Complete loss of T cells significantly increased leukemia relapse after ALL chemotherapies. (A,B,D,E,G,H) Leukemia burden is plotted as a function of time for B6 mice (blue curves) (A,D,G) and Tcra-KO mice (red curves) (B,E,H) after BCR-ABL1 leukemia cell inoculation. Animals were treated with dasatinib (A,B), dasatinib plus dexamethasone (D,E), or 6-MP (G,H). Shaded areas indicate the duration of chemotherapy. (C,F,I) Kaplan-Meier survival curves are plotted for immunocompetent B6 and Tcra-KO mice treated with dasatinib (C), dasatinib plus dexamethasone (F), or 6-MP (I). Dasatinib was administered at 10 mg/kg 2 times per day; dexamethasone was administered at 6 mg/L in drinking water for the first week and reduced to 3 mg/L thereafter; 6-MP was administered by daily intraperitoneal injection at 5 mg/kg. The differences in overall survival were detected by log-rank (Mantel-Cox) test. Of 9 immunocompetent mice receiving 6-MP, 5 were euthanized because of overall health concerns identified by the St. Jude Children’s Research Hospital Institutional Animal Care and Use Committee, at which time leukemia blast was undetectable in peripheral blood, and spleen size was normal by visual inspection. Therefore, their causes of death were likely unrelated to leukemia.