© F e r r a t a S t o r t i F o u n d a t i o nproducts inhibit replication and spread of MV. Deficiency in suppression of the type I IFN response in normal cells is part of the attenuated phenotype of vaccine strains of MV. 40,41 Many solid cancer cells are known to have a decreased IFN response (reviewed by Pitha 42 ), which can be exploited for oncolytic virotherapy. 43,44 It is unknown whether ALL cells have a deficient type I IFN response impacting on their response to viral infection, whether they harbor defects in the RIG-I/MDA-5 pathway or whether other mechanisms of increased susceptibility to attenuated MV are operative in ALL cells.In this study, we set out to investigate the hitherto unknown susceptibility of pediatric ALL to attenuated MV. Using our large collection of primary pediatric ALL propagated in immunodeficient mice 45 we show that MVEdm is remarkably effective against acute B-lineage ALL in the pre-clinical setting. Methods ALL cell lines, xenografts and patient samplesThe ALL cells lines Jurkat, CCRF-CEM, MOLT-4, REH, RS4;11 and NALM-6 were purchased. ALL cells from patients propagated in non-obese diabetic/severe combined immunodeficient mice (NOD/SCID) mice were procured from spleen tissue at a purity of above 90%. Primary patient ALL samples were obtained at diagnosis from pediatric patients with de novo ALL. Most patients were enrolled in the ALL-BFM study protocols. Patients' characteristics for the xenografts are listed in the Online Supplementary Table S1. The human study protocol was approved by the Ethical Review Board of the University Medical Center Ulm in accordance with the Declaration of Helsinki. Cell lines, xenografts and primary ALL cells were cultured in RPMI 1640 with 10% fetal calf serum, L-glutamine, penicillin and streptomycin. Human ALL NOD/SCID mouse modelNOD/SCID mice at a median age of 8-10 weeks (w) were used. Housing and treatment of animals were in accordance with state guidelines. ALL cells were injected into a lateral tail vein. After grafting, blood samples were evaluated at 1-w or 2-w intervals for human leukemia cells by determining CD45 + Ly5 -cells using flow cytometry. At necropsy cell suspensions from spleen, bone marrow and meninges were prepared and brains were procured. The presence of leukemic cells in the suspensions was determined by FACS analysis. Virus infectionEx vivo cells were infected with MV-Edm at a MOI of 1. ALL cell lines, xenografts, patient samples, peripheral blood mononuclear cells (PBMC), B and T cells were infected in serum-free RPMI 1640 medium at 37°C for 3 hours (h). For hematopoietic stem cells (HSC) serum-free IMDM medium was used. Medium was changed depending on the experiment. Replication of MV-Edm in Jurkat cells and PBMCTo quantitate viral replication in Jurkat cells, lysates were harvested 3, 24, 48 and 72 h after infection and added to Vero indicator cells. Syncytia were determined 72 h later. To compare replication in Jurkat cells to PBMC, lysates of PBMC were collected 72 h after infection and added to Vero ...