Treg cells are not the only cells that mediate peripheral tolerance. Other regulatory immune cells include T cell subsets similar to but phenotypically distinct from Treg cells, including TR1 cells that make IL-10 but do not express Foxp3, TGF-β-secreting TH3 cells, as well as other less well-characterized cells that help establish a local tolerogenic environment. B cells also express CD22, a non-specific inhibitor receptor that dampens B cell receptor activation. A subset of B regulatory cells that makes IL-10 and TGF-β also exists. Some DCs can make Indoleamine 2,3-dioxygenase (IDO) that depletes the amino acid tryptophan needed by T cells to proliferate and thus reduce responsiveness. DCs also have the capacity to directly induce anergy in T cells that recognize antigen expressed at high levels and thus presented at steady-state by DCs. In addition, FasL expression by immune privileged tissues can result in activation-induced cell death of T cells.