LUSV reduces leukemic burden in MRD in vivo models of BCP-ALL and T-ALL. (A-C) Immunodeficient mice were transplanted with PDX cells from 2 different TCF3::PBX1⁺ patients (63.0% and 89.6% CD127⁺ blasts, respectively) and treated with LUSV (5 mg/kg) or a control vehicle (n = 10, respectively) starting the day after injection, modeling an MRD-like situation (IV treatment on day +1, +3, +7, and +14 and every 14 days thereafter as described previously¹²). (A) PB of both, control and LUSV-treated animals was withdrawn when the first PDX mouse showed signs of overt leukemia and compared for the ratio of hCD45⁺/hCD19⁺/mCD45^– cells in the PB as measured via flow cytometry, unpaired 2-sided t test. Animals not showing clinical signs of overt leukemia or >70% PBBs at this time point received further treatment until reaching termination criteria. (B) Therapy-associated differences in the survival of NSG mice were determined using Kaplan-Meier log-rank statistics. (C) The experiment was terminated after 170 days, and BM samples of euthanized animals were analyzed for MRD by PCR for patient–specific Ig/T-cell receptor rearrangements. Overt ALL = mouse showed clinical signs of leukemia upon euthanization; neg, negative (below detection limit). (D-F) A phase 2-like PDX study was performed using CD127-high (≥50% CD127⁺ cells) T-ALL PDX samples (n = 8 patients) including 3 samples from R/R disease. Two NSG mice per patient were injected with PDX cells, randomly assigned into treatment groups and LUSV therapy was conducted in an MRD-like setting. (D) Experimental setup and therapy scheme. (E) Blood of both, control and LUSV-treated animals bearing the same PDX sample was withdrawn when one of the 2 PDX mice showed signs of overt leukemia and the number of hCD45⁺/hCD19⁺/mCD45^– cells in the PB was measured via flow cytometry. The waterfall plot shows the difference in PBBs between respective control and LUSV-treated mice (sorted from weakest therapy response to highest therapy response). Animals not showing clinical signs of overt leukemia or >70% PBBs at this time point received further treatment until reaching termination criteria. (F) Therapy-associated differences in the survival of NSG mice was determined using Kaplan-Meier log-rank statistics. (G-H) Cells from 1 T-ALL PDX (T-ALL PDX number 1) were injected into replicate mice (n = 3 for no-treatment group [control]; n = 5 for treatment groups), animals were subjected to chemotherapy upon 10% PBBs and LUSV immunotherapy (chemotherapy + LUSV) vs mock treatment (chemotherapy only) was initiated upon reoccurrence of PBB (termed “postchemo-MRD” model).²³ (G) Schematic depiction of the experimental setup of the postchemotherapy MRD model. (H) Therapy-associated differences in the survival of NSG mice was determined using Kaplan-Meier log-rank statistics. One mouse in the chemotherapy only group died due to procedural complications. The experiment was terminated after 130 days. ∗P < .05; ∗∗P < .01; ∗∗∗P < .001; ∗∗∗∗P < .0001. n.r., not reached.