Subsequent publications corroborated that tryptophan deprivation mediated by IDO1 represses the proliferation of intracellular pathogens auxotrophic for tryptophan, such as Chlamydia and Coxiella, arguing that IDO1-deficient subjects could be more sensitive to infections (Byrne et al., 1986b; Ganesan and Roy, 2019). Here, IDO1 is linked to infection.