IDO protein is involved in the induction of tumor tolerance. IDO is up-regulated in both tumor cells (right) and in a subset of regulatory/plasmacytoid DCs within tumor-draining lymph nodes (left). At the tumor site, tryptophan degradation impairs the effector function of antigen-specific T cells and reduces the immune-mediated control of tumor growth. In tumor-draining lymph nodes, regulatory DCs create a tolerogenic microenvironment, where the presentation of relevant tumor antigens to developing T cells may be defective, which leads to reduced numbers of antigen-specific T cells and increased numbers of T_regs.