SLAM-independent WT MV infection of II-18 cells. A, monolayers of II-18 and Vero/hSLAM cells were incubated with medium containing control mouse IgG or IPO3 (anti-SLAM mAb) and infected with IC323-Luci at an MOI of 0.01. At 3 h p.i., the fusion block peptide was added to medium, and at 24 h p.i., the Renilla luciferase activity was measured. The activity in cells treated with control mouse IgG was set to 100%. B, monolayers of II-18 cells were infected with IC323-EGFP or IC323/H(Y543S)-EGFP. At 2 days p.i., the monolayers were observed under a phase-contrast or fluorescence microscope. C, monolayers of Vero/hSLAM, II-18, NCI-H358, and HT-29 cells were infected with serially diluted virus samples of the same stock of IC323-EGFP. Titers were measured by counting the numbers of EGFP-expressing cells under a fluorescence microscope. D, monolayers of II-18 cells were infected with IC323-EGFP at an MOI of 0.01. At various days p.i., virus was harvested, and its plaque-forming units were measured in Vero/hSLAM cells. Error bars, S.D.