Indoleamine 2,3-dioxygenase 1 (IDO1) is an enzyme that oxidizes tryptophan to start the first step of kynurenine pathway product generation. IDO1 is implicated in cancer pathophysiology because of its potential to deplete tryptophan (TRP) in local tumor microenvironments (TME), thereby impairing immune cell functions. Thus, IDO1 inhibitors were developed to “reactivate” T cell proliferation and function in the TME. So far, these drugs collectively failed to show efficacy in multiple clinical trials including ovarian cancer, suggesting a more in-depth study of the molecular function of IDO1 in cancer cells is warranted.

We used single-cell RNA sequencing (scRNAseq) and deep-visual-proteomics (DVP) to study the IDO1+ tumor fraction in ovarian cancer patient tissues. We also employed a wide range of ovarian cancer cell lines to reconstitute key features of the clinical system where IDO1 expression was exogenously modulated with IFN-y in control or IDO1-deficient backgrounds. Then, stimulated cells were analyzed using life-cell imaging to determine cell death and immunoblotting to characterize the activated molecular pathways.

The results from the scRNAseq and DVP suggested that IDO1 expression is heterogenous and is specifically linked to IFN-y secreted by T cells in the ovarian TME. Subsequently, we used the beforementioned in-vitro system and investigated the response of ovarian cancer cells to IFN-y stimulation. We discovered that IDO1 induction with IFN-y results in an IDO1-dependent cell death caused by TRP starvation via activation of the GCN2 integrated stress response pathway and not by accumulation of kynurenine. The phenotype is completely abolished whenever IDO1 is knock-down or if an IDO1 inhibitor is added to the stimulated cells. To highlight the clinical relevance of our study, we detected a correlation between IDO1+ and the starvation signature in patient data, confirming that IDO1 expressing cells undergo TRP starvation in vivo.

In summary, our study provides evidence supporting a link between IFN-y and IDO1 expression in ovarian cancer patients and suggests a crucial role of TRP in promoting cancer survival. These findings may offer new insights into the molecular mechanisms underlying the detrimental effects of IDO1 inhibition in ovarian cancer. On one hand, IDO1 inhibition might reinvigorate the immune response against the tumor; on the other, IDO1 inhibition might protect the tumor cells from TRP starvation and therefore, it might undermine its therapeutic potential.