IDO2 and Encephalopathy: The important role played by the tryptophan kynurenine metabolic pathway in sepsis-associated encephalopathy may be attributed to the fact that increased inflammatory factors activate indoleamine 2,3-dioxygenase (IDO), which metabolizes and produces several neurotoxic factors, such as quinolinic acid (QA) and 3-hydroxykynurenine (3-HAA) (Stone and Darlington, 2013).