The Effect of Apoptosis Inhibitor Blockade Agents on the Third Generation CD19 CAR T Cells

Background:

Chimeric antigen receptor (CAR) engineered T cell therapy is currently revolutionizing the field of cancer immunotherapy. However, further enhancement of the function of CAR T cells either through modification of CAR structure or through combination with other therapies is intensively investigated. Resistance to apoptosis is one of the hallmarks of cancer biology. Overexpression of anti-apoptotic proteins of the Bcl-2 family proteins, such as Bcl-2, Mcl-1 and Bcl-xl is considered primarily responsible for the increased apoptosis resistance and prolonged survival of hematological tumor cells. Apoptosis inhibitor blockade agents (AIBAs) such as Bcl-2, Mcl-1 and COX inhibitors could sensitize malignant cells to apoptosis. Therefore, combining CAR T cell therapy with AIBAs might be a promising approach to increase the therapeutic response. However, the third generation of CAR T cells including a 4-1BB co-stimulatory domain which triggers a signaling cascade with the upregulation of anti-apoptotic molecules might be hampered by AIBAs. Therefore, in this study we analyzed the influence of AIBAs on third generation CD19 CAR T cells.

Materials and methods:

CD19 CAR T cells were manufactured using a 3^rd generation CAR vector containing both CD28 and 4-1BB co-stimulatory domains. The expression of anti-apoptotic Bcl-2 family members in leukemia/lymphoma cell lines and in CD19 CAR T cells have been assessed by quantitative real time polymerase chain reaction (qRT-PCR), western blot (WB) and flow cytometery. The optimal concentrations of inhibitors have been determined by CellTiter Glo™ assay. The effect of inhibitors on CAR T cells and tumor cells has been evaluated by chromium-51 (⁵¹Cr) release assay, Calcein AM™ assay and FACS-based apoptosis assay. Intracellular cytokine staining and surface marker staining were performed to determine the function of CAR T cells.

Results:

qRT-PCR and WB showed that leukemia/lymphoma cell lines 380 and U698M had the highest Bcl-2 (qRT-PCR: 1.73 ± 0.04, p = 0.001; WB: 1.82 ± 0.54, p = 0.016) and Mcl-1 (qRT-PCR: 12.39 ± 1.37, p = 0.000; WB: 1.92 ± 1.08, p = 0.142) expression levels, respectively,. These findings were confirmed by anti-apoptotic BCL-2 family protein intracellular staining. Elevated protein expression of Mcl-1, Bcl-2 and Bcl-xl was observed in CAR T cells upon 380 cell stimulation. The manufactured CD19 CAR T cells exhibited specific killing on CD19⁺ tumor cells. A slightly enhanced killing efficiency of CAR T cells was observed when 380 cells were co-cultured with CAR T cells in the presence of ABT199 (0 µM vs. 1 µM: 54.21 ± 3.23 vs. 69.06 ± 2.17, p = 0.0041). This result could be explained by a higher sensitivity of 380 cells to ABT199 than CART cells and by an increased activation of CAR T cells. Even though the killing efficiency of CAR T cells was not improved by S63845, the CD107a⁺TNF-α⁺IFN-γ⁺ CAR T cells were increased as well as the proliferation and persistence of CAR T cells were strengthened. Of note, pretreatment of tumor cells with inhibitors could significantly enhance the killing efficiency of CAR T cells via upregulation of CD19 antigen on tumor cells (ABT199-pretreated 380 cells: 0 µM vs.1 µM, 42.59 ± 5.82 vs. 54.58 ± 4.87, p = 0.0493; S63845-pretreated-U698M cells: 0 µM vs. 0.3 µM, 31.88 ± 4.77 vs. 66.35 ± 8.09, p = 0.000).

Conclusion:

The quantity of CAR T cells can be negatively affected by Apoptosis inhibitor blockade agents. However, the quality of CAR T cells could be improved. The cytotoxic effect of CAR T cells can be further enhanced by pretreatment of tumor cells with inhibitors.