Identification of CD8⁺ CAR^neg bystander T cells in patients after tisagenlecleucel infusion. (A) UMAP of scRNA-seq data from 6 pediatric patients after tisagenlecleucel, colored by patient ID. (B) UMAP with shared nearest-neighbor clustering across all patients, colored by 15 transcriptional clusters. (C) UMAP colored by cell source, as well as normalized expression of genes from bystander signature 4 (CD8, CD160, CCL5, KLRK1 [NKG2D], and KLRD1 [CD94]). The product was not cytometrically sorted into CAR^pos and CAR^neg T cells and therefore contained a mixed population of CAR^pos and CAR^neg T cells. (D) Composition of clusters by cell source, with an arrow highlighting clusters 0 and 12, which were enriched for bystander signature 1. (E) Bystander score determined by applying bystander signature 1 to each CD8⁺ CAR^neg cluster using decoupleR. (F) Composition of clusters by patient ID, with arrows highlighting clusters 0 and 12, which were enriched for bystander signature 1. (G) Shannon diversity of T-cell clones in bystander clusters 12 and 0 vs all CD8⁺ CAR T cells in each patient.