Catalyzes the first and rate limiting step of the catabolism of the essential amino acid tryptophan along the kynurenine pathway . Involved in the peripheral immune tolerance, contributing to maintain homeostasis by preventing autoimmunity or immunopathology that would result from uncontrolled and overreacting immune responses . Tryptophan shortage inhibits T lymphocytes division and accumulation of tryptophan catabolites induces T-cell apoptosis and differentiation of regulatory T-cells . Acts as a suppressor of anti-tumor immunity . Limits the growth of intracellular pathogens by depriving tryptophan . Protects the fetus from maternal immune rejection.