AuthOr’s VIew AuthOr’s VIew

In normal tissues, stem cells reside in a specialized niche that regulates their prop-erties and fates through cell-to-cell con-tacts and soluble factors including several cytokines. The components of this niche thus provide context signals to maintain stem cells in an equilibrium between self-renewal and differentiation. Tumors are generally viewed as abnormal tissues, and only a small fraction of malignant cells are considered as cancer stem cells (CSCs), owing to their superior tumor-initiating capacity. Emerging evidence shows that, similar to their normal counterparts, CSCs also reside in a specific microenvironment that is referred to as CSC niche. Thus, similar to normal stem cells, CSCs are regulated by, and in turn influence, stro-mal cells within the tumor microenviron-ment. These stromal cell compartments, including endothelial cells, infiltrating leukocytes, cancer-associated fibroblasts (CAFs), myofibroblasts and mesenchymal stem cells (MSCs), interact with CSCs directly or by secreting growth factors and cytokines.1 Among stromal cells, tumor-associated macrophages (TAMs) majorly contribute to the tumor mass. High lev-els of tumor-infiltrating macrophages are often correlated with high tumor stage and poor disease outcome.2 Although numerous studies have highlighted the links between TAMs and tumor initia-tion, proliferation, immunosuppression, Osteopontin, a possible modulator of cancer stem cells and their malignant nich