IDO1 inhibitors block septic cytokine storm by suppressing the IDO1-AHR-CYP1A1 axis

Indoleamine-2,3-dioxygenase 1 (IDO1) is the rate-limiting enzyme in tryptophan (Trp) catabolism along kynurenine (Kyn) pathway. Increased IDO1 activity has been noticed in patients with sepsis, while IDO1's involvement in sepsis, especially in the initial cytokine storm phase is not yet completely understood. Using the GEO database and clinical samples of sepsis, current study revealed that IDO1-AHR-CYP1A1 axis was significantly upregulated and closely related to cytokine storm in septic patients. With cell models of cytokine storm, it was found that IDO1 promoted cytokine storm and the apoptosis of model cells via AHR-CYP1A1, and IDO1-AHR-CYP1A1 axis correlated classic cytokine storm signal pathway including STAT3, NF-κB/STAT1, JNK/p38. With mouse models of septic cytokine storm, it was shown that IDO1 inhibitors could block the upregulated IDO1-AHR-CYP1A1 axis, reduce the enhanced inflammatory cytokine levels, decrease the phosphorylation of classic cytokine storm signal pathway, rescue organ damage, and increase survival rate. It was also found that IDO1 activation occurred after the increase of inflammatory cytokine levels. Therefore, classic cytokine storm signal pathways, inflammatory cytokines and IDO1-AHR-CYP1A1 axis form a tripartite interaction loop to promote cytokine storm. IDO1 inhibitors were able to block this process.