Figure 2. Inhibition of T cell activation.

A number of cell surface molecules controls and suppresses T cell activation by directly suppressing T cell activation (via CTLA-4 or PD-1), by directly signaling the T cell, or indirectly in the case of CTLA4Ig, or regulatory T cells by binding to DCs inducing IDO. IDO catalyzes the degradation of tryptophan to kynureine and subsequent catabolic byproducts. Local decreases in the availability of tryptophan and the presence of its catabolic byproducts inhibit T cell proliferation and may induce apoptotic death. Inhibition of clonal expansion leads to a functional state of tolerance. This pathway may be accentuated by the presence of IL-10 or TGF-β. Regulatory T cells secretion of IL-10 may induce these B7-dependent DC-signaling pathways as well.