IDO2 and long COVID-19: Furthermore, the blood KYN/Trp ratio was significantly elevated in patients with COVID-19, and IDO2 expression was induced in peripheral blood mononuclear cells (PBMCs) and in the postmortem brains of patients with post-acute sequelae of SARS-CoV-2 infection (PASC), suggesting a link between PASC and KYN metabolism [28].