Quantification of Bioluminescence Imaging

Quantification of BLI signal was performed as previously described^{52 (link)}. The Living Image software 4.4 (Caliper Life Sciences, Mainz, Germany) was used for data acquisition and quantification of light emission using a scale with a minimum of 1.8×10⁴ photons per second per cm2 per solid angle of one steradian (sr). Different regions of interest (ROI) were defined and signals were considered positive, when light emission exceeded background in each ROI. Background was measured in mice harboring GLuc negative leukemias. A ROI covering the entire animal was used (background 4×10⁶ photons per second). As an exception to determine early engraftment, a small ROI covering the femurs was used (background 6×10⁴ photons per second), as light emission became visible there first. Overt leukemia was considered above 10¹⁰ photons per second using the ROI covering the entire animal.