Involvement of Interferon Regulatory Factor 7 in Nicotine's Suppression of Antiviral Immune Responses

Nicotine, the active ingredient in tobacco smoke, suppresses antiviral responses. Interferon regulatory factors (IRFs) regulate transcription of type I interferons (IFNs) and IFN-stimulated genes (ISGs) in this response. IRF7 is a key member of the IRF family. Expression of Irf7 is elevated in the brains of virus-infected animals, including human immunodeficiency virus-1 transgenic (HIV-1Tg) rats. We hypothesized that IRF7 affects nicotine's modulation of antiviral responses. Using CRISPR/Cas9 system, IRF7-mutant cell lines were created from human embryonic kidney 293FT cells in which 16 nicotinic acetylcholine receptors (nAChRs) were detected. Decreased expression of IRF7 was confirmed at both the mRNA and protein levels, as was IRF7-regulated cell growth in two IRF7-mutant cell lines, designated IRF7-Δ7 and IRF7-Δ11. In IRF7-Δ7 cells, expression of two nAChR genes, CHRNA3 and CHRNA9, changed modestly. After stimulation with polyinosinic-polycytidylic acid (poly I:C) (0.25 μg/ml) for 4 h to mimic viral infection, 293FT wild-type (WT) and IRF7-Δ7 cells were treated with 0, 1, or 100 μM nicotine for 24 h, which increased IFN-β expression in both types of cells but elevation was higher in WT cells (p < 0.001). Expression was significantly suppressed in WT cells (p < 0.001) but not in IRF7-Δ7 cells by 24-h nicotine exposure. Poly I:C stimulation increased mRNA expression of retinoic-acid-inducible protein I (RIG-I), melanoma-differentiation-associated gene 5 (MDA5), IFN-stimulated gene factor 3 (ISG3) complex, and IFN-stimulated genes (IRF7, ISG15, IFIT1, OAS1); nicotine attenuated mRNA expression only in WT cells. Overall, IRF7 is critical to nicotine's effect on the antiviral immune response. Graphical Abstract Involvement of IRF7 in nicotine's suppression of poly I:C-induced antiviral immune responses. PAMPs, such as a synthetic viral analogue of dsRNA poly I:C attack cells, will be recognized by PRRs, and the host innate immunity against viral infection will be activated. PRRs signaling trigger phosphorylation of IRF7 and IRF3 to induce their translocation to the nucleus and result in the production of type I IFNs. Then IFNs bind to IFNAR to activate the transcription factor ISGF3, a complex consisting of STAT1, STAT2, and IRF9. Further, it induces the expression of ISGs, including IFIT1, OAS1, IRF7, ISG15, etc. Nicotine suppresses the immune responses stimulated by poly I:C. In the IRF7-mutant cells, nicotine's suppressive effects on poly I:C-stimulated immune responses were restrained.