CD8A and neoplasm: These TRCs actively import tryptophan and produce kynurenine, which is released into the tumor microenvironment where it stimulates PD-1 expression in adjacent CD8+ T-cells in a kynurenine-AhR-dependent manner to decrease the immune response 164 and has therefore attracted vast interest in the development of combination therapies including anti-PD-1 therapy 165.