AHR and COVID-19: It is likely that, in aggregate, the markedly altered levels of these AhR and PXR ligands observed in our cohort contributed to the dysregulation of the immune response in severe COVID-19; however, further studies are needed to understand the cumulative impact of oppositely altered tryptophan catabolites (which presumably also differ with respect to their AhR and PXR binding affinities) on individual immune cells.