Primary BCP-ALL cells signal to MSCs via TNTs. (A) Representative confocal images (Z-stack) showing TNT formation (white arrowhead) between a primary CD34-positive cell (DiI, yellow) and a hTERT-immortalized MSC (DiO, green) after coculture for 3 hours. White arrow indicates transfer of dye to recipient cell. (B) Graphs showing quantification of dye transfer in cocultures of BCP-ALL patient cells with primary MSCs (cultured in 4:1 ratio). Inhibition of TNTs was performed by gentle shaking of cocultures, or coculture in a 3.0-μm transwell system (red histograms). (C) Graphs showing quantification of dye transfer from DiI-stained NALM6 cells toward 10 different unstained primary MSCs (cultured in 4:1 ratio) obtained from leukemia patients (n = 5) and healthy controls (n = 5). (D) Graphs showing quantification of dye transfer from DiI-stained NALM6 cells toward 3 different primary MSCs with (red histograms) or without (gray histograms) TNT inhibition (see also supplemental Figures 4-8).