The study investigates the mechanism by which macrophage tryptophan catabolism, mediated by indoleamine 2,3-dioxygenase (IDO), suppresses T cell proliferation. Macrophages differentiated under macrophage colony-stimulating factor (MCSF) acquire the ability to rapidly and selectively deplete tryptophan, an essential amino acid, in response to T cell activation. This depletion is driven by IDO, which is induced by T cell-derived signals such as interferon-γ (IFN-γ) and CD40 ligand (CD40L). Inhibition of IDO with 1-methyl-tryptophan prevents macrophage-mediated suppression. T cells activated in tryptophan-deficient conditions can synthesize protein and enter the cell cycle but arrest at a mid-G1 checkpoint, where they require a second round of T cell receptor (TCR) signaling to progress. This arrest is stable and not resolved by simply restoring tryptophan or costimulation via CD28. The findings suggest that IDO-expressing antigen-presenting cells (APCs) regulate T cell activation by depleting tryptophan, maintaining peripheral tolerance.The study investigates the mechanism by which macrophage tryptophan catabolism, mediated by indoleamine 2,3-dioxygenase (IDO), suppresses T cell proliferation. Macrophages differentiated under macrophage colony-stimulating factor (MCSF) acquire the ability to rapidly and selectively deplete tryptophan, an essential amino acid, in response to T cell activation. This depletion is driven by IDO, which is induced by T cell-derived signals such as interferon-γ (IFN-γ) and CD40 ligand (CD40L). Inhibition of IDO with 1-methyl-tryptophan prevents macrophage-mediated suppression. T cells activated in tryptophan-deficient conditions can synthesize protein and enter the cell cycle but arrest at a mid-G1 checkpoint, where they require a second round of T cell receptor (TCR) signaling to progress. This arrest is stable and not resolved by simply restoring tryptophan or costimulation via CD28. The findings suggest that IDO-expressing antigen-presenting cells (APCs) regulate T cell activation by depleting tryptophan, maintaining peripheral tolerance.