MV infected BM-MSCs permit fusion and virus hand-off to target cells in the presence of anti-MV antibody. (A) Graphs show Vero cell syncytia quantification at 48 hours postoverlay with MV-NSe–loaded BM-MSCs or naked MV-NSe pretreated with dilutions of high titer anti-MV antibody. Total numbers of syncitia from each well of a 96-well plate were counted. Data are represented as a percentage of virus control (MV-infected cells with no serum pretreatment; x-axis) in relation to serum dilution (y-axis) for naked MV and MV-infected BM-MSCs. Mean ± SEM. N = 6 from 3 independent experiments. (B) Representative live cell confocal images of BM-MSCs at 48 hpi with MV-NSe-GFP (MOI of 1.0), cocultured with Nalm 6 cells transduced with RFP. Images show the different stages of a fusion event between a Nalm 6 cell (red) and an infected BM-MSC (green). Scale bar represents 10 μm. (C) (i) Graph shows the percentage of Nalm 6 cells fusing (dark gray bars) or not (light gray bars) after establishing contact with infected BM-MSCs within 80 minutes of coculture (first column). Columns 2 to 4 represent the test and control conditions, with dilutions of serum or FIP, as indicated. **P < .01; *P < .05; NS = P > .05. (ii) Graph shows the increase of red fluorescence in green syncytia over 80 minutes of coculture. Columns are as for 4Ci. (D) Graph shows the contact time between Nalm 6 cells and BM-MSCs before fusion occurs, within 80 minutes of coculture. When indicated, cells were also preincubated with anti-MV antibody serum 1:128. For each of the above conditions, n > 125 cells were counted, and data are from 6 independent experiments.