Interactions between cancer stem cells, immune system and some environmental components: Friends or foes?

Cancer stem cells (CSCs) are a subgroup of tumor cells that are characterized by their tumor initiating capacity, low proliferation rate, self-renewal capacity, pluripotency and chemoresistance. The immune system, including innate and adaptive immune cells, plays pro-tumorigenic and anti-tumorigenic roles in cancer biology. Immunosurveillance often initially successfully eradicates tumor cells. However, following a phenomenon referred to as immunoediting, cancer cells may ultimately evade immune destruction, thus enabling tumor progression. Here, we review how CSCs both escape immune destruction and foster establishment of an immunosuppressive tumor microenvironment through intricate interactions with and recruitment of a broad range of immune cells, including natural killer (NK) cells, myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs), tumor-associated macrophages (TAMs), cytotoxic T-lymphocytes (CTLs) and T helper (Th) cells. Further elucidation of CSC-immune cell interactions and the underlying signaling mechanisms will open up novel opportunities to improve cancer immunotherapy.