Enzymatically characterized mammalian and non-mammalian indoleamine 2,3-dioxygenases using recombinant proteins, establishing indoleamine 2,3-dioxygenase catalytic activity and the large difference in catalytic efficiency for L-tryptophan versus D-tryptophan (basis for EC 1.13.11.52 and the L-/D-Trp kinetic parameters recorded in UniProt).

Cloned the human IFN-gamma-inducible IDO cDNA (403-aa ORF); transfection of human fibroblasts conferred constitutive IDO enzymatic activity, establishing that the gene encodes the tryptophan-degrading enzyme and that expression is interferon-gamma-inducible.

In activated human renal tubular epithelial cells, IDO (assayed via the inhibitor 1-methyl-L-tryptophan) contributes to enhanced apoptosis of proliferating CD4+/CD8+ T cells; the IDO inhibitor only partially or did not restore T-cell proliferation, so the proliferation effect was not driven by IDO in this system.

Pharmacologic inhibition of IDO caused rapid T cell-induced rejection of allogeneic concepti in pregnant mice, demonstrating that IDO-mediated tryptophan catabolism at the maternal-fetal interface suppresses T-cell activity and prevents fetal rejection.

Cytosolic indoleamine 2,3-dioxygenase catalyzes the conversion of L-tryptophan and oxygen to formylkynurenine; the active human enzyme is cytosolic.