G-CSF/Plerixafor Dual-Mobilized Donor Derived CD33CAR T-Cells As Potent and Effective AML Therapy in Pre-Clinical Models

There are currently no known acute myeloid leukemia (AML) specific antigens. Genetic ablation of CD33 using CRISPR/Cas9 engineering of the hematopoietic stem cell (HSC) transplant (VOR33) represents a synthetic biology approach to generating a leukemia-specific antigen in the transplant recipient. VOR33 enables anti-CD33 CAR-T cell killing of AML cells while sparing normal myeloid lineage development and function, thereby potentially avoiding myelosuppression and increasing the therapeutic index of anti-CD33 CAR-T therapy. Mobilized leukapheresis product represents an attractive starting material for the generation of both a CD33 null HSC transplant and a complementary CD33CAR T-cell product. In this study, we sought to determine the impact of dual mobilization with Granulocyte-Colony Stimulating Factor (G-CSF) and plerixafor (mozobil) on immune cell composition, T cell phenotype, and the functionality of these T cells to control AML tumor growth upon chimeric antigen receptor (CAR) transduction.

Mobilized (mob) peripheral blood mononuclear cells (PBMCs) were collected from healthy donors injected with G-CSF (10µg/kg/day, 5 consecutive days) and plerixafor (240µg/kg, on day 4 and 5). Non-mobilized (non-mob) PBMCs, collected from the same donors, were used as controls. Cells were analyzed by flow cytometry for immunophenotyping and T cell characterization including differentiation and bone marrow homing markers, as well as responses to T cell activation with anti-CD3 (OKT3) and IL-2. Non-/mob PBMC populations were also analyzed by single-cell next generation sequencing (CITEseq) using 127 immune cell phenotypic markers in combination with extensive transcriptome and T cell receptor repertoire analysis. In addition, lentiviral transduction of anti-CD33 CAR constructs enabled functional comparisons of mob- and non-mob-CAR T-cells in AML cell co-cultures as well as AML mouse models.

Ex vivo immunophenotyping of PBMC from a total of over 30 healthy donor samples showed that mobilization decreases the overall percentage of CD3 ⁺ T cells but increases that of naïve T cells (CD45RA ⁺/CCR7 ⁺), at the expense of T effector-memory (CD45RA ^-/CCR7 ^-) and central-memory (CD45RA ^-/CCR7 ⁺) populations. Bone marrow homing factors (e.g.: CXCR4) were increased in mob compared to non-mob T cell samples. As expected, higher percentages of monocytes (CD14 ⁺) were detected in mob compared to non-mob donor samples, but this difference disappeared after culture under T cell activation conditions. T cell activation also led to similar increases in CD25, CD69 and CD137 expression, and a decrease in CD62L expression. Single cell sequencing analyses confirmed mobilization-induced increases in naïve T cells as well as changes in monocytes/macrophages, CD4 ⁺ T cells and NK cells percentages. Notably, functional in vitro cytotoxic assays demonstrated that mob-CD33-CAR T-cells are as effective as non-mob-CD33-CAR T-cells in killing CD33 ⁺ AML cells, with reduced 'bystander' activation of non-transduced T cells. Furthermore, results from in vivo AML mouse models indicate that mob-CD33-CAR T-cells are equally effective in clearing CD33 ⁺ tumors as non-mob-CD33-CAR T-cells.

Our analysis showed phenotypical ex vivo differences between mob and non-mob PBMCs, which disappeared upon activation, indicating similar responses to T cell-specific stimulation. These findings are corroborated by similar in vitro cytotoxicity profiles of non-/mob-CAR T-cells. Non-transduced T cells in the mob-CAR T-cell population showed limited 'bystander' activation, indicating a potentially favorable clinical toxicity profile. Additional in vivo assessment of mob-CAR T-cell function shows effective tumor clearance, which supports further efforts towards their clinical use in combination with engineered HSCs for the treatment of AML patients.