TDO2 and chronic kidney disease: Fourth, it is important to consider that TNF-α alone is not sufficient to induce IDO1 as shown by in vivo experiments, but it is an important potentiator of IDO1 expression in several different contexts.7,39 Finally, emerging data convincingly show that TDO also mediates immunoregulatory effects.40 Taken together, it is perhaps the combination of TDO and IDO1, not IDO1 alone, which is responsible for the accelerated metabolism of TRP in CKD.