The maternal immune system would, logically, reject the fetal tissue since it would be identified as foreign protein. This process is actively blocked by the fetal tissues by producing enzymes which rapidly uses up tryptophan. Systemic tryptophan levels falls during pregnancy.
CONCLUSION: This article demonstrates the importance of the amino acid tryptophan, in prevention of rejection of the fetus by the maternal immune system. The mammalian placenta is not a physical barrier to the maternal immune system. Inhibition of the supply of tryptophan prevents the buildup of lymphocytes against the fetus which could cause rejection. The authors conclude that the fetal tissue defends itself from maternal T cell attack by altering tryptophan levels, locally.
NOTE: Tryptophan is not used therapeutically in this article. Low tryptophan levels seem to be protective of the fetus. Use of tryptophan during pregnancy could, theoretically, cause rejection of the fetus.
It is known that cells in the region between the fetus and the mother produce an enzyme called 2,3 dioxygenase (IDO) which uses tryptophan. Tryptophan is an amino acid that is necessary for the maternal T cells of the immune system to function. This results in immune tolerance of the fetus by the mother.
There is increased tryptophan breakdown (catabolism) in cancer patients. Inhibitors of IDO may have a role in cancer chemotherapy.
Read about the role of tryptophan in the brain in depression.
Tryptophan is necessary for the cells to make nicotinamide adenine dinucleotide (NAD). NAD precursors include nicotinic acid/niacin, nicotinamide/niacinamide, and nicotinamide riboside. Autoimmune diseases can be viewed as a form of pellagra in which tryptophan deficiency causes niacin deficiency and disrupted immunity.
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