“Epitope masking” occurs in healthy T cells expressing signaling-deficient CARs. (A) Schematic representing the proposed mechanism of “epitope masking” by the CAR on antigen-expressing cells. Relative change of TCRvβ surface expression (B) or genomic DNA (C) in healthy donor T cells engineered to express TCRvβ12-CARs with or without stimulatory domains compared with NTD T cells. All experiments were run with 2 donors. Data shown are from 1 representative donor.