Optimization and validation of in vivo flow cytometry CAR‐T cell detection method using CD19his indirect staining

Abstract CD19‐targeted chimeric antigen receptor T (CAR‐T) cell therapy has shown unprecedented results in patients with B cell relapsed/refractory acute lymphoblastic leukemia (R/R‐ALL) and B cell non‐Hodgkin lymphomas where no other curative options are available. In vivo monitoring of CAR‐T cell kinetics is fundamental to understand the correlation between CAR‐T cells expansion and persistence with treatment response and toxicity development. The aim of this study was to define a robust, sensitive, and universal method for CAR‐T cell detection using flow cytometry. We set up and compared with each other three assays for CD19 CAR‐T cell detection, all based on commercially available reagents. All methods used a recombinant human CD19 protein fragment recognized by the single‐chain variable fragment of the CAR construct. The two indirect staining assays (CD19his + APC‐conjugated antihistidine antibody and CD19bio + APC‐conjugated antibiotin antibody) showed better sensitivity and specificity compared with the direct staining with CD19‐FITC, and CD19his had a better cost‐effective profile. We validated CAR detection with CD19his with parallel quantitative real‐time polymerase chain reaction data and we could demonstrate a strong positive correlation. We also showed that CD19his staining can be easily included in a multicolor flow cytometry panel to achieve additional information about the cell phenotype of CAR‐T cell positive subpopulations. Finally, this method can be used for different anti‐CD19 CAR‐T cell products and for different sample sources. These data demonstrate that detection of CAR‐T cells by CD19his flow cytometry staining is a reliable, robust, and broadly applicable tool for in vivo monitoring of CAR‐T cells.