Secondary Haemophilus parasuis infection enhances highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) infection-mediated inflammatory responses

•HP-PRRSV infection induced robust inflammatory responses and secondary bacterial infection in vivo.•HP-PRRSV and H. parasuis coinfection cell model confirmed that H. parasuis significantly enhanced HP-PRRSV infection-mediated inflammatory responses in PAMs.•H. parasuis RNA plays an important role for the inflammatory response enhancement in PRRSV-infected PAMs. Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) infection often predisposes pigs to secondary bacterial infection, which induces robust inflammatory responses. However, whether the secondary bacterial infection synergizes HP-PRRSV infection and enhances inflammatory responses is not fully understood. Here, we characterized HP-PRRSV infection-mediated secondary bacterial infection and robust inflammatory responses. HP-PRRSV infection induced higher levels of cytokines (IL-1β, IL-18, IL-6 and TNF-α) in the sera in piglets and bacterial loads of 11 bacterial species in the lung were increased after HP-PRRSV infection, including Mycoplasma hyorhinis, Haemophilus parasuis and Escherichia coli. Concurrent infection with HP-PRRSV and H. parasuis model showed that inflammatory cytokines expression and secretion in porcine alveolar macrophages (PAMs) were increased in comparison with PAMs infected with HP-PRRSV or H. parasuis alone. Additionally, we found that H. parasuis RNA plays an important role in the robust inflammatory response enhancement in HP-PRRSV-infected PAMs. Taken together, our findings suggest that bacterial RNA transfection enhanced HP-PRRSV-mediated inflammatory responses in HP-PRRSV and H. parasuis (HPS) concurrent infection, which provides an important clue for comprehensive understanding of HP-PRRSV and bacterial coinfection-mediated pathology.