As described previously [9 (link),10 (link)], WI-38 cells were grown until 100% confluent on LabTek II 4-chamber slides (Thermo-Fisher, Waltham, MA, USA). After becoming fully confluent, cells were incubated for a further 72 h to achieve growth arrest. Infections were carried out as described above with a multiplicity of infection (MOI) of 100 for dl309 [14 (link)], or dl1101 through dl1109 [15 (link)]. One hour prior to fixation, cells were pulsed with EdU for 1 h as per manufacturer’s specifications using the Click-It EdU labeling kit for microscopy (Life Technologies, Carlsbad, CA, USA). After EdU labeling, cells were fixed in 3.7% formaldehyde, stained for EdU using the Click-It kit with AlexaFluor 488 and labelled for E1A using M58 monoclonal antibody and AlexaFluor 594 conjugated secondary anti-mouse antibody (Jackson ImmunoResearch). Cells were imaged using Molecular Devices (San Jose, CA, USA) ImageXpress Micro 4 high content imager and automatically analyzed using the MetaXpress software.
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