Foxp3+ CD4+ cells are prominent immune regulatory T (Treg) cells that are most abundant in the intestine. Recent studies have suggested that intestinal Treg cells consist of thymically and extrathymically developed cells that have unique characteristics. A fraction of intestinal Treg cells express T cell receptors that recognize antigens that are derived from the gut microbiota. The presence of the gut microbiota, particularly the Clostridium species, affects the development and function of Treg cells. These intestinal bacteria-induced Treg cells are likely to play a role in the tolerance toward the gut microbiota. These recent advances provide new insight into how T cells are educated in the intestine to maintain homeostasis with the gut microbiota.
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