It has to do with the metabolism of tryptophan irrespective of whether chemo is used.
” the enzyme indoleamine 2,3-dioxygenase (IDO) has recently attracted special attention. By catabolizing tryptophan to N-formyl-kynurenine, IDO starves T cells from this important amino acid rendering them incapable of mounting appropriate immune responses.”
“Finally, evidence emerges indicating that IDO possibly promotes tumor immune escape by inducing an immunoregulatory or an anergic T cell phenotype at a systemic level.”
https://pubmed.ncbi.nlm.nih.gov/17644189/
Neither reference is talking to anything with chemo.