Gut microbiota modulates type I interferon and antibody-mediated immune responses in chickens infected with influenza virus subtype H9N2

Commensal gut microbes play a critical role in shaping host defences against pathogens, including influenza viruses. The current study was conducted to assess the role and mechanisms of action of commensal gut microbiota on the innate and antibody-mediated responses of layer chickens against influenza virus subtype H9N2. A total of 104 one-day-old specific pathogen free chickens were assigned to either of the four treatments, which included two levels of antibiotics treatment (ABX- and ABX+) and two levels of H9N2 virus infection (H9N2- and H9N2+). At day 17 of age, chickens in the H9N2+ group were infected via the oral-nasal route with 400 μl of 107 TCID_{50}/ml (200 μl/each route). Oropharyngeal and cloacal swabs at days 1, 3, 5, 7 and 9 post-infection (p.i.) for virus shedding, tissue samples at 12 h, 24 h and 36 h p.i. for mRNA measurement, and serum samples at days 7 and 14 p.i. for hemagglutination inhibition (HI) assay and IgG antibodies were collected. Virus shedding analysis showed that antibiotic treated (depleted)-H9N2 virus infected chickens showed a significantly higher oropharyngeal virus shedding at all time points, and cloacal shedding at days 3 and 5 p.i. compared to control treated (undepleted)-H9N2 infected chickens. Analysis of mRNA expression showed that infection of depleted chickens with H9N2 virus resulted in significantly down-regulated type I interferon responses both in the respiratory and gastrointestinal tracts compared to undepleted-H9N2 infected chickens. However, antibody-mediated immune response analysis showed a significantly higher HI antibody titre and IgG levels in the serum of chickens depleted with antibiotics and infected with H9N2 virus compared to undepleted-H9N2 infected chickens. In conclusion, findings from the current study suggest that the gut microbiota of chickens plays an important role in the initiation of innate responses against influenza virus infection, while the antibody-mediated immune response remains unaffected.