Evaluation of the impact of scFv on functionality and safety of 3rd generation CD123 CAR T cells

Chimeric Antigen Receptor T cells express an extracellular domain consisting of a single-chain fragment variable (scFv) targeting a surface tumor-associated antigen (TAA), it is important to select scFv regarding safety profile through evaluation of the efficacy/toxicity balance, especially in cases where the target antigen is also expressed on healthy cells. Here, we developed five CAR T cells targeting CD123 (CD123 CAR-T) by substituting only the scFv, looking for differences in terms of efficacy and on-target/off-tumor effects. Using in vitro models, we found that three of the five CD123 CAR-T were more cytotoxic against leukemic cells, without increasing lysis of monocytes or endothelial cells. Using the Incucyte system, we confirmed the low cytotoxicity of CD123 CAR-T on endothelial cells. Hematotoxicity evaluated by progenitor culture and CD34 cell lysis showed that two of the five CD123 CAR-T were less cytotoxic against HSC, and we confirmed in a humanized mouse model that CD123- cells were not eliminated by the CD123 CAR-T. These two CD123 CAR-T reduced tumor infiltration and increased overall survival of mice in three in vivo models of BPDCN. Moreover, in BPDCN aggressive model, bulk RNA-sequencing analysis showed that these CD123 CAR-T upregulated genes associated with cytotoxicity and activation/exhaustion a few days after the injection. Finally, we were able to produce CD123 CAR-T selected previously from patient T cells. Together, these results emphasize the importance of screening different scFv for the development of CAR-T constructs, to select the one with the optimal risk-benefit ratio for clinical development.