According to the report, a asthma mouse model sensitized with ovalbumin (OVA) observed that expression of IDO inhibited Th2 type airway inflammation in the lungs (74), while the IDO-expressing and Kynurenine-producing eosinophils co cultured with Th1 or Th2 cells, a preferential decrease in Th1 response and subsequent increase in Th2 cytokine production were observed (208), indicating that IDO-expressing eosinophils may create cytotoxic metabolites that maintain an imbalance between Th1 and Th2 cell populations, and the consistent results were obtained in the study of Molano et al. Here, IDO1 is linked to asthma.