CD123-Specific Chimeric Antigen Receptor Redirected T Cells Exhibit Potent Cytolytic Activity and Multiple Effector Functions Against Acute Myeloid Leukemia without Altering Normal Hematopoietic Colony Formation in Vitro

Abstract 950

Current treatment regimes for acute myeloid leukemia (AML) achieve complete remissions in only a subset of individuals and most adult patients will relapse within 5-years, emphasizing the need for novel treatment alternatives. One such therapy may be the administration of T cells engineered to express chimeric antigen receptors (CARs) specific for AML-associated antigens. CARs are typically composed of a single chain variable fragment (scFv) from a monoclonal antibody fused to the CD3ζ signaling domain and may contain one or more costimulatory endodomains. When expressed in T cells, CARs redirect T cell specificity to surface antigens on target cells in an MHC-independent manner. The interleukin 3 receptor alpha chain (IL3Rα, CD123) is a cell surface receptor which is aberrantly over-expressed on multiple hematologic malignancies including AML. Previous work has demonstrated that CD123 is not expressed on all CD34⁺/CD38⁻ hematopoietic stem cells and is restricted to cells of the myeloid lineage, making CD123 an attractive target for CAR T cell therapy. We have therefore generated two novel CD123-specific (CD123R) CARs using scFvs from previously characterized antibodies, designated 26292 and 32716, which bind two distinct epitopes on CD123. Here we demonstrate that T cells expressing CARs derived from either 26292 or 32716 effectively redirect T cell specificity against CD123⁺ cells. Healthy donor T cells (n=3) engineered to express the CD123R CARs efficiently lysed CD123⁺ cell lines LCL and KG1a while sparing the CD123⁻ cell line K562 as demonstrated by a 4 hour chromium-51 (⁵¹Cr) release assay. Additionally, both of the CD123R CAR T cells produced similar levels of IFN-γ and TNF-α and displayed comparable levels of antigen-dependent proliferation following co-culture with CD123⁺ cell lines. The potent cytolytic activity and activation of our CD123-targeting T cells was not limited to tumor cell lines. Indeed, CD123R CAR T cells, but not donor-matched CD19-specific (CD19R) CAR T cells, robustly lysed panel of primary AML samples (n=6, 3 – persistent, 1 – relapsed, 2 - untreated) (* p<0.05, ** p<0.001 using the unpaired students' t-test comparing 26292 or 32716 CAR T cells to donor-matched CD19R CAR T cells), and exhibited multiple effector functions for both CD4 and CD8 T cell subsets (ie CD107a degranulation, IFN-γ and TNF-α production, and antigen specific proliferation) when co-cultured with primary AML samples (n=3, 2 – relapsed, 1-persistent). To examine the effect our CD123-specific T cells have on normal and leukemic progenitor cells, we co-cultured CD123R CAR T cells, or donor-matched CD19-targeting T cells, with either CD34-enriched cord blood (CB, n=3) or primary AML samples (n=3, 2 – relapsed, 1 - untreated) for 4 hours (E:T 25:1) prior to plating in semisolid methylcellulose progenitor culture. CD123-targeting T cells did not significantly reduce the number of colony-forming unit granulocyte-macrophage (CFU-GM) or burst-forming unit erythroid (BFU-E) colonies from CB when compared to CD19R CAR T cells. Finally, while CD19-specific T cells had little impact on leukemic colony formation of primary AML samples, CD123-targeting T cells significantly reduced leukemic colony formation in vitro.

Collectively, our data demonstrate that CD123-specifc CARs can be expressed in primary healthy donor T cells, distinguish between CD123⁺ and CD123⁻ cells, and mediate robust anti-leukemic activity against a panel of poor-risk primary AML patient samples. Importantly, we demonstrate that CD123R CAR T cells have little impact on normal progenitor colony formation while significantly reducing the growth of clonogenic myeloid leukemic progentiors in vitro. Thus, CD123R CAR T cells are a promising candidate for future immunotherapy of AML.

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