IDO1 and COVID-19: SARS-CoV-2 spike protein is processed by transmembrane protease-serine 2 and favors binding to ACE2.1 SARS-CoV-2 can enter ACE2-expressing cells, but not cells without ACE2.2 Higher levels of ACE2 expression have been seen in lung tissues of patients with severe COVID-19.3 After entry into the cells, coronaviruses activate aryl hydrocarbon receptors (AhRs) by an indoleamine 2,3-dioxygenase (IDO1)-independent mechanism, bypassing the IDO1-kynurenine-AhR pathway.4 However, AhRs enhance their own activity through an IDO1-AhR-IDO1-positive feedback loop prolonging activation.4