IDO1 and COVID-19: In addition to assessing the expressions of the AhR, PARP1, IFN-β and IDO in suitable peripheral blood mononuclear cells and of plasma pro-and anti-inflammatory cytokines in larger patient and control samples, the Trp-metabolic status of COVID-19 patients should be established through measurements of fasting plasma Trp, Kyn and the Kyn metabolites 3-HK, 3-HAA, KA and QA.