To determine whether MSCs expressing VSV-G are better equipped to fuse with CMs than unmanipulated counterparts, vMSCs or MSCs were cocultured with CMs. To distinguish cell types in cocultures, MSCs and vMSCs were stained with Red Cell-Tracker, while CMs were stained with Green CellTracker fluorescent probes prior to being combined. Since VSV-G undergoes a conformational change from its inactive form to its active form at pH < 6.2 [54, 55] , cocultures were briefly incubated (2 min) with fusion medium of pH = 5.5. Image analysis of vMSC-CM cocultures treated with fusion medium of pH = 5.5 revealed cells with colocalization of green and red fluorescence, indicating fusion events, while MSC-CM cocultures under the same pH condition exhibited limited colocalization (Figure 2(a) ). To accurately assess the amount of cell fusion, cocultures were harvested 24 hours after seeding and analyzed via flow cytometry (double positive cells correspond to fusion events). vMSCs treated with acidic medium (pH 5.5) had significantly higher rates of fusion with CMs (4.7% ± 1.1%) than MSCs treated in the same way (i.e., spontaneous fusion, 1.4% ± 0.2%) (P < 0.05) (Figure 2(b) ). Further, the percentage of fusion products identified in vMSC-CM cocultures exposed to fusion medium of pH = 6.5 or 7.5 (maintaining VSV-G in the inactive form) did not differ from MSC-CM cultures (n = 3 replicates per sample per trial for 3 trials, Figure 2 (c)).