Lung type II alveolar epithelial cells collaborate with CCR2+inflammatory monocytes in host defense against an acute vaccinia infection in the lungs

SUMMARY The pulmonary immune system consists of a network of tissue-resident cells as well as immune cells that are recruited to the lungs during infection and/or inflammation. How the two immune components cross-talk during an acute viral infection is not well understood. Intranasal infection of mice with vaccinia virus causes lethal pneumonia and systemic dissemination. Here we report that vaccinia host range protein C7 is a critical virulence factor. Vaccinia virus with deletion of C7 (VACVΔC7L) is non-pathogenic in wild-type C57BL/6J mice, but it gains virulence in mice lacking STAT2, or IFNAR1, or MDA5/STING. We provide evidence that lung type II alveolar epithelial cells (AECs) provide first-line of defense against VACVΔC7L infection by inducing IFN-β and IFN-stimulated genes via the activation of the MDA5 and STING-mediated nucleic acid-sensing pathways. This leads to recruitment of CCR2 + inflammatory monocytes into the lungs to fight against viral dissemination.