Regulatory T Cells: Pathophysiological Roles and Clinical Applications

Inflammation and immune responses after tissue injury play pivotal roles in the resolution of inflammation, tissue recovery, fibrosis, and remodeling. Regulatory T cells (Tregs) are responsible for immune tolerance and are usually activated in secondary lymphatic tissues. Activated Tregs subsequently regulate effector T cell and dendritic cell activation. For clinical applications such as the suppression of both autoimmune diseases and the rejection of transplanted organs, methods to generate stabilized antigen-specific Tregs are required. For this purpose, transcriptional and epigenetic regulation of Foxp3 expression has been investigated. In addition to conventional Tregs, there are some Tregs that reside in tissues and are called tissue Tregs. Tissue Tregs exhibit tissue-specific functions that contribute to the maintenance of tissue homeostasis and repair. Such tissue Tregs could also be useful for Treg-based cell therapy. We recently discovered brain Tregs that accumulate in the brain during the chronic phase of ischemic brain injury. Brain Tregs resemble other tissue Tregs, but are unique in expressing neural cell-specific genes such as the serotonin receptor (Htr7); consequently, brain Tregs respond to serotonin. Here, we describe our experiences in the use of Tregs to suppress graft-versus-host disease and to promote neural recovery after stroke.