IDO1 and myalgic encephalomeyelitis/chronic fatigue syndrome: Furthermore, during infectious processes in ME/CFS patients, both increased proinflammatory cytokines and increased oxidative and nitrosative stress due to activation of microglia via TLR3 could lead to increased IDO activity in microglia, resulting in reduced tryptophan levels (increased its catabolism), increased kynurenine catabolites, and decreased 5-HT and melatonin synthesis [366, 367].