Patient 2 clinical summary, PET scans, and histological features of the CAR T-cell malignancy. (A) Timing of CAR T-cell infusions (red arrows), peripheral blood CAR T-cell quantitation (solid black line), chronic graft-versus-host disease (cGVHD), and infections (colored bars) over time. (B) PET scans showing the left para-aortic lymph node tumor seen from just prior to the third CAR T-cell infusion. (C) Morphology and immunohistochemical staining of tumor biopsy showing aberrant T cells with scanty cytoplasm and enlarged hyperchromatic nuclei, with low/negative CD3 and high CD2, CD4, and Ki67 with moderate CD56 staining. The Ki-67 proliferation index was 99%. The cells were negative for CD5, CD7, CD8, BCL-2, and CD10. Specific stains are shown above each image; original magnifications ×400 for H & E and ×50 for immunohistochemistry images. (D) Flow cytometry assessment of CAR expression on live T cells directly isolated from a biopsy of the para-aortic tumor showing that aberrant CD4⁺ T cells (purple) with reduced CD3 expression predominantly express CAR compared with a small CD8⁺ T-cell population (green) with normal CD3 expression and nil CAR expression. 7AAD, 7-amino actinomycin D; BKV, BK virus; CMV, cytomegalovirus; H. influenzae, Haemophilus influenzae; H&E, hematoxylin and eosin; HHV6, human herpesvirus 6; PCR, polymerase chain reaction.