CD19 and acute lymphoblastic leukemia: Further analyses revealed that the observed CD19-independent toxicity was attributed to the binding of the IgG1-CH2 spacer domain (which contains fragment c gamma receptor (FcγR)-binding motifs) of the CAR construct with soluble FcγR-1 of the macrophages of ALL mice, which caused off-target CAR T cell activation towards the mouse macrophages.