Chimeric Antigen Receptor T Cells Specific for CLL-1 for Treatment of Acute Myeloid Leukemia

Chimeric antigen receptor (CAR) T cells are a novel therapeutic approach which have shown good clinical outcomes in patients receiving CD19 CAR T cells for B cell acute lymphoblastic leukemia. CAR T cells are made to express a CAR that recognizes a specific surface antigen on a cell upon which they can then exert cytotoxic effects. We aim to extend the success of this therapy to acute myeloid leukemia (AML), a disease with generally poor clinical outcomes. However, due to the genetic heterogeneity characteristic of AML and the limited number of distinctive tumor markers, it has been difficult to find effective targets for CAR T cells on AML.

C-type lectin like molecule-1 (CLL-1), also known as CD371, is a transmembrane glycoprotein that is expressed on about 90% of AML patient samples. CLL-1 may function as an inhibitory signaling receptor, as it contains an intracellular immunoreceptor tyrosine based inhibitory motif (ITIM). CLL-1 is primarily expressed on myeloid lineage cells in the bone marrow and in peripheral blood. While CLL-1 has been shown to be expressed on some granulocytes in the spleen, it is not reported to be expressed in non-hematopoietic tissues or on hematopoietic stem cells, which make CLL-1 a potential therapeutic target for AML.

We generated two types of CLL-1 CARs, termed A and B, by using two different single chain variable fragments (scFvs) recognizing CLL-1. We used second generation CARs containing the scFvs, CD8 hinge and transmembrane domain, 4-1BB co-stimulatory domain, and CD3 zeta signaling domains. Using a lentiviral vector, we transferred the CAR gene into healthy donor human T cells and detected CAR expression by flow cytometry. We then tested the specific cytotoxic effects of CLL-1 CART-A and B on a CLL-1-expressing AML cell line, U937, by conducting a 4-hour chromium release assay. We found that both CAR T cells exhibited a dose-dependent killing of U937 (CLL-1 positive), while the untransduced (UTD) T cells had no cytotoxic effect (Figure 1A). We also found that U937 induces degranulation of CLL-1 CAR T cells as measured by CD107a expression by flow cytometry, while Ramos, a CLL-1 negative cell line, does not (Figure 1B).

We then proceeded to investigate the in vivo efficacy of the CAR T cells. We injected NOD/SCID/IL2RG-null (NSG) mice with 1 x 10⁶ THP-1 cells, a CLL-1 positive cell line. We confirmed engraftment by bioluminescent imaging (BLI) after 7 days and then injected 4 x 10⁶ UTD, CLL-1 CART-A or CLL-1 CART-B. Surprisingly, only one of the CAR constructs, CLL-1 CART-A, showed significant activity in vivo, although both CARs had shown comparable activity in vitro. CLL-1 CART-A treated mice had delayed tumor progression and significantly increased length of survival (85 days vs. 63 days, p = 0.0021) compared to mice injected with UTD (Figure 1C and D). While CLL-1 CART-B treated mice also exhibited slower tumor growth and a trend towards better survival (72 days vs. 63 days, p=0.0547) this was not statistically significant. Post-mortem analysis showed that human T cells that continued to express CAR were present in the tumor, bone marrow and spleen of mice treated with CLL-1 CART-A only, while the UTD and CLL-1 CART-B treated mice showed tumor in all organs and no T cells.

In summary, we show that CLL-1 CAR T cells can selectively eliminate CLL-1 positive target cells in vitro and in vivo, albeit with different degrees of efficacy modulated by the scFv. Studies are ongoing to investigate the mechanism behind the differential activity of these CAR constructs and to increase the long-term antitumor efficacy. Our results demonstrate that targeting CLL-1 using CAR T cell therapy holds promise for the treatment of AML.

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