Immunology of cancer stem cells in solid tumours. A review

Cancer stem cells (CSCs) represent a minor subpopulation of tumour cells that share some features with the normal stem cells of the tissue from which tumour derives and have the properties of self-renewal, multiple differentiation and tumour initiation (tumour-initiating cells, TICs). Thus CSCs/TICs need to survive cancer therapies in order to provide new, more differentiated, metastatic-prone tumour cells. This occurs through different signals delivered within the tumour microenvironment. The immune system of cancer patients may recognise CSCs/TICs and kill them though it is unclear whether this may occur in vivo during spontaneous tumour growth. This review summarises findings on the immunological profile of CSCs/TICs as compared with neoplastic non-stem cells and discusses the possible antigens recognised by the patients' immune system, the in vitro and the potential in vivo immunogenicity of such antigens and the ability of human CSCs/TICs to down-regulate the immune response by the release of a variety of suppressive factors. We conclude that available data on immunological characterisation of CSCs/TICs may be useful in the perspective of designing new translational immunotherapy protocols targeting CSCs/TICs.