Stroma as an Active Player in the Development of the Tumor Microenvironment

The stroma is a considerable part of the tumor microenvironment. Because of its complexity, it can influence both cancer and immune cells in their behavior and cross-talk. Aside from soluble products released by non-cancer and cancer cells, extracellular matrix components have been increasingly recognized as more than just minor players in the constitution, development and regulation of the tumor microenvironment. The variations in the connective scaffold architecture, induced by transforming growth factor beta, lysyl oxidase and metalloproteinase activity, create different conditions of ECM density and stiffness. They exert broad effects on immune cells (e.g. physical barriers, modulation by release of stored TGF-beta 1), mesenchymal cells (transition to myofibroblasts), epithelial cells (epithelial-to-mesenchymal transition), cancer cells (progression to metastatic phenotype) and stem cells (activation of differentiation addressed by the microenvironment characteristics). Physiological mechanisms of the wound healing process, as well as mechanisms of fibrosis in some chronic pathologies, closely recall aspects of cancer deregulated biology. Their elucidation can provide a better understanding of tumor microenvironment immunobiology. In the following short review, we will focus on some aspects of the fibrous stroma to highlight its active participation in the tumor microenvironment constitution, tumor progression and the local immunological network.