The challenge of standardizing CAR‐T cell monitoring: A comparison of two flow‐cytometry methods and correlation with qPCR technique

Abstract Chimeric antigen receptor (CAR) T‐cell therapy is a breakthrough in hematologic malignancies, such as acute B lymphoblastic leukemia (B‐ALL). Monitoring this treatment is recommended, although standardized protocols have not been developed yet. This work compares two flow cytometry monitoring strategies and correlates this technique with qPCR method. CAR‐T cells were detected by two different flow‐cytometry protocols (A and B) in nine blood samples from one healthy donor and five B‐ALL patients treated with Tisagenlecleucel (Kymriah®, USA). HIV‐1 viral load allowed CAR detection by qPCR, using samples from seven healthy donors and nine B‐ALL patients. CAR detection by protocol A and B did not yield statistically significant differences (1.9% vs. 11.8% CD3 + CAR+, p = 0.07). However, protocol B showed a better discrimination of the CD3 + CAR+ population. A strong correlation was observed between protocol B and qPCR ( r = 0.7, p < 0.0001). CD3 + CAR+ cells were detected by flow cytometry only when HIV‐1 viral load was above 10 4 copies/mL. In conclusion, protocol B was the most specific flow‐cytometry procedure for the identification of CAR‐T cells and showed a high correlation with qPCR. Further efforts are needed to achieve a standardized monitoring approach.