Murine alveolar macrophages limit replication of vaccinia virus

Because of concerns about zoonotic transmission of monkeypox to humans and the bioterrorism threat posed by orthopoxviruses, there is renewed interest in probing cellular and molecular mechanisms of host defense to these pathogens. In particular, it is essential to understand viral–host interactions in the respiratory tract, which is the route of infection for smallpox and a likely route of transmission for monkeypox. In this study, we analyze functions of alveolar macrophages in poxvirus infection, using a recombinant vaccinia virus expressing firefly luciferase to quantify infection in mice and cell culture. Depletion of alveolar macrophages with liposomal clodronate worsens the overall severity of infection in mice, including greater replication and systemic dissemination of vaccinia as determined by bioluminescence imaging. Absence of alveolar macrophages increases total numbers of granulocytes and granulocytes/monocyte progenitor cells in the lungs during vaccinia infection, indicating that protective effects of alveolar macrophages may be mediated in part by reducing the host inflammation. Alveolar macrophages also limit vaccinia infection in respiratory epithelium, as shown by a co-culture model of cell lines derived from alveolar macrophages and lung epithelium. Collectively, these data demonstrate that alveolar macrophages are key determinants of host defense against local and systemic infection with poxviruses.