenesis and immune-resistance in melanoma.  Support for the physiologic relevance of these findings came from analysis of processed lysates from B16 melanoma tumors, which were found to contain detectable levels of prostaglandin E2 (PGE2). These findings indicate that CD14+ bone marrow mononuclear cells are responsive to extracellular matrix mechanics, and higher stiffness induces a suppressor cell phenotype. Thus, mechanical interactions between stroma and myeloid cells may have relevant implications in tumor resistance to cancer immunotherapies.