AHR and COVID-19: In line with previous studies34,35, the host-derived tryptophan catabolites kynurenine, which is also an AhR ligand, and the potentially neurotoxic 3-hydroxykynurenine36 were strongly enriched in COVID-19 patients, with kynurenine levels showing a robust positive association with severity (ρ = 0.7, FDR-adjusted P < 0.0001; Fig. 4), in both early and late samples (Supplementary Fig. 3a).