Histone deacetylase 6 (HDAC6) is a peculiar HDAC isoform whose expression and functional alterations have been correlated with a variety of pathologies such as autoimmune disorders, neurodegenerative diseases, and cancer. It is primarily a cytoplasmic protein, and its deacetylase activity is focused mainly on nonhistone substrates such as tubulin, heat shock protein (HSP)90, Foxp3, and cortactin, to name a few. Selective inhibition of HDAC6 does not show cytotoxic effects in healthy cells, normally associated with the inhibition of Class I HDAC isoforms. Here, we describe the design and synthesis of a new class of potent and selective HDAC6 inhibitors that bear a pentaheterocyclic central core. These compounds show a remarkably low toxicity both in vitro and in vivo and are able to increase the function of regulatory T cells (Tregs) at well-tolerated concentrations, suggesting a potential clinical use for the treatment of degenerative, autoimmune diseases and for organ transplantation.