Stable pediatric kidney transplant recipients run higher urine indoleamine 2, 3 dioxygenase (IDO) levels than healthy children

Immune cells utilize the IDO enzymatic conversion of trp to kyn to determine T-cell activation vs. anergy/apoptosis. In prior studies, urine IDO levels were higher in rejecting renal allografts than in stable state. However, urine IDO levels in healthy subjects or children are unknown. As a corollary to a larger longitudinal and prospective study of serum and urine IDO levels for transplant immune monitoring, here, we analyzed the difference between urine IDO levels in stable post-transplant vs. healthy children. IDO levels were measured by tandem mass spectrometry and expressed as kyn/trp ratios. We compared one-time urine samples, from 34 well children at general pediatric clinics, to the first-month post-transplant urine samples from 18 children, while in stable state (no acute rejection or major infection event in next 30 days). Urine kyn/trp ratios were significantly higher in stable children in first-month post-kidney transplant (median 16.6, range 3.9-44.0) vs. healthy children (median 9.2, range 3.51-17.0; p = 0.0057 by nonparametric Mann-Whitney test). Higher urine IDO levels even with stable transplant suggest a continuous ongoing low-grade allorecognition/inflammatory process. Our data also provide baseline urine IDO levels in healthy subjects for use in future studies.