Pericytes, a cell type contributing to autoimmunity and immune tolerance

Pericytes have been, since their discovery, a very hard-to-define cell because of their unknown ontogeny and the lack of specific markers. As a consequence, several attempts to characterize both its molecular pattern and its metabolism have been carried out to describe the physiological role they play. Pericytes are located in the abluminal wall of small vessels and contribute to the maintenance of capillary tone and the regulation of oxygen flow to adjacent tissues, maintaining the homeostasis of the blood-brain barrier. Furthermore, they have been described as cells with immunological properties, being able to sense and secrete proinflammatory and antiinflammatory cytokines and to activate T cells, hence controlling the immune response. Interestingly, pericytes immune function might be modulated through molecular mechanisms such as chaperone-mediated autophagy, making them to convert from immunogenic to immunosuppressive cells contributing in autoimmunity and immune tolerance. The failure of the different pericytes functions which are implicated in the brain homeostasis is related with several pathologies associated to inflammation, including type 2 diabetes, multiple sclerosis, stroke, Alzheimer's disease, and cancer. In these scenarios, pericytes have always been proved as mediators of the pathology, which indicates that this barely-known type of cell might have a wide variety of unknown roles.