Chimeric antigen receptor (CAR) T-cells can eliminate acute myeloid leukaemia (AML) blasts, but also cause haematological toxicity due to presence of leukaemia associated antigens on normal haemopoietic progenitor cells. Targeting aberrant combinations of 2 or more AML antigens may improve specificity and reduce on-target, off-tumor effect. Developing optimal constructs to target individual antigens is an essential first step in this process. We developed separate CARs against CD13 and CD33 and explored the effect of varying the spacer domains on CAR expansion kinetics, phenotype, cytokine production and cytotoxicity.
Constructs encoding antibody based scFv against CD13 or CD33, combined with CD28 co-stimulatory and CD3z activation domains were synthesized and cloned into the piggyBac transposon system for gene modification. Differing lengths of an IgG1 constant region-based spacer between the scFv and the cell surface were assessed for effect on CAR function. An IgG1 hinge only spacer was compared to a spacer with hinge and CH2CH3 constant regions. CAR T-cells generated from healthy donor peripheral blood mononuclear cells (PBMCs) were enriched and expanded over 3 weeks with IL-15 and weekly exposure to irradiated PBMCs and TF-1 leukaemia cell line. At the end of culture, CAR T cells were phenotyped and analysed for specificity by measuring cytokine release following re-exposure to TF-1 cells. Cytotoxicity was assessed by release of calcein following labelling of targets with calcein-AM.
Mean expansion of 33- to 130- folds were achieved with a mean CAR expression of 67 to 87% of CD3+ cells with preponderance of naïve and memory T cells. CAR-T cells generated from all 4 constructs demonstrated antigen-specific interferon-γ and TNF-α release and cytotoxicity. The effector:target ratio required for 50% lysis of targets (ED50) are: CD13 CARs - 1.5 (CH2CH3 mutant spacer) and 2 (hinge-only spacer); CD33 CARs - 8 (CH2CH3 mutant) and 40 (hinge-only).
All 4 constructs screened were effective with specific targeting of CD13 and CD33 containing cell lines. The CH2CH3 mutant containing constructs are likely more efficacious. These could form the basis of combinatorial CAR T cells targeting more than one AML antigens to reduce off-tumour toxicity in vivo.
Clinical trial identification
Legal entity responsible for the study
Sydney West Translational Cancer Research Centre.
All authors have declared no conflicts of interest.