CAF-inducing B-ALL cells contain mtdsRNA, which is released and taken up by MSCs. (A) Photomicrographs of confocal images of cells stained with anti-dsRNA monoclonal antibody J2, Deep Red MitoTracker, and DAPI, as well as merged images, as indicated on the left. Cell types and experimental conditions are indicated above the image. (B) MFI (Y axis) after intracellular J2 staining and flow cytometry of the cells indicated on the X axis. HS27a + SD-1 or SEM indicates HS27a cells exposed to the SD-1 or SEM conditioned medium. (C) Agarose gel images showing the products of reverse transcription-PCR for 4 mitochondrial genes, and GAPDH with a 100 base pair ladder (as indicated below the bands) from the experimental conditions indicated at the top of the individual gel images: positive control (SD-1 cells), J2-immunoprecipitated RNA from SD-1 and SEM cells, and negative nontemplate control. (D) Bar chart showing percentage of ALL patient–derived xenograft (PDX) cells (Y axis) positive for dsRNA after intracellular J2 staining and flow cytometry. PDX 1, 4, and 6 are derived from BCR::ABL1+ ALL, and PDX 2, 3, and 5 from KMT2A::AFF4+ ALL. CM, conditioned medium; MFI, mean fluorescence intensity.