The rise in ABL1 mutations and dasatinib resistance in immunocompromised vs immunocompetent hosts. (A) The pie plot shows the number of mice relapsed with leukemia harboring wild-type (WT) or mutated BCR-ABL1 at the time of death; 71.4% of the relapsed immunocompetent mice had WT BCR-ABL1 B-ALL, whereas 60% to 80% of immunocompromised mice relapsed with leukemia harboring a drug-resistant BCR-ABL1 T315I mutation. (B) Immunocompromised mice relapsed much earlier than immunocompetent mice, mostly during dasatinib treatment (horizontal dash lines). (C) At relapse, ALL cells from immunocompetent hosts were more sensitive to dasatinib than those from immunocompromised mice in vitro. (D) Schematic representation of experimental design and analyses for detecting and tracking ABL1 mutations during dasatinib therapy. Dasatinib was administered at 10 mg/kg twice per day (bid). ABL1 sequence encoding the kinase domain was polymerase chain reaction amplified from genomic DNA extracted from peripheral blood and then subjected to Illumina sequencing. Leukemia burden was quantified by flow cytometry weekly. (E) BCR-ABL1 mutations were enriched shortly after dasatinib treatment was initiated relative to baseline (ie, day 14, immediately before dasatinib treatment) in both immunocompetent and Tcra-KO mice. Brown dots indicate mutations that increased in frequency during dasatinib treatment; x-axis shows the highest mutant allele frequency of each identified mutation in baseline samples, and y-axis shows the highest mutant allele frequency of each identified mutation during dasatinib treatment. (F) The frequency of ABL1 mutations was not related to leukemia growth in the absence of dasatinib treatment; x-axis shows the mutant allele frequency in leukemia samples before inoculation, and y-axis shows the highest mutant allele frequency of each identified mutation in leukemia cells immediately before dasatinib therapy was initiated. (G-H) Mutant allele frequency (left panel) and mutational burden (right panel) are plotted as functions of time in Tcra-KO mice (G) and B6 mice (H). IC₅₀, 50% inhibitory concentration; ns, not significant.