IDO1 and inflammatory bowel disease: Interestingly, a lower expression of IDO1 is observed in immune cells from IBD patients compared to healthy controls54 and reduced Kyn levels have been measured in colonic explants from non-lesional biopsies from IBD patients,23 with lower Kyn/Trp ratios in non-lesional UC patients, suggesting that a loss of tolerance caused by reduced IDO1 levels could underlie the disease.