In mice receiving allogeneic HCT using PTCy and CAR T cells on day +5, E2a-PBX relapses uniquely occurred as CD19⁺extramedullary tumors with partial loss of CD19⁺on leukemia within hematopoietic organs. (A) Flow cytometric schema for a representative mouse. (Left) Viable cells were gated off singlets. (Center) From viable cells, H2k^b+H2k^k− E2a-PBX leukemia cells were gated and (right) assessed for CD19 expression. (B-C) E2a-PBX leukemia composed nearly all cells in the extramedullary tumors wherein it was uniformly CD19⁺. By contrast, E2a-PBX was only a small fraction of cells in hematopoietic organs wherein it was CD19⁻ in a subset of cells. (D-F) Photograph and histopathology of paraspinal extramedullary tumors for the mouse in panel A with the histopathology and flow cytometric analyses all coming from different sections of the same tumor. (E) Cross section of vertebral column in the lumbosacral region, demonstrating effacement of paraspinal muscles by lymphoma. Higher magnification shows infiltration of paraspinal muscles by a uniform population of lymphoblasts and invasion of the spinal canal adjacent to spinal nerve roots. (F) Section of skeletal muscle severely infiltrated by a uniform population of lymphoblasts. Higher magnification demonstrates individual myofibers separated by a uniform population of neoplastic lymphoblasts.