Fig 2: AhR inhibition reversed the aggravated lung function induced by IFN-β or IFN-γ.a, b ICR mice were treated with IFN-β (1 μg/mouse), IFN-γ (10 μg/mouse), IFN-β + CH223191 (10 mg/kg) or IFN-γ + CH223191 through the trachea once every day for 4 days. Blood oxygen saturation was measured at day 5 after treatment (a). Rrs (resistance of the respiratory system), Ers (elastance of the respiratory system), PV-k (curvature of the upper portion of the deflation limb of the Pressure/volume loop) and Eta (tissue hysteresivity) were detected at indicated time points (b). n = 5 mice. c–e hACE2-transgenic mice were infected with SARS-CoV-2, and then treated with or without CH223191 (10 mg/kg) by tail vein once every day for 5 days. The lung tissues were fixed to perform the immunohistochemical staining of mucins 1, 5A and 5B (c, n = 6 mice) and H&E staining (d, n = 6 mice). The PO2 was also analyzed (e, n = 4 mice). Scale bars, 50 μm. The data represent means ± SD. *P < 0.05, **P < 0.01, ***P < 0.001, by one-way ANOVA (a–c and e).

Fig 3: IFN-β and IFN-γ activated the Kyn-AhR pathway for mucin expression.a PAECs or BEAS-2B cells were treated with IFN-β (1 ng/mL) or IFN-γ (10 ng/mL) for 48 h. The expression of IDO1 was determined by western blotting. b BEAS-2B cells were treated with IFN-β (1 ng/mL) or IFN-γ (10 ng/mL) for 48 h. The levels of Kyn from cell lysates or the supernatants were determined by ELISA. c PAECs were isolated from AhR–/– mice were treated with IFN-β (1 ng/mL) or IFN-γ (10 ng/mL) for 24 h. The expression of Mucs 1, 2, 5A, 6, 13, 15 and 16 was determined by real-time PCR. d BEAS-2B cells were treated with Kyn (400 μM) for 24 h. The expression of mucins (MUCs 2, 5A, 6, 7, 13, 15, 16, 17, 18 and 20) was determined by real-time PCR. e Western blot analysis of the expression of mucins 1, 5A, 5B, 13 and 16 from BEAS-2B cells or PAECs treated with Kyn (400 μM) for 48 h. The data represent means ± SD. In b–d, n = 3 biological independent samples. Representative images are from three independent experiments (a and e). *P < 0.05, **P < 0.01, ***P < 0.001, by two-tailed Student’s t-test (d) or one-way ANOVA (b and c).

Fig 4: A schematic for AhR-upregulated mucins in the hypoxia of COVID-19 patients.A normal gas exchange between the alveoli and pulmonary capillary blood is achieved through a passive diffusion of O2 and CO2. During SARS-CoV-2 infection, increased IFN-β and IFN-γ activate the AhR pathway, leading to the expression of mucins in alveolar epithelial cells. Mucins then stick to the blood–gas barrier and increase the thickness. Thickened barrier hinders O2 crossing but not CO2 at the beginning, leading to a clinical symptom of silent hypoxia. Companying the disease progression, more mucin production in combination with inflammation-induced exudate further increases barrier thickness, impeding the exchange of both O2 and CO2, leading to a critical illness. “–”, normal level.

Fig 5: Schematic diagram of AhR promoting SARS-CoV-2 replication and acting as a therapeutic target.AhR was activated by SARS-CoV-2 infection by Trp-Kyn metabolism pathway. The activated AhR inhibits the host antiviral responses mediated by IFN-I and up-regulates ACE2 expression, thus promoting viral replication. AhR antagonists block the activation of AhR, enhance the host antiviral response, and reduce the expression of ACE2 receptor, thus reducing viral replication.