Monitoring of Circulating CAR T Cells: Validation of a Flow Cytometric Assay, Cellular Kinetics, and Phenotype Analysis Following Tisagenlecleucel

Chimeric antigen receptor (CAR) T cell therapy is a potent new treatment option for relapsed or refractory hematologic malignancies. As the monitoring of CAR T cell kinetics can provide insights into the activity of the therapy, appropriate CAR T cell detection methods are essential. Here, we report on the comprehensive validation of a flow cytometric assay for peripheral blood CD19 CAR T cell detection. Further, a retrospective analysis (n = 30) of CAR T cell and B cell levels over time has been performed, and CAR T cell phenotypes have been characterized. Serial dilution experiments demonstrated precise and linear quantification down to 0.05% of T cells or 22 CAR T cell events. The calculated detection limit at 13 events was confirmed with CAR T cell negative control samples. Inter-method comparison with real-time PCR showed appreciable correlation. Stability testing revealed diminished CAR T cell values already one day after sample collection. While we found long-term CAR T cell detectability and B cell aplasia in most patients (12/17), some patients (5/17) experienced B cell recovery. In three of these patients the coexistence of CAR T cells and regenerating B cells was observed. Repeat CAR T cell infusions led to detectable but limited re-expansions. Comparison of CAR T cell subsets with their counterparts among all T cells showed a significantly higher percentage of effector memory T cells and a significantly lower percentage of naive T cells and T EMRA cells among CAR T cells. In conclusion, flow cytometric CAR T cell detection is a reliable method to monitor CAR T cells if measurements start without delay and sufficient T cell counts are given.

Automated summary

Chimeric antigen receptor (CAR) T cell therapy is a potent new treatment option for relapsed or refractory hematologic malignancies. Reliable detection methods for CAR T cells are essential for monitoring therapy activity. In this study, a flow cytometric assay was validated for peripheral blood CD19 CAR T cell detection. The assay showed precise and linear quantification, with a detection limit of 0.05% of T cells or 22 CAR T cell events. CAR T cell detection correlated well with real-time PCR, and stability testing showed decreased CAR T cell values within one day after sample collection. The study also observed long-term CAR T cell detectability and B cell aplasia in most patients, but some patients experienced B cell recovery. Repeat CAR T cell infusions led to limited re-expansions. CAR T cells showed a different distribution of T cell subsets compared to all T cells. Overall, flow cytometric CAR T cell detection is reliable if measurements start without delay and sufficient T cell counts are given.