Nonclassical human leukocyte antigen (HLA) class I molecule
HLA-G and indoleamine 2,3 dioxygenase (INDO) in humans and
mice, respectively, have been shown to play crucial immunosuppressive
roles in fetal-maternal tolerance. HLA-G inhibits
natural killer and T cell function by high-affinity interaction with
inhibitory receptors, and INDO acts by depleting the surrounding
microenvironment of the essential amino acid tryptophan,
thus inhibiting T cell proliferation. We investigated whether
HLA-G expression and INDO function were linked. Working
with antigen-presenting cell (APC) lines and monocytes, we
found that functional inhibition of INDO by 1-methyl-tryptophan
induced cell surface expression of HLA-G1 by HLA-G1-
negative APCs that were originally cell-surface negative, and
that in reverse, the functional boost of INDO by high concentrations
of tryptophan induced a complete loss of HLA-G1 cell
surface expression by APCs that were originally cell-surface
HLA-G1-positive. This mechanism was shown to be posttranslational
because HLA-G protein cell contents remained unaffected
by the treatments used. Furthermore, HLA-G cell surface
expression regulation by INDO seems to relate to INDO function,
but not to tryptophan catabolism itself. Potentia
