Anti-Spike but not control serum triggered infection of human macrophages by SARS-CoVpp. (A) Monocyte-derived macrophages were incubated for 1 hour with SARS-CoVpp carrying a luciferase reporter gene in the presence of a 1:1000 dilution of either anti-Spike or control serum and infected cells were detected by fluorescence staining of firefly luciferase (green), as described under Materials and methods. Cell nuclei were labeled with DAPI (blue). SARS-CoVpp infection of macrophages occurred only in the presence of anti-Spike serum. Images are representatives of five independent experiments using macrophages from five donors. (B) Infection was quantified by counting the percentage of immunofluorescence-positive cells in randomly chosen fields using Metamorph software. Results are shown as means ± SEM. Scale bar: 20 μm.